Low-energy electron diffraction, scanning tunneling microscopy, and photoelectron spectroscopy results from the submonolayer Sm-and Yb-induced surface structures are presented. Several similar metal-induced surface reconstructions are found to exist for Yb and Sm on Si(111)for low submonolayer coverages: 3 X 2, 5 X 1, and 7 X 1. At higher submonolayer coverage, Yb induces a 2 X 1 reconstruction while Sm induces a (&3X&3)R30'-like reconstruction. Yb is found to be divalent in all structures, whereas the Sm valence increases with increasing coverage. In the 3X2 structure only divalent Sm is present, in the 5 X 1 and 7X 1 structures a small amount of trivalent Sm appears, and, finally, in the (&3X &3)R 30 structure approximately half of the Sm atoms are trivalent. The surface Fermi-level position in the band gap for the different Sm and Yb reconstructions has been measured. The difference in valence stability between Sm and Yb is suggested to be the cause of the difference in the high-coverage structures found and the differences in pinning level for the two elements observed for the 5 X 1 and 7 X 1 structures.
Scanning tunneling microscopy has been performed on the reconstructed polar surface of substoichiometric VC08O(111). A mixture of (8X1) and (&3X&3)R30' reconstructed areas was found. The (8X1) periodicity could be determined to be the result of a square-lattice surface layer superimposed on the hexagonal substrate. As this square lattice must have its origin in strong and directed in-0 plane bonds with the relatively large length of about 2.9 A, it can be deduced to consist of vanadium atoms. Lateral positions of these vanadium surface atoms with respect to the substrate are suggested from the measured surface corrugation. The (&3X&3)R30' structure was found in small triangular areas which can, due to the measured step heights between the two reconstructions, be believed to be carbon terminated. The occurrence of a reconstructed surface with a reduced atomic concentration is in contrast to what is known for TiC(111) and NbC (111), both having stable (1X1) surfaces. A qualitative explanation for this difference is suggested.
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