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In this work, we prepared zinc stannate (ZTO) thin films by RF sputtering starting from a sintered ceramic target with Zn:Sn ratio 2:1, by varying the argon/oxygen pressures during the deposition process. Thin films were deposited on alumina substrates at 400 degrees C, and subsequently annealed at 600 degrees C in the air. The transparency typical for the as-deposited films was preserved even after their annealing at 600 degrees C in the air, where the high temperature was crucial for the improvement of the crystalline Zn2SnO4 , ZTO phase, as confirmed by Raman measurements. Additionally, the post-deposition thermal treatment resulted in porous ZTO films suitable for gas sensing application. The structural and surface morphological properties of thin films were investigated by Raman spectroscopy, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX). Finally, the gas sensing properties towards nitrogen dioxide, ethanol and acetone have been tested, showing the high potentiality of this material as the gas sensor for ethanol and acetone at 400 degrees C, and nitrogen dioxide at 200 degrees C.

An experimental approach for the detection of harmful gases in presence of humidity has been applied for gas sensors based on p-type Cu-Al-Oxide thin films. The impact of deposition conditions on the surface, morphology and sensing properties of the semiconducting oxide thin films are investigated. Cu-Al-Oxide thin film with higher resistance can be applied as p-type resistive gas sensor for the detection of pollutant gases. Thin films were characterized by X-Ray Diffraction, scanning electron microscope, and Raman spectroscopy. We observed that inert atmosphere and deposition temperature play the important role to affect the structural and surface morphology of Cu-Al-Oxide thin films. Sensitivity of nanostructured thin films towards reducing and oxidizing gas are studied as a function of gas concentration and operating temperature.

Different ZnO nanostructures were synthesized by physical vapor deposition on glass-ITO substrates. Nanowires and nanosheets were obtained by a single step process using gold nanoparticles and gold thin films as catalyst. 3D nanoarchitectures were obtained by a two-step modified process; the morphology of these structures depends on the catalyst used for the second deposition: gold nanoparticles or zinc acetate seeds. All the nanostructures were characterized by SEM and TEM analyses, which showed the different morphology under same conditions of temperature, pressure, oxide precursor and deposition time. Dye-sensitized solar cells based on these ZnO structures were successfully assembled, using N179 as sensitizer with efficiencies between 0.1% and 0.5%. In spite of the low efficiency of the cells, a novel double PVD process is presented and its integration capability into solar cell devices has been proven. © 2010 Elsevier B.V. All rights reserved., Upprättat; 2010; 20141118 (albvom)

Different ZnO nanostructures were synthesized by physical vapor deposition on glass-ITO substrates. Nanowires and nanosheets were obtained by a single step process using gold nanoparticles and gold thin films as catalyst. 3D nanoarchitectures were obtained by a two-step modified process; the morphology of these structures depends on the catalyst used for the second deposition: gold nanoparticles or zinc acetate seeds. All the nanostructures were characterized by SEM and TEM analyses, which showed the different morphology under same conditions of temperature, pressure, oxide precursor and deposition time. Dye-sensitized solar cells based on these ZnO structures were successfully assembled, using N179 as sensitizer with efficiencies between 0.1% and 0.5%. In spite of the low efficiency of the cells, a novel double PVD process is presented and its integration capability into solar cell devices has been proven. © 2010 Elsevier B.V. All rights reserved., Upprättat; 2010; 20141118 (albvom)

In this paper, the effect of electric field enhancement on Pt/nanostructured ZnO Schottky diode based hydrogen sensors under reverse bias condition has been investigated. Current-voltage characteristics of these diodes have been studied at temperatures from 25 to 620 °C and their free carrier density concentration was estimated by exposing the sensors to hydrogen gas. The experimental results show a significantly lower breakdown voltage in reversed bias current-voltage characteristics than the conventional Schottky diodes and also greater lateral voltage shift in reverse bias operation than the forward bias. This can be ascribed to the increased localized electric fields emanating from the sharp edges and corners of the nanostructured morphologies. At 620 °C, voltage shifts of 114 and 325 mV for 0.06% and 1% hydrogen have been recorded from dynamic response under the reverse bias condition. © 2010 Elsevier B.V. All rights reserved.

Pt/nanostructured ZnO/SiC Schottky contact devices were fabricated and characterized for hydrogen gas sensing. These devices were investigated in reverse bias due to greater sensitivity, which attributes to the application of nanostructured ZnO. The current-voltage (I-V) characteristics of these devices were measured in different hydrogen concentrations. Effective change in the barrier height for 1% hydrogen was calculated as 27.06 meV at 620°C. The dynamic response of the sensors was also investigated and a voltage shift of 325 mV was recorded at 620°C during exposure to 1% hydrogen in synthetic air.

The gas response mechanism of ammonia detection with Pt-based metal-insulator-semiconductor (MIS) field-effect sensors was investigated. An experimental model system was designed which compares the responses of thick continuous Pt layers with controlled morphology and surface chemical composition, with the response of thin, discontinuous layers. The surface of a thick, continuous sputter-deposited Pt film is modified, either by (i) the deposition of a thin SiO2 overlayer, (ii) reactive sputter deposition of PtOx, or (iii) co-deposition of Pt with SiO2 in Ar + O2 atmospheres. We show that the ammonia response is caused by the formation of atomic hydrogen through the dissociation of NH3 at temperatures <200 °C. It is found that the modified surfaces exhibit increased ammonia selectivity compared to a pure Pt film. Results from this work indicate that the reason for the changed selectivity is the appearance of an oxidized PtOx phase or triple phase boundaries between Pt, SiO2 and the ambient gas, rather than for solely morphological reasons. © 2002 Elsevier Science B.V. All rights reserved.

The catalytic activity at the surface of Pt based MIS field effect gas sensors is modified by the deposition of thin films of SnO2, Al2O3 and SiO2, grown by reactive sputtering. It is found that a very thin layer (<10 nm) of SiO2 and SnO2 changes the catalytic activity towards higher NH3 selectivity, but with thicker films the sensor response vanishes. Since the response mechanism for these sensors is dependent on dissociation of molecules, it is likely that at low temperatures (140 °C), neither dissociation on nor transport/diffusion through the thicker films takes place. However, with Pt in conjunction with SiO2 or SnO2, the surface reactions will be altered, with enhanced NH3 selectivity as a result. A thin film of Al2O3, on the other hand, has a much smaller influence on the gas response to the test gases used in this work. Furthermore the sputtering process is found to strongly influence the sensor responses, and specifically reduce the sensitivity of the sensor. A thin intermediate layer of evaporated Pt does not completely protect the underlying structure from sputter induced damage. © 2002 Elsevier Science B.V. All rights reserved.

Two different ways of preprocessing chemical sensor data are presented as a means to improve the interpretation and the classification ability of an electronic nose (EN). The practical problem at hand is the distinction between four commercial coffee blends - containing up to 12 types of coffees - all of which are to be consumed as Espresso. Coffee was sampled in three successive preparation phases: a beans, ground (powder) or liquid (the actual Espresso). In the case of beans, stress is put on the improved clusters visualization after the preprocessing and before the actual classification is performed. Different catalysed sensors and successive extractions were used to differentiate the response pattern towards the various coffees. The features which permitted the best samples' classification as judged from Principal Component Analysis (PCA) score plots were selected. To this end, an empirical search strategy inside the feature space is presented. Scores from PCA were subsequently utilized as inputs for a feed forward multilayer perceptron (MLP) with cross-validation resulting in 100% correct classification with just two sensors. In the case of ground coffee, a (supervised) drift compensation algorithm was developed. It essentially consists of removing the first principal component (PC) for every cluster since this is seen to be given by the drift. An 87.5% classification performance was achieved. Liquid coffee, on the other hand, was not successfully classified, probably due to the difficulty in assuring reproducible sampling conditions.

Noise measurements of conductive sensors made by either polymers and SnO2 materials, have been done at 25°C and around 300 °C respectively, in order to study their ultimate performances in terms of signal to noise ratio and the excess noise implications in multisensor artificial olfaction systems. In all the studied cases, of which only a few are here reported, excess noise spectral densities proportional to l/f were observed in the frequency range of about 1-10 kHz, even in presence of different kinds of adsorption processes. This suggests that the evaluation of the resolution values must be determined after noise characterizations have been done, especially for slow responding sensors. In fact if the time constants fall in the low frequency region where the amplitude of the noise spectral density may not be neglected. Furthermore, in presence of gas flow even l/f2 noise spectral density type (with n between 1 and 4) can be observed in Sn02, while the same kind of noise is also evident in polymer based sensors operating at room temperature at constant volatile compounds gas pressure conditions.

Spectroscopic and spectromagnetic studies were performed on SiO 2 obtained by hydrolytic condensation of tin(IV) alkoxide, Sn(OtBu) 4. Tin dioxide was produced in powder form, suitable for spectroscopic investigation, and it could also be used as base material for thin film gas sensors. Electron paramagnetic resonance (EPR) spectroscopy allowed to detect singly ionized paramagnetic oxygen vacancies (V 0*), produced by interaction of SnO 2 with carbon monoxide in the range of temperature 298-773 K. The interaction with CO at T<173 also reduced some Sn(IV) centres to Sn(II), by transfer of electrons from the oxygen vacancies to Sn(IV). This was detected by X-ray Photoelectron Spectroscopy (XPS) in the Valence Band region. Moisture too, when present in the atmosphere, was found to form paramagnetic oxygen vacancies. In the presence of air the formation of vacancies due to CO interaction, and their reaction with molecular oxygen occurred at the same time. Below 473 K the saturation of vacancies was due to chemisorbed paramagnetic O 2- , while above 473 K it was from diamagnetic O 2$