Site specific nucleation of Ni nanoclusters on S(4 × 4)/W(1 1 1)

The nucleation and growth of ultrathin nickel films on a highly corrugated S(4 × 4)/W(1 1 1) surface are studied by ultrahigh vacuum-scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). We find that at Ni coverages <0.1 ML, Ni nanoclusters are observed to nucleate preferentially at characteristic threefold hollow sites on S(4 × 4)/W(1 1 1) and exhibit self-limiting growth at room temperature. There is a clear energetic preference for one type of site over others. Thermal stability of ultrathin Ni films on the sulfided surface is investigated; the disordering of S(4 × 4) superstructure is interpreted in terms of sulfur segregation and interface formation between Ni and W. We analyze the spatial distribution of the experimentally observed site-specific nanoclusters in STM images, in comparison with random distributions based on the calculated value and a simulation. Using a statistical t-test, the difference between distributions (e.g. observed vs. random) suggests that the nucleation of site-specific nanoclusters is not a random process. Several explanations are discussed, including the possibility of long-range interactions between nucleating nanoclusters.