In Situ Study of Liquid Crystal Phase Mediated Crystal Growth in Vacuum-Deposited Thin Films of an Alkyl Perylene Diimide

Crystallization processes of N,N′-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C8) thin films, grown by vacuum deposition on SiO2/Si at substrate temperatures of ≥180 °C, are investigated in separate experiments using respective in situ characterization techniques, including two-dimensional grazing-incidence X-ray diffraction (2D-GIXD), infrared multiple-angle incidence resolution spectroscopy (IR-MAIRS), and ellipsometry. The complementary set of in situ data reveals that a high-temperature smectic liquid crystal phase first preferentially appears in vacuum deposition under substantial supercooling and then transitions directly to the crystal phase without forming any of the lower-temperature liquid crystal phases. Specifically, at a substrate temperature of 180 °C, the crystallization begins amid deposition once the film thickness reaches ∼25 nm, resulting in the formation of platelet grains over 100 μm in size. In contrast, at 195 °C, the high-temperature smectic phase persists until the thickness is even 100 nm, but upon cooling it is crystallized into either microcrystals or spherulites depending on the cooling rate. These findings highlight how strongly the substrate temperature, film thickness, and cooling conditions govern the morphological evolution of PTCDI-C8 thin films via a particular supercooled smectic phase, as directly visualized by in situ measurements.