Crystalline Polymorphs of [6,6]-Phenyl-C61-butyric Acid n-Butyl Ester (PCBNB)

The thermotropic behavior of [6,6]-phenyl-C-61-butyric acid n-butyl ester (PCBNB) in powder and thin film form was investigated using X-ray diffraction and transmission electron microscopy. Upon heating PCBNB powder above its glass-transition temperature, an amorphous-to-crystalline transition (i.e., cold crystallization) and a subsequent melting of these crystals were observed. A thin film of PCBNB was observed to order on a simple hexagonal lattice (HEX) with the c axis preferentially oriented normal to film at an annealing temperature of 180 degrees C but became disordered above 200 degrees C, consistent with the powder results. However, when annealed at 160 degrees C, the PCBNB thin film ordered on a superlattice of the HEX as indicated both by electron diffraction and high-angle annular dark field scanning TEM (HAADF-STEM) images. The formation of the HEX superlattice polymorph was independent of both solvent and substrate and could be formed both on heating from the amorphous as cast state and by cooling from the HEX structure formed at a higher temperature. HAADF-STEM shows that the superlattice corresponds to a regular deficiency of PCBNB molecules on every fifth (1 (1) over bar 0 0) plane of the HEX structure.