Scanning tunneling microscopy study of graphene on Au(111): Growth mechanisms and substrate interactions

We use scanning tunneling microscopy to study the structure of graphene islands on Au(111) grown by deposition of elemental carbon at 950 °C. Consistent with low-energy electron microscopic observations, we find that the graphene islands have dendritic shapes. The islands tend to cover depressed regions of the Au surface, suggesting that Au is displaced as the graphene grows. If small tunneling currents are used, it is possible to image simultaneously the graphene/Au moiré and the Au herringbone reconstruction, which forms underneath the graphene on cooling from the growth temperature. The delicate herringbone structure and its periodicity remain unchanged from the bare Au surface. Using a Frenkel–Kontorova model, we deduce that this striking observation is consistent with an attraction between graphene and Au of less than 13 meV per C atom. Raman spectroscopy supports this weak interaction. However, at the tunneling currents necessary for atomic-resolution imaging of graphene, the Au reconstruction is altered, implying influential tip–sample interactions and a mobile Au surface beneath the graphene.

Figure 1

(a) LEEM image (5 μm) of graphene–Au(111) grown at 950 °C. The bright features are dendritic islands of graphene. The gray background corresponds to the bare Au surface. (b) LEED (40 eV) from an area of 2 μm in diameter. (c) Raman spectrum (average of 10 spectra from separate regions) from graphene on Au(111) after background subtraction.

Figure 2

(a) STM image (500 × 500 nm) of dendritic graphene islands grown on Au(111) ($V_{\mathrm{tip}}$ = 1 V, $I_{\mathrm{t}}$ = 10 pA). (b) Same as (a), with the graphene islands shaded (blue). (c) Schematic showing how gold atoms are displaced during graphene growth, etching the Au steps. The process roughens the Au(111) surface to form hills and valleys, with graphene islands located in the flat valleys. (d) Blowup of the white-boxed region of (b), with the image contrast adjusted to make the gold herringbone under the graphene visible. The preservation of the Au(111) herringbone structure of Au(111) highlights the weak graphene–Au interaction.

Figure 3

Atomic-resolution STM images and simulated structures of graphene on Au(111). (a) STM image and (b) simulated moiré of graphene rotated 1.5° relative to Au(111) (5.9 × 5.9 nm, $V_{\mathrm{tip}}$ = 0.1 V, $I_{\mathrm{t}}$ = 300 pA). The measured periodicity and corrugation of moiré are ∼17.3 and 0.1 Å, respectively. (c) STM image and (d) simulated moiré of graphene rotated 11° (6 × 6 nm, $V_{\mathrm{tip}}$ = −0.1 V, $I_{\mathrm{t}}$ = 300 pA). The measured periodicity and corrugation of moiré are ∼10.7 and 0.8 Å, respectively. (e) Graphene rotated ∼14° (2.7 × 2.7 nm, $V_{\mathrm{tip}}$ = −0.1 V, $I_{\mathrm{t}}$ = 500 pA). (f) Graphene rotated ∼26° (2.8 × 2.8 nm, $V_{\mathrm{tip}}$ = 0.4 V, $I_{\mathrm{t}}$ = 15 pA).

Figure 4

STM images of moiré modified by the Au herringbone. (a) Graphene $\left[11\overline{2}0\right]$ rotated 11° relative to Au $\left[1\overline{1}0\right]$ (45 × 45 nm, $V_{\mathrm{tip}}$ = 1 V, $I_{\mathrm{t}}$ = 10 pA). The corrugation of the moiré is ∼0.05 Å. (b) Graphene $\left[11\overline{2}0\right]$ aligned with Au $\left[1\overline{1}0\right]$ (50 × 50 nm, $V_{\mathrm{tip}}$ = 1 V, $I_{\mathrm{t}}$ = 10 pA). The corrugation of the moiré is ∼0.2 Å. The periodicity and direction of the moiré lattice change over the herringbone stripes, because the Au atoms change stacking there.

Figure 5

(a) Schematic of the surface atoms in the $23\times \sqrt{3}$ surface unit cell of the Au surface reconstruction. Underlying Au atoms are blue, hcp binding positions are red, and fcc sites are green. The surface atoms are shaded according to their energy, as in Ref. 28. (b) Comparison of the distance between surface atoms in the $\left[1\overline{1}0\right]$ direction calculated with the LDA of density functional theory (red solid line) and with the FK model (blue dashed line) described in the text. (c) Surface energy as a function of unit cell size for the FK model of the clean Au surface (black solid line), for an attractive interaction of 13 meV per C atom between Au and graphene (blue dot-dashed line), and for a surface in which the Au-Au bond distance has been reduced by 0.001$a$ (red dashed line).

Figure 6

(a) and (b) STM images of the same area showing how aggressive scanning reorders the Au(111) herringbone pattern (100 × 100 nm). (a) First scan ($V_{\mathrm{tip}}$ = 1 V, $I_{\mathrm{t}}$ = 10 pA). (b) The same area reexamined ($V_{\mathrm{tip}}$ = 1 V, $I_{\mathrm{t}}$ = 10 pA) after aggressive tunneling (up to $V_{\mathrm{tip}}$ = −0.1 V, tunneling current $I_{\mathrm{t}}$ = 1 nA). (c) and (d) STM images of graphene over two different monatomic Au steps (8 × 8 nm, $V_{\mathrm{tip}}$ = 0.1 V, $I_{\mathrm{t}}$ = 300 pA). To make the graphene lattice visible on both sides of the step, the images are a mixture of 90$\%$ differentiated topography in the horizontal direction and 10$\%$ topography. The fuzziness of the monatomic Au step edges likely results from their motion, possibly induced by scanning.