S. Groppelli

Wo3 Sputtered Thin-Films For Nox Monitoring

We present in this paper preliminary results concerning the preparation of tungsten trioxide thin films by reactive sputtering, the characterization either of their structural properties by means of XRD measurements or of the film morphology with the AFM microscope and the electrical response of the film towards toxic and pollutant gases. WO3 thin films showed a good sensitivity towards low NOx concentrations (1–10 ppm) in the temperature interval 200–500 °C and they were also sensitive to 10 ppm NH3. These films were also selective to Nox with respect to interfering gases like CH4, CO and SO2 in the same temperature range.

A novel method for the preparation of NH3 sensors based on ZnO-In thin films

Abstract This paper reports the preparation of ZnO-In thin films that are capable of detecting low ammonia concentrations in air. Indium and zinc layers are subsequently deposited by thermal evaporation at 70 °C onto an alumina substrate; the metal-semiconductor phase transformation is obtained on thermal cycling of the films in a synthetic air flow at a maximum temperature of 500 °C. XRD analysis shows that these films are polycrystalline and are slightly oriented in the (002) direction. SEM micrographs show that the surface of the metal-oxide film presents a needle-shaped structure; the needle size ranges between 1 and 3 μm. ZnO-In thin films can detect ammonia concentrations in the range 1–10 ppm by operating between 200 and 500 °C and are selective towards reducing gases like CO and CH 4 ; a good response towards a few ppm NO x is also observed in the same temperature range. This sensor seems to be suitable for detecting some ppm of ammonia in air.

Reactively sputtered indium tin oxide polycrystalline thin films as NO and NO2 gas sensors

Abstract Indium tin oxide polycrystalline thin films (90%In 2 O 3 + 10%SnO 2 (by weigth)) have been grown by r.f. reactive sputtering from a target of the same compound. The films exhibit columnar structure with a preferential (400) orientation with a dispersion of 8° around the normal of the film plane, as obtained by rocking curve measurements. These films exhibit an increase in resistivity when exposed to small concentrations of NO or NO 2 in air. At an operating temperature of 590K they have a sensitivity S NO,NO 2 , defined as S NO,NO 2 = ( I air − I NO,NO 2 )/ I NO,NO 2 for NO and NO 2 chemisorption, of 100 and 30 respectively for gases at room temperature and atmospheric pressure at a concentration of 1000 ppm. Four zones for oxygen ion adsorption and desorption can be distinguished in the plot of conductivity activation energy vs. temperature. The absence of long-term drift and a satisfactory reproducibility even at high NO and/or NO 2 concentrations are attributed to the excellent crystallographic properties of these films and should allow the development of detectors for nitrogen oxides at concentrations in the range 1–2000 ppm.

An electronic nose for the recognition of the vineyard of a red wine

Sensor arrays have been tentatively utilized in the past as tools for the odours characterisation of particular classes of foods and beverages. In this paper a technique aiming to discriminate among similar wines is described. A sensor array of metal-oxide based gas sensors was employed for the recognition of two wines, having the same denomination (Groppello red wine) but coming from different vineyards. The performance of the array was compared with that of the standard chemical analytical approach adopted by the wines authority.

Methods for the preparation of NO, NO2 and H2 sensors based on tin oxide thin films, grown by means of the r.f. magnetron sputtering technique

Abstract Thin films based on SnO 2 (SnO 2 , SnO 2 Pd, SnO 2 (Bi 2 O 3 ), SnO 2 (In) and ITO (90% In 2 O 3 , 10% SnO 2 )) have been grown by means of the r.f. magnetron sputtering technique in order to be used as gas sensors. Sensors based on SnO 2 with additives show a high sensitivity to nitrogen oxides at temperatures close to 573 K, while they show a great sensitivity to the H 2 present in dry air in the range 623–723 K, except for the ITO thin films. The sensor with the highest response to nitrogen oxides is the SnO 2 (In) thin film, which presents a sensitivity of 20 in 20 ppm of NO at an operating temperature of 573 K. The sensors with the highest sensitivity to H 2 is the SnO 2 Pd thin film grown by the RGTO (rheotaxial growth and thermal oxidation) technique; it has a sensitivity equal to 50 in 50 ppm of H 2 at an operating temperature of 623 K. Each kind of gas sensor is characterized by XRD and SEM analysis and by its response curves to various gases; the height of the eV s barrier is also obtained for SnO 2 thin films. The mechanisms of interaction between the thin-film surface and the various gases have also been investigated.

Sub-ppm NO2 sensors based on nanosized thin films of titanium-tungsten oxides

Abstract We describe the preparation of thin films of TiWO 3 obtained by means of r.f. sputtering deposition followed by a thermal oxidation. The sensing characteristics of these thin films were obtained by measuring the response towards NO 2 in the interval 0.5–20 ppm and to other interfering gases like ethyl alcohol, CH 4 and CO; the influence of water vapour to the response towards NO 2 was also investigated. The material seems to be a promising detector of NO 2 for environmental monitoring.

Highly sensitive and selective NOx and NO2 sensor based on Cd-doped SnO2 thin films

Abstract SnO2 thin films, doped with Cd between 1 and 6 at.%, are prepared by an original method allowing us to obtain a high surface area; the adsorption of low NOx and NO2 concentrations onto the thin-film surface has been extensively studied. The thin films are grown by means of thermal evaporation, the tin being deposited at a temperature higher than its melting point (tin rheotaxial growth) while the cadmium is deposited at room temperature; the metallic film is slowly oxidized up to a maximum temperature of 520 °C. The XRD analysis shows that the thin films are polycrystalline with preferential (101) and (111) orientations for SnO2 and CdO respectively. Measurements of the thin-film electrical conductance in the presence of various gases show first that these films have a maximum sensitivity, defined as relative percentage variation of conductance, towards NOx and NO2 equal to 10 000 and 4000% respectively at a temperature of 300 °C and 10 ppm gas concentration. At the same temperature the sensitivities towards some gases like CO, CH4, C4H10 and H2, present in an air flow at a 100 ppm concentration, always remains lower than 300%.

A novel PVD technique for the preparation of SnO2 thin films as C2H5OH sensors

Abstract A sensor for ethyl alcohol, based on an SnO2 thin film, is described. This film is grown by means of sputtering with a novel technique, called RGTO (rheotaxial growth and thermal oxidation). The RGTO thin film with an ultra-thin layer of Pd shows a good sensitivity to ethyl alcohol in the temperature range 523–773 K and a high selectivity to CO, CH4, C3H8 and C4H10. The maximum value of the sensitivity, defined as the relative variation of conductance ΔG/G, is obtained at 650 K; it is equal to 150 for 1000 ppm of ethyl alcohol in synthetic air. The height of the Schottky barrier between adjacent grains is obtained by electrical measurements of the conductance stimulated by the temperature. The average height of the barrier as a function of the operating temperature shows a maximum at 773 K (eVs = 0.65 eV).

Conductivity and work function ozone sensors based on indium oxide

Abstract Indium oxide thin films were deposited on silicon substrates for work function sensing as well as on alumina (Pt) conductivity sensors. A correlation of work function and conductivity measurements due to ozone in dry and humid air reveal that a chemisorption state dominates both sensor effects at temperatures below 200°C. The influence of humidity is found to be comparatively small for these films. The work function method provides a sensitivity maximum at low concentrations of some ppb of ozone stable down to a substrate temperature of 40°C. For HSGFET work function sensors with an In 2 O 3 –Si 3 N 4 system a response of 120 mV from 100 ppb of ozone in humid air is estimated for heaterless operation at room temperature. However, thermal desorption cycles will be necessary even for a restricted use of such sensors based on sputtered In 2 O 3 films.

Bismuth-doped tin oxide thin-film gas sensors

Abstract In this paper a new method of preparing an H2 sensor, based on an SnO2(Bi2O3) thin film, is presented. The deposition process of a tin and bismuth metallic film and its rapid thermal oxidation are described. SnO2 thin films, doped with 5 at.% Bi, show a maximum sensitivity (defined as S = ΔG/G%) equal to 15 000% for an air flow plus 1000 ppm H2 at an operating temperature of 400 °C. An explanation of the mechanisms related to the formation of surface porosity, responsible for the high sensitivity of thin films to H2, is given.