FEI Tecnai F20 S/TEM: convergent beam electron diffraction (CBED)

The long-awaited video on CBED is finally here (thanks again for everyone's patience)! I discuss what CBED is (specifically compared with selected area diffraction), how to perform it (instrument settings, recording patterns with a digital camera, etc), and some of the information it can provide that selected area diffraction does not provide (by using a test specimen containing two crystal structures with and without inversion symmetry: Si and GaN).
If you have not watched the video on selected area diffraction, I recommend you watch that video first:
• FEI Tecnai F20 S/TEM: ...
Thanks for watching; if you have questions, comments, or a topic suggestion for a future video, please let me know.

[@kesh3415]

Thank you so much for this video. As usual, a very helpful, informative, and explanatory
video to guide young microscopists.

[@NicholasRudawski]

You're welcome; I have more videos planned, too!

[@renqiu3449]

Very good video! Could you plan a tutorial on parallel nano beam electron diffraction? ( don't know if you can do it on Tecnai though

[@us_visa_apply]

Thanks for the video.
How does the CBED HOLZ intensity change as a function of accelerating voltage?

[@nikhildeo1329]

Thanks a lot for the detailed video. Is there a video on interpretation of CBED patterns to determine the structure?

[@NicholasRudawski]

Hi Nikhil: you're welcome; so the interpretation of the patterns to determine the structure basically comes down to computational work and simulations. You basically build a candidate structure, then simulate the CBED pattern you would obtain from it using the known parameters of your experimental CBED pattern/sample (things like wavelength, beam semi-angle of convergence, sample thickness, etc). You basically keep adjusting the model in the simulation until the simulated CBED pattern agrees with what you are seeing (measured CBED pattern) and then you can claim to have revealed the structure. Of course, the fewer the number of parameters you need to adjust in your model, the better and more robust your results tend to be. The impact of sample thickness in CBED is very important, too, and often this ends up being another free parameter adjusted in the simulation (since it is often difficult to directly measure). I hope this helps.

[@saraa2636]

Thank you very much for all your videos. How can I obtain STEM detector map?

[@NicholasRudawski]

Do you mean imaging the STEM detector while in STEM mode? If you want to do that, make sure the specimen is out of the field of view, then just turn off the "Diffraction" button on the right hand panel while you are in STEM mode and your STEM image should now be the STEM detector.

[@saraa2636]

Excuse me, how you aligned the CBED pattern with the zone axis? You used the trackball (the condenser shift)?

[@NicholasRudawski]

Hi Sara: I used the trackball (condenser shift) to position the beam on a spot on the specimen that was aligned with the zone axis, so I didn't tilt the specimen, I just fine-tuned the beam position until I found an aligned spot. This trick works well as long as the region you are searching in is already reasonably aligned with the zone axis.

[@mdredwanulislam3727]

At the end of the video where you compared CBED with nanoprobe and microprobe. In the nanoprobe image the scale bar is a real value but in the microprobe mode it's in reciprocal value. Were you at the diffraction mode while taking nanoprobe CBED? Or in Nanoprobe you don't need to go press diffraction?

[@NicholasRudawski]

Hi Md: if the scale bar is reciprocal length, then I was in diffraction mode, so yes, when I acquired the CBED pattern I was in diffraction mode. When you switch into Nanoprobe mode, the system doesn't automatically go into diffraction mode; you have to manually push the diffraction button for that to happen.

[@Ran_HIT]

Many thanks for your professional and helpful video. Accordingly, i try to perform the CBED by the Talos 200. however, the center spot is very intensity, and even i can't see anything in the center spot. i have adjusted to the spot size to 8, and decrease the exposure time. could you give me some suggestion to solve this probelm? Thanks very much

[@NicholasRudawski]

You're welcome; I did the best I could just using a hand-held iPhone. Regarding your question, do you know approximately how much probe current you are getting with these settings? Assuming you have the beam properly focused on the sample before going into diffraction mode, the issue may be the camera length you are using (maybe it is simply too short). If you increase the camera length, this will magnify the spots more and spread the signal out over more pixels. What camera length are you using when you try to acquire the CBED patterns?

[@yuan_tu]

Nice video! @2:14, where to adjust the camera length L?

[@yuan_tu]

did the camera length mean the same with Magnification?

[@NicholasRudawski]

@@yuan_tu "Camera Length" is the term used to describe the magnification of a diffraction pattern; once you go into diffraction mode, you adjust this by using the "Magnification" knob on the right hand panel.

[@yuan_tu]

@@NicholasRudawski Thank you, Nicholas, I understand the equation Rd=Lλ. So changing the camera length is actually achieved by adjusting which lens? intermediate or objective?

[@yuan_tu]

for CBED work, the SA aperture is always inserted?

[@yuan_tu]

what did you do @21:30?

[@yuan_tu]

@28:38, why rotate the CBED pattern?

[@NicholasRudawski]

You don't need an SA aperture for CBED; the size of the beam is what effectively limits the area.

[@NicholasRudawski]

@@yuan_tu Please see the video I have on "rotation calibration"

[@yuan_tu]

@@NicholasRudawski I understand the probe size is smaller than SA aperture, but will the SA aperture limit the total electron accounts, and therefore reduce the disk pattern intensity? Anyway, I'll have a try tomorrow.