Li-intercalated graphene on SiC(0001): An STM study

We present a systematical study via scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) on the effect of the exposure of lithium on graphene on silicon carbide (SiC). We have investigated Li deposition both on epitaxial monolayer graphene and on buffer layer surfaces on the Si face of SiC. At room temperature, Li immediately intercalates at the interface between the SiC substrate and the buffer layer and transforms the buffer layer into a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. We show that intercalation occurs through the SiC step sites or graphene defects. We obtain good quantitative agreement between the number of Li atoms deposited and the number of available Si bonds at the surface of the SiC crystal. Through STM analysis, we are able to determine the interlayer distance induced by Li intercalation at the interface between the SiC substrate and the buffer layer.

Figure 1

(a) Raman map of the intensity of the 2D peak of graphene. Black areas relate to the spectrum in (b), which does not show any peak associated with graphene. These areas are BL regions. Red areas are related to EMLG regions, which show the spectrum reported in (c).

Figure 2

LEED diffraction patterns (a) for pristine graphene and after having deposited (b) $0.28\pm 0.02$ ML of Li and (c) $0.56\pm 0.04$ ML of Li. Electron energy: (a) 92.5 eV, (b) 95.5 eV, and (c) 135.6 eV.

Figure 3

(a) STM topographical image taken after deposition of 0.031 ML Li on EMLG. The moiré pattern is visible in the left and right parts of the image, while in the middle there is a flat and not reconstructed region. Scan area: $50\times 50{\mathrm{nm}}^2$. (b) STM topographical image taken after deposition of 0.047 ML Li on EMLG. The image shows that the Li-intercalated area has grown compared to (a). Scan area: $72\times 72{\mathrm{nm}}^2$. (c) Magnification taken from the area indicated by the solid square in (a). Scan area: $20\times 20{\mathrm{nm}}^2$. (d) Cross-sectional plot taken along the black line in (c). Image parameters: (a) $-500$ mV, $-170$ pA; (b) 1 V, 1 nA; (c) $-500$ mV, $-170$ pA.

Figure 4

EMLG surface after deposition of 0.28 ML Li. (a) STM topographical image. No moiré pattern is observed. Scan area: $50\times 50{\mathrm{nm}}^2$. Image parameters: $-500$ mV, $-0.51$ nA. (b) Magnification ($6\times 6{\mathrm{nm}}^2$) taken from the area indicated by the dashed square in (a). The graphene atomic lattice is resolved. Image parameters: 413 mV, 173 pA. (c) Cross-sectional plot taken along the blue line in (a).

Figure 5

EMLG surface after deposition of 0.56 ML Li. (a) STM image of this surface. Scan area: $10\times 10{\mathrm{nm}}^2$. The inset ($1\times 1{\mathrm{nm}}^2$) shows a high-resolution image from the same area with atomic resolution. Image parameters: 30 mV, 90 pA. (b) 2D-FFT image of (a) in which the graphene spots and those of the $\left(\sqrt{3}\times \sqrt{3}\right)\mathrm{R}{30}^{\circ }$ superstructure are clearly visible, highlighted by red and yellow circles, respectively. The scale bar indicates $k=1$/wavelength.

Figure 6

(a) STM image from a BL area before Li deposition. The top right shows an EMLG area. Scan area: $200\times 200{\mathrm{nm}}^2$. (b) STM image from a BL area after deposition of 0.047 ML Li. On the left, a small (nonintercalated) EMLG area is visible. Scan area: $200\times 200{\mathrm{nm}}^2$. (c) Magnification of the area indicated by the dashed square in (b). Scan area: $100\times 100{\mathrm{nm}}^2$. Island formation in the BL area is visible. (d) STM topographic image from the BL after deposition of 0.28 ML Li. Scan area: $300\times 300{\mathrm{nm}}^2$. The top left shows a small QFBLG region. Most of the BL area has been transformed to QFMLG by the Li intercalation. (e) Cross-sectional plot taken along the blue line in (b), from which we can determine the islands' height. (f) Cross-sectional plot taken along the blue line in (d). Images parameters: (a) 1 V, 1 nA; (b) and (c) $-300$ mV, $-1$ nA; (d) 0.8 V, 1 nA.

Figure 7

(a) STM image ($50\times 50{\mathrm{nm}}^2$) taken after 10 min of annealing at $400{}^{\circ }\mathrm{C}$. Image parameters: 255 mV, 150 pA. (b) The corresponding LEED pattern, taken at 95.5 eV, starts to show bright spots related to moiré in addition to graphene $\left(1\times 1\right)$ and SiC $\left(1\times 1\right)$ bright spots.

Figure 8

Schematic representation of the Li distribution at the interface (a) for the BL case and (b) for the EMLG case. (a) Li converts the BL to QFMLG. The measured value of the island height in Fig. 6, 2.6 $\text{Å}$, allows us to calculate the interlayer distance induced by the intercalated Li atoms between the substrate and the QFMLG. (b) Li converts EMLG to QFBLG. The measured value of the step height in Fig. 3, 1.6 $\text{Å}$, allows us to calculate the interlayer distance induced by the intercalated Li atoms between the substrate and the QFBLG.