Ultrapure multilayer graphene in bromine-intercalated graphite

We investigate the optical properties of bromine-intercalated highly orientated pyrolytic graphite (Br-HOPG) and provide an interpretation of the data. We observe absorption features below 620 meV which are absent in the absorption spectrum of graphite. Comparing our results with those of theoretical studies on graphite, single- and bilayer graphene, as well as recent optical studies of multilayer graphene, we conclude that Br-HOPG contains the signatures of ultrapure bilayer graphene, ultrapure single-layer graphene, and graphite. The observed supermetallic conductivity of Br-HOPG is identified with the presence of very high mobility ($\simeq$121 000 cm${}^2$V${}^{-1}$s${}^{-1}$ at room temperature and at very high carrier density), multilayer graphene components in our sample. This could provide an avenue for single- and multilayer graphene research.

Figure 1

(a) Room temperature conductivity spectra of bare HOPG and three Br-HOPGs with different intercalation times. We indicate the five differences between bare HOPG and Br-HOPG with vertical arrows (see in the text). (b) We show a schematic diagram of Br${}_2$ intercalated graphite: (upper) a top (in-plane) view and (lower) a side view (along the $c$ axis) of the $A$-$A$ line cross section. This diagram is based on results by Erbil et al.[13]

Figure 2

(a) Room temperature optical conductivity normalized at 620 meV, as described in Ref. 18. This normalization allow the extraction of an effective $c$-axis lattice constant, shown in the inset (see in the text). (b) Simulation of our data using our model calculation. (See the text for a detailed description.) In the inset, we show a schematic diagram of a band structure of bilayer graphene with an anisotropy gap, $\Delta \cong 0.44\gamma$ (not to scale). Here, the gap is caused by a built-in potential difference between the upper and lower layers. The red arrow shows the intraband Drude conductivity, the blue the interband transitions, the orange the $\gamma$ transition, and the dark green the onset of the interband transition. The gray dotted arrow shows the transition between the minimum gap, $E_{g1}$. We note that this transition would not appear at a low temperature or in a highly doped system.

Figure 3

(a) Numerical calculations based on Eqs. (1) and (2) for an equally weighted sum of single- and bilayer graphene for several chemical potentials. We include the temperature broadening in the calculation. The orange horizontal arrow shows the degenerate $\gamma$ transition, the dark green arrow the onset of the interband transition for $\mu =-1.0\gamma$. (b) We show schematic diagrams of band structures of both single- and bilayer graphene. The red arrow shows the intraband Drude transition, the blue the interband transitions, the dark green the onset of these transitions, and the orange the degenerate $\gamma$ transition.