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

The effect of surface steps on CuBr nucleation, growth, and sublimation was studied by comparing in situ variable temperature scanning tunneling micro scopy (STM) and temperature-programmed desorption (TPD) results for Cu(100) and Cu(11 1 0). Bromine chemisorption caused the Cu steps to facet paralle l 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 accumula te where the reaction took place but rather diffused across the surface, nu cleating and growing halide clusters independent of the reaction step. At r oom temperature, the halide formed flat (111)-oriented gamma-CuBr islands a t step facet corners. The islands grew by preferential addition of CuBr to {100} island edges, resulting in a triangular shape during growth that rela xed to an asymmetric hexagonal shape when growth was stopped. The halide di d not block further reaction of the underlying Cu; STM movies showed Cu ste ps being consumed beneath halide layers. High-resolution images of CuBr isl ands revealed a (1 x 1) periodicity, whereas images of multilayer films dis played a (2 x 2) periodicity attributed to an ordered vacancy structure tha t maintains neutrality of the polar (111) surface. With time, or on anneali ng, the two-dimensional islands roughened into three-dimensional clusters. Although the high step density of Cu(11 1 0) did not substantially alter th e nucleation, growth, relaxation, or structure of the CuBr, when Br-2 was d osed 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 s tructure. 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. (C) 1999 Elsevier Scie nce B.V. All rights reserved.