Adsorption of sulfur on TiC(001): Photoemission and first-principles studies

摘要

High-resolution photoemission, thermal desorption mass spectroscopy, and first-principles density-functional slab calculations were used to study the interaction of sulfur with a TiC(001) surface. At small coverages (<0.3 ML), atomic sulfur is present on the TiC(001) substrate. A saturated overlayer at 300 K contains S and molecular S_2. For 100-K deposition, a multilayer of sulfur can be grown and it contains S_8, S_4, and S_2 species. These species desorb at temperatures between 250 and 500 K, leaving only atomic sulfur on the carbide surface. The adsorption of sulfur (0.3-0.8 ML) induced small positive shifts (0.1-0.25 eV) in the Ti 2p core levels, while a big positive shift (1.0-1.3 eV) was detected for the C 1s core level. These photoemission studies indicate the existence of strong S reversible C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CTiTi hollow as the most stable site for the adsorption of sulfur. The S reversible C interactions determine the type of sulfur species present on the surface. In general, the bond between S and the TiC(001) surface is best described as covalent, with a small degree of ionic character. The deposition of sulfur induces a significant reduction in the Ti density of states near the Fermi level.