Resolving the Nanoscale Morphology and Crystallographic Structure of Molecular Thin Films: F₁₆CuPc on Graphene Oxide

Electron microscopy and diffraction are used to examine the nanoscale structure and molecular orientation in molecular films down to nominally monolayer thickness. The films studied consist of the planar n-type molecular semiconductor copper hexadecafluorophthalocyanine (F₁₆CuPc) directly deposited onto graphene oxide (GO) membranes by organic molecular beam deposition. The graphene oxide support crucially provides the strength and low background required to analyze the crystal structure and morphology of even nominally monolayer thick films and is of relevance for molecular electronic applications. The crystal structure of the F₁₆CuPc polymorph is solved by X-ray diffraction of single crystals and used to analyze the electron diffraction patterns from the thin-films, revealing that the F₁₆CuPc molecules assemble with their molecular plane oriented perpendicular to the GO. There is no evidence for changes in the unit cell with film thickness, although the thinnest films show the greatest disorder in molecular packing. Direct deposition of molecular materials on low contrast and relevant substrates combined with electron and scanning probe microscopy is thus shown to be a powerful technique for elucidating structure in nanostructured organic thin films.