Abstract
Graphene on metal substrates often shows different electronic properties from isolated graphene because of graphene-substrate interactions. One needs to remove the metals with acids and then to transfer graphene to weakly interacting substrates to recover electrical properties inherent in graphene. This process is not easy and besides causes undesirable tears, defects, and impurities in graphene. Here, we report a method to recover the electronic structure of graphene from a strongly interacting Ni substrate by spontaneous Na intercalation. In order to characterize the intercalation process, the density-functional-theory calculations and angle-resolved photoemission-spectroscopy (ARPES) and scanning-tunneling-microscopy (STM) measurements are carried out. From the density-functional-theory calculations, Na atoms energetically prefer interface intercalation to surface adsorption for the graphene/Ni(111) surface. Unlike most intercalants, Na atoms intercalate spontaneously at room temperature due to a tiny diffusion barrier, which is consistent with our temperature-dependent ARPES and core-level photoemission spectroscopy, and with our submonolayer ARPES and STM results at room temperature. As a result of the spontaneous intercalation, the electronic structure of graphene is almost recovered, as confirmed by the Dirac cone with a negligible band gap in ARPES and the sixfold symmetry in STM.
2 More- Received 24 August 2013
DOI:https://doi.org/10.1103/PhysRevX.4.031016
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Published by the American Physical Society
Popular Summary
Graphene, an arrangement of carbon atoms only one atom thick, has many proposed uses in optoelectronics and electronics. When graphene is deposited on metal substrates using, for instance, chemical vapor deposition, undesirable grain boundaries are produced. Furthermore, the electronic properties of the graphene-metal substrate differ from isolated graphene because of graphene-substrate interactions. Removing the metal substrate, however, is a difficult task that can cause impurities in the graphene. We use density-functional calculations, angle-resolved photoemission spectroscopy, and scanning-tunneling-microscopy measurements to show that the electronic structure of graphene can be recovered from a strongly interacting Ni substrate using spontaneous Na intercalation.
Graphene on a lattice-matched Ni(111) surface exhibits a band at about eV, far below the Fermi level, indicating a strong interaction with the underlying Ni substrate. We choose to intercalate the graphene/metal substrate with Na atoms that penetrate far into the graphene at , ensuring that intercalation takes place spontaneously even at room temperature. The ease of intercalating with Na may be due to the small diffusion barrier of Na atoms (as small as 0.03 eV). After intercalation, which affects the structure of the graphene/Ni(111) substrate only minimally, we find that the band of graphene returns to the state associated with pristine graphene, even at room temperature, with a negligible energy gap. Furthermore, atomic images of the graphene surface on the Ni crystal substrate after Na intercalation change from the threefold symmetry due to the Ni-induced symmetry breaking to the sixfold symmetry, direct evidence of the Na intercalation between the Ni surface and the graphene. As a result of the spontaneous intercalation, the electronic structure of graphene is almost recovered, as confirmed by the Dirac cone with a negligible band gap in angle-resolved photoemission spectroscopy and the sixfold symmetry seen with scanning tunneling microscopy.
We expect that our results will be used to create high-quality and large-area samples of graphene that do not rely on etching or transferring processes. These graphene samples can be incorporated into future electronic devices.