IN-SITU STM STUDY OF TE UPD LAYERS ON LOW-INDEX PLANES OF GOLD

The structure of well-ordered atomic layers of underpotentially deposi ted (UPD) tellurium on Au(110), Au(100) and Au(111) was studied by in- situ scanning tunneling microscopy (STM). A series of largely commensu rate structures is observed, with a close correspondence between cover ages obtained from the STM images and the cyclic voltammetry assuming complete discharge of Te4+ during adsorption. A linear dependence of t he UPD peak current and the peak potential with the square root of the scan rate is observed indicating that the adsorption phase transition is associated with a two-dimensional, instantaneous nucleation proces s. The same structures are observed in sulfuric acid and perchloric ac id supporting electrolyte. The first UPD structure formed on Au(111) c orresponds to a pseudohexagonal packing of Te (0.42 ML) (ML = monolaye r) in a single domain superlattice structure with missing atom defects . The superlattice cell is (3 root 7 x 3 root 7)R19 degrees with respe ct to the constituent Te atom hexagonal cell and is consistent with th e (12 x 12) structure on Au(111) reported previously. The second UPD s tructure on Au(111) is a pseudo-morphic (1 x 1). The Te-substrate inte raction favors site specific adsorption, and dipolar repulsion within low-coverage layers is responsible for the open packing.