Low Energy Cluster Beam Deposition Research Articles

Approach to saturation in nanocrystallized films of iron and nickel

Abstract Nanocrystallized films of iron and nickel have been elaborated using low-energy cluster beam deposition (LECBD) techniques. Grain size distribution is very narrow and close to 5 nm. The approach to saturation is interpreted in terms of the random anisotropy model. In particular, the anisotropy and inter-grain exchange coupling energies have been determined.

Nanocrystalline structures prepared by neutral cluster beam deposition

The low energy cluster beam deposition technique (LECBD) is an attractive method to produce nanocrystalline structured thin films. In this technique, clusters do not fragment at the impact on the surface of the substrate leading to a nearly random surface “pavement” when the mean cluster size is high enough to limit their diffusion. In this case, the cluster size is a key parameter which controls the nucleation and growth processes of the films. In this context, the experimental techniques to produce selected size distributions of clusters of any kind of materials, even the most refractory, are presented as well as the mass analyzing and the deposition techniques. The specific nucleation and growth process characteristic of the LECBD is discussed and compared with the results of the classical nucleation theory in order to predict and control the nanostructures of the films. Finally, to illustrate the potential applications of the cluster assembled materials, some characteristic examples are presented, such as : diamond like carbon films with a controled sp 3/sp 2 character obtained by fullerene depositions or granular transition metal films (Fe, Ni, Co) with specific magnetic properties interpretable using the correlated spin glass model in term of random anisotropy fluctuation.

Magnetic properties of nanostructured thin films of transition metal obtained by low energy cluster beam deposition

Clusters of iron, cobalt, and nickel are produced in a laser vaporization source. The size distributions of the incident clusters are checked by time-of-flight mass spectrometry before deposition at low energy. Studying the near threshold photoionization, Con and Nin clusters exhibit an icosahedral structure while for iron, no clear structure emerges. Neutral clusters were deposited on different substrates at room temperature with thicknesses up to 100 nm in view to determine their structure and magnetic properties. A limited coalescence of the clusters is observed from high-resolution transmission electron microscopy. No icosahedron has been observed but cuboctahedron and interface twins between adjacent particles have been clearly identified in Ni films. Grazing incidence x-ray diffraction experiments reveal a classical phase with grain size around 6 and 4 nm for Fe and Ni films, respectively but an anomalous fcc phase for Co films and a very low grain size of 2 nm. The density of films determined by x-ray reflectivity was estimated to represent only 60%–65% of the bulk density. Magnetic behaviors studied by ferromagnetic resonance and SQUID magnetization measurements have been interpreted using the correlated spin glass model. Mössbauer spectra performed on Fe films at zero field revealed the presence of 20% of iron in the form of thin nonmagnetic oxide skin surrounding Fe grains which allow to find 2.2 μB per magnetic iron atom in agreement with macroscopic magnetic measurements. Nevertheless we found an anomalous reduced atomic moment for Ni film.

An original way to elaborate cermet thin films: cluster deposition

The development of the Low-Energy Cluster Beam Deposition (LECBD) technique represents a new way for the synthesis of cermets. Bi-SiO x thin films have been produced by codeposition of neutral Bi clusters and SiO x molecules. A Transmission Electron Microscopy study shows that these new cermets are formed by isolated grains whose size corresponds to that of the incident clusters. This work demonstrates the main assets of this technique: homogeneity of the films, narrow size distribution of the metallic inclusions and grain size control in the nanometric range

Connectivity of diffusing particles continually deposited on a surface: relation to LECBD experiments

We generalize the conventional model of two-dimensional site percolation by including both (1) continuous deposition of particles on a two-dimensional substrate, and (2) diffusion of these particles in two-dimensions. This new model is motivated by recent thin film deposition experiments using the low-energy cluster beam deposition (LECBD) technique. Depending on various parameters such as deposition flux, diffusion constant, and system size, we find a rich range of fractal morphologies including diffusion limited aggregation (DLA), cluster-cluster aggregation (CCA), and percolation.

Stages of film growth during deposition of bismuth nanoparticles

The growth of Bi films obtained by low-energy cluster beam deposition (LECBD) is compared to those obtained by classical molecular beam deposition (MBD). For these two deposition techniques, significant differences in the very first stages of growth are presented. From transmission electron microscopy (TEM) observations, we show that the morphology of Bi thin films is sensitive to the size of incident clusters. The influence of incident cluster size on particle number density, diameter of supported particles and coverage rate is studied in the range of cluster size: 2–1100 atoms. The increase of roughness and decrease of the grain size are observed for Bi thick films (equivalent thickness t eq about 40 nm) produced by LECBD compared to those produced by MBD.

Mass selection of neutral clusters in Low-Energy Cluster Beam Deposition experiments: Is it realistic?

The crucial problem in controlling and filtering the size of free neutral clusters generated by a cluster source is studied in the particular case of the Sattler source type. The present work shows that a velocity selector can be used to control the size of neutral clusters produced in the low-energy cluster beam deposition system. This velocity selector is also used to measure the kinetic energy of free Bi clusters. The experimental results obtained for Bi clusters are compared with previous theoretical predictions.

Secondary ion emission from ultra-thin oxide layers bombarded by energetic (MeV) heavy ions: depth of origin and layer homogeneity

The escape depth of the secondary ions resulting from electronic sputtering of fast heavy ions in inorganic thin films has been investigated. Chromium layers deposited onto SiO 2 substrate as well as SiO x layers deposited onto chromium substrate have been characterised by secondary ion emission mass spectrometry (SIMS) in combination with time-of-flight (TOF) mass analysis (also referred to as HSF-SIMS [B.U.R. Sundqvist, in: Sputtering by Particle Bombardment III, eds. R. Behrisch and K. Wittmaack (Springer, Berlin, Heidelberg, 1991) p. 257]). These crossed experiments lead to a value around 1 nm for SiO x layers and 0.5 nm for Cr layers. On the other hand, HSF-SIMS can be used to correlate the intensity of the secondary ion emission to the film coverage rate (Θ) and (or) the morphology of particular films like those produced by low energy cluster beam deposition (LECBD). Using Sb deposits, the non-linear relationship between ion emission and Θ is interpreted in terms of sputtering enhancement in the individual supported clusters.

Synthesis of diamond nanocrystallites using the low-energy cluster beam deposition; An indirect proof of small fullerene existence?

Using a laser vaporization source, we have deposited low-energy neutral carbon clusters in the range C 10–C 32 in order to grow thin films ( e ≈ 100 nm). Compared to common DLC films, the first-order Raman spectrum does not give at all the same results, since it does not exhibit the usual G and D bands, but is comparable to the vibrational density of states of diamond. However, unlike amorphous covalent materials, a fine structure due to a perfect short range order is observed. Furthermore, we measure a large red shift (about 110 cm −1), which can be related to the decrease of the material force constants compared to the diamond ones. Finally, taking into account the specific nucleation and growth mechanisms involved in the low-energy cluster beam deposition as well as the crystallographic and electronic structures of our films, the hypothesis of small fullerene formation (C 10–C 32) in the nonequilibrium conditions of the laser vaporization source seems conceivable.

Mechanical properties of nanostructed diamond-like carbon films synthesized by low energy cluster beam deposition

Mechanical properties of carbon-films obtained by low energy neutral cluster beam deposition (LECBD) have been measured using the nanoindentation technique. Three selected size distributions centered around C 20, C 60 and C 900 have been deposited on various substrates at room temperature. The films with a nanostructured morphology (grain size around 15 to 25 nm) conserve a memory of the specific character of the free clusters which is sp 3 for C 20, sp 2 for C 900 and intermediate (sp 2.3) for C 60. The densities of the films (0.8 to 1.1 g.cm −3) are lower than most polymers but their hardnesses (3 to 12 GP a) are comparable to those of many DLC-films. These results are discussed taking into account the particular structures of the free clusters and the nucleation and growth mechanism specific for the LECBD technique, which corresponds nearly to a random stacking of incident clusters.

Diamondlike carbon films obtained by low energy cluster beam deposition: Evidence of a memory effect of the properties of free carbon clusters.

Free carbon clusters with various selected sizes (C 20 ,C 60 ,C 900 ) have been deposited on different substrates to obtain continuous thin films (thickness ∼100 nm). The characterization of nanostructures and electronic structures of the films have been performed using various complementary techniques (atomic force microscopy, transmission electron microscopy, electron energy loss spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy). A memory effect of the free carbon cluster properties permits the synthesis of amorphous diamond or graphite films depending on the deposited incident species (C 20 or C 900 , respectively)

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.

Low-energy cluster beam deposition : do you need it ?

Physical and chemical properties of films prepared by Low-Energy Cluster Beam Deposition (LECBD) are presented, discussed and compared to those of films obtained by other conventional deposition techniques. LECBD films are formed by the stacking of individual grains corresponding roughly to the incident free clusters. This original structure suggests that LECBD is promising for technological and theoretical applications. Film properties strongly related to the mean grain size can be controlled with this original technique. This work shows several examples showing the assets of LECBD: ultrathin sintered films, high surface-volume ratio or continuous ultrathin films, new structures of metallic deposits. Used as a tool to obtain experimental data to study fractal 2D models or to elaborate deposits to study quantum size effects, this deposition technique would be original, efficient and sometimes essential in many research areas

Low energy cluster beam deposition: a way to new materials?

Low energy cluster beam deposition allows the preparation of thin films that cannot be prepared by usual deposition techniques. We show here that low energy cluster beam deposition allows the preparation of continuous amorphous antimony thin films, contrary to the usual molecular beam deposition technique. These stable continuous amorphous films had never been obtained at room temperature. The study has been carried out by combining electrical measurements and transmission electron microscopy observations.

Films of controlled nano size grains deposited by low-energy cluster beam

Effects of low-energy cluster beam deposition (LECBD) on the growth of thin films are presented. According to TEM results, LECBD antimony films are the result of random packing of the incident clusters. We show that the “pavement of the surface substrate” previously observed in the first stage of film growth is still a valid model for thick films. With low compactness and the high roughness which characterize LECBD films, specific applications are suggested.

Evidence of the non-reactivity of samarium clusters during low-energy cluster beam deposition

Low-energy cluster beam deposition is a new and promising technique used to synthesize new materials. In addition, one important asset of this thin film deposition technique is to allow the film growth of chemically reactive materials in non-UHV vacuum conditions. The authors present obtained on samarium deposits which are very reactive with oxygen. According to transmission electron microscopy, X-ray photoemission and Auger electron spectroscopy, deposition of 3 nm mean diameter Sm clusters allows the growth of samarium films without oxidation in non-UHV vacuum conditions.

TEM and STM investigations of Antimony particles deposited on graphite by Molecular Beam Deposition and Low Energy Cluster Beam Deposition

TEM and STM investigations of Antimony particles deposited on graphite by Molecular Beam Deposition and Low Energy Cluster Beam Deposition

Can oxidation prevent nucleation studies of indium cluster deposition in a classical vacuum system?

To determine if the first stages of growth of indium deposits obtained by low-energy cluster beam deposition (LECBD) can be studied despite the oxidation of the smallest in supported aggregates, transmission electron microscopy (TEM), Auger electron spectroscopy (AES) with high spatial resolution and electron energy loss spectroscopy (EELS) experiments have been carried out. According to the analysis of the experimental results, a surface oxidation of In supported aggregates occurs during transfer into air between the deposition and the analysis systems. The oxidation of the deposits obtained after transfer through air may be neglected for a first analysis of TEM results to study nucleation and growth of indium 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.

Cluster-beam deposition of thin metallic antimony films: Cluster-size and deposition-rate effects.

Low-energy cluster-beam deposition is a technique that allows the growth of thin films via a mechanism different from the conventional molecular-beam deposition technique. These differences have been recently demonstrated in the case of antimony. This work shows that the deposition rate and the mean size of incident clusters of Sb evaporated on amorphous carbon are two crucial parameters. Crystalline aspect and coverage rate of deposits depends strongly on these two parameters. Transmission-electron-microscopy studies allow us to determine the optimum conditions under which, especially in the case of low-energy cluster-beam deposition, very thin films of high quality may be obtained.