Fine tuning of optical transition energy of twisted bilayer graphene via interlayer distance modulation

Twisted bilayer graphene (tBLG) represents a family of unique materials with optoelectronic properties tuned by the rotation angle between the two layers. The presented work shows an additional way of tweaking the electronic structure of tBLG by modifying the interlayer distance, for example by a small uniaxial out-of-plane compression. We have focused on the optical transition energy, which shows a clear dependence on the interlayer distance, both experimentally and theoretically.

Figure 1

Raman spectra of the ${}^{13}\mathrm{C}/{}^{12}\mathrm{C}$ tBLG at ambient conditions, with and without G band enhancement at the laser excitation wavelength of 488.0 nm. The spectra are normalized to the 2D band amplitude.

Figure 2

Evolution of Raman spectra of ${}^{13}\mathrm{C}/{}^{12}\mathrm{C}$ tBLG (a) with compression up to 1.6 GPa, and (b) with stress cycling of approximately ±0.4 GPa. The laser excitation wavelength is 488.0 nm. The spectra are normalized to the 2D band amplitude in each plot.

Figure 3

Evolution of the G/2D amplitude ratio with increasing stress: (a) Labeled tBLG with rotation angles of 13.0°, 12.3°, 11.9°, and 9.8° excited with 488.0, 514.5, 532.0, and 647.1 nm laser wavelengths, respectively, to achieve G band enhancement in each sample [12]. (b) ${}^{12}\mathrm{C}$ CVD BLG (squares) and ${}^{12}\mathrm{C}$/${}^{12}\mathrm{C}$ stack BLG (circles); both measured in a region resonant with 488.0 nm excitation wavelength. Dotted lines are guides for the eye.

Figure 4

Evolution of Raman spectra of ${}^{13}\mathrm{C}/{}^{12}\mathrm{C}$ tBLG with compression up to 0.6 GPa. (a) The same tBLG grain is measured with 488.0 (blue spectra) nm and 514.5 nm excitation wavelengths (green spectra). Each spectrum is normalized to the 2D band amplitude. (b) G/2D amplitude ratios from (a) as a function of increasing stress.

Figure 5

(a) Density of states of (3,2) tBLG at different interlayer distances. (b) Optical transition energy as a function of the interlayer distance for (3,2) and (1,5) tBLG, labeled with squares and circles, respectively. Highlighted points in the (3,2) tBLG curve in (b) correspond to the curves in (a) with the same color.

Figure 6

The density of states of the (3,2) tBLG, with 0.35 nm interlayer distance, as a function of the $a$ lattice parameter.