Orientation of Cobalt-Phthalocyanines on Molybdenum Disulfide: Distinguishing between Single Crystals and Small Flakes

Heterostructures consisting of transition metal dichalcogenides (TMDCs) and organic molecules are currently of enormous interest for a variety of applications. Comparably weakly interacting molecules, like phthalocyanines, exhibit a high potential for tuning the electronic properties of TMDCs. Knowledge of the molecular orientation is a prerequisite for understanding the nature and strength of the interfacial interaction. We study the molecular orientation of cobalt phthalocyanine (CoPc) and perfluorinated cobalt phthalocyanine (CoPcF16, also denoted F16CoPc), on both large molybdenum disulfide (MoS2) single crystals and small MoS2 flakes, using synchrotron-based techniques: X-ray absorption spectroscopy (XAS) at the Co L3 edge and spectromicroscopy in a photoemission electron microscope (PEEM). We show that the orientation can be radically different on both substrates. Whereas on large crystals an almost flat-lying orientation is observed, significant tilt angles were found on smaller flakes. It is proposed that the orientation depends crucially on the number of MoS2 layers and/or the smaller size of atomically flat terraces on flakes compared to bulk crystals.