Observation of Two-Dimensional Phases Associated with Defect States on the Surface of TiO2

Surface electronic states associated with defects produced by Ar-ion bombardment of Ti${\mathrm{O}}_2$ crystals have been studied by ultraviolet-photoemission, electron-energy-loss, and Auger spectroscopy and low-energy-electron diffraction. Evidence is given for three distinct phases dependent upon the extrinsic surface-defect concentration. Phase I contains isolated surface states 0.7 eV below the conduction band, phase II is associated with the creation of ${\mathrm{Ti}}^{3+}$ pairs, and phase III consists of a surface layer of ordered ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$.