Raman Spectroscopy of Carbonaceous Materials: A Concise Review

A critical review focused on the Raman spectroscopy of carbonaceous materials and of polymer-based nanocomposites that contain carbonaceous (nano) materials as fillers is presented. The origin, assignment, and parameters (position, intensity, area, width, and shape) of main Raman modes (radial breathing mode, D-mode, G-mode, G'-mode, and so forth) as well as the effect of the interactions of carbonaceous materials on the parameters of these modes is briefly discussed. The effect of dopants and of polymeric matrices on the parameters of Raman bands is succinctly analyzed. The review will provide the basic and most elementary knowledge required to understand and discuss the Raman spectra of carbonaceous materials.

Carbonaceous materials have a wide range of physical and chemical properties derived from the spatial organization of carbon atoms and their chemical covalent bonds. Diamonds, nanodiamonds, single-walled carbon nanotubes, double-walled carbon nanotubes, carbon nanofibers, and carbon fibers are characterized by an impressive mechanical strength. With the exception of diamond and nanodiamond, the Young modulus of one-dimensional carbonaceous materials decreases as their diameter is increased. This is one of the reasons that nanocomposites based on one-dimensional carbon are extremely attractive. While diamond and nanodiamond are electrical and thermal insulators, one-dimensional carbon nano- and micromaterials, graphene, and graphite are characterized by good electrical and thermal conductivity, exhibiting either semiconducting or conducting features. This wide range of physical properties explains the complex and abundant applications of carbonaceous materials.

Raman spectroscopy investigates the molecular vibrations of atoms and molecules. In a Raman experiment an incoming monochromatic electromagnetic wave (originating from a laser) excites the atoms and molecules of a sample, starting the emittance of electromagnetic radiation. The Raman spectrum is the collection of all these emitted electromagnetic waves, except for the exciting wave (the incident one). This spectrum is generally obtained by using charge-coupled device (CCD) cameras combined with an adequate optics system consisting of prisms or gratings. The position of the Raman spectrum is typically measured in wavenumbers (cm-1) starting from the frequency of the exciting radiation (that has a Raman shift of zero and is not recorded). Most spectrometers start the recording of the Raman spectrum at 50–100 cm-1. The resolution of Raman spectrometers is not critical in the analysis of carbonaceous materials because their lines are not extremely narrow. Typically, Raman spectrometers with a resolution of about 1–3 cm-1 provide excellent research data.

Raman Lines in Carbonaceous Compounds

Figure 1: Raman spectrum of single-walled carbon nanotubes.
Several Raman lines (or bands) have been reported in carbonaceous materials and polymer-based nanocomposites containing carbonaceous fillers or nanofillers. The intensity of all these lines depend on the orientation of the polarization relative to the axis of nanotubes (5), the power of the incident laser beam, and the number of accumulations. A typical Raman line for a single-walled carbon nanotubes is shown in Figure 1. The most important Raman lines are discussed in the following sections.

Radial Breathing Mode

The radial breathing mode (RBM) is related to the diameter of carbon nanotubes and was observed solely in single-walled carbon nanotubes (SWCNT) and double-walled carbon nanotubes (DWCNT) in the range 150–350 cm-1 (7) for diameters of 1–2 nm (8). Typically, RBM is a collective and symmetric movement of all atoms along the radius of the nanotube. In some multiwall carbon nanotubes (MWCNT) and carbon nanofibers (CNF) a broad line can be observed in the region of the RBM, but it is not clear if this line can be actually related to the size diameter of these structures.

The position of the RBM mode in SWCNTs and DWCNTs (ωRBM) depends on the diameter of the nanotube (D T) (8):

where A and B are constants.

For isolated nanotubes (8), B = 0 and A = 248 nm.cm-1. For bundles of nanotubes, B describes the interactions between nanotubes and is of the order of 10 cm-1, while A = 234 nm.cm-1. For narrow, single-walled carbon nanotubes (D T < 1 nm), the lattice distortions are large, thereby making simple dependencies no longer valid. For SWCNTs with a diameter equal to or larger than 2 nm, the intensity of the RBM mode is too small to be observed (8).

It was reported (4) that the RBM located at 160 cm-1 is broadened and slightly shifted toward higher positions (ω) as the isolated nanotubes collapse into a bundle. For an isolated SWCNT, the width of the RBM mode is of the order of 3 cm-1.

Table I: Most important lines in carbonaceous materials
The RBM has also been reported in fullerenes, typically below 250 cm-1 although it also was reported at 490 cm-1 (11). The position of the RBM mode in fullerenes depends on the energy (wavelength) of the incoming laser beam (12).