A non-dispersive Raman D-band activated by well-ordered interlayer interactions in rotationally stacked bi-layer Graphene

Raman measurements on monolayer graphene folded back upon itself as an ordered but skew-stacked bilayer (i.e. with interlayer rotation) presents new mechanism for Raman scattering in sp2 carbons that arises in systems that lack coherent AB interlayer stacking. Although the parent monolayer does not exhibit a D-band, the interior of the skewed bilayer produces a strong two-peak Raman feature near 1350 cm-1; one of these peaks is non-dispersive, unlike all previously observed D-band features in sp2 carbons. Within a double-resonant model of Raman scattering, these unusual features are consistent with a skewed bilayer coupling, wherein one layer imposes a weak but well-ordered perturbation on the other. The discrete Fourier structure of the rotated interlayer interaction potential explains the unusual non-dispersive peak near 1350 cm-1

Raman scattering from high frequency phonons in supported n-graphene layer films

Results of room temperature Raman scattering studies of ultrathin graphitic films supported on Si (111)/SiO2 substrates are reported. The results are significantly different from those known for graphite. Spectra were collected using 514 nm radiation on films containing from n=1 to 20 graphene layers, as determined by atomic force microscopy. Both the 1st and 2nd order Raman spectra show unique signatures of the number of layers in the film. The nGL film analog of the Raman G-band in graphite exhibits a Lorentzian lineshape whose center frequency shifts linearly relative to graphite as ~1/n (for n=1 G-band frequency ~1588 cm-1). Three weak bands, identified with disorder-induced 1st order scattering, are observed at ~ 1350, 1450 and 1500 cm-1. The 1500 cm-1 band is weak but relatively sharp and exhibits an interesting n-dependence. In general, the intensity of these D-bands decreases dramatically with increasing n. Three 2nd order bands are also observed (~2450, ~2700 and 3248 cm-1). They are analogs to those observed in graphite. However, the ~2700 cm-1 band exhibits an interesting and dramatic change of shape with n. Interestingly, for n<5 this 2nd order band is more intense than the G-band.Comment: 10 pages, 7 fig'