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

Abstract

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 ≈ 75, thus suggesting a poor screening in graphene. The optical conductivity (σ 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 π to π * at the M point and a π 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 σ 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. Graphene, a one-layer-thick carbon honeycomb structure, has recently attracted a lot of attention due its exotic quantum properties.1 In addition to the well-known electronic properties, such as ballistic electron transport, 1-3 quantum Hall effect, 4 tunable band gap, 5,6 and physics driven by many-body interactions, 7 graphene also displays very interesting optical properties. For instance, due to its low-energy excitation graphene becomes highly transparent in the visible spectral range and its infrared optical conductivity can be tuned using a gate voltage.8 These particular properties of graphene potentially result in exciting optoelectronic applications.Recent theoretical studies based on the ab initio GW and Bethe-Salpeter equation (BSE) approach by Yang et al. 9 and Trevisanutto et al. 10 have predicted the existence and have highlighted the importance of resonant excitonic effects in the optical absorption of graphene. However, there is disagreement as to the origin and position of the exciton. In Ref. 9 the calculations were done up to 7 eV and the resonant exciton was predicted to occur at 4.6 eV due to electron-hole interaction in the π * and π bands at the M point. In Ref. 10, however, the calculations were done at a much higher energy of 22 eV and the resonant exciton was predicted to appear at 8.3 eV due the background single-particle continuum of dipole forbidden transition at the the point. Despite their disagreement, both have agreed that resonant excitons play important role in elementary excitations in graphene and thus its understanding is crucial.A direct way to probe a resonant exciton in graphene is to measure its complex dielectric response in wide energy range.10 Despite reports on the optical properties of graphene over the visible energy r...

Many‐body effects in optical response of graphene‐based structures

“…This is consistent with a resonant state observed at $6.3 eV in the optical spectrum of the epitaxial graphene monolayer grown on SiC. [35] The DFT-LDA calculations confirmed a large change in the band structure of graphene with its coupling with a Ru slab. [46] Interestingly, that the p plasmon completely disappeared in a graphene-Ni interface, as it was detected experimentally and confirmed by calculations of the EEL spectra.…”

“…The contradiction could be resolved experimentally, and we find only a single paper on the measurement of optical conductivity of graphene at high energies. [35] A room-temperature stable resonant exciton at $6.3 eV was observed in the spectrum of epitaxial monolayer graphene grown on SiC. The downshift of the exciton as compared to the value of $8.3 eV predicted in Ref.…”

“…Continuing discussion about the nature of graphene resonance, we would like to draw an attention to ellipsometric spectra of substrate-transferred graphene samples, which had detected a peak around $6.2 eV [36] being very close to the resonant state reported in Ref. [35]. However, this peak had disappeared after cleaning of graphene, thus evidencing that it is not related to the intrinsic excitations in graphene.…”

Tunable optical absorption and interactions in graphene via oxygen plasma

“…In the absence of disorder, the optical conductivity of graphene -carbon atoms arranged in a two dimensional hexagonal lattice -displays many remarkable optical properties, including a broadband universal optical conductivity ( 0 = 2 /2ℎ) in the infrared-to-visible range [1][2][3][4].…”

Optical conductivity of disordered graphene beyond the Dirac cone approximation

“…11 Furthermore, the flattening of the π bands at energies away from the Dirac point is responsible for the strong peak in the spectrum at higher energies (of the order of 5 eV) which is associated with optical transitions between states of the Van Hove singularities. 8,12,13 Finally, a method to control the intermediate excited states in inelastic light scattering experiments has also been reported, revealing the important role of quantum interference in Raman scattering. 14 This intense experimental work has been accompanied by a series of theoretical studies which have treated the problem of the optical conductivity at different levels of approximation.…”