Hybridization-Induced Image Potential States with Large Effective Mass in Lead Phthalocyanine Overlayers on Graphene

Abstract

The structural and electronic properties of lead phthalocyanine (PbPc) adsorbed on graphene are studied theoretically using the van der Waals density functional method. It is revealed that an extended state analogous to an image potential state (IPS) emerges in a close-packed PbPc monolayer, hybridizing with the lowest IPS of graphene to form bonding and antibonding states at the PbPc−graphene interface. The impact of the PbPc adsorption manifests itself as increased effective masses of the hybrid IPSs as compared with those on pristine graphene. In particular, the bonding IPS exhibits a marked anisotropy in the effective mass, reflecting the uniaxially elongated unit cell adopted in the present calculations, whereas the antibonding one has an almost isotropic effective mass, which is comparable to experimental results obtained with two-photon photoemission spectroscopy of PbPc on highly oriented pyrolytic graphite and on single crystalline graphite. It is also shown that the increase in the effective masses is triggered by the hybridization with unoccupied molecular orbitals of PbPc that is localized at the Pb atom.