Dear Researchers,
Can someone explain me the detailed procedure for the preparation of a "Graphene" sample containing flakes from 5 to 50 nm thick in SEM on Si/SiO2 wafers? Do I have to contact the wafers to the SEM holder with silver paste? Which energy do you recoment for the meaurements?
Dear Vicent Lloret , I agree Khalid Mohd.
it will depend on your SEM resolution that you can see such thin films. May be the 50nm flakes will not be a problem, but measuring the 5nm flakes could be tricky. In my personal experience carbon adhesive tape works fine and if graphene is directly attached to this tape you don´t need coat it with any metallic coating, graphene is enough conductive to get good images... Due to you have your flackes attached to Si/SiO2 wafers, a good electrical contact would be necessary. I would try directly with the carbon tape and if not enough, add a silver paste contact between the stub and the wafer edge. For high resolution I would try with 10 kV or less and I would take the images in the highest resolution possible. It would be tricky, specially if your flakes edges are like vertical cliffs. If they are like a staircase may be you could count the steps as individual graphene monolayers and determine in this way its total thickness.As khalid suggest, TEM will provide you a better resolution if you can rid off the wafer.
I think that a standard AFM could do the job readily, given your flakes are discretely distributted on the wafer surface.
Another technique that may be useful is Raman spectroscopy, it allows measure the number of layers based on the Raman signal. Depth profiling would also be useful, because when the beam will reach the Si/SiO2 wafer its signal would experiment a huge increment compared with the one from graphene.
Hope it helps.
We've made such measurements for Si wafers coated with TiO2 with a layer of single graphene oxide flakes on the top. I used carbon conductive tape for joining the sample and holder. There was no need to use condcuctive paste as Si is conductive enough. Whole coating had ~60 nm thickness (TiO2 with graphene flakes). Beam energy of 5 kV was applied and the vacuum was around 10^-5 Pa (at higher energy you will burn the flakes very fast). Our SEM had field emission. I made also such measurements for single graphene oxide flakes on Si wafers and thick layer of GO flakes deposited on Si wafers. Observation of graphene flakes is difficult in the way that you can ,,burn" flakes with the beam, so I would recommend setting focus and other parameters in the different place near the one of which you would like to take picture.
In my opinion, it is necessary to clarify the terminology before discussing the issue. 5 nm (not to mention 50 nm) thick is certainly not graphene. Graphene is a monolayer of carbon. What the author of the question has is better to call it multilayer carbon (?).
"If the words are wrong, then the deeds are not done" (Confucius, Analects 1, 3).
Dear Researchers, thanks a lot for the comments, the samples are already characterized by AFM correlated to Raman and now I will try on carbon tape and also on coated TiO2 wafers for SEM. Vadim S. Gorshkov , that's why I wrote "graphene" but thank you so much for your enlightenment. I also know that TEM would be more useful but I have easy access to a SEM.
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