Ultrathin Silicon Nanowires for Optical and Electrical Nitrogen Dioxide Detection

Abstract

The ever-stronger attention paid to enhancing safety in the workplace has led to novel sensor development and improvement. Despite the technological progress, nanostructured sensors are not being commercially transferred due to expensive and non-microelectronic compatible materials and processing approaches. In this paper, the realization of a cost-effective sensor based on ultrathin silicon nanowires (Si NWs) for the detection of nitrogen dioxide (NO₂) is reported. A modification of the metal-assisted chemical etching method allows light-emitting silicon nanowires to be obtained through a fast, low-cost, and industrially compatible approach. NO₂ is a well-known dangerous gas that, even with a small concentration of 3 ppm, represents a serious hazard for human health. We exploit the particular optical and electrical properties of these Si NWs to reveal low NO₂ concentrations through their photoluminescence (PL) and resistance variations reaching 2 ppm of NO₂. Indeed, these Si NWs offer a fast response and reversibility with both electrical and optical transductions. Despite the macro contacts affecting the electrical transduction, the sensing performances are of high interest for further developments. These promising performances coupled with the scalable Si NW synthesis could unfold opportunities for smaller sized and better performing sensors reaching the market for environmental monitoring.

Keywords: gas sensing; light-emission; nitrogen dioxide; silicon nanowires.

Figure 1

(a–e) Schematic representation of the Si NW realization process by using the MACE technique. (f) Scanning Electron Microscopy of Si NWs in cross section.

Figure 2

(a) Schematic representation of the Si NW sensor in the presence of the N₂/NO₂ gas mixture. (b) RT photoluminescence spectra of the sensor exposed to 2 ppm NO₂ (red line), 90 ppm NO₂ (green line), and 180 ppm NO₂ (blue line). (c) Calibration curve of the sensor at room temperature.

Figure 3

(a) photograph of the sensor (2 × 2.3 cm²) made up of Si NWs (black color) covered by the interdigitated electrode of 100 nm Au above 5 nm of Ti. (b) Measurement of electrical resistance of the sensor exposed to 2, 5, and 30 ppm of NO₂ pulses at 115 °C. (c) Sensor recovery times after 5 min of NO₂ pulse at 2 ppm at 70, 100, and 115 °C. (d) Variation of the sensor resistance as a function of the NO₂ concentration at 70, 100, and 115 °C.