Narrow Results

Cluster deposition is a technique recently developed for the production of films of a nanometric thickness. The technique consists on driving a beam of clusters onto a substrate with an energy sufficient to obtain their fragmentation and the adhesion of the fragments to the substrate. Therefore the main mechanisms involved by this technique are (i) cluster fragmentation, (ii) cluster–substrate adhesion and (iii) coalescence of the deposited fragments.

The focus of this study is on these three mechanisms. To illustrate their peculiar features, three structures have been selected. Two of these structures, that is a cluster and a cluster dimer, are free standing. The third one represents a cluster deposited onto a substrate. The temperature response of the two free standing structures illustrates the effects of the cluster geometry and shape on the response to an external energy input. The comparison of these responses with the behavior occurring during deposition illustrates the main properties of cluster–substrate interactions.

This work presents a dynamical version of the LCAO method which includes extension variables, such as temperature and pressure. As an illustrative example, the method is applied to the temperature response of small silicon clusters. A comparison is made with molecular dynamics calculations using standard interatomic potentials.

Polarizability of small Si_n (n = 3 to 9) clusters has been calculated using density functional theory (DFT) with Vosko–Wilk–Nusair (VWN) correlation functional. The computed values of polarizability per atom tend to decrease with increasing cluster size. Frequency-dependent polarizability of Si₃, Si₄, Si₆, and Si₉ clusters demonstrates that the polarizability increases slowly with the increase in the energy for energy values less than 1.2 eV but exhibit the same size-dependent features as the static polarizability. The first resonance energy for Si₃, Si₄, Si₆, and Si₉ clusters occur at 1.25, 1.63, 2.5, and 1.98 eV, respectively. Moreover, the anisotropic polarizability is found to be the highest for Si₉ cluster.

Si_n clusters in the size range n = 4–30 have been investigated using a combination of global structure optimization methods with DFT and ab initio calculations. One of the central aims is to provide explanations for the structural transition from prolate to spherical outer shapes at about n = 25, as observed in ion mobility measurements. Firstly, several existing empirical potentials for silicon and a newly generated variant of one of them were better adapted to small silicon clusters, by global optimization of their parameters. The best resulting empirical potentials were then employed in global cluster structure optimizations. The most promising structures from this stage were relaxed further at the DFT level with the hybrid B3LYP functional. For the resulting structures, single point energies have been calculated at the LMP2 level with a reasonable medium-sized basis set, cc-pVTZ. These DFT and LMP2 calculations were also carried out for the best structures proposed in the literature, including the most recent ones, to obtain the currently best and most complete overall picture of the structural preferences of silicon clusters. In agreement with recent findings, results obtained at the DFT level do support the shape transition from prolate to spherical structures, beginning with Si₂₆ (albeit not completely without problems). In stark contrast, at the LMP2 level, the dominance of spherical structures after the transition region could not be confirmed. Instead, just as below the transition region, prolate isomers are obtained as the lowest-energy structures for n ≤ 29. We conclude that higher (probably multireference) levels of theoretical treatments are needed before the puzzle of the silicon cluster shape transition at n = 25 can safely be considered as explained.