It's confusing to see different excitation wavelength-based Raman spectroscopic research papers. Can you suggest a suitable excitation wavelength, acquisition time for g-c3n4 (prepared by calcining melamine at 550'C)?
Dear Swathi a C , Raman shift does not depend on the source's wavelength and the recorded spectra of a material using different light sources are identical in terms of peaks positions. However, since fluorescence is a critical interference in raman spectroscopy, a proper light source must be chosen that does not excite the target material to the electronic excited state. That's why in most cases it is prefered to use either a green 532 nm laser or an infreared 785 nm laser. Not to mention, NdYAG with 1064 nm will work out as well. As I mentioned above, you should choose the light source according to the fluorescence excitation spectrum of g-C3N4 which is typically around 300 nm. So, the aforementioned sources are appropriate for your case.
Best,
For metals, high energy lasers (low wavelength) like blue (488 nm) and green (532 nm) lasers are good choice. High energy is to increase the penetration depth in metals by overcoming the electron shield. These lasers also will work well for narrow band gap semiconductors like Si, Ge etc. For large bend gap semiconductors (E.g.: CH3NH3PbBr3, CsPbBr3 etc.) fluorescent effect will be a big issue. As you increase laser energy, photoluminescence (PL) will dominate over Raman effect (Raman intensity is inversely proportional to fourth power of wavelength). Raman peaks would be buried inside the PL background. Since Raman scattering is a weak inelastic scattering process (1 out of million photons involved in this process) compared to the PL (1 out of ~100/1000 molecules involved in this process), PL effect would dominate over Raman scattering. For fluorescent materials, 785nm or 1064 nm works well.
What type of wavelength need to choose depend on the type of materials (band gap). Increasing the collection time, laser wavelength or collecting few times the same spectra for the same time and average them, can increase the signal efficiency. Please ensure that the laser does not burn the sample like in chalcogen based compounds.
What wavelengths do you have available?
if you have several available, then the choice can depend on a number of factors:
(i) the Raman scattering efficiency is inversely proportional to the 4th power of the wavelength, so in principle the shorter wavelength the better.
(ii) notwithstanding (i) as the wavelength is reduced from the near IR into the visible, the likelihood of reaching a resonance of the material is increased, which can be good, as you can get resonance enhancement of the Raman scattering, but can be bad, as only the modes of the resonant moieties are enhanced, the excitation increases the possibility of photodegradation of the sample, and as Mahdi Ghamsari points out, there is the possibility of fluorescence emission, depending on the material.
(iv) the power of the lasers available
An absorption spectrum of the material can help you with your choice, but starting off resonance is usually best.
If it is a semiconductor (narrow band gap semiconductor as well), 532 nm (green) or 488 nm (blue) lasers work well
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