@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{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{miao_liu_wang_wu_huang_gubbins_nardelli_2012, title={Activation of water on the TiO2 (110) surface: The case of Ti adatoms}, volume={136}, ISSN={["0021-9606"]}, DOI={10.1063/1.3682781}, abstractNote={Using first-principles calculations we have studied the reactions of water over Ti adatoms on the (110) surface of rutile TiO2. Our results provide fundamental insights into the microscopic mechanisms that drive this reaction at the atomic level and assess the possibility of using this system to activate the water dissociation reaction. In particular, we show that a single water molecule dissociates exothermically with a small energy barrier of 0.17 eV. After dissociation, both H+ and OH− ions bind strongly to the Ti adatom, which serves as an effective reactive center on the TiO2 surface. Finally, clustering of Ti adatoms does not improve the redox activity of the system and results in a slightly higher energy barrier for water dissociation.}, number={6}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Miao, Meng and Liu, Yingchun and Wang, Qi and Wu, Tao and Huang, Liping and Gubbins, Keith E. and Nardelli, Marco Buongiorno}, year={2012}, month={Feb} }
 @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{sandin_jayasekera_rowe_kim_buongiorno nardelli_dougherty_2012, title={Multiple coexisting intercalation structures of sodium in epitaxial graphene-SiC interfaces}, volume={85}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.85.125410}, DOI={10.1103/physrevb.85.125410}, abstractNote={This article discusses multiple coexisting intercalation structures of sodium in peitaxial graphene-SiC interfaces.}, number={12}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Sandin, Andreas and Jayasekera, Thushari and Rowe, J. E. and Kim, Ki Wook and Buongiorno Nardelli, M. and Dougherty, Daniel B.}, year={2012}, month={Mar} }
 @article{borysenko_mullen_li_semenov_zavada_nardelli_kim_2011, title={Electron-phonon interactions in bilayer graphene}, volume={83}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.83.161402}, DOI={10.1103/physrevb.83.161402}, abstractNote={K. M. Borysenko, J. T. Mullen, X. Li, Y. G. Semenov, J. M. Zavada, M. Buongiorno Nardelli, 3 and K. W. Kim Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695-7911 Department of Physics, North Carolina State University, Raleigh, NC 27695-8202 CSMD, Oak Ridge National Laboratory, Oak Ridge, TN 37831 Abstract Using calculations from first principles, we demonstrate that intrinsic carrier-phonon scattering in bilayer graphene is dominated by low energy acoustic (and acoustic-like) phonon modes in a framework that bears more resemblance with bulk graphite than monolayer graphene. The total scattering rate at low/moderate electron energies can be described by a simple two-phonon model in the deformation potential approximation with effective constants Dac ≈ 15 eV and Dop ≈ 2.8 × 108 eV/cm for acoustic and optical phonons, respectively. With much enhanced acoustic phonon scattering, the low field mobility of intrinsic bilayer graphene is estimated to be significantly smaller than that of the monolayer: μ = 8.4× 104 cm2/V·s}, number={16}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Borysenko, K. M. and Mullen, J. T. and Li, X. and Semenov, Y. G. and Zavada, J. M. and Nardelli, M. Buongiorno and Kim, K. W.}, year={2011}, month={Apr} }
 @article{jayasekera_xu_kim_nardelli_2011, title={Electronic properties of the graphene/6H-SiC(0001¯) interface: A first-principles study}, volume={84}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.84.035442}, DOI={10.1103/physrevb.84.035442}, abstractNote={Using calculations from first principles, we show how the structural and electronic properties of epitaxial graphene on 6H-SiC(0001) are determined by the geometry and the chemical functionalization of the interface region. We also demonstrate that these properties can be correctly captured only if a proper treatment of the van der Waals interactions is included in the theoretical description based on density functional theory. Our results reproduce the experimentally observed n-type doping of monolayer epitaxial graphene and prove the possibility of opening a sizable (150 meV) energy gap in the bilayer case under special growth conditions. Depending on the details of the bonding at the interface, we are able to interpret recent experimental observations and provide a clear insight into the mechanisms of charge transfer and interface stability.}, number={3}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Jayasekera, Thushari and Xu, Shu and Kim, K. W. and Nardelli, M. Buongiorno}, year={2011}, month={Jul} }
 @article{jayasekera_kong_kim_buongiorno nardelli_2010, title={Band Engineering and Magnetic Doping of Epitaxial Graphene on SiC (0001)}, volume={104}, ISSN={0031-9007 1079-7114}, url={http://dx.doi.org/10.1103/PhysRevLett.104.146801}, DOI={10.1103/physrevlett.104.146801}, abstractNote={Using calculations from first principles we show how specific interface modifications can lead to a fine-tuning of the doping and band alignment in epitaxial graphene on SiC. Upon different choices of dopants, we demonstrate that one can achieve a variation of the valence band offset between the graphene Dirac point and the valence band edge of SiC up to 1.5 eV. Finally, via appropriate magnetic doping one can induce a half-metallic behavior in the first graphene monolayer. These results clearly establish the potential for graphene utilization in innovative electronic and spintronic devices.}, number={14}, journal={Physical Review Letters}, publisher={American Physical Society (APS)}, author={Jayasekera, Thushari and Kong, B. D. and Kim, K. W. and Buongiorno Nardelli, M.}, year={2010}, month={Apr} }
 @article{santagata_lakhani_davis_luo_nardelli_pearl_2010, title={Chiral Steering of Molecular Organization in the Limit of Weak Adsorbate-Substrate Interactions: Enantiopure and Racemic Tartaric Acid Domains on Ag(111)}, volume={114}, ISSN={["1932-7447"]}, DOI={10.1021/jp912124v}, abstractNote={The influence of intermolecular interactions involving molecular chiral centers on two-dimensional organization in the limit of a weak adsorbate−surface interaction has been studied with low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). A model system composed of a chiral organic molecule, tartaric acid, and an inert metallic surface, Ag(111), was employed. Dual component films formed from the serial deposition of (S,S)- and (R,R)-tartaric acid enantiomers onto this surface exhibit homochiral domain formation as revealed by molecularly resolved STM images. In contrast, a unique tartaric acid enantiomeric heteropair is experimentally and computationally verified as the basis unit of films formed via the deposition of both enantiomers simultaneously from a racemic (1:1) mixture. The molecular adsorption geometry relative to the Ag(111) lattice in both enantiomerically pure and racemic domains is determined primarily by the interaction of chiral centers between nearest ...}, number={19}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Santagata, Nancy M. and Lakhani, Amit M. and Davis, Bryce F. and Luo, Pengshun and Nardelli, Marco Buongiorno and Pearl, Thomas P.}, year={2010}, month={May}, pages={8917–8925} }
 @article{borysenko_mullen_barry_paul_semenov_zavada_nardelli_kim_2010, title={First-principles analysis of electron-phonon interactions in graphene}, volume={81}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.81.121412}, DOI={10.1103/physrevb.81.121412}, abstractNote={The electron-phonon interaction in monolayer graphene is investigated using density-functional perturbation theory. The results indicate that the electron-phonon interaction strength is of comparable magnitude for all four in-plane phonon branches and must be considered simultaneously. Moreover, the calculated scattering rates suggest an acoustic-phonon contribution that is much weaker than previously thought, revealing an important role of optical phonons even at low energies. Accordingly it is predicted, in good agreement with a recent measurement, that the intrinsic mobility of graphene may be more than an order of magnitude larger than the already high values reported in suspended samples.}, number={12}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Borysenko, K. M. and Mullen, J. T. and Barry, E. A. and Paul, S. and Semenov, Y. G. and Zavada, J. M. and Nardelli, M. Buongiorno and Kim, K. W.}, year={2010}, month={Mar} }
 @article{li_barry_zavada_nardelli_kim_2010, title={Influence of electron-electron scattering on transport characteristics in monolayer graphene}, volume={97}, number={8}, journal={Applied Physics Letters}, author={Li, X. and Barry, E. A. and Zavada, J. M. and Nardelli, M. B. and Kim, K. W.}, year={2010} }
 @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} }
 @article{paul_nardelli_2010, title={Rational computational design of optimal catalytic surfaces}, volume={97}, ISSN={["0003-6951"]}, DOI={10.1063/1.3525372}, abstractNote={Using calculations from first-principles based on density functional theory, we introduce an innovative concept for the design of catalysts based on the tuning of the surface properties by progressive layering of thin metal films on an oxide support, where the substrate is substantially exploited as an active design element for the tuning of the reactivity of the metallic surface. We demonstrate this idea by studying the adsorption and activation properties of CO2 on Pd and oxide-supported Pd thin films.}, number={23}, journal={APPLIED PHYSICS LETTERS}, author={Paul, Sujata and Nardelli, Marco Buongiorno}, year={2010}, month={Dec} }
 @article{calzolari_jin_reutt-robey_nardelli_2010, title={Substrate-Mediated Intermolecular Hybridization in Binary Phthalocyanine Superstructures}, volume={114}, ISSN={["1932-7455"]}, DOI={10.1021/jp908166j}, abstractNote={Using a combination of calculations from first principles and scanning tunneling microscopy, we have investigated the interplay between substrate-mediated and intermolecular interactions in the formation of high-density checkerboard binary superstructures from the codeposition of phenyl and perfluorophenyl Zn-phthalocyanines (ZnPc―F 16 ZnPc) on the Ag(111) surface. The analysis of the electronic structure of the interface shows the essential role played by the substrate in the formation of the molecular layer and opens the way toward the development of tailored surfaces for advanced supramolecular design.}, number={2}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Calzolari, Arrigo and Jin, Wei and Reutt-Robey, Janice E. and Nardelli, Marco Buongiorno}, year={2010}, month={Jan}, pages={1041–1045} }
 @article{li_barry_zavada_buongiorno nardelli_kim_2010, title={Surface polar phonon dominated electron transport in graphene}, volume={97}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.3525606}, DOI={10.1063/1.3525606}, abstractNote={The effects of surface polar phonons on the electronic transport properties of monolayer graphene are studied by using a Monte Carlo simulation. Specifically, the low-field electron mobility and saturation velocity are examined for different substrates (SiC, SiO2, and HfO2) in comparison to the intrinsic case. While the results show that the low-field mobility can be substantially reduced by the introduction of surface polar phonon scattering, corresponding degradation of the saturation velocity is not observed for all three substrates at room temperature. It is also found that surface polar phonons can influence graphene’s electrical resistivity even at low temperature, leading potentially to inaccurate estimation of the acoustic phonon deformation potential constant.}, number={23}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Li, X. and Barry, E. A. and Zavada, J. M. and Buongiorno Nardelli, M. and Kim, K. W.}, year={2010}, month={Dec}, pages={232105} }
 @article{chen_jayasekera_calzolari_kim_buongiorno nardelli_2010, title={Thermoelectric properties of graphene nanoribbons, junctions and superlattices}, volume={22}, ISSN={0953-8984 1361-648X}, url={http://dx.doi.org/10.1088/0953-8984/22/37/372202}, DOI={10.1088/0953-8984/22/37/372202}, abstractNote={Using model interaction Hamiltonians for both electrons and phonons and Green’s function formalism for ballistic transport, we have studied the thermal conductance and the thermoelectric properties of graphene nanoribbons (GNR), GNR junctions and periodic superlattices. Among our findings we have established the role that interfaces play in determining the thermoelectric response of GNR systems both across single junctions and in periodic superlattices. In general, increasing the number of interfaces in a single GNR system increases the peak ZT values that are thus maximized in a periodic superlattice. Moreover, we proved that the thermoelectric behavior is largely controlled by the width of the narrower component of the junction. Finally, we have demonstrated that chevron-type GNRs recently synthesized should display superior thermoelectric properties.}, number={37}, journal={Journal of Physics: Condensed Matter}, publisher={IOP Publishing}, author={Chen, Y and Jayasekera, T and Calzolari, A and Kim, K W and Buongiorno Nardelli, M}, year={2010}, month={Aug}, pages={372202} }
 @article{huang_liu_gubbins_nardelli_2010, title={Ti-decorated C-60 as catalyst for hydrogen generation and storage}, volume={96}, ISSN={["1077-3118"]}, DOI={10.1063/1.3302469}, abstractNote={First-principles calculations were carried out to study Ti–C60 nanostructures as catalysts for water dissociation to generate hydrogen and elucidate the influence of water moisture in the air on hydrogen storage capability of such systems. Our results show that both Ti atoms and dimers on C60 can act as reaction centers for water dissociation with much lower energy barriers than that for water splitting in free space (∼5 eV). After water dissociation, Ti atoms are covered with OH groups, their interaction with hydrogen is substantially reduced, and hydrogen adsorption is changed from chemisorption to physisorption. Therefore, care needs to be taken to eliminate moisture if they were designed as efficient hydrogen storage media.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Huang, Liping and Liu, Ying-Chun and Gubbins, Keith E. and Nardelli, Marco Buongiorno}, year={2010}, month={Feb} }
 @inbook{santiso_huang_gubbins_kostov_george_nardelli_2009, title={Ab initio simulations of chemical reactions in nanostructured carbon materials}, ISBN={1588831388}, booktitle={Quantum chemical calculations of surfaces and interfaces of materials}, publisher={Stevenson Ranch, Calif.: American Scientific Publishers,}, author={Santiso, E. E. and Huang, L. and Gubbins, K. E. and Kostov, M. K. and George, A. M. and Nardelli, M. B.}, editor={V. A. Basiuk and Ugliengo, P.Editors}, year={2009} }
 @article{kong_paul_nardelli_kim_2009, title={First-principles analysis of lattice thermal conductivity in monolayer and bilayer graphene}, volume={80}, ISSN={1098-0121 1550-235X}, url={http://dx.doi.org/10.1103/PhysRevB.80.033406}, DOI={10.1103/physrevb.80.033406}, abstractNote={Article on first-principles analysis of lattice thermal conductivity in monolayer and bilayer graphene.}, number={3}, journal={Physical Review B}, publisher={American Physical Society (APS)}, author={Kong, B. D. and Paul, S. and Nardelli, M. Buongiorno and Kim, K. W.}, year={2009}, month={Jul} }
 @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{huang_rocca_baroni_gubbins_nardelli_2009, title={Molecular design of photoactive acenes for organic photovoltaics}, volume={130}, ISSN={["1089-7690"]}, DOI={10.1063/1.3133361}, abstractNote={Absorption spectra of n-acenes (n from 2 to 6, for naphthalene, anthracene, tetracene, pentacene, and hexacene, respectively) have been calculated using a newly developed code based on time-dependent density-functional theory. Our calculations show that absorption spectra and charge carrier mobility of acenes not only depend on the molecular identity but also on the molecular packing. By designing the interaction between metal substrates and the first layer of acene molecules, they can be packed in a face-to-face fashion instead of the conventional herringbone (face-to-edge) arrangement. Acenes in the cofacial packing would increase the π-orbital overlap and thus enhance the charge mobility by maximizing electronic coupling between adjacent molecules. Absorption spectra of cofacially packed acenes have a better overlap with the solar spectrum, which allows harvesting more of the solar energy from red photons.}, number={19}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Huang, Liping and Rocca, Dario and Baroni, Stefano and Gubbins, Keith E. and Nardelli, Marco Buongiorno}, year={2009}, month={May} }
 @article{paul_santiso_nardelli_2009, title={Sequestration and selective oxidation of carbon monoxide on graphene edges}, volume={21}, ISSN={["1361-648X"]}, DOI={10.1088/0953-8984/21/35/355008}, abstractNote={The versatility of carbon nanostructures makes them attractive as possible catalytic materials, as they can be synthesized in various shapes and chemically modified by doping, functionalization, and the creation of defects in the nanostructure. Recent research has shown how the properties of carbon nanostructures can be exploited to enhance the yield of chemical reactions such as the thermal decomposition of water (Kostov et al 2005 Phys. Rev. Lett. 95) and the dissociation of methane into carbon and hydrogen (Huang et al 2008 J. Chem. Phys. at press). In this work, we consider the carbon-mediated partial sequestration and selective oxidation of carbon monoxide (CO), both in the presence and absence of hydrogen. Using first-principles calculations we study several reactions of CO with carbon nanostructures, where the active sites can be regenerated by the deposition of carbon decomposed from the reactant (CO) to make the reactions self-sustained. Using statistical mechanics, we also study the conditions under which the conversion of CO to graphene and carbon dioxide is thermodynamically favorable, both in the presence and in the absence of hydrogen. These results are a first step toward the development of processes for the carbon-mediated partial sequestration and selective oxidation of CO in a hydrogen atmosphere.}, number={35}, journal={JOURNAL OF PHYSICS-CONDENSED MATTER}, author={Paul, Sujata and Santiso, Erik E. and Nardelli, Marco Buongiorno}, year={2009}, month={Sep} }
 @article{pantano_nardelli_2009, title={Simulation of the Electromechanical Behavior of Multiwall Carbon Nanotubes}, volume={3}, ISSN={["1936-0851"]}, DOI={10.1021/nn900795n}, abstractNote={The enormous potential of carbon nanotubes (CNTs) as primary components in electronic devices and NEMS necessitates the understanding and predicting of the effects of mechanical deformation on electron transport in CNTs. In principle, detailed atomic/electronic calculations can provide both the deformed configuration and the resulting electrical transport behavior of the CNT. However, the computational expense of these simulations limits the size of the CNTs that can be studied with this technique, and a direct analysis of CNTs of the dimension used in nanoelectronic devices seems prohibitive at the present. Here a computationally effective mixed finite element (FE)/tight-binding (TB) approach able to simulate the electromechanical behavior of CNT devices is presented. The TB code is carefully designed to realize orders-of-magnitude reduction in computational time in calculating deformation-induced changes in electrical transport properties of the nanotubes. The FE-TB computational approach is validated in a simulation of laboratory experiments on a multiwall CNT and then used to demonstrate the role of the multiwall structure in providing robustness to conductivity in the event of imposed mechanical deformations.}, number={10}, journal={ACS NANO}, author={Pantano, A. and Nardelli, M. Buongiorno}, year={2009}, month={Oct}, pages={3266–3272} }
 @article{santiso_nardelli_gubbins_2008, title={A remarkable shape-catalytic effect of confinement on the rotational isomerization of small hydrocarbons}, volume={128}, ISSN={["1089-7690"]}, DOI={10.1063/1.2819238}, abstractNote={As part of an effort to understand the effect of confinement by porous carbons on chemical reactions, we have carried out density functional theory calculations on the rotational isomerization of three four-membered hydrocarbons: n-butane, 1-butene, and 1,3-butadiene. Our results show that the interactions with the carbon walls cause a dramatic change on the potential energy surface for pore sizes comparable to the molecular dimensions. The porous material enhances or hinders reactions depending on how similar is the shape of the transition state to the shape of the confining material. The structure of the stable states and their equilibrium distributions are also drastically modified by confinement. Our results are consistent with a doubly exponential behavior of the reaction rates as a function of pore size, illustrating how the shape of a catalytic support can dramatically change the efficiency of a catalyst.}, number={3}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Santiso, Erik E. and Nardelli, Marco Buongiorno and Gubbins, Keith E.}, year={2008}, month={Jan} }
 @article{huang_santiso_nardelli_gubbins_2008, title={Catalytic role of carbons in methane decomposition for CO- and CO(2)-free hydrogen generation}, volume={128}, ISSN={["0021-9606"]}, DOI={10.1063/1.2931456}, abstractNote={Decomposition of methane is an environmentally attractive approach to CO- and CO2-free hydrogen production. Using first principles calculations at the density functional theory level, our studies demonstrate that the defective carbons can be used as catalysts for methane decomposition, without the need for other catalysts, such as transition metals or oxides, and the catalytic sites can be regenerated by the deposition of carbon decomposed from methane, to make the hydrogen production a continuous process. Additionally, since no other gases are produced in the process, the cost of CO2 sequestration and hydrogen purification from CO contamination will be dramatically reduced.}, number={21}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Huang, Liping and Santiso, Erik E. and Nardelli, Marco Buongiorno and Gubbins, Keith E.}, year={2008}, month={Jun} }
 @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{nunez_nardelli_2008, title={Interface phase and tuning of polarization in metal-ferroelectric junctions: A theoretical study}, volume={92}, number={25}, journal={Applied Physics Letters}, author={Nunez, M. and Nardelli, M. B.}, year={2008} }
 @article{santiso_nardelli_gubbins_2008, title={Isomerization kinetics of small hydrocarbons in confinement}, volume={14}, ISSN={["1572-8757"]}, DOI={10.1007/s10450-007-9075-8}, number={2-3}, journal={ADSORPTION-JOURNAL OF THE INTERNATIONAL ADSORPTION SOCIETY}, author={Santiso, Erik E. and Nardelli, Marco Buongiorno and Gubbins, Keith E.}, year={2008}, month={Jun}, pages={181–188} }
 @article{nunez_nardelli_2008, title={Onset of ferrielectricity and the hidden nature of nanoscale polarization in ferroelectric thin films}, volume={101}, number={10}, journal={Physical Review Letters}, author={Nunez, M. and Nardelli, M. B.}, year={2008} }
 @article{calzolari_ferretti_nardelli_2007, title={Ab initio correlation effects on the electronic and transport properties of metal(II)-phthalocyanine-based devices}, volume={18}, number={42}, journal={Nanotechnology}, author={Calzolari, A. and Ferretti, A. and Nardelli, M. B.}, year={2007} }
 @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{santiso_kostov_george_nardelli_gubbins_2007, title={Confinement effects on chemical reactions - Toward an integrated rational catalyst design}, volume={253}, ISSN={["1873-5584"]}, DOI={10.1016/j.apsusc.2006.12.121}, abstractNote={Most chemical reactions of practical interest are carried out in nano-structured materials, which can enhance reactions due to their large specific surface area, their interactions with the reacting mixture and confinement effects. An experimental investigation of the role of each possible catalytic effect is challenging, since experimental measurements reflect an integration over multiple effects. In this work, we present a review of our most recent research on some of the factors that can influence a chemical reaction in confinement through the study of several model systems. We first consider the influence of steric hindrance on the equilibrium and kinetics for the rotational isomerizations of several small hydrocarbons [E.E. Santiso, M. Buongiorno Nardelli, K.E. Gubbins, Proc. Natl. Acad. Sci. U.S.A., (2007), in press]. These examples illustrate how reaction rates can vary doubly exponentially with the dimensions of the confining material (the ‘shape-catalytic’ effect). As a second example, we consider the unimolecular decomposition of formaldehyde on graphitic carbon pores of various sizes [E.E. Santiso, A.M. George, K.E. Gubbins, M. Buongiorno Nardelli, J. Chem. Phys. 125 (2006) 084711]. These results illustrate the influence of electrostatic interactions with the supporting material on the reaction mechanism and equilibrium yield for reactions involving a charge transfer. As a final example, we consider the interaction of a water molecule with a defective carbon substrate as an example of a chemical interaction that can be enhanced through a shape-catalytic effect. We first show using ab initio calculations how a vacancy site on a graphene surface can induce the thermal splitting of water at relatively low temperatures [M.K. Kostov, E.E. Santiso, A.M. George, K.E. Gubbins, M. Buongiorno Nardelli, Phys. Rev. Lett. 95 (2005) 136105]. We then examine the dissociation on a vacancy site on a nanotube surface, which shows the shape-catalytic effect of the surface curvature. These results are a first step toward the design of catalytic materials that take advantage of different enhancing effects simultaneously.}, number={13}, journal={APPLIED SURFACE SCIENCE}, author={Santiso, Erik E. and Kostov, Milen K. and George, Aaron M. and Nardelli, Marco Buongiorno and Gubbins, Keith E.}, year={2007}, month={Apr}, pages={5570–5579} }
 @article{stekolnikov_seino_bechstedt_wippermann_schmidt_calzolari_nardelli_2007, title={Hexagon versus trimer formation in in nanowires on Si(111): Energetics and quantum conductance}, volume={98}, number={2}, journal={Physical Review Letters}, author={Stekolnikov, A. A. and Seino, K. and Bechstedt, F. and Wippermann, S. and Schmidt, W. G. and Calzolari, A. and Nardelli, M. B.}, year={2007}, pages={026105} }
 @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{santiso_george_gubbins_nardelli_2006, title={Effect of confinement by porous carbons on the unimolecular decomposition of formaldehyde}, volume={125}, ISSN={["1089-7690"]}, DOI={10.1063/1.2220566}, abstractNote={As part of an effort to understand the effect of confinement by porous carbons on chemical reactions, we have carried out density functional theory calculations on the unimolecular decomposition of formaldehyde within graphitic carbons. Our results show that the interactions with the carbon walls result in a lowering of the reaction barrier. For larger pores, there is also a shift of the equilibrium towards the formation of carbon monoxide and hydrogen at low temperatures. This trend is reversed for small pore sizes.}, number={8}, journal={JOURNAL OF CHEMICAL PHYSICS}, author={Santiso, Erik E. and George, Aaron M. and Gubbins, Keith E. and Nardelli, Marco Buongiorno}, year={2006}, month={Aug} }
 @article{nunez_nardelli_2006, title={Tuning the Schottky barrier height in metal-alkaline earth oxide interfaces}, volume={243}, ISSN={["0370-1972"]}, DOI={10.1002/pssb.200666810}, abstractNote={Abstract}, number={9}, journal={PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS}, author={Nunez, Matias and Nardelli, M. Buongiorno}, year={2006}, month={Jul}, pages={2081–2084} }
 @article{lee_nardelli_marzari_2005, title={Band structure and quantum conductance of nanostructures from maximally localized wannier functions: The case of functionalized carbon nanotubes}, volume={95}, number={7}, journal={Physical Review Letters}, author={Lee, Y. S. and Nardelli, M. B. and Marzari, N.}, year={2005} }
 @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{kostov_santiso_george_gubbins_nardelli_2005, title={Dissociation of water on defective carbon substrates}, volume={95}, ISSN={["1079-7114"]}, DOI={10.1103/physrevlett.95.136105}, abstractNote={Using calculations from first principles, we found that water can dissociate over defective sites in graphene or nanotubes following many possible reaction pathways, some of which have activation barriers lower than half the value for the dissociation of bulk water. This reduction is caused by spin selection rules that allow the system to remain on the same spin surface throughout the reaction.}, number={13}, journal={PHYSICAL REVIEW LETTERS}, author={Kostov, MK and Santiso, EE and George, AM and Gubbins, KE and Nardelli, MB}, year={2005}, month={Sep} }
 @article{calzolari_buongiorno nardelli_2005, title={First principles theory of artificial metal chains on NiAl(110) surface}, volume={72}, number={4}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Calzolari, A. and Buongiorno Nardelli, M.}, year={2005} }
 @article{ferretti_calzolari_di felice_manghi_caldas_nardelli_molinari_2005, title={First-principles theory of correlated transport through nanojunctions}, volume={94}, number={11}, journal={Physical Review Letters}, author={Ferretti, A. and Calzolari, A. and Di Felice, R. and Manghi, F. and Caldas, M. J. and Nardelli, M. B. and Molinari, E.}, year={2005} }
 @article{calzolari_marzari_souza_nardelli_2004, title={Ab initio transport properties of nanostructures from maximally localized Wannier functions}, volume={69}, number={3}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Calzolari, A. and Marzari, N. and Souza, I. and Nardelli, M. B.}, year={2004} }
 @article{nardelli_walker_mckee_2004, title={Crystalline oxides on semiconductors: a future for the nanotransistor}, volume={241}, number={10}, journal={Physica Status Solidi. B, Basic Solid State Physics}, author={Nardelli, M. B. and Walker, F. J. and McKee, R. A.}, year={2004}, pages={2279–2286} }
 @article{calzolari_cavazzoni_nardelli_2004, title={Electronic and transport properties of artificial gold chains}, volume={93}, number={9}, journal={Physical Review Letters}, author={Calzolari, A. and Cavazzoni, C. and Nardelli, M. B.}, year={2004}, pages={096404} }
 @article{pantano_parks_boyce_nardelli_2004, title={Mixed finite element-tight-binding electromechanical analysis of carbon nanotubes}, volume={96}, ISSN={["1089-7550"]}, DOI={10.1063/1.1809252}, abstractNote={Electrical transport properties of carbon nanotubes can be dramatically changed by mechanical deformations that alter tube shape and the corresponding positions of the atoms comprising the tube wall. In principle, detailed atomic/electronic calculations can provide both the deformed configuration and the resulting electrical transport behavior of the tube. Here we simplify the process by refining a previously-developed nonlinear structural mechanics finite-element-based procedure for modeling mechanical behavior of carbon nanotubes to account explicitly for tube chirality. A quadrilateral element overlay procedure provides an isotropic finite element model of hexagonal cells within a graphene sheet, with the only nodal positions coincident with those of the atoms. Mechanical deformation of the nanotube structure is simulated with finite elements, and the evolving atomic [nodal] coordinates are processed within the finite element (FE) program by using a tight-binding (TB) code to calculate deformation-induced changes in electrical transport properties of the nanotube. Results of the mixed FE/TB calculations compare favorably with existing atomistic simulations of single-walled nanotubes subjected to torsion, intense lateral squeezing, and large, kink-producing bending.}, number={11}, journal={JOURNAL OF APPLIED PHYSICS}, author={Pantano, A and Parks, DM and Boyce, MC and Nardelli, MB}, year={2004}, month={Dec}, pages={6756–6760} }
 @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{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{hernandez_meunier_smith_rurali_terrones_nardelli_terrones_luzzi_charlier_2003, title={Fullerene coalescence in nanopeapods: A path to novel tubular carbon}, volume={3}, ISSN={["1530-6992"]}, DOI={10.1021/nl034283f}, abstractNote={A fascinating structural transformation occurring inside single-walled carbon nanotubes (SWNTs) is the fullerene coalescence, which is responsible for forming stable zeppelinlike carbon molecules. We report in situ transmission electron microscope (TEM) observations revealing sequences of fullerene coalescence induced by electron irradiation on pristine nanotube peapods, together with extensive theoretical investigations of the microscopic mechanism underlying this process. TEM images indicate that the merging of fullerenes results in stable but corrugated tubules (5 to 7 Angstrom in diameter) confined within SWNTs. These observations have been confirmed using a combination of theoretical approaches based on molecular dynamics, empirical potentials, tight-binding methods, Monte Carlo techniques, and first principles calculations. We have fully elucidated the coalescence mechanism of fullerenes inside SWNTs under electron irradiation and thermal annealing. The process occurs via the polymerization Of C-60 molecules followed by surface reconstruction, which can be triggered either by the formation of vacancies (created under electron irradiation) or by surface-energy minimization activated by thermal annealing. These novel tubular forms of carbon contain hexagons, pentagons, heptagons, and octagons. The stability, electronic properties, and electron conductance of the novel tubules are strongly affected by the final geometry of the coalesced fullerene complex. The possibility of forming highly conducting and semiconducting tubular structures suggests new avenues in designing carbon nanowires with specific electronic characteristics.}, number={8}, journal={NANO LETTERS}, author={Hernandez, E and Meunier, V and Smith, BW and Rurali, R and Terrones, H and Nardelli, MB and Terrones, M and Luzzi, DE and Charlier, JC}, year={2003}, month={Aug}, pages={1037–1042} }
 @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} }
 @article{mckee_walker_nardelli_shelton_stocks_2003, title={The interface phase and the Schottky barrier for a crystalline dielectric on silicon}, volume={300}, ISSN={["0036-8075"]}, DOI={10.1126/science.1083894}, abstractNote={The barrier height for electron exchange at a dielectric-semiconductor interface has long been interpreted in terms of Schottky's theory with modifications from gap states induced in the semiconductor by the bulk termination. Rather, we show with the structure specifics of heteroepitaxy that the electrostatic boundary conditions can be set in a distinct interface phase that acts as a “Coulomb buffer.” This Coulomb buffer is tunable and will functionalize the barrier-height concept itself.}, number={5626}, journal={SCIENCE}, author={McKee, RA and Walker, FJ and Nardelli, MB and Shelton, WA and Stocks, GM}, year={2003}, month={Jun}, pages={1726–1730} }
 @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_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{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{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} }
 @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{roland_nardelli_wang_guo_2000, title={Dynamic conductance of carbon nanotubes}, volume={84}, ISSN={["0031-9007"]}, DOI={10.1103/PhysRevLett.84.2921}, abstractNote={The dynamic conductance of carbon nanotubes was investigated using the nonequilibrium Green's function formalism within the context of a tight-binding model. Specifically, we have studied the ac response of tubes of different helicities, both with and without defects, and an electronic heterojunction. Because of the induced displacement currents, the dynamic conductance of the nanotubes differs significantly from the dc conductance displaying both capacitive and inductive responses. The important role of photon-assisted transport through nanotubes is revealed and its implications for experiments discussed.}, number={13}, journal={PHYSICAL REVIEW LETTERS}, author={Roland, C and Nardelli, MB and Wang, J and Guo, H}, year={2000}, month={Mar}, pages={2921–2924} }
 @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} }
 @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{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{roland_nardelli_guo_mehrez_taylor_wang_wei_2000, title={Theoretical investigations of quantum transport through carbon nanotube devices}, volume={7}, DOI={10.1016/S0218-625X(00)00077-4}, abstractNote={By combining a nonequilibrium Green's function analysis with a standard tight-binding model, we have investigated quantum transport through carbon nanotube devices. For finite-sized nanotubes, transport is dominated by resonant tunneling, with the conductance being strongly dependent on the length of the nanotubes. Turning to nanotube devices, we have investigated spin-coherent transport in ferromagnetic–nanotube–ferromagnetic devices and nanotube-superconducting devices. The former shows a significant spin valve effect, while the latter is dominated by resonant Andreev reflections. In addition, we discuss AC transport through carbon nanotubes and the role of photon-assisted tunneling.}, number={5-6}, journal={Surface Review and Letters}, author={Roland, C. and Nardelli, M. B. and Guo, H. and Mehrez, H. and Taylor, J. and Wang, J. and Wei, Y.}, year={2000}, pages={637–642} }
 @article{paulson_helser_nardelli_taylor_falvo_superfine_washburn_2000, title={Tunable resistance of a carbon nanotube-graphite interface}, volume={290}, ISSN={["0036-8075"]}, DOI={10.1126/science.290.5497.1742}, abstractNote={The transfer of electrons from one material to another is usually described in terms of energy conservation, with no attention being paid to momentum conservation. Here we present results on the junction resistance between a carbon nanotube and a graphite substrate and show that details of momentum conservation also can change the contact resistance. By changing the angular alignment of the atomic lattices, we found that contact resistance varied by more than an order of magnitude in a controlled and reproducible fashion, indicating that momentum conservation, in addition to energy conservation, can dictate the junction resistance in graphene systems such as carbon nanotube junctions and devices.}, number={5497}, journal={SCIENCE}, author={Paulson, S and Helser, A and Nardelli, MB and Taylor, RM and Falvo, M and Superfine, R and Washburn, S}, year={2000}, month={Dec}, pages={1742–1744} }
 @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{nardelli_1999, title={Electronic transport in extended systems: Application to carbon nanotubes}, volume={60}, number={11}, journal={Physical Review. B, Condensed Matter and Materials Physics}, author={Nardelli, M. B.}, year={1999}, pages={7828–7833} }
 @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{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{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} }
 @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} }