N. Donato

Room‐Temperature Hydrogen Sensing with Heteronanostructures Based on Reduced Graphene Oxide and Tin Oxide

Theres something in the air … A nanocomposite consisting of well-dispersed SnO(2) and Pt nanoparticles on reduced graphene oxide (see the high-resolution TEM image) exhibited very high responses to hydrogen at concentrations between 0.5 and 3% in air, with response times of 3-7 s and recovery times of 2-6 s. The sensor was prepared by a straightforward microwave-assisted non-aqueous sol-gel approach.

CO and NO 2 Selective Monitoring by ZnO-Based Sensors

ZnO nanomaterials with different shapes were synthesized, characterized and tested in the selective monitoring of low concentration of CO and NO2 in air. ZnO nanoparticles (NPs) and nanofibers (NFs) were synthesized by a modified sol-gel method in supercritical conditions and electrospinning technique, respectively. CO and NO2 sensing tests have demonstrated that the annealing temperature and shape of zinc oxide nanomaterials are the key factors in modulating the electrical and sensing properties. Specifically, ZnO NPs annealed at high temperature (700 °C) have been found sensitive to CO, while they displayed negligible response to NO2. The opposite behavior has been registered for the one-dimensional ZnO NFs annealed at medium temperature (400 °C). Due to their adaptable sensitivity/selectivity characteristics, the developed sensors show promising applications in dual air quality control systems for closed ambient such as automotive cabin, parking garage and tunnels.

Photoreduction of mesoporous In2O3: mechanistic model and utility in gas sensing.

A model is proposed for the drop in electronic resistance of n-type semiconducting indium oxide (In(2)O(3)) upon illumination with light (350 nm, 3.5 eV) as well as for the (light-enhanced) sensitivity of In(2)O(3) to oxidizing gases. Essential features of the model are photoreduction and a rate-limiting oxygen-diffusion step. Ordered, mesoporous In(2)O(3) with a high specific surface area serves as a versatile system for experimental studies. Analytical techniques comprise conductivity measurements under a controlled atmosphere (synthetic air, pure N(2)) and temperature-resolved in-situ Fourier transform infrared (FTIR) spectroscopy. IR measurements reveal that oxygen vacancies form a donor level 0.18 eV below the conduction band.

Microwave characterization and modeling of packaged HEMTs by a direct extraction procedure down to 30 K

The knowledge of the small-signal equivalent circuit of microwave GaAs field effect transistors (FETs) is crucial for the design of low-noise amplifiers and is very useful to support the analysis of the transistor performance. This paper reports the results of our experimental activity concerning the application of an improved procedure for the direct extraction of the model element values from scattering (S-) parameter measurements. This analytical procedure was tested on low-noise pseudomorphic high electron mobility transistors (pHEMTs) up to 6 GHz and at cryogenic temperatures without any optimization or tuning adjustment. This paper reports the behavior of the intrinsic elements versus bias condition; the experimental results were found to match the theoretical expectations. The very good agreement between the simulated and measured S-parameters confirms the validity of the proposed method. To carry out the experimental activity, a properly designed cryogenic setup operating in our laboratory, which allows performing direct current (dc) and microwave characterization down to 30 K, was employed.

CdO-based nanostructures as novel CO2gas sensors

Crystalline Cd(OH)(2)/CdCO(3) nanowires, having lengths in the range from 0.3 up to several microns and 5-30 nm in diameter, were synthesized by a microwave-assisted wet chemical route and used as a precursor to obtain CdO nanostructures after a suitable thermal treatment in air. The morphology and microstructure of the as-synthesized and annealed materials have been investigated by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and thermogravimetry-differential scanning calorimetry. The change in morphology and electrical properties with temperature has revealed a wire-to-rod transformation along with a decreases of electrical resistance. Annealed samples were printed on a ceramic substrate with interdigitated contacts to fabricate resistive solid state sensors. Gas sensing properties were explored by monitoring CO(2) in synthetic air in the concentration range 0.2-5 v/v% (2000-50 000 ppm). The effect of annealing temperature, working temperature and CO(2) concentration on sensing properties (sensitivity, response/recovery time and stability) were investigated. The results obtained demonstrate that CdO-based thick films have good potential as novel CO(2) sensors for practical applications.

Real-time monitoring of breath ammonia during haemodialysis: use of ion mobility spectrometry (IMS) and cavity ring-down spectroscopy (CRDS) techniques

BACKGROUND The diffusion of high-performance analytical technology has opened prospects for breath diagnosis as a non-invasive diagnostic tool. In this study, ion mobility spectrometry (IMS) and cavity ring-down spectroscopy (CRDS) techniques were used to analyse ammonia gas (NH3) in real-time in breath from patients undergoing haemodialysis (HD) treatment and any correlation with blood urea nitrogen (BUN) levels and Kt/V were investigated. METHODS We studied 20 patients on intermittent HD treatment. The first breath samples were taken before the start of dialysis and further breath samples were taken every hour during the treatment and after the end of the session. An evaluation was also made of 20 healthy volunteers, acting as controls [healthy subjects (HS)]. RESULTS Breath ammonia concentrations were higher in CRDS-HD (914.5±301.4 versus 280±120 parts per billion (p.p.b.), P<0.0001) and IMS-HD patients (964.4±402.4 versus 280±120 p.p.b., P<0.0001) than in HS. We assessed real-time variations in the levels of NH(3) and showed a continuous decrease in the levels of NH3. Expired NH3 correlated directly with BUN levels, both in the IMS-HD (P=0.002; r=0.84; P=0.009; r=0.76) and in the CRDS-HD group (P=0.005; r=0.80; P=0.008; r=0.77), respectively, both before and at the end of dialysis. A direct correlation with Kt/V was found in both groups studied (IMS-HD: P=0.003; r=0.82; CRDS-HD: P=0.006; r=0.79). CONCLUSIONS Breath monitoring of NH3 with IMS and CRDS techniques could be useful to assess the real-time clinical status of patients during HD. By using pre-dialysis ammonia values, an approximate calculation of the Kt/Vurea ratio can be established.

Pt-TiO2/MWCNTs Hybrid Composites for Monitoring Low Hydrogen Concentrations in Air

Hydrogen is a valuable fuel for the next energy scenario. Unfortunately, hydrogen is highly flammable at concentrations higher than 4% in air. This aspect makes the monitoring of H2 leaks an essential issue for safety reasons, especially in the transportation field. In this paper, nanocomposites based on Pt-doped TiO2/multiwalled carbon nanotubes (MWCNTs) have been introduced as sensitive materials for H2 at low temperatures. Pt-TiO2/MWNTs nanocomposites with different composition have been prepared by a simple wet chemical procedure and their morphological, microstructural and electrical properties were investigated. Resistive thick-film devices have been fabricated printing the hybrid nanocomposites on alumina substrates provided with Pt interdigitated electrodes. Electrical tests in air have shown that embedding MWCNTs in the TiO2 matrix modify markedly the electrical conductivity, providing a means to decrease the resistance of the sensing layer. Pt acts as a catalytic additive. Pt-TiO2/MWNTs-based sensors were found to be sensitive to hydrogen at concentrations between 0.5 and 3% in air, satisfying the requisites for practical applications in hydrogen leak detection devices.

Photovoltaic properties of multi-walled carbon nanotubes deposited on n-doped silicon

Multi-wall carbon nanotubes (MWCNTs), grown by catalytic chemical vapor deposition (CCVD) over Fe supported on alumina catalyst, using isobutane as feedstock, are dispersed in aqueous solutions of sodium dodecyl sulfate. Stable and highly photosensitive heterojunctions are developed by liquid-phase deposition of MWCNTs/surfactant mixtures on the top of n-doped monocrystalline silicon wafers. Electrical measurements performed in the dark and under broad-band visible light, show that the hybrid solar cells, despite their non-optimized design, have conversion energy efficiency of the order of 3%.

A robust and fast procedure for the determination of the small signal equivalent circuit of HEMTs

Abstract In this paper we present an analytical, fast, accurate and robust technique for the determination of the circuit model elements of HEMTs in the microwave range. By this method the values of the equivalent circuit parameters of the device under test are extracted using three measured scattering (S) parameter sets without any optimization. We also investigated the influence of the reverse transfer conductance Re(Y12) on the modelling by means of a gate drain resistance Rdg. The validity of this method was verified upon a set of pseudomorphic HEMTs having different gate widths tested on wafer at several bias and temperature conditions. Very good agreement between the simulated and measured S-parameters has been obtained. The procedure has been implemented in Agilent VEE language as a fully automated tool to allow an accurate, fast and complete device characterization requiring no operator supervision.

Temperature effects on DC and small signal RF performance of AlGaAs/GaAs HEMTs

We here report on the DC and microwave performance of HEMTs tested on wafer under different temperature conditions. The relevant experimental data show that the most important electrical parameters, such as the output current, the threshold voltage, the transconductance and the forward transmission coefficient, are sensibly affected by thermal phenomena. We focused our attention on the variations of the above parameters with the temperature because such a detailed knowledge is essential to establish the optimum bias point for a given application. Furthermore, we analyze the influence of the DC quiescent point degradations, due to thermal phenomena, on the small signal equivalent circuit. Since the thermal behavior of the circuit model is a function of the bias, we examine the behavior of the circuit elements vs. temperature over a wide range of bias conditions.