| AVS 60th International Symposium and Exhibition | |
| Graphene and Other 2D Materials Focus Topic | Friday Sessions |
| Session GR+EM+MS+NS+SP-FrM |
| Session: | 2D Materials: Device Physics & Applications |
| Presenter: | W. Li, National Institute of Standards and Technology (NIST) |
| Authors: | W. Li, National Institute of Standards and Technology (NIST) G. Cheng, National Institute of Standards and Technology (NIST) Y. Liang, Peking Univ., China K. Xu, NIST A. Boosalis, Univ. of Nebraska-Lincoln P.D. Ye, Purdue Univ. A.R. Hight Walker, NIST X. Liang, Peking Univ., China T. Hofmann, Univ. of Nebraska-Lincoln C.A. Richter, NIST D.J. Gundlach, NIST N.V. Nguyen, NIST |
| Correspondent: | Click to Email |
Graphene and MoS2 are among the most promising candidates for next generation of electronic and photonic devices. Accurate optical properties provide key information necessary for electronic and photonic device design. Here, we report the results of a broad band optical measurement of the dielectric function by spectroscopic ellipsometry (SE) of graphene and MoS2 grown by chemical vapor deposition (CVD). With the extended spectral range, we are able to observe new higher energy interband absorptions in MoS2 and a red shift of graphene excitons.
Monolayer graphene grown on a copper foil was transferred onto a fused silica substrate by solvent method; two and three graphene layers were formed by the sequential transfer of each monolayer. MoS2 was directly grown on sapphire. Raman spectroscopy was performed on each sample. Both the exciton peak (around 4.8 eV) and another absorption peak (around 6.3 eV) were observed from the absorption spectra of graphene. In the IR range, both refractive index (n) and k increase with longer wavelength which is consistent with the reported results.(a) It is notable that n increases whereas k decreases as the number of graphene layers increases. This is most likely due to the relatively weak interaction between transferred graphene monolayers. More importantly, we observe a red shift of the exciton peak for two layer graphene (0.04 eV shift) and three layer graphene (0.07 eV shift) relative to that of monolayer graphene, which we attribute to the interlayer screening effects. The theory predicts(b) the transmittance of monolayer graphene to depend solely on the universal fine structure constant a = e2/ħc, which is related to the graphene’s opacity. We find opacity to linearly increase for each of added layer. For example, at 550 nm the transmittance of one, two, and three layers of graphene are 96.9%, 93.6%, and 90.3%, respectively. Raman studies on MoS2 confirm the presence of 3 layers, consistent with the three layer model derived from ellipsometry. The MoS2 dielectric function exhibits the well-known A (1.86 eV) and B (2.02 eV) strong excitons which arise from the direct d-d transitions separated by a spin-orbit splitting. A series of higher energy interband transitions are clearly seen at 2.8, 3.1, 4.7 eV of which the 2.8 eV peak is the strongest absorption peak ever reported for MoS2.
(a) F. J. Nelson, et al., Appl. Phys. Lett. 97, 253110 (2010)
(b) A. B. Kuzmenko, E. van Heumen, F. Carbone, and D. van der Marel, PRL 100, 117401 (2008); R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres,2 A. K. Geim, Science320, 1308 (2008).