G. Faglia

Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts

Gas sensors have been fabricated using the single-crystalline SnO2 nanobelts. Electrical characterization showed that the contacts were ohmic and the nanobelts were sensitive to environmental polluting species like CO and NO2, as well as to ethanol for breath analyzers and food control applications. The sensor response, defined as the relative variation in conductance due to the introduction of the gas, is 4160% for 250 ppm of ethanol and −1550% for 0.5 ppm NO2 at 400 °C. The results demonstrate the potential of fabricating nanosized sensors using the integrity of a single nanobelt with a sensitivity at the level of a few ppb.

UV light activation of tin oxide thin films for NO2 sensing at low temperatures

Abstract A novel approach to operate semiconductor gas sensor at low temperatures avoiding the poisoning of the surface is described. The effects of UV light illumination on the performance of SnO 2 thin film gas sensors toward an oxidising gas of increasing interest due to environmental monitoring, like NO 2 are reported. The thin films gas sensors were prepared according to the rheotaxial growth and thermal oxidation (RGTO) technique with dc sputtering in Ar atmospheres. Results has shown that there is an enhancement of the performances with UV exposure: a decrease in the response and recovery time and no poisoning effect. This is promising for the development of a sensor for NO 2 working at room temperature.

Light enhanced gas sensing properties of indium oxide and tin dioxide sensors

Abstract The effects of UV light illumination on the performance of SnO2 and In2O3 semiconductor gas sensors toward CO and NO2 are reported. The sample were prepared with DC sputtering in Ar atmospheres by the Rheotaxial Growth and Thermal Oxidation (RGTO) technique and with reactive magnetron sputtering in Ar and O2 atmosphere, respectively. Results are quite promising for the development of sensor working at room temperature.

Investigation on the O3 sensitivity properties of WO3 thin films prepared by sol–gel, thermal evaporation and r.f. sputtering techniques

WO3 thin films have been deposited on alumina substrates provided with platinum interdigital electrodes by sol–gel (SG), r.f. sputtering (RFS), and vacuum thermal evaporation (VTE) techniques and annealed at temperatures between 500°C and 600°C for 1 to 30 h in static air. The morphology, crystalline phase and chemical composition of the films have been characterised using SEM, glancing XRD and XPS techniques. The electrical response has been measured exposing the films to O3 (10–180 ppb), NO2 (0.2–1 ppm), NOx (27 ppm NO and 1 ppm NO2) at different operating temperatures ranging between 200 and 400°C and humid air at 50% R.H. SG prepared films have shown bigger responses (S=IAir/Igas) with respect to VTE and RFS for all the investigated gases and operating temperatures. RFS prepared has resulted to be less sensitive, but faster in the response and more stable in terms of signal reproducibility. The response to O3 has been found to be at maximum at 400°C. At this temperature the response to 80 ppb of ozone has been: S=35 (SG), S=18 (VTE) and S=5 (RFS). The NO2 and NOx response reached the maximum at 200°C and becomes negligible at 400°C. Improvements on the O3 gas sensitivity and selectivity can be achieved by fixing the operating temperature of the films at 400°C.

Solid state gas sensing

Micro-Fabrication of Gas Sensors.- Electrical-Based Gas Sensing.- Capacitive-Type Relative Humidity Sensor with Hydrophobic Polymer Films.- FET Gas-Sensing Mechanism, Experimental and Theoretical Studies.- Solid-State Electrochemical Gas Sensing.- Optical Gas Sensing.- Thermometric Gas Sensing.- Acoustic Wave Gas and Vapor Sensors.- Cantilever-Based Gas Sensing.

Adsorption effects of NO2 at ppm level on visible photoluminescence response of SnO2 nanobelts

The visible photoluminescence (PL) of tin oxide nanobelts is quenched by nitrogen dioxide at ppm level in a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity and other polluting species, such as CO and NH3. We believe that adsorbed gaseous species that create surface states can quench PL by creating competitive nonradiative paths. A comparison between conductometric and PL response suggests that the two responses are ascribable to different adsorption processes.

A new technique for growing large surface area SnO2 thin film (RGTO technique)

A new technique for growing SnO2 thin films with high surface area is described, based on tin rheotaxial growth and its thermal oxidation (RGTO). Tin thin films, when grown, present a surface characterized by spheroidal agglomerates due essentially to the surface tension of the liquid metal; these agglomerates do not seem to have any electrical continuity. By means of a thermal oxidation, both the transformation of the metal into a semiconductor and the thin film continuity are obtained, owing to the volume increase during the above-mentioned phase transformation. After this annealing cycle, SnO2 thin films are slightly oriented in the (101) direction and present an electrical resistivity equal to about 102 Omega cm. So, the surfaces of SnO2 films appear to be formed by spongeous agglomerates, which are in electrical contact with the nearest agglomerates. SnO2 thin films grown by this method show a high sensitivity (defined as the relative per cent conductance variation) to H2; in fact, sensitivity to 200 PPM H2 in synthetic air at ambient pressure is equal to 400%. It seems to be possible to prepare other metal oxide semiconducting thin films with a high surface area using this technique.

Characterization of a nanosized TiO2 gas sensor

Abstract Thin films were obtained by r.f. reactive sputtering from a Ti.1W.9 target onto a Si substrate followed by annealing in air at 800 °C. The thermal treatment results in a nanosized TiO2 thin film with high surface-to-volume ratio. The nanosized structure, its stability, together with the ease of preparation, make this material suitable as a gas sensor. The sensing layer proved capable to detect 20 ppm of NO2 at a temperature suitable for monitoring of exhaust gases of engines. Its high sensitivity suggests use of this sensor for environmental purposes.

NO2 monitoring at room temperature by a porous silicon gas sensor

Abstract A study on reactivity of p + porous silicon layers (PSL) to different gas atmosphere has been carried out. Substrate doping was 5–15 mΩ cm and 0.5 Ω cm, porosity ranged from 30 to 75% and the thickness of the porous layers was 20–30 μm. Three different processes to insure good electrical contact are proposed and discussed. PSL were kept at constant bias and current variations due to interaction with different concentrations of NO 2 were monitored at constant relative humidity (R.H.). Measurements were performed at room temperature (R.T.) and at atmospheric pressure. Concentrations as low as 1 ppm were tested, but the high sensitivity of the sensor makes possible to test lower values. The recovery time of the sensor is of the order of one minute. Response to interfering gases (methanol, humidity, CO, CH 4 , NO, NO 2 ) has been examined also. In-situ FTIR spectroscopy in NO 2 atmosphere shows a fully reversible free-carrier detrapping in the IR region, confirming the validity of the models proposed in the recent past for electrical conduction in mesoporous silicon.

TiO2 Nanotubes: Recent Advances in Synthesis and Gas Sensing Properties

Synthesis—particularly by electrochemical anodization-, growth mechanism and chemical sensing properties of pure, doped and mixed titania tubular arrays are reviewed. The first part deals on how anodization parameters affect the size, shape and morphology of titania nanotubes. In the second part fabrication of sensing devices based on titania nanotubes is presented, together with their most notable gas sensing performances. Doping largely improves conductivity and enhances gas sensing performances of TiO2 nanotubes.