Scanning Tunneling Spectroscopy of Graphene on Graphite

We report low temperature high magnetic field scanning tunneling microscopy and spectroscopy of graphene flakes on graphite that exhibit the structural and electronic properties of graphene decoupled from the substrate. Pronounced peaks in the tunneling spectra develop with increasing field revealing a Landau level sequence that provides a direct way to identify graphene and to determine the degree of its coupling to the substrate. The Fermi velocity and quasiparticle lifetime, obtained from the positions and width of the peaks, provide access to the electron-phonon and electron-electron interactions.

Figure 1

Topography of the graphene layer isolated by extended defects on a graphite surface. (a) Large area topography. Two underlying defects are seen: a long ridge that runs diagonally under the top two layers and a fainter one under the first layer (dashed line). The long ridge separates a region with honeycomb structure (region $A$) above it, (e), from one with triangular structure (region $B$) below, (f). Two arrows mark positions where atomic-resolution images were taken. (b) High resolution image where the fainter ridge is visible. (c) Cross-sectional cut along line $\alpha \alpha$ in (a) showing that the separation between the top and second layer is larger than the equilibrium value (0.34 nm) near the fainter ridge. (d) Cross-sectional cut along line $\beta \beta$ showing that the height of an atomic step far from the ridges is comparable to that in Bernal-stacked graphite. (e),(f) Atomic-resolution image showing honeycomb structure in region $A$ (atoms visible at all 6 hexagon vertices) and triangular structure in $B$ (atoms seen only at 3 vertices corresponding to only one visible sublattice). A coherent honeycomb structure is seen over the entire region $A$. Set sample bias voltage and tunneling current were 300 mV and 9 pA for (a), 300 mV and 49 pA for (b), 200 mV and 22 pA for (f), 300 mV and 55 pA for (e).

Figure 2

Spectroscopic evidence for decoupled graphene layer. (a) Zero-field spectroscopy taken in regions $A$ and $B$ marked by squares in (b). Top panel shows that the tunneling conductance for decoupled graphene (region $A$) is $V$ shaped and vanishes at the DP (marked by the arrow). A typical spectrum for graphite is included for comparison. Bottom panel shows that in the more strongly coupled layer (region $B$) the differential conductance does not vanish at the DP. (b) Differential conductance ($dI/dV$) map at the DP energy as marked by arrows in (a). The scanned area is the same as in Fig. 1b. $dI/dV$ vanishes in the dark region but is finite in the bright region. (c) Field dependence of tunneling spectra in region $A$ showing a single sequence of Landau levels (LL). The peaks are labeled with LL index $n$. Each spectrum is shifted by $80\mathrm{pA}/\mathrm{V}$ for clarity. (d) LL energy showing square-root dependence on level index and field. The symbols correspond to the peaks in (c), and the solid line is a fit to Eq. (1). From the slope we obtain $v_F=0.79\times {10}^6\mathrm{m}/\mathrm{s}$ and from the intercept $E_D=16.6\mathrm{mV}$ indicating hole doping. The tunneling junctions parameters were set at a bias voltage of 300 mV and tunneling current of 20 pA for (a) and (c) and 53 pA for (b). The amplitude of the ac bias voltage modulation was 5 mV for all spectra in the figure.

Figure 3

Zero-field tunneling spectra, energy-momentum dispersion and Fermi velocity. (a) Tunnelling spectra and density of states (see text). Thick line is the DOS calculated using $v_F$ obtained in Fig. 2d. Note the consistency of spectra for tunneling resistance varying from $3.8\mathrm{G}\Omega$ to $50\mathrm{G}\Omega$. Legend shows tunneling junction settings. (b) Energy-momentum dispersion obtained as described in the text. Inset: Diagram of intervalley scattering mediated by $A_{1g}$ phonon. (c) Energy dependence of Fermi velocity.

Figure 4

LL sequence, quasiparticle lifetime and gap at the DP. (a) High resolution tunneling spectrum at 4 T showing the LL. The dashed line marks the DP obtained by fitting the nonzero LL sequence. The $n=0$ LL splits into two peaks, $0-$ and $0+$. (b) Zoom into LL sequence in hole sector. Solid squares are data points and the line represents a fit [29] with Lorentzian peaks. Inset, energy dependence of the peak width. (c) High resolution zoom into zero-field spectrum showing the appearance of a $\sim 10\mathrm{meV}$ gap at the DP and an anomaly at the Fermi energy. (d) (top) Field dependence of the $0-$ and $0+$ Landau levels. Solid symbols: data from (a); open symbols from Fig. 2c. (bottom) Field dependence of splitting. Experimental settings: [300 mV, ac 2 mV, 53 pA] in (a) and [200 mV, ac 1 mV, 53 pA] in (b).