50,000,000x Magnification

[Applause] all right so hey there everybody i am back in the microscopy suite in the materials department at ucsb and i'm going to show you some atoms so this huge like car sized box and these four monitors the huge cylinder behind those four monitors and some more equipment that's behind that back wall together constitute an instrument called a scanning transmission electron microscope or stem and today i'm going to show you exactly how this works so my last microscopy video was making a play button which was kind of a joke but today i'm going to be doing some real research i'm going to be looking for a specific defect in a crystalline material called tin selenide and i'm not sure that i'm going to be able to find it but either way i'll learn something so this is going to be like real research happening right before your eyes on youtube today [Music] now in order to actually blast electrons all the way through a sample of material in order to image its atoms that sample needs to be really really really thin imagine you know shining a piece of light through a piece of light trying to shine a light through a piece of paper you can see some light through a piece of paper but now imagine trying to shine a light through a piece of plywood it's just not going to work so they're both the same thing they're both cellulose but one is a lot thicker and a lot denser so when you're trying to make a transmission sample that you can shoot electrons all the way through it needs to more resemble a sheet of paper except a bunch of orders of magnitude thinner i didn't actually film the sample preparation but if you remember the focused ion beam from one of my previous videos that's the machine that you use to create these unreasonably impossibly thin samples for reference the sample i'm about to look at here is mounted on this tiny flake of copper but to see it we're going to have to zoom in on this flake of copper there is actually a crescent shape and in the crescent shape there are a few metal posts if you zoom in on one of those posts you can see a bump on the side that's my sample looking even closer hopefully you can appreciate how this entire sample is a vanishingly thin flag of material it's about 6 micrometers tall 14 micrometers long and between 50 and 100 nanometers thin at the spot where i'll be imaging somewhere in the range of about 150 atoms thick and this is what we'll be shooting electrons through to cast our shadows even though this copper crescent is like vaguely a hundred thousand times larger than the dimension we care about on the sample it's holding it's still small enough that it's a pain to work with so to help we've got these fancy vacuum tweezers basically a tiny hose that sucks up the sample like a piece of cardboard stuck to a shop vac this is more than a little finicky and while trying to bend around to give the camera a good view i actually dropped it the first time ah crud something like this could only happen when i tried to film it thankfully the sample itself was all right it's also worth appreciating that even this tiny sample stage is itself a miniature engineering marvel it's designed to tilt the sample in two directions without translating it in x y or z by more than a couple microns it's absurdly precise this is actually some microscope footage from a time that the holder actually broke i swear i was not pushing on the clip any harder than i should have been but you can see that the hinge here that broke isn't a normal ball bearing or something like that it's actually a ruby like literally a polished gemstone that runs in a groove so now that the sample is mounted inside of this chuck if you'll follow me around the corner in here we have the actual microscope like all electron microscopes the stem here is basically just a big multi-compartment vacuum chamber which means that this thing when i load it into the chamber needs to be able to stop all of the air from rushing in behind it when i put this in it's actually going to create a seal at this o-ring and then i'm going to have to wait for a few minutes while it pumps out the air lock before i can push it all the way into the microscope [Music] there we go that was the pump start and don't worry if i made trying to load that sample look awfully tedious just wait till i get to the part where i focus the microscope for an hour just like that air pressure pushes it right on in i don't even have to do anything a transmission electron microscope is basically casting shadows with a beam of electrons you're shooting a high energy beam of electrons all the way through a material and you're projecting whatever gets through out onto a screen and looking at that image now because it is so much easier to visualize this when you don't use electrons i'm going to be doing a demonstration with photons if i have a point source of light like this flashlight with a piece of poster board with pinhole in it any shape that i put in front of the light casts a shadow on this screen and critically the shadow is actually magnified based on the relative distances between the source object and screen huge wrench small wrench big wrench small wrench i need to get out more but if you notice the edges of this shadow are a little bit fuzzy if we try to look at something really tiny let's say these ball bearings are like atoms being imaged in a microscope this method doesn't really let us see anything one of the multiple problems here is that the pinhole isn't actually a very good point source it's almost the same size as the features that we're trying to image so light from one edge of the pinhole and light from the other edge of the pinhole actually cast very slightly different overlapping shadows and in order to accurately image the atoms then all our electrons would need to originate from a point in space that's smaller than an atom and we can't do that if we're slightly more clever we can use lenses to project a sharp image even with a less than perfect source of light here i've set a lens so that as light exits this pinhole it's actually made into a nearly parallel beam that parallel beam passes through the ball bearings and a shadow is created and then we can take that beam and if everything is lined up just right blow it up to project a larger shadow on the screen once we send it through here it gets smaller comes down to a point and then way blows up and expands out here so we have a magnified image on the screen remarkably sharp much much better than i thought i'd get with this those are our atoms just imagine that instead of a flashlight with a pinhole we have an electron gun powered by a huge high voltage power supply and instead of glass lenses we have really big electromagnets that bend that beam of electrons as it's passing through the microscope now compared to our flashlight model the electron gun is at the top of this system and it's focused by a series of magnetic lenses capable of bending the electron beam onto the sample and then through the sample and then through more lenses that expand that image into a camera now when we turn this whole shebang on we literally just get a shadow image of the copper crescent pretty cool right yeah so here you can really see that the uh the place where there is nothing like this you know you can just see bright because that's where the electrons are hitting un uninhibited this thing is the big chunk of copper that the electrons can't get through so it looks black and this is the sample which is so thin that it's electron transparent and it actually sort of looks gray because it'll let some electrons through which is which is kind of the point you can see huge bits of strain and bending in the sample as it's sort of warping around it's really interesting right now what we need to do is basically get the beam pointed at the sample and get the sample pointed at the beam getting the beam pointed at the sample sounds pretty straightforward but getting the sample pointed at the beam that's a weirder phrase you need to remember that we're dealing with a crystal this perfect 3d grid of atoms in space now earlier i used the ball bearings to emulate a single layer of atoms but what if i had two layers or three layers or 10 layers it gets a lot harder to tilt the sample so you can look straight down a column of atoms this is another reason that we like to keep the samples really super thin uh the sample looks a lot different now that's because we're actually focused on it which is you know the first step the next step is to get the sample tilted so that it's actually pointed at us the sample need not load in perfectly flat with respect to the beam the good news is that we can use the electrons passing through the sample and making diffraction patterns to actually help us line this up so we've got this awesome starburst pattern called a kakuchi pattern and basically just want to line this up so that we're looking at the center of this starburst which is called the zone axis make it look real starbursty ah not really the sample might be a little a little thin for that so i hope that explanation made sense because i'm about to rip the rug out from underneath it while it's possible to image atomic scale features using this very technique where you're basically casting shadows of atoms all these extra fringes and patterns sometimes make it difficult to interpret what you're looking at which is why at some point somebody added one extra letter high resolution transmission electron microscopy becoming high resolution scanning transmission electron microscopy linguistically almost the same physically mechanistically completely different things so as soon as i push this button right here marked stem the microscope is going to switch modes and all of my explanations so far and a lot of this talk of projected shadows is going to have to change all right so now we've switched modes where tem was pretty well represented by a flashlight and some lenses to create big parallel beams stem or stem is probably best represented by a laser mounted on a gimbal and uh that is to say it's a lot more fun so what does it mean to scan an image basically instead of taking the whole image at once you're rastering the beam back and forth taking the image one pixel at a time and in order to do that the electron beam isn't a parallel beam that covers the entire sample illuminating it at once you actually focus the beam down to a point that is smaller than an atom so that's basically what's happening here where the gimbal is guiding this laser back and forth rastering across our field of ping-pong ball atoms sometimes the laser hits an atom and causes light to scatter in all directions and sometimes the laser doesn't hit an atom goes straight through and runs into the back plate so now i'm playing part of the microscope sensor i know where the gimbal is pointed at all times and i can tell whether or not the beam is going through because i can look for the green dot on the piece of cardboard but i can't actually see the sample in reality this is because the sample is microscopic and therefore basically invisible but this is the exact procedure that the computer does when it's rastering the beam over the sample trying to find atoms you can tell that i'm making a couple mistakes in counting here apparently my counting in my head is a little bit faster than the gimbal actually does because i keep not having enough room at the end of my rows but as i fill this grid in we can start to see the shape of the 16 ping-pong balls that are forming our atoms in a real microscope what i'm doing here would be known as a bright field image i've just inverted it so after minutes of filling in grid spots i now have my picture of atoms with some scan artifacts because i can't actually count and if you imagine this happening really fast where the computer guides the tiny beam of electrons across a sample line by line over and over again you can imagine that images can get formed pretty quickly in fact that's what these scan lines are when you're you know watching any image on the screen it's going over and over and over again continuously refreshing the image one line at a time it's actually scanning the electron beam back and forth line by line to rebuild the picture the real trick now is that the beam needs to be focused to a smaller point as possible if the beam can illuminate multiple atoms at once then you just get a blurred out signal because you're always hitting something with the beam and always letting some of the beam through if i were to replace this laser with the flashlight from before you can see that no matter where the flashlight points there's always some light getting to the detector at the back which means that it can't really tell where the atoms are the flashlight light is just too wide which is why when setting up a stem measurement i take quite a long time here running through all of these different modes sort of oscillating the focus of the beam and playing all sorts of tricks in order to get it focused down to a tight as spot as possible the smaller that that beam gets the better the resolution of the images that i'll be able to form and there we go now when we focus we have a nice even beam it gets really small still a little bit of stig see and go over here to stem and now we're scanning it's exciting gotta zoom out a little bit and hopefully we will see the sample there we go spin the scan around a little bit and get it flat so while on location brian struggles to focus and zoom in let me tell you what this sample actually is if you've watched my previous video about evaporating metals and making molecular beams you may remember that our lab specializes in depositing extraordinarily thin layers of crystals crystalline materials in this big vacuum chamber that means that we end up with these multi-layer sandwich kind of structures if you slice a cross section out of one of these stacks then you get to see all of the layers and find out if your process work in this case we started with a wafer of a semiconductor called gallium arsenide deposited an extremely thin layer of lead selenide and then a reasonably thick layer of tin selenide these two layers on top are actually the weird carbon and platinum slurry that i talked about in the fib video that basically just get welded to the top of the stack to protect it while i get it ready for the microscope so you can mostly ignore those but this substrate the gallium arsenide that's the place with the highest structural quality but because we didn't make it but that means that it's the easiest place to see atoms so now if we want to see atoms all we got to do is zoom in and look at that atoms [Laughter] not very focused adams i'm moving diff shift oh wait a minute i got a line i shoot i didn't retilt iterative lots of steps need to be done more than once getting bigger getting smaller getting smaller getting bigger turn it back man this is a bendy film piece of junk who made this [Music] there we go look at that atoms these are gallium and arsenic atoms to be precise if you're wondering how this grid of dots actually corresponds to the gallium arsenide unit cell like the crystalline pattern we're actually looking at it from a diagonal like this and then if all the atoms were a little bit blurry there you go so each of these fuzzy blobs is actually two adjacent columns of atoms one column of arsenics and one column of galliums so if you're not convinced by looking at all of these really fuzzy little blobs here is a much better picture this is probably my favorite stem picture that i've ever taken and after a much longer exposure time a lot of you know waiting for all the drift to go away and stuff like that and quite a bit of processing in matlab after the fact you can very clearly see the double columns of atoms here we call them dumbbells of gallium arsenide and in the top you'll notice that the pattern is actually different it doesn't have the dumbbells this is lead selenide which forms a completely different crystal structure and what's happening in between these two weird sort of fuzzy blobs that look like dislocations but aren't really dislocations i think is really interesting but unfortunately it is outside the scope of this youtube video so when i was filming this video i was actually looking at tin selenide which is a different material and ten selenide has layers so i passed through the gallium arsenide the lead selenide and now the tin selenide let's try to find some some cool looking defects although it appears we've lost our focus this sample is not flat holy mackerel oh i had it for a second there we go check it out adams in order to get a good picture of this what i actually need to do is take a bunch of images and then stack them so it's microscopy looking at the smallest of things has a surprising overlap with astrophotography where you're looking at the largest of things so right now i'm taking a sequence so it's going to take a whole bunch of pictures and you can notice that the whole frame is shifting a little bit each time so that's the drift that you need to correct for the same way that stars in the sky move slowly and you need to correct for that so tin selenide is this layered structure so you can see once we do a little bit of the uh the correction here this is two layers of atoms that are glued together and then below it is two layers of atoms that are glued together and then below it are two layers of atoms that are glued together so you get this sort of double layer structure going through the whole material i'm looking for a location where that layered thing gets screwed up in a very particular way and that's a layer yeah this is not what i'm looking for it's cool it's atoms but it's not what i'm looking for we got a gap we got a gap we got a gap we got a gap we got a gap got a gap we got a gap we got to get all the way across okay no possibly another hyper dislocation i think those must be common which itself would be interesting so the actual defect i'm looking for is basically a hiccup in the layered structure of the material because each layer of tin selenite is two atoms tall and because of some weird stuff happening at the interface we expected that it was possible to get one region of the crystal shifted with respect to the rest and in particular we thought this shift might only be one atom tall which would actually be half of a layer meaning that the top of one layer was bonded to the bottom of the next layer and so on creating this weird zipper structure at the boundary unfortunately even if the sample is chock full of these getting one lined up in a place where i can photograph it requires quite a bit of luck and quite a bit of tedium god it's an ugly picture that is absolutely what i'm looking for okay uh unless the tilt over here is so significant that it's making this not look like what it should look like which i guess is possible i'm gonna assume that i'm not tilted right and i'm gonna pray that i can get back to this exact same spot later okay that big white dot it's next to that big white dot it's [Music] oh oh ah i got one look at this that's between two layers and then it's in the middle of a layer and then unfortunately it's right next to another defect which actually splits it apart again so this is like a this is like a two two partial dislocations right here it breaks it and then it fixes it oh it's so cool oh this will be this will be clean down here neat can't tell but i'm grinning this was annoying i think this is the first time that i've ever gone out looking for a particular defect like i want to see this thing and wasn't sure that it existed this was like three hours of tedious focusing on a really old this is like a more than a year old sample that came out of the fib and uh it's all like warped and stuff which means it's really difficult to focus on and it kept losing focus continuously but uh i did it so now i get to say in a paper that this defect exists in tensile nine because i have a picture of it that is science [Music] so [Music] you

Channel: AlphaPhoenix

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Id: eYVNZgnQ8gE

Length: 23min 39sec (1419 seconds)

Published: Mon Jun 27 2022

Reddit Comments

[u/Bananawamajama]

I can see atoms too.

Lots of them.