Abstract
The structures of the four ordered overlayers of sulfur on the Mo(100) surface were investigated by scanning tunneling microscopy (STM). In order of increasing sulfur coverage, the overlayers have c(2×2), | 12 11̄ |, c(4×2), and p(2×1) low energy electron diffraction (LEED) patterns. Only the c(2×2) overlayer geometry has been determined from LEED I(V) analysis. An examination of point and line defects and domain boundaries in STM images provides information beyond that known from LEED on the required symmetries of the ordered overlayers. Several proposed structures were shown to be incompatible with these symmetries and therefore discarded. Only one model of the c(4×2) structure was found to be consistent with the symmetry of the STM images. This structure contains sulfur atoms occupying exclusively fourfold hollow sites. Unexpectedly, STM images of the surface with the p(2×1) LEED pattern did not have p(2×1) symmetry, but consisted of small domains of the c(4×2) structure.
Highlights
Molybdenum single crystals surfaces are of interest as a model for industrial molybdenum sulfide catalysts. 12 A number of studies[1,2,3,4,5,6,7,8,9,10] have been published on the sulfidedMo( 100) surface
In our attempt to determine the structure of these over layers we considered models with sulfur atoms in bridge and hollow sites
Structure are consistent with the results obtained from dynamic low energy electron diffraction (LEED) calculations
Summary
Molybdenum single crystals surfaces are of interest as a model for industrial molybdenum sulfide catalysts. 12 A number of studies[1,2,3,4,5,6,7,8,9,10] have been published on the sulfided. These studies have determined that sulfur forms several ordered structures as a function of coverage. Except for the c(2X2) ordered overlayer, the structures of the unit cells which produce these LEED patterns are unknown. IT li, the reported LEED patterns have c(2X2), c(4 X 2), and p(2X 1) symmetry These coverages correspond to one, two, three, and two atoms per unit cell, respectively. I( V) curves has not been obtained.[13] A previous STM study[14] of this structure in air favored a model with all sulfur atoms occupying an asymmetric site between hollow and bridge. In our attempt to determine the structure of these over layers we considered models with sulfur atoms in bridge and hollow sites. Proposed struc tures could be eliminated from consideration by showing them to be incompatible with observations of the symmetry of the overlayer and of point defects and domain bound aries
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