Valeria Guglielmotti

Characterization of carbon structures produced by graphene self-assembly

Low-dimensional carbon-based materials, in particular two-dimensional graphenic carbon structures, have been produced from single-walled carbon nanotube disruption using high-shear mixing and/or treatments in sulfonitric acid mixtures at both room and high temperature. Among other two-dimensional graphenic carbon structures, colloidal dispersions of graphenic nanoflakes have been obtained. Different structural arrangements, resulting from the reorganization of carbon because of the disruption procedures applied, were observed through selected area electron diffraction (SAED) and through reflection high-energy electron diffraction (RHEED) analyses coupled to transmission and scanning electron microscopy observations. Such combined investigations in the real and reciprocal space provided structural information at the nanoscale on the clustering of graphene layers in nanoplatelets or/and on their assembly into highly ordered (single-crystal) nanosheets. Furthermore, a different carbon phase exhibiting an orthorhombic cell with Cmma symmetry has been detected by SAED and RHEED analyses. In addition, a variety of self-assemblies of hexagonal basal planes have been observed to occur as the result of their different rotational and/or translational stacking faults. Overall, the reported results contribute to define the conditions for a controlled self-assembly of graphene-based structures with tailored dimensions, which is an important technological challenge, as their structure at the nanoscale dramatically affects their electrical properties.

Nanocarbon surfaces for biomedicine

The distinctive physicochemical, mechanical and electrical properties of carbon nanostructures are currently gaining the interest of researchers working in bioengineering and biomedical fields. Carbon nanotubes, carbon dendrimers, graphenic platelets and nanodiamonds are deeply studied aiming at their application in several areas of biology and medicine. Here we provide a summary of the carbon nanomaterials prepared in our labs and of the fabrication techniques used to produce several biomedical utilities, from scaffolds for tissue growth to cargos for drug delivery and to biosensors.

Role of the Material Electrodes on Resistive Behaviour of Carbon Nanotube-Based Gas Sensors for H 2 SDetection

Miniaturized gas-sensing devices that use single-walled carbon nanotubes as active material have been fabricated using two different electrode materials, namely, Au/Cr and NbN. The resistive sensors have been assembled aligning by dielectrophoresis the nanotube bundles between 40 μm spaced Au/Cr or NbN multifinger electrodes. The sensing devices have been tested for detection of the H2S gas, in the concentration range 10–100 ppm, using N2 as carrier gas. No resistance changes were detected using sensor fabricated with NbN electrodes, whereas the response of the sensor fabricated with Au/Cr electrodes was characterized by an increase of the resistance upon gas exposure. The main performances of this sensor are a detection limit for H2S of 10 ppm and a recovery time of few minutes. The present study suggests that the mechanism involved in H2S gas detection is not a direct charge transfer between molecules and nanotubes. The hypothesis is that detection occurs through passivation of the Au surfaces by H2S molecules and modification of the contact resistance at the Au/nanotube interface.

Conducting polymer/nanodiamond composites: New opportunities and scientific challenges for material science

We present some recent results of a systematic study regarding polymer-based nanocomposites with carbon nanostructures as guest components. This paper is focused on the coupling of conductive polymers, as polyanilines and polythiophenes, with nanodiamond grains. The peculiar properties of such crystalline nanomaterial not only help in improving the mechanical and thermal properties of the host polymeric matrices, but also act on the polymerization mechanism, modifying the morphological features of the polymers. Triggered by the nanodiamond presence, the final nanocomposites show a prominent arrangement of polymer segments into tubular forms and a subsequent structural organization suitable for many technological applications.

Low temperature conductivity of carbon nanotube aggregates

We compare, over wide temperature ranges, the transport properties of single-wall carbon nanotubes arranged in the form of aligned arrays or in the form of fibres. The experimental data show that both the forms of aggregates present a crossover in the transport mechanism from three-dimensional hopping of the electrons between localized states at high temperature to fluctuation-induced tunnelling across potential barriers at low temperature. The role of the junctions formed between the bundles in the array and between the nanotubes inside the fibres is discussed on the basis of the experimental results.

Carbon Nanotubes Guides for Nickel Electrical Interconnects

We present an innovative approach for interconnections based on aligning nanosized nickel particles along single-walled carbon nanotubes bundles. The nanotubes are first deposited to connect normal gold contacts or niobium nitride superconducting pads and a successive step in fabrication, based on an electrochemical method, enables us to “coat” the nanotubes with nickel particles. This technique, as shown by field-emission SEM analysis, generates chains of nickel particles aligned along the nanotubes connecting the contact electrodes. The temperature dependence of the resistance of the chains of Ni nanoparticles evidences the metallic nature of the obtained interconnection and shows values of the resistances up to three orders of magnitudes lower than those of the bare nanotubes bundles. The ease of the fabrication process and its reliability candidate our technique as a possible solution for interconnects applications.

Preparation and Thermal Characterization of Carbon Nanotubes-Based Composites for Applications in Electronics Packaging

The thermal resistance of nanocomposite layers formed by Single Wall Carbon Nanotubes (SWCNT) dispersed in epoxy resins has been measured under conditions similar to the ones used to dissipate heat in microelectronic devices. The variation of thermal conductivity as a function of concentration of SWCNT is reported and discussed with reference to the dispersion state of SWCNT in the layers.

Carbon nanotubes field emission enhancement using a laser post treatment

This paper reports the fabrication of a cold cathode based on carbon nanotubes (CNTs) using a combined chemical vapor deposition technique and a laser processing treatment. The CNTs are exposed to a focused 514 nm continuous-wave laser at varying power, and the changes of the CNTs characteristics after each laser treatment are investigated by Raman spectroscopy and scanning electron microscopy. It is seen that a laser treatment at 70 mW produces an improved emitter, characterized by an increase of the field emitted current up to 350 times, which obtains a maximum current density of 0.13 A/cm2. The CNT deposit exhibits good time stability and a good resistance under high applied electric fields up to 33 V/μm. The results obtained in terms of current density make the laser treatment an excellent process for increasing the performance of cathodes to be used in electron guns for vacuum tubes where high current densities are required.

Current modulation of a field emission microtriode based on carbon nanotubes

Since 1960s field emission microelectronic vacuum devices have been proposed by Spindt [1] as possible trade-off between the thermionic and solid state devices. These are basically microfabricated molybdenum or silicon micro-tips arrays in gated configuration that use the local field enhancement at the apex of each emitters to lower the threshold voltage to promote the field emission [2,3].

Preparation and Functional Characterizations of Ta2O5 Deposits Organized at the Micro- and Nano-scale

The present communication deals with the coating of bundles of single wall carbon nanotube (SWCNTs) by nanocrystalline tantalum pentoxide (Ta2O5) deposits. Arrays of SWCNTs bundles are produced on Si and AlN plates by means of a Hot Filament CVD apparatus, using as reactants carbon nanopowders and in-situ produced atomic hydrogen. A subsequent process, carried out in an O2-rich atmosphere, enables to cover the bundles by a uniform deposit of Ta2O5 nanograins. The coating process does not modify the organization of the pristine nanotube deposits. A series of SWCNT/Ta2O5 samples, characterized from both the morphological and structural point of view, have been tested as cathodes in Field Emission experiments (F.E.). The F.E. experiments proved that these hybrid SWCNT/dielectric systems are good field emitters and that their properties make them suitable for the realization of cold cathodes.