IMAGING OF SULFUR OVERLAYER STRUCTURES ON THE PD(111) SURFACE

We have obtained atomic resolution STM images of both the (square-root 3 x square-root 3)R30-degrees and the (square-root 7 x square-root 7)R19-degrees sulfur overlayer lattices formed on the Pd(111) surface. The images of the (square-root 3 x square-root 3)R30-degrees overlayer are very simple and are consistent with the structure previously determined from LEED analysis. The images of the (square-root 7 x square-root 7)R19-degrees structure on the other hand are surprisingly complex. Within the unit cell which contains ten atoms (seven Pd and three S atoms) we are able to resolve seven atomic scale features. We are faced with the problem of determining which are Pd and which are S. On the basis of comparison of the positions of features in the images with positions of atoms in the Pd(111) surface we propose that two of the sulfur atoms have become incorporated into the subsurface exposing four Pd atoms.