Quasi-Free-Standing Graphene Monolayer on a Ni Crystal through Spontaneous Na Intercalation

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.

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 $\pi$ band at about $-2.7$ 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 $-100°\mathrm{C}$, 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 $\pi$ 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.

Figure 1

Employed structural models for the Na-intercalated graphene/Ni(111) surface. Large, medium, and small balls represent the Ni, Na, and C atoms, respectively. 1 ML denotes one Na atom per five C atoms. Dashed lines represent the corresponding unit cells. Arrows in (a) indicate three high-symmetry surface sites.

Figure 2

Adsorption and intercalation energies as a function of the Na coverage.

Figure 3

Dirac-point shift with reference to the Fermi level as a function of the Na coverage. All data are obtained for the majority-spin band.

Figure 4

Majority-spin band structures of (a) graphene/Ni(111), (b) Na(0.63 ML)/graphene/Ni(111), and (c) graphene/Na(0.63 ML)/Ni(111). The vertical solid line in the surface Brillouin zone represents the calculated line. The Fermi energy is set to 0, and the bands of ideal graphene are given as the reference by the thin solid lines. The filled red (open green) circles represent C-derived (Ni-derived) surface states that contain more than 20% (40%) of charge in the graphene layer (topmost Ni layer). The size of the circles is proportional to the amount of charge localized in the graphene and the Ni layer. Open blue squares represent the Na-derived surface states that contain more than 5% of charge in the Na layer.

Figure 5

Electronic band structures measured by ARPES of the (a),(c) as-grown graphene and (b),(d) Na-intercalated graphene, and (e),(f) high angular and energy-resolution measurements of the Na-intercalated graphene. (a),(b) Taken at the $\mathrm{\Gamma }$ point; (c),(d) taken at the $K$ point. The data are measured along the red lines, as indicated in the insets. The color-scale bars in (a) and (b) are the same. (e) Band structure around the $K$ point near the Fermi energy, taken along the direction perpendicular to the $\mathrm{\Gamma }\text{−}K$. (f) Photoemission intensity profiles as a function of BE (energy-distribution curves).

Figure 6

Electronic band structures of the graphene with a low coverage of Na atoms taken at (a) the $\mathrm{\Gamma }$ and (b) the $K$ points.

Figure 7

Temperature-dependent ARPES data of the graphene measured at the $K$ point (a) at $-140°\mathrm{C}$, (b) after Na deposition at $-140°\mathrm{C}$, (c) with the temperature increased to $-100°\mathrm{C}$ after the Na deposition, and (d) with the temperature increased to $+20°\mathrm{C}$ after the Na deposition.

Figure 8

Core-level PES spectra of the graphene grown on the Ni surface taken with a photon energy of 360 eV. The black, red, and green lines represent the PES spectra of the graphene/Ni(111) at $-140°\mathrm{C}$, Na/graphene/Ni(111) at $-140°\mathrm{C}$, and graphene/Na/Ni(111) at $+20°\mathrm{C}$, respectively. The intensity ratio $\mathrm{C}1s$/Na $2p$ is 3.72 for the Na/graphene/Ni(111) at $-140°\mathrm{C}$ and 4.23 for the graphene/Na/Ni(111) at $+20°\mathrm{C}$, respectively. The inset depicts the Na $2p$ PES spectra of the surface.

Figure 9

STM topography images of a graphene grown on the Ni surface. (a) Large-area scan of the graphene before Na intercalation: $2000\times 2000\AA$, $I_t=0.6\mathrm{nA}$, and $V_t=26.9\mathrm{mV}$. (b) Zoomed-in STM image of the squared region in (a): $100\times 100\AA$, $I_t\text{\hspace{0.17em}=}2.59\mathrm{nA}$, and $V_t=100\mathrm{mV}$. (c) STM image after Na intercalation: $100\times 100\AA$, $I_t=3.88\mathrm{nA}$, and $V_t=41.5\mathrm{mV}$. (d) An atomically resolved STM image of (b): $20\times 20\AA$, $I_t=2.59\mathrm{nA}$, and $V_t=42.4\mathrm{mV}$. (e) An atomically resolved STM image of (c): $30\times 30\AA$, $I_t=3.20\mathrm{nA}$, and $V_t=41.5\mathrm{mV}$.