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We present computational evidence of polaronic hole trapping and migration in lithium peroxide (Li${}_{2}$O${}_{2}$), a material of interest in lithium-air batteries. We find that the hole forms in the ${\ensuremath{\pi}}^{*}$ antibonding molecular orbitals of the peroxide (O${}_{2}^{2\ensuremath{-}}$) anion, and that this trapped hole induces significant local lattice distortion, forming a polaron. Our study finds migration barriers for the free polaron to be between 68 and 152 meV, depending on the hopping direction. This low barrier suggests that this material might not be as insulating as previously assumed, provided that the formation of carriers can be achieved. One transport limitation may arise from lithium vacancies, which we find to strongly bind to the polaron. This result, in combination with previous experimental results, suggests that electronic conductivity in this material is likely to be determined by vacancy diffusion.

The lithium-air chemistry is an interesting candidate for the next-generation batteries with high specific energy. However, this new battery technology is facing substantial challenges, such as a high overpotential upon charging, poor reversibility, and low power density. Using first-principles calculations, we study the oxygen evolution reaction (OER) on the low-index surfaces of lithium peroxide. The elementary reaction steps and the energy profile of the OER are identified on the low-index surfaces of lithium peroxide. We find that the OER processes are kinetically limited by the high energy barrier for the evolution of oxygen molecules and that the rate of the OER processes is highly dependent on the surface orientation.

Large-scale molecular-dynamics simulations are performed to study friction in nanoscale single asperity contacts. The modeling system consists of a tip made of H-terminated diamond-like carbon and an H-terminated diamond sample. Simulations are carried out using a reactive bond-order interatomic potential integrated with dispersive interactions. A quantitative agreement in contact pressures and shear strengths is achieved between our simulations and previously reported experimental studies. We show that the roughness theories capture the correct physics of deformation at the nanoscale. Our study provides a consistent explanation of the widely observed transition from a linear to sublinear dependence of the friction force on the applied load and we demonstrate that both regimes of friction are governed by the same physical phenomenon. Specifically, we show that friction is controlled by the mean number of atoms that interact chemically across the contact interface.

Friction of hydrogen-passivated surfaces can be reduced by replacing terminating hydrogen atoms with heavier deuteriums. Using molecular-dynamics simulations, we show that this experimentally observed isotope effect can be explained quantitatively by small differences in surface coverage, which are due to isotope-dependent bond stabilities. We also demonstrate that a change in vibrational frequencies alone does not account for an isotope-dependent solid friction in our model system.

The interactions between Ge, Ag, and Cu and both cleaved and sputtered (HgCd)Te substrates have been investigated using synchrotron radiation, and the results have been compared with those reported in the literature when available. Each of these overlayers forms tellurides with heats of formation near that of HgTe. Consequently, the extent of the interfacial reactions is intermediate between those occurring after deposition of reactive and unreactive metals and, in the absence of a large thermodynamic driving force for a reaction, is sensitive to the stability of the surface and the propensity of the overlayer to diffuse into the semiconductor. Ag readily diffuses into cleaved substrates, confirming earlier results. In contrast, it is trapped, at least partially, in the interfacial region of sputtered surfaces because of their increased reactivity, i.e., decreased stability. This increased reactivity of the sputtered versus cleaved surface is also demonstrated for Ge and Cu, where a greater fraction of the Hg is lost upon deposition. Our results show less Hg loss from cleaved surfaces than do the results of other work, which we attribute to differences in material or surface quality. The interactions between (HgCd)Te and intermediate overlayers are seen to serve as sensitive indicators of the surface stability.