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

Abstract

We report the study of anatase TiO(001)-oriented thin films grown by pulsed laser deposition on LaAlO(001). A combination of in situ and ex situ methods has been used to address both the origin of the Ti-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 Ti electronic configuration, homogeneously distributed over the entire film thickness. We find that an increase in the oxygen pressure corresponds to an increase in Ti only in a well-defined range of deposition pressure; outside this range, Ti and the strength of the in-gap states are reduced.

Tuning the Optical Absorption of Anatase Thin Films Across the Visible-To-Near-Infrared Spectral Region

“…In situ scanningtunneling-microscopy (STM) experiments are performed with an atomic-resolution UHV apparatus, immediately after the growth of the samples. The STM topography shows good surface quality, with an overall root-meansquare value of surface roughness of about 0.2 nm, thus implying a very low surface roughness of the grown samples, consistently with previous studies [17].…”

“…The atomic structure of the TiO 2 films is investigated by high-resolution transmission electron microscopy (HRTEM). Similarly to previously investigated as-grown samples [17,27,28], HRTEM reveals the presence of a modulated structure all through the film thickness in the case of UHV-annealed TiO 2 films also; this is tentatively associated with the occurrence of oxygen vacancies, selforganizing into regular arrays of planar defects. Importantly, no structural differences can be seen between the near-interface region and the bulk of the film, as well as no traces of spurious phases or segregation of crystalline phases other than anatase, consistently with the XRD results.…”

“…2). Such values have been proven to induce a very small amount of oxygen vacancies in TiO 2 thin films [17]. The annealing process is performed by letting the sample remain at the deposition temperature (i.e., 700 • C) in UHV (i.e., at a base pressure less than 10 −7 mbar); this sample is referred to hereafter as the annealed film.…”

“…1), performed by means of the IMD package in the XOP software [18,19], fully match the design value of 30 nm. XRR oscillations are recorded up to 2θ values of 4 • , while, above this angle, the oscillations fall below the experimental sensitivity of the x-ray diffractometer [17,20,21]. In situ scanningtunneling-microscopy (STM) experiments are performed with an atomic-resolution UHV apparatus, immediately after the growth of the samples.…”

Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2

“…Consistent results were also obtained for films grown on Nb-doped SrTiO3, indicating that the observed properties are intrinsic to the TiO2 anatase phase (see Figure S1). Details of the growth protocol and characterization results 20 Figure 1e: an outer parabola and a second (fainted) quantized sub-band related to the confinement potential at the surface 11,12 . The bottom of the two quantized states is located at ~180 meV and ~65 meV for the outer and the inner band respectively, while the Fermi momenta (kF) are at ~0.18 Å -1 and ~0.11 Å -1 .…”