Observation of Low Energy Raman Modes in Twisted Bilayer Graphene

Abstract

Two new Raman modes below 100 cm(-1) are observed in twisted bilayer graphene grown by chemical vapor deposition. The two modes are observed in a small range of twisting angle at which the intensity of the G Raman peak is strongly enhanced, indicating that these low energy modes and the G Raman mode share the same resonance enhancement mechanism, as a function of twisting angle. The ~94 cm(-1) mode (measured with a 532 nm laser excitation) is assigned to the fundamental layer breathing vibration (ZO' mode) mediated by the twisted bilayer graphene lattice, which lacks long-range translational symmetry. The dependence of this mode's frequency and line width on the rotational angle can be explained by the double resonance Raman process that is different from the previously identified Raman processes activated by twisted bilayer graphene superlattice. The dependence also reveals the strong impact of electronic-band overlaps of the two graphene layers. Another new mode at ~52 cm(-1), not observed previously in the bilayer graphene system, is tentatively attributed to a torsion mode in which the bottom and top graphene layers rotate out-of-phase in the plane.

Specific heat of twisted bilayer graphene: Engineering phonons by atomic plane rotations

“…The energies of these phonons depend on the twist angle. The rotation angle -dependent Raman peaks, associated with hybrid folded phonons have been observed experimentally [28][29][30][32][33][34][35][36][37], confirming the theoretical predictions.…”

“…Recently the attention has been shifted to twisted few-layer graphene (T-FLG), which demonstrates intriguing electron [24][25][26][27] and phonon properties [28][29][30][31][32][33][34][35][36]. Several independent experimental Raman studies of twisted graphene found series of phonon peaks in different energy regions, which are absent in FLG without the twist [28][29][30][32][33][34][35][36][37].…”

Raman modes in transferred bilayer CVD graphene

“…Among other characterization techniques such as scanning tunneling microscopy (STM), Landau level (LL) spectroscopy [6], high-resolution transmission electron microscopy (HRTEM) [10], optical microscopy and infrared spectroscopy, Raman spectroscopy has also been widely used in the studies of tBLG [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. This is because Raman spectroscopy is a very sensitive and versatile tool to probe the electron-phonon and phononphonon coupling in graphene samples [11].…”

“…In addition to the resonant G and 2D bands, the spectra also contain weak Raman peaks due to in-plane combination modes and overtones (LOTA, TOLA, LOLA, TOTA, and 2LO) [15][16][17][18]. The tBLG contains, besides the aforementioned spectral bands, out-of-plane modes such as breathing (ZO ′ and ZO [19][20][21]), rotation (R and R ′ [22][23][24][25][26]), and shear (C [27]) modes. Although these modes generally produce only weak Raman peaks compared with the G and 2D bands, they carry important information for characterizing the perfection, stacking order, and carrier transport in graphene samples.…”

“…In contrast, CVD graphene contains primarily tBLG, where the second graphene layer is rotated with respect to the rst one. In tBLG, the low-energy electronic band structure can be represented by two Dirac cones separated by a wavevector that depends on the rotational angle [10,13,19]. Band-structure modi cations and van Hove singularities in the energy dependence of the DOS (density of states) are induced when the Dirac cones of the top and bottom graphene layers are overlapping [6,10].…”