Temperature-Activated Layer-Breathing Vibrations in Few-Layer Graphene

Abstract

ABSTRACT:We investigated the low-frequency Raman spectra of freestanding few-layer graphene (FLG) at varying temperatures (400 -900 K) controlled by laser heating. At high temperature, we observed the fundamental Raman mode for the lowest-frequency branch of rigid-plane layer-breathing mode (LBM) vibration. The mode frequency redshifts dramatically from 81 cm -1 for bilayer to 23 cm -1 for 8-layer. The thickness dependence is well described by a simple model of coupled oscillators. Notably, the LBM Raman response is unobservable at room temperature, and it is turned on at higher temperature (>600 K) with a steep increase of Raman intensity. The observation suggests that the LBM vibration is strongly suppressed by molecules adsorbed on the graphene surface, but is activated as desorption occurs at high temperature.

Resonant Raman spectroscopy of twisted multilayer graphene

“…Its lower frequency (28.8 cm À 1 ) than that of the BLG C 21 and 4LG C 42 suggests a softening of the coupling between layers next to the interface. C 21 þ has a higher frequency than C 21 À because the weakly coupled middle layers vibrate out-of-phase in this case. If this coupling were as strong as in the Bernal-stacked case, Pos(C 21 þ ) would be much higher than Pos(C 21 À ), close to Pos(C 41 ) in 4LG.…”

“…C 21 þ has a higher frequency than C 21 À because the weakly coupled middle layers vibrate out-of-phase in this case. If this coupling were as strong as in the Bernal-stacked case, Pos(C 21 þ ) would be much higher than Pos(C 21 À ), close to Pos(C 41 ) in 4LG. The frequency difference between C 21 À and C 21 þ is referred to as Davydov splitting 35 , which is the frequency difference of two conjugate vibration modes.…”

“…This is 1.2 cm À 1 bigger than that (B1 cm À 1 ) measured in Bernalstacked BLG. In fact, this band can be fitted using two Lorentzians at B28.9 cm À 1 and 30.6 cm À 1 , denoted as C 21 À and C 21 þ , respectively. In t(2 þ 3)LG, besides the two C modes of its 3LG constituent, an additional mode at 30.8 cm À 1 related to its 2LG constituent is present, with a small FWHM B1 cm À 1 .…”

“…Thus, Pos(C 42 )B31 cm À 1 is equal to that of the C mode in 2LG (the latter, however, corresponding to a Raman active mode) 18 . In t(2 þ 2)LG, the displacements of the middle layers are in-phase for C 21 À . Its lower frequency (28.8 cm À 1 ) than that of the BLG C 21 and 4LG C 42 suggests a softening of the coupling between layers next to the interface.…”

“…The Raman spectrum of graphite and MLG consists of two fundamentally different sets of peaks. Those, such as D, G and 2D, present also in SLG, and due to in-plane vibrations 14,16,17 , and others, such as the shear (C) modes 18 and the layer breathing modes [19][20][21] , due to the relative motions of the planes themselves, either perpendicular or parallel to their normal. The G peak corresponds to the high-frequency E 2g phonon at G. The D peak is due to the breathing modes of six-atom rings and requires a defect for its activation 16,[22][23][24] .…”