Binding and mobility of atomically resolved cobalt clusters on molybdenum disulfide

We present the results of an ultrahigh vacuum, low-temperature scanning tun neling microscopy investigation of cobalt clusters adsorbed on the surface of single-crystal molybdenum disulfide. This system is of interest as a mod el for understanding industrial hydrotreating catalysts. We observe small m etal clusters that bind on the MoS2 basal plane, and determine from atomica lly resolved images that these clusters are composed of cobalt atoms bound exclusively atop surface sulfur atoms. Small clusters are also observed nuc leated near surface defects. Larger cobalt clusters are observed primarily bound at MoS2 surface defects, and are additionally observed near and on su rface steps. Cobalt clusters on the MoS2 surface are extremely labile and c hange their size and shape on the time scale of imaging. Individual atoms a re observed to adjoin to and detach from clusters, possibly under the influ ence of the probe tip; under appropriate tunneling conditions, the tip will sweep all clusters from a scanned area. The low barrier to diffusion for a dsorbed cobalt on MoS2 is in accord with earlier measurements of nickel on MoS2, and supports conclusions drawn about the action of promoter species i n hydroprocessing catalysis.