@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{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{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{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{naumov_bratkovsky_ranjan_2009, title={Unusual flexoelectric effect in two-dimensional noncentrosymmetric sp(2)-bonded crystals}, volume={102}, number={21}, journal={Physical Review Letters}, author={Naumov, I. and Bratkovsky, A. M. and Ranjan, V.}, year={2009} }
 @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} }