The electronic structures, adsorption geometries, chemisorption energies, and vibrational frequencies of single Cu and Ag atoms on Si(111) surfaces are determined by self-consistent total energy calculations using first principles, local density functional theory, with a numerical basis for a cluster of 20 Si atoms. The binding energy results reveal that both Cu and Ag adsorb in threefold hollow sites with equilibrium heights of 0.74 Å (Cu) and 1.48 Å (Ag) above the plane of the surface Si atoms. The adsorption energies are found to be 92 kcal/mol for Cu and 72 kcal/mol for Ag. Assuming a rigid substrate, the calculated frequencies of the perpendicular vibrational modes are 58 cm -1 for Cu and 90 cm-1 for Ag. The lateral diffusion barriers, assuming an unreconstructed rigid Si(111) surface, are found to be 12 and 8 kcal/mol for Cu and Ag, respectively. Calculations for Cu and Ag atoms being moved towards the interior of the cluster, including geometric relaxation of the nearest-neighbor Si atoms, demonstrate that Cu has a much lower vertical penetration barrier than Ag (4 vs 53 kcal/ mol). Therefore, at elevated temperatures, Cu can be expected to penetrate through the silicon surface, whereas Ag should remain above the surface Si atoms. Adsorbate-induced electron density differences indicate that Cu weakens the bonds between surface and subsurface silicon atoms, while Ag has a significantly smaller effect. Contour maps of eigenfunctions, which are associated with surface states, show that the dangling bonds of the silicon atoms at the surface interact with the metal s and d orbitals. The Cu 3d orbitals interact stronger than the Ag 4d electrons. The results suggest that the catalytic activity of Cu and the absence of activity of Ag in the syntheses of methylchlorosilanes ("direct process") is possibly due to the ability of Cu to penetrate into the surface thus forming the initial stages of a copper-silicide, whereas Ag stays at the surface and desorbs at higher temperatures.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry