A SCANNING TUNNELING MICROSCOPY STUDY OF THE INTERACTION OF S WITH THE CU(111) SURFACE

The interaction of sulfur with the Cu(111) surface has been studied with scanning tunneling microscopy in conjunction with low-energy electron diffraction. Two different phases have been observed. At low exposures, approximately 20 langmuir of H2S at room temperature, a previously unknown commensurate phase, which is present in six equivalent domain orientations, has been identified as \-4 1\1 4\ in matrix notation. At higher exposures, a phase transition to a (square-root 7 X square-root 7)R19-degrees structure takes place. During formation, Cu atoms are consumed from step edges. This indicates that the observed phases are not simple S overlayer structures, but involve a reconstruction of the Cu surface to a sulphide overlayer. By measuring the step area consumed as a function of exposure, the number of Cu atoms in the unit cells of the two phases has been determined. The concentration of the S atoms in the (square-root 7 X square-root 7)R19-degrees phase has been measured with Rutherford backscattering.