Sulfur on (formula presented) studied with resonant photoemission

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Adsorption of sulfur on (formula presented) at room temperature (RT) and (formula presented) has been studied with ultraviolet photoelectron spectroscopy. A (formula presented) (formula presented) surface with a small amount of oxygen vacancies was prepared by sputtering and annealing in ultrahigh vacuum. Oxygen vacancies induce a defect state that pins the Fermi level just below the conduction-band minimum. Sulfur adsorption at room temperature leads to the disappearance of this vacancy-related band-gap state, indicating that the surface oxygen vacancies are filled by sulfur. Sulfur-induced valence-band features are identified at binding energies of 3.4 and 8 eV. Adsorption of S at (formula presented) forms a (formula presented) superstructure at high coverages (formula presented) monolayer (ML)] that is visible with low-energy electron diffraction. In a previously proposed model for this superstructure, sulfur replaces half of the in-plane oxygen atoms and all the bridging oxygen atoms are removed. In agreement with this model, the oxygen (formula presented) peak is decreased significantly and the defect state is increased. Two additional valence features are observed: one at 2.7 eV and one at 3.9 eV. Due to those features the band gap vanishes. In resonant photoemission, these features show a similar, but weaker, resonance profile than the vacancy-related defect state. Hybridized Ti-derived states extend across the whole valence-band region. Generally, a higher resonant photon energy is found for valence-band states with lower binding energies, indicating mainly (formula presented) transitions in the upper valence band. Adsorption of sulfur reduces the strength of the resonances. © 2001 The American Physical Society.

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Physical Review B - Condensed Matter and Materials Physics

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