Using 4D STEM to probe mesoscale order in molecular glass films prepared by physical vapor deposition

TL;DR

This study employs low-dose 4D STEM to map mesoscale molecular order in vapor-deposited organic glass films, revealing domain size variations influenced by processing conditions, with implications for organic electronics.

Contribution

First application of low-dose 4D STEM to characterize mesoscale order in vapor-deposited molecular glass films, demonstrating its effectiveness for nanoscale structural analysis.

Findings

Domain sizes range from tens to hundreds of nanometers.

Processing conditions influence domain size and orientation.

Surface diffusion during deposition affects mesoscale ordering.

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 mesoscale structure in a molecular glass system which may be relevant to organic electronics.