The nature and the role of 1 to 5 nm thick TiO 2 seed layers for the growth of textured PbTiO 3 and Pb(Zr,Ti)O 3 thin films on textured Pt(111) thin film substrates have been studied. Under otherwise identical in situ sputter deposition process conditions, the PbTiO 3 texture could be turned from (100) to (111) orientation by adding the seed layer. This is demonstrated by patterning the TiO 2 film. Auger electron spectroscopy and x-ray photoemission spectroscopy showed that the seed layer was a continuous TiO 2 film. X-ray photoelectron diffraction measurements revealed epitaxial ordering in the seed layer. As there is no azimuthal order among the Pt grains, the reduced information of azimuthally averaged polar cuts is obtained. These give strong evidence for a strained rutile (110) structure. Various deposition experiments indicated that the TiO 2 is effective only when it is ordered before the PbTiO 3 nucleation starts. The epitaxial relationship between PbTiO 3 (111) and Pt(111) is thus mediated by the intermediate, epitaxial TiO 2 film, which is dissolved or transformed to PbTiO 3 afterwards. The observed growth behavior is discussed in terms of surface and interface energies.
We report first-principles GW results on the dispersion of the bulk band-gap edges in the three-dimensional topological insulator Sb 2 Te 3 . We find that, independently of the reference density-functional-theory band structure and the crystal-lattice parameters used, the one-shot GW corrections enlarge the fundamental band gap, bringing its value in close agreement with experiment. We conclude that the GW corrections cause the displacement of the valence-band maximum (VBM) to the point, ensuring that the surface-state Dirac point lies above the VBM. We extend our study to the analysis of the electron-energy-loss spectrum (EELS) of bulk Sb 2 Te 3 . In particular, we perform energy-filtered transmission electron microscopy and reflection EELS measurements. We show that the random-phase approximation with the GW quasiparticle energies and taking into account virtual excitations from the semicore states leads to good agreement with our experimental data.
Titanium dioxide exhibits superior photocatalytic properties, mainly occurring in liquid environments through molecular adsorptions and dissociations at the solid/liquid interface. The presence of these wet environments is often neglected when performing ab initio calculations for the interaction between the adsorbed molecules and the TiO 2 1 surface. In this study we consider two solvents, i.e. water and ethanol, and show that the proper inclusion of the wet environment in the methodological scheme is fundamental for obtaining reliable results. Our calculations are based on structure predictions at a density functional theory level for molecules interacting with the perfect and defective anatase (1 0 1) surface under both vacuum and wet conditions. A soft-sphere implicit solvation model is used to describe the polar character of the two solvents. As a result, we find that surface oxygen vacancies become energetically favorable with respect to subsurface vacancies at the solid/liquid interface. This aspect is confirmed by ab initio molecular dynamics simulations with explicit water molecules. Ethanol molecules are able to strongly passivate these vacancies, whereas water molecules only weakly interact with the (1 0 1) surface, allowing the coexistence of surface vacancy defects and adsorbed species. Infrared and photoluminescence spectra of anatase nanoparticles exposing predominantly (1 0 1) surfaces dispersed in water and ethanol support the predicted molecular-surface interactions, validating the whole computational paradigm. The combined analysis allows for a better interpretation of TiO 2 processes in wet environments based on improved computational models with implicit solvation features.
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