STM study of the (111) and (100) surfaces of PdAg

The (111) and (100) surfaces of the Pd67Ag33 alloy have been imaged with at omic resolution by scanning tunneling microscopy. On the (111) surface, it was found that Pd atoms appear in the images about 25 pm higher than the Ag atoms. The surface concentration of palladium was determined as a function of annealing temperature (720-920 It) and was found to vary between 5 and 11%. Analysis of the relative positions of the Pd atoms showed a tendency t owards formation of isolated palladium sites. On the (100) surface, the pal ladium concentration in the first layer is extremely low and the system has to be forced into a nonequilibrium state to find palladium atoms in the fi rst monolayer. Chemical contrast here amounts to a 60 pm apparent height di fference. (C) 1998 Elsevier Science B.V. All rights reserved.