Growth of Uniquely Small Tin Clusters on Highly Oriented Pyrolytic Graphite

Sn clusters have been grown on highly oriented pyrolytic graphite (HOPG) surfaces and investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. At low Sn coverages ranging from 0.02 to 0.25 ML, Sn grows as small clusters that nucleate uniformly on the terraces. This behavior is in contrast with the growth of transition metals such as Pd, Pt, and Re on HOPG, given that these metals form large clusters with preferential nucleation for Pd and Pt at the favored low-coordination step edges. XPS experiments show no evidence of Sn-HOPG interactions, and the activation energy barrier for diffusion calculated for Sn on HOPG (0.06 eV) is lower or comparable to those of Pd, Pt, and Re (0.04, 0.22, and 0.61 eV, respectively), indicating that the growth of the Sn clusters is not kinetically limited by diffusion on the surface. DFT calculations of the binding energy/atom as a function of cluster size demonstrate that the energies of the Sn clusters on HOPG are similar to those of Sn atoms in the bulk for Sn clusters larger than 10 atoms, whereas the Pt, Pd, and Re clusters on HOPG have energies that are 1-2 eV higher than in the bulk. Thus, there is no thermodynamic driving force for Sn atoms to form clusters larger than 10 atoms on HOPG, unlike for Pd, Pt, and Re atoms, which minimize their energy by aggregating into larger, more bulk-like clusters. In addition, annealing the Sn/HOPG clusters to 800 and 950 K does not increase the cluster size, but instead removes the larger clusters, while Sn deposition at 810 K induces the appearance of protrusions that are believed to be from subsurface Sn. DFT studies indicate that it is energetically favorable for a Sn atom to exist in the subsurface layer only when it is located at a subsurface vacancy.