Gas adsorption on phthalocyanines and its effects on electrical properties

Abstract

This review considers the factors which influence the adsorption of gases onto single crystals and films of phthalocyanines, and the consequent changes in their electrical properties, which form the basis for the use of phthalocyanines as semiconducting gas sensors. Experimental results on the structure, morphology and surface chemical composition of phthalocyanine films as a function of preparation conditions and subsequent treatment are first considered. The implications of these results and of the molecular and electronic structure of the phthalocyanine molecule for the interactions between gases and phthalocyanine surfaces are then discussed followed by a review of experimental methods for characterising adsorbed species, including infra red-, thermal desorption-, x-ray photoelectron- and Auger spectroscopies, and the study of heats of adsorption and of surface photovoltages. The effects of adsorbed gases on electrical properties of phthalocyanine single crystals and films are reviewed in detail in the light of the above information, with particular reference to the chemical and physical factors which control the magnitude, rate and reversibility of the observed changes in semiconductivity and photoconductivity. Finally, the implications of this extensive knowledge for the design and optimisation of improved chemical sensors are discussed.