Figure 2: (a) Raman spectra of a carbon nanotube sample, excited, from top to bottom, at 785 nm (1200 g/mm grating), 638 nm (1800 g/mm grating), and 532 nm (2400 g/mm grating). The spectra were acquired on ExploRA with the OD = 1 neutral density filter. (b) The low frequency region of the carbon nanotube spectra showing the RBM bands whose observed frequencies are excitation wavelength dependent.

The full Raman spectra of a sample mixture of carbon nanotubes, excited at 785, 638, and 532 nm, are shown in Figure 2a. In the low-frequency radial breathing mode (RBM) region, multiple bands are observed because the sample is composed of tubes of a variety of sizes and chirality. Figure 2b shows the expanded low-frequency region with the observed RBM frequencies, which are seen to be dependent upon excitation wavelength, because of the strong resonance of the π system with the laser.

The Raman spectra of graphene, a single layer of graphite, and a finite number of graphene layers also have been studied (1). The most striking observation is the behavior of the second-order band near 2700 cm^–1. In the spectrum of a single layer of graphene, this band is roughly four times as intense as the G band. In addition, its frequency more closely corresponds to the overtone of the D band than the G band, and it exhibits the same dispersion of frequency with excitation wavelength, and thus is termed the "2D" or D* band by many authors. In reality, when there is more than one graphene layer, the 2D band is composed of four components whose relative intensities depend upon the number of layers (up to five layers) and the excitation wavelength. Because of this, the Raman spectrum can be used to determine the number of layers of graphene in a film of five or fewer layers. The spectrum in this region also exhibits an overtone of the D' band at about 3250 cm^–1.

Figure 3 shows spectra of one, two, and three layers of graphene, excited at 532 nm. As described in the preceding paragraph, this figure shows that the G band intensity is smaller than the 2D band in the spectrum of the single layer (bottom trace). It shows that the 2D' band at 3248 cm^–1 does not change with number of layers, and it shows the changes in the 2D band with the number of layers.

Figure 3: Raman spectra, from bottom to top, of one, two, and three graphene layers, excited at 532 nm, with the OD = 3 filter. These spectra were acquired on a Lab RAM Raman microscope (focal length = 460 mm) using a 1200-g/mm grating, and the 100x LWD objective. Integration time was 2 x 30 s.

In his review, Ferrari also discusses the effects of doping on the band positions, shapes, and relative intensities of bands in the spectra. Because of the future potential to engineer graphene onto integrated circuits, the ability to estimate dopant levels with a noncontact method will be invaluable.