Pressure dependence of the large-polaron transport in anatase TiO ₂ single crystals

Abstract

Anatase is a T iO 2 polymorph which is a 3.2 eV gap semiconductor interesting for several applications, including catalysis, photocatalysis, and, especially, dye-sensitized solar cells. Surprisingly, transparent single crystals of anatase grown in our laboratory show a metallic resistivity above 60 K which origin is a shallow donor level created by oxygen vacancies. The high value of the resistivity and its T 3 temperature dependence are the result of the polaronic nature of the charge carriers which is supported by the Seebeck coefficient (S). The application of hydrostatic pressure fails to close the donor level and to extend the conducting state to the entire temperature range. Instead, we have found a non-monotonic variation of the low temperature activation energy with applied pressure which is ascribed to the change of polaron's mobility. Thermo-electric power exhibits an unconventional temperature and pressure dependence shedding an additional light on the conductivity mechanism in this compound. The pressure dependence of S is governed by the transport of the large entropy associated with the polaron formation.

Facet-dependent trapping and dynamics of excess electrons at anatase TiO2 surfaces and aqueous interfaces

“…The degree of localization of the polaronic states has been extensively investigated in rutile, for which it is widely agreed that e − ex form localized Ti 3+ species with an associated energy level in the bandgap at ∼0.8-1 eV below the Fermi level (E F ) 2,4,10-13 . For anatase, on the other hand, evidence of both small and large electron polarons has been reported 11,[14][15][16][17] . Shallow donor levels associated with large polaronic states are found in bulk anatase 7,15 as well as on the anatase (001) surface (a-001, Fig.…”

“…For anatase, on the other hand, evidence of both small and large electron polarons has been reported 11,[14][15][16][17] . Shallow donor levels associated with large polaronic states are found in bulk anatase 7,15 as well as on the anatase (001) surface (a-001, Fig. 1b) 16 , whereas a deep gap state at ∼1 eV below E F is observed on anatase (101) (a-101, Fig.…”

Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles

“…On the one hand, the presence of a moderately large electron-phonon coupling in anatase TiO 2 has often been invoked to interpret experimental results naturally pointing to the polaronic (self-trapped) picture [7][8][9][10][11][12][13]. Notable examples include the low temperature green photoluminescence (PL) due to self-trapped excitons [14][15][16][17][18][19][20] and the room temperature electron mobilities whose values are limited by strong scattering with phonons [10]. On the other hand, many-body correlations have been thought to be negligible in this material and, as such, they remained widely unexplored.…”

Tunable Polaronic Conduction in AnataseTiO2

“…In particular, anatase has been recently suggested as a candidate for replacing the In-based technology for transparent conducting oxides [9] in a wide range of applications from solar cell elements, to light-emitting devices, to flat panels, to touch-screen controls [10]. The crucial quantity for the figure of merit in these devices is conductivity, and it is therefore of major interest to understand and control the electronic properties of pristine and doped anatase.Stoichiometric anatase is an insulator with a 3.2 eV band gap [11] but oxygen vacancies, typically present with concentrations in the 10 17 cm −3 range [12,13], create a shallow donor level ∼10 meV below the conduction band (CB) [14]. Since large single crystals became available for transport studies, a better insight has been gained on the influence of these donors on the electronic response of anatase.…”