Electron Traps in Rutile TiO₂ Crystals: Intrinsic Small Polarons, Impurities, and Oxygen Vacancies

Abstract

Rutile TiO₂ is well known for its ability to “trap” photoinduced electrons at Ti⁴⁺ ions and form Ti³⁺ ions with an unpaired d¹ electron. This has been shown experimentally to result in a large family of similar, yet slightly different, Ti³⁺-related centers that include both intrinsic small polarons and donor-bound small polarons. In these latter centers, the Ti³⁺ ion is located next to an oxygen vacancy or an impurity such as fluorine, lithium, or hydrogen. These small polarons are easily formed in commercially available bulk single crystals of rutile TiO₂ by illuminating oxidized (and nominally undoped) samples at temperatures between 5 and 30 K with sub-band-gap laser light (e.g., 442 nm) or by slight reducing treatments (in the case of hydrogen). Once formed, the ground states of the defects are readily studied at low temperature with magnetic resonance (EPR and ENDOR). Single crystals of rutile TiO₂ provide complete sets of angular dependence data, and thus allow detailed information about the ground-state models of the electron traps to be extracted in the form of g matrices and hyperfine matrices. In this review, the differences and similarities of the various Ti³⁺-related trapped electron centers are described.