@article{briggs_lu_bernholc_2024, title={Adaptive finite differencing in high accuracy electronic structure calculations}, volume={10}, ISSN={["2057-3960"]}, url={https://doi.org/10.1038/s41524-024-01203-y}, DOI={10.1038/s41524-024-01203-y}, abstractNote={Abstract}, number={1}, journal={NPJ COMPUTATIONAL MATERIALS}, author={Briggs, E. L. and Lu, Wenchang and Bernholc, J.}, year={2024}, month={Jan} }
 @article{rui_bernholc_zhang_zhang_2024, title={Dilute Nanocomposites: Tuning Polymer Chain Local Nanostructures to Enhance Dielectric Responses}, volume={2}, ISSN={["1521-4095"]}, url={https://doi.org/10.1002/adma.202311739}, DOI={10.1002/adma.202311739}, abstractNote={Abstract}, journal={ADVANCED MATERIALS}, author={Rui, Guanchun and Bernholc, Jerzy and Zhang, Shihai and Zhang, Qiming}, year={2024}, month={Feb} }
 @article{liu_zhou_qin_yang_chen_li_han_wang_zhang_lu_et al._2023, title={Electro-thermal actuation in percolative ferroelectric polymer nanocomposites}, volume={5}, ISSN={["1476-4660"]}, DOI={10.1038/s41563-023-01564-7}, abstractNote={The interconversion between electrical and mechanical energies is pivotal to ferroelectrics to enable their applications in transducers, actuators and sensors. Ferroelectric polymers exhibit a giant electric-field-induced strain (>4.0%), markedly exceeding the actuation strain (≤1.7%) of piezoelectric ceramics and crystals. However, their normalized elastic energy densities remain orders of magnitude smaller than those of piezoelectric ceramics and crystals, severely limiting their practical applications in soft actuators. Here we report the use of electro-thermally induced ferroelectric phase transition in percolative ferroelectric polymer nanocomposites to achieve high strain performance in electric-field-driven actuation materials. We demonstrate a strain of over 8% and an output mechanical energy density of 11.3 J cm -3  at an electric field of 40 MV m -1  in the composite, outperforming the benchmark relaxor single-crystal ferroelectrics. This approach overcomes the trade-off between mechanical modulus and electro-strains in conventional piezoelectric polymer composites and opens up an avenue for high-performance ferroelectric actuators.}, journal={NATURE MATERIALS}, author={Liu, Yang and Zhou, Yao and Qin, Hancheng and Yang, Tiannan and Chen, Xin and Li, Li and Han, Zhubing and Wang, Ke and Zhang, Bing and Lu, Wenchang and et al.}, year={2023}, month={May} }
 @article{chen_qin_zhu_zhang_lu_bernholc_zhang_2023, title={Giant Electrostriction Enabled by Defect-Induced Critical Phenomena in Relaxor Ferroelectric Polymers}, url={https://doi.org/10.1021/acs.macromol.2c01952}, DOI={10.1021/acs.macromol.2c01952}, abstractNote={Polymers that generate large shape changes under electric stimulation are of great interest for many applications. Recently, it was shown that converting a small amount of chlorofluoroethylene (CFE) in relaxor ferroelectric poly(vinylidene fluoride–trifluoroethylene–CFE) (PVDF-TrFE-CFE) terpolymer into fluorinated alkyne (FA) creates P(VDF-TrFE-CFE-FA) tetrapolymers with giant electromechanical (EM) response at ultralow electric fields (<50 MV/m). We investigate the microscopic origin of this effect and show that converting the bulky CFE into small-size FA defects dramatically weakens the relaxor behavior. Importantly, P(VDF-TrFE-CFE-FA) tetrapolymers with near 2 mol % FA exhibit a diffused critical endpoint transition region at which the energy barriers for switching from nonpolar to polar molecular conformations become small. Consequently, a small change of the electric field induces a large electroactuation, which can enable novel applications. This work opens up a totally new approach to designing ferroelectric polymers that generate large responses at ultralow electric fields.}, journal={Macromolecules}, author={Chen, Xin and Qin, Hancheng and Zhu, Wenyi and Zhang, Bing and Lu, Wenchang and Bernholc, J. and Zhang, Q. M.}, year={2023}, month={Jan} }
 @article{chen_qin_liu_lin_zhang_lu_kim_bernholc_wang_zhang_2023, title={Interfacial origin of dielectric constant enhancement in high-temperature polymer dilute nanocomposites}, volume={122}, ISSN={["1077-3118"]}, url={https://doi.org/10.1063/5.0143938}, DOI={10.1063/5.0143938}, abstractNote={The origin of dielectric constant enhancement in high-temperature (high glass transition temperature Tg) polymer dilute nanocomposites is investigated via Infrared (IR) Spectroscopy applied through Atomic Force Microscope (AFM) and density functional theory (DFT) calculations. The dielectric constant can be greatly enhanced by trace nanofiller loadings (<0.5 vol. %) in a broad class of high-temperature polymers without affecting or even with a positive influence on breakdown strength and dielectric loss. This avenue provides attractive polymer systems for high-performance polymer-based capacitive energy storage in a wide temperature range. In the dilute nanocomposites, the interface regions between the polymers and trace nanofillers are the key to the observed dielectric constant enhancement. This Letter employs AFM-IR to study chain packing in the interface regions of polyetherimide (PEI) dilute nanocomposites. The experimental results and DFT calculations indicate that flexible linkages, i.e., ether groups in PEI, play a crucial role in inducing heterogeneous morphologies in the interface regions. These results are confirmed by studies of PI(PDMA/ODA) and other dilute polymer nanocomposites in the literature as well as by lack of dielectric constant enhancement in PI(Matrimid® 5218) that does not contain flexible linkages.}, number={21}, journal={APPLIED PHYSICS LETTERS}, author={Chen, Xin and Qin, Hancheng and Liu, Yang and Lin, Yen-Ting and Zhang, Bing and Lu, Wenchang and Kim, Seong H. H. and Bernholc, J. and Wang, Qing and Zhang, Q. M.}, year={2023}, month={May} }
 @article{han_qin_chen_xu_liu_bernholc_wang_2023, title={Microstructures and Ferroelectric Properties of Poly(vinylidene fluoride-ter-trifluoroethylene-ter-vinyl fluoride) Terpolymers}, volume={6}, ISSN={["1520-5835"]}, url={https://doi.org/10.1021/acs.macromol.3c00207}, DOI={10.1021/acs.macromol.3c00207}, abstractNote={The incorporation of comonomers as structural defects has been extensively exploited to modulate the structures and physical properties of poly(vinylidene fluoride) (PVDF)-based ferroelectric polymers. Previous studies have shown that the large-sized comonomers can induce relaxor ferroelectricity in the PVDF-based terpolymers as a result of the stabilization of the helical conformation. Here, we show that, with incorporating a small-sized defect, vinyl fluoride (VF), into the copolymer, the resulting terpolymers preferentially adopt the polar all-trans conformation, as revealed by the structural characterizations and confirmed by density functional theory simulations. Consequently, the terpolymers exhibit robust ferroelectricity with an increased ferroelectric-to-paraelectric transition temperature and a higher coercive field as the VF content increases. This work systematically investigates the compositional dependence of the microstructures, crystalline structures, and ferroelectric and electromechanical properties of the terpolymers and stresses the vital role of comonomer size in tailoring the structures and properties of ferroelectric polymers.}, journal={MACROMOLECULES}, author={Han, Zhubing and Qin, Hancheng and Chen, Xin and Xu, Wenhan and Liu, Yang and Bernholc, J. and Wang, Qing}, year={2023}, month={Jun} }
 @article{qin_lu_bernholc_2022, title={Ab initio simulations of metal contacts for graphene-based devices}, volume={131}, ISSN={["1089-7550"]}, url={https://doi.org/10.1063/5.0091028}, DOI={10.1063/5.0091028}, abstractNote={The precise atomic structure of a metal contact significantly affects the performance of nanoscale electronic devices. We use an accurate, DFT-based non-equilibrium Green’s function method to evaluate various metal contacts with graphene or graphene nanoribbons. For surface metal contacts not chemically bound to graphene, Ti contacts have lower resistance than those of Au, Ca, Ir, Pt, and Sr. However, as an edge contact, Ti has larger resistance than Au. Bridging O atoms at Ti and Au edge contacts lowers the transmission by over 30%.}, number={21}, journal={JOURNAL OF APPLIED PHYSICS}, author={Qin, Hancheng and Lu, Wenchang and Bernholc, J.}, year={2022}, month={Jun} }
 @article{zhou_xu_zhang_zhang_shen_xu_liu_bertram_bernholc_jiang_et al._2022, title={Curly-Packed Structure Polymers for High-Temperature Capacitive Energy Storage}, volume={34}, ISSN={["1520-5002"]}, url={https://doi.org/10.1021/acs.chemmater.1c04220}, DOI={10.1021/acs.chemmater.1c04220}, abstractNote={Polymer film capacitors are ubiquitous in modern electronics and electric systems, but the relatively low working temperatures of polymer dielectrics limit their application in next-generation capacitors. The currently reported high-temperature polymer dielectrics rely on the construction of nanocomposites with wide band gap fillers and cross-linked networks to achieve high breakdown strength and high efficiencies. However, generating the optimal chain structure with intrinsic great high-temperature capacitive properties using a one-component polymer is still challenging. Herein, a giant discharged energy density in neat polymer has been demonstrated in a series of linear poly(arylene ether amide) (PNFA) at 150 °C, which greatly surpass all the current free-standing dielectric polymer films measured in 10 Hz. The maximum discharged energy density with efficiency above 90% of the PNFA is 2.7 J cm–3, which is about 3 times that of the state-of-the-art commercial high-temperature polymer films. The architectures of the amorphous polymers have been identified by synchrotron X-ray diffraction combined with density functional theory calculations. The origins of superior high-temperature capacitive properties are traced to the increased packing density by the curly-packed chain structure. In addition, the reported polymer could be produced using existing industrial-grade processes, which are economical and practical for large-scale applications.}, number={5}, journal={CHEMISTRY OF MATERIALS}, publisher={American Chemical Society (ACS)}, author={Zhou, Chenyi and Xu, Wenhan and Zhang, Bing and Zhang, Yunhe and Shen, Chen and Xu, Qinfei and Liu, Xin and Bertram, Florian and Bernholc, Jerzy and Jiang, Zhenhua and et al.}, year={2022}, month={Mar}, pages={2333–2341} }
 @article{lupke_pham_zhao_zhou_lu_briggs_bernholc_kolmer_teeter_ko_et al._2022, title={Local manifestations of thickness-dependent topology and edge states in the topological magnet MnBi2Te4}, volume={105}, ISSN={["2469-9969"]}, url={https://doi.org/10.1103/PhysRevB.105.035423}, DOI={10.1103/PhysRevB.105.035423}, abstractNote={The interplay of non-trivial band topology and magnetism gives rise to a series of exotic quantum phenomena, such as the emergent quantum anomalous Hall (QAH) effect and topological magnetoelectric effect. Many of these quantum phenomena have local manifestations when the global symmetry is broken. Here, we report local signatures of the thickness dependent topology in intrinsic magnetic topological insulator MnBi$_2$Te$_4$(MBT), using scanning tunneling microscopy and spectroscopy on molecular beam epitaxy grown MBT thin films. A thickness-dependent band gap with an oscillatory feature is revealed, which we reproduce with theoretical calculations. Our theoretical results indicate a topological quantum phase transition beyond a film thickness of one monolayer, with alternating QAH and axion insulating states for even and odd layers, respectively. At an even-odd layer step, a localized gapped electronic state is observed, in agreement with an axion insulator edge state that results from a phase transition across the step. The demonstration of thickness-dependent topological properties highlights the role of nanoscale control over novel quantum states, reinforcing the necessity of thin film technology in quantum information science applications.}, number={3}, journal={PHYSICAL REVIEW B}, author={Lupke, Felix and Pham, Anh D. and Zhao, Yi-Fan and Zhou, Ling-Jie and Lu, Wenchang and Briggs, Emil and Bernholc, Jerzy and Kolmer, Marek and Teeter, Jacob and Ko, Wonhee and et al.}, year={2022}, month={Jan} }
 @article{chen_qin_qian_zhu_li_zhang_lu_li_zhang_zhu_et al._2022, title={Relaxor ferroelectric polymer exhibits ultrahigh electromechanical coupling at low electric field}, volume={375}, ISSN={["1095-9203"]}, DOI={10.1126/science.abn0936}, abstractNote={
            Electromechanical (EM) coupling—the conversion of energy between electric and mechanical forms—in ferroelectrics has been used for a broad range of applications. Ferroelectric polymers have weak EM coupling that severely limits their usefulness for applications. We introduced a small amount of fluorinated alkyne (FA) monomers (<2 mol %) in relaxor ferroelectric poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (PVDF-TrFE-CFE) terpolymer that markedly enhances the polarization change with strong EM coupling while suppressing other polarization changes that do not contribute to it. Under a low–dc bias field of 40 megavolts per meter, the relaxor tetrapolymer has an EM coupling factor (
            k
            33
            ) of 88% and a piezoelectric coefficient (
            d
            33
            ) >1000 picometers per volt. These values make this solution-processed polymer competitive with ceramic oxide piezoelectrics, with the potential for use in distinct applications.
          }, number={6587}, journal={SCIENCE}, author={Chen, Xin and Qin, Hancheng and Qian, Xiaoshi and Zhu, Wenyi and Li, Bo and Zhang, Bing and Lu, Wenchang and Li, Ruipeng and Zhang, Shihai and Zhu, Lei and et al.}, year={2022}, month={Mar}, pages={1418-+} }
 @article{qian_han_zheng_chen_tyagi_li_du_zheng_huang_zhang_et al._2021, title={High-entropy polymer produces a giant electrocaloric effect at low fields}, volume={600}, ISSN={["1476-4687"]}, DOI={10.1038/s41586-021-04189-5}, abstractNote={More than a decade of research on the electrocaloric (EC) effect has resulted in EC materials and EC multilayer chips that satisfy a minimum EC temperature change of 5 K required for caloric heat pumps1-3. However, these EC temperature changes are generated through the application of high electric fields4-8 (close to their dielectric breakdown strengths), which result in rapid degradation and fatigue of EC performance. Here we report a class of EC polymer that exhibits an EC entropy change of 37.5 J kg-1 K-1 and a temperature change of 7.5 K under 50 MV m-1, a 275% enhancement over the state-of-the-art EC polymers under the same field strength. We show that converting a small number of the chlorofluoroethylene groups in poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) terpolymer into covalent double bonds markedly increases the number of the polar entities and enhances the polar-nonpolar interfacial areas of the polymer. The polar phases in the polymer adopt a loosely correlated, high-entropy state with a low energy barrier for electric-field-induced switching. The polymer maintains performance for more than one million cycles at the low fields necessary for practical EC cooling applications, suggesting that this strategy may yield materials suitable for use in caloric heat pumps.}, number={7890}, journal={NATURE}, author={Qian, Xiaoshi and Han, Donglin and Zheng, Lirong and Chen, Jie and Tyagi, Madhusudan and Li, Qiang and Du, Feihong and Zheng, Shanyu and Huang, Xingyi and Zhang, Shihai and et al.}, year={2021}, month={Dec}, pages={664-+} }
 @article{zhang_chen_zhang_zhang_lu_chen_liu_kim_donovan_warzoha_et al._2021, title={High-temperature polymers with record-high breakdown strength enabled by rationally designed chain-packing behavior in blends}, volume={4}, ISSN={["2590-2385"]}, DOI={10.1016/j.matt.2021.04.026}, abstractNote={Polymers with high dielectric breakdown strength (Eb) over a broad temperature range are vital for many applications. The presence of weak points, such as voids and free volume, severely limit the Eb of many high-temperature polymers. Here, we present a general strategy to reduce these weak points by exploiting interchain electrostatic forces in polymer blends. We show that the strong interchain electrostatic interaction between two high-temperature polymers in blends of polyimide (PI) with poly(ether imide) (PEI) yields an extended polymer chain conformation, resulting in dense chain packing and a corresponding decrease in weak spots in the polymers. This leads to a greater than 65% enhancement of Eb at room temperature and 35% enhancement at 200°C. In conjunction with results from blends of PI/poly(1,4-phenylene ether-sulfone) (PSU) and blends of PEI/PSU, we show that this previously unexplored molecular engineering strategy is efficient and straightforward in minimizing weak points in dielectric polymers.}, number={7}, journal={MATTER}, author={Zhang, Qiyan and Chen, Xin and Zhang, Bing and Zhang, Tian and Lu, Wengchang and Chen, Zhe and Liu, Ziyu and Kim, Seong H. and Donovan, Brian and Warzoha, Ronald J. and et al.}, year={2021}, month={Jul}, pages={2448–2459} }
 @article{zhang_chen_lu_zhang_bernholc_2021, title={Morphology-induced dielectric enhancement in polymer nanocomposites}, volume={13}, ISSN={["2040-3372"]}, url={https://doi.org/10.1039/D1NR00165E}, DOI={10.1039/D1NR00165E}, abstractNote={Admixture of dilute nanoparticles into dipolar polymer dielectrics enlarges free volume and increases permittivity by over 50%.}, number={24}, journal={NANOSCALE}, publisher={Royal Society of Chemistry (RSC)}, author={Zhang, Bing and Chen, Xin and Lu, Wenchang and Zhang, Q. M. and Bernholc, J.}, year={2021}, month={Jun} }
 @article{ma_xiao_bonnesen_liang_puretzky_huang_kolmer_sumpter_lu_hong_et al._2021, title={On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations}, volume={11}, ISSN={["2041-6539"]}, DOI={10.1039/d1sc04908a}, abstractNote={Selective deuterations were exploited to synthesize graphene nanoribbons on Au(111) surface with a specific H/D pattern on edges, allowing the determination of cyclodehydrogenation reaction pathway within the framework of pericyclic reactions.}, journal={CHEMICAL SCIENCE}, author={Ma, Chuanxu and Xiao, Zhongcan and Bonnesen, Peter V and Liang, Liangbo and Puretzky, Alexander A. and Huang, Jingsong and Kolmer, Marek and Sumpter, Bobby G. and Lu, Wenchang and Hong, Kunlun and et al.}, year={2021}, month={Nov} }
 @article{zhang_cheng_kolesnikov_bernholc_lu_ramirez-cuesta_2021, title={Study of Anharmonicity in Zirconium Hydrides Using Inelastic Neutron Scattering and Ab-Initio Computer Modeling}, volume={9}, ISSN={["2304-6740"]}, url={https://doi.org/10.3390/inorganics9050029}, DOI={10.3390/inorganics9050029}, abstractNote={The anharmonic phonon behavior in zirconium hydrides and deuterides, including ϵ-ZrH2, γ-ZrH, and γ-ZrD, has been investigated from aspects of inelastic neutron scattering (INS) and lattice dynamics calculations within the framework of density functional theory (DFT). The harmonic model failed to reproduce the spectral features observed in the experimental data, indicating the existence of anharmonicity in those materials and the necessity of further explanations. Here, we present a detailed study on the anharmonicity in zirconium hydrides/deuterides by exploring the 2D potential energy surface of hydrogen/deuterium atoms and solving the corresponding 2D single-particle Schrödinger equation to obtain the eigenfrequencies, which are then convoluted with the instrument resolution. The convoluted INS spectra qualitatively describe the anharmonic peaks in the experimental INS spectra and demonstrate that the anharmonicity originates from the deviations of hydrogen potentials from quadratic behavior in certain directions; the effects are apparent for the higher-order excited vibrational states, but small for the ground and first excited states.}, number={5}, journal={INORGANICS}, author={Zhang, Jiayong and Cheng, Yongqiang and Kolesnikov, Alexander I. and Bernholc, Jerry and Lu, Wenchang and Ramirez-Cuesta, Anibal J.}, year={2021}, month={May} }
 @article{liu_zhang_xu_haibibu_han_lu_bernholc_wang_2020, title={Chirality-induced relaxor properties in ferroelectric polymers}, volume={19}, url={https://doi.org/10.1038/s41563-020-0724-6}, DOI={10.1038/s41563-020-0724-6}, abstractNote={Relaxor ferroelectrics exhibit outstanding dielectric, electromechanical and electrocaloric properties, and are the materials of choice for acoustic sensors, solid-state coolers, transducers and actuators 1-4 . Despite more than five decades of intensive study, relaxor ferroelectrics remain one of the least understood material families in ferroelectric materials and condensed matter physics 5-14 . Here, by combining X-ray diffraction, atomic force microscope infrared spectroscopy and first-principles calculations, we reveal that the relaxor behaviour of ferroelectric polymers originates from conformational disorder, completely different from classic perovskite relaxors, which are typically characterized by chemical disorder. We show that chain chirality is essential to the formation of the disordered helix conformation arising from local distortions of gauche torsional angles, which consequently give rise to relaxor properties in polymers. This study not only sheds light on the fundamental mechanisms of relaxor ferroelectrics, but also offers guidance for the discovery of new ferroelectric relaxor organic materials for flexible, scalable and biocompatible sensor and energy applications.}, number={11}, journal={Nature Materials}, publisher={Springer Science and Business Media LLC}, author={Liu, Yang and Zhang, Bing and Xu, Wenhan and Haibibu, Aziguli and Han, Zhubing and Lu, Wenchang and Bernholc, J. and Wang, Qing}, year={2020}, month={Nov}, pages={1169–1174} }
 @article{ma_xiao_puretzky_wang_mohsin_huang_liang_luo_lawrie_gu_et al._2020, title={Engineering Edge States of Graphene Nanoribbons for Narrow-Band Photoluminescence}, volume={14}, ISSN={["1936-086X"]}, DOI={10.1021/acsnano.0c01737}, abstractNote={Solid-state narrow-band light emitters are on-demand for quantum optoelectronics. Current approaches based on defect engineering in low-dimensional materials usually introduce a broad range of emission centers. Here we report narrow-band light emission from covalent heterostructures fused to the edges of graphene nanoribbons (GNRs) by controllable on-surface reactions from molecular precursors. Two types of heterojunction (HJ) states are realized by sequentially synthesizing GNRs and graphene nanodots (GNDs) and then coupling them together. HJs between armchair GNDs and armchair edges of the GNR are coherent and give rise to narrow-band photoluminescence. In contrast, HJs between the armchair GNDs and the zigzag ends of GNRs are defective and give rise to non-radiative states near the Fermi level. At low temperatures, sharp photoluminescence emissions with peak energy range from 2.03 to 2.08 eV and linewidths of 2-5 meV are observed. The radiative HJ states are uniform and the optical transition energy is controlled by the band gaps of GNRs and GNDs. As these HJs can be synthesized in a large quantity with atomic precision, this finding highlights a route to programmable and deterministic creation of quantum light emitters.}, number={4}, journal={ACS NANO}, author={Ma, Chuanxu and Xiao, Zhongcan and Puretzky, Alexander A. and Wang, Hao and Mohsin, Ali and Huang, Jingsong and Liang, Liangbo and Luo, Yingdong and Lawrie, Benjamin J. and Gu, Gong and et al.}, year={2020}, month={Apr}, pages={5090–5098} }
 @article{xiao_ma_lu_huang_liang_hong_li_sumpter_bernholc_2019, title={Ab initio investigation of the cyclodehydrogenation process for polyanthrylene transformation to graphene nanoribbons}, url={https://doi.org/10.1038/s41524-019-0228-6}, DOI={10.1038/s41524-019-0228-6}, abstractNote={Abstract}, journal={npj Computational Materials}, author={Xiao, Zhongcan and Ma, Chuanxu and Lu, Wenchang and Huang, Jingsong and Liang, Liangbo and Hong, Kunlun and Li, An-Ping and Sumpter, Bobby G. and Bernholc, Jerzy}, year={2019}, month={Sep} }
 @article{ma_xiao_huang_liang_lu_hong_sumpter_bernholc_li_2019, title={Direct writing of heterostructures in single atomically precise graphene nanoribbons}, volume={3}, ISSN={2475-9953}, url={http://dx.doi.org/10.1103/PhysRevMaterials.3.016001}, DOI={10.1103/PhysRevMaterials.3.016001}, abstractNote={Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with an STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double barrier quantum dot structure with atomic precision for novel functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.}, number={1}, journal={Physical Review Materials}, publisher={American Physical Society (APS)}, author={Ma, Chuanxu and Xiao, Zhongcan and Huang, Jingsong and Liang, Liangbo and Lu, Wenchang and Hong, Kunlun and Sumpter, Bobby G. and Bernholc, J. and Li, An-Ping}, year={2019}, month={Jan} }
 @article{liu_zhang_haibibu_xu_han_lu_bernholc_wang_2019, title={Insights into the Morphotropic Phase Boundary in Ferroelectric Polymers from the Molecular Perspective}, volume={123}, ISSN={1932-7447 1932-7455}, url={http://dx.doi.org/10.1021/acs.jpcc.9b01220}, DOI={10.1021/acs.jpcc.9b01220}, abstractNote={Significantly enhanced electromechanical responses are inherent to piezoelectric materials at the morphotropic phase boundary (MPB). Here we reveal that conformational competition between the trans-planar and 3/1-helical phases of poly(vinylidene fluoride–trifluoroethylene) P(VDF-TrFE) occurs intramolecularly rather than intermolecularly to induce the formation of MPB. We attribute significantly enhanced piezoelectric properties observed near MPB to the polarization rotation between energetically degenerate trans-planar and 3/1-helical phases. Our results offer design principles to search for new MPB polymers from a molecular perspective.}, number={14}, journal={The Journal of Physical Chemistry C}, publisher={American Chemical Society (ACS)}, author={Liu, Yang and Zhang, Bing and Haibibu, Aziguli and Xu, Wenhan and Han, Zhubing and Lu, Wenchang and Bernholc, J. and Wang, Qing}, year={2019}, month={Mar}, pages={8727–8730} }
 @article{zhang_cheng_lu_briggs_ramirez-cuesta_bernholc_2019, title={Large-Scale Phonon Calculations Using the Real-Space Multigrid Method}, volume={15}, url={https://doi.org/10.1021/acs.jctc.9b00802}, DOI={10.1021/acs.jctc.9b00802}, abstractNote={Phonons are fundamental to understanding the dynamical and thermal properties of materials. However, first-principles phonon calculations are usually limited to moderate-size systems due to their high computational requirements. We implemented the finite displacement method (FDM) in the highly parallel real-space multigrid (RMG) suite of codes to study phonon properties. RMG scales from desktops to clusters and supercomputers containing thousands of nodes, fully supports GPUs, including multiple GPUs per node, and is very suitable for large-scale electronic structure calculations. It is used as the core computational kernel to calculate the force constants matrix with FDM. By comparing with other widely-used DFT packages and experimental data from inelastic neutron scattering, we demonstrate that RMG is very accurate in calculating forces at small displacements from equilibrium positions. The calculated phonon band structures and vibrational spectra for a variety of different systems are in very good agreement with plane-wave-based density functional theory codes, Quantum ESPRESSO, CASTEP and VASP, and these results have been validated comparing with inelastic neutron scattering experimental data measured at the VISION spectrometer at the Spallation Neutron Source.}, number={12}, journal={Journal of Chemical Theory and Computation}, publisher={American Chemical Society (ACS)}, author={Zhang, Jiayong and Cheng, Yongqiang and Lu, Wenchang and Briggs, Emil and Ramirez-Cuesta, Anibal J. and Bernholc, J.}, year={2019}, month={Dec}, pages={6859–6864} }
 @article{ma_xiao_lu_huang_hong_bernholc_li_2019, title={Step edge-mediated assembly of periodic arrays of long graphene nanoribbons on Au(111)}, volume={55}, url={https://doi.org/10.1039/C9CC05273A}, DOI={10.1039/C9CC05273A}, abstractNote={Step edges on Au(111) surface are found to promote the assembly of compact polymer arrays with enhanced interchain π–π interactions, which in turn lead to periodic arrays of graphene nanoribbons with enhanced average length near step edges.}, number={79}, journal={Chemical Communications}, publisher={Royal Society of Chemistry (RSC)}, author={Ma, Chuanxu and Xiao, Zhongcan and Lu, Wenchang and Huang, Jingsong and Hong, Kunlun and Bernholc, J. and Li, An-Ping}, year={2019}, pages={11848–11851} }
 @article{xiao_ma_huang_liang_lu_hong_sumpter_li_bernholc_2018, title={Design of Atomically Precise Nanoscale Negative Differential Resistance Devices}, volume={2}, ISSN={2513-0390 2513-0390}, url={http://dx.doi.org/10.1002/adts.201800172}, DOI={10.1002/adts.201800172}, abstractNote={Abstract}, number={2}, journal={Advanced Theory and Simulations}, publisher={Wiley}, author={Xiao, Zhongcan and Ma, Chuanxu and Huang, Jingsong and Liang, Liangbo and Lu, Wenchang and Hong, Kunlun and Sumpter, Bobby G. and Li, An‐Ping and Bernholc, Jerzy}, year={2018}, month={Dec}, pages={1800172} }
 @article{liu_aziguli_zhang_xu_lu_bernholc_wang_2018, title={Ferroelectric polymers exhibiting behaviour reminiscent of a morphotropic phase boundary}, volume={562}, ISSN={0028-0836 1476-4687}, url={http://dx.doi.org/10.1038/s41586-018-0550-z}, DOI={10.1038/s41586-018-0550-z}, abstractNote={Piezoelectricity-the direct interconversion between mechanical and electrical energies-is usually remarkably enhanced at the morphotropic phase boundary of ferroelectric materials 1-4 , which marks a transition region in the phase diagram of piezoelectric materials and bridges two competing phases with distinct symmetries 1,5 . Such enhancement has enabled the recent development of various lead and lead-free piezoelectric perovskites with outstanding piezoelectric properties for use in actuators, transducers, sensors and energy-harvesting applications 5-8 . However, the morphotropic phase boundary has never been observed in organic materials, and the absence of effective approaches to improving the intrinsic piezoelectric responses of polymers 9,10  considerably hampers their application to flexible, wearable and biocompatible devices. Here we report stereochemically induced behaviour in ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) copolymers, which is similar to that observed at morphotropic phase boundaries in perovskites. We reveal that compositionally tailored tacticity (the stereochemical arrangement of chiral centres related to the TrFE monomers 11,12 ) can lead to intramolecular order-to-disorder evolution in the crystalline phase and thus to an intermediate transition region that is reminiscent of the morphotropic phase boundary, where competing ferroelectric and relaxor properties appear simultaneously. Our first-principles calculations confirm the crucial role of chain tacticity in driving the formation of this transition region via structural competition between the trans-planar and 3/1-helical phases. We show that the P(VDF-TrFE) copolymer with the morphotropic composition exhibits a longitudinal piezoelectric coefficient of -63.5 picocoulombs per newton, outperforming state-of-the-art piezoelectric polymers 10 . Given the flexibility in the molecular design and synthesis of organic ferroelectric materials, this work opens up the way for the development of scalable, high-performance piezoelectric polymers.}, number={7725}, journal={Nature}, publisher={Springer Nature}, author={Liu, Yang and Aziguli, Haibibu and Zhang, Bing and Xu, Wenhan and Lu, Wenchang and Bernholc, J. and Wang, Qing}, year={2018}, month={Oct}, pages={96–100} }
 @article{ma_xiao_puretzky_baddorf_lu_hong_bernholc_li_2018, title={Oxidization stability of atomically precise graphene nanoribbons}, volume={2}, ISSN={2475-9953}, url={http://dx.doi.org/10.1103/PhysRevMaterials.2.014006}, DOI={10.1103/physrevmaterials.2.014006}, abstractNote={The stability of graphene nanoribbons (GNRs) against oxidation is critical for their practical applications. Here we study both the thermal stability and the oxidation process of the ambient-exposed armchair GNRs with a width of seven carbon atoms (7-aGNR), grown on an Au(111) surface. The atomic scale evolution of the armchair edges and the zigzag ends of the aGNRs after annealing at different temperatures are revealed by using scanning tunneling microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and first-principles calculations. We observe evidence that the zigzag ends start to be oxidized and decomposed at 180 \ifmmode^\circ\else\textdegree\fi{}C, while the armchair edges are intact at 430 \ifmmode^\circ\else\textdegree\fi{}C but become oxidized at 520 \ifmmode^\circ\else\textdegree\fi{}C. Two different oxygen species are identified at the armchair edges, namely the hydroxyl pair and the epoxy bonding motif with one oxygen bonded to two edge carbons. These oxidization species modify the electronic properties of the pristine 7-aGNRs, with a band-gap reduction from 2.6 to 2.3 eV and 1.9 eV for the hydroxyl pair- and epoxy-terminated edges, respectively. These findings demonstrate the oxidation stability of both the zigzag and armchair edges of GNRs, and they provide an opportunity to harness the high density of edge atoms in applications such as GNR-based high-temperature oxygen sensors.}, number={1}, journal={Physical Review Materials}, publisher={American Physical Society (APS)}, author={Ma, Chuanxu and Xiao, Zhongcan and Puretzky, Alex A. and Baddorf, Arthur P. and Lu, Wenchang and Hong, Kunlun and Bernholc, J. and Li, An-Ping}, year={2018}, month={Jan} }
 @book{bernholc_2018, title={Theoretical Investigations of Nano and Bio Structures}, url={https://doi.org/10.2172/1467564}, DOI={10.2172/1467564}, abstractNote={The PI was supported by the DOE grant entitled “Theoretical Investigations of Nano and Bio Structures,” 3/1/13-2/28/17, $440,000, and a one-year no cost extension, 3/1/17-2/28/18. Its main emphasis was on nonequilibrium electron transport and novel nanoscale materials and devices, such as graphene nanoribbons, nanotube-based sensors, nanocircuits able to detect DNA replication, and DNA nanowires. We have also made major enhancements to our RMG suite of electronic structure codes, leading to much increased scalability, excellent parallelization of across both CPU and GPU nodes, and cleaner, refactored code base. For example, RMG performs excellently on Summit, DOE’s flagship supercomputer and the fastest supercomputer in the world. It scales to over 100,000 CPU cores and 20,000 GPUs, and can use multiple GPUs per node. It performs 80x faster per node than on DOE’s previous flagship supercomputer, Titan. This is the final report for grant DE-FG02-98ER45685.}, author={Bernholc, Jerzy}, year={2018}, month={Aug} }
 @article{dolgova_yu_cvitkovic_hodak_nienaber_summers_cotelesage_bernholc_kaminski_pickering_et al._2017, title={Binding of Copper and Cisplatin to Atox1 Is Mediated by Glutathione through the Formation of Metal–Sulfur Clusters}, volume={56}, ISSN={0006-2960 1520-4995}, url={http://dx.doi.org/10.1021/acs.biochem.7b00293}, DOI={10.1021/acs.biochem.7b00293}, abstractNote={Copper is an essential nutrient required for many biological processes involved in primary metabolism, but free copper is toxic due to its ability to catalyze formation of free radicals. To prevent toxic effects, in the cell copper is bound to proteins and low molecular weight compounds, such as glutathione, at all times. The widely used chemotherapy agent cisplatin is known to bind to copper-transporting proteins, including copper chaperone Atox1. Cisplatin interactions with Atox1 and other copper transporters are linked to cancer resistance to platinum-based chemotherapy. Here we analyze the binding of copper and cisplatin to Atox1 in the presence of glutathione under redox conditions that mimic intracellular environment. We show that copper(I) and glutathione form large polymers with a molecular mass of approximately 8 kDa, which can transfer copper to Atox1. Cisplatin also can form polymers with glutathione, albeit at a slower rate. Analysis of simultaneous binding of copper and cisplatin to Atox1 under physiological conditions shows that both metals are bound to the protein through copper-sulfur-platinum bridges.}, number={24}, journal={Biochemistry}, publisher={American Chemical Society (ACS)}, author={Dolgova, Natalia V. and Yu, Corey and Cvitkovic, John P. and Hodak, Miroslav and Nienaber, Kurt H. and Summers, Kelly L. and Cotelesage, Julien J. H. and Bernholc, Jerzy and Kaminski, George A. and Pickering, Ingrid J. and et al.}, year={2017}, month={Jun}, pages={3129–3141} }
 @article{ma_xiao_zhang_liang_huang_lu_sumpter_hong_bernholc_li_et al._2017, title={Controllable conversion of quasi-freestanding polymer chains to graphene nanoribbons}, volume={8}, ISSN={2041-1723}, url={http://dx.doi.org/10.1038/ncomms14815}, DOI={10.1038/ncomms14815}, abstractNote={Abstract}, number={1}, journal={Nature Communications}, publisher={Springer Science and Business Media LLC}, author={Ma, Chuanxu and Xiao, Zhongcan and Zhang, Honghai and Liang, Liangbo and Huang, Jingsong and Lu, Wenchang and Sumpter, Bobby G. and Hong, Kunlun and Bernholc, Jerzy and Li, An-Ping and et al.}, year={2017}, month={Mar} }
 @article{thakur_zhang_iacob_yang_bernholc_chen_runt_zhang_2017, title={Enhancement of the dielectric response in polymer nanocomposites with low dielectric constant fillers}, volume={9}, ISSN={2040-3364 2040-3372}, url={http://dx.doi.org/10.1039/C7NR01932G}, DOI={10.1039/c7nr01932g}, abstractNote={An enhanced dielectric response in nanocomposite films with low dielectric constant nanofillers at a very low volume content is achieved.}, number={31}, journal={Nanoscale}, publisher={Royal Society of Chemistry (RSC)}, author={Thakur, Yash and Zhang, Tian and Iacob, C. and Yang, Tiannan and Bernholc, J. and Chen, L. Q. and Runt, J. and Zhang, Q. M.}, year={2017}, pages={10992–10997} }
 @article{thakur_zhang_dong_lu_iacob_runt_bernholc_zhang_2017, title={Generating high dielectric constant blends from lower dielectric constant dipolar polymers using nanostructure engineering}, volume={32}, ISSN={2211-2855}, url={http://dx.doi.org/10.1016/j.nanoen.2016.12.021}, DOI={10.1016/j.nanoen.2016.12.021}, abstractNote={It is a great challenge in dielectric polymers to achieve a high dielectric constant while maintaining low dielectric loss and high operating temperatures. Here we report that by blending two glassy state dipolar polymers i.e., poly(arylene ether urea) (PEEU, K=4.7) and an aromatic polythiourea (ArPTU, K=4.4) to form a nanomixture, the resulting blend exhibits a very high dielectric constant, K=7.5, while maintaining low dielectric loss (<1%). The experimental and computer simulation results demonstrate that blending these dissimilar dipolar polymers causes a slight increase in the interchain spacing of the blend in its glassy state, thus reducing the barriers for reorientation of dipoles in the polymer chains along the applied electric field and generating a much higher dielectric response than the neat polymers.}, journal={Nano Energy}, publisher={Elsevier BV}, author={Thakur, Yash and Zhang, Bing and Dong, Rui and Lu, Wenchang and Iacob, C. and Runt, J. and Bernholc, J. and Zhang, Q.M.}, year={2017}, month={Feb}, pages={73–79} }
 @article{ma_liang_xiao_puretzky_hong_lu_meunier_bernholc_li_2017, title={Seamless Staircase Electrical Contact to Semiconducting Graphene Nanoribbons}, volume={17}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/acs.nanolett.7b02938}, DOI={10.1021/acs.nanolett.7b02938}, abstractNote={Electrical contact to low-dimensional (low-D) materials is a key to their electronic applications. Traditional metal contacts to low-D semiconductors typically create gap states that can pin the Fermi level (EF). However, low-D metals possessing a limited density of states at EF can enable gate-tunable work functions and contact barriers. Moreover, a seamless contact with native bonds at the interface, without localized interfacial states, can serve as an optimal electrode. To realize such a seamless contact, one needs to develop atomically precise heterojunctions from the atom up. Here, we demonstrate an all-carbon staircase contact to ultranarrow armchair graphene nanoribbons (aGNRs). The coherent heterostructures of width-variable aGNRs, consisting of 7, 14, 21, and up to 56 carbon atoms across the width, are synthesized by a surface-assisted self-assembly process with a single molecular precursor. The aGNRs exhibit characteristic vibrational modes in Raman spectroscopy. A combined scanning tunneling microscopy and density functional theory study reveals the native covalent-bond nature and quasi-metallic contact characteristics of the interfaces. Our electronic measurements of such seamless GNR staircase constitute a promising first step toward making low resistance contacts.}, number={10}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Ma, Chuanxu and Liang, Liangbo and Xiao, Zhongcan and Puretzky, Alexander A. and Hong, Kunlun and Lu, Wenchang and Meunier, Vincent and Bernholc, J. and Li, An-Ping}, year={2017}, month={Sep}, pages={6241–6247} }
 @article{li_hodak_lu_bernholc_2017, title={Selective sensing of ethylene and glucose using carbon-nanotube-based sensors: an ab initio investigation}, volume={9}, ISSN={2040-3364 2040-3372}, url={http://dx.doi.org/10.1039/C6NR07371A}, DOI={10.1039/c6nr07371a}, abstractNote={Functionalized carbon nanotubes have great potential for nanoscale sensing applications, yet many aspects of their sensing mechanisms are not understood. Here, two paradigmatic sensor configurations for detection of biologically important molecules are investigated through ab initio calculations: a non-covalently functionalized nanotube for glucose detection and a covalently functionalized nanotube for ethylene detection. Glucose and ethylene control key life processes of humans and plants, respectively, despite of their structural and chemical simplicity. The sensors' electrical conductance and transmission coefficients are evaluated at the full density-functional theory level via the non-equilibrium Green's function method. We also investigate the effects of the density of the receptors, the band gaps of the nanotubes, the source-drain voltages, and the atomic modification of the receptor on detection sensitivities. A clear atomistic picture emerges about the mechanisms involved in glucose and ethylene sensing. While semiconducting nanotubes exhibit good sensitivities in both cases, the current through metallic nanotubes is only weakly affected by analyte attachment. These quantitative results could guide the design of improved sensors.}, number={4}, journal={Nanoscale}, publisher={Royal Society of Chemistry (RSC)}, author={Li, Yan and Hodak, Miroslav and Lu, Wenchang and Bernholc, J.}, year={2017}, pages={1687–1698} }
 @article{dong_ranjan_buongiorno nardelli_bernholc_2016, title={First-principles simulations of PVDF copolymers with high dielectric energy density: PVDF-HFP and PVDF-BTFE}, volume={94}, ISSN={2469-9950 2469-9969}, url={http://dx.doi.org/10.1103/PhysRevB.94.014210}, DOI={10.1103/physrevb.94.014210}, abstractNote={Phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers with hexafluoropropylene (HFP) and bromotrifluoroethylene (BTFE) are investigated via first-principles simulations and compared to previously studied P(VDF-chlorotrifluoroethylene) (CTFE) data. We find that a nonpolar to polar phase transition induced by an electric field also occurs in HFP and BTFE copolymers and the results for P(VDF-HFP) show good agreement with existing experiments. For P(VDF-BTFE) we show that its nonpolar phase remains the ground state for a substantially larger range of concentrations than for P(VDF-CTFE) and P(VDF-HFP), and predict that a high BTFE concentration copolymer will achieve a significantly higher energy density at low field than P(VDF-CTFE) 9%. The transition pathways connecting the polar and nonpolar phases are also calculated and the energy barriers for the transitions turn out to be similar for the three copolymers, even at different co-monomer concentrations. The similarity of barriers indicates that a mixture of these and related copolymers can be used to optimize the properties of the dielectric, such as energy density, processability, and cost.}, number={1}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Dong, Rui and Ranjan, V. and Buongiorno Nardelli, Marco and Bernholc, J.}, year={2016}, month={Jul} }
 @article{li_hodak_lu_bernholc_2016, title={Mechanisms of NH3 and NO2 detection in carbon-nanotube-based sensors: An ab initio investigation}, volume={101}, ISSN={0008-6223}, url={http://dx.doi.org/10.1016/j.carbon.2016.01.092}, DOI={10.1016/j.carbon.2016.01.092}, abstractNote={The mechanisms of NH3 and NO2 detection by single-walled carbon nanotube-based devices are investigated by ab initio calculations and the non-equilibrium Greens function (NEGF) methodology. While both NH3 and NO2 can physisorb to a pristine carbon nanotube, we show that their adsorption only results in small current changes through the device. For a carbon nanotube (CNT) attached to gold nanowire leads, the most sensitive detection site is at the CNT near the CNT-Au contact, where chemisorption occurs. The resulting change in electron transmission and low-bias current can lead to over 30% sensitivity. While both NH3 and NO2 can also chemisorb at the Au electrodes, their adsorption results in only a small change in the plurality of the conducting levels of the gold layers, and thus a small effect on current. In order to enhance the detection sensitivity, it is thus beneficial to mask the electrodes to prevent chemisorption. Furthermore, the length of the pure CNT segment does not strongly affect the relative sensitivity. Our results suggest that a short-CNT device with exposed contact regions and masked electrodes would have the greatest sensitivity.}, journal={Carbon}, publisher={Elsevier BV}, author={Li, Yan and Hodak, Miroslav and Lu, Wenchang and Bernholc, J.}, year={2016}, month={May}, pages={177–183} }
 @article{dong_ranjan_buongiorno nardelli_bernholc_2015, title={Atomistic simulations of aromatic polyurea and polyamide for capacitive energy storage}, volume={92}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.92.024203}, DOI={10.1103/physrevb.92.024203}, abstractNote={Materials for capacitive energy storage with high energy density and low loss are desired in many fields. We investigate several polymers with urea and amide functional groups using density functional theory and classical molecular dynamics simulations. For aromatic polyurea (APU) and para-aramid (PA), we find several nearly energetically degenerate ordered structures, while meta-aromatic polyurea (mAPU) tends to be rotationally disordered along the polymer chains. Simulated annealing of APU and PA structures results in the formation of hydrogen-bonded sheets, highlighting the importance of dipole-dipole interactions. In contrast, hydrogen bonding does not play a significant role in mAPU, hence the propensity to disorder. We find that the disordered structures with misaligned chains have significantly larger dielectric constants, due to significant increase in the free volume, which leads to easier reorientation of dipolar groups in the presence of an electric field. Large segment motion is still not allowed below the glass transition temperature, which explains the experimentally observed very low loss at high field and elevated temperature. However, the degree of disorder needs to be controlled, because highly entangled structures diminish the free dipoles and decrease permittivity. Among the considered materials, mAPU is the most promising dielectric for capacitive energy storage, but the concept of increasing permittivity while maintaining low loss through disorder-induced free volume increase is generally applicable and provides an alternative pathway for the design of high-performance dielectrics for capacitive energy storage.}, number={2}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Dong, Rui and Ranjan, V. and Buongiorno Nardelli, Marco and Bernholc, J.}, year={2015}, month={Jul} }
 @article{tan_hodak_lu_bernholc_2015, title={Charge transport in DNA nanowires connected to carbon nanotubes}, volume={92}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.92.075429}, DOI={10.1103/PhysRevB.92.075429}, abstractNote={DNA is perhaps the worlds most controllable nanowire, with potential applications in nano-electronics and sensing. However, understanding of its charge transport (CT) properties remains elusive with experiments reporting a wide range of behaviors from insulating to superconductive. We report extensive first-principle simulations that account for DNA’s high flexibility and its native solvent environment. The results show that the CT along the DNA’s long axis is strongly dependent on DNA’s instantaneous conformation varying over many orders of magnitude. In high CT conformations, delocalized conductive states extending over up to 10 base pairs are found. Their low exponential decay constants further indicate that coherent CT, which is assumed to be active only over 2-3 base pairs in the commonly accepted DNA CT models, can act over much longer length scales. We also identify a simple geometrical rule that predicts CT properties of a given conformation with high accuracy. The effect of mismatched base pairs is also considered: while they decrease conductivities of specific DNA conformations, thermally-induced conformational fluctuations wash out this effect. Overall, our results indicate that an immobilized partially dried poly(G)-poly(C) B-DNA is preferable for nanowire applications.}, number={7}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Tan, Bikan and Hodak, Miroslav and Lu, Wenchang and Bernholc, J.}, year={2015}, month={Aug} }
 @article{li_hodak_bernholc_2015, title={Enzymatic Mechanism of Copper-Containing Nitrite Reductase}, volume={54}, ISSN={0006-2960 1520-4995}, url={http://dx.doi.org/10.1021/bi5007767}, DOI={10.1021/bi5007767}, abstractNote={Copper-containing nitrite reductases (CuNiRs) catalyze the reduction of nitrite to nitric oxide, a key step in the denitrification process that maintains balance between organic and inorganic nitrogen. Despite their importance, their functioning is not well understood. In this work, we carry out first-principles calculations and show that the available structural data are consistent only with a single mechanism. For this mechanism, we determine the activation energies, transition states, and minimum energy pathways of CuNiR. The calculations lead to an updated enzymatic mechanism and resolve several controversial issues. In particular, our work identifies the origins of the two protons necessary for the enzymatic function and shows that the transformation from the initial O-coordination of substrate to the final N-coordination of product is achieved by electron transfer from T1 copper to T2 copper, rather than by the previously reported side-on coordination of a NO intermediate, which only takes place in the reduced enzyme. We also examine the role of structural change in the critical residue Asp(98), reported in one experimental study, and find that while the structural change affects the energetics of substrate attachment and product release at the T2 copper reaction center, it does not significantly affect the activation energy and reaction pathways of the nitrite reduction process.}, number={5}, journal={Biochemistry}, publisher={American Chemical Society (ACS)}, author={Li, Yan and Hodak, Miroslav and Bernholc, J.}, year={2015}, month={Jan}, pages={1233–1242} }
 @article{thakur_dong_lin_wu_cheng_hou_bernholc_zhang_2015, title={Optimizing nanostructure to achieve high dielectric response with low loss in strongly dipolar polymers}, volume={16}, ISSN={2211-2855}, url={http://dx.doi.org/10.1016/j.nanoen.2015.06.021}, DOI={10.1016/j.nanoen.2015.06.021}, abstractNote={Advances in modern electronics require the development of polymer-based dielectric materials with high dielectric constant, low dielectric loss, and high thermal stability. Fundamental dielectric theory suggests that strongly dipolar polymers have the potential to realize a high dielectric constant. In order to achieve high thermal stability, these polymers should also possess a high glass transition temperature Tg. However, it has been observed that in many dielectric polymers the dielectric constant decreases markedly at temperatures below Tg due to dipole freezing. This study shows, through combined theoretical and experimental investigations, that nano-structure engineering of a weakly-coupled strongly-dipolar polymer can result in a high energy density polymer with low loss and high operating temperature. Our studies reveal that disorder creates a significantly larger free volume at temperatures far below Tg, enabling easier reorientation of dipoles in response to an electric field in aromatic urea and thiourea polymers. The net result is a substantial enhancement in the dielectric constant while preserving low dielectric loss and very high breakdown field. These results here pave the way for engineering the nanostructure to create high energy density polymers with low loss and high operating temperature.}, journal={Nano Energy}, publisher={Elsevier BV}, author={Thakur, Yash and Dong, Rui and Lin, Minren and Wu, Shan and Cheng, Zhaoxi and Hou, Ying and Bernholc, J. and Zhang, Q.M.}, year={2015}, month={Sep}, pages={227–234} }
 @book{briggs_kelley_bernholc_2015, title={Parallel implementation of electronic structure eigensolver using a partitioned folded spectrum method}, url={http://arxiv.org/pdf/1502.07806.pdf}, author={Briggs, E.L. and Kelley, C.T. and Bernholc, J.}, year={2015} }
 @article{li_owens_han_sumpter_lu_bernholc_meunier_maksymovych_fuentes-cabrera_pan_et al._2013, title={Self-Organized and Cu-Coordinated Surface Linear Polymerization}, volume={3}, ISSN={2045-2322}, url={http://dx.doi.org/10.1038/srep02102}, DOI={10.1038/srep02102}, abstractNote={We demonstrate a controllable surface-coordinated linear polymerization of long-chain poly(phenylacetylenyl)s that are self-organized into a "circuit-board" pattern on a Cu(100) surface. Scanning tunneling microscopy/spectroscopy (STM/S) corroborated by ab initio calculations, reveals the atomistic details of the molecular structure, and provides a clear signature of electronic and vibrational properties of the poly(phenylacetylene)s chains. Notably, the polymerization reaction is confined epitaxially to the copper lattice, despite a large strain along the polymerized chain that subsequently renders it metallic. Polymerization and depolymerization reactions can be controlled locally at the nanoscale by using a charged metal tip. This control demonstrates the possibility of precisely accessing and controlling conjugated chain-growth polymerization at low temperature. This finding may lead to the bottom-up design and realization of sophisticated architectures for molecular nano-devices.}, number={1}, journal={Scientific Reports}, publisher={Springer Science and Business Media LLC}, author={Li, Qing and Owens, Jonathan R. and Han, Chengbo and Sumpter, Bobby G. and Lu, Wenchang and Bernholc, Jerzy and Meunier, V. and Maksymovych, Peter and Fuentes-Cabrera, Miguel and Pan, Minghu and et al.}, year={2013}, month={Jul}, pages={2102} }
 @article{ranjan_nardelli_bernholc_2012, title={Electric Field Induced Phase Transitions in Polymers: A Novel Mechanism for High Speed Energy Storage}, volume={108}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.108.087802}, DOI={10.1103/physrevlett.108.087802}, abstractNote={Using first-principles simulations, we identify the microscopic origin of the nonlinear dielectric response and high energy density of polyvinylidene-fluoride-based polymers as a cooperative transition path that connects nonpolar and polar phases of the system. This path explores a complex torsional and rotational manifold and is thermodynamically and kinetically accessible at relatively low temperatures. Furthermore, the introduction of suitable copolymers significantly alters the energy barriers between phases providing tunability of both the energy density and the critical fields.}, number={8}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Ranjan, V. and Nardelli, Marco Buongiorno and Bernholc, J.}, year={2012}, month={Feb} }
 @article{li_han_fuentes-cabrera_terrones_sumpter_lu_bernholc_yi_gai_baddorf_et al._2012, title={Electronic Control over Attachment and Self-Assembly of Alkyne Groups on Gold}, volume={6}, ISSN={["1936-086X"]}, DOI={10.1021/nn303734r}, abstractNote={Self-assembled monolayers are the basis for molecular nanodevices, flexible surface functionalization, and dip-pen nanolithography. Yet self-assembled monolayers are typically created by a rather inefficient process involving thermally driven attachment reactions of precursor molecules to a metal surface, followed by a slow and defect-prone molecular reorganization. Here we demonstrate a nonthermal, electron-induced approach to the self-assembly of phenylacetylene molecules on gold that allows for a previously unachievable attachment of the molecules to the surface through the alkyne group. While thermal excitation can only desorb the parent molecule due to prohibitively high activation barriers for attachment reactions, localized injection of hot electrons or holes not only overcomes this barrier but also enables an unprecedented control over the size and shape of the self-assembly, defect structures, and the reverse process of molecular disassembly from a single molecule to a mesoscopic length scale. Electron-induced excitation may therefore enable new and highly controlled approaches to molecular self-assembly on a surface.}, number={10}, journal={ACS NANO}, author={Li, Qing and Han, Chengbo and Fuentes-Cabrera, Miguel and Terrones, Humberto and Sumpter, Bobby G. and Lu, Wenchang and Bernholc, Jerry and Yi, Jieyu and Gai, Zheng and Baddorf, Arthur P. and et al.}, year={2012}, month={Oct}, pages={9267–9275} }
 @inproceedings{bernholc_ranjan_tan_hodak_lu_buongiorno nardelli_2012, title={High Power Density Energy Storage and Bio-Inspired Devices}, author={Bernholc, J. and Ranjan, V. and Tan, B. and Hodak, M. and Lu, W. and Buongiorno Nardelli, M.}, year={2012} }
 @inproceedings{moore_briggs_hodak_lu_bernholc_lee_2012, place={Piscataway, NJ}, title={Scaling the RMG Quantum Mechanics Code}, booktitle={BW-XSEDE '12: Proceedings of the Extreme Scaling Workshop}, publisher={IEEE}, author={Moore, S. and Briggs, E. and Hodak, M. and Lu, W. and Bernholc, J. and Lee, C.W.}, year={2012} }
 @article{li_han_horton_fuentes-cabrera_sumpter_lu_bernholc_maksymovych_pan_2012, title={Supramolecular Self-Assembly of pi-Conjugated Hydrocarbons via 2D Cooperative CH/pi Interaction}, volume={6}, ISSN={["1936-086X"]}, DOI={10.1021/nn203952e}, abstractNote={Supramolecular self-assembly on well-defined surfaces provides access to a multitude of nanoscale architectures, including clusters of distinct symmetry and size. The driving forces underlying supramolecular structures generally involve both graphoepitaxy and weak directional nonconvalent interactions. Here we show that functionalizing a benzene molecule with an ethyne group introduces attractive interactions in a 2D geometry, which would otherwise be dominated by intermolecular repulsion. Furthermore, the attractive interactions enable supramolecular self-assembly, wherein a subtle balance between very weak CH/π bonding and molecule-surface interactions produces a well-defined "magic" dimension and chirality of supramolecular clusters. The nature of the process is corroborated by extensive scanning tunneling microscopy/spectroscopy (STM/S) measurements and ab initio calculations, which emphasize the cooperative, multicenter characters of the CH/π interaction. This work points out new possibilities for chemical functionalization of π-conjugated hydrocarbon molecules that may allow for the rational design of supramolecular clusters with a desired shape and size.}, number={1}, journal={ACS NANO}, author={Li, Qing and Han, Chengbo and Horton, Scott R. and Fuentes-Cabrera, Miguel and Sumpter, Bobby G. and Lu, Wenchang and Bernholc, Jerry and Maksymovych, Petro and Pan, Minghu}, year={2012}, month={Jan}, pages={566–572} }
 @article{jiang_turnbull_lu_boguslawski_bernholc_2012, title={Theory of nitrogen doping of carbon nanoribbons: Edge effects}, volume={136}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.3673441}, DOI={10.1063/1.3673441}, abstractNote={Nitrogen doping of a carbon nanoribbon is profoundly affected by its one-dimensional character, symmetry, and interaction with edge states. Using state-of-the-art ab initiocalculations, including hybrid exact-exchange density functional theory, we find that, for N-doped zigzag ribbons, the electronic properties are strongly dependent upon sublattice effects due to the non-equivalence of the two sublattices. For armchair ribbons, N-doping effects are different depending upon the ribbon family: for families 2 and 0, the N-induced levels are in the conduction band, while for family 1 the N levels are in the gap. In zigzag nanoribbons, nitrogen close to the edge is a deep center, while in armchair nanoribbons its behavior is close to an effective-mass-like donor with the ionization energy dependent on the value of the band gap. In chiral nanoribbons, we find strong dependence of the impurity level and formation energy upon the edge position of the dopant, while such site-specificity is not manifested in the magnitude of the magnetization.}, number={1}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Jiang, Jie and Turnbull, Joseph and Lu, Wenchang and Boguslawski, Piotr and Bernholc, J.}, year={2012}, month={Jan}, pages={014702} }
 @inproceedings{bernholc_hodak_lu_rose_2011, title={Hybrid quantum simulations of biomolecules: the role of copper in neurodegenerative diseases}, booktitle={Proceedings of the 2010 Scientific Discovery through Advanced Computing (SciDAC) Conference}, year={2011}, pages={12–22} }
 @article{rose_hodak_bernholc_2011, title={Mechanism of copper(II)-induced misfolding of Parkinson's disease protein}, volume={1}, ISSN={2045-2322}, url={http://dx.doi.org/10.1038/srep00011}, DOI={10.1038/srep00011}, abstractNote={α-synuclein (aS) is a natively unfolded pre-synaptic protein found in all Parkinson's disease patients as the major component of fibrillar plaques. Metal ions and especially Cu(II), have been demonstrated to accelerate aggregation of aS into fibrillar plaques, the precursors to Lewy bodies. In this work, copper binding to aS is investigated by a combination of quantum and molecular mechanics simulations. Starting from the experimentally observed attachment site, several optimized structures of Cu-binding geometries are examined. The most energetically favorable attachment results in significant allosteric changes, making aS more susceptible to misfolding. Indeed, an inverse kinematics investigation of the configuration space uncovers a dynamically stable β-sheet conformation of Cu-aS that serves as a nucleation point for a second β-strand. Based on these findings, we propose an atomistic mechanism of copper-induced misfolding of aS as an initial event in the formation of Lewy bodies and thus in PD pathogenesis.}, number={1}, journal={Scientific Reports}, publisher={Springer Science and Business Media LLC}, author={Rose, Frisco and Hodak, Miroslav and Bernholc, Jerzy}, year={2011}, month={Jun} }
 @article{kim_bernholc_kwon_2010, title={Band gap control of small bundles of carbon nanotubes using applied electric fields: A density functional theory study}, volume={97}, ISSN={["0003-6951"]}, DOI={10.1063/1.3478237}, abstractNote={Electrostatic screening between carbon nanotubes (CNTs) in a small CNT bundle leads to a switching behavior induced by electric field perpendicular to the bundle axis. Using a first-principles method, we investigate the electronic structures of bundles consisting of two or three CNTs and the effects of the electric field applied perpendicular to the bundle axis. The applied field causes band gap closure in semiconducting bundles, while a gap opening occurs in metallic ones, which enables considerable modulation of bundle conductivity. The modulation effect originates from symmetry breaking due to electrostatic screening between the adjacent tube walls.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Kim, Gunn and Bernholc, J. and Kwon, Young-Kyun}, year={2010}, month={Aug} }
 @article{saha_lu_bernholc_meunier_2010, title={Electron transport in multiterminal molecular devices: A density functional theory study}, volume={81}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.81.125420}, abstractNote={The electron transport properties of a four-terminal molecular device are computed within the framework of density functional theory and nonequilibrium Keldysh theory. The additional two terminals lead to new properties, including a pronounced negative differential resistance not present in a two-terminal setup, and a pseudogating effect. In general, quantum interference between the four terminals and the central molecule leads to a complex nonlinear behavior of the current, which depends on the alignment of individual molecular states under bias and their coupling to the leads.}, number={12}, journal={PHYSICAL REVIEW B}, author={Saha, Kamal K. and Lu, Wenchang and Bernholc, J. and Meunier, Vincent}, year={2010}, month={Mar} }
 @article{hodak_bernholc_2010, title={Insights into prion protein function from atomistic simulations}, volume={4}, ISSN={["1933-690X"]}, DOI={10.4161/pri.4.1.10969}, abstractNote={Computer simulations are a powerful tool for studies of biological systems. They have often been used to study prion protein (PrP), a protein responsible for neurodegenerative diseases, which include “mad cow disease” in cattle and Creutzfeldt-Jacob disease in humans. An important aspect of the prion protein is its interaction with copper ion, which is thought to be relevant for PrP’s yet undetermined function and also potentially play a role in prion diseases. For studies of copper attachment to the prion protein, computer simulations have often been used to complement experimental data and to obtain binding structures of Cu-PrP complexes. This paper summarizes the results of recent ab initio calculations of copper-prion protein interactions focusing on the recently discovered concentration-dependent binding modes in the octarepeat region of this protein. In addition to determining the binding structures, computer simulations were also used to make predictions about PrP’s function and the role of copper in prion diseases. The results demonstrate the predictive power and applicability of ab initio simulations for studies of metal-biomolecular complexes.}, number={1}, journal={PRION}, author={Hodak, Miroslav and Bernholc, Jerzy}, year={2010}, pages={13–19} }
 @article{zheng_lu_abtew_meunier_bernholc_2010, title={Negative Differential Resistance in C60-Based Electronic Devices}, volume={4}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/nn101902r}, DOI={10.1021/nn101902r}, abstractNote={Unlike single-C(60)-based devices, molecular assemblies based on two or more appropriately connected C(60) molecules have the potential to exhibit negative differential resistance (NDR). In this work, we evaluate electron transport properties of molecular devices built from two C(60) molecules connected by an alkane chain, using a nonequilibrium Green function technique implemented within the framework of density functional theory. We find that electronic conduction in these systems is mediated by the lowest unoccupied molecular orbitals (LUMOs) of C(60), as in the case of a single-C(60)-based device. However, as the positions of the LUMOs are pinned to the chemical potentials of their respective electrodes, their relative alignment shifts with applied bias and leads to a NDR at a very low bias. Furthermore, the position and magnitude of the NDR can be tuned by chemical modification of the C(60) molecules. The role of the attached molecules is to shift the LUMO position and break the symmetry between the forward and reverse currents. The NDR feature can also be controlled by changing the length of the alkane linker. The flexibility and richness of C(60)-based molecular electronics components point to a potentially promising route for the design of molecular devices and chemical sensors.}, number={12}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Zheng, Xiaohong and Lu, Wenchang and Abtew, Tesfaye A. and Meunier, Vincent and Bernholc, Jerry}, year={2010}, month={Nov}, pages={7205–7210} }
 @article{ranjan_yu_nakhmanson_bernholc_nardelli_2010, title={Polarization effects and phase equilibria in high-energy-density polyvinylidene-fluoride-based polymers}, volume={66}, ISSN={["2053-2733"]}, DOI={10.1107/s0108767310026358}, abstractNote={Using first-principles calculations, the phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers under an applied electric field are studied and phase transitions between their nonpolar alpha and polar beta phases are discussed. The results show that the degree of copolymerization is a crucial parameter controlling the structural phase transition. In particular, for tetrafluoroethylene (TeFE) concentration above 12%, PVDF-TeFE is stabilized in the beta phase, whereas the alpha phase is stable for lower concentrations. As larger electric fields are applied, domains with smaller concentrations (< or = 12%) undergo a transition from the alpha to the beta phase until a breakdown field of approximately 600 MV m(-1) is reached. These structural phase transitions can be exploited for efficient storage of electrical energy.}, journal={ACTA CRYSTALLOGRAPHICA A-FOUNDATION AND ADVANCES}, author={Ranjan, V. and Yu, L. and Nakhmanson, Serge and Bernholc, Jerry and Nardelli, M. Buongiorno}, year={2010}, month={Sep}, pages={553–557} }
 @inproceedings{bernholc_ranjan_zheng_jiang_lu_abtew_boguslawski_nardelli_lu_nardelli_et al._2010, title={Properties of High-Performance Capacitor Materials and Nanoscale Electronic Devices}, ISBN={9781612849867}, url={http://dx.doi.org/10.1109/hpcmp-ugc.2010.76}, DOI={10.1109/hpcmp-ugc.2010.76}, abstractNote={Recent advances in theoretical methods combined with the advent of massively parallel supercomputers allow one to reliably simulate the properties of complex materials and device structures from first principles. We describe applications in two general areas: i) novel polymer composites for ultra-high-density capacitors, necessary for pulsed-power applications, such as electric rail guns, power conditioning, and dense electronic circuitry, and ii) electronic properties in nanoelectronic devices, such as graphene nanoribbons and C60-based devices. For capacitor materials, polyvinylidene fluoride (PVDF) with a small concentration of chlorotrifluoroethylene (CTFE) has been observed to store very high energy as compared to currently used polymers. We have recently suggested that the ultra-high energy storage is due to an electric-field-induced phase transition from the non-polar a to the polar b-PVDF. We have now extended our investigations to multi-component polymers and also to the initial stages of kinetics. Turning to nanoelectronic materials, we show that devices based on two C60 molecules can display negative differential resistance at low voltages. We also explore the electronic structure and spin polarization of nitrogen-doped carbon nanoribbons, which are candidate materials for ultra-high-speed nanodevices. We find enhanced N segregation in zigzag nanoribbons, due to interplay between impurity states in the valence bands and the edge states. Spin distribution is significantly affected, even at edges that are quite far from the dopant. We also find that the three armchair nanoribbons (ARs) families, defined by mod (n, 3), behave differently in doping.}, booktitle={2010 DoD High Performance Computing Modernization Program Users Group Conference}, publisher={IEEE}, author={Bernholc, J. and Ranjan, V. and Zheng, X. H. and Jiang, J. and Lu, W. and Abtew, T. A. and Boguslawski, P. and Nardelli, M. Buongiorno and Lu, W. and Nardelli, M. Buongiorno and et al.}, editor={Bernholc, J. and Ranjan, V. and Han, C. and Lu, W. and Nardelli, M.BuongiornoEditors}, year={2010}, month={Jun}, pages={195–200} }
 @article{saha_nikolić_meunier_lu_bernholc_2010, title={Quantum-Interference-Controlled Three-Terminal Molecular Transistors Based on a Single Ring-Shaped Molecule Connected to Graphene Nanoribbon Electrodes}, volume={105}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.105.236803}, DOI={10.1103/physrevlett.105.236803}, abstractNote={We study molecular transistors where graphene nanoribbons act as three metallic electrodes connected to a ring-shaped 18-annulene molecule. Using the nonequilibrium Green function formalism combined with density functional theory, recently extended to multiterminal devices, we show that these nanostructures exhibit exponentially small transmission when the source and drain electrodes are attached in a configuration with destructive interference of electron paths around the ring. The third electrode, functioning either as an attached infinite-impedance voltage probe or as an "air-bridge" top gate covering half of molecular ring, introduces dephasing that brings the transistor into the "on" state with its transmission in the latter case approaching the maximum limit for a single conducting channel device. The current through the latter device can also be controlled in the far-from-equilibrium regime by applying a gate voltage.}, number={23}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Saha, Kamal K. and Nikolić, Branislav K. and Meunier, Vincent and Lu, Wenchang and Bernholc, J.}, year={2010}, month={Dec} }
 @inproceedings{bernholc_yu_ranjan_nardelli_lu_saha_meunier_2009, title={Electronic Properties of High-Performance Capacitor Materials and Nanoscale Multiterminal Devices}, ISBN={9781424457687}, url={http://dx.doi.org/10.1109/hpcmp-ugc.2009.51}, DOI={10.1109/hpcmp-ugc.2009.51}, abstractNote={Recent advances in theoretical methods combined with the advent of massively-parallel supercomputers allow one to reliably simulate the properties of complex materials and device structures from first principles. We describe applications in two general areas: (i) novel ferroelectric oxide-polymer composites for ultrahigh power density capacitors, necessary for pulsed power applications, such as electric discharges, power conditioning, and dense electronic circuitry, and (ii) electron transport properties of ballistic, multi-terminal molecular devices, which could form the basis for ultraspeed electronics and spintronics. For capacitor materials, we investigate the dielectric properties of PbTiO3 slabs and polypropylene/PbTiO3 nanocomposites. We evaluate both the optical and static local dielectric permittivity profiles for isolated PbTiO3 slabs and across the polypropylene/PbTiO3 interface. For thin ferroelectric slabs, we find that in order to maintain the ferroelectric structure, it is necessary to introduce compensating surface charges. Our results show that: (i) the surface-and interface-induced modifications to dielectric permittivity in polymer/metal-oxide composites are localized to only a few atomic layers; (ii) the interface effects are mainly confined to the metal-oxide side; and (iii) metal-oxide particles larger than a few nanometers retain the average macroscopic value of bulk dielectric permittivity. Turning to nanoelectronic devices, we investigate ballistic electron transport through a paradigmatic four-terminal molecular electronic device. In contrast to a conventional two-terminal setup, the same organic molecule placed between four electrodes exhibits new properties, such as a pronounced negative differential resistance.}, booktitle={2009 DoD High Performance Computing Modernization Program Users Group Conference}, publisher={IEEE}, author={Bernholc, J. and Yu, L. and Ranjan, V. and Nardelli, M. Buongiorno and Lu, W. and Saha, K. and Meunier, V.}, year={2009}, month={Jun} }
 @article{yu_ranjan_nardelli_bernholc_2009, title={First-principles investigations of the dielectric properties of polypropylene/metal-oxide interfaces}, volume={80}, ISSN={["2469-9969"]}, DOI={10.1103/PhysRevB.80.165432}, abstractNote={Article on first-principles investigations of the dielectric properties of polypropylene/metal-oxide interfaces. Nanoscale-resolved dielectric properties of polypropylene/metal-oxide (alumina, PbTiO₃) interfaces and of the corresponding surfaces are investigated via first-principles calculations.}, number={16}, journal={PHYSICAL REVIEW B}, author={Yu, Liping and Ranjan, V. and Nardelli, M. Buongiorno and Bernholc, J.}, year={2009}, month={Oct} }
 @article{saha_lu_bernholc_meunier_2009, title={First-principles methodology for quantum transport in multiterminal junctions}, volume={131}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.3247880}, DOI={10.1063/1.3247880}, abstractNote={We present a generalized approach for computing electron conductance and I-V characteristics in multiterminal junctions from first-principles. Within the framework of Keldysh theory, electron transmission is evaluated employing an O(N) method for electronic-structure calculations. The nonequilibrium Green function for the nonequilibrium electron density of the multiterminal junction is computed self-consistently by solving Poisson equation after applying a realistic bias. We illustrate the suitability of the method on two examples of four-terminal systems, a radialene molecule connected to carbon chains and two crossed-carbon chains brought together closer and closer. We describe charge density, potential profile, and transmission of electrons between any two terminals. Finally, we discuss the applicability of this technique to study complex electronic devices.}, number={16}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Saha, Kamal K. and Lu, Wenchang and Bernholc, J. and Meunier, Vincent}, year={2009}, month={Oct}, pages={164105} }
 @article{hodak_chisnell_lu_bernholc_2009, title={Functional implications of multistage copper binding to the prion protein}, volume={106}, ISSN={0027-8424 1091-6490}, url={http://dx.doi.org/10.1073/pnas.0903807106}, DOI={10.1073/pnas.0903807106}, abstractNote={
            The prion protein (PrP) is responsible for a group of neurodegenerative diseases called the transmissible spongiform encephalopathies. The normal function of PrP has not yet been discovered, but indirect evidence suggests a linkage to its ability to bind copper. In this article, low-copper-concentration bindings of Cu
            2+
            to PrP are investigated by using a recently developed hybrid density functional theory (DFT)/DFT method. It is found that at the lowest copper concentrations, the binding site consists of 4 histidine residues coordinating the copper through ε imidazole nitrogens. At higher concentrations, 2 histidines are involved in the binding, one of them in the axial position. These results are in good agreement with existing experimental data. Comparison of free energies for all modes of coordination shows that when enough copper is available, the binding sites will spontaneously rearrange to accommodate more copper ions, despite the fact that binding energy per copper ion decreases with concentration. These findings support the hypothesis that PrP acts as a copper buffer in vivo, protecting other proteins from the attachment of copper ions. Using large-scale classical molecular dynamics, we also probe the structure of full-length copper-bound PrP, including its unfolded N-terminal domain. The results show that copper attachment leads to rearrangement of the structure of the Cu-bonded octarepeat region and to development of turns in areas separating copper-bound residues. These turns make the flexible N-terminal domain more rigid and thus more resistant to misfolding. The last result suggests that copper binding plays a beneficial role in the initial stages of prion diseases.
          }, number={28}, journal={Proceedings of the National Academy of Sciences}, publisher={Proceedings of the National Academy of Sciences}, author={Hodak, Miroslav and Chisnell, Robin and Lu, Wenchang and Bernholc, J.}, year={2009}, month={Jun}, pages={11576–11581} }
 @inproceedings{tomov_lu_bernholc_moore_dongarra_2009, title={Performance evaluation for petascale quantum simulation tools"}, booktitle={Proceedings of Cray User Group 2009 (CUG09): Compute the Future}, author={Tomov, S. and Lu, W. and Bernholc, J. and Moore, S. and Dongarra, J.}, year={2009} }
 @article{ribeiro_lu_bernholc_2008, title={Doping-Dependent Negative Differential Resistance in Hybrid Organic/Inorganic Si−Porphyrin−Si Junctions}, volume={2}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/nn800252b}, DOI={10.1021/nn800252b}, abstractNote={Quantum transport properties of porphyrin-bridged p-n junctions with Si leads are investigated by ab initio calculations. It is shown that this system exhibits strong negative differential resistance (NDR) peaks, whose magnitude and position can be controlled by the doping levels of the leads and by changing the central transition metal atom of the porphyrin. These results are explained by bias-induced on-off switching of resonant tunneling channels associated with specific molecular orbitals. The predicted behavior is general and should be observable for other organic molecules bridging doped semiconducting leads.}, number={8}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Ribeiro, Filipe J. and Lu, Wenchang and Bernholc, Jerzy}, year={2008}, month={Jul}, pages={1517–1522} }
 @article{jiang_lu_bernholc_2008, title={Edge States and Optical Transition Energies in Carbon Nanoribbons}, volume={101}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.101.246803}, DOI={10.1103/physrevlett.101.246803}, abstractNote={The edge states and optical transition energies in carbon nanoribbons are investigated with density-functional calculations. While the ground state of zigzag ribbons is spin polarized, defects at the edges destroy spin polarization and lead to a nonmagnetic ground state. Scanning tunneling spectroscopy will thus show different features depending on edge quality. Optical transition energies in nanoribbons Eii are strongly affected by the edges and confinement, which introduce a term inversely proportional to their width. After removing that term, the scaling of Eii is quantitatively similar to that in carbon nanotubes.}, number={24}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Jiang, J. and Lu, W. and Bernholc, J.}, year={2008}, month={Dec} }
 @article{kim_wang_lu_buongiorno nardelli_bernholc_2008, title={Effects of end group functionalization and level alignment on electron transport in molecular devices}, volume={128}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.2822180}, DOI={10.1063/1.2822180}, abstractNote={The effect of metal-molecule coupling on electron transport is examined in the prototypical case of alkane chains sandwiched between gold contacts and bridged by either amine or thiol groups. The results show that end group functionalization plays a crucial role in controlling electron transport, and that the symmetries and spatial extent of orbitals near the Fermi level control the conductivity rather than the strength of the bonding. For amine/Au and thiol/Au junctions, a crossover in conductivity with increasing bias is predicted.}, number={2}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Kim, Gunn and Wang, Shuchun and Lu, Wenchang and Buongiorno Nardelli, Marco and Bernholc, J.}, year={2008}, month={Jan}, pages={024708} }
 @article{kim_wang_lu_nardelli_bernholc_2008, title={Effects of end group functionalization and level alignment on electron transport in molecular devices}, volume={128}, number={2}, journal={Journal of Chemical Physics}, author={Kim, G. and Wang, S. C. and Lu, W. C. and Nardelli, M. B. and Bernholc, J.}, year={2008} }
 @article{yu_ranjan_lu_bernholc_nardelli_2008, title={Equivalence of dipole correction and Coulomb cutoff techniques in supercell calculations}, volume={77}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.77.245102}, abstractNote={Article on the equivalence of dipole correction and Coulomb cutoff techniques in supercell calculations. In this work, the authors compare the dipole correction and Coulomb cutoff methods under the same conditions in the framework of plane-wave based density-functional theory.}, number={24}, journal={PHYSICAL REVIEW B}, author={Yu, Liping and Ranjan, V. and Lu, W. and Bernholc, J. and Nardelli, M. Buongiorno}, year={2008}, month={Jun} }
 @article{hodak_lu_bernholc_2008, title={Hybrid ab initio Kohn-Sham density functional theory/frozen-density orbital-free density functional theory simulation method suitable for biological systems}, volume={128}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.2814165}, DOI={10.1063/1.2814165}, abstractNote={A hybrid computational method intended for simulations of biomolecules in solution is described. The ab initio Kohn-Sham (KS) density functional theory (DFT) method is used to describe the chemically active part of the system and its first solvation shells, while a frozen-density orbital-free (FDOF) DFT method is used to treat the rest of the solvent. The molecules in the FDOF method have fixed internal structures and frozen electron densities. The hybrid method provides a seamless description of the boundary between the subsystems and allows for the flow of molecules across the boundary. Tests on a liquid water system show that the total energy is conserved well during molecular dynamics and that the effect of the solvent environment on the KS subsystem is well described. An initial application to copper ion binding to the prion protein is also presented.}, number={1}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Hodak, Miroslav and Lu, Wenchang and Bernholc, J.}, year={2008}, month={Jan}, pages={014101} }
 @inproceedings{bernholc_jiang_ranjan_yu_nardelli_lu_2008, title={Multiscale Simulations of High Performance Capacitors and Nanoelectronic Devices}, ISBN={9780769535159 9781424433230}, url={http://dx.doi.org/10.1109/dod.hpcmp.ugc.2008.78}, DOI={10.1109/dod.hpcmp.ugc.2008.78}, abstractNote={Recent advances in theoretical methods combined with the advent of massively-parallel supercomputers allow one to reliably simulate the properties of complex materials and device structures from first principles. We describe applications in two general areas: (i) novel polymer composites for ultrahigh density capacitors, necessary for pulsed power applications, such as electric rail guns, power conditioning, and dense electronic circuitry, and (ii) ballistic electron transport in novel molecule-on-semiconductor structures exhibiting negative differential resistance. The phase diagram of P(VDF-CTFE), which has an usually high energy density, is determined as a function of the electric field. The calculations explain the origin of the observed ultra-high capacitance and suggest a systematic route, not limited to polymers, for obtaining nanostructured materials with high energy density. Turning to molecular electronics, we investigated porphyrins between Si leads, which are candidates for molecular memories and logic. We show that they exhibit tunable negative differential resistance (NDR). In some cases, huge peak-to-valley ratios are obtained, which should result in excellent switching behavior.}, booktitle={2008 DoD HPCMP Users Group Conference}, publisher={IEEE}, author={Bernholc, J. and Jiang, J. and Ranjan, V. and Yu, L. and Nardelli, M. Buongiorno and Lu, W.}, year={2008} }
 @article{bernholc_hodak_lu_2008, title={Recent developments and applications of the real-space multigrid method}, volume={20}, ISSN={0953-8984 1361-648X}, url={http://dx.doi.org/10.1088/0953-8984/20/29/294205}, DOI={10.1088/0953-8984/20/29/294205}, abstractNote={The salient features of the real-space multigrid method and its recent applications are described. This method is suitable for very large scale, massively parallel calculations of atomic and electronic structure, as well as quantum molecular dynamics. Its nearly O(N) implementation provides a compact, variationally optimized basis that is also very useful for fully O(N) calculations of quantum transport. Recently, we also developed a hybrid method for simulating biomolecules in solution, in which most of the solvent is inexpensively treated using an approximate density-functional method, while the biomolecule and its first solvation shells are described at the full Kohn–Sham level. Our calculations show excellent parallel efficiency and scaling on massively parallel supercomputers.}, number={29}, journal={Journal of Physics: Condensed Matter}, publisher={IOP Publishing}, author={Bernholc, J and Hodak, Miroslav and Lu, Wenchang}, year={2008}, month={Jun}, pages={294205} }
 @inbook{boguslawski_bernholc_2008, title={Theoretical Approach to Polarization Effects in Semiconductors}, ISBN={9780387368313 9780387683195}, url={http://dx.doi.org/10.1007/978-0-387-68319-5_1}, DOI={10.1007/978-0-387-68319-5_1}, booktitle={Polarization Effects in Semiconductors}, publisher={Springer US}, author={Boguslawski, Piotr and Bernholc, J.}, year={2008}, pages={2–25} }
 @article{sabir_lu_roland_bernholc_2007, title={Ab initio simulations of H-2 in Li-doped carbon nanotube systems}, volume={19}, number={8}, journal={Journal of Physics. Condensed Matter}, author={Sabir, A. K. and Lu, W. C. and Roland, C. and Bernholc, J.}, year={2007} }
 @article{sabir_lu_roland_bernholc_2007, title={Ab initio simulations of H2 in Li-doped carbon nanotube systems}, volume={19}, ISSN={0953-8984 1361-648X}, url={http://dx.doi.org/10.1088/0953-8984/19/8/086226}, DOI={10.1088/0953-8984/19/8/086226}, abstractNote={Because of their unique structure, it has been proposed that carbon nanotube ropes may well provide an ideal container for the storage of molecular hydrogen. Indeed, there has been some experimental evidence of enhanced hydrogen uptake in doped Li and other alkali metal systems (Chen et al 1999 Science 285 91). We have therefore addressed this issue of hydrogen storage in Li-doped graphite and carbon nanotube systems theoretically with ab initio simulations. Our results find no evidence for such enhanced storage, based on the induced structural changes. In addition, we have also investigated the diffusion barriers for hydrogen to enter into nanotube interiors, both in the presence and absence of topological defects. Even if nanotube interiors are made accessible, the hydrogen uptake remains modest, i.e., less than 3.5 wt%. Mechanically or chemically processing nanotubes is therefore not likely to lead to greatly increased hydrogen storage. Even with its limitation, because of the use of the LDA and GGA approaches, the current investigation must be regarded as a means in the search for proper media that can successfully enhance hydrogen storage.}, number={8}, journal={Journal of Physics: Condensed Matter}, publisher={IOP Publishing}, author={Sabir, A K and Lu, Wenchang and Roland, Christopher and Bernholc, Jerzy}, year={2007}, month={Feb}, pages={086226} }
 @article{bernholc_lu_nakhmanson_hahn_meunier_nardelli_schmidt_2007, title={Atomic scale design of nanostructures}, volume={105}, ISSN={["1362-3028"]}, DOI={10.1080/00268970701189186}, abstractNote={Recent advances in theoretical methods and high performance computing allow for reliable first-principles predictions of complex nanostructured materials and devices. This paper describes three examples: (i) non-equilibrium electron transport through molecular junctions, as a stepping stone for the design of molecular-scale devices and for integration of biomolecules with Si technology; (ii) polarization and piezoelectric properties of PVDF and related polymers; and (iii) the many-body optical spectrum of water. For the molecular junction, our results provide a qualitative picture and quantitative understanding of the mechanism leading to negative differential resistance for a large class of small molecules. For ferroelectric polymers, the calculations show that their polarization is described by cooperative, quantum-mechanical interactions between polymer chains. Nevertheless, the ab initio results lead to a simple parameterization of polarization as a function of copolymer concentration. Finally, our calculations explain the well-known redshift in the fundamental absorption of water as due to exciton delocalization upon aggregation.}, number={2-3}, journal={MOLECULAR PHYSICS}, author={Bernholc, J. and Lu, W. and Nakhmanson, S. M. and Hahn, P. H. and Meunier, V. and Nardelli, M. Buongiorno and Schmidt, W. G.}, year={2007}, pages={147–156} }
 @article{hodak_wang_lu_bernholc_2007, title={Implementation of ultrasoft pseudopotentials in large-scale grid-based electronic structure calculations}, volume={76}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.76.085108}, abstractNote={An implementation of Vanderbilt ultrasoft pseudopotentials in real-space grid-based electronic structure calculations is presented. Efficient utilization of these pseudopotentials requires the use of different grids for i wave functions, ii charge density, and iii sharply peaked operators within the atomic core radii. High-order interpolation between the various grids is important for accuracy, as is high-order discretization for the differential operators. However, efficiency is also of paramount importance, especially when parallelizing over hundreds or thousands of processors. We describe algorithms and procedures used to achieve an effective implementation in the real-space multigrid code, and provide test results for first-row diatomics, bulk transition metals, and energy-conserving quantum molecular dynamics of water. The code parallelizes efficiently over several thousands of processors on modern parallel supercomputers, such as the Cray XT3 and XT4.}, number={8}, journal={PHYSICAL REVIEW B}, author={Hodak, Miroslav and Wang, Shuchun and Lu, Wenchang and Bernholc, J.}, year={2007}, month={Aug} }
 @inproceedings{bernholc_rajan_ribeiro_lu_yu_nardelli_2007, title={Multiscale Simulations of High Performance Capacitors and Nanoelectronic Devices}, url={http://dx.doi.org/10.1109/hpcmp-ugc.2007.54}, DOI={10.1109/hpcmp-ugc.2007.54}, abstractNote={Recent advances in theoretical methods combined with the advent of massively-parallel supercomputers allow one to reliably simulate the properties of complex materials and device structures from .first principles. We describe applications in two general areas: (i) novel polymer composites for ultrahigh density capacitors, necessary for pulsed power applications, such as electric discharges, power conditioning, and dense electronic circuitry, and (ii) electronic properties of graphene nanoribbons, which are candidate materials for ultraspeed electronics and spintronics. Polypropylene is an excellent capacitor dielectric that, however, has already reached its energy density limit. We investigate polypropylene-alumina nanocomposites and show that the dielectric permittivity of the composite can be significantly enhanced. For sufficiently dispersed alumina, the composite should still exhibit the excellent stability and high breakdown .field of pure polypropylene. We have also investigated Poly-Vinylidene Fluoride (PVDF)-based copolymers and show that in addition to co-polymer of VDF-chlorotrifluoroethylene [P(VDFCTFE)], P(VDF-TeFE) can also be a high energy density material, provided an appropriate TeFE concentration and dispersion is achieved. We present a composition range where its energy density would be similar to that of P(VDF-CTFE). For graphene nanoribbons, we report the results of extensive ab initio investigations of the properties of edge states, .nding that the edge-related peaks in the local density of states depend on spin polarization, and that defects at zigzag edges and/or higher-index edges can switch o. The polarization, leading to a non-magnetic ribbon. Our results explain differences between disparate scanning tunneling spectroscopy experiments as due to spin-polarized and unpolarized edges, respectively.}, booktitle={2007 DoD High Performance Computing Modernization Program Users Group Conference}, publisher={IEEE}, author={Bernholc, J. and Rajan, V. and Ribeiro, F. and Lu, W. and Yu, L. and Nardelli, M. Buongiorno}, year={2007}, month={Jun} }
 @article{ranjan_yu_nardelli_bernholc_2007, title={Phase Equilibria in High Energy Density PVDF-Based Polymers}, volume={99}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.99.047801}, DOI={10.1103/physrevlett.99.047801}, abstractNote={The phase diagrams of polyvinylidene fluoride (PVDF) and its copolymers with chlorotrifluoroethylene (CTFE) are investigated by first-principles calculations. Both PVDF and dilute P(VDF-CTFE) prefer nonpolar structures at zero field, but transform to a polar phase below the breakdown field. The critical field decreases with increasing CTFE content, facilitating the transformation. In disordered P(VDF-CTFE), a distribution of concentrations leads to a range of polar transitions, resulting in ultrahigh energy density. These results explain well experimental observations of very high-energy density in P(VDF-CTFE).}, number={4}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Ranjan, V. and Yu, L. and Nardelli, Marco Buongiorno and Bernholc, J.}, year={2007}, month={Jul} }
 @article{meunier_lu_sumpter_bernholc_2006, title={Density functional theory studies of quantum transport in molecular systems}, volume={106}, ISSN={["1097-461X"]}, DOI={10.1002/qua.21197}, abstractNote={Abstract}, number={15}, journal={INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY}, author={Meunier, Vincent and Lu, Wenchang and Sumpter, Bobby G. and Bernholc, Jerry}, year={2006}, month={Dec}, pages={3334–3342} }
 @article{bogusławski_szwacki_bernholc_2006, title={Interfacial Segregation and Electrodiffusion of Dopants inAlN/GaNSuperlattices}, volume={96}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.96.185501}, DOI={10.1103/physrevlett.96.185501}, abstractNote={A first-principles theory of interfacial segregation of dopants and defects in heterostructures is developed and applied to GAN/A1N superlattices. The results indicate that the equilibrium concentrations of a dopant at two sides of an interface may differ by up to a few orders of magnitude, depending on its chemical identity and charge state, and that these cannot be obtained from calculations for bulk constituents alone. In addition, the presence of an internal electric field in polar heterostructures induces electro-migration and accumulation of hydrogen at the appropriate interfaces.}, number={18}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Bogusławski, P. and Szwacki, N. Gonzalez and Bernholc, J.}, year={2006}, month={May} }
 @inproceedings{bernholc_nardelli_lu_ranjan_wang_yu_2006, title={Multiscale Simulations of Quantum Structures}, ISBN={0769527973}, url={http://dx.doi.org/10.1109/hpcmp-ugc.2006.49}, DOI={10.1109/hpcmp-ugc.2006.49}, abstractNote={Recent advances in theoretical methods and parallel supercomputing allow for reliable ab initio simulations of the properties of complex materials. We describe two current applications: (i) electron transport properties of a Si/organic-molecule/Si junction and (ii) polar properties of polyvinylidene fluoride (PVDF) and its copolymers. For the molecular junction, our results provide a qualitative picture and quantitative understanding of the mechanism leading to negative differential resistance for a large class of small molecules. For ferroelectric polymers, the calculations show that their polarization described by cooperative, quantum-mechanical interactions between polymer chains, which cannot be viewed as a superposition of rigid dipoles. Nevertheless, the ab initio results lead to a simple parameterization of polarization in each monomer unit as a function of copolymer concentration.}, booktitle={2006 HPCMP Users Group Conference (HPCMP-UGC'06)}, publisher={IEEE}, author={Bernholc, J. and Nardelli, M. Buongiorno and Lu, W. and Ranjan, V. and Wang, S. and Yu, L.}, year={2006}, month={Jun} }
 @inproceedings{bernholc_lu_nardelli_2006, title={Multiscale Simulations of Quantum Structures}, ISBN={0769524966}, url={http://dx.doi.org/10.1109/dodugc.2005.43}, DOI={10.1109/dodugc.2005.43}, abstractNote={Recent advances in theoretical methods and parallel supercomputing allow for reliable ab initio simulations of the properties of complex materials. We describe two current applications: (i) electron transport properties of a Si/organic-molecule/Si junction and (ii) polar properties of polyvinylidene fluoride (PVDF) and its copolymers. For the molecular junction, our results provide a qualitative picture and quantitative understanding of the mechanism leading to negative differential resistance for a large class of small molecules. For ferroelectric polymers, the calculations show that their polarization described by cooperative, quantum-mechanical interactions between polymer chains, which cannot be viewed as a superposition of rigid dipoles. Nevertheless, the ab initio results lead to a simple parameterization of polarization in each monomer unit as a function of copolymer concentration.}, booktitle={2005 Users Group Conference (DOD-UGC'05)}, publisher={IEEE}, author={Bernholc, J. and Lu, W. and Nardelli, M.B.}, year={2006}, month={Oct} }
 @article{boguslawski_bernholc_2006, title={Properties of wurtzite w-MnN and of w-MnN inclusions in (Ga,Mn)N}, volume={88}, ISSN={["1077-3118"]}, DOI={10.1063/1.2180874}, abstractNote={Mn ions in (Ga,Mn)N exhibit a tendency to form pairs, which is enhanced by ferromagnetic Mn–Mn coupling. We also find that segregation of Mn to form MnN clusters in the wurtzite structure of host GaN is energetically favorable. However, our first-principles calculations show that both w-MnN inclusions stretched to fit the lattice constant of GaN, and relaxed ones with the equilibrium lattice constant of MnN are antiferromagnetic, and thus they are not responsible for the high temperature ferromagnetism reported for (Ga,Mn)N. Incorporation of Mn on the N sites is prohibited by the high formation energy of MnN.}, number={9}, journal={APPLIED PHYSICS LETTERS}, author={Boguslawski, P and Bernholc, J}, year={2006}, month={Feb} }
 @article{wang_lu_zhao_bernholc_2006, title={Resonant coupling and negative differential resistance in metal/ferrocenyl alkanethiolate/STM structures}, volume={74}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.74.195430}, abstractNote={Recent experimental studies demonstrated that self-assembled molecules sandwiched between metallic contacts can perform logic functions based on negative differential resistance NDR. To understand the mechanism of NDR, the electronic structure and transport properties of one such junction, ferrocenyl alkanethiolate attached to a gold surface and probed with a scanning tunneling microscope tip, are investigated by large scale ab initio calculations. The I-V characteristics show strong NDR features at both positive and negative biases, in good agreement with the experimental data. The voltage-dependent transmission, potential drop profile, and molecular level alignment under bias suggest that the ferrocenyl group acts like a quantum dot and that the NDR features are due to resonant coupling between the highest occupied molecular orbital and the density of states of gold leads. The strength of the individual NDR peaks can be tuned by changing the tunneling distance or using suitable spacer layers.}, number={19}, journal={PHYSICAL REVIEW B}, author={Wang, Shuchun and Lu, Wenchang and Zhao, Qingzhong and Bernholc, J.}, year={2006}, month={Nov} }
 @article{zhao_buongiorno nardelli_lu_bernholc_2005, title={Carbon Nanotube−Metal Cluster Composites:  A New Road to Chemical Sensors?}, volume={5}, ISSN={1530-6984 1530-6992}, url={http://dx.doi.org/10.1021/nl050167w}, DOI={10.1021/nl050167w}, abstractNote={Novel carbon nanotube-metal cluster structures are proposed as prototype systems for molecular recognition at the nanoscale. Ab initio calculations show that already the bare nanotube cluster system displays some specificity because the adsorption of ammonia on a carbon nanotube-Al cluster system is easily detected electrically, while diborane adsorption does not provide an electrical signature. Since there are well-established procedures for attaching molecular receptors to metal clusters, these results provide a "proof-of-principle" for the development of novel, high-specificity molecular sensors.}, number={5}, journal={Nano Letters}, publisher={American Chemical Society (ACS)}, author={Zhao, Q. and Buongiorno Nardelli, M. and Lu, W. and Bernholc, J.}, year={2005}, month={May}, pages={847–851} }
 @article{nakhmanson_nardelli_bernholc_2005, title={Collective polarization effects in beta-polyvinylidene fluoride and its copolymers with tri- and tetrafluoroethylene}, volume={72}, DOI={10.1103/physrevb.72.115210}, abstractNote={Article on collective polarization effects in β-polyvinylidene fluoride and its copolymers with tri- and tetrafluoroethylene.}, number={11}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Nakhmanson, S. M. and Nardelli, M. B. and Bernholc, Jerzy}, year={2005} }
 @article{edmonds_boguslawski_wang_campion_novikov_farley_gallagher_foxon_sawicki_dietl_et al._2005, title={Comment on "Mn interstitial diffusion in (Ga,Mn)As'' - Reply}, volume={94}, number={13}, journal={Physical Review Letters}, author={Edmonds, K. W. and Boguslawski, P. and Wang, K. Y. and Campion, R. P. and Novikov, S. V. and Farley, N. R. S. and Gallagher, B. L. and Foxon, C. T. and Sawicki, M. and Dietl, T. and et al.}, year={2005} }
 @article{meunier_lu_bernholc_sumpter_2005, title={Electron transport in molecular systems}, volume={16}, ISSN={1742-6588 1742-6596}, url={http://dx.doi.org/10.1088/1742-6596/16/1/039}, DOI={10.1088/1742-6596/16/1/039}, abstractNote={Large-scale quantum electronic structure calculations coupled with nonequilibrium Green function theory are employed for determining quantum conductance on practical length scales. The combination of state-of-the-art quantum mechanical methods, efficient numerical algorithms, and high performance computing allows for realistic evaluation of properties at length scales that are routinely reached experimentally. Two illustrations of the method are presented. First, quantum chemical calculations using up to 104 basis functions are used to investigate the amphoteric doping of carbon nanotubes by encapsulation of organic molecules. As a second example, we investigate the electron transport properties of a Si/organic molecule/Si junction using a numerically optimized basis.}, journal={Journal of Physics: Conference Series}, publisher={IOP Publishing}, author={Meunier, Vincent and Lu, Wenchang and Bernholc, Jerry and Sumpter, Bobby G}, year={2005}, month={Jan}, pages={283–286} }
 @article{boguslawski_bernholc_2005, title={Fermi-level effects on the electronic structure and magnetic couplings in (Ga,Mn)N}, volume={72}, number={11}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Boguslawski, P. and Bernholc, J.}, year={2005} }
 @article{yi_bernholc_2005, title={Hydrogenation effect on the structural transition of C-60}, volume={403}, ISSN={["1873-4448"]}, DOI={10.1016/j.cplett.2005.01.035}, abstractNote={Abstract The effects of hydrogenation on the structural transitions in C 60 and its isomers are investigated from first principles. C 60 H 2 and C 60 H 60 are used as paradigmatic examples. We find that hydrogenation enhances the stability of the isomers and it reduces the barriers for structural transitions through Stone–Wales bond rotations. The reduction is due to sp 2 bonding of the rotating pair of carbon atoms at the transition state. The calculated barriers for the in-plane Stone–Wales process are 6.03 and 5.30 eV for C 60 H 2 and C 60 H 60 , respectively.}, number={4-6}, journal={CHEMICAL PHYSICS LETTERS}, author={Yi, JY and Bernholc, J}, year={2005}, month={Feb}, pages={359–362} }
 @article{lu_meunier_bernholc_2005, title={Nonequilibrium Quantum Transport Properties of Organic Molecules on Silicon}, volume={95}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.95.206805}, DOI={10.1103/physrevlett.95.206805}, abstractNote={Electron transport properties of a Si/organic-molecule/Si junction are investigated by large-scale nonequilibrium Green function calculations. The results provide a qualitative picture and quantitative understanding of the importance of self-consistent screening, broadening of quasimolecular orbitals under large bias, and enhancement of transmission, which occurs when the broadened lowest unoccupied molecular orbital aligns with the conduction band edge of the negative lead. The varying coupling can lead to negative differential resistance for a large class of small molecules.}, number={20}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Lu, Wenchang and Meunier, Vincent and Bernholc, J.}, year={2005}, month={Nov} }
 @article{hahn_schmidt_seino_preuss_bechstedt_bernholc_2005, title={Optical Absorption of Water: Coulomb Effects versus Hydrogen Bonding}, volume={94}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.94.037404}, DOI={10.1103/physrevlett.94.037404}, abstractNote={The optical spectrum of water is not well understood. For example, the main absorption peak shifts upwards by 1.3 eV upon condensation, which is contrary to the behavior expected from aggregation-induced broadening of molecular levels. We investigate theoretically the effects of electron-electron and electron-hole correlations, finding that condensation leads to delocalization of the exciton onto nearby hydrogen-bonded molecules. This reduces its binding energy and has a dramatic impact on the line shape. The calculated spectrum is in excellent agreement with experiment.}, number={3}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Hahn, P. H. and Schmidt, W. G. and Seino, K. and Preuss, M. and Bechstedt, F. and Bernholc, J.}, year={2005}, month={Jan} }
 @inproceedings{bogusławski_bernholc_2005, title={Theory of Electronic Structure and Magnetic Interactions in (Ga,Mn)N and (Ga,Mn)As}, volume={772}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.1994603}, DOI={10.1063/1.1994603}, abstractNote={Using density functional theory we investigate the electronic structure of Mn ions and the magnetic Mn‐Mn coupling in (Ga,Mn)N and (Ga,Mn)As. Our calculations show that the magnetic interactions critically depend on the position of the Fermi level and thus not only on Mn concentration but also on the presence of dopants.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={Bogusławski, P. and Bernholc, J.}, year={2005}, pages={1331} }
 @article{nakhmanson_nardelli_bernholc_2004, title={Ab Initio Studies of Polarization and Piezoelectricity in Vinylidene Fluoride and BN-Based Polymers}, volume={92}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.92.115504}, DOI={10.1103/physrevlett.92.115504}, abstractNote={Highly piezoelectric and pyroelectric phases of boron-nitrogen-based polymers have been designed from first principles. They offer excellent electrical and structural properties, with up to 100% improvement in the piezoelectic response and an enhanced thermal stability with respect to polyvinylidene fluoride (PVDF). Since methods for their synthesis are readily available, these polymers are extremely promising for numerous technological applications, rivaling the properties of ferroelectric ceramics and superseding PVDF-based materials in high-performance devices.}, number={11}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Nakhmanson, S. M. and Nardelli, M. Buongiorno and Bernholc, J.}, year={2004}, month={Mar}, pages={115504} }
 @article{fleischer_chandola_esser_richter_mcgilp_schmidt_wang_lu_bernholc_2004, title={Atomic indium nanowires on Si(111): the (4 × 1)–(8 × 2) phase transition studied with reflectance anisotropy spectroscopy}, volume={234}, ISSN={0169-4332}, url={http://dx.doi.org/10.1016/j.apsusc.2004.05.114}, DOI={10.1016/j.apsusc.2004.05.114}, abstractNote={Abstract Despite much experimental and theoretical effort, the electronic and the structural properties of the (4 × 2)/(8 × 2)-low-temperature phase of in nanowires on Si(1 1 1) are still under discussion. In our combined experimental and theoretical work we utilize the surface selectivity of reflectance anisotropy spectroscopy (RAS) to analyze the electronic (surface electronic states) properties. The RAS response is directly compared with \emph{ab-initio} density functional theory (DFT)–local density approximation (LDA) calculations of the optical anisotropy of different surface models.}, number={1-4}, journal={Applied Surface Science}, publisher={Elsevier BV}, author={Fleischer, K and Chandola, S and Esser, N and Richter, W and McGilp, J.F and Schmidt, W.G and Wang, S and Lu, W and Bernholc, J}, year={2004}, month={Jul}, pages={302–306} }
 @article{schmidt_seino_hahn_bechstedt_lu_wang_bernholc_2004, title={Calculation of surface optical properties: from qualitative understanding to quantitative predictions}, volume={455}, ISSN={["0040-6090"]}, DOI={10.1016/j.tsf.2003.11.263}, abstractNote={In the last couple of years there has been much methodological and computational progress in the modeling of optical properties from first principles. While the calculation of non-linear optical coefficients is still hampered by numerical limitations—demonstrated here for the case of bulk GaAs—linear optical spectra can now be calculated accurately and with true predictive power, even for large and complex surface structures. This allows on one hand for a much better understanding of the origin of specific features such as surface optical anisotropies. We find that in particular microscopic electric fields at the surface induce slight deformations of bulk-like wavefunctions and thus give rise to optical anisotropies even from sub-surface layers. On the other hand, from the comparison of measured and calculated spectra, one can now confidently reach conclusions on the surface geometry. This short review focuses on the simulation of reflectance anisotropy spectroscopy. The clean, hydrogenated and uracil-covered Si(001) surface is used to illustrate the microscopic origin of surface optical anisotropies and the present state-of-the-art in computational modeling of optical spectra.}, number={2004 May 1}, journal={THIN SOLID FILMS}, author={Schmidt, WG and Seino, K and Hahn, PH and Bechstedt, E and Lu, W and Wang, S and Bernholc, J}, year={2004}, month={May}, pages={764-+} }
 @article{fleischer_chandola_esser_richter_mcgilp_schmidt_wang_lu_bernholc_2004, title={Determination of the electron inelastic mean free path in some binary alloys for application in quantitative surface analysis}, volume={235}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2004.05.117}, abstractNote={Quantitative surface- and thin film analysis by electron spectroscopies (AES, XPS, EELS) requires the knowledge of one of the most important parameters of the electron transport such as the inelastic mean free path (IMFP) of electrons. Numerous data on the IMFP have been already published, mainly for elemental solids and some inorganic and organic compounds. However, the IMFPs for some binary alloy systems are still lacking. Although IMFP values for complex solids can be calculated from predictive formulae, IMFPs can be also measured experimentally by elastic peak electron spectroscopy (EPES). The present work is dealing with the experimental determination of the IMFP in selected binary alloys, i.e. AuxCu100−x (x=25, 75 at.%), AuxPd100−x (x=10, 90 at.%) and AuxNi100−x (x=5 at.%) within the 200–2000 eV range. The relative EPES experiments have been performed using a double-pass CMA spectrometer and the Ni standard. The measured IMFPs were compared with IMFPs calculated from the TPP-2M predictive formula. The RMS deviation from IMFP values calculated from the TPP-2M equation was 0.6–2.5 Å depending on the alloy surface composition, and it was smallest for the Au5Ni95 alloy. The mean percentage deviation from the TPP-2M IMFPs was 4.3–17.6%.}, number={1-2}, journal={APPLIED SURFACE SCIENCE}, author={Fleischer, K. and Chandola, S. and Esser, N. and Richter, W. and McGilp, J. F. and Schmidt, W. G. and Wang, S. and Lu, W. and Bernholc, Jerzy}, year={2004}, month={Jul}, pages={15–20} }
 @inproceedings{bernholc_nardelli_lu_nakhmanson_meunier_2004, title={Large-Scale Quantum-Mechanical Simulations of Nanoscale Devices and New Materials}, ISBN={0769522599}, url={http://dx.doi.org/10.1109/dod_ugc.2004.26}, DOI={10.1109/dod_ugc.2004.26}, abstractNote={Recent advances in theoretical methods and high performance computing allow for reliable first-principles investigations of nanoscale devices and complex materials. Using large scale O(N) real-space-based ab initio calculations, we carried out a theoretical study of carbon nanotube-cluster composites as prototype systems for molecular sensing at the nanoscale. Dramatic changes in the electrical conductance of the composite are predicted when gas molecules are adsorbed onto the metal clusters. The observed sensitivity and selectivity might suggest new avenues for the design and production of nanotube-based molecular sensors. The second part of this article focuses on calculating and predicting the properties of piezoelectrics, and on "designing" new materials with enhanced piezoelectric response. We consider polymers in the polyvinylidene fluoride (PVDF) family and show that our calculations not only reproduce well the existing experimental data, but also provide a much improved understanding of their polar properties, which leads to a "design" of novel polymers with a BN backbone. The new polymers are predicted to have up to 100% better piezoelectric response and an enhanced thermal stability with respect to their PVDF analogs. Since methods for their synthesis are readily available, they offer a promising avenue for improving ferro and piezoelectric devices.}, booktitle={2004 Users Group Conference (DOD_UGC'04)}, publisher={IEEE}, author={Bernholc, J. and Nardelli, M.B. and Lu, W. and Nakhmanson, S. and Meunier, V.}, year={2004} }
 @article{edmonds_bogusławski_wang_campion_novikov_farley_gallagher_foxon_sawicki_dietl_et al._2004, title={Mn Interstitial Diffusion in(Ga,Mn)As}, volume={92}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.92.037201}, DOI={10.1103/physrevlett.92.037201}, abstractNote={We present a combined theoretical and experimental study of the ferromagnetic semiconductor (Ga,Mn)As which explains the remarkably large changes observed on low-temperature annealing. Careful control of the annealing conditions allows us to obtain samples with ferromagnetic transition temperatures up to 159 K. Ab initio calculations, in situ Auger spectroscopy, and resistivity measurements during annealing show that the observed changes are due to out diffusion of Mn interstitials towards the surface, governed by an energy barrier of 0.7-0.8 eV. Electric fields induced by Mn acceptors have a significant effect on the diffusion.}, number={3}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Edmonds, K. W. and Bogusławski, P. and Wang, K. Y. and Campion, R. P. and Novikov, S. N. and Farley, N. R. S. and Gallagher, B. L. and Foxon, C. T. and Sawicki, M. and Dietl, T. and et al.}, year={2004}, month={Jan} }
 @article{schmidt_fuchs_hermann_seino_bechstedt_passmann_wahl_gensch_hinrichs_esser_et al._2004, title={Oxidation- and organic-molecule-induced changes of the Si surface optical anisotropy: ab initio predictions}, volume={16}, ISSN={["1361-648X"]}, DOI={10.1088/0953-8984/16/39/007}, abstractNote={In the last couple of years there has been much methodological and computational progress in the modelling of optical properties from first principles. Reflectance anisotropy spectra (RAS) can now be calculated with true predictive power and can thus be used to draw conclusions directly on the surface geometry. In the present work we study two potentially very interesting applications for RAS: the oxidation of Si(001) and the functionalization of the Si surface with organic molecules. Our calculations confirm experimental indications that the polarity of the interface-induced optical anisotropy is reversed layer by layer with increasing oxide thickness. The oscillation of the RAS amplitude should thus allow for the quantitative monitoring of the vertical progression of the oxidation. Our results for Si(001) surfaces modified by cyclopentene and 9,10-phenanthrenequinone adsorption show a strong sensitivity of the RAS signal with respect to the adsorption geometry. Comparison with experimental data shows that cyclopentene most probably adsorbs via a cycloaddition reaction with the Si surface dimers, while phenanthrenequinone seems to adsorb across two Si dimers.}, number={39}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Schmidt, WG and Fuchs, F and Hermann, A and Seino, K and Bechstedt, F and Passmann, R and Wahl, M and Gensch, M and Hinrichs, K and Esser, N and et al.}, year={2004}, month={Oct}, pages={S4323–S4334} }
 @inproceedings{bernholc_buongiorno nardelli_lu_meunier_nakhmanson_zhao_2004, title={Simulations of nanotube-based structures and devices}, volume={367}, booktitle={Annual Conference Proceedings of the Foundations of Nanoscience: Self-assembled architectures and devices}, publisher={ScienceTechnica}, author={Bernholc, J. and Buongiorno Nardelli, M. and Lu, W. and Meunier, V. and Nakhmanson, S. and Zhao, Q.}, year={2004} }
 @article{bernholc_nakhmanson_nardelli_meunier_2004, title={Understanding and enhancing polarization in complex materials}, volume={6}, ISSN={["1521-9615"]}, DOI={10.1109/mcse.2004.78}, abstractNote={Recent advances in theoretical methods and high-performance computing allow for reliable first-principles investigations of complex materials. This article focuses on calculating and predicting the properties of piezoelectrics and "designing" new materials with enhanced piezoelectric responses. This paper considers two systems: boron-nitride nanotubes (BNNTs) and polymers in the polyvinylidene fluoride (PVDF) family.}, number={6}, journal={COMPUTING IN SCIENCE & ENGINEERING}, author={Bernholc, J and Nakhmanson, SM and Nardelli, MB and Meunier, V}, year={2004}, pages={12–21} }
 @article{lu_schmidt_bernholc_2003, title={Cycloaddition reaction versus dimer cleavage at the Si(001): C5H8 interface}, volume={68}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.68.115327}, abstractNote={The interface formed between an ordered monolayer of cyclopentene and silicon is studied by first-principles density-functional calculations. Several different structural models of the interface are considered and their reflectance anisotropy spectra are calculated. The spectra turn out to be highly structure dependent and can therefore be used to monitor the interface formation. We also find that coadsorption of hydrogen, which leads to dimer cleavage, can stabilize the interface by saturating the dangling bonds and releasing the high strain energy.}, number={11}, journal={PHYSICAL REVIEW B}, author={Lu, WC and Schmidt, WG and Bernholc, J}, year={2003}, month={Sep} }
 @article{kang_bernholc_2003, title={First-principle study of molecular springs under shear deformation}, volume={107}, ISSN={["1089-5639"]}, DOI={10.1021/jp0267353}, abstractNote={Recently synthesized tunable molecular springs are investigated theoretically using massively parallel density functional simulations with the generalized gradient approximation. The springs are salts of organosilver complexes that crystallize in structures with monoclinic symmetry. For springs with NO 3 - (N-spring) and ClO 4 - (Cl-spring) ions as negative balancers, we are able to refine their X-ray structures. Our calculations of total energies as functions of the nonorthogonal lattice angle β correctly reproduce the experimental equilibrium values of the angle for both the N- and Cl-springs. For the N-spring, our calculations reveal that the nitrate ions undergo concerted propeller rotations in the clockwise direction as the angle increases by 10° around the experimental value. For the Cl-spring, the rotations of chlorate ions are more enhanced in a limited range of the β angle, but they move in the counterclockwise direction. For the N-spring, the potential energy curve is symmetric and the shear modulus is about 0.01TPa. Calculations of the electronic density of states show that both springs are semiconductors.}, number={9}, journal={JOURNAL OF PHYSICAL CHEMISTRY A}, author={Kang, HS and Bernholc, J}, year={2003}, month={Mar}, pages={1377–1383} }
 @article{pristovsek_tsukamoto_ohtake_koguchi_orr_schmidt_bernholc_2003, title={Gallium-rich reconstructions on GaAs(001)}, volume={240}, ISSN={["1521-3951"]}, DOI={10.1002/pssb.200301885}, abstractNote={Abstract}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Pristovsek, M and Tsukamoto, S and Ohtake, A and Koguchi, N and Orr, BG and Schmidt, WG and Bernholc, J}, year={2003}, month={Nov}, pages={91–98} }
 @article{wang_lu_schmidt_bernholc_2003, title={Nanowire-induced optical anisotropy of the Si(111)-In surface}, volume={68}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.68.035329}, abstractNote={Ab initio calculations of the reflectance anisotropy of Si(111)-In surfaces are presented. A very pronounced optical anisotropy around 2 eV is predicted for the structural model proposed by Bunk et al. [Phys. Rev. B 59, 12 228 (1999)] for the (4 × 1) reconstructed surface. The (4 X 2)/(8 × 2) reconstructed surface, induced by a slight distortion of the indium chains, is shown to result in a splitting of the 2 eV peak. The calculated results are in excellent agreement with recent polarized reflectance data acquired during the (4×1)→(4×2)/ (8×2) phase transition.}, number={3}, journal={PHYSICAL REVIEW B}, author={Wang, SC and Lu, WC and Schmidt, WG and Bernholc, J}, year={2003}, month={Jul} }
 @article{meunier_nardelli_bernholc_zacharia_charlier_2003, title={Response to “Comment on ‘Intrinsic electron transport properties of carbon nanotube Y junctions’ ” [Appl. Phys. Lett. 83, 1674 (2003)]}, volume={83}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.1604949}, DOI={10.1063/1.1604949}, abstractNote={Related Articles Quantized charge pumping through a carbon nanotube double quantum dot Appl. Phys. Lett. 100, 143104 (2012) Electrical characteristics of multiwalled carbon nanotube arrays and influence of pressure AIP Advances 2, 022103 (2012) Percolation threshold and electrical conductivity of a two-phase composite containing randomly oriented ellipsoidal inclusions J. Appl. Phys. 110, 123715 (2011) Tuning the electrical transport properties of double-walled carbon nanotubes by semiconductor and semi-metal filling J. Appl. Phys. 110, 123708 (2011) Electronic and transport properties of achiral carbon nanotubes with di-vacancy pairs J. Appl. Phys. 110, 113702 (2011)}, number={8}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Meunier, Vincent and Nardelli, Marco Buongiorno and Bernholc, J. and Zacharia, Thomas and Charlier, Jean-Christophe}, year={2003}, month={Aug}, pages={1676–1677} }
 @article{nakhmanson_calzolari_meunier_bernholc_nardelli_2003, title={Spontaneous polarization and piezoelectricity in boron nitride nanotubes}, volume={67}, ISSN={["1098-0121"]}, DOI={10.1103/physrevb.67.235406}, abstractNote={Ab initio calculations of the spontaneous polarization and piezoelectric properties of boron nitride nanotubes show that they are excellent piezoelectric systems with response values larger than those of piezoelectric polymers. The intrinsic chiral symmetry of the nanotubes induces an exact cancellation of the total spontaneous polarization in ideal, isolated nanotubes of arbitrary indices. Breaking of this symmetry by intertube interaction or elastic deformations induces spontaneous polarization comparable to those of wurtzite semiconductors. order of magnitude weaker than those of PZT. 3 In this paper, we examine spontaneous polarization and piezoelectricity in boron nitride nanotubes ~BNNT's! in order to estimate their potential usefulness in various pyroelectric and piezoelectric device applications, and to understand the interplay between symmetry and polarization in nanotubular systems. BNNT's, broadly investigated since their initial predic- tion 4 and succeeding discovery, 5 are already well known for their excellent mechanical properties. 6 However, unlike car- bon nanotubes ~CNT's !, most of BN structures are noncen- trosymmetric and polar, which might suggest the existence of nonzero spontaneous polarization fields. Recently, these properties have been partially explored by Mele and Kral, using a model electronic Hamiltonian. 7 They predicted that BNNT's are piezoelectric and pyroelectric, with the direction of the spontaneous electric field that changes with the index of the tubes. The ab initio calculations presented in this pa- per provide a much fuller description and show that BNNT systems are indeed excellent lightweight piezoelectrics, with comparable or better piezoelectric response and superior me- chanical properties than in piezoelectric polymers. However, contrary to the conclusions of Ref. 7, our combined Berry phase and Wannier function ~WF! analysis demonstrates that electronic polarization in BNNT's does not change its direc- tion but rather grows monotonically with the increasing di- ameter of the tube. Furthermore, the electronic and ionic spontaneous polarizations in BNNT's cancel exactly and these systems are pyroelectric only if their intrinsic helical symmetry is broken by, e.g., intertube interactions or elastic distortions. The rest of this paper is organized as follows: Sec. II briefly reviews the formulation of the modern polarization theory in terms of Berry phases or Wannier functions. It also presents the details of the numerical techniques that were used to compute polarization. In Sec. III we discuss the re- sults and the complementary nature of the two techniques to compute the spontaneous polarization. Finally, Sec. IV pre- sents the summary and conclusions.}, number={23}, journal={PHYSICAL REVIEW B}, author={Nakhmanson, SM and Calzolari, A and Meunier, V and Bernholc, J and Nardelli, MB}, year={2003}, month={Jun} }
 @inbook{bernholc_buongiorno nardelli_orlikowski_roland_zhao_2002, title={ATOMIC TRANSFORMATIONS, STRENGTH, PLASTICITY, AND ELECTRON TRANSPORT IN STRAINED CARBON NANOTUBES}, ISBN={9780080441047}, url={http://dx.doi.org/10.1016/b978-008044104-7/50016-2}, DOI={10.1016/b978-008044104-7/50016-2}, abstractNote={Nanotubes are hollow cylinders consisting of ‘rolled-up’ graphitic sheets. They form spontaneously in the same apparatus as the famed C60 molecule, and have been predicted and/or observed to have even more spectacular properties than C60, including extremely high strength and flexibility, ability to form nanoscale electronic devices consisting entirely of carbon, strong capillary effects, cold cathode field emission, etc. Carbon nanotubes have also been theoretically predicted to be among the strongest materials known. Their strength, which has already been verified experimentally, may enable unique applications in many critical areas of technology. While very high strain rates must lead to tube breakage, nanotubes with (n,m) indices, where n,m < 14, can display plastic flow under suitable conditions. This occurs through the conversion of four hexagons to a 5–7–7–5 defect, which then splits into two 5–7 pairs. The index of the tube changes between the 5–7 pairs, potentially leading to metal–semiconductor junctions. Furthermore, carbon adatoms-induced transformations in strained nanotubes can lead to the formation of quantum dots. The high-strain conditions can be imposed on the tube via, e.g., AFM tip manipulations, and we show that such procedures can lead to intratube device formation. The defects and the index changes occurring during the mechanical transformations also affect the electrical properties of nanotubes. The computed quantum conductances of strained defective and deformed tubes show that the defect density and the contacts play key roles in reducing the conductance at the Fermi energy. The role of bending in changing the electrical properties was also explored. It was found that mechanical deformations do not significantly affect the conductance of bent armchair nanotubes up to substantial bending angles, while a conductivity gap is induced by the bending of chiral nanotubes. These results are in good agreement with recent experimental data.}, booktitle={Fiber Fracture}, publisher={Elsevier}, author={Bernholc, J. and Buongiorno Nardelli, M. and Orlikowski, D. and Roland, C. and Zhao, Q.}, year={2002}, pages={357–376} }
 @article{meunier_kephart_roland_bernholc_2002, title={Ab initio investigations of lithium diffusion in carbon nanotube systems}, volume={88}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.88.075506}, abstractNote={Li-nanotube systems can substantially improve the capacity of Li-ion batteries by utilizing both nanotube exteriors and interiors. Our ab initio simulations show that while Li motion through the sidewalls is forbidden, Li ions can enter tubes through topological defects containing at least nine-sided rings, or through the ends of open-ended nanotubes. Once inside, their motion is not diffusion limited. These results suggest that "damaging" nanotube ropes by either chemical or mechanical means will yield superior material for electrochemical storage.}, number={7}, journal={PHYSICAL REVIEW LETTERS}, author={Meunier, V and Kephart, J and Roland, C and Bernholc, J}, year={2002}, month={Feb} }
 @article{meunier_roland_bernholc_nardelli_2002, title={Electronic and field emission properties of boron nitride/carbon nanotube superlattices}, volume={81}, ISSN={["1077-3118"]}, DOI={10.1063/1.1491013}, abstractNote={BN/C nanotube superlattices are quasi one-dimensional heterostructures that show unique physical properties derived from their peculiar geometry. Using state-of-the-art ab initio calculations, we show that BN/C systems can be used for effective band-offset nanodevice engineering, polarization-based devices, and robust field emitters with an efficiency enhanced by up to two orders of magnitude over carbon nanotube systems.}, number={1}, journal={APPLIED PHYSICS LETTERS}, author={Meunier, V and Roland, C and Bernholc, J and Nardelli, MB}, year={2002}, month={Jul}, pages={46–48} }
 @article{meunier_buongiorno nardelli_shelton_roland_bernholc_zacharia_2002, title={Field Emission Properties of BN/C and BN@C Hybrid Nanotubes}, volume={739}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-739-h5.7}, DOI={10.1557/proc-739-h5.7}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Meunier, Vincent and Buongiorno Nardelli, Marco and Shelton, William and Roland, Christopher and Bernholc, Jerry and Zacharia, Thomas}, year={2002} }
 @article{schmidt_bechstedt_bernholc_2002, title={GaAs(001) surface reconstructions: geometries, chemical bonding and optical properties}, volume={190}, ISSN={["0169-4332"]}, DOI={10.1016/S0169-4332(01)00862-5}, abstractNote={We re-examine the GaAs(0 0 1) surface by means of first-principles calculations based on a real-space multigrid method. The c(4×4),(2×4)and(4×2) surface reconstructions minimize the surface energy for anion-rich, stoichiometric and cation-rich surfaces, respectively. Structural models proposed in the literature to explain the Ga-rich GaAs(0 0 1) (4×6) surface are dismissed on energetic grounds. The electronic properties of the novel ζ(4×2) structure are discussed in detail. We calculate the reflectance anisotropy of the energetically most favoured surfaces. A strong influence of the surface geometry on the optical anisotropy is found.}, number={1-4}, journal={APPLIED SURFACE SCIENCE}, author={Schmidt, WG and Bechstedt, F and Bernholc, J}, year={2002}, month={May}, pages={264–268} }
 @article{schmidt_bechstedt_lu_bernholc_2002, title={Interplay of surface reconstruction and surface electric fields in the optical anisotropy of GaAs(001)}, volume={66}, number={8}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Schmidt, W. G. and Bechstedt, F. and Lu, W. and Bernholc, J.}, year={2002}, pages={085334–1} }
 @article{meunier_nardelli_bernholc_zacharia_charlier_2002, title={Intrinsic electron transport properties of carbon nanotube Y-junctions}, volume={81}, ISSN={["0003-6951"]}, DOI={10.1063/1.1533842}, abstractNote={The electron transport properties of three-terminal carbon-nanotube junctions are investigated within the Landauer theory of quantum conductance. Using a realistic tight-binding Hamiltonian, we demonstrate that the experimentally observed rectifying behavior is not an intrinsic property of the junction, but rather of the contact geometry. When semiconducting nanotubes are connected to metallic leads, nontransmitting states are induced at the nanotube–metal interface, leading to asymmetric transmission curves and potentially rectifying behavior of the nanodevice.}, number={27}, journal={APPLIED PHYSICS LETTERS}, author={Meunier, V and Nardelli, MB and Bernholc, J and Zacharia, T and Charlier, JC}, year={2002}, month={Dec}, pages={5234–5236} }
 @misc{bernholc_brenner_nardelli_meunier_roland_2002, title={Mechanical and electrical properties of nanotubes}, volume={32}, ISSN={["1531-7331"]}, DOI={10.1146/annurev.matsci.32.112601.134925}, abstractNote={▪ Abstract  We review the recent progress in our understanding of the mechanical and electrical properties of carbon nanotubes, emphasizing the theoretical aspects. Nanotubes are the strongest materials known, but the ultimate limits of their strength have yet to be reached experimentally. Modeling of nanotube-reinforced composites indicates that the addition of small numbers of nanotubes may lead to a dramatic increase in the modulus, with only minimal crosslinking. Deformations in nanotube structures lead to novel structural transformations, some of which have clear electrical signatures that can be utilized in nanoscale sensors and devices. Chemical reactivity of nanotube walls is facilitated by strain, which can be used in processing and functionalization. Scanning tunneling microscopy and spectroscopy have provided a wealth of information about the structure and electronic properties of nanotubes, especially when coupled with appropriate theoretical models. Nanotubes are exceptional ballistic conductors, which can be used in a variety of nanodevices that can operate at room temperature. The quantum transport through nanotube structures is reviewed at some depth, and the critical roles played by band structure, one-dimensional confinement, and coupling to nanoscale contacts are emphasized. Because disorder or point defect–induced scattering is effectively averaged over the circumference of the nanotube, electrons can propagate ballistically over hundreds of nanometers. However, severe deformations or highly resistive contacts isolate nanotube segments and lead to the formation of quantum dots, which exhibit Coulomb blockade effects, even at room temperature. Metal-nanotube and nanotube-nanotube contacts range from highly transmissive to very resistive, depending on the symmetry of two structures, the charge transfer, and the detailed rehybridization of the wave functions. The progress in terms of nanotube applications has been extraordinarily rapid, as evidenced by the development of several nanotube-based prototypical devices, including memory and logic circuits, chemical sensors, electron emitters and electromechanical actuators.}, number={2002}, journal={ANNUAL REVIEW OF MATERIALS RESEARCH}, author={Bernholc, J and Brenner, D and Nardelli, MB and Meunier, V and Roland, C}, year={2002}, pages={347-+} }
 @article{seino_schmidt_bechstedt_bernholc_2002, title={Structure and energetics of Ga-rich GaAs(001) surfaces}, volume={507}, ISSN={["0039-6028"]}, DOI={10.1016/S0039-6028(02)01278-5}, abstractNote={The atomic structures and energies of Ga-rich GaAs(0 0 1) surface reconstructions are examined by means of first-principles total-energy calculations based on a real-space multigrid method. Our calculations confirm the existence of the novel ζ(4×2) structure suggested by Lee et al. [Phys. Rev. Lett. 85 (2000) 3890]. (4×6) surface reconstructions suggested to explain STM experiments are found to be unstable. The calculations indicate that the adsorption of Ga adatoms in the trenches of the ζ(4×2) surface could possibly explain the observed structures. The diffusion of Ga/As adatoms on the Ga-rich GaAs surface is predicted to be anisotropic and should preferably take place parallel to the [1 1 0]/[1 1̄ 0] direction, respectively.}, number={2002 Jun 1}, journal={SURFACE SCIENCE}, author={Seino, K and Schmidt, WG and Bechstedt, F and Bernholc, J}, year={2002}, month={Jun}, pages={406–410} }
 @article{boguslawski_bernholc_2002, title={Surface segregation of Ge at SiGe(001) by concerted exchange pathways}, volume={88}, ISSN={["0031-9007"]}, DOI={10.1103/physrevlett.88.166101}, abstractNote={The segregation of Ge during growth on SiGe(001) surfaces was investigated by ab initio calculations. Four processes involving adatoms rather than ad-dimers were considered. The two most efficient channels proceed by the concerted exchange mechanism and involve a swap between an incorporated Ge and a Si adatom, or between Si and Ge in the first and the second surface layers, respectively. The calculated activation energies of approximately 1.5 eV explain well the high-temperature experimental data. Segregation mechanisms involving step edges are much less efficient.}, number={16}, journal={PHYSICAL REVIEW LETTERS}, author={Boguslawski, P and Bernholc, J}, year={2002}, month={Apr} }
 @article{zhao_nardelli_bernholc_2002, title={Ultimate strength of carbon nanotubes: A theoretical study}, volume={65}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.65.144105}, abstractNote={The ultimate strength of carbon nanotubes is investigated by large-scale quantum calculations. While the formation energy of strain-induced topological defects determines the thermodynamic limits of the elastic response and of mechanical resistance to applied tension, it is found that the activation barriers for the formation of such defects are much larger than estimated previously. The theoretical results indicate a substantially greater resilience and strength, and show that the ultimate strength limit of carbon nanotubes has yet to be reached experimentally. Nanotubes are indeed the strongest material known.}, number={14}, journal={PHYSICAL REVIEW B}, author={Zhao, QZ and Nardelli, MB and Bernholc, J}, year={2002}, month={Apr} }
 @article{schmidt_bechstedt_fleischer_cobet_esser_richter_bernholc_onida_2001, title={GaAs(001): Surface structure and optical properties}, volume={188}, ISSN={["0031-8965"]}, DOI={10.1002/1521-396x(200112)188:4<1401::aid-pssa1401>3.0.co;2-1}, abstractNote={The optical anisotropy of differently reconstructed GaAs(001) surfaces has been analysed both theoretically and experimentally. The atomic structures and RAS spectra are calculated from first principles for the As-rich c(4 × 4) and β2(2 × 4) as well as for the stoichiometric α2(2 × 4) and the Ga-rich ζ(4 × 2) surface phases. These results are compared with spectra recorded at low temperature (40 K). We find good agreement between the calculated and measured data, in particular for the As-rich surface phases. In marked contrast to earlier calculations we find the peak near the E1 critical point energy, characteristic of the β2(2 × 4) surface, to originate from electronic transitions in bulk layers. The experimental data for the Ga-rich (4 × 2) surface phase are less well reproduced, possibly due to surface defects or structural deviations from the ζ(4 × 2) model for the surface geometry.}, number={4}, journal={PHYSICA STATUS SOLIDI A-APPLIED RESEARCH}, author={Schmidt, WG and Bechstedt, F and Fleischer, K and Cobet, C and Esser, N and Richter, W and Bernholc, J and Onida, G}, year={2001}, month={Dec}, pages={1401–1409} }
 @inbook{bernholc_briggs_nardelli_fattebert_ramamoorthy_schmidt_sullivan_2001, place={Amsterdam}, series={NATO science series: Computer and Systems Sciences}, title={Large-scale, multilevel solutions of Kohn-Sham equations: methodology and applications}, booktitle={Multiscale Computational Methods in Chemistry and Physics}, publisher={IOS Press}, author={Bernholc, J. and Briggs, E.L. and Nardelli, M. and Fattebert, J.-L. and Ramamoorthy, M. and Schmidt, W.G. and Sullivan, D.J.}, editor={Brandt, A. and Bernholc, J. and Binder, K.Editors}, year={2001}, collection={NATO science series: Computer and Systems Sciences} }
 @article{meunier_kephart_roland_bernholc_2001, title={Li Uptake in Carbon Nanotube Systems: A First Principles Investigation}, volume={706}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-706-z8.8.1}, DOI={10.1557/proc-706-z8.8.1}, abstractNote={Abstract}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Meunier, Vincent and Kephart, Jeremy and Roland, Christopher and Bernholc, Jerry}, year={2001} }
 @inbook{brandt_bernholc_binder_2001, place={Amsterdam}, series={NATO Science Series}, title={Multiscale Computational Methods in Chemistry and Physics}, volume={Series III}, booktitle={Computer and Systems Sciences}, publisher={IOS Press}, year={2001}, collection={NATO Science Series} }
 @article{nardelli_fattebert_bernholc_2001, title={O(N) real-space method for ab initio quantum transport calculations: Application to carbon nanotube-metal contacts}, volume={64}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.64.245423}, abstractNote={We present an ab initio O(N) method that combines an accurate optimized-orbital solution of the electronic structure problem with an efficient Green's function technique for evaluating the quantum conductance. As an important illustrative example, we investigate carbon nanotube-metal contacts and explain the anomalously large contact resistance observed in nanotube devices as due to the spatial separation of their conductance eigenchannels. The results for various contact geometries and strategies for improving device performance are discussed. The study of the electrical properties of nanostructures has seen intense activity in the last decade, due to the prom- ise of novel technological applications for nanoscale quan- tum electronic devices. The theoretical study of quantum conductance in such structures has thus become of primary interest and has been addressed by a variety of techniques. 1 Due to the complexity of describing an ''open'' system of a nanoscale device in contact with effectively infinite leads, most of the current approaches rely on phenomenological tight binding models, which for many systems may not pro- vide a sufficiently reliable and accurate description. There are only few examples of ab initio calculations of quantum conductance and the field is still in a critical phase of development. The existing methods are based on the so- lution of the quantum scattering problem for the electronic wave functions through the conductor using a number of related techniques: Lippman-Schwinger and perturbative Green's function methods have been used to study conduc- tance in metallic nanowires and recently in small molecular nanocontacts; 2,3 conduction in nanowires, junctions, and nanotube systems has been addressed using local 4 or nonlocal 5,6 pseudopotential methods and through the solution of the coupled-channel equations in a scattering-theoretic approach. 7-9 These methods are based on a plane wave rep- resentation of the electronic wave functions, which imposes severe restrictions on the size of the system because of the large number of basis functions necessary for an accurate description of the electron transmission process. Therefore, structureless jellium leads, which do not provide a micro- scopic description of the conductor-metal contact, had to be assumed in most cases for computational reasons. Only re- cently have real-space approaches been considered for a more efficient solution of the electronic transport problem. They are based on the use of linear combination of atomic orbitals 10 ~LCAO! or Gaussian 11 orbital bases. These are combined with either a scattering state solution for the transmission 10 or Green's function-based techniques. 11 In this paper we present an approach based on a real- space optimized-orbital solution of the electronic structure problem, combined with an efficient Green's function-based technique for the evaluation of the electron transmission probability. Both the ab initio and the transport algorithms scale essentially linearly with the size of the system, thus extending greatly the range of applicability of our method. This method has been already successfully applied to de- scribe quantum conductance in ideal and defective carbon nanotubes. 12 Following a brief overview of the methodology, we address the problem of contacts in a metal-carbon nano- tube assembly, which is very important in the design of effi- cient nanotube-based devices. Contact resistances of the or- der of MV are typically observed in most of the prototypical nanotube-based devices realized so far, 13-16 whereas from simple band structure arguments one would expect resis- tances of the order of a few tenths of kV, 17 because the fundamental resistance of a single ballistic channel is 12.9 kV. The results of our calculations provide an explanation for this pathologically high contact resistance and suggest strategies to improve the performance of nanotube-metal contacts. It is important to stress that this problem requires self-consistent ab initio methodology, in order to accurately describe the highly inhomogeneous environment of a nanowire-metal junction and to account for the charge trans- fer occurring at the interface between the two dissimilar ma- terials.}, number={24}, journal={PHYSICAL REVIEW B}, author={Nardelli, MB and Fattebert, JL and Bernholc, J}, year={2001}, month={Dec} }
 @inbook{bogusławski_bernholc_2001, title={Point Defects and Impurities in Silicon Carbide and Group III-Nitrides}, ISBN={9780080431529}, url={http://dx.doi.org/10.1016/b0-08-043152-6/01263-8}, DOI={10.1016/b0-08-043152-6/01263-8}, booktitle={Encyclopedia of Materials: Science and Technology}, publisher={Elsevier}, author={Bogusławski, P. and Bernholc, J.}, year={2001}, pages={7131–7139} }
 @article{nardelli_fattebert_bernholc_2001, title={Quantum Transport in Nanotube-Based Structures}, volume={706}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-706-z8.2.1}, DOI={10.1557/proc-706-z8.2.1}, abstractNote={Abstract}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Nardelli, M. Buongiorno and Fattebert, J.-L. and Bernholc, J.}, year={2001} }
 @article{meunier_nardelli_roland_bernholc_2001, title={Structural and electronic properties of carbon nanotube tapers}, volume={64}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.64.195419}, abstractNote={Since their initial discovery in 1990 by Iijima, 1 carbon nanotubes have come under ever increasing scientific scrutiny. They not only have outstanding mechanical and electrical properties, but also show considerable technological potential as field emitters and electrochemical storage devices. 2 In the emerging field of nanotechnology, carbon nanotubes are playing a crucial role by providing a suitable ‘‘test bed’’ or ‘‘laboratory’’ for materials properties at the nanometer length scale. Single-wall carbon nanotubes are formed when a graphene sheet is curled up into a cylinder and the carbon atoms are joined seamlessly to each other. Nanotubes are therefore characterized by their length, diameter, and helicity. The latter is a measure of the orientation of the graphene sheet as it is folded to form nanotubes. Following the notation of Hamada et al., 3 the structure of a nanotube is described by a pair of integers ( l,m), which give the coordinates of its circumference vector in the basis of the primitive lattice vector of graphene. The helicity is important because it determines both the mechanical and electrical properties of}, number={19}, journal={PHYSICAL REVIEW B}, author={Meunier, V and Nardelli, MB and Roland, C and Bernholc, J}, year={2001}, month={Nov} }
 @article{schmidt_bechstedt_bernholc_2001, title={Terrace and step contributions to the optical anisotropy of Si(001) surfaces}, volume={63}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.63.045322}, abstractNote={The contributions of atomically flat terraces as well as of SA , SB , andDB steps to the optical anisotropy of Si~001! surfaces have been calculated using a real-space multigrid method together with ab initio pseudopotentials. Our results for ideal (1 32), p(232), andc(234) reconstructed surfaces show a distinct influence of the dimer arrangement on the optical spectra. The calculated spectrum for the Si(001) c(234 surface agrees best with the signal measured for atomically smooth terraces. The significant optical anisotropy around 3 eV observed for vicinal surfaces, however, is induced by surface steps. Both electronic transitions directly at the surface as well as in deeper layers contribute to the optical anisotropy. We identify two mechanisms causing anisotropy signals from layers beneath the surface: the influence of the anisotropic surface potential on the bulk wave functions as well as minor contributions from atomic relaxations caused by surface-induced stress.}, number={4}, journal={PHYSICAL REVIEW B}, author={Schmidt, WG and Bechstedt, F and Bernholc, J}, year={2001}, month={Jan} }
 @article{schmidt_bernholc_bechstedt_2000, title={(001) Surfaces of GaP and InP: structural motifs, electronic states and optical signatures}, volume={166}, ISSN={["0169-4332"]}, DOI={10.1016/S0169-4332(00)00406-2}, abstractNote={We present ab initio calculations on the energetics and geometry, as well as electronic and optical properties of InP and GaP(001) surfaces. Cation-rich conditions lead to the formation of asymmetric cation–anion dimers on top of a (2×4) reconstructed cation-terminated surface. Anion-rich surfaces form c(4×4) reconstructions. Based on the DFT-LDA electronic structure, we compute the reflectance anisotropy of the energetically favoured (2×4) reconstructions. Strong anisotropies in the low-energy region arise from transitions between σ-like cation–cation bonding states and empty dangling bonds. Transitions involving P dimer states and surface modified bulk wave functions contribute at higher energies. The application of GW corrections leads to non-uniform shifts of characteristic peaks and changes the line shape, considerably improving the agreement with experiment.}, number={1-4}, journal={APPLIED SURFACE SCIENCE}, author={Schmidt, WG and Bernholc, J and Bechstedt, F}, year={2000}, month={Oct}, pages={179–184} }
 @article{bungaro_rapcewicz_bernholc_2000, title={Ab initio phonon dispersions of wurtzite AlN, GaN, and InN}, volume={61}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.61.6720}, abstractNote={Phonon excitations play an important role in electronic transport, nonradiative electron-relaxation processes, and other properties of interest for materials characterization, device engineering, and design. We have calculated the phonon dispersions and density of states for wurtzite AlN, GaN, and InN using state-of-the-art density-functional perturbation theory. The calculations are in good agreement with the existing experimental data for zone-center modes and predict the full phonon dispersions throughout the Brillouin zone. In particular, it is found that the three-phonon decay of the LO phonon in two acoustic phonons is not allowed in GaN and InN, since the LO frequency is much larger than the acoustic frequencies over the entire spectrum. The substantial potential of the group-III nitrides and their alloys for applications in optoelectronic and high speed devices has attracted a great deal of interest. AlN, GaN, and InN have direct energy gaps which span a substantial range, from the visible to the ultraviolet region of the spectrum. Consequently, their alloys have direct gaps that can be tuned to any value within this range simply by varying the alloy composition. This tunability offers many possibilities for device engineering. In particular, InGaN alloys were used in the realization of light emitting diodes and laser diodes operating in the blue and UV spectral region. Under ambient conditions, the III nitrides crystallize in a hexagonal, wurtzite ~2H! structure, although thin films having a cubic, zincblende ~3C! structure have also been grown. 1 Properties of interest for device engineering and design, such as electronic transport, nonradiative electron relaxation processes, lattice specific heat, etc., are strongly influenced by phonon excitations. Furthermore, a number of nondestructive experimental techniques of sample characterization, for instance, Raman spectroscopy or IR reflectivity, involve phonon measurements. A characterization of the phonon dispersions and densities of states for the group-III nitrides is therefore desirable. However, since it is very challenging to grow single crystals of suitable size for neutron-scattering experiments, there are no experimental data for the phonon dispersions of these compounds. Only very recently have the phonon density of states for AlN and GaN been obtained from time-of-flight neutron spectroscopy using bulk powders. 2,3 In addition, numerous studies of the zone-center phonons in GaN and AlN films have been conducted using Raman and IR spectroscopy. Due to the lack of latticematched substrates, these samples are affected by the high density of defects and strain present in the films and it is therefore unclear how well these data represent the true bulk values. The least studied of the three nitrides is InN for which there are only few Raman studies. 4,5}, number={10}, journal={PHYSICAL REVIEW B}, author={Bungaro, C and Rapcewicz, K and Bernholc, J}, year={2000}, month={Mar}, pages={6720–6725} }
 @article{frisch_vogt_visbeck_hannappel_willig_braun_richter_bernholc_schmidt_esser_et al._2000, title={Angle resolved photoemission spectroscopy of the InP(001) surface}, volume={166}, ISSN={["1873-5584"]}, DOI={10.1016/S0169-4332(00)00431-1}, abstractNote={The P-rich (2×1)/(2×2) and the In-rich (2×4) reconstructions of InP(001) are studied by Angle Resolved Photoemission Spectroscopy (ARPES). Experimental results for the In-rich (2×4) InP(001) surface show two bound surface states and one surface resonance, located at −1.6 eV binding energy. This agrees well with ab-initio calculations. For the P-rich (2×1)/(2×2) InP(001) surface, neither theoretical nor experimental work regarding the electronic surface structure exists until now. We identify two possible surface states showing no discernible dispersion along the Γ&#x0304;–J̄ and Γ&#x0304;–J̄′ directions of the surface Brillouin zone (SBZ). The energetically highest state, located close to the valence band maximum (VBM), is only observed along the [1&#x0304;10] direction and is most likely a P-dimer-bond state. The second state, located at −4.4 eV binding energy, is assigned to a surface resonance.}, number={1-4}, journal={APPLIED SURFACE SCIENCE}, author={Frisch, A. M. and Vogt, P. and Visbeck, S. and Hannappel, T. and Willig, F. and Braun, W. and Richter, W. and Bernholc, Jerzy and Schmidt, W. G. and Esser, N. and et al.}, year={2000}, month={Oct}, pages={224–230} }
 @misc{gilman_2000, title={Calculating buckyballs and nanotubes}, volume={53}, ISSN={["0031-9228"]}, DOI={10.1063/1.1292500}, number={7}, journal={PHYSICS TODAY}, author={Gilman, JJ}, year={2000}, month={Jul}, pages={76–76} }
 @article{bernholc_briggs_bungaro_nardelli_fattebert_rapcewicz_roland_schmidt_zhao_2000, title={Large-scale applications of real-space multigrid methods to surfaces, nanotubes, and quantum transport}, volume={217}, ISSN={["1521-3951"]}, DOI={10.1002/(sici)1521-3951(200001)217:1<685::aid-pssb685>3.0.co;2-3}, abstractNote={The development and applications of real-space multigrid methods are discussed. Multigrid techniques provide preconditioning and convergence acceleration at all length scales, and therefore lead to particularly efficient algorithms. When using localization regions and optimized, non-orthogonal orbitals, calculations involving over 1000 atoms become practical on massively parallel computers. The applications discussed in this chapter include: (i) dopant incorporation and ordering effects during surface incorporation of boron, which lead to the formation of ordered domains at half-monolayer coverage; (ii) incorporation of Mg into GaN during growth, and in particular the conditions that would lead to maximum p-type doping; (iii) optical fingerprints of surface structures for use in real-time feedback control of growth: and (iv) mechanisms of stress release and quantum transport properties of carbon nanotubes.}, number={1}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Bernholc, J and Briggs, EL and Bungaro, C and Nardelli, MB and Fattebert, JL and Rapcewicz, K and Roland, C and Schmidt, WG and Zhao, Q}, year={2000}, month={Jan}, pages={685–701} }
 @misc{ceder_2000, title={Materials science needs and is getting quantitative methods}, volume={53}, ISSN={["0031-9228"]}, DOI={10.1063/1.882979}, number={2}, journal={PHYSICS TODAY}, author={Ceder, G}, year={2000}, month={Feb}, pages={75–76} }
 @article{nardelli_fattebert_orlikowski_roland_zhao_bernholc_2000, title={Mechanical properties, defects and electronic behavior of carbon nanotubes}, volume={38}, ISSN={["1873-3891"]}, DOI={10.1016/S0008-6223(99)00291-2}, abstractNote={Using state-of-the-art classical and quantum simulations, we have studied the mechanical and electronic response of carbon nanotubes to external deformations, such as strain and bending. In strained nanotubes the spontaneous formation of double pentagon–heptagon defect pairs is observed. Tubes containing these defects are energetically preferred to uniformly stretched tubes at strains greater than 5%. These defects act as nucleation centers for the formation of dislocations in the originally ideal graphitic network and constitute the onset of further deformations of the carbon nanotube. In particular, plastic or brittle behaviors can occur depending upon the external conditions and tube symmetry. We have also investigated the effects that the presence of addimers has on strained carbon nanotubes. The main result is the formation of a new class of defects that wrap themselves about the circumference of the nanotube. These defects are shown to modify the geometrical structure and to induce the formation of nanotube-based quantum dots. Finally, we computed transport properties for various ideal and mechanically deformed carbon nanotubes. High defect densities are shown to greatly affect transport in individual nanotubes, while small diameter bent armchair nanotubes mantam thier basic electrical properties even in presence of large deformations with no defects involved.}, number={11-12}, journal={CARBON}, author={Nardelli, MB and Fattebert, JL and Orlikowski, D and Roland, C and Zhao, Q and Bernholc, J}, year={2000}, pages={1703–1711} }
 @article{lu_schmidt_briggs_bernholc_2000, title={Optical Anisotropy of the SiC(001)-(3×2) Surface: Evidence for the Two-Adlayer Asymmetric-Dimer Model}, volume={85}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.85.4381}, DOI={10.1103/physrevlett.85.4381}, abstractNote={The structure of the ( 3x2) reconstruction of beta-SiC(001) surface has been identified by comparing reflectance anisotropy spectra calculated from first principles with recent measurements. Only the calculations for the two-adlayer asymmetric-dimer model agree with experiment. The two prominent peaks at 3.6 and 5.0 eV found experimentally are assigned to electronic transitions between surface and bulklike electronic states. A further pronounced anisotropy at 2.0 eV, due to transitions between surface states, is predicted.}, number={20}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Lu, Wenchang and Schmidt, W. G. and Briggs, E. L. and Bernholc, J.}, year={2000}, month={Nov}, pages={4381–4384} }
 @article{schmidt_bernholc_2000, title={Step-induced optical anisotropy of Si(111): H surfaces}, volume={61}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.61.7604}, abstractNote={The step-induced optical anisotropy of the hydrogenated Si(111) surface has been calculated using a real-space multigrid method together with ab initio pseudopotentials. The investigated step configurations give rise to relatively similar anisotropy spectra with maxima close to the ${E}_{1}$ and ${E}_{2}$ critical points of the silicon bulk band structure. The spectrum for $〈112\ifmmode\bar\else\textasciimacron\fi{}〉$ steps resembles the reflectance anisotropy for Si(110):H and is in good agreement with measurements for Si(111):H vicinally cut toward $[112\ifmmode\bar\else\textasciimacron\fi{}].$ Our results for $〈1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2〉$ steps, however, are qualitatively different from the corresponding measurements for surfaces miscut toward $[1\ifmmode\bar\else\textasciimacron\fi{}1\ifmmode\bar\else\textasciimacron\fi{}2].$ This apparent discrepancy can be resolved if the formation of staircaselike structures of monohydride terminated steps is assumed.}, number={11}, journal={PHYSICAL REVIEW B}, author={Schmidt, WG and Bernholc, J}, year={2000}, month={Mar}, pages={7604–7608} }
 @article{boguslawski_rapcewicz_bernholc_2000, title={Surface segregation and interface stability of AlN/GaN, GaN/InN, and AlN/InN {0001} epitaxial systems}, volume={61}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.61.10820}, abstractNote={segregation energies are similar and an order of magnitude larger than in the arsenides or Si/Ge systems. The largest segregation energy of about 3.6 eV is found for the AlN/InN surface. In contrast, segregation effects for the N-adatom 232 reconstruction on the ~0001! surfaces are strongly suppressed. At AlN/GaN and GaN/InN interfaces the segregation effects are very weak and should not affect their morphology. Our results suggest the choice of optimal conditions of growth that lead to sharp interfaces and longer lifetimes of carriers in III-nitride heterosystems.}, number={16}, journal={PHYSICAL REVIEW B}, author={Boguslawski, P and Rapcewicz, K and Bernholc, JJ}, year={2000}, month={Apr}, pages={10820–10826} }
 @article{orlikowski_nardelli_bernholc_roland_2000, title={Theoretical STM signatures and transport properties of native defects in carbon nanotubes}, volume={61}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.61.14194}, abstractNote={We have investigated theoretically the STM images and conductance signatures of defective carbon nanotubes. The defects considered are those that form on nanotubes under tension, both in the absence and presence of additional carbon atoms. The most prominent features observed in the STM images are a set of bright rings, whose positions correlate with the location of the pentagons within the defect. These features are useful, as they enable the ready identification of many of the defects. By contrast, most of the defects have only a relatively modest effect on the transport properties of the nanotubes. While there is a general decrease in the conductance of the nanotube due to scattering effects, there appears to be no unique feature that can be associated with any of the defects investigated. This unfortunately precludes the use of transport measurements as a means of defect identification. The STM images and conductances of nanotube heterojunctions, as well as tubes under different bias voltages, have also been explored.}, number={20}, journal={PHYSICAL REVIEW B}, author={Orlikowski, D and Nardelli, MB and Bernholc, J and Roland, C}, year={2000}, month={May}, pages={14194–14203} }
 @article{roland_bernholc_brabec_nardelli_maiti_2000, title={Theoretical investigations of carbon nanotube growth}, volume={25}, ISSN={["0892-7022"]}, DOI={10.1080/08927020008044108}, abstractNote={Abstract The growth of carbon nanotubes was investigated using a variety of complementary simulation techniques. Currently, a number of experimental methods are used to synthesize carbon nanotubes suggesting that different mechanisms play a role in their formation. However, it has been shown that growth of nanotubes takes place primarily at the open-ended tips of nanotubes. Ab initio simulations show that the high electric fields present at the nanotube tips in carbon arc discharges cannot be responsible for keeping the tubes open. Rather, the opening and closing of tubes is controlled by the formation of curvature-inducing defects such as adjacent pentagon pairs. On narrow tubes, the formation of such defects is favored leading to the rapid closure of the tubes. By contrast, the formation of hexagons, which lead to straight open-ended growth is favored on large-diameter tubes, with an estimated crossover radius of about 3 nm. Large-scale molecular dynamics and kinetic Monte Carle simulations have been used to verify these ideas. We have also explored the role of the so-called lip–lip interactions during growth. Such an interaction is important in producing multiwalled nanotubes, where the interaction between two open nanotube tips leads to the formation of a network of bonds. Simulations show that such an interaction is indeed significant, but does not provide the additional stabilization required for straight, open-ended, multiwalled nanotube growth. Finally, we consider the formation of nanotubes in the presence of large and small catalytic particles. In the former case, growth is believed to take place via a root-growth mechanism, while the direct adsorption and extrusion of carbon from the vapor dominates the latter. Both mechanisms lead to the formation of small-diameter, single-wall nanotubes.}, number={1-2}, journal={MOLECULAR SIMULATION}, author={Roland, C and Bernholc, J and Brabec, C and Nardelli, MB and Maiti, A}, year={2000}, pages={1–12} }
 @article{fattebert_bernholc_2000, title={Towards grid-based O(N) density-functional theory methods: Optimized nonorthogonal orbitals and multigrid acceleration}, volume={62}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.62.1713}, abstractNote={We have formulated and implemented a real-space ab initio method for electronic structure calculations in terms of nonorthogonal orbitals defined on a grid. A multigrid preconditioner is used to improve the steepest descent directions used in the iterative minimization of the energy functional. Unoccupied or partially occupied states are included using a density matrix formalism in the subspace spanned by the nonorthogonal orbitals. The freedom introduced by the nonorthogonal real-space description of the orbitals allows for localization constraints that linearize the cost of the most expensive parts of the calculations, while keeping a fast convergence rate for the iterative minimization with multigrid acceleration. Numerical tests for carbon nanotubes show that very accurate results can be obtained for localization regions with radii of 8 bohr. This approach, which substantially reduces the computational cost for very large systems, has been implemented on the massively parallel Cray T3E computer and tested on carbon nanotubes containing more than 1000 atoms.}, number={3}, journal={PHYSICAL REVIEW B}, author={Fattebert, JL and Bernholc, J}, year={2000}, month={Jul}, pages={1713–1722} }
 @article{schmidt_bechstedt_bernholc_2000, title={Understanding reflectance anisotropy: Surface-state signatures and bulk-related features}, volume={18}, ISSN={["1071-1023"]}, DOI={10.1116/1.1305289}, abstractNote={Using a real-space multigrid method and ab initio pseudopotentials we calculated the reflectance anisotropy spectra for (001) growth structures of GaP and InP as well as for stepped Si(111):H and hydrogenated Si(110) surfaces. Our results, obtained within DFT-LDA, show good qualitative agreement with recent experiments. Further improvement is achieved by using a numerically efficient GW approach with approximations for local-field effects and dynamical screening. We find the contributions to the anisotropy signal related either directly to surface states or to transitions between surface-perturbed bulk wave functions. Our results demonstrate the high sensitivity of RAS to the surface structure and chemistry and show that the optical transitions causing the anisotropy signal take place mainly in a few uppermost atomic layers of the substrate.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Schmidt, WG and Bechstedt, F and Bernholc, J}, year={2000}, pages={2215–2223} }
 @article{schmidt_esser_frisch_vogt_bernholc_bechstedt_zorn_hannappel_visbeck_willig_et al._2000, title={Understanding reflectance anisotropy: Surface-state signatures and bulk-related features in the optical spectrum of InP(001)(2X4)}, volume={61}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.61.r16335}, abstractNote={A detailed analysis based on first-principlescalculations with self-energy corrections is combined with photoemission spectroscopy to determine the origin of features observed in reflectance anisotropy spectroscopy ~RAS! at semiconductor surfaces. Using the InP(001)(2 34) surface as a model case we obtain quantitative agreement between slab calculations and low-temperature RAS measurements. We find the contributions to the anisotropy signal related either directly to surface states or to transitions between surface perturbed bulk wave functions. Our results demonstrate the high sensitivity of RAS to the surface structure and chemistry and show that the absorption processes causing the anisotropy signal take place in the uppermost few atomic layers of the substrate.}, number={24}, journal={PHYSICAL REVIEW B}, author={Schmidt, WG and Esser, N and Frisch, AM and Vogt, P and Bernholc, J and Bechstedt, F and Zorn, M and Hannappel, T and Visbeck, S and Willig, F and et al.}, year={2000}, month={Jun}, pages={16335–16338} }
 @article{frisch_schmidt_bernholc_pristovsek_esser_richter_1999, title={(2x4) GaP(001) surface: Atomic structure and optical anisotropy}, volume={60}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.60.2488}, abstractNote={We have investigated the microscopic structure and optical anisotropy of $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstructed GaP(001) surfaces. Optical and electron spectroscopy from GaP(001) surfaces prepared in ultrahigh-vacuum conditions were combined with first-principles calculations of the energetics and reflectance anisotropy. Symmetry, composition and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, photoemission spectroscopy and reflectance anisotropy spectroscopy. In contrast to most earlier reports, we find that the stable Ga-rich surface corresponds to a $(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction. No $(4\ifmmode\times\else\texttimes\fi{}2)$ reconstruction could be observed, independent of the preparation method. Depending on the Ga coverage, however, two distinct line shapes in the reflection anisotropy spectra occur, indicating the existence of at least two different surface phases with $(2\ifmmode\times\else\texttimes\fi{}4)$ periodicity. This agrees with our total-energy calculations: Four $(2\ifmmode\times\else\texttimes\fi{}4)$ structural models may be stable depending on the chemical potentials of the surface constituents. All considered $(4\ifmmode\times\else\texttimes\fi{}2)$ structures, however, are unstable. Based on the comparison between calculated reflectance anisotropy spectra and measured data we suggest mixed Ga-P dimers on top of the Ga-terminated substrate as ground-state geometry for the cation-rich phase of GaP(001)$(2\ifmmode\times\else\texttimes\fi{}4)$. Our results indicate the formation of P dimers at the surface for the more anion-rich phase of GaP(001)$(2\ifmmode\times\else\texttimes\fi{}4)$.}, number={4}, journal={PHYSICAL REVIEW B}, author={Frisch, AM and Schmidt, WG and Bernholc, J and Pristovsek, M and Esser, N and Richter, W}, year={1999}, month={Jul}, pages={2488–2494} }
 @article{orlikowski_nardelli_bernholc_roland_1999, title={Ad-dimers on strained carbon nanotubes: A new route for quantum dot formation?}, volume={83}, ISSN={["0031-9007"]}, DOI={10.1103/PhysRevLett.83.4132}, abstractNote={Among the many remarkable properties of single-walled carbon nanotubes, it is their mechanical and electronic properties that stand out [1]. The excellent resistance of carbon nanotubes to bending [2 –7] should lead to future applications of the tubes as a high-strength, lightweight material. Turning to their electronic properties, we find that, at ambient temperatures, single-walled carbon nanotubes may be either metallic or semiconducting, depending upon their helicity [8– 12]. It has been shown [13] that tubes with different helicities may be joined together with one or more pentagon-heptagon (5-7) defects to form different electronic heterojunctions, thereby opening up the intriguing possibility of forming all-carbon based microelectronic devices [14,15]. Some of these remarkable theoretical predictions have recently been confirmed by scanning tunneling microscopy (STM) experiments [16]. However, if the dream of producing an all-carbon-based microelectronics is ever to be realized, then different methods will be needed to produce a variety of devices. Here, we present the results of a large-scale study of the mechanical transformations of strained nanotubes in the presence of ad-dimers and show that this combination may well turn out to be a natural route for the formation of all-carbon nanotube-based quantum dots. Ad-dimers are likely to be present in small amounts on as-grown carbon nanotubes, or they may be deposited there with a STM tip or other methods. The formation of quantum dots with ad-dimers is particularly favorable for the n ,0 zigzag tubes. Specifically, we show that ad-dimers induce plastic behavior on tubes that are otherwise brittle. Before discussing the simulations, we briefly review the}, number={20}, journal={PHYSICAL REVIEW LETTERS}, author={Orlikowski, D and Nardelli, MB and Bernholc, J and Roland, C}, year={1999}, month={Nov}, pages={4132–4135} }
 @article{bernholc_buongiorno nardelli_fattebert_orlikowski_roland_rose_zhao_1999, place={Cambridge, UK}, title={Atomic Transformations and Quantum Transport in Carbon Nanotubes}, volume={593}, DOI={10.1557/proc-593-547}, abstractNote={ABSTRACT}, journal={MRS Online Proceedings: Symposium U – Amorphous & Nanostructured Carbon}, publisher={Cambridge/Materials Research Society}, author={Bernholc, J. and Buongiorno Nardelli, M. and Fattebert, J.-L. and Orlikowski, D. and Roland, C. and Rose, F. and Zhao, Q.}, editor={Robertson, J. and Sullivan, J.P. and Zhou, O. and Allen, T.B. and Coll, B.F.Editors}, year={1999} }
 @article{ramamoorthy_briggs_bernholc_1999, title={Chemical and strain effects on Boron-doped Si(100)}, volume={59}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.59.4813}, abstractNote={Theoretical investigations uncover complex strain-induced modes of boron ordering at the Si(100) surface, which are highly anomalous in comparison with other group III impurities. The fundamental units of the clustering process are subsurface pairs of B atoms. The structural relaxations around segregated B impurities are substantial and the induced strain fields couple with the $(2\ifmmode\times\else\texttimes\fi{}1)$ dimer reconstruction of the surface to stabilize complex, zigzag modes of ordering. Impurity configurations exist that are strongly bound with respect to isolated subsurface impurity pairs up to the critical doping level of 0.5 monolayer. Above this doping level, however, all modes of ordering of the impurities at the surface are repulsive. A number of experimental observations are explained and interesting structures are predicted.}, number={7}, journal={PHYSICAL REVIEW B}, author={Ramamoorthy, M and Briggs, EL and Bernholc, J}, year={1999}, month={Feb}, pages={4813–4821} }
 @article{bernholc_1999, title={Computational materials science: The era of applied quantum mechanics}, volume={52}, ISSN={["1945-0699"]}, DOI={10.1063/1.882840}, abstractNote={For many centuries, materials were discovered, mined, and processed in a largely serendipitous way. However, the characterization of the atom and the progress made in x-ray diffraction during the early years of this century started a quest for a theory of materials in terms of their atomic constituents. Later decades saw scientists developing many qualitative and semi-quantitative models that explained the principles of atomic cohesion and the basic properties of semiconductors, metals, and salts. Considering their simplicity, some of the models were surprisingly accurate and led to remarkable progress. However, for most materials of current interest, the interatomic interactions are intricate enough to require fairly elaborate models. Fortunately, we are entering an era in which high-performance computing is coming into its own, allowing true predictive simulations of complex materials to be made from information on their individual atoms.}, number={9}, journal={PHYSICS TODAY}, author={Bernholc, J}, year={1999}, month={Sep}, pages={30–35} }
 @article{ramamoorthy_briggs_bernholc_1999, title={Defect energetics and impurity incorporation mechanisms at the arsenic-passivated Si(100) surface}, volume={60}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.60.8178}, abstractNote={Theoretical calculations show that defect properties of the Si~100! and Si~100!:As surfaces are completely different. Large atomic relaxations around vacancies near the Si~100! surface cause chemical rebonding and defect healing that greatly lowers their formation energies. However, passivation of the surface by a monolayer of As induces substantial structural rigidity in the near-surface region. This reduces atomic relaxations and raises vacancy formation energies to high values, inhibiting vacancy mediated processes near the surface. The formation energies of silicon interstitials near the As-passivated surface are significantly lower than those of vacancies, which favors an interstitial mode of arsenic incorporation into the bulk during in diffusion. These results explain the observed uniformity of the Si~100!:As surface and the high level of electrical activation of in-diffused As. @S0163-1829~99!00835-8#}, number={11}, journal={PHYSICAL REVIEW B}, author={Ramamoorthy, M and Briggs, EL and Bernholc, J}, year={1999}, month={Sep}, pages={8178–8184} }
 @article{esser_schmidt_bernholc_frisch_vogt_zorn_pristovsek_richter_bechstedt_hannappel_et al._1999, title={GaP(001) and InP(001): Reflectance anisotropy and surface geometry}, volume={17}, ISSN={["1071-1023"]}, DOI={10.1116/1.590810}, abstractNote={We have investigated the optical anisotropy of GaP(001) and InP(001) surfaces. The samples were prepared by homoepitaxial metalorganic vapor phase epitaxy growth and either directly transferred into ultrahigh vacuum (UHV) or in situ capped and, after transfer, decapped in UHV by thermal desorption of a P/As capping layer. Symmetry, composition, and surface optical anisotropy were characterized by low-energy electron diffraction, Auger electron spectroscopy, and reflectance anisotropy spectroscopy. We observe (2×1)/(2×2)-like reconstructions for the very P-rich and (2×4) reconstructions for the more cation-rich surfaces. No (4×2) reconstruction could be prepared, independent of the preparation method. A comparison of the reflectance anisotropy between GaP(001) and InP(001) surfaces shows similar line shapes for the very cation-rich (2×4) surfaces. For less cation-rich surfaces, however, we observe distinct differences between the spectra of the two systems. In both cases, different line shapes in the reflection anisotropy spectra occur for the (2×4) periodicity, suggesting the existence of different (2×4) geometries. The experimental results are discussed on the background of atomic structures, total energies and reflectance anisotropy spectra obtained ab initio from density-functional theory local-density approximation calculations.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Esser, N and Schmidt, WG and Bernholc, J and Frisch, AM and Vogt, P and Zorn, M and Pristovsek, M and Richter, W and Bechstedt, F and Hannappel, T and et al.}, year={1999}, pages={1691–1696} }
 @inproceedings{schmidt_bernholc_bechstedt_1999, place={Singapore}, title={InP(001)(2x4): A new type of dimer reconstruction}, booktitle={24th International Conference on the Physics of Semiconductors : Jerusalem, Israel, August 2-7,1998}, publisher={World Scientific}, author={Schmidt, W.G. and Bernholc, J. and Bechstedt, F.}, editor={Gershoni, D.Editor}, year={1999} }
 @article{nardelli_bernholc_1999, title={Mechanical deformations and coherent transport in carbon nanotubes}, volume={60}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.60.r16338}, abstractNote={Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202~Received 23 July 1999; revised manuscript received 6 October 1999!We have investigated the conductance of carbon nanotubes under mechanical distortions likely to occurwhen forming nanoscale electronic devices. Using a realistic tight-binding Hamiltonian, several structure-dependent classes of electrical behavior in deformed nanotubes have been discovered. Bending, defects, andtube-tube contacts are shown to strongly modify transport in individual nanotubes, and to induce, in somecases, metal-semiconductor transitions. These results provide a clear interpretation of recent experimentalfindings and suggest avenues for their use in devices.@S0163-1829~99!51748-7#}, number={24}, journal={PHYSICAL REVIEW B}, author={Nardelli, MB and Bernholc, J}, year={1999}, month={Dec}, pages={R16338–R16341} }
 @article{boguslawski_bernholc_1999, title={Segregation effects at vacancies in AlxGa1-xN and SixGe1-x alloys}, volume={59}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.59.1567}, abstractNote={It is shown by quantum molecular-dynamics calculations that the formation energies of vacancies in semiconductor alloys strongly depend on the chemical identities of their nearest-neighbor atoms. For example, in ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys the Ga-terminated vacancy is lower in energy than the Al-terminated one by 2.8 eV, while the corresponding difference in ${\mathrm{Si}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ is 1 eV. This leads to unexpectedly strong preferences for vacancy termination in semiconductor alloys. The results also predict that local segregation will occur at grain boundaries and dislocations.}, number={3}, journal={PHYSICAL REVIEW B}, author={Boguslawski, P and Bernholc, J}, year={1999}, month={Jan}, pages={1567–1570} }
 @article{schmidt_fattebert_bernholc_bechstedt_1999, title={Self-energy effects in the optical anisotropy of GaP(001)}, volume={6}, ISSN={["0218-625X"]}, DOI={10.1142/S0218625X99001281}, abstractNote={ We calculate the reflectance anisotropy for GaP(001)(2×4) surfaces using a real-space multigrid method and ab initio pseudopotentials. Our results obtained within DFT-LDA show good qualitative agreement with recent experiments. This holds in particular for the stoichiometric trends. A strong negative anisotropy at low photon energies is linked to the formation of Ga–Ga bonds along the [110] direction. There are discrepancies, however, with respect to the line shape and the energetic positions of characteristic peaks. Substantial improvement is achieved by using a numerically efficient GW approach with approximations for local-field effects and dynamical screening. We find that the spectral features related to transitions between surface perturbed bulk wave functions are more strongly affected by self-energy corrections than anisotropies directly linked to surface electronic states. }, number={6}, journal={SURFACE REVIEW AND LETTERS}, author={Schmidt, WG and Fattebert, JL and Bernholc, J and Bechstedt, F}, year={1999}, month={Dec}, pages={1159–1165} }
 @article{schmidt_briggs_bernholc_bechstedt_1999, title={Structural fingerprints in the reflectance anisotropy spectra of InP(001)(2x4) surfaces}, volume={59}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.59.2234}, abstractNote={The reflectance anisotropy has been calculated from first principles for a series of recently proposed structural models of the $\mathrm{InP}(001)(2\ifmmode\times\else\texttimes\fi{}4)$ surface. The features of the calculated spectra are related to specific surface bonding configurations. We find a pronounced negative anisotropy around 1.7 eV linked to transitions between \ensuremath{\sigma}-like In-In bonding states and empty dangling bonds localized at the surface cations. The strength of that anisotropy is directly related to the number of In-In bonds at the surface. This explains the gradual change of the corresponding measured anisotropy in that energy region, depending on the growth conditions. Positive anisotropies at higher energies arise from transitions between P-P dimer related states and surface resonances. Additionally we find derivativelike features at the energy of the ${E}_{1}$ peak that depend only weakly on the surface structure and stoichiometry. In conjunction with the experimental data, our results indicate that the (2\ifmmode\times\else\texttimes\fi{}4) reconstructed InP(001) surface features In-In bonds along [110] and P-P dimers parallel to $[11\ifmmode\bar\else\textasciimacron\fi{}0],$ respectively. The relative number of these bonds varies with the growth conditions.}, number={3}, journal={PHYSICAL REVIEW B}, author={Schmidt, WG and Briggs, EL and Bernholc, J and Bechstedt, F}, year={1999}, month={Jan}, pages={2234–2239} }
 @article{bungaro_rapcewicz_bernholc_1999, title={Surface sensitivity of impurity incorporation: Mg at GaN (0001) surfaces}, volume={59}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.59.9771}, abstractNote={The interplay of surface termination, reconstruction patterns, and availability of species involved determines the incorporation of impurities during growth. We study ab initio this interplay for Mg at the GaN ~0001! surfaces and find that optimal incorporation conditions strongly depend upon surface orientation and cannot be predicted using bulk stoichiometric arguments. With reasonable assumptions on the kinetics, high densities of Mg can be achieved in the absence of hydrogen and the Ga surface displays superior incorporation characteristics to that of the N surface. @S0163-1829~99!02615-6# Controlled incorporation of impurities is critical to semiconductor science and technology. Specific dopant concentrations are necessary to achieve desired conductivities, recombination rates in light emitting devices, or other electrical characteristics. In general, doping of bulk materials can be performed in three different ways: ~i! by in-diffusion following growth, ~ii! by ion implantation, and ~iii! during growth. The in-diffusion of impurities is limited by the maximum solubility, which is a thermodynamic quantity that can be determined from either experiments or calculations. Ion implantation has no such limit, but in many materials the implantation damage is severe and cannot be annealed out, limiting the usefulness of this technique. During growth, the incorporation of an impurity occurs at the surface and depends sensitively upon the surface and its environment. For example, Tersoff 1 has emphasized that impurity energetics and consequently its solubility in the near-surface region can be significantly different from that in the bulk. This is because under typical growth conditions, diffusion at the surface is much faster than in the bulk and the impurity density of the near-surface region can be frozen in as the film grows,}, number={15}, journal={PHYSICAL REVIEW B}, author={Bungaro, C and Rapcewicz, K and Bernholc, J}, year={1999}, month={Apr}, pages={9771–9774} }
 @article{nardelli_yakobson_bernholc_1998, title={Brittle and ductile behavior in carbon nanotubes}, volume={81}, ISSN={["1079-7114"]}, DOI={10.1103/PhysRevLett.81.4656}, abstractNote={The field of carbon nanotubes has seen an explosive growth in recent years due to the substantial promise of these quasi-1D structures for potential uses as highstrength, light-weight materials, super-strong fibers, novel nanometer-scale electronic and mechanical devices, catalysts, and energy storage media. Despite the potential impact that new composites based on carbon nanotubes could have in many areas of science and industry, a full characterization of their mechanical properties, and ultimately of their strength, is still lacking. Carbon nanotubes have already demonstrated exceptional mechanical properties: Their excellent flexibility during bending has been observed experimentally and studied theoretically [1 ‐ 3]. Their high stiffness combines with resilience and the ability to buckle and collapse in a reversible manner: even largely distorted configurations (axially compressed, twisted) can be due to elastic deformations with virtually no atomic defects involved. [1,2,4,5] In this Letter we focus on the occurrence of mechanical failure in carbon nanotubes under a tensile load, which leads to the emergence of novel, unforeseen patterns in plasticity and breakage. Because of its hexagonal symmetry, a graphite sheet (graphene), the basic constituent of carbon nanotubes, has three equivalent directions with respect to the application of an external planar tension. We call “longitudinal” the tension that is applied parallel to one of the C-C bond directions, and “transverse” the one that is applied normal to it. Once the planar sheet is rolled into a nanotube, the case of the transverse tension corresponds to the application of tensile strain to an armchair tube, while the longitudinal case corresponds to the application of tensile strain to a zigzag tube. Our study, based on the extensive use of classical, tight-binding and ab initio molecular dynamics simulations, shows that the different orientations of the carbon bonds with respect to the strain axis lead to completely different scenarios: ductile or brittle behaviors can be observed in nanotubes of different symmetry under the same external conditions. Furthermore, the behavior of nanotubes under large tensile strain strongly depends on their symmetry and diameter. Several modes of behavior are identified, and a full map of their ductile-vs-brittle behavior is presented. Beyond a critical value of the tension, an armchair nanotube in “transverse” tension releases its excess strain via spontaneous formation of topological defects. A transverse tension finds a natural release in the rotation}, number={21}, journal={PHYSICAL REVIEW LETTERS}, author={Nardelli, MB and Yakobson, BI and Bernholc, J}, year={1998}, month={Nov}, pages={4656–4659} }
 @article{ramamoorthy_briggs_bernholc_1998, title={Chemical trends in impurity incorporation into Si(100)}, volume={81}, ISSN={["1079-7114"]}, DOI={10.1103/PhysRevLett.81.1642}, abstractNote={Theoretical investigations of the adsorption, incorporation, and segregation of a number of common impurities at the (2 3 1)-Si(100) surface reveal two qualitatively distinct classes of behavior. Some impurities prefer to adsorb in trenches between surface dimer rows. Their incorporation into the surface is highly unfavorable. Other impurities prefer to adsorb on top of surface dimer rows. Their incorporation into the surface is either energetically favorable or only marginally unfavorable. The results explain a number of experimental observations. [S0031-9007(98)06884-7]}, number={8}, journal={PHYSICAL REVIEW LETTERS}, author={Ramamoorthy, M and Briggs, EL and Bernholc, J}, year={1998}, month={Aug}, pages={1642–1645} }
 @article{rapcewicz_chen_yakobson_bernholc_1998, title={Consistent methodology for calculating surface and interface energies}, volume={57}, ISSN={["1550-235X"]}, DOI={10.1103/physrevb.57.7281}, abstractNote={A consistent approach to the calculation of the surface energy valid for all crystal systems is presented. Voronoi polyhedra are introduced and used in conjunction with the energy-density formalism of Chetty and Martin @Phys. Rev. B 45, 6074 ~1992!; 45, 6089 ~1992!# to provide a methodology for the determination of surface energies. The surface energies of the unrelaxed, unreconstructed GaAs ~001! and ~111! surfaces are calculated as a test. As an example of the application of the formalism to a low symmetry system, the energies of selected ~0001! surfaces of the wide-gap semiconductors GaN and SiC are determined. @S0163-1829~98!02012-8#}, number={12}, journal={PHYSICAL REVIEW B}, author={Rapcewicz, K and Chen, B and Yakobson, B and Bernholc, J}, year={1998}, month={Mar}, pages={7281–7291} }
 @inproceedings{boguslawski_bernholc_1998, title={Doping and segregation effects in AlGaN systems}, ISBN={0780343549}, url={http://dx.doi.org/10.1109/sim.1998.785114}, DOI={10.1109/sim.1998.785114}, abstractNote={Doping properties of substitutional C, Si, Ge, Be, and Se impurities in wurtzite GaN and AlN are studied by quantum molecular dynamics. We investigate effects that potentially quench doping efficiency, namely the self-compensation and the compensation by native defects. We also predict a strong interfacial segregation of Se/sub N/, C/sub N/, and nitrogen vacancies at the AlN/GaN heterointerface towards the GaN layer.}, booktitle={Semiconducting and Insulating Materials 1998. Proceedings of the 10th Conference on Semiconducting and Insulating Materials (SIMC-X) (Cat. No.98CH36159)}, publisher={IEEE}, author={Boguslawski, P. and Bernholc, J.}, year={1998} }
 @article{nardelli_brabec_maiti_roland_bernholc_1998, title={Lip-lip interactions and the growth of multiwalled carbon nanotubes}, volume={80}, DOI={10.1103/physrevlett.80.313}, abstractNote={Using a realistic many-body potential, we have simulated the properties of double-walled carbon nanotubes with the aim of investigating the role of lip-lip interactions on nanotube growth. Surprisingly, the lip-lip interaction by itself does not stabilize open-ended growth, but rather facilitates tube closure by mediating the transfer of atoms between inner and outer shells. A simulation of growth on a wide double-wall nanotube leads to considerable deviations from the ideal structure, in contrast to corresponding simulations for single-wall tubes, which result in nearly perfect structures. [S0031-9007(97)04989-2]}, number={2}, journal={Physical Review Letters}, author={Nardelli, M. B. and Brabec, C. and Maiti, A. and Roland, C. and Bernholc, Jerzy}, year={1998}, pages={313–316} }
 @article{nardelli_yakobson_bernholc_1998, title={Mechanism of strain release in carbon nanotubes}, volume={57}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.57.r4277}, abstractNote={Static and dynamical properties of carbon nanotubes under uniaxial tension have been investigated via quantum and classical simulations. In strained nanotubes at high temperatures we observe the spontaneous formation of double pentagon-heptagon defect pairs. Tubes containing these defects are energetically preferred to uniformly stretched tubes at strains greater than 5%. These topological defects act as nucleation centers for the formation of dislocations in the originally ideal graphite network, and they constitute the onset of a plastic deformation of the carbon nanotube. The mechanism of formation of such defects, their energetics, and transformations are described. @S0163-1829~98!50208-1# Since their discovery in 1991, 1 carbon nanotubes have attracted much interest due to their peculiar character at a crossroad between traditional carbon fibers and fullerenes. They hold substantial promise for use as superstrong fibers, catalysts, and as components of novel electronic devices. Despite the potential impact that new composites based on carbon nanotubes would have in many areas of science and industry, very little is known about the microscopic origin of their strength and a complete theoretical understanding of their behavior is desirable. The excellent resistance of carbon nanotubes to bending has already been observed experimentally and studied theoretically. 2‐4 The remarkable flexibility of the hexagonal network allows the system to sustain very high bending angles, kinks, and highly strained regions. In addition, nanotubes are observed to be extremely resilient, suggesting that even largely distorted configurations ~axial compression, twisting! can be due to elastic deformations with no atomic defects involved. 2,3,5,6}, number={8}, journal={PHYSICAL REVIEW B}, author={Nardelli, MB and Yakobson, BI and Bernholc, J}, year={1998}, month={Feb}, pages={R4277–R4280} }
 @article{nardelli_roland_bernholc_1998, title={Theoretical bounds for multiwalled carbon nanotube growth}, volume={296}, ISSN={["0009-2614"]}, DOI={10.1016/S0009-2614(98)01068-9}, abstractNote={The role of the lip–lip interaction on the non-catalytic growth of multiwalled carbon nanotube is explored with classical molecular dynamics simulations. The simulations show that the lip–lip interaction actually provides a natural mechanism for the closing of the different nanotube shells, irrespective of the number of shells making up the nanotube. Based on an extensive annealing study, we estimate the mean time to closure and an upper bound length for nanotubes that can be formed, as a function of nanotube diameter and temperature. The extrapolated results are in good agreement with the existing experimental data.}, number={5-6}, journal={CHEMICAL PHYSICS LETTERS}, author={Nardelli, MB and Roland, C and Bernholc, J}, year={1998}, month={Nov}, pages={471–476} }
 @article{bernholc_brabec_nardelli_maiti_roland_yakobson_1998, title={Theory of growth and mechanical properties of nanotubes}, volume={67}, ISSN={["1432-0630"]}, DOI={10.1007/s003390050735}, number={1}, journal={APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING}, author={Bernholc, J and Brabec, C and Nardelli, MB and Maiti, A and Roland, C and Yakobson, BI}, year={1998}, month={Jul}, pages={39–46} }
 @inbook{roland_wensell_zhang_boguslawski_bernholc_1997, place={New York}, title={Ab initio simulations of the Si (100) surface: steps and melting}, booktitle={Dynamics of Crystal Surfaces and Interfaces}, publisher={Plenum Press}, author={Roland, C. and Wensell, M.G. and Zhang, Q.-M. and Boguslawski, P. and Bernholc, J.}, editor={Duxbury, P.M. and Pence, T.J.Editors}, year={1997}, pages={135–145} }
 @article{boguslawski_bernholc_1997, title={Doping properties of C, Si, and Ge impurities in GaN and AlN}, volume={56}, ISSN={["0163-1829"]}, DOI={10.1103/physrevb.56.9496}, abstractNote={Doping properties of substitutional C, Si, and Ge impurities in wurtzite GaN and AlN were studied by quantum molecular dynamics. We considered incorporation of impurities on both cation and anion sublattices. When substituting for cations, Si and Ge are shallow donors in GaN, while Ge becomes a deep donor in AlN. Both impurities are deep acceptors on the N site. Substitutional C cation is a shallow donor in GaN, but a deep one in AlN; C N is a relatively shallow acceptor in both materials. Two effects that potentially quench doping efficiency were investigated. The first one is the transition of a donor from a substitutional position to a DX-like configuration. In crystals with a wurtzite symmetry, there are two possible variants of a DX-like state, and they have substantially different properties. In GaN, DX 2 states of both Si and Ge are unstable, or metastable, and thus they do not affect doping efficiency. In contrast, they are stable in AlN, and therefore neither Si nor Ge is a dopant in this material. Estimates obtained for Al xGa 12xN alloys show that the crossover composition for DX stability is much lower for Ge ( x.0.3) than for Si ( x.0.6). The second effect quenching the doping efficiency is self-compensation, i.e., simultaneous incorporation of impurity atoms on both cation and anion sublattice. This effect may be enhanced by the formation of nearest-neighbor donor-acceptor pairs. The calculated binding energies of such pairs are large, about 1 eV, influencing self-compensation in some cases. Finally, the computed formation energies are used to identify growth conditions under which all these impurities may be efficient dopants in wide-band-gap nitrides. @S0163-1829~97!02340-0#}, number={15}, journal={PHYSICAL REVIEW B}, author={Boguslawski, P and Bernholc, J}, year={1997}, month={Oct}, pages={9496–9505} }
 @article{yakobson_campbell_brabec_bernholc_1997, title={High strain rate fracture and C-chain unraveling in carbon nanotubes}, volume={8}, ISSN={["0927-0256"]}, DOI={10.1016/S0927-0256(97)00047-5}, abstractNote={Nanotube behavior at high rate tensile strain (~ 1 MHz) is studied by molecular dynamics using a realistic many-body interatomic potential. The simulatins performed for single- and double-walled nanotubes of different helicities, and at different temperatures, show that nanotubes have an extremely large breaking strain. It decreases somewhat with increasing temperature and smaller strain rate, while the influence of helicity is very weak. At later stages of fracture, the nanotube fragments are connected by a set of unraveling monoatomic chains. The chains ‘compete’ with each other for carbon atoms popping out of the original tube segments. The interaction between chains eventually leads to a single chain, which grows up to hundreds of atoms in length before its breakage.}, number={4}, journal={COMPUTATIONAL MATERIALS SCIENCE}, author={Yakobson, BI and Campbell, MP and Brabec, CJ and Bernholc, J}, year={1997}, month={Sep}, pages={341–348} }
 @article{maiti_brabec_bernholc_1997, title={Kinetics of metal-catalyzed growth of single-walled carbon nanotubes}, volume={55}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.55.r6097}, abstractNote={Molecular-dynamics and total-energy calculations using a realistic three-body potential for carbon reveal the basic atomic processes by which single-shelled nanotubes can grow out of metal-carbide particles by the root growth mechanism. We find that nanometer-sized protrusions on the metal-particle surface lead to the nucleation of very narrow tubes. Wide bumps lead to a strained graphene sheet and no nanotube growth. The results also explain the absence of multishelled tubes in metal-catalyzed growth. @S0163-1829~97!51210-0#}, number={10}, journal={PHYSICAL REVIEW B}, author={Maiti, A and Brabec, CJ and Bernholc, J}, year={1997}, month={Mar}, pages={R6097–R6100} }
 @inbook{yakobson_nardelli_campbell_brabec_bernholc_1997, place={Pennington, NJ}, title={Modeling of Structural Stability and Tensile Strength of Carbon Nanotubes}, booktitle={Proceedings of the Symposium on Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials}, publisher={Electrochemical Society}, author={Yakobson, B.I. and Nardelli, M.Buongiorno and Campbell, M.-P. and Brabec, C.J. and Bernholc, J.}, editor={Kadish, K.M. and Ruoff, R.S.Editors}, year={1997}, pages={907} }
 @misc{bernholc_roland_yakobson_1997, title={Nanotubes}, volume={2}, ISSN={["1879-0348"]}, DOI={10.1016/s1359-0286(97)80014-9}, abstractNote={The field of nanotubes is undergoing an explosive growth, fueled by brakethroughs in synthesis and the promise of unique applications. Highly unusual properties and devices have been predicted and/or observed, including extremely high strength and flexibility, nanoscale electronic devices consisting entirely of carbon, and strong capillary effects leading to the production of exceptionally thin wires, cold cathode field emission and other effects.}, number={6}, journal={CURRENT OPINION IN SOLID STATE & MATERIALS SCIENCE}, author={Bernholc, J and Roland, C and Yakobson, BI}, year={1997}, month={Dec}, pages={706–715} }
 @article{nardelli_rapcewicz_bernholc_1997, title={Polarization field effects on the electron-hole recombination dynamics in In0.2Ga0.8N/In1-xGaxN multiple quantum wells}, volume={71}, DOI={10.1063/1.120269}, abstractNote={The effect of the polarization field in wurtzite In0.2Ga0.8N/In1−xGaxN (x>0.8) multiple quantum wells is studied from first principles. The pyroelectric and piezoelectric fields naturally present in the system due to its wurtzite structure are strong enough to reduce the interband recombination rate in an ideal quantum well. We suggest that composition fluctuations, observed in the active region of actual devices, provide the necessary confinement for an improved recombination rate and lasing.}, number={21}, journal={Applied Physics Letters}, author={Nardelli, M. B. and Rapcewicz, K. and Bernholc, Jerzy}, year={1997}, pages={3135–3137} }
 @article{bernholc_briggs_sullivan_brabec_nardelli_rapcewicz_roland_wensell_1997, title={Real space multigrid methods for large scale electronic structure problems}, volume={65}, DOI={10.1002/(SICI)1097-461X(1997)65:5<531::AID-QUA18>3.0.CO;2-5}, abstractNote={We describe the development and applications of a new electronic structure method that uses a real-space grid as a basis. Multigrid techniques provide preconditioning and convergence acceleration at all length scales and therefore lead to particularly efficient algorithms. The salient points of our implementation include: (i) new compact discretization schemes in real space for systems with cubic, orthorhombic, and hexagonal symmetry and (ii) new multilevel algorithms for the iterative solution of Kohn–Sham and Poisson equations. The accuracy of the discretizations was tested by direct comparison with plane-wave calculations, when possible, and the results were in excellent agreement in all cases. These techniques are very suitable for use on massively parallel computers and in O(N) methods. Tests on the Cray-T3D have shown nearly linear scaling of the execution time up to the maximum number of processors (512). The above methodology was tested on a large number of systems, such as the C60 molecule, diamond, Si and GaN supercells, and quantum molecular dynamics simulations for Si. Large-scale applications include a simulation of surface melting of Si and investigations of electronic and structural properties of surfaces, interfaces, and biomolecules. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 531–543, 1997}, number={5}, journal={International Journal of Quantum Chemistry}, author={Bernholc, Jerzy and Briggs, E. L. and Sullivan, D. J. and Brabec, C. J. and Nardelli, M. B. and Rapcewicz, K. and Roland, C. and Wensell, M.}, year={1997}, pages={531–543} }
 @article{nardelli_rapcewicz_bernholc_1997, title={Strain effects on the interface properties of nitride semiconductors}, volume={55}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.55.r7323}, abstractNote={the range from the deep ultraviolet to the visible. 2 Not surprisingly the potential technological importance of these materials has elicited the interest of a number of theoretical groups. 3‐7 In spite of this, the strained interfaces of these lattice mismatched materials have not been studied. We find that strain effects are significant, inducing changes of 20% to 40% in the value of the band offset and that these changes increase with decreasing in-plane lattice parameter. The AlN/GaN/InN interfaces are all of type I, while the Al 0.5Ga 0.5N/AlN zinc-blende ~001! interface is found to be of type II. Finally, we studied the GaN/AlN wurtzite interface, where qualitatively new features, namely pyroelectric and piezoelectric effects, appear due to the low symmetry of the wurtzite lattice. The standard ab initio plane-wave pseudopotential method 8‐10 was employed in the calculations. The energy cutoff for the plane-wave expansion was 50 Ry to ensure convergence of the nitrogen pseudopotential. We used the equivalent of ten k points for bulk and superlattice calculations in the zinc-blende structure 11 and six k points for calculations of the wurtzite structure. 12 Convergence both in the size of the plane-wave basis and in the number of special points has been carefully checked. The Perdew-Zunger parametrization 13 of the Ceperley-Alder form 14 of the exchange-correlation energy was used. For interface calculations, we employed 414 superlattices ~16 atoms! along the ~001! and ~0001! directions. Nonlocal, norm-conserving pseudopotentials 15‐17 were included using the KleinmanBylander approach. 18 For nitrogen, we used a neutral con}, number={12}, journal={PHYSICAL REVIEW B}, author={Nardelli, MB and Rapcewicz, K and Bernholc, J}, year={1997}, month={Mar}, pages={R7323–R7326} }
 @article{nardelli_rapcewicz_bernholc_1997, title={Theory of interfaces and surfaces in wide-gap nitrides}, volume={15}, DOI={10.1116/1.589429}, abstractNote={A selection of the results of a theoretical investigation of the properties of interfaces and surfaces of the wide-gap III–V nitride semiconductors is reviewed. The electronic properties of wurtzite heteroepitaxial interfaces of AlN and GaN, incorporating the effects of strain, are discussed. In particular, we find that this interface is of type I and have calculated the valence-band offset to be −0.57 eV. The surface energies and atomic geometries of the 2×2 reconstructions of the (0001) face of GaN are also presented. In conditions which are rich in a given species, an adatom reconstruction of that species is found to be the most energetically favorable: for gallium-rich conditions, the reconstruction with a gallium adatom on a T3 site is the most stable, while for nitrogen-rich conditions the reconstruction with a nitrogen adatom on the H3 site is energetically the most favorable.}, number={4}, journal={JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B}, author={Nardelli, MB and Rapcewicz, K and Bernholc, J}, year={1997}, pages={1144–1147} }
 @article{rapcewicz_buongiorno nardelli_bernholc_1997, title={Theory of surface morphology of wurtzite GaN (0001) surfaces}, volume={56}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.56.r12725}, DOI={10.1103/physrevb.56.r12725}, abstractNote={The influence of growth conditions and surface polarity upon the morphology of (0001) GaN surfaces is studied from first principles. The charge transfer between the Ga and N atoms in GaN and the very large electronegativity of nitrogen are found to play decisive roles in determining the stable reconstructions. Further, hydrogen stabilizes the ideally cleaved surface irrespective of polarity. For both polarities, adsorption of 3/4 of a monolayer of hydrogen results in a very stable surface with a 2\ifmmode\times\else\texttimes\fi{}2 symmetry.}, number={20}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Rapcewicz, Krzysztof and Buongiorno Nardelli, Marco and Bernholc, J.}, year={1997}, month={Nov}, pages={R12725–R12728} }
 @inproceedings{bernholc_boguslawski_brabec_biggs_maiti_roland_sullivan_wensell_yakobson_zhang_et al._1996, place={Singapore}, title={Ab Initio Simulations of Materials}, ISBN={0789814531320}, booktitle={Proceedings of the Science And Technology Of Atomically Engineered Materials}, publisher={World Scientific Publishing Co. Pte. Ltd.}, author={Bernholc, J. and Boguslawski, P. and Brabec, C. and Biggs, E.L. and Maiti, A. and Roland, C. and Sullivan, D.J. and Wensell, M. and Yakobson, B. and Zhang, Q.-M. and et al.}, editor={Jena, P. and Khanna, S.N. and Rao, B.K.Editors}, year={1996}, pages={193–200} }
 @article{bogusl/awski_briggs_bernholc_1996, title={Amphoteric properties of substitutional carbon impurity in GaN and AlN}, volume={69}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.117934}, DOI={10.1063/1.117934}, abstractNote={Electronic structure and formation energies of substitutional carbon impurities in hexagonal GaN and AlN were studied by quantum molecular dynamics. Substitutions on both cation and anion sites were considered. Ccation is a shallow donor, while CN is a shallow acceptor. A DX-like configuration is metastable for CGa and CAl0, and stable for CAl−. The solubility of carbon is excellent, but it is accompanied by efficient self-compensation of carbon acceptors by carbon donors. This is due to two factors: (i) the large energy gain induced by electron transfer from Ccation to CN, which is close to the band gap energy, and (ii) the large binding energy (∼1 eV) of nearest neighbor Ccation+-CN− pairs.}, number={2}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Bogusl/awski, P. and Briggs, E. L. and Bernholc, J.}, year={1996}, month={Jul}, pages={233–235} }
 @article{boguslawski_bernholc_1996, title={Doping Properties of Amphoteric C, Si, and Ge Impurities in GaN and AlN}, volume={90}, ISSN={0587-4246 1898-794X}, url={http://dx.doi.org/10.12693/aphyspola.90.735}, DOI={10.12693/aphyspola.90.735}, abstractNote={Electronic structure of substitutional group-IV impurities C, Si, and Ge in hexagonal GaN and AlN were studied by quantum molecular dynamics. Cis is a very shallow acceptor, and thus a promising p-type dopant. Both Si and Ge are excellent donors in GaN. However, in AlGaN alJoys the DX configurations are stable for a sufficiently high Al content, which quenches the doping efficiency. Electronic structure of nearest-neighbor Χcatiοn—ΧΝ pairs is also discussed. PACS numbers: 71.55.—i}, number={4}, journal={Acta Physica Polonica A}, publisher={Institute of Physics, Polish Academy of Sciences}, author={Boguslawski, P. and Bernholc, J.}, year={1996}, month={Oct}, pages={735–738} }
 @article{yakobson_brabec_bernholc_1996, title={Nanomechanics of carbon tubes: Instabilities beyond linear response}, volume={76}, ISSN={["0031-9007"]}, DOI={10.1103/PhysRevLett.76.2511}, abstractNote={Carbon nanotubes subject to large deformations reversibly switch into different morphological patterns. Each shape change corresponds to an abrupt release of energy and a singularity in the stress-strain curve. These transformations, simulated using a realistic many-body potential, are explained by a continuum shell model. With properly chosen parameters, the model provides a remarkably accurate ``roadmap'' of nanotube behavior beyond Hooke's law.}, number={14}, journal={PHYSICAL REVIEW LETTERS}, author={Yakobson, BI and Brabec, CJ and Bernholc, J}, year={1996}, month={Apr}, pages={2511–2514} }
 @article{briggs_sullivan_bernholc_1996, title={Real-space multigrid-based approach to large-scale electronic structure calculations}, volume={54}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.54.14362}, DOI={10.1103/physrevb.54.14362}, abstractNote={We describe a set of techniques for performing large scale ab initio calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods provide effective convergence acceleration and preconditioning on all length scales, thereby permitting efficient calculations for ill-conditioned systems with long length scales or high energy cutoffs. We discuss specific implementations of multigrid and real-space algorithms for electronic structure calculations, including an efficient multigrid-accelerated solver for Kohn-Sham equations, compact yet accurate discretization schemes for the Kohn-Sham and Poisson equations, optimized pseudopotentials for real-space calculations, efficacious computation of ionic forces, and a complex-wave-function implementation for arbitrary sampling of the Brillouin zone. A particular strength of a real-space multigrid approach is its ready adaptability to massively parallel computer architectures, and we present an implementation for the Cray-T3D with essentially linear scaling of the execution time with the number of processors. The method has been applied to a variety of periodic and nonperiodic systems, including disordered Si, a N impurity in diamond, AlN in the wurtzite structure, and bulk Al. The high accuracy of the atomic forces allows for large step molecular dynamics; e.g., in a 1-ps simulation of Si at 1100 K with an ionic step of 80 a.u., the total energy was conserved within 27 \ensuremath{\mu}eV per atom. \textcopyright{} 1996 The American Physical Society.}, number={20}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Briggs, E. L. and Sullivan, D. J. and Bernholc, J.}, year={1996}, month={Nov}, pages={14362–14375} }
 @article{iijima_brabec_maiti_bernholc_1996, title={Structural flexibility of carbon nanotubes}, volume={104}, ISSN={["0021-9606"]}, DOI={10.1063/1.470966}, abstractNote={We report high resolution electron microscope (HREM) observations and atomistic simulations of the bending of single and multi-walled carbon nanotubes under mechanical duress. Single and multiple kinks are observed at high bending angles. Their occurrence is quantitatively explained by the simulations, which use a realistic many-body potential for the carbon atoms. We show that the bending is fully reversible up to very large bending angles, despite the occurrence of kinks and highly strained tube regions. This is due to the remarkable flexibility of the hexagonal network, which resists bond breaking and bond switching up to very high strain values.}, number={5}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Iijima, S and Brabec, C and Maiti, A and Bernholc, J}, year={1996}, month={Feb}, pages={2089–2092} }
 @article{yakobson_brabec_bernholc_1996, title={Structural mechanics of carbon nanotubes: From continuum elasticity to atomistic fracture}, volume={3}, ISSN={0928-1045 1573-4900}, url={http://dx.doi.org/10.1007/bf01185652}, DOI={10.1007/bf01185652}, number={1-3}, journal={Journal of Computer-Aided Materials Design}, publisher={Springer Science and Business Media LLC}, author={Yakobson, B. I. and Brabec, C. J. and Bernholc, J.}, year={1996}, month={Aug}, pages={173–182} }
 @article{bernholc_boguslawski_briggs_nardelli_chen_rapcewicz_zhang_1996, title={Theory of Defects, Doping, Surfaces and Interfaces in Wide Gap Nitrides}, volume={423}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-423-465}, DOI={10.1557/proc-423-465}, abstractNote={Abstract}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Bernholc, J. and Boguslawski, P. and Briggs, E. L. and Nardelli, M. Buongiorno and Chen, B. and Rapcewicz, K. and Zhang, Z.}, year={1996} }
 @inproceedings{boguslawski_bernholc_1996, place={Singapore}, title={Theory of Group-IV Impurities in Wide Gap Nitrides}, booktitle={23rd International Conference on the Physics of Semiconductors : Berlin, Germany, July 21-26, 1996}, publisher={World Scientific}, author={Boguslawski, P. and Bernholc, J.}, editor={Scheffler, Matthias and Zimmermann, RolandEditors}, year={1996}, pages={2889} }
 @article{nardelli_rapcewicz_briggs_bungaro_bernholc_1996, title={Theory of Interfaces in Wide-Gap Nitrides}, volume={449}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-449-893}, DOI={10.1557/proc-449-893}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Nardelli, M. Buongiorno and Rapcewicz, K. and Briggs, E. L. and Bungaro, C. and Bernholc, J.}, year={1996} }
 @inproceedings{rapcewicz_buongiorno nardelli_chen_zhang_bernholc_1996, place={Pennington, NJ, USA}, title={Theory of Surfaces and Interfaces in Wide-Gap Nitrides}, ISBN={1566771633}, booktitle={Proceedings of the First Symposium on III-V Nitride Materials and Processes}, publisher={Electrochemical Society}, author={Rapcewicz, K.L. and Buongiorno Nardelli, M. and Chen, B. and Zhang, Z. and Bernholc, J.}, editor={Moustakas, T.D. and Dismukes, J.P. and Pearton, S.J.Editors}, year={1996}, pages={76–86} }
 @article{nemanich_benjamin_bozeman_bremser_king_ward_davis_chen_zhang_bernholc_1995, title={(Negative) Electron Affinity of AlN and AlGaN Alloys}, volume={395}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-395-777}, DOI={10.1557/proc-395-777}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Nemanich, R.J. and Benjamin, M.C. and Bozeman, S.P. and Bremser, M.D. and King, S.W. and Ward, B.L. and Davis, R.F. and Chen, B. and Zhang, Z. and Bernholc, J.}, year={1995} }
 @inproceedings{roland_boguslawski_zhang_zhang_bernholc_1995, place={Singapore}, title={Ab Initio Studies of Si (100) Steps of Monoatomic Height: Structure and Diffusion Barriers}, booktitle={22nd International Conference on the Physics of Semiconductors : Vancouver, Canada, August 15-19, 1994}, publisher={World Scientific}, author={Roland, C. and Boguslawski, P. and Zhang, Q.M. and Zhang, Z. and Bernholc, J.}, editor={Lockwood, D.J.Editor}, year={1995}, pages={493} }
 @article{zhang_roland_bogusławski_bernholc_1995, title={Ab Initio Studies of the Diffusion Barriers at Single-Height Si(100) Steps}, volume={75}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.75.101}, DOI={10.1103/physrevlett.75.101}, abstractNote={The binding sites and diffusion barriers for a Si adatom moving over a buckled Si(100) surface and single-height steps were calculated using ab initio methods. Surprisingly, the diffusion barriers may be sensitive to the local buckling of the dimers. The adatom binding energies and escape barriers at step edges depend strongly on the type of the step. The ${S}_{A}$ step edge is a relatively poor sink for adatoms, while adatoms can be easily trapped at the ${S}_{B}$ edge. These results explain the fast growth observed at the ends of the dimer rows.}, number={1}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Zhang, Q.-M. and Roland, C. and Bogusławski, P. and Bernholc, J.}, year={1995}, month={Jul}, pages={101–104} }
 @article{bogusławski_zhang_zhang_roland_bernholc_1995, title={Ab initio studies of single-height Si(001) steps}, volume={30}, ISSN={0921-5107}, url={http://dx.doi.org/10.1016/0921-5107(94)09011-4}, DOI={10.1016/0921-5107(94)09011-4}, abstractNote={Abstract We have studied the Si(001) surface with single-height steps by ab initio molecular dynamics simulations. Surface dimers were found to be unstable with respect to buckling for all geometries considered. However, the ground state reconstruction depends on the type of step. For the SA step, the c(2 × 4) geometry is induced by the step edge, while, for the SB step, the p(2 × 2) reconstruction is more stable. The binding sites and diffusion barriers for a single Si adatom were investigated via the adiabatic trajectory method. In agreement with other studies of the flat surface, fast diffusion takes place along the dimer rows. The local changes to buckling induced by the adatom are sizable and lead to changes in the activation barriers for diffusion, in particular for the path perpendicular to the dimer rows. We also investigated the diffusion of the adatom over the rebonded SB step. The calculations show that there is no additional barrier for the arrival of the adatom at the edge from the upper terrace, while a barrier of at least 1 eV exists for the arrival of the adatom from the lower edge. In step flow growth involving the rebonded SB step, most of the adatoms will thus arrive from the upper terrace.}, number={2-3}, journal={Materials Science and Engineering: B}, publisher={Elsevier BV}, author={Bogusławski, R. and Zhang, Q.-M. and Zhang, Z. and Roland, C. and Bernholc, J.}, year={1995}, month={Mar}, pages={167–173} }
 @article{sullivan_briggs_brabec_bernholc_1995, title={Electronic Structure Calculations on a Real-Space Mesh with Multigrid Acceleration}, volume={408}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-408-145}, DOI={10.1557/proc-408-145}, abstractNote={Abstract}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Sullivan, D. J. and Briggs, E. L. and Brabec, C. J. and Bernholc, J.}, year={1995} }
 @article{wensell_zhang_bernholc_1995, title={Fluorine-Based Mechanisms for Atomic-Layer-Epitaxial Growth on Diamond (110)}, volume={74}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.74.4875}, DOI={10.1103/physrevlett.74.4875}, abstractNote={Atomic-layer-epitaxy (ALE) processes for the growth of diamond are evaluated using ab initio molecular dynamics. It is shown that the Cl-based ALE cycle on diamond (111) is not advantageous, due to strong steric hindrances. New ALE processes are proposed and shown to be energetically favorable, self-limiting, and sterically unhindered. They exploit the large heat of formation and the small size of the HF molecule and are based on alternating exposure of the diamond (110) surface to hydrocarbons and fluorocarbons. Alternatively, H and F gases can be used in parts of the cycle.}, number={24}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Wensell, M. G. and Zhang, Z. and Bernholc, J.}, year={1995}, month={Jun}, pages={4875–4878} }
 @article{brabec_maiti_roland_bernholc_1995, title={Growth of carbon nanotubes: a molecular dynamics study}, volume={236}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/0009-2614(95)00194-9}, DOI={10.1016/0009-2614(95)00194-9}, abstractNote={Molecular dynamics with realistic many-body atomic potentials was used to study the growth of carbon nanotubes. Analysis of the bond switching and ring migration processes has led to an identification of tube growth mechanisms. Wide tubes, initially open, were found to grow straight maintaining an all-hexagonal structure, while narrow tubes were found to develop permanent pentagonal rings that lead to tube closure upon further deposition. Continued deposition on the top of a closed tube yields a disordered cap structure, implying that open tubes are critical for defect-free growth.}, number={1-2}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Brabec, C.J. and Maiti, A. and Roland, C. and Bernholc, J.}, year={1995}, month={Apr}, pages={150–155} }
 @inproceedings{bernholc_sullivan_briggs_1995, place={San Diego, California}, title={Large Scale Electronic Structure Calculations Using Multigrid Methods}, booktitle={High performance computing 1995 - "grand challenges in computer simulation" : proceedings of the 1995 Simulation Multiconference, April 9-13, 1995, The Crescent Hotel at Koll Center, Phoenix, Arizona.}, publisher={The Society for Computer Simulation}, author={Bernholc, J. and Sullivan, D.J. and Briggs, E.L.}, editor={Tentner, A.Editor}, year={1995}, pages={153} }
 @article{briggs_sullivan_bernholc_1995, title={Large-scale electronic-structure calculations with multigrid acceleration}, volume={52}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.52.r5471}, DOI={10.1103/physrevb.52.r5471}, abstractNote={We have developed a set of techniques for performing large-scale {ital ab} {ital initio} calculations using multigrid accelerations and a real-space grid as a basis. The multigrid methods permit efficient calculations on ill-conditioned systems with long length scales or high-energy cutoffs. The technique has been applied to systems containing up to 100 atoms, including a highly elongated diamond cell, an isolated C{sub 60} molecule, and a 32-atom cell of GaN with the Ga {ital d} states in valence. The method is well suited for implementation on both vector and massively parallel architectures.}, number={8}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Briggs, E. L. and Sullivan, D. J. and Bernholc, J.}, year={1995}, month={Aug}, pages={R5471–R5474} }
 @article{brabec_maiti_roland_bernholc_1995, place={Cambridge, UK}, title={Molecular Dynamics Studies of Nanotube Growth in a Carbon Arc}, volume={359}, DOI={10.1557/PROC-359-235}, abstractNote={ABSTRACT}, journal={MRS Online Proceedings: Symposium G – Science and Technology of Fullerene Materials}, publisher={Cambridge/Materials Research Society}, author={Brabec, C.J. and Maiti, A. and Roland, C. and Bernholc, J.}, year={1995}, pages={235} }
 @article{boguslawski_briggs_bernholc_1995, title={NATIVE DEFECTS IN GALLIUM NITRIDE}, volume={51}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.51.17255}, abstractNote={The results of an extensive theoretical study of native defects in hexagonal GaN are presented. We have considered cation and anion vacancies, antisites, and interstitials. The computations were carried out using ab initio molecular dynamics in supercells containing 72 atoms. N vacancy introduces a shallow donor level, and may be responsible for the n-type character of as-grown GaN. Due to the wide gap of nitrides, self-compensation effects strongly reduce both n-type and p-type doping efficiencies due to the formation of gallium vacancy and interstitial Ga, respectively.}, number={23}, journal={PHYSICAL REVIEW B}, author={BOGUSLAWSKI, P and BRIGGS, EL and BERNHOLC, J}, year={1995}, month={Jun}, pages={17255–17258} }
 @article{roland_zhang_boguslawski_bernholc_gilmer_1995, title={Simulations of SI(100) Growth: Step Flow and Low Temperature Growth}, volume={399}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-399-511}, DOI={10.1557/proc-399-511}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Roland, C. and Zhang, Q.M. and Boguslawski, P. and Bernholc, J. and Gilmer, G.H.}, year={1995} }
 @article{zhang_wensell_bernholc_1995, title={Surface structures and electron affinities of bare and hydrogenated diamond C(100) surfaces}, volume={51}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.51.5291}, DOI={10.1103/physrevb.51.5291}, abstractNote={The structure and electronic properties of bare and H-terminated C(100)-2\ifmmode\times\else\texttimes\fi{}1 surfaces have been studied by ab initio molecular dynamics. Stable 2\ifmmode\times\else\texttimes\fi{}1 structures were found for both clean and monohydride-terminated surfaces. The 1\ifmmode\times\else\texttimes\fi{}1 dihydride phase is energetically unstable relative to ${\mathrm{H}}_{2}$ desorption, although a local minimum, consisting of canted dihydride pairs, was found. The bare diamond (100)-2\ifmmode\times\else\texttimes\fi{}1 surface has a positive electron affinity, while a negative electron affinity is obtained for both the 2\ifmmode\times\else\texttimes\fi{}1:H and 1\ifmmode\times\else\texttimes\fi{}1:2H surfaces. The existence of negative electron affinity for the diamond (100)-2\ifmmode\times\else\texttimes\fi{}1:H surface is in good agreement with recent experimental data.}, number={8}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Zhang, Z. and Wensell, M. and Bernholc, J.}, year={1995}, month={Feb}, pages={5291–5296} }
 @inproceedings{bernholc_chen_zhang_wang_kajihara_sullivan_1995, place={Singapore}, title={Theory Of Diffusion In Gaas/AlAs Superlattices}, booktitle={22nd International Conference on the Physics of Semiconductors : Vancouver, Canada, August 15-19, 1994}, publisher={World Scientific}, author={Bernholc, J. and Chen, B. and Zhang, Q.-M. and Wang, C. and Kajihara, S.K. and Sullivan, D.}, editor={Lockwood, D.J.Editor}, year={1995}, pages={2259} }
 @inproceedings{maiti_brabec_roland_bernholc_1995, place={Singapore}, title={Theory Of Growth Of Graphitic Nanotubes}, booktitle={22nd International Conference on the Physics of Semiconductors : Vancouver, Canada, August 15-19, 1994}, publisher={World Scientific}, author={Maiti, A. and Brabec, C.J. and Roland, C.M. and Bernholc, J.}, editor={Lockwood, D. J.Editor}, year={1995}, pages={2065} }
 @inproceedings{maiti_brabec_roland_bernholc_1995, place={Pennington, NJ}, title={Theory and simulations of nanotube growth}, volume={95}, booktitle={Electrochemical Society Proceedings}, publisher={Electrochemical Society}, author={Maiti, A. and Brabec, C.J. and Roland, C. and Bernholc, J.}, year={1995}, pages={108} }
 @inproceedings{boguslawski_briggs_bernholc_1995, place={Singapore}, title={Theory of Native Defects and of Carbon Doping in Wurtzite GaN}, booktitle={22nd International Conference on the Physics of Semiconductors : Vancouver, Canada, August 15-19, 1994}, publisher={World Scientific}, author={Boguslawski, P. and Briggs, E. and Bernholc, J.}, editor={Lockwood, D.J.Editor}, year={1995}, pages={2331} }
 @article{maiti_brabec_roland_bernholc_1995, title={Theory of carbon nanotube growth}, volume={52}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.52.14850}, DOI={10.1103/physrevb.52.14850}, abstractNote={The kinetics of carbon nanotube growth under arc discharge conditions were investigated over different length and time scales using complementary numerical techniques. Relaxation by ab initio molecular dynamics (Car-Parrinello method) shows that large electric fields present at the tube tips are not the critical factor responsible for the open-ended growth observed experimentally. Classical molecular-dynamics simulations using realistic many-body carbon-carbon potentials show that wide tubes that are initially open can continue to grow straight and maintain an all-hexagonal structure. However, tubes narrower than a critical diameter, estimated to be about \ensuremath{\sim}3 nm, readily nucleate curved, pentagonal structures that lead to tube closure with further addition of atoms. Very narrow tubes can be grown, however, if a small metal particle prevents tube closure. This effect was simulated explicitly by kinetic Monte Carlo methods. Monte Carlo simulations were also used to study nanotube growth over longer time scales. The resulting structures are in agreement with the above growth scenario, and provide an estimate for the lowest tube tip temperature necessary for the growth of nanotubes.}, number={20}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Maiti, A. and Brabec, C. J. and Roland, C. and Bernholc, J.}, year={1995}, month={Nov}, pages={14850–14858} }
 @article{perlin_suski_teisseyre_leszczynski_grzegory_jun_porowski_bogusławski_bernholc_chervin_et al._1995, title={Towards the Identification of the Dominant Donor in GaN}, volume={75}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/physrevlett.75.296}, DOI={10.1103/physrevlett.75.296}, abstractNote={We analyze optical absorption, transmission, luminescence, and Raman scattering in $n$-type GaN at hydrostatic pressures up to 30 GPa. The results show freeze-out of free electrons above $\ensuremath{\sim}20\mathrm{GPa}$ due to trapping at levels that are resonant at ambient pressure but become gap states at high pressures. Ab initio calculations show that both the N vacancy and the Ga interstitial undergo this transition at $\ensuremath{\sim}20\mathrm{GPa}$, but the vacancy should be more abundant. The pressure dependence of the yellow luminescence indicates that a transition between a shallow donor and a deep acceptor is involved.}, number={2}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Perlin, P. and Suski, T. and Teisseyre, H. and Leszczynski, M. and Grzegory, I. and Jun, J. and Porowski, S. and Bogusławski, P. and Bernholc, J. and Chervin, J. C. and et al.}, year={1995}, month={Jul}, pages={296–299} }
 @article{maiti_brabec_roland_bernholc_1994, title={Growth Energetics of Carbon Nanotubes}, volume={73}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.73.2468}, DOI={10.1103/physrevlett.73.2468}, abstractNote={The growth energetics of carbon nanotubes during arc discharge conditions are investigated. Ab initio molecular dynamics calcualtions show that the electric field alone cannot stabilize the growth of open metallic tubes. The addition of atoms and small clusters to tubes were studied using realistic atomic potentials. Deposition on tubes narrower than \AA{}3 nm leads to nucleation of curved defects (adjacent pentagon pairs) and eventual tube closure, while deposition on wider tubes favors the formation of hexagons and isolated pentagons, thereby promoting open-ended growth.}, number={18}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Maiti, A. and Brabec, C. J. and Roland, C. M. and Bernholc, J.}, year={1994}, month={Oct}, pages={2468–2471} }
 @article{boguslawski_briggs_white_wensell_bernholc_1994, title={Native Defects in Wurtzite GaN And AlN}, volume={339}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-339-693}, DOI={10.1557/proc-339-693}, abstractNote={Abstract}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Boguslawski, P. and Briggs, E. and White, T. A. and Wensell, M. G. and Bernholc, J.}, year={1994} }
 @article{van der weide_zhang_baumann_wensell_bernholc_nemanich_1994, title={Negative-electron-affinity effects on the diamond (100) surface}, volume={50}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.50.5803}, DOI={10.1103/physrevb.50.5803}, abstractNote={A negative electron affinity (NEA) was found both experimentally by photoemission spectroscopy and theoretically by ab initio calculations for the 2\ifmmode\times\else\texttimes\fi{}1 reconstructed diamond (100) surface. This surface is the dominant growth surface for thin diamond films and can be obtained by chemical-vapor deposition. Various surface-preparation methods which result in a NEA are described. Theoretical results indicate that the observed NEA is associated with a monohydride-terminated surface, while the hydrogen-free surface exhibits a positive electron affinity.}, number={8}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={van der Weide, J. and Zhang, Z. and Baumann, P. K. and Wensell, M. G. and Bernholc, J. and Nemanich, R. J.}, year={1994}, month={Aug}, pages={5803–5806} }
 @article{chen_zhang_bernholc_1994, title={Si diffusion in GaAs and Si-induced interdiffusion in GaAs/AlAs superlattices}, volume={49}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.49.2985}, DOI={10.1103/physrevb.49.2985}, abstractNote={Various microscopic mechanisms for Si diffusion in GaAs and Si-induced interdiffusion in GaAs/AlAs superlattices are investigated by ab initio molecular dynamics. The dominant mechanism involves the motion of negatively charged ${\mathrm{Si}}_{\mathrm{III}\mathrm{\ensuremath{-}}}$${\mathit{V}}_{\mathrm{III}}$ pairs through second-nearest-neighbor jumps. This mechanism explains both the ability of Si to disorder superlattices regardless of whether it was introduced during growth or in-diffused afterwards and the suppression of interdiffusion by compensation doping. The computed activation energies are in very good agreement with the experimental data.}, number={4}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Chen, B. and Zhang, Q.-M. and Bernholc, J.}, year={1994}, month={Jan}, pages={2985–2988} }
 @article{brabec_maiti_bernholc_1994, title={Structural defects and the shape of large fullerenes}, volume={219}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/0009-2614(94)00119-7}, DOI={10.1016/0009-2614(94)00119-7}, abstractNote={Motivated by the recent experimental observation of nearly spherical multi-layer fullerenes, we investigate theoretically the role of thermally generated structural defects on the shape and stability of large fullerenes by equilibrium statistical mechanics methods. Low energy defects are generated via atom insertions and bond rotations and the resulting structures are relaxed using realistic atomic potentials. The lowest defect formation energies are about 1 eV. A free energy analysis shows that large fullerenes will contain a substantial number of such defects at high temperatures. The presence of these defects effectively spreads out the sharp “kinks” associated with the faceted ground state structure, leading to a more spherical cage of a uniform curvature.}, number={5-6}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Brabec, C.J. and Maiti, A. and Bernholc, J.}, year={1994}, month={Mar}, pages={473–478} }
 @article{bernholc_brabec_maiti_yi_1994, title={Structural transformations, reactions, and electronic properties of fullerenes, onions, and buckytubes}, volume={2}, ISSN={0927-0256}, url={http://dx.doi.org/10.1016/0927-0256(94)90086-8}, DOI={10.1016/0927-0256(94)90086-8}, abstractNote={We describe the results of extensive quantum molecular dynamics calculations of the properties of fullerenes and microtubules. The topics to be discussed include: (i) stability of C60 isomers and barriers to isomerization; (ii) reactivity of C60 and C58 with C2 and C3, and its implications on the formation and growth of fullerenes; and (iii) atomic and electronic structure and doping of semiconducting microtubules. We also discuss the structures, stabilities and atomic transformations of large multishell fullerenes and offer an explanation for the formation of spheroidal “onions” under high fluence electron irradiation conditions. The last results, which involved calculations for up to ∼ 15 000 atoms, were obtained using classical three-body potentials.}, number={3-4}, journal={Computational Materials Science}, publisher={Elsevier BV}, author={Bernholc, J. and Brabec, C.J. and Maiti, A. and Yi, J.-Y.}, year={1994}, month={Jul}, pages={547–556} }
 @article{bogusławski_zhang_zhang_bernholc_1994, title={Structure of monatomic steps on the Si(001) surface}, volume={72}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.72.3694}, DOI={10.1103/physrevlett.72.3694}, abstractNote={The atomic and electronic structure of the stepped Si(001) surface is investigated by ab initio molecular dynamics. All configurations considered by us are unstable with respect to the buckling of the surface dimers. The ground state reconstruction depends on the type of the step and, in some cases, is different from that of the flat surface. The observed differences between the filled-state and the empty-state scanning tunneling microscopy images are due to differences in wave function shapes rather than tip-induced reconstruction.}, number={23}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Bogusławski, P. and Zhang, Q.-M. and Zhang, Z. and Bernholc, J.}, year={1994}, month={Jun}, pages={3694–3697} }
 @article{zhang_bernholc_1993, title={AsGa-XI complexes as models for the EL2 center in GaAs}, volume={47}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.47.1667}, DOI={10.1103/physrevb.47.1667}, abstractNote={Several As antisite-interstitial complexes are considered as models for the EL2 center in GaAs. Large-scale electronic-structure calculations show that the binding energy of the complex is insufficient to account for the annealing properties of EL2. The electronic structure of the complexes is also in disagreement with the majority of the experimental data. The isolated AsG, remains thus the best candidate for the EL2 center. Some of the antisites may be associated with interstitials, but the salient properties of EL2 are due to the antisite rather than to the interstitial or the complex}, number={3}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Zhang, Q.-M. and Bernholc, J.}, year={1993}, month={Jan}, pages={1667–1670} }
 @article{yi_bernholc_1993, title={Atomic structure and doping of microtubules}, volume={47}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.47.1708}, DOI={10.1103/physrevb.47.1708}, abstractNote={The atomic and electronic structures of microtubules of graphitic carbon have been investigated by ab initio molecular dynamics. Fully optimized atomic structures of two microtubules, a reflection symmetric and a chiral, are found to differ little from their ideal, graphite-derived geometries. Both are semiconductors with direct band gaps at the \ensuremath{\Gamma} point and along the \ensuremath{\Delta} direction, respectively. It is shown by direct calculations in large supercells that substitutional N and B dope microtubules n and p type.}, number={3}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Yi, Jae-Yel and Bernholc, J.}, year={1993}, month={Jan}, pages={1708–1711} }
 @article{kajihara_antonelli_bernholc_1993, title={Impurity incorporation and doping of diamond}, volume={185}, ISSN={0921-4526}, url={http://dx.doi.org/10.1016/0921-4526(93)90228-x}, DOI={10.1016/0921-4526(93)90228-x}, abstractNote={Electronic applications require the ability to dope diamond p- and n-type. Boron is well known to dope both natural and synthetic diamond p-type. N-type doping, however, has proven exceedingly difficult. In this work, the suitability of several impurities for n-type doping is investigated theoretically. We also examine the well-known nitrogen deep impurity as well as the effect of simultaneous doping with N and B on the thermodynamic equilibrium between diamond and graphite. The calculations were carried out using local density theory, the pseudopotential formalism, and the Car- Parrinello method. The impurities were embedded in a large supercell and atomic relaxations were computed using ab initio forces. The impurities Li, Na and P are shown to be shallow donors, but they have very low solubilities. This makes their incorporation via in-diffusion difficult and leaves ion implantation and possibly incorporation during growth as the only alternatives. Once incorporated, Li is found to be a fast diffuser whereas Na will be stable up to moderate temperatures. The most suitable shallow donor is Na, which occupies an interstitial site. It is particularly appropriate for ion implantation, since no self-implantation step to create vacant sites is necessary.}, number={1-4}, journal={Physica B: Condensed Matter}, publisher={Elsevier BV}, author={Kajihara, S.A. and Antonelli, A. and Bernholc, J.}, year={1993}, month={Apr}, pages={144–149} }
 @article{bernholc_chen_zhang_wang_yakobson_1993, title={Impurity-Enhanced Disordering in Superlattices}, volume={143-147}, ISSN={1662-9752}, url={http://dx.doi.org/10.4028/www.scientific.net/msf.143-147.593}, DOI={10.4028/www.scientific.net/msf.143-147.593}, journal={Materials Science Forum}, publisher={Trans Tech Publications}, author={Bernholc, J. and Chen, B. and Zhang, Q. and Wang, C. and Yakobson, B.}, year={1993}, month={Oct}, pages={593–598} }
 @inbook{bernholc_oh_sullivan_yi_1993, place={Amsterdam}, title={Quantum Molecular Dynamics Studies of the Structure and Dynamics of Metal Clusters}, ISBN={9780444890221}, url={http://dx.doi.org/10.1016/b978-0-444-89022-1.50018-3}, DOI={10.1016/b978-0-444-89022-1.50018-3}, abstractNote={This chapter discusses quantum molecular dynamics (QMD) studies of the structure and dynamics of metal clusters. What is present in the chapter focuses on metal clusters with Al chosen as a paradigm. The structures of the clusters will play an important role in the assembly process and influence the properties of the final material. The chapter discusses the structures of larger Al clusters, looking at the icosahedral to fee phase transition via the first-principles Car-Parrinello (quantum molecular dynamics) approach. The results of such calculations can be used for the development of empirical potentials that would be suitable for classical molecular dynamics simulations of much larger clusters, cluster fusion, and the assembly of cluster materials. The chapter describes Car-Parrinello calculations that are focused on (1) investigating the transition between icosahedral and bulk (fee) cluster structures in Al, as a function of cluster size, and (2) on developing a database of first-principles results for determining an embedded atom potential for clusters.}, booktitle={On Clusters and Clustering: From atoms to fractals}, publisher={North Holland/Elsevier}, author={Bernholc, J. and Oh, D. and Sullivan, D.J. and Yi, Jae-Yel}, editor={Reynolds, Peter J.Editor}, year={1993}, pages={127–142} }
 @article{bernholc_yi_zhang_brabec_anderson_davidson_kajihara_1993, title={Quantum molecular dynamics simulations of fullerenes and graphitic microtubules}, volume={26}, ISSN={0178-7683 1434-6079}, url={http://dx.doi.org/10.1007/bf01429110}, DOI={10.1007/bf01429110}, number={1-4}, journal={Zeitschrift f�r Physik D Atoms, Molecules and Clusters}, publisher={Springer Science and Business Media LLC}, author={Bernholc, J. and Yi, Jae-Yel and Zhang, Q. -M. and Brabec, C. J. and Anderson, E. B. and Davidson, B. N. and Kajihara, S. A.}, year={1993}, month={Mar}, pages={74–78} }
 @article{yi_bernholc_1993, title={Reactivity, stability, and formation of fullerenes}, volume={48}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.48.5724}, DOI={10.1103/physrevb.48.5724}, abstractNote={The energetics and reactions of fullerenes have been investigated by ab initio molecular dynamics. ${\mathrm{C}}_{60}$ and other fullerenes of similar sizes are reactive towards ${\mathrm{C}}_{2}$, which can insert into the fullerene cage without an activation barrier. The computed stability and reactivity patterns suggest that the increase of abundance of ${\mathrm{C}}_{60}$ and other locally stable fullerenes upon high-temperature annealing is due to atom and fragment exchange with other carbon clusters.}, number={8}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Yi, Jae-Yel and Bernholc, J.}, year={1993}, month={Aug}, pages={5724–5727} }
 @inproceedings{hwang_chen_boguslawski_bernholc_1993, title={Reconstructions at Si- and C-Terminated Surfaces of 2H-SiC: an ab Initio Molecular Dynamics Study}, volume={327}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-327-293}, DOI={10.1557/proc-327-293}, abstractNote={Abstract}, booktitle={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Hwang, Yong Gyoo and Chen, Bin and Boguslawski, Piotr and Bernholc, J.}, year={1993} }
 @article{maiti_brabec_bernholc_1993, title={Structure and energetics of single and multilayer fullerene cages}, volume={70}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.70.3023}, DOI={10.1103/physrevlett.70.3023}, abstractNote={Minimum energy structures for large carbon cages are investigated using realistic atomic potentials. Above a critical size of ∼6000 there is a stability crossover from single to multilayer cages. The lowest energy atomic configuration of a cage of any size consists only of hexagons and pentagons and is polyhedral in shape. Lowest energy defects involve either rearrangements between pentagons and hexagons or 5-7-5 triplets. Simulated transmission electron microscopy images show that a polyhedral onion appears spheroidal when viewed along its highest symmetry axis, and polyhedral when viewed from a more general direction}, number={20}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Maiti, A. and Brabec, C. J. and Bernholc, J.}, year={1993}, month={May}, pages={3023–3026} }
 @inproceedings{zhang_yi_brabec_anderson_davidson_kajihara_bernholc_1993, place={Singapore}, title={Structure, Dynamics, and Formation of Carbon and Aluminum Clusters}, ISBN={9789810210847}, booktitle={Clusters and fullerenes : proceedings of the Adriatico research conference, Trieste, Italy, June 23-26, 1992}, publisher={World Scientific}, author={Zhang, Q.M. and Yi, J.Y. and Brabec, C.J. and Anderson, E.B. and Davidson, B.N. and Kajihara, S.A. and Bernholc, J.}, editor={Kumar, V. and Martin, T.P. and Tosatti, E.Editors}, year={1993}, pages={83} }
 @article{maiti_brabec_bernholc_1993, title={ZERO AND FINITE TEMPERATURE STUDY OF SINGLE FULLERENE CAGES AND CARBON “ONIONS” — GEOMETRY AND SHAPE}, volume={07}, ISSN={0217-9849 1793-6640}, url={http://dx.doi.org/10.1142/s0217984993001892}, DOI={10.1142/s0217984993001892}, abstractNote={ Scaling arguments are used to show that above a critical size of several thousand atoms, there is a stability crossover from single to multilayer cages. Conjugate gradient minimization using a classical three-body interatomic potential, as well as tight-binding electronic structure calculations yield ground-state configurations for large fullerene shells that are polyhedral with clearly faceted geometry. The structure, energetics and configurational entropy associated with low-energy defects are calculated and the number of defects estimated as a function of temperature. The role of these thermally generated defects on the shape of large fullerenes is investigated in order to explain the nearly spherical shapes of the newly discovered carbon “onions”. }, number={29n30}, journal={Modern Physics Letters B}, publisher={World Scientific Pub Co Pte Lt}, author={Maiti, A. and Brabec, C.J. and Bernholc, J.}, year={1993}, month={Dec}, pages={1883–1895} }
 @article{yi_bernholc_1992, title={Atomic structure of Al-GaAs(110) interfaces}, volume={69}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.69.486}, DOI={10.1103/physrevlett.69.486}, abstractNote={Atomic structures of Al on GaAs(110) are studied by ab initio molecular dynamics for coverages of 1/8 to 1 monolayer (ML). A single chemisorbed Al atom resides at the center of a triangle of one Ga and two As atoms. Al dimers have very long bond lengths and bind due to substrate-mediated interactions. Epitaxial growth of 1 ML of Al is less stable than the formation of islands. Preformed clusters bond strongly to the substrate, which shows that the absence of Fermi-level pinning in samples grown by cluster deposition is due to suppression of reactivity rather than lack of interactions}, number={3}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Yi, Jae-Yel and Bernholc, J.}, year={1992}, month={Jul}, pages={486–489} }
 @article{davidson_lucovsky_bernholc_1992, title={Defect States and Structural Disorder in a-Si.}, volume={258}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-258-263}, DOI={10.1557/proc-258-263}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Davidson, B. N. and Lucovsky, G. and Bernholc, J.}, year={1992} }
 @article{yi_bernholc_1992, title={Isomerization of C60 fullerenes}, volume={96}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.462269}, DOI={10.1063/1.462269}, abstractNote={Car–Parrinello calculations show that the transformation between C60 isomers involves barriers in excess of 5 eV, although the energetic difference between the buckyball and the next lowest isomer is only 1.6 eV. C60 isomerization thus requires high temperatures and long annealing times.}, number={11}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Yi, Jae‐Yel and Bernholc, J.}, year={1992}, month={Jun}, pages={8634–8636} }
 @inproceedings{bernholc_kajihara_antonelli_1992, place={London}, title={N-Type and P-Type Doping of Diamond - Past, Present, and Future}, ISBN={9781907747687}, booktitle={Surface modification technologies V : proceedings of the Fifth International Conference held in Birmingham, UK, 2-4 September 1991}, publisher={The Institute of Materials}, author={Bernholc, J. and Kajihara, S.A. and Antonelli, A.}, editor={Sudarshan, T.S. and Braza, J.F.Editors}, year={1992}, pages={273} }
 @article{brabec_anderson_davidson_kajihara_zhang_bernholc_tománek_1992, title={Precursors toC60fullerene formation}, volume={46}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.46.7326}, DOI={10.1103/physrevb.46.7326}, abstractNote={It is shown that the free energy of C 20 clusters favors a transformation from a closed cage (fullerene) structure at low temperatures to a more open corrannulene structure and then to a ring structure as the temperature is increased. The corrannulene structure can be thought of as a fragment of C 60 and is an efficient precursor to its formation. These results are applicable to other carbon clusters of similar sizes and are consistent with the experimental findings that high temperatures are necessary for the formation of substantial quantities of C 60}, number={11}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Brabec, C. J. and Anderson, E. B. and Davidson, B. N. and Kajihara, S. A. and Zhang, Q.-M. and Bernholc, J. and Tománek, D.}, year={1992}, month={Sep}, pages={7326–7328} }
 @inbook{bernholc_yi_zhang_sullivan_brabec_kajihara_anderson_davidson_1992, title={Quantum Molecular Dynamics of Clusters}, ISBN={9789401726474 9789401726450}, url={http://dx.doi.org/10.1007/978-94-017-2645-0_35}, DOI={10.1007/978-94-017-2645-0_35}, booktitle={Physics and Chemistry of Finite Systems: From Clusters to Crystals}, publisher={Springer Netherlands}, author={Bernholc, J. and Yi, Jae-Yel and Zhang, Q.-M. and Sullivan, D. J. and Brabec, C. J. and Kajihara, S. A. and Anderson, E. B. and Davidson, B. N.}, year={1992}, pages={287–297} }
 @article{bernholc_kajihara_antonelli_1992, title={Theory of Doping of Diamond}, volume={242}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-242-323}, DOI={10.1557/proc-242-323}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Bernholc, J. and Kajihara, S. A. and Antonelli, A.}, year={1992} }
 @article{zhang_wang_bernholc_1992, title={Theory of Zn-Enhanced Disordering in GaAs/AlAs Superlattices}, volume={83-87}, ISSN={1662-9752}, url={http://dx.doi.org/10.4028/www.scientific.net/msf.83-87.1351}, DOI={10.4028/www.scientific.net/msf.83-87.1351}, journal={Materials Science Forum}, publisher={Trans Tech Publications}, author={Zhang, Q.M. and Wang, C. and Bernholc, J.}, year={1992}, month={Jan}, pages={1351–1356} }
 @article{wang_zhang_bernholc_1992, title={Theory of Zn-enhanced disordering in GaAs/AlAs superlattices}, volume={69}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.69.3789}, DOI={10.1103/physrevlett.69.3789}, abstractNote={The microscopic mechanisms for Zn diffusion in GaAs and Zn-induced interdiffusion in GaAs/AlAs superlattices are investigated by ab initio molecular dynamics. Among the various proposed mechanisms for Zn diffusion, kick-out by Ga interstitials has the lowest activation energy. Zn in-diffusion generates nonequilibrium group-III interstitials, which are bound to Zn by Coulomb forces. The interstitials follow the Zn diffusion front and disorder the superlattice. The calculated activation energies for these processes are in good agreement with the experimental data}, number={26}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Wang, C. and Zhang, Q.-M. and Bernholc, J.}, year={1992}, month={Dec}, pages={3789–3792} }
 @article{bernholc_kajihara_wang_antonelli_davis_1992, title={Theory of native defects, doping and diffusion in diamond and silicon carbide}, volume={11}, ISSN={0921-5107}, url={http://dx.doi.org/10.1016/0921-5107(92)90222-u}, DOI={10.1016/0921-5107(92)90222-u}, abstractNote={The properties of native defects and impurities in diamond and SiC are investigated via large-scale band structure and Car-Parrinello calculations. In diamond, the activation energy for self-diffusion is very high in the intrinsic material (9 eV) but decreases by up to 3 eV in either p- or n-type material. Phosphorus, lithium and sodium are shallow donors, but their solubilities are very low, which makes them unsuitable for incorporation into diamond via in-diffusion. Instead, kinetic trapping during growth or ion implantation must be used. Considering the stability at the dopant site, substitutional phosphorus is expected to diffuse by the vacancy mechanism and to have a high activation energy by analogy to self-diffusion. Both lithium and sodium diffuse through the interstitial channel. Lithium is a relatively fast diffuser while sodium should be stable up to moderately high temperatures. For nitrogen in diamond, the well known (111) distortion is found to be due to the interaction of the fully occupied nitrogen lone pair with the dangling bond of the C(111) atom. The single electron associated with the center resides in an antibonding orbital formed from the dangling hybrid and the nitrogen lone pair. This orbital has most of its amplitude on the carbon atom. In SiC, the lowest energy defect in n-type and intrinsic material is the electrically inactive silicon antisite, while the lowest energy defect in p-type SiC is the doubly positive carbon vacancy. The electrons released by the vacancies compensate acceptor dopants, leading to strong self-compensation effects when doping occurs during crystal growth. In carbon-rich SiC, the dominant defect for all Fermi level positions is the electrically inactive carbon antisite. In boron-doped SiC, BC is preferred for silicon-rich material while, in carbon-rich SiC, BC and BSi have similar formation energies.}, number={1-4}, journal={Materials Science and Engineering: B}, publisher={Elsevier BV}, author={Bernholc, J. and Kajihara, S.A. and Wang, C. and Antonelli, A. and Davis, R.F.}, year={1992}, month={Jan}, pages={265–272} }
 @inproceedings{nelson_averback_bernholc_1991, title={Clusters and Cluster-Assembled Materials}, volume={206}, booktitle={Materials Research Society Symposium Proceedings}, publisher={Materials Research Society}, author={Nelson, D.L. and Averback, R.S. and Bernholc, J.}, year={1991} }
 @article{berry_bernholc_salamon_1991, title={Disappearance of grain boundaries in sintering}, volume={58}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.105222}, DOI={10.1063/1.105222}, abstractNote={An exactly solvable model of grain boundary annealing in sinters is introduced and studied for a range of particle sizes. The model explains well the cluster fusion events observed by high-resolution transmission electron microscopy.}, number={6}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Berry, R. Stephen and Bernholc, J. and Salamon, Peter}, year={1991}, month={Feb}, pages={595–597} }
 @inproceedings{davidson_lucovsky_bernholc_1991, title={Effect of the Local Disorder in a-Si on the Electronic Density of States at the Band Edges.}, volume={219}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-219-581}, DOI={10.1557/proc-219-581}, abstractNote={ABSTRACT}, booktitle={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Davidson, B. N. and Lucovsky, G. and Bernholc, J.}, year={1991} }
 @article{davidson_lucovsky_bernholc_1991, title={Effect of the local disorder in a-Si on the electronic density of states near the band edges}, volume={137-138}, ISSN={0022-3093}, url={http://dx.doi.org/10.1016/s0022-3093(05)80117-6}, DOI={10.1016/s0022-3093(05)80117-6}, abstractNote={We have systematically investigated the formation of electronic states in the region of the conduction and valence band edges of a-Si as functions of variations in the bond and dihedral angle distributions. Local Densities of States (LDOS) for Si atoms in disordered environments have been calculated using the cluster Bethe lattice method with a Hamiltonian containing both first and second nearest neighbor interaction terms. We conclude that the changes in orbital overlap, incurred from rotations about the axes defining the dihedral angle distortions, are the origin of the effect of dihedral angle disorder on the electronic states near the band gap.}, journal={Journal of Non-Crystalline Solids}, publisher={Elsevier BV}, author={Davidson, B.N. and Lucovsky, G. and Bernholc, J.}, year={1991}, month={Jan}, pages={307–310} }
 @inbook{bernholc_kajihara_antonelli_1991, place={Pittsburgh}, title={N-type Doping and Donor Incorporation in Diamond}, booktitle={New diamond science and technology : proceedings of the second international conference, Washington, DC, September 23-27-1990}, publisher={Materials Research Society}, author={Bernholc, J. and Kajihara, A. and Antonelli, A.}, editor={Messier, R. and Glass, J.T. and Butler, J.E. and Roy, R.Editors}, year={1991}, pages={923} }
 @article{kajihara_antonelli_bernholc_car_1991, title={Nitrogen and potentialn-type dopants in diamond}, volume={66}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.66.2010}, DOI={10.1103/physrevlett.66.2010}, abstractNote={Potential n-type dopants in diamond are investigated via ab initio methods. The well-known distortions around the deep donor N are found to arise from the interaction of the N lone pair with a C dangling bond. P, Li, and Na are all shallow dopants, but their solubilities are muh too low for doping via in-diffusion. Li is a relatively fast diffuser, Na is stable up to moderate temperatures, while P should remain immobile even at high temperatures. Na, being an interstitial dopant, is particularly suitable for ion implantation, since there is no need to displace host atoms.}, number={15}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Kajihara, S. A. and Antonelli, A. and Bernholc, J. and Car, R.}, year={1991}, month={Apr}, pages={2010–2013} }
 @article{bernholc_antonelli_wang_davis_1991, title={Self-Diffusion Mechanisms in Diamond, SiC, Si and Ge}, volume={38-41}, ISSN={1662-9752}, url={http://dx.doi.org/10.4028/www.scientific.net/msf.38-41.713}, DOI={10.4028/www.scientific.net/msf.38-41.713}, journal={Materials Science Forum}, publisher={Trans Tech Publications}, author={Bernholc, J. and Antonelli, A. and Wang, C. and Davis, Robert F.}, year={1991}, month={Jan}, pages={713–718} }
 @article{basa_schön_berry_bernholc_jellinek_salamon_1991, title={Shapes of wetted solids and sinters}, volume={43}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.43.8113}, DOI={10.1103/physrevb.43.8113}, abstractNote={The first stage of wetting of porous media and sintering of powders is characterized by the condition that the wetted contact regions between the constituent particles do not overlap, thus reducing the global problem to a set of independent local problems. The local problem can often be approximated by the wetting and/or sintering of overlapping spheres of different sizes. The last problem has axial symmetry and can conveniently be modeled as the wetting of solid cores by a local mobile pool. We have developed a wetting algorithm which determines the shape of the contacts, assuming the separability of the time scales between the local equilibration of neck shapes between the particles (fast) and the supply of liquid material by various diffusive processes (slow). The algorithm minimizes the free energy of the system, subject to the constraint of a fixed liquid volume. The problem may be naturally divided into several cases, and we present efficient numerical procedures for calculating neck shapes for each case.}, number={10}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Basa, P. and Schön, J. C. and Berry, R. S. and Bernholc, J. and Jellinek, J. and Salamon, P.}, year={1991}, month={Apr}, pages={8113–8122} }
 @article{yi_bernholc_salamon_1991, title={Simulated annealing strategies for molecular dynamics}, volume={66}, ISSN={0010-4655}, url={http://dx.doi.org/10.1016/0010-4655(91)90066-t}, DOI={10.1016/0010-4655(91)90066-t}, abstractNote={We present a convenient implementation of constant-thermodynamic-speed simulated annealing for molecular dynamics calculations. Sample results are shown for finding the ground states of 13-Al and 55-Al clusters by classical molecular dynamics and by ab initio quantum molecular dynamics using the Car-Parrinello formalism.}, number={2-3}, journal={Computer Physics Communications}, publisher={Elsevier BV}, author={Yi, Jae-Yel and Bernholc, J. and Salamon, Peter}, year={1991}, month={Sep}, pages={177–180} }
 @article{yi_oh_bernholc_1991, title={Structural distortions in metal clusters}, volume={67}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.67.1594}, DOI={10.1103/physrevlett.67.1594}, abstractNote={The energetics and structures or metal clusters are studied by the Car-Parrinello method with Al chosen as a paradigm. For the 13- and SS-atom clusters which can assume perfect icosahedral and cubic structures, the distortions from these ideal structures are substantial. For the SS-atom cluster, several inequivalent but energetically nearly degenerate structures are round. The origin or this near degeneracy is traced to the short range or the effective interatomic interactions, which provides an ab initio justification for their parametrization in terms or short-range potentials}, number={12}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Yi, Jae-Yel and Oh, Dirk J. and Bernholc, J.}, year={1991}, month={Sep}, pages={1594–1597} }
 @article{bernholc_yi_sullivan_1991, title={Structural transitions in metal clusters}, volume={92}, ISSN={1359-6640 1364-5498}, url={http://dx.doi.org/10.1039/fd9919200217}, DOI={10.1039/fd9919200217}, abstractNote={The methods for first-principles calculation of the structure and dynamics of clusters have now progressed to a point where clusters containing ca. 50 non-transition-metal atoms can be studied. As a paradigm, we studied the energetics of structural transformations in 13- and 55-atom Al clusters, which can assume both perfect icosahedral and cuboctahedral structures. Using the Car–Parrinello (quantum molecular dynamics) formalism, we found Al13 has a unique structure, a slightly distorted icosahedron, but Al55 has several inequivalent but energetically nearly degenerate structures. The degeneracy in Al55 is due to the short range of the effective interatomic interactions in a metallic cluster and should lead to floppiness at finite temperatures. A new accurate procedure for calculating ionization potentials (Ei) and electron affinities (Eea) within the Car–Parrinello formalism was developed and applied to Al clusters. Unfortunately, it appears that at least for some clusters, most notably for Al55, the Ei and Eea are very similar for different structural models of this cluster. A formulation and the first tests of a new multigrid-based method for real space electronic structure calculations are briefly described. This method should make possible calculations similar to the above for clusters containing transition metal and/or first-row atoms in the fairly near future.}, journal={Faraday Discussions}, publisher={Royal Society of Chemistry (RSC)}, author={Bernholc, J. and Yi, Jae-Yel and Sullivan, D. J.}, year={1991}, pages={217} }
 @article{zhang_yi_bernholc_1991, title={Structure and dynamics of solid C60}, volume={66}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.66.2633}, DOI={10.1103/physrevlett.66.2633}, abstractNote={Results of ab initio molecular-dynamics simulations of the properties of solid C 60 are reported. The soccer-ball structure of molecular C 60 is well preserved in the solid. The T=0 structure consists of alternating single and double bonds, but this distinction becomes blurred at relatively low temperatures. The intermolecular interactions are so weak that the individual C 60 can rotate at relatively low temperatures. At high temperatures vibrations cause large distortions, but the cage structure is still preserved. The present results are in excellent agreement with photoemission and NMR data}, number={20}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Zhang, Q.-M. and Yi, Jae-Yel and Bernholc, J.}, year={1991}, month={May}, pages={2633–2636} }
 @article{faux_hall_bernholc_1990, title={A New Monte Carlo Simulation Technique for the Study of Epitaxial Crystal Growth}, volume={4}, ISSN={0892-7022 1029-0435}, url={http://dx.doi.org/10.1080/08927029008022400}, DOI={10.1080/08927029008022400}, abstractNote={Abstract A new type of Monte Carlo simulation is described which models epitaxial crystal growth on non-lattice-matched substrates. The model allows adatoms to immediately occupy positions of local potential minima which are defined by the particle-particle interactions. Adatoms make discrete hops from one minimum to another. This model therefore combines some of the flexibility of continuous-space Monte Carlo with the speed associated with lattice-like simulations. Details of the Monte Carlo procedure are presented for a two-dimensional system where the particle-particle interactions are assumed to be of the Lennard-Jones type. Some results are presented for the case where the adsorbate crystal lattice is larger than the substrate lattice by 2% and 8%.}, number={6}, journal={Molecular Simulation}, publisher={Informa UK Limited}, author={Faux, D. A. and Hall, C. K. and Bernholc, J.}, year={1990}, month={May}, pages={361–369} }
 @article{faux_gaynor_carson_hall_bernholc_1990, title={Computer simulation studies of the growth of strained layers by molecular-beam epitaxy}, volume={42}, ISSN={0163-1829 1095-3795}, url={http://dx.doi.org/10.1103/physrevb.42.2914}, DOI={10.1103/physrevb.42.2914}, abstractNote={Ces nouvelles simulations sont plus economiques de ressources de l'ordinateur que la dynamique moleculaire au Monte Carlo de l'espace continu. Chacune des 2 simulations est appliquee a l'etude de la croissance sur un substrat en deux dimensions a l'aide des potentiels interatomiques de Lennard-Jones}, number={5}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Faux, D. A. and Gaynor, G. and Carson, C. L. and Hall, C. K. and Bernholc, J.}, year={1990}, month={Aug}, pages={2914–2922} }
 @inproceedings{bernholc_kajihara_antonelli_1990, place={Singapore}, title={Donor Impurities, N-type Doping, and Impurity Diffusion in Diamond}, booktitle={20th International Conference on the Physics of Semiconductors : Thessaloniki, Greece, August 6-10, 1990}, publisher={World Scientific}, author={Bernholc, J. and Kajihara, S.A. and Antonelli, A.}, editor={Anastassakis, E.M. and Joannopoulos, J.D.Editors}, year={1990}, pages={332} }
 @article{carson_bernholc_faux_hall_1990, title={Efficient techniques for computer simulations of heteroepitaxial growth}, volume={56}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.103014}, DOI={10.1063/1.103014}, abstractNote={A new discrete Monte Carlo technique suitable for simulations of the kinetics of heteroepitaxial crystal growth has been developed and tested on a 103 atom system. The technique offers sizable speed advantages over previous simulation methods and allows for realistic three-dimensional studies of the kinetics of both pseudomorphic and misfit growth modes and of the transformation between them. Elements of the method are of general utility and can also be used to substantially improve the efficiency of continuous-space Monte Carlo and molecular dynamics simulations of growth and other atomic transformations.}, number={20}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Carson, C. L. and Bernholc, J. and Faux, D. and Hall, C. K.}, year={1990}, month={May}, pages={1971–1973} }
 @book{antonelli_bernholc_1990, place={Pittsburgh, PA}, series={Materials Research Society Symposium Proceedings}, title={Impurities, defects, and diffusion in semiconductors :
bulk and layered structures : symposium held November 27-December 1, 1989, Boston, Massachusetts, U.S.A}, volume={163}, ISSN={0272-9172}, url={http://dx.doi.org/10.21236/ada229590}, DOI={10.21236/ada229590}, abstractNote={Abstract : This volume of proceedings contains manuscripts from Symposium G, entitled "Impurities, Defects, and Diffusion in Semiconductors: Bulk and Layered Structures." Historically, Symposium G was the seventh in a series of MRS- sponsored symposia which focused on various aspects of defects and defect properties in semiconducting materials. This symposium was conceived from the view that impurities, defects, and diffusion play key roles in modern-day research and development of semiconducting materials, structures, and devices. Recent breakthroughs in materials preparation with monolayer control, in diversity and sensitivity of characterization techniques, and in new theoretical methods, have collectively led to great advances in the understanding of defect- and impurity-related phenomena.}, journal={Impurities, Defects and Diffusion in Semiconductors: Bulk and Layered Structures}, publisher={Materials Research Society}, author={Antonelli, A. and Bernholc, Jerzy}, editor={Wolford, D.J. and Bernholc, J. and Haller, E.E.Editors}, year={1990}, pages={523}, collection={Materials Research Society Symposium Proceedings} }
 @inproceedings{zhang_yi_bernholc_1990, title={Quantum Molecular Dynamics of Solid C60}, volume={206}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-206-727}, DOI={10.1557/proc-206-727}, abstractNote={ABSTRACT}, booktitle={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Zhang, Q. and Yi, Jae-Yel and Bernholc, J.}, year={1990} }
 @article{yi_oh_bernholc_car_1990, title={Structural transitions in aluminum clusters}, volume={174}, ISSN={0009-2614}, url={http://dx.doi.org/10.1016/s0009-2614(90)87180-y}, DOI={10.1016/s0009-2614(90)87180-y}, abstractNote={The energetics of transitions between icosahedral and bulk-like structures is studied theoretically via the Car—Parrinello (CP) method. Total energy calculations show that the energy differences between icosahedral and cuboctahedral structures for 13-, 19-, and 55-atom clusters are surprisingly small, indicating that the transition between icosahedral and bulk structures may occur very early in Al clusters. Bulk embedded atom potentials are found useful for estimates of atomic relaxations for a given structure, but they do not reproduce well the energy differences between different structures. Ionization potentials and electron affinities were calculated within the CP formalism and found to differ by 0.4–1.0 eV from the experimental results.}, number={5}, journal={Chemical Physics Letters}, publisher={Elsevier BV}, author={Yi, Jae-Yel and Oh, Dirk J. and Bernholc, J. and Car, R.}, year={1990}, month={Nov}, pages={461–466} }
 @inproceedings{bernholc_yi_oh_sullivan_1990, title={Structure and Dynamics of Aluminum Clusters}, volume={206}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-206-209}, DOI={10.1557/proc-206-209}, abstractNote={ABSTRACT}, booktitle={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Bernholc, J. and Yi, Jae-Yel and Oh, Dirk J. and Sullivan, D. J.}, year={1990} }
 @article{salamon_bernholc_berry_carrera‐patiño_andresen_1990, title={The wetted solid—A generalization of the Plateau problem and its implications for sintered materials}, volume={31}, ISSN={0022-2488 1089-7658}, url={http://dx.doi.org/10.1063/1.529031}, DOI={10.1063/1.529031}, abstractNote={A new generalization of the Plateau problem that includes the constraint of enclosing a given region is introduced. Physically, the problem is important insofar as it bears on sintering processes and the structure of wetted porous media. Some primal and dual characterizations of the solutions are offered and aspects of the problem are illustrated in one and two dimensions in order to clarify the combinatorial elements and demonstrate the importance of numerous local minima.}, number={3}, journal={Journal of Mathematical Physics}, publisher={AIP Publishing}, author={Salamon, Peter and Bernholc, J. and Berry, R. Stephen and Carrera‐Patiño, Martin E. and Andresen, Bjarne}, year={1990}, month={Mar}, pages={610–615} }
 @inbook{bernholc_antonelli_wang_davis_pantelides_1989, title={Defect Abundances and Diffusion Mechanisms in Diamond, SiC, Si and Ge}, ISBN={9781468457056 9781468457032}, url={http://dx.doi.org/10.1007/978-1-4684-5703-2_4}, DOI={10.1007/978-1-4684-5703-2_4}, booktitle={Atomistic Simulation of Materials}, publisher={Springer US}, author={Bernholc, J. and Antonelli, A. and Wang, C. and Davis, R. F. and Pantelides, S. T.}, year={1989}, pages={33–39} }
 @inproceedings{kajihara_antonelli_bernholc_1989, title={N-Type Doping and Diffusion of Impurities in Diamond}, volume={162}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-162-315}, DOI={10.1557/proc-162-315}, abstractNote={ABSTRACT}, booktitle={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Kajihara, S. A. and Antonelli, A. and Bernholc, J.}, year={1989} }
 @article{wang_bemholc_davis_1989, title={Native Defects, Diffusion, Self-Compensation, and Boron Doping in Cubic Silicon Carbide}, volume={162}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-162-475}, DOI={10.1557/proc-162-475}, abstractNote={ABSTRACT}, journal={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Wang, C. and Bemholc, J. and Davis, R. F.}, year={1989} }
 @article{antonelli_bernholc_1989, title={Pressure effects on self-diffusion in silicon}, volume={40}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.40.10643}, DOI={10.1103/physrevb.40.10643}, abstractNote={Les effets sont etudies par calculs d'energie totale parametre. Les trois mecanismes microscopiques, echange concerte, interstitiel et lacune, qui ont des energies d'activation similaires dans Si, presentent des variations differentes avec la pression}, number={15}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Antonelli, A. and Bernholc, J.}, year={1989}, month={Nov}, pages={10643–10646} }
 @article{babić_rabii_bernholc_1989, title={Structural and electronic properties of arsenic chalcogenide molecules}, volume={39}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.39.10831}, DOI={10.1103/physrevb.39.10831}, abstractNote={Local-density cluster pseudopotential calculations have been carried out for arsenic chalcogenide molecules ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$, ${\mathrm{As}}_{4}$${\mathrm{S}}_{5}$, ${\mathrm{As}}_{4}$${\mathrm{S}}_{6}$, and ${\mathrm{As}}_{4}$${\mathrm{Se}}_{4}$. Based on the results of the calculations, a model has been developed that explains the relative stability of the arsenic sulfide molecules. Arsenic-arsenic bonds in ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ and ${\mathrm{As}}_{4}$${\mathrm{S}}_{5}$ are suggested as the main reasons for the stability of these molecules. The ${\mathrm{As}}_{4}$${\mathrm{S}}_{4}$ and ${\mathrm{As}}_{4}$${\mathrm{Se}}_{4}$ molecules represent well the electronic properties of the corresponding solids. A randomly packed network of either ${\mathrm{As}}_{4}$${\mathrm{S}}_{5}$ and ${\mathrm{As}}_{4}$${\mathrm{S}}_{6}$ molecules appears to be an unlikely model for the local structure of the amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$. Excellent transferability of the arsenic-sulfur bond charge density among the molecules has been found.}, number={15}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Babić, Davorin and Rabii, Sohrab and Bernholc, Jerzy}, year={1989}, month={May}, pages={10831–10838} }
 @article{amar_bernholc_berry_jellinek_salamon_1989, title={The shapes of first‐stage sinters}, volume={65}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.342674}, DOI={10.1063/1.342674}, abstractNote={The shape of first-stage sinters is derived within a framework in which a solid skeleton is wetted by a mobile‘‘liquid’’ pool. Several lattices are considered as possible solid skeletons, corresponding to various degrees of surface versus grain boundary and volume transport; the effect of a nonzero dihedral angle at the interface between crystal faces is also treated. In some cases, significant differences exist between shapes calculated by minimizing the free energy and the shapes assumed by previous workers.}, number={8}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Amar, Francois and Bernholc, J. and Berry, R. Stephen and Jellinek, Julius and Salamon, Peter}, year={1989}, month={Apr}, pages={3219–3225} }
 @article{wang_bernholc_davis_1988, title={Formation energies, abundances, and the electronic structure of native defects in cubic SiC}, volume={38}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.38.12752}, DOI={10.1103/physrevb.38.12752}, abstractNote={The relative abundance of native point defects in cubic SiC has been studied via ab initio calculations as a function of composition and the Fermi-level position. For Si-rich cubic SiC, the ${\mathrm{Si}}_{\mathrm{C}}$ antisite is the dominant defect in $n$-type material, while the carbon vacancy, which is a double donor, dominates in $p$-type material. These results explain the experimentally observed low doping efficiencies of acceptors and the strong self-compensation effects in Si-rich cubic SiC. In C-rich SiC, the dominant defect is the electrically inactive ${\mathrm{C}}_{\mathrm{Si}}$ antisite, regardless of the position of the Fermi level. The slightly C-rich cubic SiC is thus a more suitable material for $p$-type doping.}, number={17}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Wang, C. and Bernholc, J. and Davis, R. F.}, year={1988}, month={Dec}, pages={12752–12755} }
 @article{bernholc_antonelli_del sole_bar-yam_pantelides_1988, title={Mechanism of self-diffusion in diamond}, volume={61}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.61.2689}, DOI={10.1103/physrevlett.61.2689}, abstractNote={Le processus d'autodiffusion est domine par un mecanisme de lacune independamment de la position du niveau de Fermi}, number={23}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Antonelli, A. and Del Sole, T. M. and Bar-Yam, Y. and Pantelides, S. T.}, year={1988}, month={Dec}, pages={2689–2692} }
 @inproceedings{bernholc_antonelli_wang_davis_1988, place={Warsaw, Poland}, title={Native Defects and Self-Diffusion in Diamond and SiC}, booktitle={19th International Conference on the Physics of Semiconductors : Warsaw, Poland, August 15-19, 1988}, publisher={institute of Physics, Polish Academy of Sciences}, author={Bernholc, J. and Antonelli, A. and Wang, C. and Davis, R.F.}, editor={Zawadzki, W.Editor}, year={1988}, pages={1115} }
 @inproceedings{antonelli_wang_bemholc_davis_1988, title={Native Defects in Diamond, Sic, and Si: Energetics and Self-Diffusion}, volume={141}, ISSN={0272-9172 1946-4274}, url={http://dx.doi.org/10.1557/proc-141-249}, DOI={10.1557/proc-141-249}, abstractNote={Abstract}, booktitle={MRS Proceedings}, publisher={Cambridge University Press (CUP)}, author={Antonelli, A. and Wang, C. and Bemholc, J. and Davis, R. F.}, year={1988} }
 @inbook{bernholc_salamon_berry_1987, place={Boston, MA}, title={Annealing of Fine Powders: Initial Shapes and Grain Boundary Motion}, ISBN={9781475703597 9781475703573}, url={http://dx.doi.org/10.1007/978-1-4757-0357-3_6}, DOI={10.1007/978-1-4757-0357-3_6}, booktitle={Physics and Chemistry of Small Clusters}, publisher={Springer US}, author={Bernholc, J. and Salamon, Peter and Berry, R. Stephen}, editor={Jena, P. and Bao, B.K. and Khanna, S.N.Editors}, year={1987}, pages={43–48} }
 @article{bernholc_horsley_murrell_sherman_soled_1987, title={Broensted acid sites in transition metal oxide catalysts: modeling of structure, acid strengths, and support effects}, volume={91}, ISSN={0022-3654 1541-5740}, url={http://dx.doi.org/10.1021/j100290a047}, DOI={10.1021/j100290a047}, abstractNote={The trends and origin of Broensted acidity of dispersed, ..gamma..-alumina-supported transition metal oxides are investigated via a new theoretical method. The method is based on first-principles quantum mechanical calculations for the acid site and includes a realistic model of the acid-base interaction. Both free and supported transition metal (TM) oxide clusters of Ti, Nb, and W are considered. For tetrahedrally coordinated unsupported TM oxide clusters, it is found that the Broensted acidity depends strongly on the number of terminal oxygen atoms (terminal O effect). For supported TM oxides, the calculations show that the charge delocalization extends into the support, leading to a strong additional increase in the Broensted acidity. This novel inductive effect of the support together with the terminal O effect is responsible for the high Broensted acidity of the catalysts. The strong support effect suggests that support modifications may significantly alter the catalytic properties of supported transition metal oxides. The calculated trends are in good agreement with the experimental measurements of relative Broensted acidity of the catalysts.}, number={6}, journal={The Journal of Physical Chemistry}, publisher={American Chemical Society (ACS)}, author={Bernholc, J. and Horsley, J. A. and Murrell, L. L. and Sherman, L. G. and Soled, S.}, year={1987}, month={Mar}, pages={1526–1530} }
 @article{liang_bernholc_pan_hughes_stiefel_1987, title={Electronic structures of cluster compounds of molybdenum sulfide (MoS42-, Mo3S92-) and nickel molybdenum sulfide (Ni(MoS4)22-) by XPS studies}, volume={26}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/ic00256a019}, DOI={10.1021/ic00256a019}, abstractNote={XPS studies were performed on cluster compounds (TEA)/sub 2/MoS/sub 4/, (TEA)/sub 2/Mo/sub 3/S/sub 9/, and (TEA)/sub 2/Ni(MoS/sub 4/)/sub 2/ (TEA = tetraethylammonium). The results are analyzed on the basis of formal charge and molecular orbital considerations. The major features of the valence band spectra of (TEA)/sub 2/MoS/sub 4/ and (TEA)/sub 2/Mo/sub 3/S/sub 9/ are in good agreement with theoretical densities of state of recent calculations using the local density pseudopotential method. However, the XPS data show that the multiple excitation process is quite significant in the simple tetrahedral MoS/sub 4//sup 2 -/. The XPS of (TEA)/sub 2/Ni(MoS/sub 4/)/sub 2/ reveals a significant difference from the XPS of the other two compounds in the top valence band, which is attributed to the additional d electrons in Ni (formally d/sup 8/) compared to Mo (formally d/sub 2/).}, number={9}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Liang, K. S. and Bernholc, J. and Pan, W. H. and Hughes, G. J. and Stiefel, E. I.}, year={1987}, month={May}, pages={1422–1425} }
 @article{wong_lucovsky_bernholc_1987, title={Intrinsic localized defect states in a-Se associated with dihedral angle distortions}, volume={97-98}, ISSN={0022-3093}, url={http://dx.doi.org/10.1016/0022-3093(87)90279-1}, DOI={10.1016/0022-3093(87)90279-1}, abstractNote={Studies of the transport properties in a-Se have shown that the hole and electron drift mobilities are controlled by shallow traps associated with intrinsic bonding defects. We propose a model for these defects that is based on a distortion of the normal dihedral angle in which the lone-pair orbitals on a pair of neigboring Se atoms in the defect configuration are parallel, rather than nearly perpendicular as in the normal bonding arrangement.}, journal={Journal of Non-Crystalline Solids}, publisher={Elsevier BV}, author={Wong, C.K. and Lucovsky, G. and Bernholc, J.}, year={1987}, month={Dec}, pages={1171–1174} }
 @inproceedings{bernholc_phillips_1987, place={Singapore}, title={Microclusters of Carbon, Silicon and Germanium}, booktitle={Proceedings of the 18th International Conference on the Physics of Semiconductors : Stockholm, Sweden, August 11-15, 1986}, publisher={World Scientific}, author={Bernholc, J. and Phillips, J.C.}, editor={Engstrom, O.Editor}, year={1987}, pages={1855} }
 @article{bernholc_stiefel_1986, title={Comparison of the electronic properties of bis(disulfido)dioxidi-.mu.-sulfidodimolybdate(2-) and bis(disulfido)disulfidodi-.mu.-sulfidodimolybdate(2-)}, volume={25}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/ic00242a009}, DOI={10.1021/ic00242a009}, abstractNote={A detailed theoretical analysis is presented of the similarities and differences of the electronic structures of Mo/sub 2/O/sub 2/(..mu..-S)/sub 2/(S/sub 2/)/sub 2//sup 2 -/ (1) and Mo/sub 2/S/sub 2/(..mu..-S)/sub 2/(S/sub 2/)/sub 2//sup 2 -/ (2). The analysis is based on self-consistent, first-principles electronic structure calculations for both systems. The replacement of the O atoms in 1 with S atoms in 2 leaves the higher filled MOs unaffected, although it removes the lower lying molecular orbitals corresponding to the Mo-O triple bond. The LUMOs, which in 1 are the Mo-O antibonding orbitals, are in 2 the Mo-S(terminal) antibonding orbitals at lower energies, decreasing the LUMO-HOMO gap in accordance with experimental observations. The energies of other antibonding orbitals are shifted somewhat to lower energies in going from 1 to 2. The results suggest a similar reactivity pattern for reactions involving primarily the HOMO S/sub 2/ ..pi.. orbitals, i.e., reactions with electrophiles. For reactions with nucleophilic components, somewhat lower activation energies should be expected due to the lowering of the HOMO-LUMO gap in 2 compared to that in 1.}, number={22}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Bernholc, J. and Stiefel, E. I.}, year={1986}, month={Oct}, pages={3876–3879} }
 @article{bernholc_phillips_1986, title={Kinetics of aggregation of carbon clusters}, volume={33}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.33.7395}, DOI={10.1103/physrevb.33.7395}, abstractNote={Gas-phase cluster aggregation is simulated by a model which includes the simultaneous growth of neutral and charged clusters in a laser vaporization source. Electronic-structure calculations determine the kinetic parameters of the model. Results for carbon clusters are in good agreement with experimental data, explain the magic numbers in the ${\mathrm{C}}_{10}$--${\mathrm{C}}_{25}$ region, and demonstrate the importance of both thermodynamic and kinetic effects on the cluster growth process.}, number={10}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Phillips, J. C.}, year={1986}, month={May}, pages={7395–7398} }
 @article{bernholc_phillips_1986, title={Kinetics of cluster formation in the laser vaporization source: Carbon clusters}, volume={85}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.450996}, DOI={10.1063/1.450996}, abstractNote={A general model of cluster formation in the laser vaporization source starting from the atomic vapor is developed and applied to carbon clusters. Two limiting cases of cluster growth exist: the diffusion-limited regime, in which the cluster distribution is essentially featureless, and the reaction-limited regime, in which the most stable clusters show as magic numbers in the cluster distribution. An approximate theory of the aggregation kernel allows for calculation of the cluster distribution in the reaction-limited regime from the formation energies of the reacting clusters. Heat released in cluster fusion allows small and medium size clusters to attain their lowest or almost lowest energy configurations in both the diffusion- and reaction-limited regimes. For larger clusters, crystalline structures are expected for transition metals, while network-forming materials are likely to exibit a multitude of structures. An application to carbon clusters in the n=1–25 range reproduce the experimentally observed cluster distributions and the magic numbers in the n=10–25 regime. The equilibrium structures of the small carbon clusters formed in the nozzle are found to be chains and monocyclic rings. At the reaction temperature, the transition between the chain and ring structures occurs around n=10 for the neutral and the positively charged clusters.}, number={6}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Bernholc, J. and Phillips, J. C.}, year={1986}, month={Sep}, pages={3258–3267} }
 @article{bernholc_stiefel_1985, title={Electronic structure of thiomolybdates (MoS42- and Mo3S92-}, volume={24}, ISSN={0020-1669 1520-510X}, url={http://dx.doi.org/10.1021/ic00203a012}, DOI={10.1021/ic00203a012}, abstractNote={ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTElectronic structure of thiomolybdates (MoS42- and Mo3S92-Jerzy Bernholc and E. I. StiefelCite this: Inorg. Chem. 1985, 24, 9, 1323–1330Publication Date (Print):April 1, 1985Publication History Published online1 May 2002Published inissue 1 April 1985https://doi.org/10.1021/ic00203a012RIGHTS & PERMISSIONSArticle Views207Altmetric-Citations37LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts}, number={9}, journal={Inorganic Chemistry}, publisher={American Chemical Society (ACS)}, author={Bernholc, Jerzy and Stiefel, E. I.}, year={1985}, month={Apr}, pages={1323–1330} }
 @article{bernholc_1985, title={Impurity Induced States in Amorphous Hydrogenated Silicon}, volume={14a}, journal={Journal of Electronic Materials}, author={Bernholc, J.}, year={1985}, pages={781} }
 @article{bernholc_1985, title={Modeling of defects in amorphous semiconductors: Threefold coordination in a-Si:H}, volume={75}, ISSN={0022-3093}, url={http://dx.doi.org/10.1016/0022-3093(85)90235-2}, DOI={10.1016/0022-3093(85)90235-2}, abstractNote={Defects in a-Si:H are modeled by embedding a small “defect cluster” in a crystalline Si matrix. Electronic structure calculations for threefold coordinated Si, P, B, Ga and Al are carried out using the Green's function method. The electronic U for the Si dangling bond is in good agreement with the experimental data for a dangling bond related defect. The threefold coordinated P is found to be electrically inactive, while the threefold coordinated B, Ga and Al all have a level in the gap. The positions their respective levels, suggest that Ga or Al may be better acceptor dopants than B.}, number={1-3}, journal={Journal of Non-Crystalline Solids}, publisher={Elsevier BV}, author={Bernholc, J.}, year={1985}, month={Oct}, pages={305–310} }
 @article{scheffler_bernholc_lipari_pantelides_1984, title={Electronic structure and identification of deep defects in GaP}, volume={29}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.29.3269}, DOI={10.1103/physrevb.29.3269}, abstractNote={We report a theoretical study of the Ga vacancy, the P antisite defect (i.e., a P atom occupying a normally Ga site), and carbon impurities at Ga sites. The analysis employs a modified version of the self-consistent Green s-function method that was previously used to describe deep centers in Si. In Rll three cases, the nuInber, orbital content, and relative energy positions of the localized states can be understood in terms of simple models, as in the case of corresponding centers in Si. In particular, we find that, in all cases, the gap state consists primarily of atomiclike p orbitals centered on the four nearest neighbors, revealing that deviations from sp3-hybrid character are possible even without any relaxation of the surrounding lattice. In the case of the antisite defect, we find an 3 ~ bound state in the gap which is consistent with the EPR spectxa that led to the identification of the defect. The calculated ionization energies are in agreement with optical data and more recent deeplevel-txansient-spectroscopy data. In the case of the Ga vacancy, we find a Tq level in the gap above an 3 & xesonance inside the valence bands, and in the case of carbon, we find a deep 3 1 level in the gap. These results contradict key assumptions that had led to the identification of the NRL-1 EPR signal as the V' charge state of the Ga vacancy. More recent experiments redetermined some of the paxameters of NRL-1 and reassigned the center to the neutral state of the Ga vacancy. Here, wc discuss thc interplay between experiment Rnd thcoI'y that lcd to thc ncw ldcntlflcRtlon Rnd dcscribc cxlstlng difficulties thRt limit our Rblllty to construct R dctallcd model w41ch ls conslstcnt with all experiments and with theory.}, number={6}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Scheffler, M. and Bernholc, J. and Lipari, N. O. and Pantelides, S. T.}, year={1984}, month={Mar}, pages={3269–3282} }
 @article{bernholc_holzwarth_1984, title={Local density pseudopotential calculations for molecules: O2 and Mo2O2S2(S2)2−2}, volume={81}, ISSN={0021-9606 1089-7690}, url={http://dx.doi.org/10.1063/1.448139}, DOI={10.1063/1.448139}, abstractNote={We report a development of a self-consistent, local density pseudopotential method for calculation of the electronic properties of large molecules. Angular momentum dependent, first-principles ionic pseudopotentials are used to describe the valence-core interactions. Average correlation effects are included in the local density sense. The wave functions, the charge density and the exchange-correlation potential are expanded in atom-centered Gaussian orbitals of s, p, and d symmetry. These atomic basis sets account for nonspherical (non-muffin tin) effects and are easily transferable to different geometries and other environments. The total energy of the system is accurately evaluated. The method is tested on the O2 molecule and found to give the orbital energies, the equilibrium distance and the vibrational frequency in good agreement with previous all-electron calculations. As the first application to a large system, we have investigated the electronic structure of the Mo2O2S2(S2)2−2 anion. In this anion, the molybdenum atoms are fivefold coordinated: to one oxygen atom, two bridging sulfur atoms, and a molecular S2. From the energy level diagram, the oxygen atoms are found to form approximately triple bonds to the molybdenum atoms, the Mo–S and Mo–S2 bonds being successively weaker. The molybdenum atoms form a single Mo–Mo bond and are formally in the Mo(V) oxidation state. The highest occupied molecular orbitals are, however, the S2 π* orbitals, oriented perpendicular to the Mo-S2 planes and located almost 1 eV above the Mo–Mo bond energy. The lowest unoccupied molecular orbitals are the relatively closely spaced π* antibonding Mo–O, Mo–Mo, and Mo–S2 orbitals.}, number={9}, journal={The Journal of Chemical Physics}, publisher={AIP Publishing}, author={Bernholc, J. and Holzwarth, N. A. W.}, year={1984}, month={Nov}, pages={3987–3995} }
 @article{bernholc_holzwarth_1983, title={Local Spin-Density Description of Multiple Metal-Metal Bonding: Mo2 and Cr2}, volume={50}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.50.1451}, DOI={10.1103/physrevlett.50.1451}, abstractNote={Bonding and dissociation properties of multiply bonded ${\mathrm{Mo}}_{2}$ and ${\mathrm{Cr}}_{2}$ molecules are examined with use of the local spin-density theory and ab initio pseudopotentials. The equilibrium properties are in good agreement with experimental data. The core-polarization shifts are large; pseudopotentials give accurate results for multiply bonded systems only if outer-core $p$ electrons are included in the calculations. The results confirm the applicability of pseudopotential techniques to these systems.}, number={19}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Holzwarth, N. A. W.}, year={1983}, month={May}, pages={1451–1454} }
 @article{divincenzo_bernholc_brodsky_1983, title={Localized states and the electronic properties of a hydrogenated defect in amorphous silicon}, volume={28}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.28.3246}, DOI={10.1103/physrevb.28.3246}, abstractNote={We study the electronic properties of a hydrogen-dressed silicon monovacancy as a model of H centers in hydrogenated amorphous silicon ($a\ensuremath{-}\mathrm{Si}:\mathrm{H}$). Using the self-consistent Green's-function technique, we obtain total and local densities of states for the defect, as well as the charge density of defect eigenstates. The hydrogenated vacancy has no states in the band gap and reduces the Si host density of states at both the valence- and conduction-band edge. The Si---H bonding character of the defect eigenstates is greatest 4.5 eV below the valence-band edge; however, no sharp H-induced resonance occurs anywhere in the valence or conduction bands. We discuss the importance of including the self-consistent rearrangement of charge around the H atom in obtaining accurate results for the hydrogenated vacancy. Within the context of a recently proposed quantum-well model, we relate the results of our Green's-function calculation to the properties of $a\ensuremath{-}\mathrm{Si}:\mathrm{H}$. Using several simple models (percolation theory, the Anderson model, effective-mass theory), we obtain estimates for the valence- and conduction-band mobility edges. We find that the valence band is much more strongly affected than the conduction band by H disorder, and that H disorder is more important than dihedral-angle disorder in valence-band edge localization.}, number={6}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={DiVincenzo, D. P. and Bernholc, J. and Brodsky, M. H.}, year={1983}, month={Sep}, pages={3246–3257} }
 @article{bernholc_lipari_pantelides_scheffler_1982, title={Electronic structure of deep sp-bonded substitutional impurities in silicon}, volume={26}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.26.5706}, DOI={10.1103/physrevb.26.5706}, abstractNote={The recently-developed Green's-function method for defects in semiconductors is used to investigate the electronic properties of deep substitutional $\mathrm{sp}$-bonded impurities in Si. These impurities are grouped according to the value ${\ensuremath{\Delta}}_{z}={z}_{\mathrm{imp}}\ensuremath{-}{z}_{\mathrm{host}}$ where $z$ is the chemical valence. We choose a typical representative impurity from each group: sulfur with $\ensuremath{\Delta}z=2$ (double donor), zinc with $\ensuremath{\Delta}z=\ensuremath{-}2$ (double acceptor), and hydrogen with $\ensuremath{\Delta}z=\ensuremath{-}3$ (triple acceptor). In each case we obtain the impurity-induced state-density changes in the valence bands, and the energy positions and wave functions of the bound states in the forbidden gaps. These results are compared with earlier results for the vacancy, which may be viewed as a $\ensuremath{\Delta}z=\ensuremath{-}4$ nominal quadruple acceptor. We describe a simple physical model which explains the chemical trends as a function of $\ensuremath{\Delta}z$ and compare with available experimental data.}, number={10}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Lipari, N. O. and Pantelides, S. T. and Scheffler, M.}, year={1982}, month={Nov}, pages={5706–5715} }
 @inproceedings{divincenzo_bernholc_brodsky_lipari_pantelides_1981, title={Hole conductivity through neighboring Si-H bonds in hydrogenated silicon}, ISSN={0094-243X}, url={http://dx.doi.org/10.1063/1.33066}, DOI={10.1063/1.33066}, abstractNote={The spatially‐resolved electronic states of a model representation of Si‐H bonds on vacancies in hydrogenated amorphous silicon (a‐Si:H) have been calculated using a Greens’s‐funciton technique. We find no gap states in the host Si bandgap. In the vicinity of the defect, states near the valence‐band edge and, to a lesser extent, near the conduction‐band edge are removed. We relate our results to a model of the Si‐H bond as a barrier to band edge carriers. Parameters are derived that pertain to localization due to randomly located hydrogenated defects.}, booktitle={AIP Conference Proceedings}, publisher={AIP}, author={DiVincenzo, D. P. and Bernholc, J. and Brodsky, M. H. and Lipari, N. O. and Pantelides, S. T.}, year={1981} }
 @article{scheffler_pantelides_lipari_bernholc_1981, title={Identification and Properties of Defects in GaP}, volume={47}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.47.413}, DOI={10.1103/physrevlett.47.413}, abstractNote={We report an application of self-consistent pseudopotential Green's-function calculations to the problem of identification of defects in GaP. The results are in agreement with and support the EPR-based identification of the P antisite defect, but contradict the assumptions that led to the identification of the Ga vacancy. We reexamine other possibilities using the results of our calculations and find that C at Ga sites is the only center whose properties are consistent with the available data.}, number={6}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Scheffler, M. and Pantelides, S. T. and Lipari, N. O. and Bernholc, J.}, year={1981}, month={Aug}, pages={413–416} }
 @article{divincenzo_bernholc_brodsky_1981, title={THE ELECTRONIC STRUCTURE OF A MODEL DEFECT IN HYDROGENATED AMORPHOUS SILICON}, volume={42}, ISSN={0449-1947}, url={http://dx.doi.org/10.1051/jphyscol:1981426}, DOI={10.1051/jphyscol:1981426}, abstractNote={We calculate the electronic properties of a model defect for hydrogen in hydrogenated amorphous Si. Our model is a vacancy in crystal Si with four H's satisfying the dangling bonds. Using a Green's function technique, we find the change in the density of states caused by the defect, as well as the local density of states for the Si-H bond and surrounding bonds. From several approaches, we extract information on band edge localization. Each approach gives a mobility edge of order tenths of an eV, therefore we conclude that compositional disorder has an effect comparable to that previously estimated for topological disorder. Conduction band effects are calculated to be similar but smaller.}, number={C4}, journal={Le Journal de Physique Colloques}, publisher={EDP Sciences}, author={DiVincenzo, D. P. and Bernholc, J. and Brodsky, M. H.}, year={1981}, month={Oct}, pages={C4–137-C4–140} }
 @article{bernholc_pantelides_lipari_baldereschi_1981, title={The electronic structure of deep SP-bonded acceptor impurities in semiconductors}, volume={37}, ISSN={0038-1098}, url={http://dx.doi.org/10.1016/0038-1098(81)91082-6}, DOI={10.1016/0038-1098(81)91082-6}, abstractNote={Abstract This paper reports self-consistent calculations of the electronic structure of a series of deep impurities in Si. For the first time, these calculations provide a detailed description of gap states as well as the resonances and antiresonances within the band continua with the same accuracy as that of bulk-crystal calculations. The analysis of charge densities and wavefunctions in terms of simple models provides an understanding of the chemical bonding, the degree of localization, and the relevance of ideas based on effective-mass theory.}, number={9}, journal={Solid State Communications}, publisher={Elsevier BV}, author={Bernholc, J. and Pantelides, S.T. and Lipari, N.O. and Baldereschi, A.}, year={1981}, month={Mar}, pages={705–708} }
 @inproceedings{bernholc_lipari_pantelides_scheffler_1981, place={Bristol}, title={Theory of Point Defects and Deep Impurities in Semiconductors}, ISBN={9780854981502}, booktitle={Defects and radiation effects in semiconductors, 1980 : invited and contributed papers from the eleventh International Conference on Defects and Radiation Effects in Semiconductors held in Oiso, Japan, 8-11 September 1980}, publisher={Institute of Physics}, author={Bernholc, J. and Lipari, N.O. and Pantelides, S.T. and Scheffler, M.}, editor={Hasiguti, Ryukiti R.Editor}, year={1981} }
 @article{bernholc_lipari_pantelides_1980, title={Scattering-theoretic method for defects in semiconductors. II. Self-consistent formulation and application to the vacancy in silicon}, volume={21}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.21.3545}, DOI={10.1103/physrevb.21.3545}, abstractNote={A self-consistent-field method for calculation of the electronic structure of localized defects in semiconductors is described. The method is based on Green's-function theory and follows the original idea of Koster and Slater and its developments by Callaway and coworkers. The Wannier functions of the original formulations are, however, replaced by a more flexible set of linear combination of atomic orbitals. This choice and an accurate evaluation of the perfect-crystal Green's function bring this method to the level of sophistication, accuracy, and rigor characteristic of state-of-the-art band-structure and surface calculations. The efficiency of the method stems largely from the fact that it exploits both the translational symmetry of the host crystal and the short range of the defect potential. Thus, all bulk properties (e.g., band gaps, bandwidths, etc.) are built in from the start via a band-structure calculation and are preserved. One then focuses on the changes produced by the defect potential, so that the interpretation of the results is straightforward and unambiguous. In this paper, we report an application of this method to an isolated vacancy in Si assuming no lattice relaxation. The unrelaxed vacancy introduces a bound state of ${T}_{2}$ symmetry at 0.7 eV above the valence-band edge and a number of resonances and antiresonances within the valence bands. A detailed analysis of these states in terms of their origin, orbital content, and of state and charge densities is presented. We find that, while many of these states are individually quite extended, they combine destructively to produce a very localized net change in the charge density. We also find that the resulting localized potential can be well approximated by a negative of an atomic silicon potential extracted from a self-consistent bulk calculation. Finally, we compare the relative merits of the three increasingly more sophisticated, but also more costly, approaches to the defect problem, namely, (1) tight-binding, (2) non-self-consistent, and (3) self-consistent calculations.}, number={8}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Lipari, Nunzio O. and Pantelides, Sokrates T.}, year={1980}, month={Apr}, pages={3545–3562} }
 @article{pantelides_lipari_bernholc_1980, title={The effective-mass nature of deep-level point-defect states in semiconductors}, volume={33}, ISSN={0038-1098}, url={http://dx.doi.org/10.1016/0038-1098(80)90314-2}, DOI={10.1016/0038-1098(80)90314-2}, abstractNote={The basic premise of Effective-Mass Theory (EMT) is that bound-state wavefunctions are constructible from Bloch functions in a small region or regions of k space. In contrast, deep-level wavefunctions are believed to involve Bloch functions from the entire Brillouin zone and several bands. In this paper we analyse the wavefunction of the deep vacancy level in Si obtained recently by self-consistent Green's-function calculations. We find that this wavefunction has a strong EMT character in that it is composed primarily of Bloch functions from the nearest bands and the corresponding coefficients, i.e. the envelope functions, are peaked about the band extrema. As a further check, we have used a spherical average of the self-consistent vacancy potential in the acceptor EMT equations. The resulting energy level is at Ev+0.9 eV, as compared with the Green's-function-theoretic value of Ev+0.8 eV. The resulting wavefunction, on the other hand, does not have the correct form. A check of the correction terms left out by the standard EMT equations reveals that their contributions to the energy level are large and tend to cancel one another.}, number={10}, journal={Solid State Communications}, publisher={Elsevier BV}, author={Pantelides, S.T. and Lipari, N.O. and Bernholc, J.}, year={1980}, month={Mar}, pages={1045–1049} }
 @inproceedings{pantelides_bernholc_lipari_1980, place={Tokyo, Japan}, title={Theory of Deep Impurities and Defects in Semiconductors}, booktitle={Proceedings of the 15th International Conference on the Physics of Semiconductors, Kyoto, Sept. 1st-5th, 1980}, publisher={Physical Society of Japan,}, author={Pantelides, S.T. and Bernholc, J. and Lipari, N.O.}, editor={Tanaka, Shoji and Toyozawa, YutakaEditors}, year={1980}, pages={235} }
 @article{lipari_bernholc_pantelides_1979, title={Electronic Structure of the Jahn-Teller Distorted Vacancy in Silicon}, volume={43}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.43.1354}, DOI={10.1103/physrevlett.43.1354}, abstractNote={This Letter reports self-consistent\char22{}field calculations of the electronic structure of the Jahn-Teller distorted vacancy in Si. With use of the tetragonal atomic displacements estimated by Watkins, it is found that the Jahn-Teller splitting of the sixfold-degenerate bound state in the gap is of the order of 0.5 eV. This, together with small breathing-mode displacements, results in a fully occupied doublet in the lower part of the band gap, in agreement with experimental observations.}, number={18}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Lipari, N. O. and Bernholc, J. and Pantelides, S. T.}, year={1979}, month={Oct}, pages={1354–1357} }
 @inproceedings{pantelides_lipari_bernholc_1979, place={Bristol, England}, series={Institute of Physics Conference Series}, title={The Electronic Structure of Point Defects in Semiconductors - A Self-Consistent Scattering-Theoretic Approach}, booktitle={Physics of semiconductors, 1978 : invited and contributed papers from the 14th International Conference on the Physics of Semiconductors held in Edinburgh, 4-8 September 1978}, publisher={Institute of Physics}, author={Pantelides, S.T. and Lipari, N.O. and Bernholc, J.}, editor={Wilson, B.L.H.Editor}, year={1979}, pages={429}, collection={Institute of Physics Conference Series} }
 @article{pantelides_bernholc_pollmann_lipari_1978, title={Green's function scattering-theoretic methods for point defects, surfaces, and interfaces in solids}, volume={14}, ISSN={0020-7608 1097-461X}, url={http://dx.doi.org/10.1002/qua.560140845}, DOI={10.1002/qua.560140845}, abstractNote={We have recently developed novel ways to describe the electronic structure of point defects, surfaces, and interfaces in semiconductors, making use of the original Koster-Slater ideas for localized perturbations in otherwise periodic solids. In this article we give a brief review of the method and our applications to the vacancy, free surfaces, and interfaces of several tetrahedral semiconductors and SiO2, using both empirical tight-binding and self-consistent Hamiltonians.}, number={S12}, journal={International Journal of Quantum Chemistry}, publisher={Wiley}, author={Pantelides, S. T. and Bernholc, J. and Pollmann, J. and Lipari, N. O.}, year={1978}, month={Mar}, pages={507–521} }
 @article{bernholc_pantelides_1978, title={Scattering-theoretic method for defects in semiconductors. I. Tight-binding description of vacancies in Si, Ge, and GaAs}, volume={18}, ISSN={0163-1829}, url={http://dx.doi.org/10.1103/physrevb.18.1780}, DOI={10.1103/physrevb.18.1780}, abstractNote={A technique for an accurate and efficient calculation of point-defect-induced changes in the electronic structure of an otherwise perfect crystal is described. It is based on the Green's-function method introduced by Koster and Slater and developed further by Callaway and coworkers, and achieves its efficiency and convenience by avoiding the use of Wannier functions. The efficiency and accuracy of the method is exhibited by calculating the states of a widely studied model system, namely, the ideal vacancy in covalent solids, using semiempirical, but realistic, host energy-band structures. Results for the vacancy in Si, Ge, and GaAs compare favorably with those obtained previously. In addition, a wealth of new information is obtained. It is argued that the present method is the most efficient technique available for the study of deep-level impurities and defects in semiconductors---the efficiency stemming from an exploitation of both the short-range nature of the defect potential and the translational symmetry of the host crystal.}, number={4}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Pantelides, Sokrates T.}, year={1978}, month={Aug}, pages={1780–1789} }
 @article{bernholc_lipari_pantelides_1978, title={Self-Consistent Method for Point Defects in Semiconductors: Application to the Vacancy in Silicon}, volume={41}, ISSN={0031-9007}, url={http://dx.doi.org/10.1103/physrevlett.41.895}, DOI={10.1103/physrevlett.41.895}, abstractNote={We report the development of a method to calculate self-consistently the electronic structure of neutral point defects in semiconductors. The method is an adaptation of the original Koster-Slater idea. Calculations become feasible, practical, and accurate at the level of current band-structure and surface calculations when an LCAO basis set is used instead of Wannier functions. A detailed study of the isolated vacancy is Si is used to illustrate the method.}, number={13}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Lipari, Nunzio O. and Pantelides, Sokrates T.}, year={1978}, month={Sep}, pages={895–899} }
 @article{bernholc_pantelides_1977, title={Theory of binding energies of acceptors in semiconductors}, volume={15}, ISSN={0556-2805}, url={http://dx.doi.org/10.1103/physrevb.15.4935}, DOI={10.1103/physrevb.15.4935}, abstractNote={Acceptor binding energies are calculated using the full 6 \ifmmode\times\else\texttimes\fi{} 6 effective-mass acceptor Hamiltonian for both shallow and deep levels. It is found that for homopolar semiconductors (Si and Ge) the method is valid for both shallow and deep acceptors, in agreement with a similar result obtained previously by Pantelides and Sah for donors in Si. Screening the impurity potential by the dielectric function $\ensuremath{\epsilon}(q)$ is again found to be very important. The point-charge model is found to be adequate for shallow single acceptors and the relatively shallow double acceptors in Ge, but completely inadequate for the deep double acceptors in Si and triple acceptors in Ge. Use of model potentials, however, which reflect the chemical nature of individual impurities, is shown to be capable of reproducing the observed values. When model potentials from the literature are used, agreement with experiment is only modest, due to a sensitivity of the calculation on the core part of the impurity potential. Similar calculations in heteropolar semiconductors (GaP, GaAs, etc.) show that good agreement is obtained only for very shallow levels. For deeper levels, effects due to differences in the anion and cation sites are dominant. It is argued that these effects lie outside the effective-mass theory.}, number={10}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Bernholc, J. and Pantelides, Sokrates T.}, year={1977}, month={May}, pages={4935–4947} }
 @inproceedings{pantelides_bernholc_1976, place={Bristol}, series={Institute of Physics Conference Series}, title={Theory of Photoionization Cross Sections of Impurities in Semiconductors}, booktitle={Radiation Effects in Semiconductors, 1976 : Invited and contributed papers from the International Conference on Radiation Effects in Semiconductors held in Dubrovnik, 6-9 September 1976}, publisher={Institute of Physics}, author={Pantelides, S.T. and Bernholc, J.}, editor={Urli, N.B. and Corbett, J.W.Editors}, year={1976}, pages={465}, collection={Institute of Physics Conference Series} }