Preferential Nucleation and Self-Limiting Growth of Cu Nanoclusters on S(4 × 4)/W(111)

Abstract

The interaction of Cu with the highly ordered S(4 x 4)/W(111) surface has been studied by means of Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) at roomtemperature. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 x 4) periodicity characterized by broad, planar terraces (30 nm in width). We find that fractional monolayers of vapor-deposited Cu grow homogeneously as clusters on the S(4 x 4)/W(111) surface over a wide coverage range. At low Cu coverages (<0.1 ML), Cu nanoclusters are observed to nucleate preferentially at characteristic 3-fold hollow sites on the S(4 x 4)/W(111) surface; there is a clear energetic preference for one type of site over others. The formed Cu nanoclusters are uniform in size (0.6 nm) as coverage increases, indicating self-limiting growth. Evidence for long-range interactions that may affect the nucleation of nanoclusters is discussed. For coverages ≥0.1 ML, additional sites are populated, and the whole surface is decorated by Cu clusters. STM data are supported by LEED and AES measurements. The results are interpreted in terms of heat of formation, relative reactivity, surface corrugation, surface diffusion, lattice mismatch, and long-range interactions in the nanoscale regime.