Nanoporous Silicon in Gas Preconcentration and Sensing

Abstract

This thesis addresses new developments in the sensing of chemical compounds in the gas phase, a topic which is of great importance in almost every sector. Volatile organic compounds, or VOCs, are emitted by many processes, and have significant effects on human health and the environment. Importantly, they have a variety of structures over a wide range of vapour-phase concentrations, making accurate and selective detection difficult. This work responds to the increasing demand for real-time, online, rapid and location-specific gas sensing by small, low-cost and portable devices. In particular, this research focusses on improving sensitivity and/or selectivity for several important applications. To that end, the potential of nanoporous silicon as a ‘preconcentrator’ to solve some of these challenges is investigated, by adsorbing molecules inside the pores before releasing them in a pulse into a gas detector, improving its effective sensitivity. This technique is applied to vapour-phase explosives detection, a challenging area of research due to their low vapour pressures. The effectiveness of a nanoporous preconcentrator is demonstrated for several sensing systems, and enhancements of 12-16 times are achieved for an explosive ‘taggant’. A compact detection device including the nanoporous preconcentrator and a small VOC sensor is developed and systematically tested for various VOCs, achieving enhancement of the sensor’s signal by up to 80 times, a substantial improvement. Surface modifications of the nanoporous silicon are found to better enhance the adsorption of different types of VOCs, and initial research into composite formation with metal-organic structures provides promising early results for improving VOC selectivity. However, preparation of the composites presents a challenge to be investigated further in future. The application of controlled heating to cause temperature-resolved desorption of different VOCs from the nanoporous preconcentrator is also explored, as a means of adding selectivity to a total VOC detector. Individual VOCs were selectively detected from a binary mixture at low detection limits < 10 ppb by this low-cost, compact device, which is a significant achievement. This work successfully demonstrates sensitivity and selectivity improvements across a range of challenging VOC detection applications, by the use of nanoporous silicon based technologies.