@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{thakur_dong_lin_wu_cheng_hou_bernholc_zhang_2015, title={Optimizing nanostructure to achieve high dielectric response with low loss in strongly dipolar polymers}, volume={16}, ISSN={2211-2855}, url={http://dx.doi.org/10.1016/j.nanoen.2015.06.021}, DOI={10.1016/j.nanoen.2015.06.021}, abstractNote={Advances in modern electronics require the development of polymer-based dielectric materials with high dielectric constant, low dielectric loss, and high thermal stability. Fundamental dielectric theory suggests that strongly dipolar polymers have the potential to realize a high dielectric constant. In order to achieve high thermal stability, these polymers should also possess a high glass transition temperature Tg. However, it has been observed that in many dielectric polymers the dielectric constant decreases markedly at temperatures below Tg due to dipole freezing. This study shows, through combined theoretical and experimental investigations, that nano-structure engineering of a weakly-coupled strongly-dipolar polymer can result in a high energy density polymer with low loss and high operating temperature. Our studies reveal that disorder creates a significantly larger free volume at temperatures far below Tg, enabling easier reorientation of dipoles in response to an electric field in aromatic urea and thiourea polymers. The net result is a substantial enhancement in the dielectric constant while preserving low dielectric loss and very high breakdown field. These results here pave the way for engineering the nanostructure to create high energy density polymers with low loss and high operating temperature.}, journal={Nano Energy}, publisher={Elsevier BV}, author={Thakur, Yash and Dong, Rui and Lin, Minren and Wu, Shan and Cheng, Zhaoxi and Hou, Ying and Bernholc, J. and Zhang, Q.M.}, year={2015}, month={Sep}, pages={227–234} }
 @article{dong_calzolari_felice_el-shafei_hussain_nardelli_2014, title={Optical Enhancement in Heteroleptic Ru(II) Polypyridyl Complexes Using Electron-Donor Ancillary Ligands}, volume={118}, ISSN={["1932-7455"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84899829963&partnerID=MN8TOARS}, DOI={10.1021/jp409733a}, abstractNote={Organic dyes are a viable alternative to silicon for energy conversion. Using simulations from first-principles, we show that chemical manipulation is a powerful tool for tuning the optical absorption spectra of a special class of dyes in a way that is convenient for exploitation in dye-sensitized solar cells. Specifically, we have carried out density functional theory calculations on three Ru(II) polypyridyl complexes with electron-donor ancillary ligands. These complexes were recently developed to study how different electron-donor ancillary ligands affect the photophysical and electrochemical properties of these dyes for light harvesting and photon-to-electron conversion efficiency. We found that the electron-donor ancillary ligands significantly enhance the light harvesting in the visible and the near-infrared regions relative to the reference dye N3. Furthermore, we detected a decrease in the ionization potential, which improves the energy alignment with the redox potentials of the electrolyte. These...}, number={17}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Dong, Rui and Calzolari, Arrigo and Felice, Rosa and El-Shafei, Ahmed and Hussain, Maqbool and Nardelli, Marco Buongiorno}, year={2014}, month={May}, pages={8747–8755} }