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 (100)/SiO₂ substrates are reported. The results are significantly different from those known for graphite. Spectra were collected using 514.5 nm radiation on films containing from n = 1 to 20 graphene layers, as determined by atomic force microscopy. Both the first- and second-order Raman spectra show unique signatures of the number of layers in the film. The nGL film analogue of the Raman G-band in graphite exhibits a Lorentzian line shape whose center frequency shifts linearly relative to graphite as ∼1/n (for n = 1 ω_G ≈ 1587 cm^-1). Three weak bands, identified with disorder-induced first-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 second-order bands are also observed (∼2450, ∼2700, and 3248 cm^-1). They are analogues 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 second-order band is more intense than the G-band.