An STM, AES and LEED study of the segregated sulfur on Pd(111)

At elevated temperatures, 700 K, sulfur segregates to the Pd(111) surface. The coverages as measured by Auger electron spectroscopy (AES) are in the range of 0.1 to 0.25 monolayers. The LEED pattern represents a clear (root 7 x root 7) R 19.1-S degrees structure. With the scanning tunneling microscope (STM) the root 7 structure is found on parts of the surface only. The position of the adsorbed S atoms is the fee threefold hollow site. Beside the root 7 structure, part of the surface is covered by a disordered layer of sulfur. A sizeable part of the surface is covered by sulfur in a (2 x 2) structure. The adsorbate in that structure is mesoscopically ordered in triangular 2D clusters with a rather narrow size distribution. The triangles contain between 15 to 66 sulfur atoms. The [110] edges of the Pd(111) surface are 'decorated' with sulfur, also in a (2 x 2) structure. These structures run parallel to the steps with a width of 4 to 5 atoms.