The presence of nanoparticles in industrial processes dates back well before the advent of nanotechnology  : carbon nanoparticles as rubber additives for tires  or titania nanoparticles for pigmentary applications  represent a paradigm of mass production of nanoscale objects. The success of manufacturing approaches based on the assembling of nanoparticles is largely determined by the established infrastructure and customer base with cost reduction as a general benchmark. Several techniques for the synthesis of metal oxides have been developed: sol-gel processing,  chemical vapor deposition,  hydrolysis process.  Among different processes for the production of nanoparticles, gas-phase routes are very popular both for large scale production and for fundamental studies.  Producing the nanoclusters in the gasphase makes it for instance possible to obtain clean particles without contaminants. With the so called low-energy cluster beam deposition, it is possible to produce gas-phase clusters in vast amounts! For a deposition area of 10×10 cm for a thin film of 30 nm thickness, the deposition time is around 10 minutes.
What is low-energy cluster beam deposition?
Nanostructured materials are defined as systems composed of single or multiple phases such that at least one of them has characteristic dimensions in the nanometer range (1-100 nm).  Low-Energy Cluster Beam Deposition (LECBD) [9,10] is a technique of choice for the fabrication of nanostructured systems, since it allows the deposition on a substrate of neutral particles with low kinetic energy (E kin< 1eV/atom) produced in gas phase and maintaining their properties even after deposition. This have been proven to be a powerful bottom-up approach for the engineering of nanostructured thin films with tailored properties, since it allows in principle the control of physical and chemical characteristics of the building blocks. [11,12]
How do we produce gas phase clusters?
Gas phase synthesis is an established and well-developed process able to produce large scale quantities of nanoparticles with a high level of control on particle physic-chemical properties such as phase and composition.  In gas phase synthesis, nanoparticles are made by ‘building ’ them from individual atoms or molecules up to the desired size. Cluster embryos are formed either by physical means such as condensation of a supersaturated vapor or by chemical reaction of gaseous precursors.
What is supersonic cluster beam deposition?
Among different approaches to LECBD, supersonic cluster beam deposition (SCBD)  presents several advantages in terms of deposition rates, lateral resolution compatible with planar microfabrication techniques and neutral particle mass selection by exploiting aerodynamic focusing effects. One of the most relevant properties of cluster-assembled materials is the surface morphology. The morphology of cluster-assembled materials is characterized by hierarchical arrangements of small units in larger and larger features up to a certain critical length-scale, ingeneral determined by the duration of the deposition process.  The cluster-assembled film morphology is characterized by high specific area and porosity at the nano and sub-nanometer scale, extending in the bulk of the film. Surface pores and surface specific area, as well as rms roughness, depend on film thickness, and increase with it. [14,15]
How does a SCBD system looks like?
Nanostructured thin films can be deposited on different substrates by depositing a supersonic seeded beam of a metal oxide (TiOx, ZrOx, etc.) clusters produced by a microplasma cluster source (PMCS) under high vacuum conditions. The source (PMCS), located in the first chamber called the ‘expansion chamber’, is composed by a ceramic hollow body inside of which is obtained a cylindrical body (volume of a few cm3) at whose end is a solenoid pulsed valve which regulates the inlet of a high pressure gas. The opposite end is connected to a second chamber (called ‘deposition chamber’) through a low conductance nozzle and it represents the cluster beam exit. In the cavity of the source is inserted a cylindrical metal rod that acts as a cathode, Fig. (1).
The PMCS operation principle is based on the ablation of a metal rod by an argon plasma jet, ignited by a pulsed electric discharge. After the ablation, the metal ions thermalize with argon and condense to form clusters. The mixture of clusters and inert gas is then extracted in vacuum through a nozzle to form a seeded supersonic beam, which is collected on a substrate located in the beam trajectory. The use of supersonic beams of clusters allows a stronger control of the cluster mass distribution and kinetic energy with the possibility to obtain very high deposition rates [16,17] and very collimated beams. 
Author: Alexandra Raileanu
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