The effect of surface topography on the sensitivity of lead phthalocyanine thin films to nitrogen dioxide

Optical absorption spectroscopy is used to characterize the topography of thin films of lead phthalocyanine (PbPc) evaporated onto quartz substrates held at temperatures between −45 and 250 °C, and to determine the temperature at which optimum smoothness, for device applications, occurs, X-ray diffraction confirms that the change from the monoclinic phase to the triclinic phase begins between 50 and 75 °C. The smoothest films, determined by scanning electron microscopy (SEM) and atomic force microscopy (AFM), are produced at −35 °C. X-ray diffraction indicates that these films are completely amorphous. The response and recovery of these films to nitrogen dioxide is examined by observing the change in optical absorbance of the films (without prior exposure to atmospheric air). It is found that the films produced at −35 °C and operated at room temperature using light of 725 nm wavelength display the greatest response to NO2, but these films show low recovery on heating to 100 °C in vacuum. However, films deposited at 100 °C and operated at room temperature using light of 515 nm show a lower response to the gas, but recover completely on heating to 100 °C in vacuum.