The search for efficient photoelectrochemically active materials has led to several studies on BiVO 4 , which has shown higher photocurrent than most typical semiconductors under photoelectrochemical (PEC) operation. BiVO 4 can be stabilized in different crystal structures, yet high PEC efficiency is found only for n-type doped monoclinic phase. The theoretical description of the monoclinic polymorph is difficult because of spontaneous transformation to a tetragonal phase. The cause of such instability of the monoclinic phase has yet to be resolved. Using first-principles calculations based on density functional theory, we explore how the concentration of excess electrons in this material affects its phase stability. We analyze the unit-cell structure, local bonding, and band structure of BiVO 4 at different concentrations of excess electrons. We find that as the concentration of excess electrons increases, the tetragonal phase spontaneously transforms into the monoclinic phase, suggesting a crucial role of doping in the structure and, thus, the photoelectrochemical performance of BiVO 4 .

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As a prototypical Mott insulator with ferromagnetic ordering, YTiO3 (YTO) is of great interest in the study of strong electron correlation effects and orbital ordering. Here we report the first molecular beam epitaxy (MBE) growth of YTO films, combined with theoretical and experimental characterization of the electronic structure and charge transport properties. The obstacles of YTO MBE growth are discussed and potential routes to overcome them are proposed.DC transport and Seebeck measurements on thin films and bulk single crystals identify p-type Arrhenius transport behavior, with an activation energy of ~ 0.17 eV in thin films, consistent with the energy barrier for small hole polaron migration from hybrid density functional theory (DFT) calculations. Hard X-ray photoelectron spectroscopy measurements (HAXPES) show the lower Hubbard band (LHB) at 1.1 eV below the Fermi level, whereas a Mott-Hubbard band gap of ~1.5 eV is determined from photoluminescence (PL) measurements. These findings provide critical insight into the electronic band structure of YTO and related materials.

Germanium-based oxides such as rutile GeO2 are garnering attention owing to their wide band gaps and the prospects of ambipolar doping for application in high-power devices. Here, we present the use of germanium tetraisopropoxide (GTIP), a metal-organic chemical precursor, as a source of germanium for the demonstration of hybrid molecular beam epitaxy for germanium-containing compounds. We use Sn1-xGexO2 and SrSn1-xGexO3 as model systems to demonstrate our synthesis method. A combination of high-resolution X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy confirms the successful growth of epitaxial rutile Sn1-xGexO2 on TiO2(001) substrates up to x = 0.54 and coherent perovskite SrSn1-xGexO3 on GdScO3(110) substrates up to x = 0.16. Characterization and first-principles calculations corroborate that germanium occupies the tin site, as opposed to the strontium site. These findings confirm the viability of the GTIP precursor for the growth of germanium-containing oxides by hybrid molecular beam epitaxy, thus providing a promising route to high-quality perovskite germanate films.

By employing first principles DFT calculations, we propose a new stable model for Mo6S9 nanowires (NWs) obtained by condensing tetrahedral Mo4S6 clusters rather than octahedral Mo6S8 clusters, which are known as magic clusters in the Mo-S polyhedral cluster family. The pristine NW is found to be metallic and its local structure and physical properties can be tuned by doping of iodine atoms. This doping increases the number of Mo-Mo bonds in the NW, thus, Mo4 tetrahedra are initially fused to the Mo6 octahedron, and then, to the Mo8 dodecahedron. Further, a close correlation among the Mo-Mo bonding in the local structure, mechanical and electronic properties, is observed from our study. Finally, the stability of the pristine and iodine doped Mo8S12-xIx NW structures obtained from condensation of Mo4 tetrahedra are found to be quite comparable with that of already reported Mo6S9-xIx NWs with Mo6 octahedra as building blocks.

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