Observation of room-temperature high-energy resonant excitonic effects in graphene

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3 eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers $N\approx 75$, thus suggesting a poor screening in graphene. The optical conductivity (${\sigma }_1$) of a resonant exciton scales linearly with the number of graphene layers (up to at least 8 layers), implying the quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4 eV, which is a mixture of interband transitions from $\pi$ to ${\pi }^{*}$ at the M point and a $\pi$ plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for $N⩽$ 28 the ${\sigma }_1$ is dominated by the resonant exciton, while for $N>$ 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite.

Figure 1

Room-temperature experimental results of (a) reflectivity and (b) loss function, Im (${\varepsilon }^{-1}$). The inset of (a) shows the experimental geometry while the insets of (b) shows (${\varepsilon }^{-1}$) on an expanded scale from 4.5 to 6.5 eV and from 5 to 10 eV.

Figure 2

The optical conductivity (${\sigma }_1$) of (a) substrate and graphene ($N=1,2,8$) and (b) graphene ($N=22,75$) and graphite shows three peaks at 5.4 eV (label A), 6.3 eV (label B), and 14.1 eV (label C). Inset 1 shows the comparison of ${\sigma }_{1,1}$ between experimental data and theoretical calculations. Insets 2 and 3 show the ${\sigma }_1$ on an expanded scale at various energy ranges. (c) The value of ${\sigma }_1$ at A, B, and C as a function of $N$. (d) The partial spectral weight ($W$) as a function of $N$.

Figure 3

(a) Comparison between experimental data and calculated ${\sigma }_1$. (Top inset) ${\sigma }_1$ spectra of graphite. (Bottom inset) ${\sigma }_1$ spectra of the substrate. (b) The background single-particle continuum transitions ($\Delta$) at the $\Gamma$ point as a function of nominal charge transfer ($n_e$) from substrate to graphene for various graphene layer ($N$). The red arrow shows the observed charge transfer in our sample. (c) A proposed model of optical absorption of exciton in graphene and graphene on substrate as function of $N$. RE stands for the resonant excitonand (${\Delta }_{\mathrm{exc}}$) is excitation energy.