Gas sensors using organic thin films at room temperature

Abstract

The thesis describes work on the thin film deposition, characterization and gas sensing of three groups of organic materials: polyaniline, lutetium bisphthalocyanine and metallotetraphenylporphyrins. These materials were deposited by evaporation, spinning and using the Langmmr-Blodgetttechnique. Film characterization by dc conductivity, scanning electronmicroscopy, surface plasmon resonance, ultraviolet/visible and infrared spectroscopy is described. Gas-induced changes in devices coated with the thinorganic films were monitored at the molecxilar level using Fourier transform infrared spectroscopy and at the macroscopic level using a chemiresistor and surface plasmon resonance. The gases used have included NO(_x), H(_2)S, SO(_2), COand CH(_4) at room temperature and pressure. Devices made with the thinorganic films were reversibly sensitive to some of the gases at low concentrations (<10ppm), but the reactions were irreversible at high concentrations (>100ppm). Polyaniline is shown to exhibit a different chemical structure, reflecting the conditions of its deposition. Spun films were found to be in the emeraldine baseform, the evaporated films were physically and chemically similar toleucoemeraldine base while the Langmuir-Blodgett films were closer topemigraniline, the completely oxidized form of polyaniline. All the types of polyaniline films were sensitive to H(_2)S and NO(_x) concentrations down to4ppm. Only spun and evaporated films were responsive to SO(_2). The spun layers were most sensitive to SO2 and least sensitive to NO(_x)- In contrast, the evaporated layers were most sensitive to NO(_x)- The changes recorded in these measurements occurred after a delay time of few seconds, but were complete lyirreversible for exposure to high gas concentrations. However, no response was observed when exposed to CO or CH(_4) even at very high concentrations. Infrared spectroscopy studies indicated the irreversible changes, for example when exposed to NO(_x), are due oxidation of benzoid rings into quinoid structures. Lutetium bisphthalocyanine has been deposited by the Langmuir-Blodgett technique. Using changes in conductivity, LB layers of lutetium bisphthalocyanine showed responses to NO(_x), H(_2)S and SO(_2) at concentrations below l00ppm. However, using surface plasmon resonance, only the NO(_x), induced changes could be measured at concentrations below l00ppm. These responses- occurred after a delay time of a few seconds and were partially reversible. Cobalt (II) methoxyltetraphenyl porphyrin has been deposited mainly by the Langmuir-Blodgett technique. Gas induced changes in the film were monitored using changes in dc conductivity and surface plasmon resonance. In all cases, NO(_x) was reversibly detected, but at a much higher concentration (>100ppm)than for the other materials in this thesis.