S. Botti

Carbon nanotubes and nanowires grown from spherical carbon nano-particles

Abstract In this Letter, we report a new method for the fabrication of carbon nanotubes (CNT) and nanowires where amorphous hydrogenated carbon nano-particles were used as precursor, without metal catalyst addition. In particular, depending on the process parameters, we obtained single walled nanotubes (SWNTs) with mean diameter 1.2 nm and carbon nanowires with mean diameter 250 nm. A discussion on the possible growth mechanism is also reported.

Self-assembled carbon nanotubes grown without catalyst from nanosized carbon particles adsorbed on silicon

Carbon nanotubes films have been prepared by low-velocity spraying of carbon nanosized particles on heated Si substrate. Studies reveal that by properly choosing the deposition temperature, well-aligned carbon nanotubes are self-assembled from the particles without a catalyst. Raman scattering and reflection high-energy electron diffraction show that the tubes are bundles of single-wall nanotubes.

Second- and third- harmonic generation in single-walled carbon nanotubes at nanosecond time scale

Second- and third-harmonic generation in single-walled carbon nanotube films is experimentally investigated with the fundamental 1064nm radiation from a Q-switched Nd:YAG laser. Measurements were performed both on commercially available carbon nanotubes and on samples of carbon nanotubes grown with a catalyst-free method. Third-harmonic generation was observed in both samples while only the sample grown with a catalyst-free method generated a second-harmonic signal. The quantum yield of second- and third-harmonic signal was proven to scale, respectively, with the second and third power of the pump pulse energy up to intensities of 109W∕cm2.

Growth of single-walled carbon nanotubes by a novel technique using nanosized graphite as carbon source

Abstract Single-walled carbon nanotubes (SWCN) have been produced treating nanosized graphitic powders with atomic hydrogen fluxes in a purpose-designed reactor. The Fe-catalyzed synthesis process yielded rather dense networks of nanotubes forming continuous layers adherent to the substrates. Raman spectroscopy in the 140–250 cm −1 region, high-resolution transmission electron microscopy and reflection electron diffraction in selected area configuration have been used to define the structural details of the nanotubes. The analysis evidenced a nanotubes population belonging prevalently to the ( n ,0) achiral symmetry subclass. This new experimental approach for SWCNs growth provides promising conditions for maximizing yields and generating selected tubular configurations.

Thermal shock tests of β-sic pellets prepared from laser synthesized nanoscale sic powders

Abstract Nanoscale SiC powders, produced by laser synthesis from gaseous precursors, have been successfully used to prepare sintered pellets. All the sintered samples showed the low temperature β-SiC structures and presented an enhanced thermal conductivity (> 20%) with respect to materials prepared from commercial powders. Samples hardness and toughness, comparable with commercial products, confirmed the good samples quality. The thermal shock tests have been performed by irradiating the pellets with a Nd-YAG pulsed laser (pulse duration and energy: 0.25 μs and 0.18 J or 0.4 ms and 0.65 J). The laser fluence (power density) was increased by reducing the spot size, up to the appearance of a visible surface damage. The threshold values for the structural damage were quantified using a heat flux parameter φabs√tp. The measured threshold value rose from ca. 20 MW/m2 √s for the best materials prepared from commercial SiC powders to ca. 24 MW/m2√s for the newly developed β-SiC.

Surface-enhanced Raman spectroscopy characterisation of functionalised multi-walled carbon nanotubes

Multi-walled nanotube (MWNT) functionalisation was investigated by surface-enhanced Raman spectroscopy (SERS). The MWNTs were deposited as dilute dispersions on SERS-active substrates. We used nano-structured gold surfaces with various morphologies for our measurements. The surface enhancement effect was used to amplify the Raman signal from functional molecules bound to the nanotube walls. The recorded spectral features allowed for discrimination between the differently functionalised MWNTs. Although the present study is limited to a few examples, our measurements indicate the higher specificity obtained by the SERS approach and its possible use for a systematic study of functionalisation effects on MWNT structures.

Photoluminescence from silicon nanoparticles in a diamond matrix

Composite polycrystalline diamond layers containing light-emitting Si nanoparticles (mean diameter: 3.6 and 6.2 nm) have been prepared by using a chemical-vapor deposition technique coupled with a powder-flowing apparatus. The room-temperature photoluminescence behavior of such material in the 1.6–2.5 eV energy range is found characterized by a strong increase of the emission efficiency from the Si nanostructures embedded in the diamond matrix. Moreover, the studies reveal that it is possible to tune the luminescence frequency in the yellow-green spectral region by selecting the size of the inserted Si grains.

IR multiple-photon excitation of polyatomic molecules: a route towards nanostructures

The availability of high power IR laser sources in the 1980s paved the way to new attractive experiments for driving chemical reactions, based on the selective excitation of vibrational modes in polyatomic molecules up to and above the dissociation threshold. The process was first studied in the collisionless regime for applications to laser isotope separation and selective chemistry, and later in collision-assisted conditions leading to the synthesis of nanostructures. In particular, the mechanisms of IR laser pyrolysis of SiH4 and hydrocarbons were investigated in order to control the production of silicon- and carbon-based nanoaggregates. For this purpose, different on-line diagnostics were utilized to monitor the gas-phase reaction intermediates and the process of particle nucleation and growth. The basic principles of the process of multiple-photon excitation of polyatomic molecules and its applications to the synthesis of nanostructures will be reviewed in this paper. Peculiar optical properties of silicon nanoparticles and carbon nanotubes obtained as final products of the developed processes will be described in relation to remarkable applications in several fields.

Application of micro-Raman spectroscopy for fight against terrorism and smuggling

Abstract. We report the results of Raman measurements on some common military explosives and explosives precursors deposited on clothing fabrics, both synthetic and natural, in concentration comparable to those obtained from a single fingerprint or mixed with similar harmless substances to detect illegal compounds for smuggling activities. Raman spectra were obtained using an integrated portable Raman system equipped with an optical microscope and a 785-nm laser in an analysis of <1  min. The spectral features of each illicit substance have been identified and distinguished from those belonging to the substrate fabric or from the interfering compound. Our results show that the application of Raman spectroscopy (RS) with a microscope-based portable apparatus can provide interpretable Raman spectra for a fast, in-situ analysis, directly from explosive particles of some μm3, despite the contribution of the substrate, leaving the sample completely unaltered for further, more specific, and propedeutic laboratory analysis. We also show how the RS is suitable for detecting illegal compounds mixed with harmless substances for smuggling purposes or for counterfeiting activities.

Raman spectroscopy for the detection of explosives and their precursors on clothing in fingerprint concentration: a reliable technique for security and counterterrorism issues

In this work we report the results of RS measurements on some common military explosives and some of the most common explosives precursors deposited on clothing fabrics, both synthetic and natural, such as polyester, leather and denim cotton at concentration comparable to those obtained from a single fingerprint. RS Spectra were obtained using an integrated portable Raman system equipped with an optical microscope, focusing the light of a solid state GaAlAs laser emitting at 785 nm. A maximum exposure time of 10 s was used, focusing the beam in a 45 μm diameter spot on the sample. The substances were deposited starting from commercial solutions with a Micropipetting Nano-Plotter, ideal for generating high-quality spots by non-contact dispensing of sub-nanoliter volumes of liquids, in order to simulate a homogeneous stain on the fabric surface. Images acquired with a Confocal Laser Scanning Microscope provided further details of the deposition process showing single particles of micrometric volume trapped or deposited on the underlying tissues. The spectral features of each substance was clearly identified and discriminated from those belonging to the substrate fabric or from the surrounding fluorescence. Our results show that the application of RS using a microscope-based apparatus can provide interpretable Raman spectra in a fast, in-situ analysis, directly from explosive particles of some μm3 as the ones that it could be found in a single fingerprint, despite the contribution of the substrate, leaving the sample completely unaltered for further, more specific and propaedeutic laboratory analysis. The same approach can be envisaged for the detection of other illicit substances like drugs.