Alain, I believe the changes in the intensity when yousing longer acquisitions are coming from heating effects espcally when you use high power. The instrument doesn't drift so much during the measurement. I though also about a saturation effect on the detector but I'm not sure. Just saying. I don't have an answer about the infuence of grating but I'm really interested to see others opinion. 

Dear Alain, what is the dynamic range of your CCD?

Is it near the max intensity of the highest lines when you are using higher acquisition times?

Under the conditions of higher resolution, was change in G/D ration favoring G or D? I mean did the ratio increase or decrease? Heating could result in more intensity of disorder peaks.

Have you tried to use the same integration time with both types of gratings? In this way you can discard heating effects (or confirm them).

Dear Alain,

There are two main disadvantages to grating. First is related to the presence of artifacts (the so-called dropouts): big changes of diffraction efficiency in a localized zone of the spectrum. Moreover, a little change in the grating angle cause the double change in the dispersed output: high sensitivity to angle change. The second (for me the most important when working with carbon materials) is the sample contamination and cleaning a grating is, in my opinion, almost impossible.  

I was wondering if you are interested in this equipment in order to buy one. Most of the existing RAMAN equipment are grating. Fourier based Raman are more expensive. However, for carbon materials are more appropriate, because need less time for acquisition and thus, carbon materials are not degraded by heating as with grating equipment.

Dear All,

Thank you so much for your comments, much appreciated. Please find attached a picture of what happened with gratings of 1200 and 2400 l/mm. The effect on the D band at 1360 cm-1 is impressive. How, in these conditions, is it possible to deduce anything from the IG/ID ratio ?

To George: I did more experiments today, and even at 10% of the laser power, a hole can sometimes be seen after the experiment at the place of the laser spot. More trustful experiments should therefore be obtained with lower acquisition times, but what is the limit ? In my case, the acquisition time was much higher for the 2400 grating, leading to an increase of D band. Is it logical ?

To Hamed: As you can see on this picture, the maximum intensity is far from the saturation, which corresponds to 60.000 counts.

To Vijay: D/G increased, so it might indeed confirm the effect of heating, right ?

To  Satyaprakash: I have never seen any effect on the polarisation modes, and now I wonder why I purchased this option ! 10% of my laser is still 4.5 mW, do you think it is already too high ? Decreasing it further dramatically decreases the signal/noise ratio, and at the end the position and the intensity of the peaks are not so accurate. This is another problem ...

To Enrique: Yes, I jut tried today, there is an effet of acquisition time. Now I believe that many published papers reported wrong conclusions, as this effect is never discussed ... I have observed that I can obtain very different D/G ratios for a same material, depending on the laser power, time and grating. So finally, what do these results mean ? I feel very embarrassed now !

To Maite: I didn't know these problems. I have already an Xplora apparatus (Horiba), and these questions come from my use of it.

To All: if it is indeed just a problem of heating, should the samples be investigated in water or in other medium ?

Thank you again for your comments, if you have more, don't hesitate to post them here.

Alain

Dear Alain, 

If your instrument is with intergarted microscope then try to have your laser spot little overfocused. You 'll still get spectra and doing this may avoid any damage from the laser (and increase of the D band).

About the spectra you show: Is this measurment from the same spot? Because if you have measured let's say with the one grating and had also some thermal effect from the laser then the difference shouldn't caused by the change of grating but from the damage that already exists from the previous exitation.  

Yes, it seems that using a larger spot minimises the problem which, however, is difficult to avoid completely. I have to check everytime that a hole didn't appear. And maybe modification of the carbon already exists without visual evidence ...

I confirm that in the spectra were obtained without moving the sample. Possibly indeed the sample was already modified after the 1st trial.

Alain

Alain, I think first we must have a dry test of grating alone. Do mamima show shift with laser power or time and how much without allowing the beam through the Carbon material. I mean let us check the consistency of grating material.

Dear Vijay,

Sorry, I didn't understand your recommendation. Please give me more details on what should be done.

Best regards,

Alain

So the changes you see in the spectra arise from the laser effect on your sample and not the grating.  Try different lens, focal length, power and acquisition time and I'm sure that you will find the optimum conditions. What else can you do is make a concentrated aqueous solution of your material (if it is soluble) and place a drop under the laser. Haven't tried this before but since water is low scatterer it won't affect your spectra 

Alain, I mean hope the grating material on which some thousand lines per mm make up this grating should be tested if that does not alter significantly the grating element under exposure to laser beam conditions.

Further,  when you start afresh and try to repeat your experiment do you go through same repetitiveness in the dynamics of intensity of G/D ratio on increase of acquisition times?

Dear All,

Thank you for your valuable comments and suggestions.

It is clear that I now have various trials to do before concluding anything. I'll keep you posted.

Best regards,

Alain

For high resolution spectra, grating with 2400lines/mm is preferred. However, the gratings are also polarization dependent. You can consider collecting only parallel component of your Raman signal and compare results with  different  gratings. Of course careful intensity calibration is very important for compensating variation(s).

D/G ratio gives information ONLY about the size of sp2 domains >ID/IG -->

Please, Alain

A previous question: in your experiments about the intensity ratio G/D, is this ratio increasing when you change the grating from 600 to 2400?

Dear All,

Thank you for your additional comments.

The calibration is quite good, and it really seems that there is a problem of heating. These samples are really sensitive, even at 1% of the initial power, and now I guess that many wrong things are published in the literature, because such effects are not taken into account, as far as I could read (except for CNTs).

In my last trials, I used carbon aerogels, and I could observe "nice" holes in the material exactly at the position of the laser beam, whereas the surface was perfect before switching the laser on. Powers of 0.1% had to be used to not see holes, and obtain reproducible results. However, even like this, I have no guarantee that no heting occurred. Indeed, at 0.1%, the signal is so low that using gratings higher than 600 l/mm is just no more possible.

A very difficult matter indeed !

Alain

Dear All,

I have posted a new question related to this one.

Please follow the link, maybe you can suggest something ?

Best regards,

Alain

https://www.researchgate.net/post/How_best_using_the_polarisation_kit_of_a_Horiba_XPlora_Raman_spectrometer_for_investigating_carbon_materials

I am getting blue shift in peak when moving from 2400 to 1200 grating l/mm. 

Can anyone suggest me what is the problem???