Formation of two-dimensional sulfide phases on Al(111): an STM study

Abstract

The structure and growth of incommensurate phases, formed by dissociation of H2S on Al(111) at 350 or 570 K, were studied by scanning tunneling microscopy. Various hexagonal structures were found, with lattice constants decreasing from 3.55 to 3.47 Å and rotational angles increasing from 0° to 8° with respect to the [11̄0] direction of the substrate. This points to rotational epitaxy, indicative of a flat interaction potential in relation to strong interactions within the adlayer. The exact structure of this phase varies slightly across the surface, local variations are introduced by edge dislocations or other defects. The adlayer nucleates at steps, after which triangular islands with very stable edges grow on the terraces. The initial nuclei are distinctly different from the islands, appearing much higher in the STM images. Larger coverages initiate restructuring and facetting of the Al substrate, indicating mass transport during formation of the adlayer. Our observations provide strong evidence for a composite, sulfide-like structure with strong internal interactions which involves a reconstruction of the substrate. The misfit of this hexagonal multilayer structure and the substrate leads to the moiré pattern observed by STM. The relatively flat interaction potential of this composite adlayer with the substrate allows various rotational orientations, depending on the exact, local coverage and adlayer lattice parameters.