A. Hoareau

Diffusion and aggregation of large antimony and gold clusters deposited on graphite

Abstract We present a study of the first stages of growth of thin films produced by low-energy cluster beam deposition (LECBD) on graphite. Our experiments are analyzed in the framework of new models including three physical ingredients, which are the deposition, the diffusion and the aggregation of the clusters. The comparison between computer simulations of the model and the experimental structures reveals that only the incident clusters diffuse on the graphite, the clusters stick irreversibly upon contact, and allow us to quantify the diffusion of clusters on graphite. Two kinds of metallic cluster films are studied: thin films produced by deposition of antimony clusters (containing 2300 and 250 atoms) and others by deposition of gold clusters (containing about 250 atoms). In both cases, we find that the clusters, in spite of their large size, diffuse very rapidly on the surface. The different microscopic diffusion mechanisms proposed in the literature are investigated, but none is compatible with our experimental results. Finally, we suggest a collective mechanism where the cluster rotates on the surface as a rigid entity to explain our results.

Time-of-flight spectroscopy of a supersonic beam of mercury. Intensities and appearance potentials of Hgx aggregates

Molecular clusters Hgx (2<or=x<or=25) have been detected in mercury vapour. They were produced in a supersonic nozzle type molecular beam, ionised by electronic impact and analysed in a time-of-flight mass spectrometer. The appearance potentials of Hgx (2<or=x<or=12) have been measured. The ions resulted from direct ionisation of the corresponding neutral molecular aggregates.

Continuous amorphous antimony thin films obtained by low-energy cluster beam deposition

We have prepared continuous amorphous antimony thin films by low‐energy cluster beam deposition. Contrary to the antimony films prepared by molecular beam deposition, this new technique allows preparation of continuous amorphous films which are stable at room temperature. This study has been carried out by combining electrical measurements and transmission electron microscopy observations.

Study of the crystallization of antimony thin films by transmission electron microscopy observations and electrical measurements

We have correlated electrical measurements and transmission electron microscopy observations of ultrathin antimony films. The obtained results allow us to understand the crystallization of antimony. It seems that, at room temperature, the critical thickness below which no complete crystallization is possible corresponds to the percolation threshold.

Low-energy Bi cluster beam deposition

Electrical and chemical properties of Bi films produced by low-energy cluster beam deposition (LECBD) are studied and compared with those of films made by molecular beam deposition (MBD). For LECBD Bi films, increase of roughness and decrease of film grain size explain increase of resistivity and chemical reactivity to oxygen. Two phenomena are proposed to play a role in LECBD film growth: low surface diffusion of Bi clusters (relative to Bi2) and a high barrier to cluster coalescence. The present experiments show that growth of LECBD crystallized Bi films and the film grain size can be controlled by the size distribution of incident clusters.

Crystallization of thin antimony deposits on amorphous carbon

Abstract The systematic study of the first stages of antimony growth on amorphous carbon substrates is inhibited by the oxidation of the deposit during transfer through air from the evaporating chamber to the analysis system. To prevent the oxidation of the deposit, the sample is coated in situ with carbon. Under these conditions, transmission electron microscopy and high resolution transmission electron microscopy show that antimony deposits on amorphous carbon supports are amorphous in the first stages of growth. This work shows for the first time that, even for a very low thickness where antimony is considered amorphous, some crystalline antimony aggregates exist on the support. These aggregates are assumed to act as nucleation centres for the crystallization of continuous antimony films which occurs when the equivalent thickness is larger than a critical thickness.

Generation of silver cluster using the inert-gas condensation technique-observation of doubly charged clusters

Mass spectra of Agn aggregates (2 I(Ag2n+). Doubly charged clusters have also been detected and evidence exists for a Coulomb explosion mechanism.

Atomic rearrangements of supported Bi particles observed by high-resolution electron microscopy

The morphology of supported Bi particles is studied using the technique of profile imaging at atomic resolution. These particles show a crystallized Bi core surrounded by an amorphous (or liquid) material. For small supported particles (diameter of the Bi core, about 6-9 nm) observed by high-resolution electron microscopy at room temperature, the lattice fringe orientation changes very rapidly during the observation. This fluctuating evolution is attributed to atomic rearrangements of the particle induced by the microscope electron beam. During this evolution, a decrease in the size of the crystallized Bi core is sometimes observed for the smaller particles.

Experimental achievement of 2D percolation and cluster-cluster aggregation models by cluster deposition

We show that cluster deposition, contrary to atomic or molecular deposition, makes it possible to experimentally approximate to several 2D theoretical models (percolation, cluster-cluster aggregation). Concerning percolation, molecular deposition seems to simulate bond correlated percolation whereas free-cluster deposition is close to geometrical site percolation. The low mobility of the clusters can explain these features. In different experimental conditions, cluster deposition allows to perform 2D cluster-cluster aggregation experiments.

Electronic structure of selenium aggregates by a self-consistent method including 4d orbitals

The electronic structure of small selenium aggregates Sen > (2 < n < 10) has been calculated using the CNDO method on a complete basis 4s, 4p, 4d. The energy per bond, the ionization potentials (IP) and the electronic affinities (EA) are given as a function of the aggregate size. The IP and EA converge to a common value which approaches the work function of bulk metal. The calculated gap is in good agreement with recent experimental determinations.