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Metal nanoclusters exhibit unusual chemical and physical properties
different from those of the bulk material or of the atoms, and have a
number of fascinating potential applications in heterogeneous catalysis,
micro- and nanoelectronics and opto-electronics
devices. Therefore, the understanding of their thermal, structural and
dynamical properties is a topic of intense interest from the scientific and
technological viewpoint. One of the important factors of the nanosize
regime is the presence of a large percentage of surface atoms. In
addition, when nanoclusters are deposited on surfaces, their physical and
chemical properties are strongly dependent not only on their particle size
and chemical composition, but also on the structure of the surface and that
of the metal/substrate interface.
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• Nanosize transition metal clusters
melt at
much lower temperature than the
bulk
metal; the melting temperature
depends
on cluster size, shape, and
chemical
composition.
• Wetting behavior (island formation on
the
substrate) can be induced by
thermal
treatments or by substrate
chemical and
physical modifications.
• Bimetallic systems that exhibit
surface
segregation behavior melt in two
stages:
surface diffusion followed by
total
melting.
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An
improved understanding of heterogeneous catalysis is emerging thanks both
to significant advances in surface science techniques as well as to the
insights provided by the application of first-principles theoretical
methods. It is crucial to be able to elucidate and predict the
structure and composition of materials used as catalysts, and it is now
well recognized that chemical, thermal, and mechanical treatments may
significantly affect the structure of the exposed faces, and therefore
their catalytic activity.
A detailed understanding of the melting process of metal nanoclusters is
one important factor to understand their special behavior. It is known that
the solid-liquid transition in nanoclusters differs from that in bulk
materials, and many studies have addressed the melting point change
variation with the nanocluster size. At low temperatures, the cluster
structure is solid-like, and as the temperature increases the structure
acquires liquid features, passing through an intermediate state, called
dynamic coexistence, where the structure fluctuates between liquid and
solid behavior. Recent experiments and molecular simulations have concluded
that the nanoclusters melting temperature depends
on their size, shape, composition, and in most cases is lower than that of
bulk melting. As the cluster size is decreased, the melting temperature
generally decreases. The phenomenon can be understood considering that a
large percentage of atoms residing on the surface of cluster are weakly
bonded and less constrained in their thermal motion. However, quantum
effects in clusters below a certain size can be important yielding a
non-monotonic behavior for the variation of the melting temperature vs.
cluster size at small sizes.
Molecular dynamics techniques provide useful insights that contribute to
the understanding of complex microscopic phenomena such as mixing and
growth in bimetallic systems, and the statistical thermodynamics of small
systems on which such simulations are based has been discussed
extensively. We employ MD simulations to study the structure
and dynamics of several mono and bimetallic systems, including Cu-Ni and
Pt-Au nanoclusters with various compositions and cluster sizes, both in
vacuum and deposited on a graphite surface. The many-body Sutton-Chen
potential is used for the metal-metal interactions, and Lennard-Jones
potentials describe the interaction between nanocluster and
substrate. Deformation parameters are defined to investigate shape
changes in the nanoclusters, and changes in the structural and dynamical
properties are discussed as a function of temperature. The diffusivity of
the nanocluster as a whole in the solid and liquid phases is analyzed.
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