S. Orlanducci

Carbon nanotubes for gas detection: materials preparation and device assembly

An efficient sensing device for NH3 and NOx detection has been realized using ordered arrays of single-walled C nanotubes deposited onto an interdigitated electrode platform operating at room temperature. The sensing material has been prepared using several chemical–physical techniques for purification and positioning of the nanotubes inside the electrode gaps. In particular, both DC and AC fields have been applied in order to move and to align the nanostructures by electrophoresis and dielectrophoresis processes. We investigated the effects of different voltages applied to a gate contact on the back side of the substrate on the performances of the device and found that for different gas species (NH3, NOx) a constant gate bias increases the sensitivity for gas detection. Moreover, in this paper we demonstrate that a pulsed bias applied to the gate contact facilitates the gas interaction with the nanotubes, either reducing the absorption times or accelerating the desorption times, thus providing a fast acceleration and a dramatic improvement of the time dependent behaviour of the device.

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.

Flip-Cathode Design for Carbon Nanotube-Based Vacuum Triodes

In this letter, we report on the realization of a carbon nanotube-based field emission triode with a new flip-chip design of the cathode. Our fabrication method is highly versatile and allows the use of various emitting materials, overcoming at the same time the severe limitations of material choices for the nanotriode fabrication. The obtained device shows a good modulation behavior and low losses. This technology opens a new way for the realization of highly efficient and scaled vacuum field emission devices.

Piezoresistive behaviour of single wall carbon nanotubes

Correlation between mechanical deformation and conductivity behaviour of free-standing membranes of single walled carbon nanotubes (SWCNTs) has been successfully investigated. The aim of this study is proposing this kind of material as strain and pressure sensors. The nanotubes gauge factor of piezoresistivity has been evaluated by comparing the electrical responses induced by deformation in SWCNT membrane and in Si substrate: it proved to be factor 2.3-2.5 larger than that of the Si substrate.

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.

Quartz crystal nano-balance for hydrogen sensing at room temperature using carbon nanotubes aggregates

The gas sensor based on carbon nanotubes are presently receiving considerable attention because of the outstanding properties, such as faster response, higher sensitivity, lower operating temperature and robustness of the nanotubes in comparison with the other types of sensing materials. In the present research, we demonstrate detection of hydrogen at room temperature using a Quartz Crystal Nano-balance (QCN) and as sensing material, Single-Walled Carbon Nanotubes (SWCNTs) dispersed in a polythiophene matrix. The experimental determination of H2 in H2/N2 mixtures has been performed by using a counter frequency and observing the frequency shifts induced in a quartz crystal resonator by H2 adsorption and consequent mass variation of the active layer deposited on the quartz. The high sensitivity of the realized nano-balance allows us to observe mass variations up to few nanograms /Hertz and to detect up to 1% of H2. The good sensing performances of the nanotube-based material make unnecessary the use of any catalyst species for H2 detection. Moreover this QCN device is able to work with good efficiency at 23 °C and 1 Atm.

Realization of a carbon nanotube-based triode

We report the design and realization of a carbon nanotube-based integrated triode. Patterned Si/SiO2/Nb/Nb2O5multilayer was successfully realized by means of a photolithographic process. Such structure constitutes the patterned substrate of the successive Hot Filament Chemical Vapour Deposition (HFCVD) process. Selective growth of highly oriented SWCNT arrays was obtained in the predefined locations while survival of the entire structure was achieved. Field emission measurements of such materials were carried out both on a diode and in a triode configuration. Good and reproducible field emission behaviour has been observed in several realized structures. In order to validate the experimental data a first simulation of the behaviour of the integrated vacuum triode with a single field emission CNT was also simulated.

Nanodiamonds: The ways forward

We present here a short overview of the main classes of methods used to generate diamond nanostructures. The described methodologies, namely the CVD techniques, the explosive reactions, the laser-induced processes and the plasma treatments, offer the feasibility to produce nanosized diamonds in forms of powders or films, to modulate size, shape and structure of individual nanograins or of nanodiamond aggregates, to build complex architectures. A proper design and a subsequent controlled production of diamond structures at the nanoscale are strict requirements for the transition from fundamental material research to real-world applications.

Carbon Nanotubes Films for Sensing Applications: From Piezoresistive Sensor to Gas Sensing

In this work we present the study of the sensing properties of Carbon nanotube films. We show the realization of deformation sensors based on composite CNT-polymer films realized by electrochemical deposition starting from a dispersion of SWCNTs and EDOT monomer. The gauge factor (GF) of the sensors has been found to be up to 3–4 times higher than that of the commercial strain gauge. We show also that the CNT have gas sensing properties, that can be applied to different gases, included H2O in gaseous form. Detection can be optimized by means of electrostatic polarization of the film substrate.

Photoemission from nano-structured a-C/diamond layers irradiated by intense Nd:YAG laser harmonics

We investigated the influence of the surface state of diamond layers on the characteristics of the photoemission induced by 4.7 eV photons. A series of diamond samples grown by CVD under slightly different conditions have been analysed. Polycrystalline diamond layers with nanoscale graphitic patches embedded at the grain boundaries are found to exhibit unusually high efficiency of electron photoemission. The photoemitting properties of the different samples are rationalized by considering the electron emission process located at the a-C/diamond/vacuum triple border and the quantum efficiency (Q.E). governed by the ratio of amorphous sp2-C to crystalline sp3-C. At 4.7 eV values of quantum efficiency up to 1.5 x 10-5 have been measured and the trends of the experimental Q vs J curves indicate that photoemission occurred mainly under one-photon regimes.