DOPING EFFECTS AND REVERSIBILITY STUDIES ON GAS-EXPOSED ALPHA-SEXITHIOPHENE THIN-FILMS

The electronic effects produced by controlled gas exposures on alpha-s exithiophene thin films have been investigated using x-ray photoelectr on spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) . Incremental exposures of alpha 6T films to NO2, O-2, NH3 in N-2, and water vapor in N-2 were performed in ultrahigh vacuum, while repeated doses of O-2, H-2 in N-2, and methanol vapor in N-2 were performed ex situ at ambient pressure. In both conditions XPS spectra of gas-expos ed films showed no evidence of chemical changes. However, the features in both XPS and UPS spectra were observed to shift as a function of g as dosage, with the magnitude dependent on each particular gas. These effects could be reversed by heating the films to temperatures around 100 K. This behavior is interpreted in terms of doping by weakly bonde d gas species within the near-surface region of the alpha 6T films. Gr eater doping effects were observed for films dosed at ambient pressure . We discuss possible gas adsorption models that may explain the diffe ring gas sensitivities and their dependences on pressure, temperature, and exposure lime. Finally, temperature programmed desorption was use d to study the reversibility and resistance to reaction of a6T films d uring this gas adsorption/desorption process. The films were found to be chemically stable to gas exposure and subsequent thermal desorption in the dosage ranges explored. (C) 1998 American vacuum Society. [S07 34-2101 (98)00705-2].