Effective mass and inner potential of hot electrons in organic thin films.

采用电子能低于15 eV的方法，测定了不同晶序酞菁铜（CuPc）薄膜的低能电子透射光谱（LEET）。LEET特征高度依赖于晶体顺序，表明它们不是来自于单个分子中的电子态，而是来自于反映晶体结构的电子态。对于晶体bc面平行于表面的α型CuPc定向多晶薄膜，发现LEET最小值的能量位置与假设垂直于晶体bc面方向的抛物型导带所得到的带隙的能量位置非常一致。我们进一步估计了热电子的有效质量m^*为2.2_2m_0，其中m_0为自由电子质量，在垂直于α型CuPc的bc面方向上的内势v_0为-1.3_3 eV。结果表明，入射电子在带隙处的镜面反射是影响LEET特性的主要因素，即使在真空水平附近，抛物线色散也能很好地描述热电子态。

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 bc 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 bc plane. Further we estimated the effective mass of hot electron m^* to be 2.2_2m_0, where m_0 is the free-electron mass, and the inner potential v_0 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 states can be well described by the parabolic dispersion even near the vacuum level.