Thin film sensors: The role of defects and impurity sites in controlling sensor response and selectivity

Abstract

Thin film gas sensors are discussed which are produced from polycrystalline organic dyes, such as the phthalocyanines, perylenes, etc. or from metal oxides, such as SnO2, ZnO, etc. Both types of materials are typically used to detect gas phase analytes by virtue of the surface conductance changes which accompany chemisorption of the analyte. In the case of the thin film oxides this process is understood in terms of the change in surface charge density and band bending which accompanies chemisorption of electron donors or acceptors. Defects in the oxide structure, or intentionally added impurities in the near surface region, can enhance the charge density at the surface and provide specific chemisorption sites for various analytes, which are the main means of introducing selectivity to these sensors. In the case of the organic dye thin films, hopping processes, rather than movement of charge through valence and conduction bands, control the flow of charge. Significantly higher defect and impurity densities are anticipated, versus the metal oxide thin films, and movement of charge through trap sites accounts for a significant fraction of the charge transfered. Illumination of certain organic thin films can enhance the sensitivity of the sensor, by a factor of up to 104, depending upon the ratio of dark to photoconductivity in the solid. Addition of electron or hole traps in the near surface region of the thin film can introduce chemisorption sites which interact more selectively with the analyte, and further increase sensitivity. Examples of this approach are given, for the detection of NH3 and NO2, using selected photoconductive phthalocyanine thin films.