Question

Research and then name the fundamental Raman modes of SWCNTs and describe briefly their energy and source in the nanotubes.

Answer 1

Raman spectroscopy has good spatial resolution (~0.5 micrometers) and sensitivity (single nanotubes); it requires only minimal sample preparation and is rather informative. Consequently, Raman spectroscopy is probably the most popular technique of carbon nanotube characterization. Raman scattering in SWCNTs is resonant, i.e., only those tubes are probed which have one of the bandgaps equal to the exciting laser energy.

The fundamental Raman modes of SWCNTs are as follows:

1. Radial Breathing Mode(RBM) :- It corresponds to radial expansion    and contraction of the nanotubes.Therefore, its frequency νRBM (in cm−1) depends on the nanotube diameter d as, νRBM= A/d + B   (where A and B are constants dependent on the environment in which the nanotube is present.For example, B=0 for individual nanotubes.)

2. Bundling Mode :- This mode originates from the collective vibration in the bundle of SWCNTs. This mode is a special form of RBM.

3. G Mode :- This mode corresponds to planar vibrations of carbon atoms and is present in most graphite-like materials.G band in SWCNT is shifted to lower frequencies relative to graphite and is split into several peaks. The splitting pattern and intensity depend on the tube structure and excitation energy; they can be used, though with much lower accuracy compared to RBM mode, to estimate the tube diameter and whether the tube is metallic or semiconducting.

4. D mode:- This is present in all graphite-like carbons and originates from structural defects.The ratio of the G/D modes is conventionally used to quantify the structural quality of carbon nanotubes. High-quality nanotubes have this ratio significantly higher than 100. The G/D ratio remains almost unchanged and gives an idea of the functionalisation of a nanotube.

5. G' Mode:- This mode is usually the second strongest after the G mode. However, it is actually the second overtone of the defect-induced D mode (and thus should logically be named D'). Its intensity is stronger than that of the D mode due to different selection rules. In particular, D mode is forbidden in the ideal nanotube and requires a structural defect, providing a phonon of certain angular momentum, to be induced. In contrast, G' mode involves a "self-annihilating" pair of phonons and thus does not require defects. The spectral position of G' mode depends on diameter, so it can be used roughly to estimate the SWCNT diameter.In particular, G' mode is a doublet in double-wall carbon nanotubes, but the doublet is often unresolved due to line broadening.