In situ investigation of the early stage of TiO2 epitaxy on (001) SrTiO3

Evidence of a 2D Electron Gas in a Single-Unit-Cell of Anatase TiO2 (001)

The formation and the evolution of electronic metallic states localized at the surface, commonly termed 2D electron gas (2DEG), represents a peculiar phenomenon occurring at the surface and interface of many transition metal oxides (TMO). Among TMO, titanium dioxide (TiO2 ), particularly in its anatase polymorph, stands as a prototypical system for the development of novel applications related to renewable energy, devices and sensors, where understanding the carrier dynamics is of utmost importance. In this study, angle-resolved photo-electron spectroscopy (ARPES) and X-ray absorption spectroscopy (XAS) are used, supported by density functional theory (DFT), to follow the formation and the evolution of the 2DEG in TiO2 thin films. Unlike other TMO systems, it is revealed that, once the anatase fingerprint is present, the 2DEG in TiO2 is robust and stable down to a single-unit-cell, and that the electron filling of the 2DEG increases with thickness and eventually saturates. These results prove that no critical thickness triggers the occurrence of the 2DEG in anatase TiO2 and give insight in formation mechanism of electronic states at the surface of TMO.

Role of Oxygen Deposition Pressure in the Formation of Ti Defect States in TiO2(001) Anatase Thin Films

We report the study of anatase TiO₂(001)-oriented thin films grown by pulsed laser deposition on LaAlO₃(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti³⁺-localized states and their relationship with the structural and electronic properties on the surface and the subsurface. Localized in-gap states are analyzed using resonant X-ray photoelectron spectroscopy and are related to the Ti³⁺ electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti³⁺ only in a well-defined range of deposition pressure; outside this range, Ti³⁺ and the strength of the in-gap states are reduced.

Atomically Precise Lateral Modulation of a Two-Dimensional Electron Liquid in Anatase TiO2 Thin Films

Engineering the electronic band structure of two-dimensional electron liquids (2DELs) confined at the surface or interface of transition metal oxides is key to unlocking their full potential. Here we describe a new approach to tailoring the electronic structure of an oxide surface 2DEL demonstrating the lateral modulation of electronic states with atomic scale precision on an unprecedented length scale comparable to the Fermi wavelength. To this end, we use pulsed laser deposition to grow anatase TiO₂ films terminated by a (1 × 4) in-plane surface reconstruction. Employing photostimulated chemical surface doping we induce 2DELs with tunable carrier densities that are confined within a few TiO₂ layers below the surface. Subsequent in situ angle-resolved photoemission experiments demonstrate that the (1 × 4) surface reconstruction provides a periodic lateral perturbation of the electron liquid. This causes strong backfolding of the electronic bands, opening of unidirectional gaps and a saddle point singularity in the density of states near the chemical potential.

Anatase TiO₂ films with dominant {001} facets fabricated by direct-current reactive magnetron sputtering at room temperature: oxygen defects and enhanced visible-light photocatalytic behaviors

A TiO2 film with dominant anatase {001} facets is directly prepared by direct-current reactive magnetron sputtering at room temperature without using morphology-controlling agents. The formation mechanism of anatase TiO2 films with dominant {001} facets is explained by the competition between thermodynamics and ion impinging in the deposition process. The crystalline TiO2 film shows a superior photocatalytic efficiency for the degradation of Rhodamine B under UV-visible (λ > 250 nm) lights. Furthermore, a comparable photodegradation of Rhodamine B is also found on the TiO2 film surface by using visible (λ > 420 nm) lights. During film growth, the surface bombarded by high energy of ions yields plenty of oxygen defects, which can enhance the photocatalytic activity of the films irradiated under visible light.

Nanostructure of buried interface layers in TiO₂ anatase thin films grown on LaAlO₃ and SrTiO₃ substrates

TiO(2) anatase thin films grown by pulsed laser deposition are investigated by high resolution transmission electron microscopy and high angle annular dark field scanning transmission electron microscopy. The analyses provide evidence of a peculiar growth mode of anatase on LaAlO(3) and SrTiO(3) characterized by the formation of an epitaxial layer at the film/substrate interface, due to cationic diffusion from the substrate into the film region. Pure TiO(2) anatase growth occurs in both specimens above a critical thickness of about 20 nm. The microstructural and chemical characterization of the samples is presented and discussed in the framework of oxide interface engineering.