P. Piseri

Cluster beam deposition: a tool for nanoscale science and technology

Gas phase nanoparticle production, manipulation and deposition is of primary importance for the synthesis of nanostructured materials and for the development of industrial processes based on nanotechnology. In this review we present and discuss this approach, introducing cluster sources, nanoparticle formation and growth mechanisms and the use of aerodynamic focusing methods that are coupled with supersonic expansions to obtain high intensity cluster beams with a control on nanoparticle mass and spatial distribution. The implication of this technique for the synthesis of nanostructured materials is also presented and applications are highlighted.

Raman spectroscopy characterization of titania nanoparticles produced by flame pyrolysis: The influence of size and stoichiometry

A systematic study of the shift and linewidth of the Eg Raman peak at 144cm−1 of anatase TiO2 nanopowders, produced by a flame aerosol technique, is here presented. The analysis was performed as a function of the crystal domain size and of the degree of oxidation. In the nanopowders, a clear contribution of the stoichiometry defects to the peak shift was evidenced, while the peak width seems to be less affected by the oxygen content. The Raman peak behavior due to size reduction has been interpreted in the framework of a phonon quantum confinement model. A critical review of the different approaches to this model, adopted in the literature to explain the behavior of the anatase Raman spectra as a function of the domain size, is presented. In particular, the hypothesis of a three-dimensional isotropic model for the dispersion relations is discussed. This analysis evidences general limits in the application of the phonon confinement model to the study and characterization of nanoparticles and nanostructured...

Cluster-Beam Deposition and in situ Characterization of Carbyne-Rich Carbon Films

Nanostructured carbon films produced by supersonic cluster beam deposition have been studied by in situ Raman spectroscopy. Raman spectra show the formation of a sp2 solid with a very large fraction of sp-coordinated carbyne species with a long-term stability under ultrahigh vacuum. Distinct Raman contributions from polyyne and cumulene species have been observed, as well as different stabilities under gas exposure. Our experiments confirm theoretical predictions and demonstrate the possibility of producing a carbyne-rich pure carbon solid. The stability of the sp2-sp network has important implications for astrophysics and for the production of novel carbon-based systems.

A pulsed microplasma source of high intensity supersonic carbon cluster beams

A novel, pulsed, supersonic, cluster-beam source based on microplasma ablation has been realized and tested. Its intensity and stability allows one to overcome the limitations encountered in laser vaporization and pulsed arc cluster sources. Supersonic carbon cluster beams have been obtained and characterized. Target ablation processes and cluster growth have been investigated, showing the presence of mechanisms substantially different from those observed in other plasma-based sources. In particular, we show that the precise confinement of the ablation plasma in the source is of fundamental importance for the production of cluster beams with high intensity and stability. This opens new opportunities for the study of free and supported clusters and for the synthesis of nanostructured materials.

Supercapacitors based on nanostructured carbon electrodes grown by cluster-beam deposition

Nanostructured carbon films have been grown at room temperature by supersonic cluster beam deposition. Due to a structure based on nanotube embryos and a porosity with grain sizes of a few tens of nanometers, these films have a highly accessible surface area needed for electrochemical applications such as supercapacitors. Films with a density of 1 g/cm3 show, in the dc regime, a specific capacitance per electrode of 75 F/g on a single-cell device with polycarbonate as the organic electrolyte. The resulting energy and power densities of cluster-assembled carbon electrodes are 76 Wh/kg and 506 kW/kg. The possibility of depositing nanostructured films over a large area on a variety of substrates makes cluster-beam deposition very interesting for the realization of supercapacitors.

Near-edge x-ray absorption fine structure and Raman characterization of amorphous and nanostructured carbon films

Amorphous and nanostructured carbon films were grown by using two different techniques: ion sputtering and cluster beam deposition. The films were studied by near-edge x-ray absorption fine structure (NEXAFS) and Raman spectroscopy. Depending on the precursors, atoms, or clusters, the films are characterized by a different sp2/sp3 ratio which influences the mechanical and the electronic properties. Due to the sensitivities of NEXAFS (local order) and Raman (medium-range order), we have characterized and compared the structure of the films over different length scales. The complementarity of NEXAFS and Raman techniques for the characterization of disordered forms of carbon is here presented and discussed. We also present an original method of NEXAFS spectra calibration allowing a better determination of peak positions.

Production and characterization of highly intense and collimated cluster beams by inertial focusing in supersonic expansions

Intense and collimated supersonic cluster beams have been produced by exploiting inertial focusing effects. To this goal we have developed and tested a novel focusing nozzle (focuser). Using this device with a pulsed microplasma cluster source we have obtained cluster beams with a divergence of 10 mrad and average densities of 3×1010 atoms/cm3 (2×1012 atoms/cm3 pulsed) corresponding to deposition rates of 2 nm/s at 300 mm distance from the source nozzle. With a focusing nozzle cluster thermal relaxation and mass distribution in a supersonic expansion can be controlled. We have measured the cluster transverse velocities, with extremely high precision, by characterizing the cluster beam deposition on a substrate by an atomic force microscope. Besides the relevance for the understanding of relaxation processes in expanding jets, the inertial focusing of clusters has several important consequences for the synthesis of nanostructured films with controlled structure and for all the experimental techniques requi...

The influence of the precursor clusters on the structural and morphological evolution of nanostructured TiO2 under thermal annealing

We have produced nanostructured titanium dioxide thin films by supersonic cluster beam deposition. The as-deposited films have a nanocrystalline or amorphous structure depending on the mass distribution of the precursor clusters. This can be controlled by aerodynamic separation effects typical of supersonic expansions. On thermal annealing at temperatures from 400 to 800 °C in ambient atmosphere, amorphous-to-anatase and anatase-to-rutile phase transitions have been observed. The nanostructure and microstructure evolution of the film upon annealing has been characterized by atomic force microscopy and transmission electron microscopy. The influence of the precursor clusters in the evolution of the film nanostructure at high temperatures has been demonstrated. This observation opens up new perspectives for batch fabrication of devices based on cluster-assembled materials.

Engineering the nanocrystalline structure of TiO2 films by aerodynamically filtered cluster deposition

We have produced nanocrystalline titanium dioxide films with different structures (anatase or rutile) by depositing mass selected clusters from the gas phase. Nanoparticles are produced by a pulsed microplasma cluster source and are selected by aerodynamic separation effects. We have characterized nanocrystalline films by Raman spectromicroscopy and transmission electron microscopy, showing that the films assembled with very small clusters have a predominant rutile phase, whereas larger clusters form films with anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks.

Influence of cumulenic chains on the vibrational and electronic properties of s p-s p2 amorphous carbon.

We report the production and characterization of a form of amorphous carbon with s p-s p(2) hybridization (atomic fraction of sp hybridized species > or =20%) where the predominant sp bonding appears to be (=C=C=)(n) cumulene. Vibrational and electronic properties have been studied by in situ Raman spectroscopy and electrical conductivity measurements. Cumulenic chains are substantially stable in high vacuum conditions for temperatures lower than 250 K and they influence the electrical transport properties of the s p-s p(2) carbon through a self-doping mechanism by pinning the Fermi level closer to one of the mobility gap edges. Upon heating above 250 K the cumulenic species decay to form graphitic nanodomains embedded in the s p(2) amorphous matrix thus reducing the activation energy of the material. This is the first example of a pure carbon system where the s p hybridization influences bulk properties.