Scanning Tunneling Microscopy Visualization of Polaron Charge Trapping by Hydroxyls on TiO(110).

Using scanning tunneling microscopy (STM), we investigate the spatial distribution of the bridging hydroxyl (OH) bound excess electrons on the rutile TiO(110) surface and its temperature dependence. By performing simultaneously recorded empty and filled state imaging on single OHs at different temperatures in STM, we determine that the spatial distribution of the OH bound excess electrons retains a symmetric four-lobe structure around the OH at both 78 and 7 K. This indicates that OHs are much weaker charge traps compared to bridging O vacancies (O-vac). In addition, by sequentially removing the capping H of each OH using voltage pulses, we find that the annihilation of each OH is accompanied by the disappearance of some lobes in the filled state STM, thus verifying the direct correlation between OHs and their excess electrons.