Surface Structures of Ultrathin TiOx Films on Au(111)

Ultrathin films of titanium oxide were grown on (22 x root 3)-reconstructed Au(111) surfaces by Ti evaporation in ultrahigh vacuum (UHV). Following Ti deposition onto room temperature Au substrates, the samples were oxidized at 600 degrees C in 10(-6) Pa O-2. Scanning tunneling microscopy (STM) images show that three different ordered TiOx film structures form as the amount of deposited Ti is increased. Auger electron spectroscopy (AES) was used to measure the stoichiometry of the films. The first structure occurs for Ti surface coverages of <0.5 monolayer (ML), and exhibits a (2 x 2) reconstruction that has 8 hexagonal symmetry and is termed the honeycomb structure. This (2 x 2) structure has Ti2O3 stoichiometry. The second structure arises after depositing 0.5 ML - 1.8 MLs of Ti and resembles a pinwheel shape. The pinwheel structure forms a (root 67 x root 67)R12.2 degrees Moire pattern, and it tends to coexist with the (2 x 2) reconstruction. The third structure occurs for >0.5 ML Ti depositions and forms triangular shaped islands with a measured stoichiometry of TiO1.3. Further increasing the amount of deposited Ti causes film growth to continue via the coalescence of the triangular islands. Atomic models are proposed for the honeycomb and pinwheel structures.