E. Barborini

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.

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.

Negatively curved spongy carbon

We describe the production and characterization of a form of nanostructured carbon consisting of fully connected, three-dimensional (3D) sp2 networks. This form of carbon is characterized by interconnected thin layers forming a spongy structure with meso- and macroporosity. It is produced by a pulsed microplasma cluster source in the presence of a very low concentration of metallorganic catalyst. The unique 3D structure and topology of the spongy carbon are very interesting for catalysis and electrochemical applications and for the investigation of negatively curved forms of carbon such as schwarzites.

Cluster beam microfabrication of patterns of three-dimensional nanostructured objects

This letter describes the use of supersonic cluster beam deposition (SCBD) through a stencil mask for the fabrication of patterns of cluster-assembled objects. Using carbon cluster beams, micrometer-size pillars and tips have been produced on a variety of substrates. SCBD is characterized by high deposition rates, high lateral resolution, and low temperature processing. Nanostructured objects can be produced with high aspect ratio and controlled shapes. Micropatterning with SCBD can be of interest for applications requiring the integration of cluster-assembled structures with microelectronic or micromechanical devices.

SELF-AFFINE PROPERTIES OF CLUSTER-ASSEMBLED CARBON THIN FILMS

Surface morphologies of nanostructured carbon films, produced by supersonic cluster beam deposition, have been investigated by atomic force microscopy. From topographical images, we evaluated the surface roughness corresponding to different scan lengths and thicknesses, and deduced that the images examined are self-affine. Experimental scaling exponents and the hypothesis of non-destructive primeval cluster aggregation suggest that quenched noise effects may be important in the process of growth of the nanostructured surfaces.