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Previous studies of the gas sensitivity of CuPC (copper phthalocyanine) and ZnPC have been extended to include CoPC and NiPC, with particular reference to ammonia detection. Ammonia sensitivity is found to depend on the crystal structure of the films, ammonia concentration and temperature. The alpha - beta phase transition is found to give different film morphologies for ZnPC and NiPC. alpha -NiPC is the only PC studied that shows an increase in resistance on exposure to air. The other PCs show enhanced conductivity resulting from hole injection associated with oxygen adsorption. Surface conduction dominates bulk properties for all the PCs. Ammonia adsorption decreases conductivity due to the catalytic removal of surface oxygen but the response of the various PCs varies by two orders of magnitude. The low response of ZnPC is thought to be associated with the formation of a ZnPC-NH3 surface complex. CoPC and NiPC offer the highest response for low ammonia concentrations in air and the improved linearity of the response-concentration characteristics with SiO2 sublayers suggests that these may be viable materials for the fabrication of an integrated NH3 detector.

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