Atmospheric effect of air, N_2, O_2, and water vapor on the ionization energy of titanyl phthalocyanine thin film studied by photoemission yield spectroscopy

摘要

The effect of atmospheric gases on the ionization energy (I) of titanyl phthalocyanine thin film was investigated by an apparatus of photoemission yield spectroscopy developed for the measurements of I for the same specimen both in vacuum and under gaseous atmosphere. It was found that the value of I is affected by the exposure to various ambient gases (i.e., air, nitrogen, and oxygen of 1 atm, and water vapor corresponding to 27% relative humidity at 300 K), and that the effect strongly depends on the gas. The ionization energies in vacuum could be determined as the onset I_0 of the cube-root plot of the photoemission yield as a function of photon energy. When the sample was exposed to gases, the cube-root plot still gives an onset, but often a long tail at the low-energy side with another onset I, was also observed. The first exposure to air did not affect both I_0 and I_t much, while the following evacuation-exposure cycles caused mostly reversible decrease and increase of I_0 and I_t by about 0.2 and 0.4 eV, respectively. Among the examined constituent gases of air, nitrogen was found to hardly affect both I_0 and I_t except for the small temporal decrease by 0.06 eV at the first exposure. Similar trends of the change of I_0 and I_t with those for air were found for oxygen, with the amplitudes of mostly reversible change being 0.1 eV for I_0 and 0.2 eV for I_t. Overlapped with these changes, a slow decrease of both I_0 and I_t, was also observed with a slope of 0.01 eV per day. For water, the trends were mostly similar with O_2, except that (1) the first exposure to water vapor showed small and large changes in I_0 (0.03 eV increase) and I_t (0.4 eV decrease) and (2) the amplitudes of the mostly reversible change of I_0 (0.3 eV) and I_t (0.05 eV) were much larger and smaller than those for O_2, respectively. A long-term decrease was also observed with a slope of 0.04 eV per day. These results indicate that the atmospheric effect by ambient air on I is mainly caused by water vapor, although oxygen also makes significant contribution. Since the observed trends are rather complex, there seem to be multiple factors affecting I, which is the energy difference between the vacuum level and the highest occupied molecular orbita. Possible microscopic mechanisms of the observed variation of I on these levels are also discussed in terms of the energy change in these levels.