The structure of sulfur adlayers at Cu(110) surfaces: an STM and XPS study

Abstract

A detailed understanding of the sulfur phases formed by the dissociation of hydrogen sulfide and methyl mercaptan at Cu(110) surfaces has been obtained in a combined STM/X-ray photoelectron spectroscopy study. At low concentrations adsorbed sulfur atoms are mobile at room temperature, forming the ordered c(2×2) structures only when the concentration of sulfur reaches approximately 3.8×10¹⁴ cm⁻². Surface oxygen inhibits sulfur diffusion and ordered c(2×2) sulfur structures are formed at much lower sulfur concentrations in its presence, the sulfur also influences the oxygen structure inhibiting the formation of large oxygen islands. Sulfur concentration (σ_s) limits for the c(2×2) (σs<4.4×10¹⁴ cm⁻²), p(5×2) (4.4×10¹⁴ cm⁻²<σ_s<6.6 ×10¹⁴ cm⁻²) and the p(3×2) (σ_s=7.1×10¹⁴ cm⁻²) sulfur phases have been established. Models, involving surface buckling, have been suggested for these structures which reconcile the previous LEED, STM and SEXAFS data. The adsorption of methyl mercaptan at 295 K results in mercaptide formation but no ordered structures; a herringbone type structure is, however, observed when methyl mercaptan is adsorbed at 70 K and then warmed to room temperature. On heating to 450 K mercaptide dissociates with the complete desorption of carbon as hydrocarbons, providing an alternative route to high chemisorbed sulfur concentration structures. On the basis of XP spectra of methyl mercaptan and a correlation between the XP data and the STM images, the total photoionisation cross-section of the S 2p orbital reported in the literature has been shown to be in error and a new value (2.10, relative to the C 1s cross-section) derived.