PARABOLIC DISPERSION AND EFFECTIVE MASS OF HOT-ELECTRONS IN ORIENTED THIN-FILMS OF COPPER PHTHALOCYANINE DETERMINED BY MEANS OF LOW-ENERGY-ELECTRON TRANSMISSION

Low-energy-electron transmission (LEET) spectra were measured for thin films of copper phthalocyanine (CuPc) with different crystalline order using electron energies below 15 eV. The LEET features were highly dependent on the crystalline order, indicating that they do not originate from the electronic states localized in the individual molecules, but from those which reflect the crystal structure. For oriented polycrystalline films of alpha form CuPc with the crystal be plane parallel to the surface, the energy positions of the LEET minima were found to be in excellent agreement with those of the band gaps obtained by assuming a parabolic conduction band in the direction perpendicular to the crystal be plane. Further, we estimated the effective mass of hot electron m* to be 2.2(2)m0, where m0 is the free-electron mass, and the inner potential V0 to be -1.3(3) eV in the direction perpendicular to the bc plane of alpha-form CuPc. The results show that the specular reflection of the incident electron at the band gap is a main factor for the LEET features and the hot-electron state can be well described by the parabolic dispersion even near the vacuum level.