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Metal oxide nanowires have been deposited on 100nm SiO 2 /Si substrates and SnO 2 and ZnO single nanowire devices have been fabricated by electron beam lithography technique. Gas sensing tests have been carried out, in order to detect ammonia, CO and NO 2 under standard humidity conditions. The devices have been also tested in either dark condition or exposed to ultraviolet light at room temperature. SnO 2 nanowires are able to detect ammonia at concentration of the order of few ppm also in dark conditions, they are sensitive to NO 2 and cannot detect CO. ZnO devices show good sensitivity to 5ppm of ammonia in 30% relative humidity atmosphere. The samples have been mounted on a TO-39 case in a field effect transistor configuration in order to study the influence of the back-gate voltage on the single nanowire devices gas sensing performances. [ABSTRACT FROM AUTHOR]

It is almost a decade since the first presentation of metal oxide nanowires as chemical sensors. Significant advances have been made both in terms of preparation procedures and their integration into functional sensing devices, whilst the progress in their fundamental understanding of functional properties has been slow. In fact, the full integration still remains a challenge that has been wisely approached in different ways. In this article we review the most recent developments in bottom up and top down approaches for applications of chemical sensors. [Copyright &y& Elsevier]

The present paper compares three different kinds of semiconductor gas sensing materials: metal oxides (MOX), hydrogenterminated diamond (HD), and hydrogenated amorphous silicon (a-Si:H). Whereas in MOX materials oxygen is the chemically reactive surface species, HD and a-Si:H are covalently bonded semiconductors with hydrogenterminated surfaces. We demonstrate that these dissimilar semiconductor materials exhibit the same kind of low-temperature gas response. This low-temperature response-mechanism is mediated by a thin layer of adsorbed water with the semiconductor materials themselves acting as pH sensors. In this adsorbate-limited state the gas sensitivity is limited to molecular species that can easily dissolve in H2O and subsequently undergo electrolytic dissociation. At higher temperatures, where a closed layer of adsorbed water can no longer exist, the gas response is limited by direct molecule-semiconductor interactions. In this latter mode of operation, MOX gas sensors respond to adsorbed gases according to their different oxidising or reducing properties. Hydrogenated amorphous silicon (a-Si:H), on the other hand, exhibits a significantly different cross sensitivity profile, revealing that gas-surface interactions may largely be restricted to analyte molecules with lone-pair and electron-deficient three-centre orbitals. [ABSTRACT FROM AUTHOR]

Quasi-one-dimensional nanostructures of tin oxide were produced in controlled conditions through condensation from the vapor phase. The preparation was assisted by noble metal catalysts and uniform single-crystalline nanowires were produced. The nucleation of nanowires was achieved at 470 °C, owing to the vapor–liquid–solid growth mechanism activated by the catalytic Pt clusters. The peculiar microstructural properties of these semiconducting metal oxide nanostructures will be summarized. The high aspect ratio and the high degree of crystallinity achieved for the nanowires foresee their functional exploitation. [ABSTRACT FROM AUTHOR]

Abstract: We perform feature selection (FS) on an electronic nose (EN) dataset composed of 30 features, obtained by extracting 5 diverse features from the response curves of six metal oxide sensors. The 5 features are: the classical relative change in resistance R/R 0; the curve integral both over the gas adsorption and desorbtion process and the phase space integral, again over adsorption and desorbtion. The phase space integral is a novel feature introduced in . We show that performance (in terms of the cross validated test error of a three nearest neighbour classifier) is always significantly better for the best selected features than for all 30 features. Moreover – for some of the 5 features types – performance with all 30 features is worse than performance with just the 6 features of a single type. Results are not univocal regarding the best feature type. Yet, on average over the four datasets in which the complete dataset can be decomposed, the phase integral calculated over the desorption wins. Also, the features (phase and integral) calculated on the desorbtion seem to consistently give higher performance than the corresponding features calculated during adsorption. The standard R/R 0 stands in the lower part of the ranking. [Copyright &y& Elsevier]

Abstract: We report on measurements of the gas response time of pure and platinum (Pt) catalysed SnO2 films using the moving gas outlet method. In this mode of operation changing gas flows are forced directly onto the sensor surface to avoid the formation of a stagnant layer and to reveal gas response times unaffected by the usual diffusive delays. We find that the intrinsic response and recovery time constants τ follow an exponential temperature dependence τ(T)= τ 0exp(E a/k B T), with E a standing for the activation energies for adsorption or desorption, respectively. We find that catalytic enhancement lowers the activation energies both for adsorption and desorption, with the more pronounced changes generally taking place in the case of the adsorption time constants. We further show that catalyst-induced changes in E a are accompanied by changes in the prefactor τ 0 which partly compensate the accelerating effect of a lower E a. [Copyright &y& Elsevier]

Abstract: We present an analysis of roasted coffee ripening performed by the novel Electronic Olfactory System EOS835, manufactured by the Italian company Sacmi Imola s.c.a.r.l., which is based on thin film semiconductor metal oxide gas sensors. We focused our analysis on: (1) exploratory data analysis for systematically investigating the outcomes of different sampling conditions and therefore selecting advantageous settings; (2) feature selection for improving classification performance and ranking the contribution of the different sensors and feature types. Exploratory analysis, via the successive generation of PCA plots, showed that the main factors influencing discrimination between diverse ripening times are headspace generation time (HGT, i.e. time elapsed between vial filling and measurement performing) and sample preparation. A relatively long HGT (18h) allows to follow the ripening progression of the coffee blend over time and to correctly classify the best coffee ripening (as determined by an expert taster). In forming the feature vector, we added a feature calculated in the phase space to the standard features. Feature selection showed that, the phase space feature consistently lead to improved classification and that, of the three sensor types constituting the array, the two indium–tin oxide sensors perform better for our application. [Copyright &y& Elsevier]

We observe the change of aromatic compounds in the headspace of white truffles (Alba’s Truffle) after storage at +4 °C over a period of a few days. Measurements have been performed using SPME-GC-MS technique and the Pico2-electronic nose (EN) developed at Sensor Lab. in Brescia. The EN shows a very high sensitivity towards the truffle’s relevant molecules such as 2,4-dithiapentane and is able to detect gasses from truffle samples up to a mass of the order of 10 mg. As for truffle aging, results obtained with both techniques are strongly correlated and confirm that there is a variation of the truffle’s headspace after circa 5 days. [Copyright &y& Elsevier]

Abstract: The proof of principle of the successful application of spray deposition to the SILAR (successive ionic layer absorption and reaction) technique, one of the most effective strategies to sensitized TiO2 scaffold with QDs, is demonstrated. Systematically improved optical features of the materials (higher optical density together with reduced nanocrystal sizes) as well as of the functional performances of QD solar cells (photoconversion efficiency, fill factor, short circuit current, open circuit voltage) sensitized via SD-SILAR, with respect to traditional SILAR sensitization based on impregnation, are demonstrated. [Copyright &y& Elsevier]

Abstract: This work is the continuation of two previous works: in the former we developed a WO3 based gas sensor featuring high sensitivity to NO2 but with the drawbacks of low conductance values (below 1nS) and long recovery times (over 20min); in the latter we developed a low-cost control electronic circuit suited to measure a wide range of conductance values (10pS–100μS). Here, we use this electronic system to control this WO3 sensor according to temperature profile protocols, with the aim to show the possibility to handle such low conductance devices by means of cheap instrumentation, featuring at the same time reduce response times and a degree of selectivity arising from the temperature profile protocol. In particular, we focus on two target applications: detection of NO2 and detection of reducing gases, namely ethanol and methane, in different humidity conditions, showing the usefulness of time constants extrapolated from the response dynamics for the purpose. [Copyright &y& Elsevier]

Abstract: In this work we studied deposition conditions by RF sputtering of ternary oxides of Cd and Sn, starting from Cd2SnO4 target and varying substrate temperature, sputtering power and deposition gas (from inert Ar to oxidizing 50% Ar–50% O2 atmosphere). The aim of this study was to obtain thin films for use as Transparent Conducting Oxide (TCO). TCOs are oxides that couples low sheet resistance and high transparency that find application in many fields like solar cells, light emitting diodes and transparent thin film transistors. Thin films functional properties were characterised by means of sheet resistance and transmittance measurements in the visible region, and film composition and structure were investigated by total reflection X ray fluorescence and glancing incidence X ray diffraction. Morphology was studied by Atomic Force Microscopy and Scanning Electron Microscope and showed very smooth surface suitable for solar cells application. Composition and phase analysis allowed us to discuss possible correlation of film structure with functional properties. Deposition in inert atmosphere at 400°C substrate temperature was selected for its low sheet resistance and high transparency that are comparable to the ones of commercial TCOs like indium tin oxide or SnO2: F. The thin film obtained in these conditions was amorphous, and it crystallized into CdSnO3 ilmenite phase when annealed at 700°C; segregation of Sn3O4 was also observed. Since sheet resistance of thin films increases after annealing treatments, amorphous thin film was selected for future applications. [Copyright &y& Elsevier]

Abstract: In this study a metal oxide sensor array is exposed to a time-dependent amount of gas inside the sensor chamber of negligible “dead” volume. Special parameters of the response kinetics are used for multi-parametric featuring of volatile organic compounds (VOCs). The composition of the atmosphere in the chamber varies due to the time-dependent release of the VOCs from a solid phase micro-extraction (SPME) fiber into the flow of synthetic air. Four types of volatile compounds, namely acetone, acetic acid, acetaldehyde and butyric acid, that are known being frequently emitted from infected wounds, are tested in this study. The explorative data analysis (EDA) of the features is performed for the sensor outputs obtained at different carrying gas flow rates and the VOC amounts. Influence of specific aspects of the SPME based sampling on the sensor outputs is estimated. It is demonstrated by the PCA results that the target compounds cannot be distinguished below 3–4ppm if only the sensor outputs based on the signal magnitudes are used for the featuring of VOCs (static-compatible features). The dynamic features add significant information and allow a better discrimination of the volatile compounds. The classification of the target volatile compounds can additionally be improved by precise control of the VOCs expansion in the chamber in the dynamic exposure approach. [Copyright &y& Elsevier]

Abstract: Pt/WO3/SiC devices based Schottky diodes have been fabricated and their hydrogen and hydrocarbon gas sensing performance investigated. The semiconducting metal oxide WO3 films were 100nm thick and prepared using r.f. magnetron sputtering. A Pt layer was deposited on the top of the WO3 forming the Schottky diode. The I–V characteristics have been analyzed, from which the change in the barrier height, subject to the introduction of the analyte gas, was calculated. The dynamic properties of these sensors were evaluated at constant biasing currents of 9 and 90μA, at temperatures between 300 and 700°C. The results show the sensors have extremely stable baseline, with all responses being repeatable. Voltages shifts in excess of 1V were observed. [Copyright &y& Elsevier]

Abstract: Quasi-one-dimensional nanostructures (so-called nanobelts or nanowires) of tin dioxide, obtained by vapour phase deposition, have been investigated for gas sensing. The physical quantity for signal transduction could be either electrical conductance or photoluminescence. Tin dioxide nanobelts/nanowires could be used as conductometric gas sensors for oxygen and environmental polluting species like CO and NO2 as well as ethanol. The visible photoluminescence is quenched by nitrogen dioxide at hundreds of ppb in a fast (time scale order of seconds) and reversible way. The response is highly selective towards humidity and other polluting species like CO and NH3. The differences between conductometric and optical sensor are discussed. [Copyright &y& Elsevier]

The kinetics of the resistance response to a steep change in gas composition is experimentally and theoretically investigated in SnO2-based thin film sensors modified by different catalytic metals. H2 and CO gases were used in the tests. Based on the phenomenological model, the transients of the resistance response are described by a combination of the characteristics of both the layer and the surface chemical reaction. The time constants are introduced for characterisation of the response kinetics and are defined by the phenomenology using the rates of the reaction. An influence of the type and density of metallic impurities on the time constants and the rates of the surface processes is discussed. [Copyright &y& Elsevier]