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

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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.