Local state analysis of organic semiconductor thin films by spatially and angular resolved electron energy-loss spectroscopy

通过电子能量损失谱与透射电子显微镜相结合对有机薄膜进行局域态分析。通过EELS测量了7,7,8,8,-四氰基醌二甲烷(TCNG)及其氟衍生物(F_4TCNQ)的能量损失近边结构(ELNES)并进行了理论计算分析。空间分辨的 ELNES 是通过在分子中的独立碳位点包含一个核心孔来计算的。然后通过未占据的分子轨道解释ELNES中的结构，并显示由于氟化效应导致的电子结构的变化。将核心孔纳入计算对于解释空间分辨 EELS 非常有用，因为分子中的激发位点是明确的。为了研究有机晶体的辐射损伤机制，详细检查了卤化金属酞菁薄膜的 ELNES 变化。至于卤化效应，研究表明，氯和溴原子的取代增强了分子对电子辐照的抵抗力，而氟化并没有太大改善，这被解释为晶体中的分子堆积效应。作为角分辨EELS在有机晶体薄膜中的应用，开发了一种确定外延生长在衬底上的平面分子取向的新方法。在碳K边ELNES中，共轭分子晶体通常观察到具有分子面外方向的1s→π^*跃迁峰。新方法是通过能量过滤透射电子显微镜直接测量1s→π^*强度的二维角分布。角度分布使得可以确定分子的倾斜角度以及倾斜轴的方向。在本研究中，已论证了其在氯化铜酞菁薄膜中的应用。

Local state analysis on organic thin films has been performed by electron energy-loss spectroscopy combined with transmission electron microscopy. The energy-loss near-edge structure (ELNES) was measured from 7,7,8,8,-tetracyanoquinodimethane (TCNG) and its fluorine derivative (F_4TCNQ) by the EELS and analyzed by theoretical calculation. The spatially resolved ELNES was calculated by including a core-hole at independent carbon sites in the molecules. Then the structure in the ELNES was interpreted by unoccupied molecular orbitals and the change of electronic structure due to fluorination effects was shown. The including a core-hole into the calculation is very useful to interpret spatially resolved EELS because an excited site is explicit in a molecule.In order to investigate the mechanism of radiation damage of organic crystals the changes of ELNES was examined in detail for halogenated metal-phthalocyanine thin films. As for the halogenations effect, it was reveal that the substitution by chlorine and bromine atoms reinforced the molecules against electron irradiation, while the fluorinations did not improved so much, which was interpreted as the molecular packing effect in the crystals.As an application of angular resolved EELS for organic crystal thin films, a new method to determine the orientation of a planar molecule grown on a substrate epitaxially has been developed. In the carbon K-edge ELNES, 1s→π^* transition peak with a directivity to out of plane of a molecule is generally observed for conjugated molecular crystals. The new method is to measure the two dimensional angular distribution of 1s→π^* intensity directly by an energy filtering transmission electron microscopy. The angular distribution makes it possible to determine the inclining angle of molecule as well as the direction of tilting axis. In the present study, the application to chlorinated cupper-phthalocyanine thin film has been demonstrate.