INVESTIGATION OF THE STRUCTURES OF SULFUR ON MO(100) BY SCANNING-TUNNELING-MICROSCOPY

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 X 2), /2/1 1/1BAR/, c(4 X 2), and p(2 X 1) low energy electron diffraction (LEED) patterns. Only the c(2 x 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 X 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 X 1) LEED pattern did not have p(2 X 1) symmetry, but consisted of small domains of the c(4 X 2) structure.