210 research outputs found
'Governo parlamentare': nascita di una categoria politica nella cultura costituzionale italiana tra Ottocento e Novecento
The effect of a modulated flux on the growth of thin films
Thin films are usually obtained by depositing atoms with a continuous flux.
We show that using a chopped flux changes the growth and the morphology of the
film. A simple scaling analysis predicts how the island densities change as a
function of the frequency of the chopped flux in simple cases where aggregation
is irreversible. These predictions are confirmed by computer simulations. We
show that the model can be used to obtain information on the diffusion or the
evaporation of the adatoms. The model is also useful to understand the growth
of thin films prepared by pulsed sources.Comment: 10 pages, Revte
Diffusion of gold nanoclusters on graphite
We present a detailed molecular-dynamics study of the diffusion and
coalescence of large (249-atom) gold clusters on graphite surfaces. The
diffusivity of monoclusters is found to be comparable to that for single
adatoms. Likewise, and even more important, cluster dimers are also found to
diffuse at a rate which is comparable to that for adatoms and monoclusters. As
a consequence, large islands formed by cluster aggregation are also expected to
be mobile. Using kinetic Monte Carlo simulations, and assuming a proper scaling
law for the dependence on size of the diffusivity of large clusters, we find
that islands consisting of as many as 100 monoclusters should exhibit
significant mobility. This result has profound implications for the morphology
of cluster-assembled materials
Different W cluster deposition regimes in pulsed laser ablation observed by in situ Scanning Tunneling Microscopy
We report on how different cluster deposition regimes can be obtained and
observed by in situ Scanning Tunneling Microscopy (STM) by exploiting
deposition parameters in a pulsed laser deposition (PLD) process. Tungsten
clusters were produced by nanosecond Pulsed Laser Ablation in Ar atmosphere at
different pressures and deposited on Au(111) and HOPG surfaces. Deposition
regimes including cluster deposition-diffusion-aggregation (DDA), cluster
melting and coalescence and cluster implantation were observed, depending on
background gas pressure and target-to-substrate distance which influence the
kinetic energy of the ablated species. These parameters can thus be easily
employed for surface modification by cluster bombardment, deposition of
supported clusters and growth of films with different morphologies. The
variation in cluster mobility on different substrates and its influence on
aggregation and growth mechanisms has also been investigated.Comment: 12 pages (3 figures); Surface Science (accepted
Fast diffusion of a Lennard-Jones cluster on a crystalline surface
We present a Molecular Dynamics study of large Lennard-Jones clusters
evolving on a crystalline surface. The static and the dynamic properties of the
cluster are described. We find that large clusters can diffuse rapidly, as
experimentally observed. The role of the mismatch between the lattice
parameters of the cluster and the substrate is emphasized to explain the
diffusion of the cluster. This diffusion can be described as a Brownian motion
induced by the vibrationnal coupling to the substrate, a mechanism that has not
been previously considered for cluster diffusion.Comment: latex, 5 pages with figure
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