Scanning tunneling microscopy of graphene on Ru(0001)

After prolonged annealing of a Ru(0001) sample in ultrahigh vacuum a superstructure with a periodicity of $\sim 30\mathrm{\AA }$ was observed by scanning tunneling microscopy (STM). Using x-ray photoelectron spectroscopy it was found that the surface is covered by graphitic carbon. Auger electron spectroscopy shows that between 1000 and $1400\mathrm{K}$ carbon segregates to the surface. STM images recorded after annealing to increasing temperatures display islands of the superstructure, until, after annealing to $T⩾1400\mathrm{K}$, it covers the entire surface. The morphology of the superstructure shows that it consists of a single graphene layer. Atomically resolved STM images and low-energy electron diffraction reveal an $(11\times 11)$ structure or incommensurate structure close to this periodicity superimposed by $12\times 12$ graphene cells. The lattice mismatch causes a moiré pattern. Unlike the common orientational disorder of adsorbed graphene, the graphene layer on Ru(0001) shows a single phase and very good rotational alignment. Misorientations near defects in the overlayer only amount to $\sim 1°$, and the periodicity of $\sim 30\mathrm{\AA }$ is unaffected. In contrast to bulk graphite both carbon atoms in the graphene unit cell were resolved by STM, with varying contrast depending on the position above the Ru atoms. The filled and empty state images of the moiré structure differ massively, and electronic states at $-0.4$ and $+0.2\mathrm{V}$ were detected by scanning tunneling spectroscopy. The data indicate a significantly stronger chemical interaction between graphene and the metal surface than between neighboring layers in bulk graphite. The uniformity of the structure and its stability at high temperatures and in air suggest an application as template for nanostructures.

Figure 1

STM image of Ru(0001) after annealing for $90\mathrm{s}\text{to}1400\mathrm{K}$, taken at room temperature. The quasihexagonal lattice is a graphene overlayer with a lattice constant of about $30\mathrm{\AA }$. $1000\mathrm{\AA }\times 1000\mathrm{\AA }$, $I_t=3\mathrm{nA}$, $V_{\text{sample}}=-0.85\mathrm{V}$.

Figure 2

(a) XPS of clean Ru(0001) and (b) of Ru(0001) after $90\mathrm{s}$ annealing to $1400\mathrm{K}$, showing the binding energy region of the Ru $3d_{3∕2}$ and Ru $3d_{5∕2}$ states. The fit of spectrum (b) with the parameters of the clean Ru spectrum (a) (branching ratio, energies and widths of the Ru 3d doublet peaks, and background kept fixed) reveals two states under the left peak, the Ru $3d_{3∕2}$ state at $284.1\mathrm{eV}$ and an additional component at $284.8\mathrm{eV}$ caused by the C $1s$ state of graphitic carbon.

Figure 3

Determination of the carbon coverage by AES. (a) $dN∕dE$ AES of clean Ru (Ru $MNN$ transition at $273\mathrm{eV}$) and (b) of the graphene covered surface after annealing. The enhanced asymmetry of the $273\mathrm{eV}$ peak is caused by the coinciding, strongly asymmetric C $KLL$ transition). (c) Carbon coverage, determined from the ratio of the negative and positive going parts of the $273\mathrm{eV}$ peak in the $dN∕dE$ spectra. The increase between 800 and $1400\mathrm{K}$ shows the segregation of carbon.

Figure 4

STM images recorded after annealing at increasing temperatures. (a) After $120\mathrm{s}$ at $1000\mathrm{K}$ graphene has formed islands at the steps. $1000\mathrm{\AA }\times 1000\mathrm{\AA }$, $I_t=1\mathrm{nA}$, $V_{\text{sample}}=-0.5\mathrm{V}$. (b) After $90\mathrm{s}$ at $1470\mathrm{K}$ the surface is fully covered by graphene. $500\mathrm{\AA }\times 500\mathrm{\AA }$, $I_t=1\mathrm{nA}$, $V_{\text{sample}}=-0.2\mathrm{V}$. (c) Height profile along the line displayed in (b). The steps are $\sim 2.1\mathrm{\AA }$ high and thus represent steps of the Ru(0001) substrate. (b) shows that the Ru step edges are aligned along the main directions of the overlayer, indicating a restructuring of the underlying Ru surface.

Figure 5

(Color online) Atomically resolved images of the graphene overlayer. (a) The image shows three different levels of apparent heights, namely four bright maxima, a dark minimum in the center between the three maxima on the right hand side, and a less dark minimum between the three maxima on the left hand side. $50\mathrm{\AA }\times 40\mathrm{\AA }$, $I_t=1\mathrm{nA}$, $V_{\text{sample}}=-0.05\mathrm{V}$. (b) Section of (a) superimposed by the honeycomb lattice of graphene. Details from the three marked areas are enlarged. The two carbon atoms in the graphene unit cell, A and B, are displayed as dots and crosses, respectively. At the moiré maximum both atoms appear bright, so that the complete graphene hexagons are visible. In the darker half-cell the B atoms appear bright, in the less dark half-cell the A atoms. (c) STM image of a slightly rotated graphene layer. Using $\beta \cong \frac{a_{\mathrm{Ru}}-a_{\mathrm{C}}}{a_{\mathrm{Ru}}}\alpha$ the measured angle $\alpha =\sim 10°$ between the moiré pattern and the graphene lattice can be transformed into the rotational angle $\beta =1°$ between the ruthenium and the graphene lattice ($a_{\mathrm{Ru}}$ and $a_{\mathrm{C}}$ are the lattice constants of the Ru(0001) surface and of graphene). (Ref. 23) $40\mathrm{\AA }\times 80\mathrm{\AA }$, $I_t=3\mathrm{nA}$, $V_{\text{sample}}=-0.05\mathrm{V}$.

Figure 6

LEED pattern of Ru(0001) after annealing to $1770\mathrm{K}$. The bright substrate spots are surrounded by satellites caused by the graphene overlayer. Beam voltage $86\mathrm{eV}$.

Figure 7

Model of the overlayer structure of graphene on the Ru(0001) surface. The first layer Ru atoms are the light gray spheres, the second layer Ru atoms dark gray, and the graphene layer is the honeycomb net. The $\left[10\overline{1}0\right]$ direction of graphene is aligned with the $\left[10\overline{1}0\right]$ direction of ruthenium. The figure shows a commensurate $(11\times 11)$ structure (with $12\times 12$ graphene unit cells superimposed), but an incommensurate structure with very close periodicity is also possible.

Figure 8

STM images taken on the same area at varying bias voltages at $55\mathrm{K}$. The sign refers to the sample, the tunneling current is the same in all images $(I_t=1\mathrm{nA})$. The appearance of the moiré changes from a hexagonal pattern of maxima for the filled states to bright rings for the empty states. The truncated appearance of the rings at intermediate voltages is probably an effect of an asymmetric tip and was not present in other measurements. $100\mathrm{\AA }\times 100\mathrm{\AA }$.

Figure 9

STS measurements at the edge of a graphene island and on the surrounding metal surface. (a) Topographic image with superimposed grid where the spectra were taken. $I_t=1\mathrm{nA}$, $V_{\text{offset}}=-0.4\mathrm{V}$, $100\mathrm{\AA }\times 100\mathrm{\AA }$. (b) Averaged $dI∕dV$ spectra on the graphene area and on the metal. The graphene spectrum shows two peaks at $-0.4\mathrm{V}$ and $+0.2\mathrm{V}$.