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

Abstract

Two-dimensional (2D) metallic states induced by oxygen vacancies at oxide surfaces and interfaces provide new opportunities for the development of advanced applications, but the ability to control the behavior of these states is still limited. We used Angle Resolved Photoelectron Spectroscopy combined with density functional theory to study the reactivity of states induced by the oxygen vacancies at the (001)(14) surface of anatase TiO2, where both 2D metallic and deeper lying in-gap states (IGs) are observed. Remarkably, the two states exhibit very different evolution when the surface is exposed to molecular O2: while IGs are almost completely quenched, the metallic states are only weakly affected. The energy scale analysis for the vacancy migration and recombination resulting from the DFT calculations confirms indeed that only the IGs originate from and remain localized at the surface, whereas the metallic states originate from subsurface vacancies, whose migration and recombination at the surface is energetically less favorable rendering them therefore insensitive to oxygen dosing. ARTICLE TEXTI.

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

“…7(b)] shows an outer parabola and a subband at a relative separation of approximately 100 meV. This subband is known to arise from the confinement potential at the surface and suggests that the metallic state is a two-dimensional electron gas (2DEG) [54][55][56]. The parabolic dispersive bands allow one to derive the effective mass of free carriers m * , as this is reflected in their curvature through the formula…”

“…Therefore the distribution of the oxygen vacancies (i.e., of the excess carriers) in the samples is uniform throughout the material (i.e., in the bulk and surface). At the surface (i.e., in the probing depth accessible by ARPES), the excess electrons associated with those vacancies manifest themselves as a 2DEG state [56]. In this respect, Luttinger's theorem [59] relates the free-carrier density to the area of the Fermi surface.…”

Dual behavior or coexistence of trapped and free states in reducible rutile TiO2

“…The picture is all the more general that similar charge distributions are also evidenced for n-doping with hydrogen 16,17 or aliovalent cations and anions 18 , electron injection 19 and UV irradiation 17 , in line with theoretical predictions 10,14,20,21 . Now, the apparent deep character of the BGS seems to run counter the high bulk mobility observed in the n-type TiO 2 22 , which raises the hotly debated issue of the nature of carriers in titania [9][10][11][22][23][24] . The conductivity was first suggested to stem from polaron hopping mechanism with a diffusion barrier of few tens of meV 9,11 .…”

Angle, Spin, and Depth Resolved Photoelectron Spectroscopy on Quantum Materials

“…reconstruction is observed, [39][40][41] and a sketch of this rearrangement of the surface is shown in the figure . The origin of such reconstruction has been proposed, and experimentally confirmed, to arise from a structural reorganisation able to release the large surface stress of the unreconstructed anatase surface. 38 This observation differs from other transition metal oxide systems that display various surface reconstructions for small changes in the density of the oxygen vacancies.…”

“…by oxygen vacancies and/or cation off-stoichiometry) often produces both the stabilization of a dispersive bi-dimensional electron gas (2DEG) at the surfaces, and the formation of localized non-dispersive states, so called 'defect' in-gap (IG) states. 39,45,48,[238][239][240][241] Starting from the first observation of 2DEG at the LAO/STO interface and at STO single crystals, a rich variety of such states have been reported both in parent materials and in engineered heterostructures. 47,48,242 Different behavior and character are found: some analyses reveal that in STO these states are deep-level traps associated with defects resulting from interstitial oxygen suggesting defect interconversion, 41 while other reports highlight in LAO/STO the importance of acceptors and donors states as well as the difference between crystalline and amorphous systems.…”