Probe microscope observation of platinum atoms deposited on the TiO2(110)-(1 x 1) surface.

Titanium dioxide (TiO2) (110) surfaces with Pt adatoms were examined using a noncontact atomic force microscope (NC-AFM) and a Kelvin probe force microscope (KPFM). Topographic images with NC-AFM identify Pt atoms adsorbed at three different sites. These sites are on the Ti atom rows, on the O atom rows, and in O atom vacancies. Most Pt adatoms were observed on Ti atom rows. Successively recorded images show that the Pt adatoms on Ti atom rows (adatoms A) and O atom rows (adatoms C) are mobile while the adatoms in the O atom vacancies (adatoms B) are not. Adatoms A and adatoms B were identified in KPFM images. However, adatoms C were not visualized in KPFM images because they moved quickly or were swept out by the tip. The KPFM measurements reveal that the work function on adatoms A are lower than that on the surrounding (1 x 1) surface by 0.24 eV whereas adatoms B reduced the work function by 0.26 eV. The work function decrease is interpreted with an electric dipole moment directed toward the vacuum, as a result of electron transfer from the adatoms to the TiO2 substrate. In an O atom vacancy, the adatom B is in contact with two Ti atoms and therefore the electron transfer can be enhanced.