Abstract
Metal phthalocyanines (MPcs) have been demonstrated to be useful materials for gas sensing application due to their favorable transducer response, high resistance to oxygen and moisture interference, and good thermal stability. They have been used widely for organic thin film transistor (OTFT) gas sensors. In this thesis, we use newly completed ambient pressure X-ray photoelectron spectroscopy (AP-XPS) endstation to investigate the nitrogen dioxide (NO2) sensing mechanism of titanyl phthalocyanine (TiOPc) and derive at how the electronic structure of TiOPc is modified during the gas sensing process. An abrupt lower binding energy shift of 0.4 eV is noted when the NO2 backing pressure is increased above 0.025 mbar, in accord with the detection threshold of 25 ppm determined from OTFT measurements. And the binding energy shift is interpreted as an evidence of hole-doping effect by NO2 on TiOPc film. With XPS curve fitting analysis, a variety of N-containing surface species is characterized under NO2 rich conditions. The surface adsorbed NO2 might undergo a disproportionation process to produce NO3- and NO+. Reduced nitrogenated species could bond to titanium central atom and increase its binding energy due to their electron withdrawing nature. Simultaneously, the formation of the ionic bond modifies the electronic structure of unoccupied orbitals of metal ion through the changed crystal field. In addition, the molecular orientation of TiOPc is found to change after the exposure of NO2. The angle dependent near edge X-ray adsorption fine structure (NEXAFS) data reveal that the aromatic plane of printine TiOPc thin film are oriented preferentially along the substrate surface plane, but turns into random orientation after the dose of NO2. Same experimental approach is extended to the investigation of copper phthalocyanine (CuPc) thin film to address whether different molecular configuration and different central metal atom in CuPc indeed influence the NO2 absorption/reaction behavior and govern its possible application in NO2 sensing. With real time information available from AP-XPS techniques, one can fully explore the the interaction between NO2 gases and TiOPc thin film, and further understanding the principle of the toxic gas detection.