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 saturatio n of the chemisorbed layer, high adatom mobility prevented the c(2 x 2 ) structure from being imaged with STM. As the chemisorbed layer appro ached saturation, the surface steps faceted to align along (100) direc tions. Continued halogen exposure resulted in the formation of Cu(I) h alides. For both halogens, the reaction was found to cause step bunchi ng, suggesting that the reaction consumes Cu atoms from the steps caus ing them to recede. For Cl-2, a staggered pattern was observed in the receding steps that was attributed to the reaction starting at the cor ners of the step facets and then continuing by removing Cu atoms from kinks or jogs in the step position. This picture was supported by Mont e 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 hal ogen was exposed to the other halogen were performed to investigate th e origin of this difference between Cl-2 and Br-2. The results suggest ed that relaxation of the atomic positions adjacent to the steps can c reate sites that are highly reactive to Br-2, but that the reaction wi th Cl-2 is insensitive to such changes. (C) 1998 Elsevier Science B.V. All rights reserved.