Anatase TiO₂—A Model System for Large Polaron Transport

Abstract

Large polarons have been of significant recent technological interest as they screen and protect electrons from point-scattering centers. Anatase TiO2 is a model system for studying large polarons as they can be studied systematically over a wide range of temperature and carrier density. The electronic and magneto transport properties of reduced anatase TiO2 epitaxial thin films are analyzed considering various polaronic effects. Unexpectedly, with increasing carrier concentration, the mobility increases, which rarely happens in common metallic systems. We find that the screening of the electron–phonon (e–ph) coupling by excess carriers is necessary to explain this unusual dependence. We also find that the magnetoresistance could be decomposed into a linear and a quadratic component, separately characterizing the carrier transport and trapping as a function of temperature, respectively. The various transport behaviors could be organized into a single phase diagram, which clarifies the evolution of large polaron in this material.

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

“…The e-polarons preferentially locate at specific Ti3d orbitals, and distort the local Ti-O octahedral geometry that is crucial for forming a small e-polaron [182]. The excess electrons can delocalize over several Ti sites to exist in large polaron or partially delocalized states [183]. Many studies were devoted to e-polarons and had gave a clear physical picture for spatial population of them.…”

Formation of Plasmonic Polarons in Highly Electron-Doped Anatase TiO2

“…For instance, the formation of large polarons in A-TiO 2 can effectively screen and protect electrons from local scattering centers, which is expected to be a way to engineer its catalytic activities. 12 The enhanced electron−phonon (e−ph) coupling can induce high-T c superconductivity at FeSe/A-TiO 2 interface. 13 These versatile properties from catalysis to superconductivity make A-TiO 2 a fascinating system to study the electron-boson couplings.…”

“…Defects in transition metal oxide (TMO), such as the anatase titanium dioxide (A-TiO 2 ), are closely related to the activity in catalytic and photocatalytic reactions. − More and more studies have been focused on the electron doping introduced by defects, where the band bending potential confines the electron to create two-dimensional (2D) or three-dimensional (3D) electron liquid with complex many-body interactions. − As a result, various quasiparticles may be involved via different electron-boson couplings, which can dramatically affect the electronic properties of A-TiO 2 . For instance, the formation of large polarons in A-TiO 2 can effectively screen and protect electrons from local scattering centers, which is expected to be a way to engineer its catalytic activities . The enhanced electron–phonon (e−ph) coupling can induce high- T c superconductivity at FeSe/A-TiO 2 interface .…”

Optical absorption induced by small polaron formation in transition metal oxides: The case ofCo3O4

“…Unlike the distinct signature of SPs on the carrier conduction discussed above, the effect of polarons on electronic structure and optical transitions in TMOs is rather complex and difficult to elucidate. For example, in several TMOs, such as WO 3 [22,23], TiO 2 [24][25][26] and SrTiO 3 [27,28], the presence of large polarons that are delocalized over several unit cells may lead to a strong band gap renormalization through electron-phonon coupling. By contrast, the formation of SPs may introduce isolated gap states away from the band edges due to their spatially localized nature, which could be easily misinterpreted as band edges that define the fundamental band gap.…”