NO oxidation rates over La 1−x Sr x CoO 3 (x = 0−0.3) perovskite catalysts are reported as a function of Sr doping in the absence and presence of NO 2 in the feed. Sr substitution is found to increase the rate of oxidation and to diminish the inhibitory influence of NO 2 . Temperature programmed desorption and isotopic exchange (TPIE) experiments were used to identify surface species and oxygen exchange processes expected to correlate with NO oxidation activity. Oxygen exchange in the LaCoO 3 perovskites occurred primarily through a heteroexchange process that was enhanced by doping with Sr. Density functional theory (DFT) calculations were used to further investigate the oxygen exchange processes on (100) facets of undoped and doped LaCoO 3 . Vacancy formation is predicted to be more facile on CoO 2 -terminated than LaO-terminated surfaces. The Sr dopant segregates to the LaO-terminated surface and diminishes oxygen bonding consistent with the TPIE results. The results suggest a model in which multiple oxygen species contribute to low-and high-temperature oxygen exchange.

Perovskites are of interest as low-cost replacements for Pt-based NO oxidation catalysts. While the mechanism of Pt-catalyzed NO oxidation is fairly well understood, such is not the case for the oxides. The perovskite LaCoO 3 itself has been shown to have good NO oxidation activity, and Sr substitution improves NO oxidation rates and reduces NO 2 inhibition. In this work, we report density functional theory (DFT) results for the adsorption of NO x (x = 1, 2) to the undoped and Sr doped (100) LaO and CoO 2 terminated LaCoO 3 . Further, we used first-principles thermodynamic models to determine the most common surface species under NO oxidation conditions. Nitrates and adsorbed NO are most stable on the LaO and CoO 2 terminations respectively. We explored the relative free energies of surface and vacancy-mediated pathways for NO oxidation. The vacancy-mediated pathways suffer from energetically costly removal of surface oxygen, while the surface pathway is most feasible for NO oxidation to occur.The perovskite surface free energy reaction pathway is compared to RuO 2 , MgO, and Pt. The CoO 2 termination surface pathway is energetically most similar to that of Pt and is considered to be the most plausible for NO oxidation.

Compact optical interconnects require efficient lasers and modulators compatible with silicon. Ab initio modeling of Ge1−xCx (x = 0.78%) using density functional theory with HSE06 hybrid functionals predicts a splitting of the conduction band at Γ and a strongly direct bandgap, consistent with band anticrossing. Photoreflectance of Ge0.998C0.002 shows a bandgap reduction supporting these results. Growth of Ge0.998C0.002 using tetrakis(germyl)methane as the C source shows no signs of C-C bonds, C clusters, or extended defects, suggesting highly substitutional incorporation of C. Optical gain and modulation are predicted to rival III–V materials due to a larger electron population in the direct valley, reduced intervalley scattering, suppressed Auger recombination, and increased overlap integral for a stronger fundamental optical transition.

Dilute germanium carbides (Ge 1-x C x ) offer a direct bandgap for compact silicon photonics, but widely varying results on its properties have been reported. This work uses ab initio simulations with HSE06 hybrid density functionals and spin-orbit coupling to study the Ge 1-x C x band structure in the absence of defects. Contrary to Vegard's law, the conduction band minimum at k=0 is consistently found to decrease with increasing C content, while L and X valleys change much more slowly. A vanishing bandgap was observed for all alloys with x>0.017. Conduction bands deviate from a constant-potential band anticrossing model except near the center of the Brillouin zone.

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