Effects of heat treatment on oxidative gas adsorption for lead naphthalocyanine thin films

Abstract Thin films of Pb-naphthalocyanine (330 nm thick) prepared by vacuum sublimation were heat-treated at 250°C in air atmosphere for various periods. As estimated from the conductance versus temperature correlations, the adsorption of NO 2 and O 2 on the films at room temperature, which was only slight after the heat-treatment for 30 min, increased very remarkably with prolonging heat-treatment time up to 10 h. The 10 h treated film was found to exhibit completely reversible changes in conductance upon exposure to 5 ppm NO 2 in N 2 at 210°C. Visible light absorption spectra indicated a change from a largely amorphous state of the as-deposited film to a largely crystalline state of the film treated for 2 h. X-Ray diffraction analysis showed that the as-deposited film consisted of N -oriented particles, while the proportion of P -oriented particles increased with prolonging treatment time up to 10 h. The enhancement of gas adsorption was thus attributed to the reconstruction of films during the heat treatment. It appears that a reconstruction-assisted increase in porosity not only facilitates gas diffusion inside the film, but also increases the number of Pb-naphthalocyanine molecules accessible NO 2 or O 2 .