Why is the resolution of TEMs and SEMs not smaller?

[ccrook]

I was wondering if someone could offer an explanation as to why TEMs and SEMs have practical resolutions several orders of magnitude less than what is predicted by the Rayleigh Criterion. This of course comes from my own calculations of the Rayleigh Criterion assuming an electron is accelerated by 300kV and 10 kV field respectively and correcting for relativity in the electron's speed. In the TEM case, I found the electron wavelength to be approximately 2e-12 m and the SEM 1.2e-11 m, which are much smaller than 0.5 angstrom and 50-100 nm observed. Thank you!

 

[BvU]

Perhaps page 10 here ? Abbe's equation ##\ d = 0.61 \lambda/{\rm NA}\ ##. The numerical aperture of the objective comes in too.

 

[Andy Resnick]

I was wondering if someone could offer an explanation as to why TEMs and SEMs have practical resolutions several orders of magnitude less than what is predicted by the Rayleigh Criterion. This of course comes from my own calculations of the Rayleigh Criterion assuming an electron is accelerated by 300kV and 10 kV field respectively and correcting for relativity in the electron's speed. In the TEM case, I found the electron wavelength to be approximately 2e-12 m and the SEM 1.2e-11 m, which are much smaller than 0.5 angstrom and 50-100 nm observed. Thank you!

One reason is that the magnetic lenses used suffer from large amounts of spherical aberration and astigmatism. There has been some good progress:

http://rsta.royalsocietypublishing.org/content/367/1903/3637
https://en.wikipedia.org/wiki/Transmission_Electron_Aberration-Corrected_Microscope