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 (CoPcF$_{16}$), also
denoted F$_{16}$CoPc), on both large molybdenum disulfide (MoS$_2$)
single crystals and small MoS$_2$ flakes, using synchrotron-based
techniques: X-ray absorption spectroscopy (XAS) at the Co L$_3$
edge and spectromicroscopy in a photoemission electron micro-
scope (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 MoS$_2$ layers and/or the smaller size of atomically flat terraces on flakes compared to bulk crystals.