Identification of adatoms on metal surfaces by STM: experiment and theory

The possibility of identifying the chemical nature of atomic adsorbates (C, N, O, F and S) on metal surfaces has been explored both theoretically and experimentally. It is shown that, although the tip and the surface may markedly influence the appearance of adatoms in STM images, atomic size and electronegativity are the dominant factors that determine the contrast. In this work, low-temperature (<40 K) STM is used to suppress adatom diffusion on the Pd(lll) surface and to image isolated adsorbates at low coverages. It is found experimentally that isolated carbon and sulfur atoms on the Pd(ll) surface scanned by a platinum tip appear as bumps with a height of about 0.3-0.4 Angstrom and 0.8 Angstrom, respectively, whereas oxygen atoms are characterized by a depression with a negative corrugation of -0.35 Angstrom. To simulate STM images of adatoms on metal surfaces, we have applied Green's function formalism to solve the Schrodinger equation in the tight binding approximation. It is shown that highly electronegative atoms such as O and F always appear as depressions. As the electronegativity decreases (from F to C), the depression transforms into a shallow dimple with a slight bump in the middle in the case of N, and into a well-pronounced bump for C. Third-row adsorbates (Na through Cl) are also calculated to produce bumps, with a corrugation that does not correlate simply with the adatom-surface spacing. The shape and size of the image corrugations predicted theoretically are in good agreement with the experimental data. (C) 1998 Published by Elsevier Science B.V. All rights reserved.