Crystallization and Epitaxy Study of Organic Molecules on Graphene from 4D Scanning Transmission Electron Microscopy

Organic electronics have recently attracted attention thanks to distinct favorable properties, such as lightweight, flexibility, and printability. However, it is hard to control the molecular packing and morphology. Recently, we demonstrated using vertical vaper transportation to achieve the epitaxial growth of single-crystalline p-n junctions on a graphene substrate using two organic small molecules: Zinc phthalocyanine (ZnPc) as a donor-type material and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) as an acceptor-type material. We have shown through synchrotron X-ray experiments that these planer-shaped molecules pack parallel to the substrate through π-π interactions, either by themselves only, or as stacks on top of each other. Nevertheless, local structure information is still lacking because direct observation of organic molecular nanocrystals and their epitaxy using electron microscopy is difficult, as organic materials are radiation-sensitive and form complex local morphologies. Here, we show how 4D-scanning transmission electron diffraction (4D-STEM) directly observes the orientation and strain distribution of ZnPc and PTCDA crystals on graphene, and thereby elucidating the local structural relationship between stacked ZnPc/ PTCDA heterojunctions. Preliminary data shows PTCDA has more random distributed crystal orientations than ZnPc. We hypothesize different growth kinetics of two molecules on graphene, as the competition between intramolecular and molecule-graphene interactions are different for ZnPc and PTCDA. Additionally, by statistically analyzing many nanodiffraction patterns, we show that ZnPc and PTCDA establish line-on-line organic weak epitaxy, where the PTCDA lattice rotates to a certain degree, such that it coincides with the ZnPc lattice. This finding agrees with previous results obtained from synchrotron X-ray experiments.<br/>This straightforward observation can better explain the growth mechanism and packing/orientation of organic molecules growing on the graphene substrate, guiding organic molecules and electronics design in the future.