Fig. 1: Optical and Raman characteristics of CVD-grown tBLG.
From: Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene
a Microscope image of CVD-grown graphene transferred onto a 300 nm SiO2/Si substrate. MLG covers most of the silica area with locally distributed star-shaped tBLG centred at the seeds (the darkest spots in the image). b Enlarged microscope image of the tBLG indicated by the black downward arrow in (a). Colour circles represent individual tBLG regions according to the twist angles. The black-circled region represents MLG. c Raman spectra of tBLG marked with the same colour as the circles in (b). Inset: Raman spectrum showing the full G- and 2D-bands in the green-circled tBLG in (b), where Raman G-resonance occurs from the interaction with the 532 nm excitation laser, indicating that the twist angle should be 12 ± 1° (refs. 29,33) (the critical angle, θc, regime). Various Raman signals measured in 63 tBLG regions (d 2D/G peak ratio, and e 2D-band area normalized by that of MLG) were used to classify the twist angle of tBLG. For clarity, tBLG regions with θ < θc, θ ∼ θc, and θ > θc are represented by orange-, green- and purple-coloured sections, respectively. f Linear absorption contrast spectra in tBLG on a 100 nm SiO2/Si substrate
