Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for ${\mathrm{In}}_2{\mathrm{O}}_3$ is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of ${\mathrm{In}}_2{\mathrm{O}}_3$ exhibit a small dispersion and the conduction band minimum is positioned at $\Gamma$. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV.

NiO is a p-type wide bandgap semiconductor of use in various electronic devices ranging from solar cells to transparent transistors. Understanding and improving its optical and transport properties have been of considerable interest. In this work, we have investigated the effect of Li doping on the electronic, optical and transport properties of NiO epitaxial thin films grown by pulsed laser deposition. We show that Li doping significantly increases the p-type conductivity of NiO, but all the films have relatively low room-temperature mobilities (<0.05 cm² V⁻¹ s⁻¹). The conduction mechanism is better described by small-polaron hoping model in the temperature range of 200 K < T < 330 K, and variable range hopping at T < 200 K. A combination of X-ray photoemission and O K-edge X-ray absorption spectroscopic investigations reveal that the Fermi level gradually shifts toward the valence band maximum (VBM) and a new hole state develops with Li doping. Both the VBM and hole states are composed of primarily Zhang-Rice bound states, which accounts for the small polaron character (low mobility) of hole conduction. Our work provides guidelines for the search for p-type oxide materials and device optimization.

Surface impurity species, most notably Li₂CO₃, that develop on layered oxide positive electrode materials with atmospheric aging have been reported to be highly detrimental to the subsequent electrochemical performance. LiNi_0.8Co_0.15Al_0.05O₂ (NCA) was used as a model layered oxide compound to evaluate the growth and subsequent electrochemical impact of H₂O, LiHCO₃, LiOH and Li₂CO₃. Methodical high temperature annealing enabled the systematic removal of each impurity specie, thus permitting the determination of each specie's individual effect on the host material's electrochemical performance. Extensive cycling of exposed and annealed materials emphasized the cycle life degradation and capacity loss induced by each impurity, while rate capability measurements correlated the electrode impedance to the impurity species present. Based on these characterization results, this work attempts to clarify decades of ambiguity over the growth mechanisms and the electrochemical impact of the specific surface impurity species formed during powder storage in various environments.

The origin of electron accumulation at wurtzite InN surfaces is explained in terms of the bulk band structure. Ab initio calculations of the electronic structure of wurtzite InN reveal an unusually low conduction band minimum at the $\Gamma$-point. As a result, the branch point energy, ${\mathrm{E}}_B,$ which is the crossover point from donor-type to acceptor-type surface states, is located in the conduction band at the $\Gamma$-point. This allows donor-type surface states to exist in the conduction band. The donor-type surface states emit their electrons into the conduction band, thus giving rise to electron accumulation at the surface. Experimental measurements, probing the conduction band electron plasma, confirm the existence of electron accumulation at InN surfaces, with a surface Fermi level location in agreement with the predictions of the ab initio theory.