I want to do compositional analysis of doped colloidal CsPbBr3 nanocrystals, by STEM-EDX.
However, even for short exposures, the electron beam induces carbon buildup on the sample, which dominates the EDX signal, and it becomes impossible to measure anything else.
Can someone indicate if and how they managed to do this type of measurement? Is the carbon buildup due to the organic ligands which are not attached to the nanocrystals? is there a simple way to remove them form the solution, to enable the measurement? Please note that I am not doing the measurement myself, so my knowledge of the technique is not very extensive.
Dear Claudiu,
carbon contaminations are a "traditional" problem of electron beam excitation. Usually, the carbon layers are due to a vacuum generated with pumps using organic fuels. This can be forepumps or diffusion pumps. Such conventional systems are unsufficient for chemical analysis in the nanorange. The pumpoil is cracked by the electron beam and deposites on the sample surface.
Nevertheless, the perovskites ought to be detected. But you must know that the counting rate is very low because the electron beam penetrates the sample if its thickness is smaller than 1 µm. Due to this, you need high counting times and the carbon layer grows. Whereas the measurement the count rate of the sample decreases and the carbon count rate increases. Consequently, the count rate of the sample vanishes in the background.
It must be pointed out: Nanotechnology is a very pure technology. For exact measurements you need ultrahigh vacuum.
With Regards
R. Mitdank
Dear Rüdiger,
Thank you for the reply. Would the trace amounts of oil in the vacuum result in such a thick carbon film, that effectively blocks all the signal from the sample within 30 seconds of exposure?
Kind regards,
Claudiu
I would expect that you can detect the elements in the sample. Especially because you have high atomic numbers, the X-rays are hardly absorbed by carbon. The other condition is the energy used. To excite the sample volume you need low energies. To excite the elements you need high energies. I assume, you try to detect the M-lines or L-lines. The best chance, I believe is to look for the Pb-lines due to their high absorption you need a lower sample thickness. The M-alfa energy is 2,34 keV transition energy (so you need only 5 keV excitation) or L-alfa (10,5 keV). This is well to excite with 15 keV electrons. With such energies you penetrate the carbon layer and the X-rays do so. Perhaps it helpes.
With Regards
R. Mitdank
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