In vertical aligned CNT forests, many types of tubes can coexist. If only MWCNTs are present, the Raman spectrum does not show the RBM mode, the heights of the D and G peaks are almost the same and there is a unique 2G peak. However when some SWCNT are in the forest some RBM modes appears (corresponding to tubes of different diameters), the G peak increases and the 2G peak becomes double (appearing a new term at lower wavenumbers).
My question is if RBM modes show a predominant one, the G peak is near ten times higher than the D one and the 2D peak is unique and at the lower wavenumber, Could one affirm that the quantity of SWCNT is much greater than that of MWCNT?
Are there any other features in Raman spectrum that can quantify in any way the purity of SWCNT?
Thanks so much for your answers!
May be a wild guess, but i would expect less broadening and a slight shift to the higher frequency of the peaks especially on the G peak which will be more effected then other peaks by more intense inter-molecular interaction between the aligned SWCNT's with respect to randomly oriented bulk.
Best Regards.
SWCNT have strong resonant modes and it is sometimes possible to get the RBM signal if there is only SWCNT in the whole lot...the signal intensity does not necessarily give the information about the quantity of SWCNT and MWCNT...One possiblity is to do Scanning Raman instead of point measurements...Many such measurements will give a rough estimate of the CNT distribution...
As Sivasankaran Harish comented one option could be Scanning Raman spectroscopy. Doing this measurement you can basically obtain a map with the D/G ratio of the nanotubes, SWCNTs will show lower intensities for the ratio than MWCNTs. It is even possible to get an indication of the interaction between both nanotubes, maybe SWCNTs are aggregated together forming "islands" or on the other way around. If this is like that, you could quantify the number of spectra that correspond to each type but it might change in different spots of the same sample. Then, several map measurements would be needed.
I agree with both, Sivasankaran and Vicent. This situation is possible. But the question is what happens when an extended light spot is used.
See an example in the next figure. These are Raman spectra of CNT forest grown on AlN thin films at different conditions with roughly the same thickness (20 to 30 um). Note the behaviour of the 2G band. Is quite significate.
The question is, may I affirm that the green spectrum corresponds to a forest with almost only SWCNT?
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