4D-STEM orientation mapping in anisotropic molecular glasses*

We have used four dimensional – scanning transmission electron microscopy (4D – STEM) to study the molecular orientation domain structure of vapor-deposited thin film glasses of a phenanthroperylene ester, a discotic mesogen known to form an equilibrium columnar hexagonal phase. Previously, grazing-incidence wide-angle X-ray diffraction and ellipsometry experiments have shown that substrate temperature and deposition rate control the average orientation of the molecules and the hexagonal packing order at different growth conditions. 4D STEM mapping of diffraction from the intracolumnar disc stacking provides an image of the local molecular orientation with sub-nanometer spatial resolution. We find that the in-plane columnar orientation persists within domains whose length scale varies from 10s of nanometers for films deposited at 12 K below the glass transition temperature T_g to several microns for films annealed at T_g + 65 K. Our analysis allows us to detect domain boundaries via automated identification of shifts in column orientations across regions.  Defects in the orientation maps are also visible. These results probe the structure of the films directly at the length scales relevant to intracolumn charge transport in optoelectronic devices.