Control of two-dimensional electronic states at anatase TiO2 (001) surface by K adsorption

The nature of the intriguing metallic electronic structures appearing at the surface of anatase titanium dioxide (a-TiO2) remains to be elucidated, mainly owing to the difficulty of controlling the depth distribution of the oxygen vacancies generated by photoirradiation. In this study, K atoms were adsorbed onto the (001) surface of a-TiO2 to dope electrons into the a-TiO2 and to confine the electrons in the surface region. The success of the electron doping and its controllability were confirmed by performing in situ angle-resolved photoemission spectroscopy as well as core-level measurements. Clear subband structures were observed in the surface metallic states, indicating the creation of quasi-two-dimensional electron liquid (q2DEL) states in a controllable fashion. With increasing electron doping (K adsorption), the q2DEL states exhibited crossover from polaronic liquid states with multiple phonon-loss structures originating from the long-range Frohlich interaction to "weakly correlated metallic" states. In the q2DEL states in the weakly correlated metallic region, a kink due to short-range electron-phonon coupling was clearly observed at about 80 +/- 10 meV. The characteristic energy is smaller than that previously observed for the metallic states of a-TiO2 with three-dimensional nature (similar to 110 meV). These results suggest that the dominant electron-phonon coupling is modulated by anisotropic carrier screening in the q2DEL states.