Structural and Electronic Properties of Pb- Intercalated Graphene on Ru(0001)

The Pb intercalation at the interface of monolayer graphene (MG) and Ru(0001) is studied by means of low temperature scanning tunneling microscopy (LT-STM) and Raman spectroscopy. Despite being covered by MG, the atomic structures of the Pb layer formed between MG and Ru(0001) have been directly imaged using LT-STM. The Pb layer intercalated underneath MG exhibits a √7 × √7-R19° superstructure with respect to the Ru(0001) surface. STM and Raman spectroscopy measurements and density functional theory calculations reveal that the epitaxial MG are effectively decoupled from the Ru(0001) substrate and recover its intrinsic electronic property after Pb intercalation. ■ INTRODUCTION As a single layer of sp-bonded carbon atoms with a honeycomb lattice, graphene has attracted considerable attention because of its unusual physical properties, such as the massless charge carriers and the quantum Hall effect. Graphene is intrinsically nonsuperconductive. Meanwhile, when a superconductor contacts with graphene, the coupling between the Cooper pairs and the relativistic massless Dirac Fermions may give rise to new quantum physics, for example, bipolar supercurrent and specular Andreev reflection. Such graphene/superconductor heterostructures are very promising for developing novel devices with entirely new concepts. It is highly desirable to “deposit” large-area and high-quality graphene with intrinsic electronic properties on superconductors. In the past few years, the epitaxial growth of large-area uniform graphene with low defect density has been achieved on various transition metal substrates. However, the electronic coupling between graphene and metal substrates may damage the characteristic Dirac cone band structure of graphene, which impedes many potential applications of epitaxial graphene. For instance, high-quality single-crystalline monolayer graphene (MG) can be epitaxially grown on Ru(0001), but its π band is strongly hybridized with the Ru 4d state, resulting in a dramatic modification of the density of states (DOS) near the Fermi level and an n-doped feature of the thermoelectrical property. In order to decouple the epitaxial graphene from the metal substrate and regain the intrinsic electronic properties of graphene, a buffer layer can be introduced at the interface of the graphene layer and its substrate. In recent years, it has been shown that intercalation of foreign materials, such as Pt, Co, and Si, can efficiently weaken the interaction between MG and the Ru(0001) substrates, leading to the recovery of the intrinsic linear energy band dispersion of free-standing MG. However, intercalation of a superconducting layer at the interface of MG and substrate and its impact on the electronic property of MG have been rarely addressed to date. The atomic structures of the intercalated buffer layers are unclear, as they are covered by graphene, usually inaccessible for scanning tunneling microscopy (STM). Clarification of the atomic structure of the foreign materials intercalated at the interface of graphene and substrate remains a great challenge, but might be very helpful for tuning the electronic and transport properties of MG on metal substrate. Previously, we investigated the growth and structural property of Pb on MG/Ru(0001). In this work, we reported on the structural and electronic properties of Pb-intercalated MG on Ru(0001) by means of STM, scanning tunneling spectroscopy (STS) and Raman spectroscopy. Although the Pb atoms are covered by MG, we managed to obtain special tip states, so that the MG seems to be transparent, allowing direct imaging the atomic structures of Pb layer intercalated between graphene and Ru(0001) by STM. The Pb atoms intercalated underneath MG form a close-packed layer exhibiting a √7 × √7-R19° superstructure with respect to the Ru(0001) surface, which can effectively decouple MG from Ru(0001) and restore the intrinsic electronic property of MG, as revealed by STM/ STS and Raman spectroscopy measurements and density functional theory (DFT) calculations. Received: January 18, 2015 Revised: March 16, 2015 Published: March 20, 2015 Article