Direct observation of room temperature high-energy resonant excitonic effects in graphene

TL;DR

This study observes a high-energy resonant exciton at room temperature in epitaxial graphene with many layers, revealing poor screening and quantum electron behavior, and compares it to graphite's electronic structure.

Contribution

First direct observation of a high-energy resonant exciton at room temperature in multilayer graphene, highlighting differences from graphite and elucidating electron interactions.

Findings

Resonant exciton at 6.3 eV observed at room temperature in graphene.

Optical conductivity scales linearly with the number of graphene layers.

Crossover in electronic structure from graphene to graphite occurs at around 28 layers.

Abstract

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3eV 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 resonant exciton scales linearly with number of graphene layer (up to \emph{at least} 8 layers) implying quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4eV, 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 $N \leq$ 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.