Application of Raman Spectroscopy in Carbon Materials

There are a wide variety of different carbon nanostructures, however they all have a few basic things in common.

The range of these materials starts with the well known allotropes of diamond and graphite, and continues on to encompass fullerenes, graphene and more complex structures  such as carbon nanotubes.

From a molecular perspective, these materials are all entirely composed of C-C bonds, although the orientation of these bonds is different in the different materials.  

To characterize their molecular structure in a meaningful manner, it is necessary to have a technique which is highly sensitive to even slight changes in  orientation of C-C bonds.

Raman is highly sensitive to morphology. Raman spectroscopy is particularly well suited to molecular morphology characterization of carbon materials.

A typical Raman system has four main components:

• Excitation source (Laser)
• Sample lighting system and scattered light collection system.
• Wavelength selector (filter or spectrometer)
• Probe (photodiode string probe, CCD or PMT)

The spectrum of diamond is easily distinguished from the spectra of silicon and germanium by the frequency (cm-1 position) of the band even though they share the same tetrahedral crystal configuration. The heavier atoms of silicon and germanium slow the vibrational frequency and shift the corresponding Raman band to lower frequency as well. From that, it is easy to distinguish Diamond and Graphite even though both are composed entirely of C-C bonds.

The graphite spectrum has several bands in the spectrum and the main band has shifted from 1332 cm-1 in diamond to 1582 cm-1 in graphite.

The small crystal size of nanocrystalline diamond results in a  finite-size effect in which the lattice is  somewhat distorted. This is manifested in the Raman spectrum by a slightly  downshifted tetrahedral sp3   band.

The additional band at 1620 cm-1 and the shoulders on the 1620 cm-1 and  tetrahedral sp3   band are also indicative  of sp2   bonded carbon that represents  surface defect modes, which would be  insignificant in larger diamond crystals.

Raman is capable of determining layer thickness at atomic layer resolution for graphene layer thicknesses of less than four layers (i.e. at thicknesses that are of interest to the present field of graphene research).

The intensity of the Raman spectral peaks also shows the existence of oxide functional groups in the Graphene film material.

Raman spectroscopy of Carbon nanotubes

• Carbon nanotubes are essentially rolled up graphene sheets that have been sealed to form hollow tubes.
• Single-wall carbon nanotubes (SWCNT) are cylindrical tubes with a single outer  wall with diameters that are usually only 1 – 2 nm. There are also double-wall carbon nanotubes (DWCNT) which have a second layer of graphene wrapped around an inner single-wall carbon nanotube

The Raman spectrum of a SWCNT bears a lot of similarity to  graphene, which is not too surprising as it is simply a rolled  up sheet of graphene.

There is another a series of bands appearing at the low frequency end of the spectrum known as Radial Breathing Mode or RBM  bands. The RBM bands are unique to SWCNTs and as their name suggests, correspond to the expansion and contraction  of the tubes.