Using 4D STEM to Probe Mesoscale Order in Molecular Glass Films Prepared by Physical Vapor Deposition

Abstract

Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor-deposited and liquid-cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface-diffusion-mediated ordering during film deposition. These results demonstrate the ability of low-dose 4D STEM to characterize a mesoscale structure in a molecular glass system which may be relevant to organic electronics.

The slow Arrhenius process in small organic molecules

“…Growing experimental evidence shows, however, that systems can evolve towards more stable, less energetic states also through faster pathways, characterized by a constant activation barrier. For instance, as shown by dewetting, 3 physical aging, 4–7 crystallization, 8,9 shear relaxation 10 experiments and surface diffusion measurements, 11,12 materials can equilibrate within timescales much shorter than that predicted by models based on the α-relaxation only.…”