SCANNING-TUNNELING-MICROSCOPY STUDY OF THE REACTION OF BR-2 WITH CU(100)

The adsorption of Br-2 on Cu(100) and the further reaction of Br-2 wit h Cu(100) to form CuBr was studied using scanning tunneling microscopy (STM). Although low energy electron diffraction indicated the formati on of a c(2 x 2) layer immediately upon the dissociative adsorption of Br-2, thermal fluctuations in the adsorbed layer prevented this struc ture from being observed until the coverage exceeded roughly 80% of sa turation. As the c(2 x 2) layer saturated, the substrate steps were ob served to rotate to align parallel to the (100) close-packed direction s of the adlayer. The rotated steps exhibited thermal fluctuations tha t STM movies showed were due to kink diffusion. Further Br-2 exposure greatly decreased the step fluctuations and resulted in the formation of bulk CuBr. The former was due to a decreased kink density that allo wed more Br atoms to be accommodated in the c(2 x 2) layer. The [100] steps were the predominant source of Cu atoms for the reaction to CuBr . As the reaction progressed the steps receded until they reached anot her step and formed a {110} facet. Defects such as kinks and domain bo undaries were not observed to contribute significantly to the reactivi ty of the steps. The reaction at the steps was not uniform; certain se gments of the steps were found to be far more reactive than others. An nealing the c(2 x 2) layer increased the lengths of the reactive segme nts, consistent with previous temperature programmed desorption data t hat showed that annealing increases the reactivity of the surface. Com parison of STM images of reactive and unreactive sections of the steps suggested that the variation in reactivity is due to a thermally driv en relaxation of the atomic positions around the step. The CuBr formed at the steps was mobile and aggregated into clusters at locations tha t showed no obvious correlation with the locations of changes in the s urface structure associated with consumption of the Cu surface, The im plications of these results on oxidation and etching reactions of meta l surfaces are discussed. (C) 1998 Elsevier Science B.V.