Synthesis of differently shaped C70 nano/microcrystals by using various aromatic solvents and their crystallinity-dependent photoluminescence

We report a simple synthesis of C70 microcrystals by a fast solvent-assisted evaporation method by selectively using aromatic solvents with halogen radicals as a controller. In a detailed analysis we show that depending on solvent, C70 concentration in the solution, and synthesis temperature both C70 microrods and microcubes can be produced. The samples were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. From meta-type solvents microrods or microcrystal cubes grow with a single orthorhombic or a monoclinic structure or a mixture of both. These crystals show traces of solvents in the structure. From para-type solvents cubic (or rectangular) crystals grow with a hexagonal structure and with no trace of solvents. Room temperature photoluminescence (PL) of the C70 crystal samples reveals that the PL efficiency of the crystals increases with crystalline order and that the best crystalline C70 cube crystals show 10 times higher PL efficiency than that of pristine C70 polycrystalline samples.