Metal to Insulator Transition in Epitaxial Graphene Induced by Molecular Doping

The capability to control the type and amount of charge carriers in a material and, in the extreme case, the transition from metal to insulator, is one of the key challenges of modern electronics. By employing angle-resolved photoemission spectroscopy we find that a reversible metal to insulator transition and a fine-tuning of the charge carriers from electrons to holes can be achieved in epitaxial bilayer and single layer graphene by molecular doping. The effects of electron screening and disorder are also discussed. These results demonstrate that epitaxial graphene is suitable for electronics applications, as well as provide new opportunities for studying the hole doping regime of the Dirac cone in graphene.

Figure 1

Dispersions through the $K$ point for the as-grown single layer graphene (a) and for the sample with the highest ${\mathrm{NO}}_2$ doping (c). Panels (b) and (d) show the angle integrated spectra for (a) and (c), respectively. In panels (c) and (d), note the appearance of the ${\mathrm{NO}}_2$ states at $\approx 5$ and 11 eV below $E_F$.

Figure 2

(a) Dispersions of as-grown bilayer graphene for a cut through the $K$ point (see red line in the inset). (b) Dispersions taken after 0.6 Langmuir ${\mathrm{NO}}_2$ adsorption. (c) Data taken after ${\mathrm{NO}}_2$ desorption by synchrotron beam. (d) EDCs at $k_1$, $k_2$, $k_3$, and $k_4$ as labeled on the top of panels (a) and (b). The red dots are the raw data points, and the red lines are the smooth data as a guide for the eye. The black and red arrows point to the midpoint of the leading edge, which is shifted to higher binding energy after ${\mathrm{NO}}_2$ doping. (e) Symmetrized EDC with respect to $E_F$. The absence of a peak at $E_F$ in the red curves shows the insulating property of the sample in panel (b). (f) Zoom-in of data shown in panel (b). The white line is a guide to the eye for the dispersions. (g) Momentum distribution curve (MDC) at the energy labeled by the dotted black line in panel (f). The dots are the raw data and the solid line is the fit using three Lorentzian peaks simulating the cross section of the hatlike dispersion in panel (e).

Figure 3

(a)–(f) Dispersions for single layer graphene through the $K$ point, for the as-grown sample (a) and for various dopings after ${\mathrm{NO}}_2$ adsorption (b)–(f). The white lines are the dispersions extracted from fitting the MDC peaks. When the MDC peaks cannot be clearly resolved, the extrapolated dispersion is shown [see dotted lines in panels (c)–(d)]. (g) EDCs taken in the momentum regions (indicated by a small tick mark on top of each panel) where the bands are closest to $E_F$. The symbols are the raw data, and the solid lines are guides for the eye. (h) Symmetrized EDCs for data shown in panel (g) with respect to $E_F$.

Figure 4

(a), (b) Plot of $E_D$ and Fermi velocity as a function of the carrier concentration for data taken on single layer epitaxial graphene. In panel (b), the data are extracted by fitting the dispersion between $E_F$ and $-0.3\mathrm{eV}$ (circles) and between $E_F$ and $-0.1\mathrm{eV}$ (diamonds). The open symbols are extracted from the dispersions on the left and the filled symbols from the dispersions on the right.