Atomic-scale mechanisms of the halogenation of Cu(100)

The adsorption of Cl-2 on Cu(100) and the subsequent reaction to CuCl was studied using scanning tunneling microscopy (STM) and the results were compared with recent results for Br-2. Adsorption of Cl-2 and Br-2 was essentially identical. Low exposures of either halogen resulted in the formation of a c(2 x 2) low energy electron diffraction (LEED) pattern due to chemisorbed Cl or Br. Below similar to 70% of saturation of the chemisorbed layer, high adatom mobility prevented the c(2 x 2) structure from being imaged with STM. As the chemisorbed layer approached saturation, the surface steps faceted to align along (100) directions. Continued halogen exposure resulted in the formation of Cu(I) halides. For both halogens, the reaction was found to cause step bunching, suggesting that the reaction consumes Cu atoms from the steps causing them to recede. For Cl-2, a staggered pattern was observed in the receding steps that was attributed to the reaction starting at the corners of the step facets and then continuing by removing Cu atoms from kinks or jogs in the step position. This picture was supported by Monte Carlo simulations. In contrast, for Br-2 there was no evidence that kinks were more reactive than other sites along the steps. Sequential exposure experiments in which a saturated chemisorbed layer of one halogen was exposed to the other halogen were performed to investigate the origin of this difference between Cl-2 and Br-2. The results suggested that relaxation of the atomic positions adjacent to the steps can create sites that are highly reactive to Br-2, but that the reaction with Cl-2 is insensitive to such changes. (C) 1998 Elsevier Science B.V. All rights reserved.