Raman spectroscopy of twisted bilayer graphene grown by CVD
Twisted bilayer graphene (tBLG) is of interest and potential for novel applications. The physical properties of tBLG largely depend on the relative twist (rotation) angle and interlayer coupling of successive graphene layers. TBLG is generally characterized by two Dirac cones displaced by a wavevector in momentum space and band crossings at high energies. The interlayer coupling near band crossings is said to be enhanced due to the presence of van Hove singularities (VHS) in the electronic density of states (DOS). To study them, we employed Raman spectroscopy and resonantly excited the VHS in CVD-grown tBLG grains (at a critical twist angle of ~12°) using a 532-nm laser source under ambient conditions.
Observation of low frequency interlayer Raman modes
We discovered two new Raman modes below 100 cm^-1 (the fundamental layer breathing mode ZO’_L and the torsion mode X, which are not found in the Bernal-stacked bilayer) shown in the right figure, revealing the unique interlayer coupling of tBLG and the same resonance enhancement origin as observed in the G Raman mode. The inset is a schematic of the layer breathing (ZO’) vibration in tBLG.
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R. He*, T. F. Chung* et al., Nano Lett. 13, 3594 (2013)
Electric control of electron-phonon coupling
We found a striking splitting of the G Raman mode and strong modulation of the Raman scattering intensities using an electrolyte gate (presented in the left figure), enabling us to perform the first measurement of electron-phonon coupling parameter and to demonstrate electro-optic control in tBLG. The inset schematically shows the effect of gate voltage (V_TG) on the tBLG energy band. Vertically displaced Dirac cones suppress the interband direct transitions (green dashed arrow) between the saddle points (VHS), resulting in a significant decrease in the resonance G Raman modes.
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T. F. Chung et al., Nano Lett. 15, 1203 (2015)