Scanning tunneling microscopy atomic resolution images of sulfur overlayers on Fe(111)

Abstract

We report the first atomic resolution scanning tunneling microscopy (STM) images of S overlayers on the Fe(111) surface. S overlayers were obtained by annealing the Fe(111) crystal to elevated temperatures to induce the segregation of S from the bulk. STM images of the (1×1)-S structure are consistent with the proposed model of one “geometric” monolayer of S atoms occupying on-top three-fold hollow sites of the Fe(111) surface. The STM data also revealed the presence of nanoscopic triangular pits on the (1×1)-S surface. These pits are only one atom deep. Increased segregation of S results in the formation of a (2√3 ×1)R30° structure and an increase in the size and depth of the triangular pits. This new structure corresponds to S coverage corresponding to more than one “geometric” monolayer of S based on one geometric monolayer coverage for the (1×1)-S structure. STM images obtained within large pits reveal a periodic “staircase” topography consisting of terraces with (111) orientation. These terraces are made up of five atomic rows (14 Å) separated by monatomic steps. Images obtained on flat areas in between large pits reveal surface buckling. Two different packing arrangements of surface buckling were observed both consisting of vertically displaced atomic rows with a 14 Å periodicity, identical to the terrace widths of the staircase surface found inside large triangular pits. We propose that additional segregation of S to the (1×1)-S phase to form the (2√3 ×1)R30° structure involves the segregation of S to the subsurface three-fold hollow sites on the Fe(111) surface. The close proximity of S atoms located at on-top and subsurface three-fold hollow sites can result in strong S–S repulsive interactions which consequently drives the surface to undergo structural changes, similar to other reported adsorbate-induced faceting of bcc(111) surfaces.