Safer Batteries through Coupled Multiscale Modeling

The recently synthesized freestanding fouratom-thick double-layer sheet of ZnSe holds great promise as an ultraflexible and transparent photoelectrode material for solar water splitting. In this work, we report theoretical studies on a novel three-atom-thick single layer sheet of ZnSe that demonstrates a strong quantum confinement effect by exhibiting a large enhancement of the band gap (2.0 eV) relative to the zinc blende (ZB) bulk phase. Theoretical optical absorbance shows that the largest absorption of this ultrathin single-layer sheet of ZnSe occurs at a wavelength similar to its four-atomthick double-layer counterpart, suggesting a comparable behavior on incident photon-to-current conversion efficiency for solar water splitting, among a wealth of potential applications. The results presented herein for ZnSe may be generalized to other group II-VI analogues. Alexander A. Puretzky, Liangbo Liang, Xufan Li, Kai Xiao, Kai Wang, Masoud Mahjouri-Samani, Leonardo Basile, Juan Idrobo, Bobby G. Sumpter, Vincent Meunier, David B. Geohegan, Low-frequency Raman spectroscopy reveals stacking patterns of twodimensional transition metal dichalcogenides, ACS Nano DOI: 10.1021/acsnano.5b01884 (2015). Abstract: The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks; however, fast optical characterization techniques are essential for rapid development of the field. Here, using twoand three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low frequency (LF) Raman modes (<50 cm_1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries. A. Kreisel, Peayush Choubey, T. Berlijn, W. Ku, B. M. Andersen, P. J. Hirschfeld, Interpretation of Scanning Tunneling Quasiparticle Interference and Impurity States in Cuprates, Phys. Rev. Lett. 114, 217002 (2015). Cover of Journal The tunable optoelectronic properties of stacked two-dimensional (2D) crystal monolayers are determined by their stacking orientation, order, and atomic registry. Atomic-resolution Z-contrast scanning transmission electron microscopy (AR-Z-STEM) and electron energy loss spectroscopy (EELS) can be used to determine the exact atomic registration between different layers, in few-layer 2D stacks; however, fast optical characterization techniques are essential for rapid development of the field. Here, using twoand three-layer MoSe2 and WSe2 crystals synthesized by chemical vapor deposition, we show that the generally unexplored low frequency (LF) Raman modes (<50 cm_1) that originate from interlayer vibrations can serve as fingerprints to characterize not only the number of layers, but also their stacking configurations. Ab initio calculations and group theory analysis corroborate the experimental assignments determined by AR-Z-STEM and show that the calculated LF mode fingerprints are related to the 2D crystal symmetries. A. Kreisel, Peayush Choubey, T. Berlijn, W. Ku, B. M. Andersen, P. J. Hirschfeld, Interpretation of Scanning Tunneling Quasiparticle Interference and Impurity States in Cuprates, Phys. Rev. Lett. 114, 217002 (2015). Cover of Journal Abstract: We apply a recently developed method combining first principles based Wannier functions with solutions to the Bogoliubov–de Gennes equations to the problem of interpreting STM data in cuprate superconductors. We show that the observed images of Zn on the surface of Bi2Sr2CaCu2O8 can only be understood by accounting for the tails of the Cu Wannier functions, which include significant weight on apical O sites in neighboring unit cells. This calculation thus puts earlier crude “filter” theories on a microscopic foundation and solves a long-standing puzzle. We then study quasiparticle interference phenomena induced by out-ofplane weak potential scatterers, and show how patterns long observed in cuprates can be understood in terms of the interference of Wannier functions above the surface. Our results show excellent agreement with experiment and enable a better understanding of novel phenomena in the cuprates via STM imaging. We apply a recently developed method combining first principles based Wannier functions with solutions to the Bogoliubov–de Gennes equations to the problem of interpreting STM data in cuprate superconductors. We show that the observed images of Zn on the surface of Bi2Sr2CaCu2O8 can only be understood by accounting for the tails of the Cu Wannier functions, which include significant weight on apical O sites in neighboring unit cells. This calculation thus puts earlier crude “filter” theories on a microscopic foundation and solves a long-standing puzzle. We then study quasiparticle interference phenomena induced by out-ofplane weak potential scatterers, and show how patterns long observed in cuprates can be understood in terms of the interference of Wannier functions above the surface. Our results show excellent agreement with experiment and enable a better understanding of novel phenomena in the cuprates via STM imaging. Muralikrishna Raju, P Ganesh, Paul RC Kent, Adri CT van Duin, Reactive Force Field Study of Li/C Systems for Electrical Energy Storage, Journal of Chemical Theory and Computation, 11, 2156-2166. Abstract: Graphitic carbon is still the most ubiquitously used anode material in Li-ion batteries. In spite of its ubiquity, there are few theoretical studies that fully capture the energetics and kinetics of Li in graphite and related nanostructures at experimentally relevant length, timescales, and Li-ion concentrations. In this paper, we describe the development and application of a ReaxFF reactive force field to describe Li interactions in perfect and defective carbon-based materials using atomistic simulations. We develop force field parameters for Li–C systems using van der Waals-corrected density functional theory (DFT). Grand canonical Monte Carlo simulations of Li intercalation in perfect graphite with this new force field not only give a voltage profile in good agreement with known experimental and DFT results but also capture the in-plane Li ordering and interlayer separations for stage I and II compounds. In defective graphite, the ratio of Li/C (i.e., the capacitance increases and voltage shifts) both in proportion Publications of Note Graphitic carbon is still the most ubiquitously used anode material in Li-ion batteries. In spite of its ubiquity, there are few theoretical studies that fully capture the energetics and kinetics of Li in graphite and related nanostructures at experimentally relevant length, timescales, and Li-ion concentrations. In this paper, we describe the development and application of a ReaxFF reactive force field to describe Li interactions in perfect and defective carbon-based materials using atomistic simulations. We develop force field parameters for Li–C systems using van der Waals-corrected density functional theory (DFT). Grand canonical Monte Carlo simulations of Li intercalation in perfect graphite with this new force field not only give a voltage profile in good agreement with known experimental and DFT results but also capture the in-plane Li ordering and interlayer separations for stage I and II compounds. In defective graphite, the ratio of Li/C (i.e., the capacitance increases and voltage shifts) both in proportion Publications of Note