Au Enrichment And Vertical Relaxation Of The Cu3au(111) Surface Studied By Normal-Incidence X-Ray Standing Waves
PHYSICAL REVIEW B(2016)
摘要
We have investigated the Cu3Au(111) surface, prepared under ultrahigh vacuum conditions by sputtering and annealing, by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and normal incidence x-ray standing waves (NIXSW). We find the surface to be depleted with Cu and enriched with Au at the same time, yielding a nominal Cu: Au ratio of 0.61:0.39 in the topmost layer. The STM images reveal that the first layer is nearly closely filled with atoms and contains a small amount of vacancies with an area concentration of about 5%. Together with the Au enrichment, these cause local short-range disorder of the Au p(2 x 2) reconstruction. From this data, the average stoichiometry of the p(2 x 2) surface unit cell is estimated at Cu2.22Au1.44 square(0.20) (instead of Cu3.00Au1.00 square(0.00) of the ideal surface; square denotes an atomic vacancy site). From NIXSW we find a significant outward relaxation of both the Cu and Au atoms of the topmost layer by 0.28 angstrom and 0.33 angstrom, which corresponds to 13% and 15% of the (111) bulk layer spacing of Cu3Au. We suggest that this originates from a widening of the first/second layer spacing, by 6.8% and 8.8% for the Cu and Au atoms, respectively, plus an additional rigid increase in the second/third layer spacing by 6.2%. We explain this by steric repulsions between Au atoms of the topmost layer, replacing smaller Cu atoms, and Au atoms in the second layer in combination with disorder. Finally, a lateral reconstruction, similar to that on the Au(111) surface, but with a much larger periodicity of 290 angstrom, is identified from LEED.
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