Radiation damage and structural studies: Halogenated phthalocyanines

Radiation damage makes it more difficult to accomplish structural studies of beam-sensitive materials. Nevertheless, we report here a 500 kV high-resolution study of halogenated phthalocyanines. Epitaxially grown molecular crystals of fully brominated and fully chlorinated phthalocyanines were found to be isomorphous and had only the α-type polymorph structure. Despite the sensitivity of these crystals to beam damage, the characteristics of thin film growth and details of structural defects could be determined, provided that molecular image resolution was available. The effect of electron beam damage was a degradation of crystallinity which varied considerably according to local morphology and depended on the ease with which the displaced halogen could diffuse away from the parent molecule and thereby avoid recombination. Our findings demonstrate the feasibility of similar structural studies of other beam-sensitive materials.