Three-Ni-atom cluster formed by sulfur adsorption on Ni(1 1 1)

Sulfur adsorption on Ni(1 1 1) at room temperature has been studied by scanning tunneling microscopy. Irregularly-shaped islands, which are aggregates of small particles, appear with troughs on (1 1 1) terraces. Estimating the number of small particles in the irregular islands and that of Ni atoms ejected through the formation of troughs, we find that each small particle contains three Ni atoms. A three-Ni-atom cluster is proposed, where three S atoms are adsorbed on 4-fold hollow sites formed on the sides of the cluster. The Ni3S3 cluster can assume two orientations on the (1 1 1) surface. We show that the apparently-irregular distribution of small particles is well understood by assuming the alternate orientation of neighboring Ni3S3 clusters and the slight energy difference between the two orientations of a single Ni3S3 cluster. Real-time observation of a Ni(1 1 1) surface under H2S ambience reveals that small islands composed of three or four Ni3S3 clusters change their positions rapidly, preventing the simple growth of islands at the initially-nucleated positions.