FEI Tecnai F20 S/TEM: 2-beam imaging
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- เผยแพร่เมื่อ 3 ต.ค. 2024
- This is a demonstration of the operation of an FEI Tecnai F20 scanning/transmission electron microscope by Dr. Nicholas Rudawski of the University of Florida; this demonstration covers 2-beam imaging in TEM (conventional) mode including:
1. Basic considerations for 2-beam imaging
2. Setting up a 2-beam condition
3. Bright-field imaging from a 2-beam condition
4. Centered (axial) dark-field imaging from a 2-beam condition
5. Weak-beam dark-field imaging from a 2-beam condition
I cannot believe this is a free video...
Ha ha, no problem; I have this channel as a free service to the electron microscopy community.
Dear Nicholas, thank you very much for this video. It's very helpful.
You're welcome, I'm glad you found it helpful.
Thanks Nicholas! Your videos are super helpful to the TEM course I'm taking. Great work!
Hi Zhitao: thanks, I'm glad to hear my videos help you!
Great work! Kudos from Northwestern University! Thanks a lot!
Thank you! More demos covering other topics with be coming soon.
Nice video !!! It would be great if you would demonstrate weak beam imaging of dislocations .... it looks quite nine as well !
Glad you liked it; I have a lot of specimens with these small loops, but I don't really have any with nice, long, linear dislocations, but those would be good to use in a remake.
Hi Nicho, thanks very much for the great video. It is really very illustrative! I am just wondering if you could recommend some practical reading materials. Many thanks in advance
Hi Dr. Nicholas, I am confused between centered-dark field (CDF) and weak-beam dark field. Beam is tilted to adjust +g spot to the center for both case, is it right?
Hi Sumi: the difference between CDF and WBDF is how the beam is tilted. Starting from a 2-beam condition, you tilt to move the direct beam to the position of +g for a CDF image; for a WBDF image, you tilt to move to move the direct beam to the position of -g (opposite direction of what you did for a CDF image).
Thank you for your reply. I have one more question! At 12:30 and 19:30, why both direct spot and bragg spot are located at the exact same position is important? It is just for 16:50 situation? or It is for the better image condition? (CDF image). If you select bragg spot directly using objective aperture, It shows poor focus because it is not a CDF image? I think I get a answer while I give you a question ha ha 😀. When I do like this, beam is blocked by aperture while I switch the BF and DF image.
In this case, same position is possible because direct beam is moved to (+G) diffracted beam condition and (-G) diffracted beam condition is moved to direct beam condition. but some cases like 3G condition, I think exact same position is difficult because the position of objective aperture is same, but the direct beam spot and diffracted beam spot position is different. In this case, multifunction X and Y is work only for dark field beam tilt. I use Talos F200X model, but I suffer from get a nice focus at the bright field image...
Thank you so much for making this video. I wonder how could you be sure the left beam is direct beam and the right beam is diffracted beam at 8:45. Like 9:36, the left beam could be a diffracted beam (-g) condition. I couldn't find excess of deficient line at that moment
You're welcome; you are right, it can be tough sometimes to distinguish between the two beams just visually. The best thing to avoid confusion is to make sure the direct beam is centered on the screen when you still know it is the direct beam and before you do any tilting of the beam. If you look at timestamps you mentioned, you will see the direct beam was always in the middle of the screen.
@ 12:33 you already brought the -G bragg spot at the location of the direct spot. So, I don't get why you are doing this again for centering the Objective aperture.
Hi Manvi: basically, this is just fine tuning the beam tilt so the Bragg beam is perfectly along the optic axis. If you start by centering the objective aperture precisely around the direct beam, then you can use the center of the objective aperture as the reference point to make sure when you tilt the Bragg beam, it goes precisely to the same position. If you do this without the aid of the objective aperture, the Bragg beam may be off from the optic axis when you form the image. I hope this helps.
Fantastic video! One question - what is the reasoning behind using Spot Size 1 for two-beam imaging?
Hi Kyle: thank you; at spot size 1, you have the most beam current. Since you are already forming the image with a "weak" beam, this helps to improve the signal to noise and keep your exposure times reasonable. Sometimes, particularly if imaging wide areas at lower magnifications, the C2 aperture can also be increased in size to further boost the beam current and further improve the signal to noise.
Nice video. Can you give some numbers to get an idea about the alpha and beta tilt needed to go from zone to 2 beam condition? Of course it will depend on the sample, zone and g vector. But knowing the ball park figure will be useful while setting up.
Hi Anuj: I'm glad you liked the video; you are indeed correct that giving exact values for the tilts isn't really practically possible. That being said, assuming you prepared the sample so you are close to the zone axis of interest, the amount of tilting you would need to do for either axis shouldn't be more than few degrees (at most).
please make video on kikuchi alingn
Hi Nicholas. Thanks for uploading this helpful tutorial. At 6:40, the sample is under 2-beam approximation. I have a slight concern on it. There are two spots(one on the left side of T beam and the other is on the right side) are close to their own Kikuchi lines. Is this still safe to say the sample is under 2-beam approximation? If the sample have some slight local deformation, e.g. bending , more wedged shape, high density of dislocations, etc., it is very likely to let one of these two spots sit on its own Kikuchi line, which we may actually have three beam approximation.
Hi Yao: glad you found the video helpful. Remember that when you illuminate an area of the specimen and you then go into diffraction mode, you are seeing a contribution from everywhere the beam is illuminating in the resulting pattern so you can sort of think of that as like an "average" for the whole area. It is entirely possible if the sample has deformed or defective parts that there may be points in the image that are not actually at the two-beam condition you are seeing, which is exactly the case here for those little dislocation loops and that is precisely why 2-beam imaging works. If the material is more defective than non-defective, I can see how what you are talking about may be an issue, but I have never encountered case like that; there is pretty much always more non-defective than defective material even in material that is considered "highly defective".
thanks for the great description. However, at 5.36, you plan to creat a 220 two beam condition as you said, but then the established one looks like 002 two beam condition. Is it right?
Hi Pan: it is indeed a 220 2-beam condition; the zone axis is [001] and the material is Si (diamond cubic) so the closest 4 spots are 220-type.
Thank you for this great video. It is really helpful for me.
And I have a question. Can I ask the meaning of switch on the dark-field button and control the MF nob? I think it is to tilt the incident beam, is it right?
Thank you, I'm glad you found the video helpful! The dark-field button essentially is a toggle switch that switches between two different beam tilt conditions; when activated, it uses the tilt condition for dark-field. The MF knobs, as you correctly stated, tilt the incident beam.
Hello, I am your fans. Recently, I am learning to operate a TEM. About how to get a 2-beam condition, I am still confused. Normally, we get a diffraction pattern with SAED. In this case, there are only spots, not Kikuchi lines. As far as I know, we should tilt the specimen to make the middle Kikuchi lines coincide with 0 and g spot under the zone axis (may be deviate to the g to get S>0). I don’t know how to make the line and spot appear together. Do not use the selection aperture? This will appear polycrystalline ring. Or stop down to concentrate light on a single grain? This will only appear Kikuchi lines? I guessed. So confused. In addition, there are not Kikuchi lines in the thin sample or the sample with many defects. What should we do to get 2-beam condition?
Hi Deng: getting the spots and lines to appear simultaneously is about having the right sample thickness. If the sample is too thin, you won't see any lines, but if it's too thick, the spots will be harder to see; if you aren't seeing both the lines and spots at the same time, the best thing to do is try and analyze a different area.
Thank you so much for making this video. I just have a general question. That is, even without setting up a 2 beam condition, we can still capture a centered dark field image by going to dark field mode and bringing any one diffracted Bragg spot to the position of the direct beam, right?
Thank you! You are correct, you can capture a centered dark field image without setting up a 2 beam condition, but it is generally implied that a 2 beam condition was used without any additional specification.
@@NicholasRudawski Thanks again. Hope to see more such videos on your channel.
Very good video! I just have two quick questions about the accelerating voltage and sample thickness. (a) Is it much better to use lower high tension, say 80 kV, for 2-beam/WBDF experiments? My samples are rather thin, and it's very difficult to tilt it till the real 2-beam condition with 300 kV. There are always some extra spots, especially the systematic ng spots. (b) I rarely see any Kikuchi lines, which may be due to my thin samples. Is there any way to tell the excitation error in this case? Thank you!
Hi Ren, I'm glad you liked the video. If you can't see any Kikuchi lines, it can be pretty tough to accurately determine the excitation error. Thinner is usually always better in TEM, but with WBDF this can be problematic because you will lose the Kikuchi lines; if you have some thicker spots on your samples (just thick enough to see the Kikuchi lines), I would give those a try before dropping the voltage.
Hi again, is there a freeware to calculate holder tilt (position) to get to two beam condition? What I mean is say we are at a zone axis at cetain (a, b) holder tilt. can we use the diff pattern, import it to a software and calculate the holder position for certain two beam condition? is there any available freeware to do that?
Hi Anuj: you can determine how far away from the zone axis you are tilted to get the 2 beam condition by measuring the radius of the Laue circle (the circle the spots that goes through the direct beam); the larger the radius, the farther you are tilted. You don't need software to do this as long as you know information about the d spacing of the spots in the zone axis you are near. This would actually be a good topic for a video, so thanks for the suggestion.
Hi Nicholas, great video! Do you have any recommended readings for learning more about 2 beam imaging and g.b analysis?
Thanks,
Nigel
Thank you! Williams and Carter: sections 22.5, 25.5, 26.1 - 26.6, and 27.1 - 27.5; Fultz and Howe: sections 7.7, 7.8, and 7.10 - 7.12; I hope this helps.
@@NicholasRudawski Thank you, I will check those out.
why do we need 2beam ?
You don't technically need a 2-beam condition to image faults; however, the main reason why it is useful is because a 2-beam condition isolates diffraction from a single set of crystallographic planes and makes it possible to analyze faults in the sample using a g dot b analysis (which I didn't cover in the video); this is actually a good topic for a future video, but I need a suitable sample for this (the sample used in this video isn't good for this because the faults are very small).
@@NicholasRudawski Many thanks for your response