Molecule−Substrate Coupling between Metal Phthalocyanines and Epitaxial Graphene Grown on Ru(0001) and Pt(111)

Self-assembly of metal phthalocyanine (MPc) molecules on monolayer graphene (MG) epitaxially grown on Ru(0001) and Pt(111) is investigated by means of lowtemperature scanning tunneling microscopy. At low coverage, dispersive single molecules, dispersive molecular chains, and small patches of Kagome lattice are observed for iron phthalocyanine (FePc), manganese phthalocyanine (MnPc), nickel phthalocyanine (NiPc), and phthalocyanine (H2Pc) on MG/Ru(0001). In contrast, although MG/Pt(111) exhibits various domains with different moire ́ patterns and corrugations, FePc molecules always form densely packed twodimensional islands with a square lattice on MG/Pt(111) at submonolayer coverage. The different self-assembling behaviors of MPc molecules on MG/Ru(0001) and MG/Pt(111) originate from a subtle balance between molecule−molecule and molecule−substrate interactions tuned by central metal ions of the MPc molecules and the metal substrates. ■ INTRODUCTION Graphene, a single layer of sp-bonded carbon atoms with a honeycomb lattice, has been attracting great interest because of its outstanding physical properties and potential applications. To fulfill the requirement of the forthcoming graphene-based technology, it is vital to incorporate other materials into graphene and understand their interfacial structures and coupling. It has been found that the electronic structures and transport properties of graphene can be tuned by the metal substrates that the graphene sheets were epitaxially grown on due to different graphene−substrate interactions. For instance, the strong interaction between graphene sheets and Ru(0001) and Ni(111) substrates results in a dramatic modification of the density of states (DOS) near the Fermi level and a n-doped feature of the thermoelectrical property, whereas the graphene sheets grown on Pt(111) preserve the intrinsic “V”-shaped DOS of free-standing graphene because of a weak graphene−substrate interaction. Adsorption of organic molecules on graphene is also an issue of special importance. Because of charge transfer between molecular adsorbates and graphene, the graphene sheets are shown to be doped and their electronic structures are greatly modified by the organic molecules, providing a promising method to tailor the electronic and transport properties of graphene-based devices. Meanwhile, the molecule− graphene interaction also plays a key role in the molecular self-assembly on graphene, since the final self-assembly is essentially governed by the subtle balance between molecule− substrate and molecule−molecule interactions. Metal phthalocyanine molecules (MPcs), each consisting of a central metal ion and a macrocycle of alternating carbon and nitrogen atoms (Figure 1a), have been attracting considerable interest because of their potential applications in organic electronic and spintronic devices. Formation of densely packed monolayer on graphite, NaCl, Au(111), Ag(111), and Cu(111) surfaces and Kagome lattice on metal surfaces was revealed by various scanning tunneling microscopy (STM) studies. Recently, we adopted the moire ́ pattern of monolayer graphene (MG) that originates from the lattice mismatch between MG and Ru(0001) surface as a template and fabricated regular Kagome lattices of MPcs. We revealed that the site-specific anchoring of FePc molecules on the moire ́ pattern of MG/Ru(0001) is driven by the lateral Received: April 27, 2012 Revised: June 1, 2012 Published: June 2, 2012 Figure 1. (a) Chemical structure of MPc molecules. (b) Large-scale STM image of graphene grown on Ru(0001), showing the hexagonal moire ́ pattern due to the lattice mismatch between graphene and Ru substrate. (c) Zoom-in STM image with atomic resolution, showing the unit cell of the moire ́ pattern. Atop, fcc, and hcp regions are indicated by the circle, solid triangle, and dashed triangle, respectively. Scanning parameters: (b) sample bias U = −1 V, tunneling current I = 0.03 nA; (c) U = −0.2 V, I = 0.5 nA. Article