Gas-sensing properties of semiconducting films of crown-ether-substituted phthalocyanines

Abstract

The effects of nitrogen dioxide and ammonia on the semiconductivity properties of solution-deposited thin films of the tetra-substituted 15-crown-5, 18-crown-6 and 21-crown-7 metal-free and copper phthalocyamnes are reported. The 15-crown-5 films showed good reversible conductivity changes at room temperature in NO₂ concentrations up to 5 ppm. The sensor characteristics worsen as the size of the crown-ether ring increases and this is consistent with the effect of greater separation of adjacent adsorbed species as molecular size increases. Lateral repulsions, which are believed to play a dominant role in controlling the response and reversal processes, are reduced by this increase. Treatment of the films with aqueous KCl solution led to dramatic changes in gas-sensing properties and, for the 15-crown-5 derivative, in film morphology. After KCl treatment the film conductivity decreased very rapidly and reversibly on exposure to NO₂, even at room temperature, and the 15-crown-5 film changed from a polycrystalline needle structure to an extremely smooth structure. These effects are tentatively ascribed to pronounced changes in the molecular assembly induced by the interaction of the potassium ions with the crown-ether moieties, and to consequent changes in the porosity of the film to small gas molecules. Above 3 ppm NO₂ the response begins to saturate, suggesting depletive chemisorption on an n-type material. However, similar responses are also observed for the electron-donor gas ammonia, indicating the presence of both donor and acceptor impurities in the materials. The gas-sensing properties of the KCl treated films at room temperature are the best of any organic semiconductor film yet reported.