AVS 60th International Symposium and Exhibition
    Spectroscopic Ellipsometry Focus Topic Wednesday Sessions
       Session EL+AS+EM+SS+TF-WeA

Paper EL+AS+EM+SS+TF-WeA2
A Physical Model Dielectric Function for Graphene from the THz to the UV

Wednesday, October 30, 2013, 2:20 pm, Room 101 A

Session: Spectroscopic Ellipsometry: Perspectives and Novel Applications
Presenter: A. Boosalis, University of Nebraska-Lincoln
Authors: A. Boosalis, University of Nebraska-Lincoln
W. Li, National Institute of Standards and Technology (NIST)
R. Elmquist, National Institute of Standards and Technology (NIST)
M. Real, National Institute of Standards and Technology (NIST)
N.V. Nguyen, National Institute of Standards and Technology (NIST)
M. Schubert, University of Nebraska-Lincoln
R. Yakimova, Linköping University, Sweden
V. Darakchieva, Linköping University, Sweden
R.L. Myers-Ward, Naval Research Laboratory
C. Eddy, Naval Research Laboratory
D.K. Gaskill, Naval Research Laboratory
T. Hofmann, University of Nebraska-Lincoln
Correspondent: Click to Email
Graphene has been the focus of much recent research due to its unique electronic and optical properties, with potential for high performance electronics, tunable ultra-fast lasers, and transparent electrodes. Further development of graphene requires a complete understanding of graphene’s optical properties. Once thought to be trivially related to the lattice constant, it has become clear that graphene’s dielectric response contains distinct absorption features at ~4.5 and ~6 eV. However, the scientific community currently lacks consensus as to the origin of each feature [1,2].

In order to determine the physical origin of both absorption features, we have carried out spectroscopic ellipsometry measurements from 0.75 to 9 eV on graphene grown by CVD on Cu and by high-temperature Si sublimation from SiC. CVD grown graphene was transplanted to a fused silica substrate prior to measurement, while measurements conducted on SiC included 3C and 6H SiC polymorphs, before and after hydrogen intercalation.

Experimental data were analyzed with a biaxial model dielectric function which is dependent on the graphene joint density of states and modified by the Fano configuration interaction to account for exciton absorption [3]. Physical parameters include the electron next-neighbor hopping energy, the exciton resonant energy, the exciton absorption affinity, and the graphene optical thickness. All parameters are varied until the lowest mean squared error between model dielectric function and experimental spectra is achieved.

Our results show that the absorption ~4.5 eV is excitonic, while the absorption ~6 eV is an interband transition arising from the saddle point at the M position in the graphene band structure, a similar result to optical properties predicted by density functional theory [4]. The strain in the graphene lattice can be estimated from the next-neighbor hopping energy, and our results demonstrate relaxation in the graphene lattice after hydrogen intercalation of graphene on SiC. Epitaxial graphene on SiC also shows a higher affinity for exciton production and a lower exciton binding energy than graphene grown by CVD.

References:

[1] Mak et al., Phys. Rev. B. 106, 046401 (2011)

[2] Santoso et al., Phys. Rev. B. 84, 081403 (2011)

[3] Chae et al., Nano. Lett. 11, 1379 (2011)

[4] Yang et al., Phys. Rev. Lett. 103, 186802 (2009)