Electron–phonon coupling and atomic reconstruction in 2D twisted bilayer materials. (a) and (b) Schematics of interlayer electron–phonon process where a phonon with momentum ℏ q 1 connects the states k and k ′ of different layer (a) and intralayer electron–phonon process where both states k and k ′ are from the same layer. (c) and (d) Raman spectra of twisted bilayer graphene with θ = 6 ° and 13° measured with the 2.18 eV and 2.41 eV laser lines, respectively. The vertical coordinate corresponds to the ratio of the peak intensities of the Raman spectra in twisted bilayer graphene and monolayer graphene. (c) The peak around 1620 cm⁻¹ is called L_a and stems from intralayer electron–phonon scattering process and (d) the peak at 1480 cm⁻¹ is called T_e and stems from the interlayer electron–phonon scattering process. (e) and (f) Excitation Raman maps of twisted bilayer graphene with θ = 6 ° (e) and 13° (f) measured under different laser energy excitations. Reprinted with permission from Ref. [54]. Copyright 2019, Nature Publishing Group. (g) TEM dark-field images of twisted bilayer graphene with different twist angles obtained by selecting diffraction peak ( g = 10 1 ¯ 0 ) . Different contrast stands for different stacking order (AB/BA). Reprinted with permission from Ref. [45]. Copyright 2019, Nature Publishing Group.