Hi Hai,

Raman is very powerful to determine CNT type and Quality.

SWNT are resonant and have an unique raman Fingerprint. Very sharp G band, very low D band and From the breathing mode (100-300 cm-1) you can even determine the Diameter. Another important factor is the D* band (2600 cm-1). This is a 2nd order Transition which is enhanced in SWNT due to the perfect isolated electronic structure. So low D, sharp G, strong D* and RBM will tell you it is a SWNT.

DWNT, if of good Quality, will have similar Features of SWNT, as they could be seen as 2 concentric tubes. However in this case D* is basically forbidden, as the exciton will be transferred to the 2nd tube instead of being excited by a second photon. So low D, sharp G, RBM (inner and outer tube) and no D* is a Fingerprint of DWNT.

In theory you can go Forward, however as you start adding more walls, the presence of defects increases and the interaction between the wall too... So you start losing the Features of SWNT (perfect isolated electronic structure). It is very difficult to measure the number of walls for MWNT by raman. D/G will tell you only the amount of defects. You can have very thin (few wall) defective CNT with a D band stronger than perfectly graphitized MWNT with 50-60 walls.

However if you have good material, looking at the RBM is always useful. Please note, that as you increase the Diameter, the RBM will Approach the laser line, so you will Need a good spectrometer with a double/triple notch filter to see them.

Giulio

Thank you very much Guilio.

The reason I asked this question is relating to my thermal treatment for MWCNTs in Argon environment. When I checked with Raman, I see ID/IG got lower clearly (~40%). But when I checked with TEM, many walls of CNTs removed and remained on CNT surfaces; many MWCNTs became DWCNTs. Thus I am not sure D band got lower because defects got healed by annealing effect or because MWCNTs turned to DWCNTs.

Could you have any idea on this?

Hi 

when wall number in CNT increases the D band also increases, D band indicating the defects and disorder in carbon atoms hexagonal structure. It is impossible to place second wall on exactly the first wall hence the more the walls more will be the defects and disorder in carbon atoms networks. After thermal annealing your Id/Ig value reduces indicating reduction in defects and disorder in carbon atom structure which you already confirms by TEM your MWCNTs converts to DWCNTs.

Hi Hai,

The main effect you see is due to annealing of the defects. This reduce the D/G Ratio. You can observe the same effect in many other annealing experiments where the number of walls does not Change.

In your case it is associated to a reduction of the wall number. Of course a DWNT will have a much lower D/G as the probability of having a defect there is lower (being the wall less).

To demonstrate this I suggest to repeat your Experiment in complete Absence of oxidating. If you use Ar from the bottle you will have some ppm O2 and lot of H2O which will burn your carbon (or you have a small leak). You should purify your Ar thorugh an O2 scavenger (there are filters you can buy which contain some alkali metals to completely remove them) and work absolutely dry. In this case you will not have wall number reduction.