Sensors

Senior investigator: Prof. Elisabetta Comini, Prof. Guido Faglia

This activity together with the one on biosensors deals with applications of materials as sensors

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Electrical and optical chemical gas sensing

Nanowires and thin films of nanostructured metal-oxide semiconductors oxides target the development of gas sensors for environmental, agroalimentary, safety and industrial applications.

Electrical gas sensing

The electrical and functional properties of developed layers are studied with AC, DC and work function measurements towards environmental and polluting gaseous species. Chemoresistive properties of the nanostructures are also studied both analytically and numerically from the fundamental point of view of surface-gas interaction. The objective is to use these sensors to monitor the air quality with sensitivity, selectivity and reliability comparable to that of conventional analytical devices. They can also be employed inside an electronic nose; therefore, they fit perfectly integration of measurements of outdoor pollutants and confined and semi confined ambient applications, from both fixed and mobile web-enabled stations.

Optical Gas Sensing

Nanowires of ZnO and SnO2 show photoluminescence (PL) spectra at room temperature. Reversible modification of static photoluminescence efficiency was obtained upon exposure to low concentrations of nitrogen dioxide. We first demonstrated this effect in 2005 for SnO2 nanowires: few ppm of NO2 significantly quenched the visible PL emission of nanowires, not affected by variation of relative humidity, at 120°C. This allows the development of all optical gas sensor , where contacts are not needed.

Figure 1. Setup for functional characterization of gas sensors

As for SnO2, the PL spectrum of ZnO is quenched by interaction with NO2 in a reversible way, with optimum results at room temperature. In this case the cross interference of other gases like ethanol and relative humidity is not negligible and must be taken in account, while no response was observed for CO. The effect of humidity should be ascribed to room temperature operation of the sensor. The possibility to use the sensor at room temperature is interesting when dealing with gas detection in explosive environment. The lowest concentration detected was 0.1ppm, the attention level for outdoor detection. UV LED was successfully used as alternative cheap and low power light source to excite PL signal.

Figure 2. (left) PL emission of ZnO nanowires in dry air; (right) PL area variation in 2 ppm of NO2.

NO2 interaction with gases has been studied more in detail at room temperature by mean of Time Resolved Photoluminescence (TRPL) studies. While the decrease in PL intensity is linearly proportional to surface density of adbsorbed molecules (assuming a Langmuir–like adsorption of gas molecule on the semiconductor surface), the recombination rates are not significantly affected by interaction with NO2. This suggests that NO2 acts as static quenchers, determining a change in the number of states acting as radiative recombination centers.

References

G. Faglia, C. Baratto, G. Sberveglieri, M. Zha and A. Zappettini, Appl. Phys. Lett. 86, 011923 (2005)

Comini E, Faglia G, Sberveglieri G, Pan Z, Wang Z (2002) Appl. Phys. Lett. 81:1869-1871

Lettieri S., Amato L.S., Maddalena P., Comini E., Baratto C., Todros S.,  Recombination dynamics of deep defect states in zinc oxide nanowires, Nanotechnology, Volume 20, Issue 17, Apr 2009, 175706

Lettieri S., Setaro A., Bismuto A., Maddalena P., Baratto C., Comini E., Todros, S., Sberveglieri, G., Light Emission Properties of SnO2 Nanowires for Applications in Gas Sensing, Sensor Letters, Volume 6, Number 4, August 2008, pp. 596-600(5)

C. Baratto, S. Todros, G. Faglia, E. Comini, G. Sberveglieri, S. Lettieri, L. Santamaria, P. Maddalena, Luminescence response of ZnO nanowires to gas adsorption, Sensors and Actuators B: Chemical, Volume 140, Issue 2, pp. 461-466, (2009)

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Theoretical research activity

The development of nanostructured devices for advanced functional applications cannot be carried out only by a trail-and-error approach, leaving the study of operation models out of consideration.
In particular for chemical sensing the basis for the operation models which has been proposed until now lies in the awareness that the chemisorption processes of the gaseous species on the semiconducting metal oxides are mediated by the oxygen concentration and they also modify the density of the surface charge. The study of the gas species adsorption on the charge transport properties of quasi 1D structures will be carried out to optimize the preparation of the devices and to choose the most suitable geometries.