S-induced faceting transformations on Fe(111)

Abstract

We report previously unobserved structural transformations on the Fe(111) surface with respect to segregation of S studied by a combination of STM, LEED and AES. An evolution of different surface topographies and LEED patterns was observed as a function of S coverage. At low coverages of S and other impurities of N and O, a complex “clock” LEED pattern was observed along with a faceted surface consisting of triangular terraces, mounds and sawtooth structures. The terraces display a typical width of ∼1400 Å separated by multi-height steps as high as ∼50 Å. The clock pattern appears only after annealing the crystal to temperatures above 520 K. Annealing of the crystal at higher temperatures (≥700 K) induced increased segregation of bulk S resulting in the transformation to a (1×1)-S LEED pattern. In contrast to the multi-height steps observed for the clock surface, STM measurements on the (1×1)-S phase revealed a planar topography consisting of monatomic steps spaced at approximately 100 Å from each other. Upon further annealing of the crystal, additional S was driven to the (1×1)-S surface resulting in the transformation to a new LEED pattern corresponding to a (2 3 ×1)R30° structure. STM images for the (2 3 ×1)R30° structure revealed that the steps have coalesced to reform the multi-height steps and wide terraces similar to that observed on the clock surface. In addition to the coalescence of the steps, an unusual faceting behavior was observed for this high-S coverage phase, consisting of numerous triangular pits of varying sizes with edges aligned along the 〈110〉 directions of the surface. Within large triangular pits, a very periodic “staircase” topography was observed. The pits are as deep as 50 Å with edges as long as 800 Å. Our data show that S, at varying coverages, interacts very strongly with the steps of the Fe(111) surface.