Abstract
In this thesis, we report on the fabrication of a high-performance titanyl phthalocyanine (TiOPc) organic field effect transistor (OFET) on a chemically modified dielectric layer, and the utilization of the OFET for gas sensing application via the current amplification capability of OFET. The gas sensors often needs to operate at higher temperature for a continuous operation and thus the material choices of semiconductors and dielectrics could be stringent. TiOPc was selected as the active OFET material because of its good thermal stability and its potential for having high carrier mobility, thanks to its rather short stacking distance (3.14 Å). The inorganic substrate of Al2O3 film (3 nm thickness) was first grown on SiO2 (300 nm thickness) via atomic layer epitaxial deposition, and the self-assembled monolayer of n-octadecylphosphonic acid (ODPA) noted for its higher thermal stability was then prepared on Al2O3 via solution immersion. The aluminum oxide layer plays a critical role in improving the interfacial bonding of phosphonate group and achieving a near saturated surface coverage of ODPA. A plethora of techniques have been used to characterize organic thin films. The morphology, hydrophilicity, ODPA coverage, molecular orientation, and crystalline state of ODPA and TiOPc thin films can be characterized by atomic force microscopy (AFM), contact angle analyzer, X-ray photoemission spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray diffraction (XRD), respectively. Moreover, the change of electronic structure induced by NO2 adsorption on TiOPc can be investigated by XPS and ultraviolet photoemission spectroscopy (UPS). XPS result shows that the initial surface coverage of ODPA monolayer is over 90% and the ODPA monolayer remains structurally stable at least up to 120 °C, evidenced by XPS and NEXAFS data. The TiOPc thin film crystallizes into -phase on top of ODPA, as revealed by XRD patterns, and the nearly planar TiOPc molecules are found to be tilted from the surface by ~65° via NEXAFS, in accord with the crystallographic data. The TiOPc OFET achieves a modest mobility of ~1.45×10–1 cm2/Vs and possesses a sensing capability for NO2 with a detection limit approaching 25 ppm at 1 atm. XPS and UPS data indicate that NO2 interact weakly with TiOPc, resulting in no observable feature change in the spectra of Ti 2p, C 1s, and N 1s core levels as well as the HOMO band other than a shift toward lower binding energy by a maximum 0.3 eV. This binding energy shift is interpreted as an evidence of hole doping by NO2 on TiOPc semiconducting film. In conclusion, the present research shows that crystalline TiOPc thin film can be grown on ODPA/Al2O3/SiO2 substrate and the organic films exhibit a good thermal stability up to 120 °C. The resultant TiOPc OFET possess an improved mobility and is capable of detecting NO2 gas at a minimum sensitivity of 25 ppm.