NO2 sensors with room temperature operation and long term stability using copper phthalocyanine thin films

Effect of substrate temperature on microstructure of copper phthalocyanine (CuPc) thin films deposited on quartz and polycrystalline alumina substrates by vacuum sublimation technique has been studied. The films undergo a transition from α phase at lower substrate temperatures to β phase for temperatures above 200 °C. A simultaneous change in morphology from granular to nano-fibrous films was observed. Resistance of α phase films increased by more than one order of magnitude on initial application of electric field requiring stabilization for several hours, while β phase films did not show any drift. This behavior was attributed to oxygen absorption/desorption under applied field. Sensitivity of films for detection of NO2 gas at room temperature was found to be higher for films in the α-phase with nano-fibrous morphology. Methodology was devised for measurement of NO2 concentration in ppm range with good response and recovery times. With this methodology, the sensors showed very good stability in response (at room temperature) for a period of 1 year.