Noninvasive coupling of PbPc monolayers to epitaxial graphene on SiC(0001)

Abstract

Proximity coupling is discussed as a promising approach to tune further graphene’s properties. Organic π-conjugated molecules couple per se weakly to surfaces and the class of phthalocyanines provides manifold possibilities for molecular functionalization. We have investigated the adsorption of lead phthalocyanine (PbPc) on epitaxial graphene by means of scanning tunneling microscopy and in-situ electronic transport measurements in order to quantify both a molecular charge transfer and modifications of the carrier mobility in graphene. Compared to other metallic surfaces, monolayer structures of PbPc adsorbed on graphene solely pointing upwards, apparently, due to the antibonding character of the Pb-states with the pz-orbitals of graphene. The squared lattice of the molecular layer is not commensurate with the honeycomb lattice of graphene and the PbPc molecules are slightly tilted, thus forming a chiral monolayer structure. Despite this interaction with graphene, the charge state of the molecule as well as the mobility of the charge carriers in graphene are not altered. Therefore, monolayer structures of Pb molecules are promising candidates for proximity studies in graphene.