Subsurface impurities in Pd(111) studied by scanning tunneling microscopy

Low concentrations of three distinct impurity species beneath the Pd(111) s urface are studied by STM. The subsurface impurities are distinguished by t heir image contrast, diffusion properties, and interactions with adsorbed m olecules. Isolated subsurface impurities appear at low gap resistance (less than or similar to5 M Omega) as three-fold symmetric modulations of the Pd 1x1 surface corrugation. One impurity type is found to occupy substitution al sites in the layer below the surface. Based on Auger spectroscopy this s pecies is identified as sulfur. The other two species are found to occupy o ctahedral interstitial sites immediately below the surface layer. Two-dimen sional diffusion of the interstitial impurities occurs below room temperatu re. The onset temperature for diffusion is lowered dramatically in the pres ence of surface adsorbates. Quantitative measures of the diffusion barriers are consistent with surface facilitated diffusion of interstitial oxygen a nd carbon atoms. The mobile impurities interact with adsorbed atoms and mol ecules, limiting surface diffusion, nucleating island growth, and serving a s active sites for surface reactions. (C) 2001 American Institute of Physic s.