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

Abstract

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 microscope (PEEM). We show that the orientation can be radically different on both substrates. Whereas on large crystals an almost flatlying 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.

Tuning the Interfacial Electronic Structure of MoS2 by Adsorption of Cobalt Phthalocyanine Derivatives

“…The displayed feature A represents transitions in states orthogonal to the molecular plane, while the B features are transitions within the molecular plane. The orientation of CoPc and CoPcF 16 on MoS 2 was extensively studied in a previous work . In summary, it can be said that the molecules adopt a flat, lying orientation relative to the MoS 2 surface.…”

“…One aspect that must be considered is the possibility that the orientation of molecules relative to the MoS 2 surface may vary, which can influence substrate–molecule interactions . In previous, more detailed X-ray absorption studies, it was demonstrated that in CoPc­(F 16 ), a flat, face-on orientation is adopted on MoS 2 …”

Toward Gigahertz Photodetection with Transition Metal Dichalcogenides

“…Such an increase in the resistance is detrimental to the RC time and the expected response time. In a similar context, Haizmann et al have recently shown that substrate roughness and its effect on thin TMDC layers also alter the orientation of small molecules adsorbed to the TMDC surface . While the exact consequences of this alteration remain to be explored, it is likely that this will affect the chemical interactions at the interface and, thus, the efficiency with which such small molecules saturate surface defects.…”