What are sample differences in curves?

It may be repumping the energy into the higher mode (nonlinear effect).

Hello Dr.Eugene F Kislyakov

Thank you very much for your attention. the differences in the curves are 

1. Change in the position and intensity of  2D band ( at 2720 nm)

2. Increase in the intensity of D band (at 1350 nm)

3. Increase in the peak intensity of D' band ( shoulder peak beside to G band)

  i am a chemist, could you please explain it in detail. 

It seems you have destroyed your sample (due to local heating)  if all spectra were recorded in the same point. The spectrum at the bottom is a signature of defects with broken C-C bonds.

Hi Santosh K. Tiwari

Thank you very much for your suggestion.

Raman Spectroscopy is very sensitive technique for this kind of study. Agree with Dr. Yuzyuk that "It seems you have destroyed your sample (due to local heating) if all spectra were recorded in the same point." Second point is don't confuse with 2D and D, although 2D is the overtone of D. The origin of those bands are quite different. It was nice to see the variation in the bands in terms of position and intensity. But with respect to what (x),  you got these changes? I think you will get other clues for those changes if X can be correlate. Have you checked with FWHM of Raman bands or crystalline size? And it is better to compare the Raman spectra of films with normalized intensity.

Good luck.

may be formation of multilayer graphene. if graphene is few layer then it gives 2D peak

Dear Friend, 

This link may help you. http://journals.aps.org/prb/abstract/10.1103/PhysRevB.80.233407

Dear Munaiah,

I asked not about diferences in curves (they are evident), but about diferences in measurement conditions. What were the samples, what were other conditions of measurements?

Hello Munaiah,

first of all, the 2D is NOT an overtone of D band - while you need a defect to activate the D band (plus one TO phonon, in a simplified explanation), no defect is needed to activate the 2D band (just two TO-derived phonons). It is a bit confusing there... So, in the end, the more defects you have, the more intense the D band will be (and all other defect-induced bands, like D' and D''), but the less intense all the other bands will be, which is especially visible on the 2D band. In highly disordered sp2 carbons, you'll see only the D, G and D' (plus some of the others more or less in the background in that frequency region), all of them broadened, and the 2D will be almost invisible (and very broad too). That is what is happening in your experiment.

What is the origin of 2D band, Otakar?

Thank You in advance,

Eugene.

Was your graphene functionalization a process of gradual oxidation of graphite?

The first and last curves seem to be corresponding to the Raman spectra of pristine graphite and graphene oxide respectively. The others can be assigned to the intermediate product during the process.

Dear Eugene,

the 2D mode has an origin in the excitation of two phonons from the TO branch, which have opposite momenta, in the vicinity of the K and K' points, respectively (in the Brillouin zone). The mode has a A' symmetry - like a breathing mode of the benzene ring, but it is dangerous (in terms of interpretation) to look at the mode just in terms of simple symmetry. Since it is doubly resonant (or even triply, to be more precise), the momentum space is more important for the interpretation. Also a final note, some people refer to the mode as G', so it is not confused as an overtone of D (but in my opinion , G' is confusing too, in a different way :)). There is a couple of good recent papers about the doubly resonant modes, e.g. Venezuela et al., PRB 2011: http://arxiv.org/abs/1103.4582v2

If the last sample, presumed as GO, is further reduced chemically, the intensity of the 2D band is most likely to be increased to a certain extent. 

However, the reduction treatment will not largely change the D and G band morphology and intensity, with D band possibly increased slightly due to the size reduction effect.

Ps: sp2 domain size

Thank You, Otakar.

Hello Dr Eugene F Kislyakov,

I did synthesize graphene by continuous treatment of graphite rod with plasma. the attached spectrums are corresponding to graphene collected at different time intervals. I knew that crystalline structure of graphene has damaged but i just wanted to know what is the physics behind it.  Could you please explain the way that i can understand or i will appreciate if you suggest any reference for it.

Thank you in advance

Different time intervals of treatment, Munaiah?

yes. Dr. Eugene F kyislyakov

And other conditions of measurements were not changed?

You have different materials, Munaiah. Try to determine the difference between them also by some other method. Only Raman is not enough.

Regards,

Eugene.

Now your question has clarity that you synthesized carbon nanomaterials from graphite target using plasma. With Eugene, "seeing is beleiving". Optical image is must for confirming the material with Raman spectroscopy. After seeing the Raman spectra evaluation with respect to times, its looks like the transformation of nanographite (contains a-C, defect etc.) to nanographene or multilayer graphene or vertical graphene nanosheets or carbon nanowalls. But has to ensure with optical characterization like SEM or AFM. You can go through following references

1.Ghosh et al. Evolution and defect analysis of vertical graphene nanosheets. J. Raman Spectrosc. 45, 642-649 (2014).

2. Hazra et al. Thinning of multilayer graphene to monolayer graphene in a plasma environment. Nanotechnology 22, 025704 (2011).

3. Yoshimura et al. Atomic force microscopy and Raman spectroscopy study of the early stages of carbon nanowall growth by dc plasma-enhanced chemical vapor deposition. Carbon 50, 2698-2702 (2012).

4.Zhang et al. Catalyst-free growth of nanographene films on various substrates. Nano Research 4, 315-321 (2011).

5. Kim et al. Fabrication of graphene flakes composed of multi-layer graphene sheets using a thermal plasma jet system. Nanotechnology 21, 095601 (2010).

If you have doubt, contact without any hesitation.

Best of luck.

Hi guys, for your reference below

Two possible etching mechanisms include

isotropic etching and anisotropic etching. If the single

sheet removal of graphenes by oxygen plasma is

isotropic, the etching rate should be uniform everywhere

on a graphene sheet regardless of graphene defects

and edges. Initiatively, when a single atomic layer

is removed from the top, a single or a couple of chains

of atoms at the edge will be removed. This is because

removing an atom from the top will need sufficient energy

to break three bonds, while at the edges, only one

or two bonds need to be broken due to the existence

of dangling bonds. That means the etching cannot be

an isotropic process. For graphene sheets, there are

two possible preferred directions for anisotropic etching:

vertical (top surface) and horizontal (edges). Experimentally,

it is possible to detect a removed single

layer using Raman or optical microscopy, but the loss

of a couple of chains of atoms at the edge is beyond

the resolution of these instruments. Consequently, the

sizes of the graphene sheet and the defective regions will

not change during the etching in the case of anisotropically

vertical etching, but will be projected to the bottom

graphene sheet after the etching process.

What substrate are you using for synthesis?

Amorphous materials generally yields such spectra. When I  was working with silica glass I usually had such spectra...

This link is really useful,

http://www.instanano.com/characterization/experimental/raman-graphene/