Scanning tunneling microscopy study of halide nucleation, growth, and relaxation on singular and vicinal Cu surfaces

Abstract

The effect of surface steps on CuBr nucleation, growth, and sublimation was studied by comparing in situ variable temperature scanning tunneling microscopy (STM) and temperature-programmed desorption (TPD) results for Cu(100) and Cu(11 1 0). Bromine chemisorption caused the Cu steps to facet parallel to 〈100〉 directions. The faceted steps supplied Cu atoms for the reaction with Br 2 to form CuBr. STM movies showed that the halide did not accumulate where the reaction took place but rather diffused across the surface, nucleating and growing halide clusters independent of the reaction step. At room temperature, the halide formed flat (111)-oriented γ-CuBr islands at step facet corners. The islands grew by preferential addition of CuBr to {100} island edges, resulting in a triangular shape during growth that relaxed to an asymmetric hexagonal shape when growth was stopped. The halide did not block further reaction of the underlying Cu; STM movies showed Cu steps being consumed beneath halide layers. High-resolution images of CuBr islands revealed a (1×1) periodicity, whereas images of multilayer films displayed a (2×2) periodicity attributed to an ordered vacancy structure that maintains neutrality of the polar (111) surface. With time, or on annealing, the two-dimensional islands roughened into three-dimensional clusters. Although the high step density of Cu(11 1 0) did not substantially alter the nucleation, growth, relaxation, or structure of the CuBr, when Br 2 was dosed at 325 K, a low temperature CuBr TPD peak was observed for Cu(11 1 0) but not Cu(100). This low temperature peak disappeared when the CuBr films were roughened by increasing the reaction temperature to 380 K, suggesting that the low temperature peak is associated with the two-dimensional film structure. Thus, the low temperature peak was attributed to the steps on the Cu(11 1 0) surface introducing defects into the CuBr films because of the large mismatch between the Cu and CuBr step heights.