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

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.

Automated summary

Physical vapor deposition is used to create stable organic glass systems with nanoscale orientational and translational ordering of molecules. Using low-dose 4D STEM and a 2 nm probe, we have mapped columnar order in glassy samples of a discotic mesogen. Both vapor-deposited and liquid-cooled glassy films show similar domains of orientation, with size variations depending on processing. These results demonstrate the potential of low-dose 4D STEM for characterizing mesoscale structures in molecular glass systems relevant to organic electronics.