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When a non-conductive sample is viewed with an electron microscope, it can happen that some electrons from the beam can be trapped (absorbed) by the sample so it acquires a charge. This is called "charging".

To prevent charging, the non-conductive sample is covered with a thin coat of a conductive material.

I've read that this prevents charging, but I don't understand how the coating works.

If the sample holder is conductive, after depositing the coating the sample will be inside a conductive body (if the coating is in contact with the sample holder). But I'm stuck here because I can't see what this would imply on the charging effect: electrons could break through the coating and reach the non-conductive sample, charging it?

Edit: The coating is usually a few nanometers thick. The electrons from the beam have typically energies of a few keV. In these conditions, I think the energy of those electrons is enough to penetrate some tenths or hundreds of nanometers (it also depends on the material but for metals the order of magnitude is similar), so they can reach the sample below the coating.

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  • $\begingroup$ How far do energetic electrons penetrate a material? And why do you worry about a Faraday cage effect? $\endgroup$
    – Jon Custer
    Commented Sep 16, 2018 at 16:23
  • $\begingroup$ Jon Custer: Thanks for your reply. I've changed the question so it may be more complete now. I've also noticed that the Faraday cage does not play any role in this case. $\endgroup$
    – falgenint
    Commented Sep 17, 2018 at 9:10

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Yes, electrons can and do penetrate inside the material, but if a conductive coating is used, those electrons either diffuse out of the material or, if they don't, they don't deflect incoming electrons anyway.

This is worth some further explanation. When a sample is illuminated with electrons, some of the electrons cause secondary charge emission - this causes the sample to become positively charged. Some of the electrons embed themselves inside the material. This causes the sample to become negatively charged. It can often be possible to find a balance of energies where these two effects cancel out and samples can be imaged even without conductive coating. But this requires the electron gun to be precisely tuned for the material to be imaged. This is usually not feasible, so a conductive coating is used. This coating helps a great deal in reducing charge buildup, because any stray charges are either captured (and conducted back to ground) or shielded. Shielding happens because, for instance, if there is a negative charge inside the material, a corresponding positive charge will appear on the inside of the conductive coating (think of a capacitor). The net effect will be that an electron moving towards the outside surface won't 'see' the charge and won't be deflected.

See this article.

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  • $\begingroup$ Al Nejati: Thank you for your answer. I understand that if the coating is not grounded and a negative charge is hosted inside the sample, the coating will polarize. So I guess that the coating must be always grounded to prevent the negative charge to appear on the outer surface in order to only preserve the positive charge on the inner side of the coating? $\endgroup$
    – falgenint
    Commented Sep 25, 2018 at 17:27
  • $\begingroup$ Yes that's correct. $\endgroup$
    – A Nejati
    Commented Sep 25, 2018 at 20:08

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