the most Raman spectrometers use LASER wavelengths of 533 and 633 during characterizing Graphene. When a LASER of wavelength 785 nm is used the D & G peaks in the spectrum are greatly shifted and the 2D peak does not exist.
Dear Amr Hessein, I found out this paper which discuss the spectra behavior of graphene under 514 nm and 785 nm lasers excitation. This is not the focus of the paper, but I believe the discussion can help you to understand and give you some references to explore. Hope it help.
Thanks Grazielle.
some more reference
You're probably using to much power of 785 nm laser, the thin layer gets thermally damaged and it gives Raman lines shift. We're routinely using 0.05% of 60 mW laser (i.e 30 mkW) and it is very near of damage threshold for thin carbon structures.
@R.P.wang sir is it necessary to have Eg-wavelength matched laser to be used or higher wavelength can be used in general as well in case of iron oxide material.
We've been using 785 nm and can see a nice 2D peak when the quality of the graphene is good.
Our spot size is 100 um (very big) and the laser is 250 mW.
We scan for 1 second and repeat 120 times.
We tried many combinations of scanning parameters (time, intensity, repetition) before we managed to get a decent Raman spectrum. Reducing the intensity didn't help. Presumably the laser was optimised correctly for the spot size? About 2.5 mW/um?
Our instrument has a manually initiated raman focus option... and seems to work best if we do that prior to commencing a scan. We also do a background scan using the glass slide the graphene is on prior to commencing.
The 2D peak only shows up clearly as you move towards few layer graphene. Prior to that it's very noisy and easy to think you haven't got one. Once you know where the peak is it is much easier to find the 2D peak in noisy samples. A bit of smoothing usually helps especially with the noisier samples. Too much smoothing will misrepresent the peaks NB. You should measure your peaks before smoothing to avoid errors.
Just after the 2D peak it gets even more noisy. Deleting the noisy data above the 2D peak makes the spectrum less confusing.
Refer Czajka 2015 SECM Preparation of graphene and inclusion in composites...
We're using a Perkin Elmer Raman Station 400F
The 785 nm signal can be boosted significantly with gold. Refer Lee 2011 Surface enhanced Raman scattering...
785 nm is not a common graphene scanning frequency... but can work well. The signal you end up with depends on your instrument, settings and persistence.
As sensitivity improves on newer Raman instruments the spectra keep improving.
:-)
This link is really useful,
http://www.instanano.com/characterization/experimental/raman-graphene/
can we use 515nm laser for graphene characterization in composite hydrogel
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