SCANNING-TUNNELING-MICROSCOPY ATOMIC-RESOLUTION IMAGES OF SULFUR OVERLAYERS ON FE(111)

We report the first atomic resolution scanning tunneling microscopy (S TM) images of S overlayers on the Fe(111) surface. S overlayers were o btained by annealing the Fe(111) crystal to elevated temperatures to i nduce the segregation of S from the bulk. STM images of the (1 x 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 F e(111) surface. The STM data also revealed the presence of nanoscopic triangular pits on the (1 x 1)-S surface. These pits are only one atom deep. Increased segregation of S results in the formation of a (2 roo t 3 x 1)R30 degrees structure and an increase in the size and depth of the triangular pits. This new structure corresponds to S coverage cor responding to more than one ''geometric'' monolayer of S based on one geometric monolayer coverage for the (1 x 1)-S structure. STM images o btained within large pits reveal a periodic ''staircase'' topography c onsisting of terraces with (111) orientation, These terraces are made up of five atomic rows (14 Angstrom) separated by monatomic steps. Ima ges obtained on fiat areas in between large pits reveal surface buckli ng. Two different packing arrangements of surface buckling were observ ed both consisting of vertically displaced atomic rows with a 14 Angst rom periodicity, identical to the terrace widths of the staircase surf ace found inside large triangular pits. We propose that additional seg regation of S to the (1 x 1)-S phase to form the (2 root 3 x 1)R30 deg rees structure involves the segregation of S to the subsurface three-f old hollow sites on the Fe(111) surface, The close proximity of S atom s located at on-toy and subsurface three-fold hollow sites can result in strong S-S repulsive interactions which consequently drives the sur face to undergo structural changes, similar to other reported adsorbat e-induced faceting of bcc(111) surfaces. (C) 1998 American Vacuum Soci ety.