Interfaces between cobalt phthalocyanines and MoSe$_2$ : conditions for charge transfer and the nature of gap states

The recently increasing research efforts devoted to layered transition metal dichalcogenides (TMDCs) derive from their novel electronic properties, which might be further tuned by the adsorption of molecules. However, the nature of interactions is often not fully understood, especially for comparably weakly interacting molecules, such as transition metal phthalocyanines (TMPcs). We studied interface properties between
cobalt phthalocyanine (CoPc) or perfluorinated CoPcF$_{16}$ and molybdenum diselenide (MoSe2) using photoelectron spectroscopy (PES), angle resolved photoelectron spectroscopy (ARPES), X-ray absorption spectroscopy (XAS) and photoluminescence spectroscopy. The interaction between MoSe$_2$ and cobalt phthalocyanines crucially depends on both the ionization potential of the molecule and the doping level of the substrate. Only for CoPcF$_{16}$ interfaces, a redistribution of electrons of the Co ion is observed, which can be associated with interfacial charge transfer. These effects are significantly more pronounced, if the MoSe$_2$ substrate is nominally n-doped. The results are discussed in the context of integer and fractional charge transfer, the latter associated with the formation of states within the band gap