The nucleation and growth of metals are being studied on a highly-corrugated S(4x4)/W(111) surface by UHV-scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). The motivation for this work is related to the use of Ni, Co on MoS₂ or WS₂ as catalysts for hydrodesulfurization (HDS), an important process in petroleum refining. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4x4) periodicity, characterized by broad, planar terraces. We find that fractional monolayers of vapor-deposited metal, i.e. Ni, Cu grow as clusters on the S(4x4)/W(111) surface over a wide coverage range. At low metal coverages (<0.2 ML), metal nanoclusters (~0.5 nm) are observed to nucleate preferentially and exhibit self-limiting growth at characteristic 3-fold hollow sites on the S(4x4)/W(111) surface; there is a clear energetic preference for one type of site over others. As coverage increases, additional sites are populated and metal clusters grow homogeneously on the whole surface. Thermochemistry of metals on S(4x4)/W(111) is studied using AES.

For the preferentially nucleated nanoclusters at low coverages (<0.1 ML), we simulate the distribution of nanoclusters using a simplistic random-generation model. Hence, statistics of nanoclusters from the experimental observations and simulations can be analyzed and compared in terms of isolated nanoclusters (highlighted in yellow on the STM image) and row structures (highlighted in red on the STM image). The results suggest that there may be long-range interactions (>1 nm) between metal atoms/nanoclusters and existing nanoclusters that affect nucleation processes.

References:

1.

Qifei Wu, Wenhua Chen, and Theodore E. Madey

Preferential Nucleation and Self-Limiting Growth of Cu Nanoclusters on S(4x4)/W(111)

J. Phys.Chem.B., 2002, V.106,  P.6419-6430