A study of partially irreversible characteristics in a TTF–TCNQ gas sensing system

The irreversible gas sensing behavior is investigated for TTF–TCNQ complex in the presence of NO2 or O2. Reactions with the gas molecules are the main reason to cause the irreversibility, which has been reported and supported by analytical data. The totally irreversible behavior was noticed when a TTF–TCNQ thin film was brought in contact with NO2 gas. As for O2 gas, however, only partially irreversible phenomenon was observed. For a TTF–TCNQ thin film that interacts with NO2 or O2 gas, the degree of irreversibility is determined by the competition ratio of the desorption rate to that of the reaction rate. A theory based on the competition concept was proposed and a general expression for three possible behaviors (totally reversible, totally irreversible, and partially reversible cases) in a gas sensing system was obtained. The O2 sensing data match the theoretical prediction when the reaction and the adsorption are limited at the surface.