@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 A multi-order Adaptive Finite Differencing (AFD) method is developed for the kinetic energy operator in real-space, grid-based electronic structure codes. It uses atomic pseudo orbitals produced by the corresponding pseudopotential codes to optimize the standard finite difference (SFD) operators for improved precision. Results are presented for a variety of test systems and Bravais lattice types, including the well-known Δ test for 71 elements in the periodic table, the Mott insulator NiO, and borax decahydrate, which contains covalent, ionic, and hydrogen bonds. The tests show that an 8th-order AFD operator leads to the same average Δ value as that achieved by plane-wave codes and is typically far more accurate and has a much lower computational cost than a 12th-order SFD operator. The scalability of real-space electronic calculations is demonstrated for a 2016-atom NiO cell, for which the computational time decreases nearly linearly when scaled from 18 to 144 CPU-GPU nodes.}, number={1}, journal={NPJ COMPUTATIONAL MATERIALS}, author={Briggs, E. L. and Lu, Wenchang and Bernholc, J.}, year={2024}, month={Jan} }
 @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_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{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{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 (k33) of 88% and a piezoelectric coefficient (d33) >1000 picometers per volt. These values make this solution-processed polymer competitive with ceramic oxide piezoelectrics, with the potential for use in distinct applications. Description Polymer piezo The best-performing piezoelectric materials are oxide ceramics, which are widely used for sensors and actuators. X. Chen et al. added two additional components to poly(vinylidene difluoride) trifluoroethylene to improve the electromechanical coupling (see the Perspective by Wang and Liao). The resulting tetrapolymer has piezoelectric properties that are dramatically improved and it appears to be competitive with traditional oxides. The pliability and relative ease of fabrication of this tetrapolymer piezoelectric makes it attractive for a range of interesting applications. —BG An improved polymer has properties that make it competitive with commercially available ceramic piezoelectrics.}, 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{liu_mao_zhang_wang_yang_lu_2021, title={A monolayer of Pd on ZrC(001) speeds up O-2 dissociation: An ab initio study}, volume={537}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsuse.2020.148050}, journal={APPLIED SURFACE SCIENCE}, author={Liu, Ning and Mao, Jianjun and Zhang, Xilin and Wang, Yan and Yang, Zongxian and Lu, Wenchang}, year={2021}, month={Jan} }
 @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={The mechanism of the recently discovered enhancement of dielectric properties in dilute polymer-nanoparticle composites is investigated by experiments and computer simulations. We show that the weakening of the hydrogen bonds between the nanoparticles and the polymer chains reduces the polymer-nanoparticle composite's dielectric enhancement. The subsequent multiscale simulations investigate the attachment of solvated highly dipolar polymers to oxide nanoparticles, which leads to deposition of nanoparticle-polymer blobs during solution casting and a reduced density compared to a neat polymer film. Coarse-grained simulations of nanocomposite morphology are followed by molecular dynamics and density functional theory calculations of permittivities. The increased free volume in the nanocomposite enables easier reorientation of monomer dipoles with an applied electric field, and thus a higher dielectric permittivity. The numerical results are in excellent agreement with experimental data for PEEU and PEI nanocomposites.}, 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={Understanding the reaction mechanisms of dehydrogenative Caryl–Caryl coupling is the key to directed formation of π-extended polycyclic aromatic hydrocarbons. Here we utilize isotopic labeling to identify the exact pathway of cyclodehydrogenation reaction in the on-surface synthesis of model atomically precise graphene nanoribbons (GNRs). Using selectively deuterated molecular precursors, we grow seven-atom-wide armchair GNRs on a Au(111) surface that display a specific hydrogen/deuterium (H/D) pattern with characteristic Raman modes. A distinct hydrogen shift across the fjord of Caryl–Caryl coupling is revealed by monitoring the ratios of gas-phase by-products of H2, HD, and D2 with in situ mass spectrometry. The identified reaction pathway consists of a conrotatory electrocyclization and a distinct [1,9]-sigmatropic D shift followed by H/D eliminations, which is further substantiated by nudged elastic band simulations. Our results not only clarify the cyclodehydrogenation process in GNR synthesis but also present a rational strategy for designing on-surface reactions towards nanographene structures with precise hydrogen/deuterium isotope labeling patterns.}, 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{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{yang_lu_cheng_wang_zhang_yang_lu_2020, title={Identification of Efficient Single-Atom Catalysts Based on V2CO2 MXene by ab Initio Simulations}, volume={124}, ISSN={["1932-7455"]}, DOI={10.1021/acs.jpcc.9b09912}, abstractNote={The first-principles simulations are used to search and identify a potential candidate for the single-atom catalysts (SACs). By exploring the stability, clustering tendency, and catalytic activity ...}, number={7}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Yang, Xinwei and Lu, Zhansheng and Cheng, Cheng and Wang, Yan and Zhang, Xilin and Yang, Zongxian and Lu, Wenchang}, year={2020}, month={Feb}, pages={4090–4100} }
 @article{liu_yang_zhang_williams_lin_li_zhou_lu_kim_chen_et al._2020, title={Structural Insight in the Interfacial Effect in Ferroelectric Polymer Nanocomposites}, volume={32}, ISSN={["1521-4095"]}, DOI={10.1002/adma.202005431}, abstractNote={Both experimental results and theoretical models suggest the decisive role of the filler–matrix interfaces on the dielectric, piezoelectric, pyroelectric, and electrocaloric properties of ferroelectric polymer nanocomposites. However, there remains a lack of direct structural evidence to support the so‐called interfacial effect in dielectric nanocomposites. Here, a chemical mapping of the interfacial coupling between the nanofiller and the polymer matrix in ferroelectric polymer nanocomposites by combining atomic force microscopy–infrared spectroscopy (AFM–IR) with first‐principles calculations and phase‐field simulations is provided. The addition of ceramic fillers into a ferroelectric polymer leads to augmentation of the local conformational disorder in the vicinity of the interface, resulting in the local stabilization of the all‐trans conformation (i.e., the polar β phase). The formation of highly polar and inhomogeneous interfacial regions, which is further enhanced with a decrease of the filler size, has been identified experimentally and verified by phase‐field simulations and density functional theory (DFT) calculations. This work offers unprecedented structural insights into the configurational disorder‐induced interfacial effect and will enable rational design and molecular engineering of the filler–matrix interfaces of electroactive polymer nanocomposites to boost their collective properties.}, number={49}, journal={ADVANCED MATERIALS}, author={Liu, Yang and Yang, Tiannan and Zhang, Bing and Williams, Teague and Lin, Yen-Ting and Li, Li and Zhou, Yao and Lu, Wenchang and Kim, Seong H. and Chen, Long-Qing and et al.}, year={2020}, month={Dec} }
 @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{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} }
 @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_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{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{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{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} }
 @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} }
 @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{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{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} }
 @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={Jul}, pages={11576–11581} }
 @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} }
 @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} }
 @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{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} }
 @article{lu_wechsler_2007, title={The radiation damage database: Section on helium cross section}, volume={361}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2006.12.005}, abstractNote={Abstract A radiation damage database with emphasis on spallation interactions is described. Currently, the database contains damage energy, displacement, helium, and hydrogen cross sections for 23 elemental targets irradiated by proton and neutron projectiles up to 3.2 GeV. In this paper, the focus is on proton-induced helium cross sections, but it is shown that for high energies (above about 500 MeV) proton- and neutron-induced helium cross sections are almost equal. The cross section calculations were run on the Cascade–Exciton Model code (no options) and also on the Bertini code with three nuclear level-density models and multistage pre-equilibrium model on and off. Calculation and experimental results are compared. For various targets, we tried to determine which code and options give best agreement with experiment. In some cases, such determinations are uncertain, partly because of limited and conflicting experimental information and partly perhaps because of the need for modifications in the codes.}, number={2-3}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Lu, W. and Wechsler, M. S.}, year={2007}, month={Apr}, pages={282–288} }
 @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{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{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{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} }
 @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} }
 @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{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{lu_wechsler_dai_2003, title={Radiation damage at the aluminum entrance window of the SINQ Target 3}, volume={318}, ISSN={["1873-4820"]}, DOI={10.1016/S0022-3115(03)00082-5}, abstractNote={Calculations are underway to determine radiation damage (displacement, helium, and hydrogen production) at the entrance window of the SINQ Target 3 (Mark II Type). Damage production rates were determined in two ways. In Method 1, the displacement, He, and H cross-sections were folded into the proton and neutron fluxes to give the three defect production rates separately for protons and neutrons. In Method 2, MCNPX with a computer model of SINQ was used directly to calculate the three production rates due to the combined effects of protons and neutrons. The production rates at the central tip of the target by Method 1 are 4.1 and 0.47 dpa/yr per mA for protons and neutrons, respectively, giving a total of 4.6 dpa/yr per mA. By Method 2 using several approaches, we obtain a range of production rates from 3.6 to 4.1 dpa/yr per mA. For helium, the production rates are calculated to be about 1000 and a range from 950 to 1580 appmHe/yr per mA, respectively. LAHET calculations indicate that the helium is completely retained in the irradiated aluminum. For hydrogen, the calculations indicate that perhaps only about half of the hydrogen produced is retained. Method 1 gives about 3500 appmH/yr per mA retained and Method 2 gives a range from 3500 to 4400 appmH/yr per mA.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Lu, W and Wechsler, MS and Dai, Y}, year={2003}, month={May}, pages={176–184} }
 @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} }