EFFECT OF ELECTRODE GEOMETRY ON GAS SENSITIVITY OF LEAD PHTHALOCYANINE THIN-FILMS

The effect of electrode geometry on the gas sensitivity of lead phthalocyanine (PbPc) thin films has been studied. The material PbPc has been chosen for this study because it has a high sensitivity (approximately ppm) to oxidizing gases and may be suitable for use in a commercial solid-state chemical sensor. PbPc films of varying thickness (up to 1.5 mum) are vacuum sublimed across ultrathin (0.1 pm) coplanar pairs of gold electrodes lying on a sapphire substrate. The change of the electrical conductance of these devices in atmospheric nitrogen dioxide (NO2) is measured, over a range of electrode separations for several film thicknesses and temperatures, by a computer-controlled automated system. The temperatures are chosen around 160-degrees-C, at which a peak sensitivity of PbPc to nitrogen dioxide has been observed. The transient and steady-state responses of these devices are analysed in terms of diffusion-rate limited and reaction-rate limited models modified from previous work on thick porous metal oxide semiconducting films. Experimental results show that the response time of the device at temperatures from 130 to 190-degrees-C is insensitive to both the electrode separation and film thickness. Furthermore, the response time, typically 120 s. decreases with increasing nitrogen dioxide concentration (1-9 ppm). These observations are consistent with the reaction-rate limited rather than the diffusion-rate limited model of mass transport in thin PbPc films. The activation energy of the PbPc films is determined from the temperature dependence of the device conductance, and found to be (0.15 +/- 0.1) eV; as expected, it is independent of electrode separation and film thickness. The film conductance in air increases with increasing reciprocal electrode separation, but does not follow the linear relationship predicted from the model. Furthermore, the steady-state response (defined as the fractional change in conductance) of the PbPc films to atmospheric nitrogen dioxide shows a gradual but systematic increase with electrode separation, whereas the model predicts a constant value. The deviation of the experimental results from theory suggests the presence of considerable film inhomogeneity. The nature of this deviation is consistent with a film in which both the electrical conductivity and density of adsorption sites are considerably lower near the substrate surface than elsewhere. This observed heterogeneity in the gas sensitivity of thin sublimed PbPc films makes it more difficult to characterize their behavior and may reduce their commercial viability as chemical sensors.