Elisabetta Comini

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.

Ultrasensitive and highly selective gas sensors using three-dimensional tungsten oxide nanowire networks

The performance of a solid-state gas sensor is characterized by its sensitivity, stability, and selectivity. The working principle relies on modulation of electrical conductivity due to surface oxidation reduction caused by gas exposure. Because only the surface layer is affected by such reactions, the sensitivity is strongly dependent on the surface-to-volume ratio of the material used. This purpose has been pursued by synthesizing layers with a porous morphology to enhance the material surface area. Porosity is enhanced by means of the thick film synthesis approach typically adopted in the gas sensing field. Such high porosity is not easy to achieve by thin film approach. Another approach largely used in the field is the rheotaxial growth and its thermal oxidation RGTO method, which allows synthesizing a porous thin film mono

TiO2 thin films by a novel sol–gel processing for gas sensor applications

Novel thin films of titanium dioxide dispersed in a polymeric matrix have been prepared by a chemically modified sol–gel technique. Nanostructured films of pure TiO2 in the anatase form are obtained after annealing at 500°C. SEM, TEM and TG/DTA are used for the structure characterisation of TiO2 films. The role of the polymer in controlling the microstructure is confirmed. The first application of this technique in gas sensor field is presented in this work. Ethanol and methanol sensing properties are tested and reported. TiO2 sensors can detect very well concentration required for breath analysers.

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.

p- and n-type Fe-doped SnO2 gas sensors fabricated by the mechanochemical processing technique

Fe-doped SnO2 sensors were fabricated using micromechanical synthesis technique. The Fe-doped sensor was compared to pure SnO2. Fe-doped SnO2 responded as a p-type semiconductor to oxygen concentrations of up to 10% at 300 °C. As the temperature increased to 400°C, the material responded as an n-type semiconductor. Furthermore, a higher surface area and smaller grains size diameters were achieved when doping SnO2 with Fe. This translated into improved dynamic gas sensing properties and also improved responses to gases such as ethanol.

Metal oxide nanowires as chemical sensors

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.

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.