Highly $n$-doped graphene generated through intercalated terbium atoms

We obtained highly $n$-type doped graphene by intercalating terbium atoms between graphene and SiC(0001) through appropriate annealing in ultrahigh vacuum. After terbium intercalation angle-resolved-photoelectron spectroscopy (ARPES) showed a drastic change in the band structure around the $K$ points of the Brillouin zone: the well-known conical dispersion band of a graphene monolayer was superposed by a second conical dispersion band of a graphene monolayer with an electron density reaching ${10}^{15}{\mathrm{cm}}^{-2}$. In addition, we demonstrate that atom intercalation proceeds either below the buffer layer or between the buffer layer and the monolayer graphene. The intercalation of terbium below a pure buffer layer led to the formation of a highly $n$-doped graphene monolayer decoupled from the SiC substrate, as evidenced by ARPES and x-ray photoelectron spectroscopy measurements. The band structure of this highly $n$-doped monolayer graphene showed a kink (a deviation from the linear dispersion of the Dirac cone), which has been associated with an electron-phonon coupling constant one order of magnitude larger than those usually obtained for graphene with intercalated alkali metals.

Figure 1

XPS spectra of the Tb3d core level. (a) Just after deposition. After annealing (b) at ${600}^{\circ }\mathrm{C}$ and (c) at ${800}^{\circ }\mathrm{C}$. (d) Oxidized Tb.

Figure 2

XPS spectra of the C1s core level (a) of pristine monolayer graphene and (b) after terbium intercalation. (c) Scheme of ML graphene on SiC(0001).

Figure 3

STM images and corresponding LEED pattern of (a) pristine graphene on SiC (0001) before intercalation experiments and (b) graphene intercalated with terbium atoms and annealed at ${800}^{\circ }\mathrm{C}$. (c) STM image of a terrace after terbium intercalation, where the further zoom-in shows all six carbons of graphene in (e). (d) STM image of a recessed area after terbium intercalation, where the further zoom-in shows all six carbons of graphene in (f).

Figure 4

(a) Schemes of the graphene reciprocal lattice and the $K$ point where the ARPES spectra are acquired. ARPES spectra (b) of pristine graphene and after deposition of Tb and annealing at ${800}^{\circ }\mathrm{C}$ (c) for 5 min and (d) for 30 min. In (c) and (d), line scans along different energies (near $E_f$ and near $-2.4$ eV) are represented, in order to show clearly the existence of a new band structure, in particular, in the lowest part of the ARPES spectra. Inset in (d): A short window represented by the dashed-line rectangle in order to increase the contrast locally. (e–g) Corresponding schemes showing an increase in the Fermi surface of the new band with the annealing time, as evidenced by the line scan near $E_f$ in (d), where green arrows represent the positions of the new band in the line scan near $E_f$ in (c).

Figure 5

C1s core level spectra (a) of pristine buffer layer and (b) after Tb intercalation.

Figure 6

C1s core level spectra of pristine graphene samples with varying graphene–to–buffer layer (BL) ratios: (a) 80/20; (b) 50/50; (c) 30/70; (d) 0/100. (e–h) Resulting ARPES spectra after terbium intercalation on these samples. This clearly shows that the new highly $n$-doped band structure results from the decoupling of BL by intercalated terbium atoms.