Interfacial electronic structure at a metal-phthalocyanine/graphene interface: Copper-phthalocyanine versus iron-phthalocyanine

The energy level alignment of CuPc and FePc on single-layer graphene/Ni(111) (SLG/Ni) substrate was investigated by using ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). The highest occupied molecular orbitals (HO-MOs) in a thick layer of CuPc and FePc lie at 1.04 eV and 0.90 eV, respectively, below the Fermi level of the SLG/Ni substrate. Weak adsorbate-substrate interaction leads to negligible interfacial dipole at the CuPc/SLG/Ni interface, while a large interfacial dipole (0.20 eV) was observed in the case of FePc/SLG/Ni interface, due to strong adsorbate-substrate coupling. In addition, a new interfacial electronic feature was observed for the first time in the case of FePc on SLG/Ni substrate. This interfacial state can be attributed to a charge transfer from the SLG/Ni substrate to unoccupied orbitals of FePc.