I just encountered a very puzzling Raman spectroscopy issue. I have the same specimen measured at the same spot. One used 735nm laser, and the other 532nm.
As it can be seen clearly from the attach graph that not only the peak heights are very different, but also the peak position: 735nm: D band at 1312cm-1, G Band at 1603 cm-1, but 532nm: D band shifted to 1343cm-1 (+31cm-1), and G Band shifted to 1578 cm-1 (-25cm-1). Such intensity differences and peak shift is very hard to explain.
Does this mean that there is no "standard" Raman spectra for the same specimen on the same spot?
Can anyone help out? Appreciated.
Timothy D. Huang
I don't think that just changing the laser wavelength should per se result in shifts in the Raman shift (this should be independent of the incident wavelength). However, if you shifted from larger to smaller incident wavelength you will deposit more energy onto your sample, which might damage the surface or induce chemical reactions, which might induce an additional Raman shift (especially when you measure in the open air). Have you checked for this?
@ Johannes
Thanks, but I don't think the surface was damaged or chemical reactions induced. It was repeated several times yielding similar results.
yes, but this doesn't really tell you anything because if you have the same setup with the same parameters and the same kind of surface, damages or chemical reactions on the surface should also be the same and, hence, leading to similar results. You could, for example mark the area on the surface where you do your Raman measurements, e.g. by encircling it and then do XPS where the spot was and somewhere else, which gives you the chemical composition, you can also use optical microscopy, TEM, etc... for surface analysis.
However, I should add that I have worked with Raman spectroscopy frequently but I am not a real expert - maybe someone else who is more familiar with the topic has a better idea, where the shifts come from. From my physical understanding a change in surface chemistry or morphology in the atomic structure of your samples is a likely explanation.
But if the vibrational mode is coupled to the electron state, for example, there is resonance effect that can happen.
Some phonon patterns in two-dimensional materials are wavelength dependent.
This is what I got from a Chinese scholar.
I assume that your measurements were taken in identical spatial arrangements. But how about the sample temperature? It may differ, even when both lasers are nominally of the same power.
Please, consider that any small change in your spatial arrangements will affect your Raman spectrum. This is very important.
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