Structural and electronic properties of Li-intercalated graphene on SiC(0001)

We investigate the structural and electronic properties of Li-intercalated monolayer graphene on SiC(0001) using combined angle-resolved photoemission spectroscopy and first-principles density functional theory. Li intercalates at room temperature both at the interface between the buffer layer and SiC and between the two carbon layers. The graphene is strongly $n$-doped due to charge transfer from the Li atoms and two $\pi$ bands are visible at the $\overline{K}$ point. After heating the sample to $300{}^{\circ }\mathrm{C}$, these $\pi$ bands become sharp and have a distinctly different dispersion to that of Bernal-stacked bilayer graphene. We suggest that the Li atoms intercalate between the two carbon layers with an ordered structure, similar to that of bulk ${\mathrm{LiC}}_6$. An AA stacking of these two layers becomes energetically favourable. The $\pi$ bands around the $\overline{K}$ point closely resemble the calculated band structure of a ${\mathrm{C}}_6{\mathrm{LiC}}_6$ system, where the intercalated Li atoms impose a superpotential on the graphene electronic structure that opens gaps at the Dirac points of the two $\pi$ cones.

Figure 1

Band dispersion recorded from monolayer graphene samples before [(a)–(c)] and after [(d)–(f)] Li deposition, and after heating to $300{}^{\circ }\mathrm{C}$ [(g)–(i)]. The dashed lines are guides to the eye for illustrating the location of the first and second $\overline{K}$ points or an initial state energy of 0 eV and $-1.4$ eV. The Fermi energy is located at 0 eV. (j) The 2D Brillouin zone of graphene showing the $\overline{\mathrm{\Gamma }}, \overline{M}$, and $\overline{K}$ high-symmetry points. The $A$ and $A^{\prime }$ directions are also shown.

Figure 2

Band dispersion recorded in the vicinity of the $\overline{K}$ point (top two panels) and angular distributions extracted at fixed energies close to the Dirac point. The six energies are shown as white dashed lines through the band dispersions in the top panels. The Fermi energy is located at 0 eV. An incident photon energy of 33 eV was used.

Figure 3

Calculated band structure of (a) AB-stacked clean bilayer graphene, (b) AA-stacked clean bilayer graphene (c) AB-stacked ${\mathrm{C}}_8{\mathrm{LiC}}_8$, (d) AA-stacked ${\mathrm{C}}_8{\mathrm{LiC}}_8$, (e) AB-stacked ${\mathrm{C}}_6{\mathrm{LiC}}_6$, and (f) AA-stacked ${\mathrm{C}}_6{\mathrm{LiC}}_6$. The high-symmetry labels are those of the graphene unit cell.

Figure 4

Band structure of free-standing ${\mathrm{C}}_6{\mathrm{LiC}}_6$ as calculated with DFT (red solid lines) and using a tight-binding model (blue dashed lines), compared to the experimental ARPES spectra [cf. Fig. 1]. The calculated band structures have been shifted rigidly in energy to match the top ${\pi }^{*}$ band.

Figure 5

Structure of ${\mathrm{C}}_6{\mathrm{LiC}}_6$. (a) Top view showing the graphene lattice vectors ${\mathit{a}}_1$ and ${\mathit{a}}_2$ and the lattice vectors of ${\mathrm{C}}_6{\mathrm{LiC}}_6, {\mathit{R}}_1$ and ${\mathit{R}}_2$. ${\mathit{R}}_1$ and ${\mathit{R}}_2$ are rotated by $30{}^{\circ }$ with respect to ${\mathit{a}}_1$ and ${\mathit{a}}_2$ and are $\sqrt{3}$ longer. The two carbon sublattices are denoted with gray and white circles and Li atoms are shown as green circles. The two types of bonds associated with the Kekulé textured graphene are shown as thin and thick solid lines. The two different intralayer hopping parameters between carbon atoms associated with these two bonds are denoted $t$ and $t^{\prime }$. The six atoms belonging to the ${\mathrm{C}}_6{\mathrm{LiC}}_6$ unit cell of one of the graphene layers are labeled 1–6, and are used to generate the Hamiltonian of the system. (b) Side view showing the interlayer coupling parameters, ${\gamma }_1$ between carbon atoms directly on top of one another, and ${\gamma }_2$ and ${\gamma }_2^{\prime }$ describing the textured skew coupling terms.

Figure 6

Electronic band structure of (a) the Li intercalated zero layer graphene on SiC(0001) system and (b) Li intercalated monolayer graphene on SiC(0001). The Li concentration corresponds to ${\mathrm{LiC}}_8$ and ${\mathrm{LiC}}_8{\mathrm{LiC}}_8$, respectively. The insets show the relaxed structure of both configurations. The Fermi energy is located at 0 eV.

Figure 7

Side view of a $2\times 1$ unit cell of SiC(0001) that includes two layers of graphene (16 C atoms in each) and three Li atoms. Isosurfaces of charge density difference show how charge is transferred from a Li atom inserted (a) between the two carbon layers and (b) at the interface between SiC and the bottom carbon layer. A purple (orange) isosurface refers to a gain (loss) in charge density. The isosurface value is 0.0005 e/a.u.${}^3$.