I usually use Raman spectroscopy to characterize graphene peak and I have issues about Raman peak shift during the analysis. I have two problems during the Raman work.
1. I usually collect data several times (2-3times) at the same spot just to make sure the datas I collected are trustworthy. But every data at the same spot with nothing (power, location, etc,.) changed has different wavenumber.
2. I mostly used the spectroscopy to track the state of the graphene. Like the peak shift of graphene due to acetone (which is known to cause p-doping), PMMA spin coated graphene and lastly PMMA removed graphene because as I did some research about them, the cleaner the graphene is, red shift must occur (towards the prisitne graphene). But the theory doesnt work for me. It does not have any tendency.
I wonder if anybody has same issues like i have.
I am guessing this problems are caused due to
1) old device (Witec alpha 300M+)
2) bad focusing of laser
3) I use exfoliated graphene which is composed of many different layers around the spot i focused on (I dont know whether it matters)
4) Lastly, the power. The device's power management of laser is done through nobe not by giving numbers to the program and therefore the power won't be exactly the same. (I raise power where the peak saturation occurs through oscilloscope)
every time getting different wavenumber may be you are focusing the size which is less than even nanometer. IF the focusing of laser is less than nanometer size then it is natural because particle with in this much small area is not present at one place it may be oscillating or moving in small area which may not be detectable by our observation .Heisenberg's Uncertainty Principle and Some Quantum laws also says this
I've had that with photochemically active material more than one time. If the laser induces structural changes in the material, which may even manifest in changes of morphology visible in optical microscopy, random shifts or appearance of peaks may happen.
Since you are just inspecting graphene, the amount of photochemistry should be limited for large flakes, but if you have nano-sized flakes whose termination would allow photopolymerization, that might be what's happening.
You'd better list all the factors that can lead to the peak shift of your studied samples. Then design the experiments to eliminate those irrelevant from significant attributes and understand experiment conditions too. It does not look like it is a multivariate attributes case that results in the shift.
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