1 research outputs found
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