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 similar to 1/n (for n = 1 omega(G) approximate to 1587 cm(-1)). Three weak bands, identified with disorder-induced first-order scattering, are observed at similar to 1350, 1450, and 1500 cm(-1). The similar to 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 (similar to 2450, similar to 2700, and 3248 cm(-1)). They are analogues to those observed in graphite. However, the similar to 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.