Multiparametric sensor for air pollutants based on a porous silicon optical microcavity
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Multiparametric sensor for air pollutants based on a porous silicon optical microcavity Z. Gaburro1, G. Faglia2, C. Baratto2, G. Sberveglieri2, L. Pavesi1 1 INFM and Department of Physics, University of Trento, via Sommarive 14, I-38050 Povo (Tn) Italy 2 INFM and Department of Chemistry and Physics, University of Brescia, via Valotti 9, I-25133 Brescia, Italy ABSTRACT We experimentally demonstrate that porous silicon optical microcavities can be effectively used as multi-parametric gas sensors. As known, the photoluminescence intensity and electrical conduction of porous silicon are strongly dependent on environmental properties, such as the dipole moment of molecules of surrounding gases. The sensitivity is large due to the large surface/volume ratio of porous silicon. While these effects can be observed in any porous silicon structure, microcavities of porous silicon allow an additional sensing parameter, i.e. the spectral position of the resonance cavity peak. The position of the peak depends on the index of refraction of the environment, and gives independent additional information. Moreover, we show that the dynamic response of the peak shift is much faster comparing the other sensing parameters. The combined effects on the peak position, luminescence intensity and electrical conduction can allow discrimination between different substances, and therefore porous silicon optical microcavities can work as multi-parametric optical/electrical sensors. We report detection of 1 ppm of NO2 and 500 ppm of ethanol at room temperature. With NO2, the electrical conduction increases and PL quenches, but the peak does not shift, whereas the peak shifts with ethanol (no significant PL quenching is observed at 500 ppm). This suggests that discrimination between different species can be achieved. INTRODUCTION For pollution control, and, more in general, for environmental monitoring, multiparametric sensors are needed. In fact, several gases in unknown mixtures and concentrations are normally present in any practical situation. Independent sensing parameters are therefore necessary to collect enough information to discriminate between all the present species. Porous silicon (PS) is a promising material for multiparametric sensors, because its electrical and optical properties are affected by surrounding gases, due to its large internal interface area [1]. The electrical conductivity changes with the environment [2]. For example, in presence of NO2, an hazardous air pollutant, the relative variation of the conductance ∆G/G is 30 at 3 ppm [3]. The photoluminescence (PL) is normally quenched by surrounding gases [1, 4]. Other optical sensing parameters are reflectivity [5, 6, 7], and optical waveguiding [8]. The above sensing parameters can effectively work at room temperature, which is a big advantage for integration and power consumption. However, more parameters are desirable to solve discrimination in realistic situations. We have demonstrated that high quality PS microcavities (PSM’s) have one additional sensing optical parame
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