Study On Optical Sensing Techniques Based On Porous Silicon For Monitoring Sulfur Dioxide In Environment

Sulphur dioxide (SO2) is a main contaminant in air and also the pollutant primarily associated with acid rain. The principal sources of SO2 are from the combustion of fossil fuel in domestic premises, and more importantly, non-nuclear power station. In many countries, the economical losses resulting from sulphur dioxide and acid rain are very great, which makes monitoring SO2 in environment is a critical part in pollution prevention, industrial and agricultural contamination regulations. At present, several methods and instruments are available to monitor SO2 concentration both in gaseous and in liquid media using different systems including colorimetric, amperometric, conductometric, gas chromatography, flame photometry, surface acoustic wave (SAW) gas sensor, tin dioxide gas sensor as well as electrochemical sensors based on high temperature solid electrolyte, liquid electrolyte, solid polymer electrolyte, etc. But these techniques have their own characteristics and limits, new approaches to the detection and analysis of SO2 appear to be in dire need. As a novel functional material, porous silicon is constituted by a nano-crystalline skeleton (quantum sponge) immersed in a network of pore, and has a very large internal surface area and good bio-compatibility. So far, chemical properties, physical properties and optical properties of the material have been studied extensively. Because porous silicon can be easily synthesized directly from the same single-crystal silicon wafers, it seems ideal for Si-based opto-electronic devices, bio- and chemical sensors, mass spectrometry, new material support, biocompatible materials and in vivo electronics etc. In this dissertation, porous silicon is formed by electrochemical anodization of p- or n-type single-crystal silicon materials. In order to hinder oxidization process of porous silicon in air, either hydrosilylation or photochemical oxidization is used to stabilize porous silicon, whose principle is based on silicon hydride bonds of porous silicon are replaced by silicon alkyls or silicon dioxide. Furthermore, its structure, properties and sensing process are studied experimentally. The main points of this dissertation are as following: 1) The effect of dopant of single-crystal silicon, anodization parameters, stabilization methods on porous silicon formation is studied extensively. By fluorescent spectrometer, SEM, AFM, FT-IR, etc., the properties of porous silicon such as porosity, thickness, structure, morphology, photoluminescence, anti-corrosion ability, are measured, thecorresponding optimal conditions of fabrication and stabilization for porous silicon are obtained. Analyzed these experimental results, it is proved that porous silicon generating from n-type silicon wafer is more stable than that of p-type silicon. 2) According to the experimental results of porous silicon sensing SO2 qualitatively and quantitatively, it is discovered that photoluminescence quenching values of porous silicon oxidized by n- or p-type single-crystal silicon wafers is positively correlated with concentration of SO2. Moreover, stabilization method based on white-promoted hydrosilylation is superior to photochemical oxidization.3) The corresponding Stern-Volmer constant KS is also calculated according to the experimental results of photoluminescent quenching process. Within the concentration range from 0 to 0.25mL·L-1, the quenching process is in accord with Stern-Volmer equation. The Stern-Volmer conatant KS of UV oxidation porous silicon is equal to 0.5,and that of hydrosilylation porous silicon is 0.7. Within high concentration range, the quenching process isn't in accord with Stern-Volmer equation.4) The other gas such as N2, COx, NOx in monitoring environment cann't disturb the sensing process of SO2.