Massimiliano Lucci

Indentation modulus and hardness of viscoelastic thin films by atomic force microscopy: A case study

We propose a nanoindentation technique based on atomic force microscopy (AFM) that allows one to deduce both indentation modulus and hardness of viscoelastic materials from the force versus penetration depth dependence, obtained by recording the AFM cantilever deflection as a function of the sample vertical displacement when the tip is pressed against (loading phase) and then removed from (unloading phase) the surface of the sample. Reliable quantitative measurements of both indentation modulus and hardness of the investigated sample are obtained by calibrating the technique through a set of different polymeric samples, used as reference materials, whose mechanical properties have been previously determined by standard indentation tests. By analyzing the dependence of the cantilever deflection versus time, the proposed technique allows one to evaluate and correct the effect of viscoelastic properties of the investigated materials, by adapting a post-experiment data processing procedure well-established for standard depth sensing indentation tests. The technique is described in the case of the measurement of indentation modulus and hardness of a thin film of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate), deposited by chronoamperometry on an indium tin oxide (ITO) substrate.

Quantitative measurement of indentation hardness and modulus of compliant materials by atomic force microscopy

An atomic force microscopy (AFM) based technique is proposed for the characterization of both indentation modulus and hardness of compliant materials. A standard AFM tip is used as an indenter to record force versus indentation curves analogous to those obtained in standard indentation tests. In order to overcome the lack of information about the apex geometry, the proposed technique requires calibration using a set of reference samples whose mechanical properties have been previously characterized by means of an independent technique, such as standard indentation. Due to the selected reference samples, the technique has been demonstrated to allow reliable measurements of indentation modulus and hardness in the range of 0.3-4.0 GPa and 15-250 MPa, respectively.

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.

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.

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.

Superconductive niobium films coating carbon nanotube fibers

Superconducting niobium (Nb) has been successfully obtained by sputter deposition on carbon nanotube fibers. The transport properties of the niobium coating the fibers are compared to those of niobium thin films deposited on oxidized Si substrates during the same deposition run. For niobium films with thicknesses above 300 nm, the niobium coating the fibers and the thin films show similar normal state and superconducting properties with critical current density, measured at T = 4.2 K, of the order of 105 A cm−2. Thinner niobium layers coating the fibers also show the onset of the superconducting transition in the resistivity versus temperature dependence, but zero resistance is not observed down to T = 1 K. We evidence by scanning electron microscopy (SEM) and current-voltage measurements that the granular structure of the samples is the main reason for the lack of true global superconductivity for thicknesses below 300 nm.

Characterization of anomalous pair currents in Josephson junction networks

Measurements performed on superconductive networks shaped in the form of planar graphs display anomalously large currents when specific branches are biased. The temperature dependences of these currents evidence that their origin is due to Cooper pair hopping through the Josephson junctions connecting the superconductive islands of the array. The experimental data are discussed in terms of theoretical models which predict, for the system under consideration, an inhomogeneous Cooper pair distribution on the superconductive islands of the network as a consequence of a Bose-Einstein condensation phenomenon.

Anodization-based process for the fabrication of all niobium nitride Josephson junction structures

We studied the growth and oxidation of niobium nitride (NbN) films that we used to fabricate superconductive tunnel junctions. The thin films were deposited by dc reactive magnetron sputtering using a mixture of argon and nitrogen. The process parameters were optimized by monitoring the plasma with an optical spectroscopy technique. This technique allowed us to obtain NbN as well as good quality AlN films and both were used to obtain NbN/AlN/NbN trilayers. Lift-off lithography and selective anodization of the NbN films were used, respectively, to define the main trilayer geometry and/or to separate electrically, different areas of the trilayers. The anodized films were characterized by using Auger spectroscopy to analyze compounds formed on the surface and by means of a nano-indenter in order to investigate its mechanical and adhesion properties. The transport properties of NbN/AlN/NbN Josephson junctions obtained as a result of the above described fabrication process were measured in liquid helium at 4.2 K.

Mechanical characterisation of the four most used coating materials for optical fibres

Optical multimode fibres have a wide variety of applications ranging from industrial to medical use. Therefore, even if they are just used as waveguides or sensors, it is important to characterise the whole fingerprint, including the optical and mechanical properties of such fibres. Since the stiffness/elasticity of a material could influence the optical output of a fibre due to micro-bendings, in this paper we report the calculated Youngs Modulus of acrylate, fluorinated acrylate, polyimide and silicone, which are the four most used coating materials for such optical components. The results demonstrate that Youngs Modulus does have an impact on the attenuation of propagating light along the optical fibre. However, the refractive index of the coating materials still has a significant impact on the performance of optical fibres.