Ultrahigh\nConcentration Hydrogen Doping into TiO<sub>2</sub>

We\ninvestigate ultrahigh concentration doping of hydrogen (H) into\nrutile-TiO<sub>2</sub> (100) single crystals by low-energy (2.5 keV)\nhydrogen ion beam irradiation at low temperature (LT). While the hydrogen\nconcentration was limited to H<sub>0.3</sub>TiO<sub>2</sub> at 300\nK, <i>in situ</i> nuclear reaction analysis (NRA) revealed\nultrahigh concentration doping of hydrogen up to H<sub>1.2</sub>TiO<sub>2</sub> by the LT irradiation at 50 K. The large (∼8.2%) expansion\nof the out-of-plane lattice constant suggests that hydrogen occupies\ninterstitial sites in rutile TiO<sub>2</sub>. Hydrogens of early stage\nirradiation act as electron donors and induce a large increase in\nconductivity, which is consistent with theoretical studies in the\ndilute limit. The nature of excess H was investigated <i>in situ</i> by transport and photoemission measurements. After LT excess H doping\nand postannealing to room temperature, unusual electrical transport\nproperties were observed while maintaining the ultrahigh H concentration. <i>In situ</i> photoemission measurements show that the excessively\ndoped hydrogens by LT irradiation generate a deeper in-gap state (IGS)\nof metastable nature. Density functional theory predicts the formation\nof double neighboring interstitial hydrogens as a possible mechanism\nfor the deeper IGS.