Physical vapor deposition of a wide range of materials on rare-gas solids leads to spontaneous cluster formation. Desorption of the rare-gas buffer causes the clusters to aggregate, a process known as buffer-layerassisted growth. We have studied the extent of aggregation and the size distribution of Au nanostructures as a function of the buffer composition ͑Xe, Kr, and Ar͒ and thickness, using transmission electron microscopy to image them after buffer desorption and delivery to amorphous carbon substrates. For small compact Au nanostructures ͑less than ϳ5 nm mean radius, р3ϫ10 4 atoms͒, the diffusivity varies strongly with size and even increases with average size in a limited range. This enhanced diffusion phenomenon is attributed to self-heating during coalescence. It is most important for small particles and is more evident on Kr than on Xe because of weaker interface coupling. In the limit of large ramified Au nanostructures ͑exceeding ϳ10 nm mean radius, у2ϫ10 5 atoms͒, the diffusivity scales as the inverse of the contact area, in agreement with molecular dynamics simulations of fast slip diffusion of nanocrystals on incommensurate surfaces. Motion is driven by phonons of the cluster and substrate, and is controlled by friction between a cluster facet and the buffer surface. A simple model is proposed that explains the observed exponential dependence of cluster size on buffer thickness. In this model, the growth kinetics are controlled by competition between the rate of cluster diffusion and the rate of buffer depletion.
Physical vapor deposition onto rare gas buffer layers leads to the spontaneous formation of clusters. During the thermal desorption of the buffer, these clusters diffuse and aggregate into larger structures, a process known as buffer-layer-assisted growth and desorption assisted coalescence. We studied the effect of buffer thickness and the rate of buffer desorption on the extent of this aggregation for Ag, Au, Cu, Pd, Co, and Ni particles on a solid Xe surface. On the basis of these experiments, results from Monte Carlo simulations and the existing theoretical models for cluster-cluster aggregation, we report for the first time the Arrhenius parameters for nanoparticle slip-diffusion. The effective activation energies range from 0.12 for small Ag clusters (few hundred atoms) to 0.60 eV for ramified Ni islands (millions of atoms), and the giant pre-exponential factors were found to differ by many orders of magnitude. Significantly, the pre-exponential factors follow a Meyer-Neldeltype dependence on the corresponding effective activation energy, with a characteristic Meyer-Neldel energy of 6.9 meV. This energy is associated with the phononic excitations in solid Xe that are responsible for nanostructure mobility. This dependence should be a characteristic feature of nanoparticle diffusion.
The deposition of Au onto thin Xe films at a low temperature leads to cluster formation. The subsequent Xe sublimation results in cluster aggregation and delivery to the substrate in a process known as buffer-layerassisted growth. Previously, this process was described in terms of a diffusion-limited cluster-cluster aggregation process during layer-by-layer desorption of the buffer. Instead, significant diffusion, restructuring, and dewetting of the Xe occur prior to desorption, and this leads to cluster aggregation. Cluster motion and aggregation are driven by capillary forces as the dewetting film retreats and sublimes. Kinetic Monte Carlo simulations reproduce the experimentally observed particle shapes and size distributions, and they provide additional insight into the interaction of the particles with the dewetting front. The presence of nanoscale particles on the film inhibits dewetting and significantly alters the shape of the front.
II-VI semiconductor nanostructures exhibit interesting optical properties due to quantum confinement of their charge carriers. Here, we discuss the assembly of nanostructures of CdS, CdSe, and CdTe using bufferlayer-assisted growth with Xe buffers. Both compact clusters and ramified wires can be synthesized by varying the Xe buffer layer thickness. Analysis of the nanostructure size distributions and densities makes it possible to calculate their diffusion parameters on the desorbing Xe. Clear differences in the effective activation energies for diffusion for CdS, CdSe, and CdTe can be attributed to differences in London dispersion interactions. Photoluminescence measurements indicate changes from 3D to 2D confinement as compact particles are replaced by ramified wires. Laser power dependent measurements yield the low temperature exciton lifetime, and temperature dependent measurements indicate that optical phonons play a dominant role in the decay of the signal above 50 K and defect states play a dominant role below 50 K.
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