Underappreciated and Very Useful: The Case for Microprobe-STEM

Hello, fellow EM aficionados! When you switch your Themis Z S/TEM from TEM to STEM mode, the instrument will automatically switch the illumination system from microprobe (uP) to nanoprobe (nP) mode, but did you also know you can perform STEM in uP mode, too? To be sure, you absolutely need to use nP-STEM mode if wanting to perform any type of imaging/analysis that is at (or very near) the atomic level, but there are many scenarios where uP-STEM will actually be more effective and appropriate.
So, let's talk about uP-STEM mode and how it is different compared to nP-STEM while presenting lots of interesting data to demonstrate just how useful it can be when implemented appropriately.
Thank you for your support, my fellow EM aficionados! Please like, subscribe, and share and leave any questions or comments you may have and I will do my best to reply as soon as possible. Video topic requests are always welcome and appreciated; I enjoy making these videos and wish I could make them more frequently, but the demands of my job make it tough to do so; I’m in charge of three S/TEMs, two dual FIB/SEM systems, and half of an SEM and this keeps me very busy!
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[@adrianzavodov6745]

Nick, thank you very much for this video and for remembering me that current in microprobe is much bigger. Earlier, on Tecnai G2, 90 % of time i worked in STEM microprobe. The main reason, i used mProbe - strong diffraction contrast in BF, so useful in visualisation of metals and alloys. There is a big overlapping of central and diffraction spots in nProbe, and as a sequence brightfield images are awful, as in TEM without objective diaphragm. But in mProbe this problem is absents. And especially thanks for table with alignments procedures for both modes!

[@NicholasRudawski]

You're welcome; yes uP-STEM is definitely advantageous when you are trying to avoid disc overlaps in the DPs and form true BF or DF STEM images to improve contrast. I am planning to do another MSA webinar in mid November specifically about STEM imaging so that would be a great opportunity to discuss this (as well as the comparison with nP-STEM mode I did here).

[@thelmaswan814]

Dear Nicholas, THANK you for your nice video, and I hope you and your loved ones will remain safe during this Florida storm.

[@NicholasRudawski]

You're welcome, and thanks so much for your well wishes. We were actually quite lucky and spared anything remotely close to serious and we never lost power. The good news of the university being closed was that it gave me an opportunity to record and upload this video!

[@electronmicroscopist1986]

Hello Nicholas, thanks for bringing awareness to µP-STEM! It's a great mode for applications where one can get away with non-atomic-resolution (which are plenty!). The usage with the increased depth-of-field for the nanoparticles is very useful. One other important feature: The ability to acquire "useful" diffraction patterns (in the sense of - often - separated Bragg spots) by simply dragging the beam around with the beam position marker tool in the same mode as STEM imaging and EDS! On the contrary, I think "nP TEM" is even less appreciated and might be an idea for a video (parallel illumination for smallest possible area)? Then you have all combinations of TEM/STEM µP/nP complete. :-)

[@jeroenhoi98]

Thank you for the helpful video. One question I hope you could clarify is wether the depth of field and the resolution would be the same for the microscope aligned in microprobe mode at 3mrad and one aligned in nanoprobe mode with an aperture limiting the convergence angle to 3mrad? And would the probe currents be the same? Again, thank you for making all these amazing videos and for showing the cool results.

[@NicholasRudawski]

You're welcome, glad to hear you found my video helpful. Your question is a very interesting one indeed. Should there be any difference between nP and uP if the convergence semi-angle and probe currents are the same in both cases? Well, in principle, there should be no difference because angles are angles and current is current, right? With the Themis being a 3 condenser lens system, it has the capability to arbitrarily set the value of alpha in either mode, but I don't know if nP has the range to obtain alpha < 3 mrad. It actually may be easier to make alpha larger in uP mode to be comparable to alpha in nP mode. Even with alpha being the same, I believe there would be a discrepancy with the currents in a 3 condenser system because of the "zoom" between C2 and C3 which causes the current to decrease as alpha is decreased.

[@tymofiipieshkov3819]

Hm... on our Titan people use microprobe only for 4D-STEM with EMPAD that we have, I did not know that it actually has some benefits :) Although, knowing that we have some histeresis after switching the modes and the need to realign everything every time seems like too much just because of better depth of field. I'll try it next time I'll need BF, though. The other thing is if you're doing CBED, how much would that influence the resulting pattern? I saw some people saying that the discs won't be overlapping?

[@NicholasRudawski]

Yes, you are correct about the discs not overlapping (or at least overlapping less) in uP-STEM versus nP-STEM. I actually discussed this in a video about CBED I did several years ago using our Tecnai. You are also correct that true BF-STEM is not really possible in nP because of the discs overlapping, so uP also has an advantage here, too. It would be interesting to do an image comparison of BF-STEM in nP versus uP using a material with lots of dislocations. This would definitely be another good video to do at some point.

[@Pkshah420]

So when you are turning the focus Knob, whose focal length it changes: upper or lower objective?

[@NicholasRudawski]

Great question! While the OL effectively acts like two lenses, these cannot be controlled independently, so you are changing the strength (focal length) of both at the same time. This is why the MC coil is present in the upper pole piece, to reduce the effect of the OL pre-field when it (the MC) is (optically) on, which corresponds to microprobe mode. Otherwise, the system would only have nanoprobe mode.