Competition between Itineracy and Localization of Electrons Doped into the Near-Surface Region of Anatase TiO2

The competition between itineracy and localization of electrons doped into the near-surface region of anatase TiO2 (a-TiO2) is examined by photoemission spectroscopy. Since a-TiO2 samples used in this study are naturally grown single crystals, some electrons derived from oxygen vacancies are inherently present within the bulk and result in both a metallic state just below the Fermi level (E-F) and a localized state tightly trapped by the oxygen vacancies (deep-trap state) within the energy gap. Additional electrons were doped to the surface by in situ deposition of potassium and by irradiation with high-intensity synchrotron radiation (SR). The potassium deposition does not cause a direct disturbance in the conduction pathway of the doped electrons, whereas the SR radiation produces oxygen vacancies in the pathway. Most of the doped electrons produced by depositing potassium contribute to a metallic state just below E-F and behave as the carriers responsible for the surface conductivity. The remaining fraction of electrons doped at the outermost a-TiO2 surface are loosely trapped by the deposited potassium ions and produce a quasi-localized state in the band gap (shallow-trap state). The shallow-trap state has a finite spectral intensity at E-F, so that it is considered to contribute in part to the surface conductivity. In the case where the number of oxygen vacancies generated by SR irradiation remains below a critical threshold, doped electrons are transferred into the nascent metallic state, in addition to the localized state within the band gap. The doped electrons in the vicinity of the oxygen vacancies near the surface are found to be more tightly trapped compared with those trapped by the potassium ions and hardly contribute to the conductivity. As the oxygen vacancies further increase, all the doped electrons near the surface are trapped and localized. Since such microscopic electronic behaviors of the doped electrons are closely related to the macroscopic conduction properties such as the mobility and dopability of electron carriers, our results on the electronic behavior of electrons doped into the near-surface region of a-TiO2 will be expected to contribute to improving the functionality of existing oxide devices and/or developing innovative technologies, which utilizes the change in physical properties near the surface.