Hi Jan,

I am not an expert of Raman spectroscopy, but I did have some experiences of utilising such a technique to study graphene.

According to previous literatures [A.C. Ferrari...etc], the D band arises from the point-defect (nD ~ 1/LD), while the 2D band is the double resonance (DR) of G band. Since the G band is an indicator for the topological disorder in sp2 carbon, the width (FWHM) broads with the sp2 carbon become more and more disorder.

In my experience. a highly damaged graphene (high nD, or high concentration of heteroatoms in the sp2 structure ) will result to a broad G band and very low intensity of 2D band. while the D band will be very obvious. Therefore, I guess your graphene sample might be defective or presence a significant amount of heteroatoms. Maybe take a well known graphite sample as a reference to your graphene sample would be a good idea.

Regards,

James

Dear Jan Mischke ,

you can read something about interpretation of different modes for disordered graphite-based nanomaterials in my work:

Article Structure, temperature and frequency dependent electrical co...

Best regards

Thank you @Li-Shang and @Adrian for your responses. Maybe I can clarify my problem a little bit more:

I have two different raman spectra. The first one (Raman1) has a typicall graphene spectrum with a high D-Peak. The ID/IG ratio is slightly above 1 (~1.2). The other Raman spectrum (Raman2) hast nearly the same ID/IG ratio but shows no distinct 2D-Peak.

This leaves me a little bit puzzled:

--> If the D and 2D band have the same origin, namely the interaction of the electron/hole with iTO phonons, then why is it possible to sometimes only see a D-Peak and no 2D-Peak, while the ratio between ID/IG is nearly the same. If I see a D-Peak, I know that there need to be iTO phonons at the K-point present. Thus it should be possible to get double or triple resonance processes for the 2D band.

Basically I just want to know the correct origin of the 2D-Peak in respect to the D-Peak (because both are very similar, yet very different)

This is interesting Jan! Are the spectra you show real experimental data? and not computer generated? how did you make your graphene samples? if it is not a big deal, could you please normalize the two spectra to the G band intensity and plot them together?

Based on my experience,

D and 2D band have the same origin is that DR and interaction with iTO phonon. D band is activated when the defect presence in the sample. But the fact is that if you D-band, it is not necessary to have 2D band too. That's why the peak at 2700 cm-1 is assigned as G' peak in order to avoid the confusion in many literature. The origin of both peaks will be more clear from the E-K band diagram as attached. taken from Physics Reports 473 (2009) 51–87 . I recommend this article to you

In addition, I also have similar question as Dr. Rodrigues asked

If you look carefully at the spectra posted above you'll notice that the 2D peak shape is not Lorentzian, it is not even symmetric. Also, it is not clear if the 2D peak completely disappears since there seems to be a cutoff in the lower intensity scale.

Keep in mind that although both peaks (D and 2D) are double resonance processes originating from the breathing mode of the hexagonal carbon ring in graphene, the Raman activation mechanism is different: the D peak requires defects while the 2D peak ensures momentum conservation by involving two phonons with opposite momentum. This said, the intensity given by the matrix elements in the double resonance process are different.

My (poorly) educated guess, if the spectra above are real, is that the drastic decrease in 2D intensity can be linked to different doping levels.

Thank you all for your contribution. The spectra I showed are of course not real! They were just a quick help to clearify my question. Sorry for the confusion