I am trying to understand the contrast difference between 2D-Materials (namels graphene) and the underlying susbtrate (e.g. SiO2 or germanium). Using an SE2-detector you get topographical information of the sample. With an In-Lens detector you get the material contrast. Now as graphene is only one atomic layer the In-Lense detector must be somehow extremely surface sensitive as the PE beam will penetrate deeper than just one atomic layer. I get that only low energy electrons are colected by the In-Lense detector. But how does the contrast (graphene is dark, the susbtrate is bright) get so clear that you can see graphene in an In-Lense detector?
It must have something to do with the interaction of the graphene with the PE electrons. Is the delocalized electron cloud of the graphene interacting more strongly with PE electrons than the surrounding substrate and hence I get a very distinctive contrast?
Thank you for your help
To my opinion, It is because the signal you get (and give you your black/white image) corresponds to the quantity of electrons going back to the detector. The electrons going back to the detector are generated when the beam hits and interacts with the underlying material (graphene is transparent to high energy electrons so incident electrons are not Impacted by graphene). The electrons resulting from the interaction with the substrate (SE or BSE) and being »probed » by the detectors are less energetic and are screened by graphene. The thicker the graphene, the stronger will be your screening effect, thus reducing the number of electrons reaching the detector and making that region of your image darker. The conducting nature of graphene can also play a role...especially on insulating materials like SiO2 or sapphire. And yeah, the in-lens detector is way more sensitive than the regular E-T detector.
I hope it helps!
Thank your Benjamin Huet for your response. I thought about the screening effect of graphene as well. But as I understand it, graphene is also transparent for low energy electrons (at least to a certain amount).Article Transparency of graphene for low-energy electrons measured i...
So the contrast information would have to be extremely sensitive to small transparency variations due to number of layers and/or interaction witht the underlying substrate.
If it is correct (that in the end it all comes to the low energy electrons that are influenced by the graphene layers), what would you think about layers with a very high amorphous amount? If I have a graphene grain and enxt to it is amorphous graphene. There is a distincitve contrast difference. And if it is interaction with low energy electrons then I would assume that the higher the amorhpous amount, the less transparent it is for those electrons. So it should appear darker in der SEM image, right? Or does it absorb most of the icoming high energy electrons?
I think Bejamin is right, and the screenin effect of graphene is the main reason for that dark appearance of graphene compared with SiO2. This is something you can check easily by comparing the contrast of 1 layer and multi-layer graphene. The contrast should increase with the number of layers, but not in a linear way, it would be probably exponential.
About the "amorphous" domains in your graphene sample: The main component is still C, if we assume that it is still 1 atom thick, and we don´t take into account the electronic cloud over the layer, the contrast should be similar to that of graphene 1 layer, both because the screening effect and the atomic number of the atoms implied. If your "amorphous" domains contain different bonding than sp2, probably you are lossing the monoatomic thick structure and therefore the contrast should be higher (you have more C atoms piled).
The screening happens in some extention for both primary electrons going through the graphene and for the SE, but the main effect of screening happens over the SE. It is a matter of energy: Even if you are working at low acceleration voltages (ie. 1KV) your primary electrons have a lot of energy, therefore they pass through the graphene easily. They reach the substrate (say SiO2) and because of their "low" energy, they produce few SE electrons from the SiO2. SE have energies well below 50eV, so at least 2 orders of magnitude less energetic than your incoming electrons. These SE (
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