The interaction between gold and sulfur is an important topic in catalysis, electrochemistry, and also in nanotechnology. On Au(111) surface, which is one of the most popular substrates in surface science, several investigations have been carried out to clarify the adsorption process of sulfur [1]. It has been well known that the adsorption of very small amount of sulfur under UHV condition causes destruction of "herringbone" structure. However, the detail of initial stage of sulfur adsorption is still not clear. This is partly because sulfur atoms on Au(111) surfaces are mobile at room temperature [2].
We have applied scanning tunneling microscopy (STM) and local barrier height (LBH) measurements on the initial stage of sulfur adsorption on Au(111) at 77K. Beside single sulfur atoms adsorbed on fcc stacking region, we found the several types of clusters. The LBH at these clusters decreases ~0.1eV, which indicates that the electric dipoles pointing outward are induced at these clusters. Since the interaction between gold and sulfur results in partial charge transfer from gold to sulfur, the results of LBH measurements probably indicate that these clusters include gold adatoms that are interacting with sulfur atoms. We count the number of clusters and compare with the number of gold atoms that should be ejected from the surface during the transition from "herringbone" structure to 1x1. The estimated number of gold atoms in these clusters is about three in average, which reasonably agree with the STM image of these clusters.
Interestingly, we found that the diffusion process of sulfur atoms can be observed by taking consecutive STM images at 77K. The obtained STM images clearly show that the diffusion of sulfur is accompanied with local modification of dislocation lines in the herringbone structure. On Au(111) reconstructed surface, it is believed that the dislocation lines and the adsorbates can interact each other through the strain field. A detailed analysis for the sulfur diffusion on Au(111) surface will be presented.
[1] J. Rodriguez et al., J. AM. CHEM. SOC., 125 (2003) 285.
[2] M Biener et al., Langmuir, 21 (2005) 1668.
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