Microstructure evolution during crystallization of vapor-deposited hexanitroazobenzene films

Vapor-deposited hexanitroazobenzene (HNAB) films were observed to form a dense amorphous structure that crystallizes to a mixture of the HNAB-II polymorph and an unidentified structure over a period ranging from hours to weeks depending on the ambient temperature. Films crystallized at various temperatures were characterized using scanning electron microscopy, atomic force microscopy, X-ray diffraction, and Raman spectroscopy to measure the impact of crystallization temperature on resultant microstructure. Crystallization temperature was observed to have different effects on film microstructure over two temperature regimes. At temperatures below approximately 65°C, increases in temperature led to a greater fraction of the film forming the HNAB-II polymorph and caused subtle changes in morphology. However, at higher temperatures, a thin surface layer was observed to form prior to crystallization, which led to films composed primarily of the unknown crystal structure with conspicuous differences in morphology.