WEBVTT 00:00:00.950 --> 00:00:03.320 The following content is provided under a Creative 00:00:03.320 --> 00:00:04.710 Commons license. 00:00:04.710 --> 00:00:06.920 Your support will help MIT OpenCourseWare 00:00:06.920 --> 00:00:11.010 continue to offer high quality educational resources for free. 00:00:11.010 --> 00:00:13.580 To make a donation or to view additional materials 00:00:13.580 --> 00:00:17.540 from hundreds of MIT courses, visit MIT OpenCourseWare 00:00:17.540 --> 00:00:18.420 at ocw.mit.edu. 00:00:22.928 --> 00:00:24.470 MICHAEL SHORT: Anyway, today is going 00:00:24.470 --> 00:00:27.380 to be a lot lighter than the past few days, which have been 00:00:27.380 --> 00:00:28.760 heavy on theory and new stuff. 00:00:28.760 --> 00:00:30.530 And I want to focus today on what can you 00:00:30.530 --> 00:00:33.580 do with the photon and ion interactions with matter. 00:00:33.580 --> 00:00:35.330 So we're going to go through a whole bunch 00:00:35.330 --> 00:00:37.940 of different analytical and materials 00:00:37.940 --> 00:00:40.580 characterization techniques that use the stuff that we've 00:00:40.580 --> 00:00:42.745 been learning and see what you can actually do. 00:00:42.745 --> 00:00:44.120 And I'll be drawing from examples 00:00:44.120 --> 00:00:46.340 from the open literature, from textbooks, 00:00:46.340 --> 00:00:47.430 and from my own work. 00:00:47.430 --> 00:00:50.470 So stuff I was doing here on my PhD thesis 00:00:50.470 --> 00:00:54.620 is actually a direct result of what do we do here in 22.01. 00:00:54.620 --> 00:00:57.433 So a quick review just to get it all on the board of what 00:00:57.433 --> 00:00:58.350 we've been looking at. 00:01:02.540 --> 00:01:05.459 So I don't hit anyone on the way in. 00:01:05.459 --> 00:01:15.350 We talked about different photon interactions, which include 00:01:15.350 --> 00:01:16.580 the photoelectric effect. 00:01:19.560 --> 00:01:22.110 Let's say this will be the energy 00:01:22.110 --> 00:01:26.910 of the scattered whatever, and this will be its cross section. 00:01:26.910 --> 00:01:31.430 We talked about Compton scattering. 00:01:31.430 --> 00:01:33.297 We talked about pair production. 00:01:36.220 --> 00:01:41.500 For the photoelectric effect, the energy of the photoelectron 00:01:41.500 --> 00:01:44.620 comes off like the energy of the gamma 00:01:44.620 --> 00:01:48.220 ray minus some very small difference, the binding 00:01:48.220 --> 00:01:50.050 energy of the electron. 00:01:50.050 --> 00:01:52.730 Let's just call it Eb. 00:01:52.730 --> 00:01:56.523 And this effect starts when you hit what's 00:01:56.523 --> 00:01:57.565 called the work function. 00:02:00.713 --> 00:02:02.380 I'm just going to put this all up there, 00:02:02.380 --> 00:02:04.390 so when we explain the analytical techniques, 00:02:04.390 --> 00:02:06.430 we can point to different bits of this 00:02:06.430 --> 00:02:09.189 and explain why we use these different things. 00:02:12.340 --> 00:02:16.480 The cross-section, I made sure to keep this handy, 00:02:16.480 --> 00:02:18.830 so I don't want to lose it. 00:02:18.830 --> 00:02:20.930 Strongly proportional with z. 00:02:20.930 --> 00:02:25.350 So the cross-section comes out of another line. 00:02:25.350 --> 00:02:26.540 What was it proportional to? 00:02:26.540 --> 00:02:29.330 Oh yeah, this is nuts. 00:02:29.330 --> 00:02:35.160 It's like z to the fifth over energy to the 7/2, 00:02:35.160 --> 00:02:36.860 which says that for higher z materials, 00:02:36.860 --> 00:02:39.800 the photoelectron yield is much, much stronger, 00:02:39.800 --> 00:02:43.340 and it's way more likely that way lower energy. 00:02:43.340 --> 00:02:45.140 So you can imagine if you wanted to use 00:02:45.140 --> 00:02:47.240 this in an analytical technique, and you 00:02:47.240 --> 00:02:49.610 want to study which photoelectrons come from which 00:02:49.610 --> 00:02:53.570 elements, you might think to use a low energy photon to excite 00:02:53.570 --> 00:02:57.410 them, not a high energy photon, because like we had done 00:02:57.410 --> 00:02:59.180 a couple of times before, if we draw 00:02:59.180 --> 00:03:05.295 our energy versus major cross-section range, 00:03:05.295 --> 00:03:07.670 we had a graph that looks something like this, where this 00:03:07.670 --> 00:03:08.878 was the photoelectric effect. 00:03:12.410 --> 00:03:15.230 This was Compton scattering. 00:03:15.230 --> 00:03:19.240 This is pair production. 00:03:19.240 --> 00:03:21.007 And so by knowing what energy-- 00:03:21.007 --> 00:03:21.590 oh, I'm sorry. 00:03:21.590 --> 00:03:23.850 That's supposed to be z. 00:03:23.850 --> 00:03:27.840 And this would give you the dominant process 00:03:27.840 --> 00:03:29.990 that each the combination of energy and z. 00:03:29.990 --> 00:03:32.540 So if you know what energy photons you've got 00:03:32.540 --> 00:03:36.120 and what you're looking for, well, there you go. 00:03:36.120 --> 00:03:36.620 Let's see. 00:03:36.620 --> 00:03:42.367 What was the energy of the Compton electron? 00:03:42.367 --> 00:03:43.700 Remember the wavelength formula. 00:03:47.850 --> 00:03:54.470 It was like alpha 1 minus cosine theta over-- 00:03:54.470 --> 00:03:55.145 let's see. 00:03:59.090 --> 00:04:03.170 Another 1 minus cosine theta. 00:04:03.170 --> 00:04:05.320 In came the gamma ray energy. 00:04:05.320 --> 00:04:07.378 What was the part that came beforehand? 00:04:07.378 --> 00:04:09.170 That's why I have this here because I don't 00:04:09.170 --> 00:04:10.378 want to write anything wrong. 00:04:12.855 --> 00:04:14.605 It's good to have it all up there at once. 00:04:19.450 --> 00:04:22.290 1. 00:04:22.290 --> 00:04:24.090 Yeah. 00:04:24.090 --> 00:04:25.200 That's all I was missing. 00:04:25.200 --> 00:04:26.370 Cool. 00:04:26.370 --> 00:04:29.160 And the cross-section for Compton scattering 00:04:29.160 --> 00:04:33.300 scaled something like z over energy, something pretty 00:04:33.300 --> 00:04:38.090 simple, not nearly as strong as pair production 00:04:38.090 --> 00:04:39.740 or photoelectric effect, so you can 00:04:39.740 --> 00:04:42.050 think Compton scattering happens much more 00:04:42.050 --> 00:04:44.360 dominantly at low z or the other two 00:04:44.360 --> 00:04:45.945 don't really happen that much at low 00:04:45.945 --> 00:04:48.540 z, whichever way you want to think of it. 00:04:48.540 --> 00:04:50.900 And for pair production, you get a whole mess of stuff. 00:04:50.900 --> 00:04:53.210 You get positrons coming out. 00:04:53.210 --> 00:04:57.860 You get a bunch of 511 keV gamma rays 00:04:57.860 --> 00:05:00.350 and all sorts of other things you can detect. 00:05:00.350 --> 00:05:01.820 And the cross-section, this one's 00:05:01.820 --> 00:05:04.460 got the funny scaling term. 00:05:09.530 --> 00:05:11.440 This one, yeah. 00:05:11.440 --> 00:05:12.800 It's like z squared log. 00:05:16.830 --> 00:05:23.470 Energy over mec squared, so some z squared kind of dependence. 00:05:23.470 --> 00:05:26.580 So let's keep those up for now. 00:05:26.580 --> 00:05:30.250 Let's get the electron ones in. 00:05:30.250 --> 00:05:32.440 AUDIENCE: [INAUDIBLE] mez squared? 00:05:32.440 --> 00:05:33.860 MICHAEL SHORT: Was it z squared? 00:05:33.860 --> 00:05:35.600 Let me check. 00:05:35.600 --> 00:05:36.830 No, that's a c. 00:05:36.830 --> 00:05:37.750 AUDIENCE: [INAUDIBLE] 00:05:37.750 --> 00:05:38.583 MICHAEL SHORT: Yeah. 00:05:38.583 --> 00:05:41.670 Yeah, just make sure that's clearly a c squared. 00:05:44.350 --> 00:05:46.860 So now let's call it charged particle, 00:05:46.860 --> 00:05:50.520 or just more generally ion electron interactions. 00:05:54.080 --> 00:05:56.030 Since these are more fresh in our head, 00:05:56.030 --> 00:05:57.770 what are the three ways in which charged 00:05:57.770 --> 00:06:03.730 particles can interact with matter that we talked about? 00:06:03.730 --> 00:06:05.110 Just rattle off any one of them. 00:06:08.278 --> 00:06:09.320 AUDIENCE: Bremsstrahlung? 00:06:09.320 --> 00:06:11.362 MICHAEL SHORT: Yeah, Bremsstrahlung or radiative. 00:06:23.090 --> 00:06:24.968 What else? 00:06:24.968 --> 00:06:25.860 AUDIENCE: [INAUDIBLE] 00:06:25.860 --> 00:06:26.160 MICHAEL SHORT: Is what? 00:06:26.160 --> 00:06:27.035 AUDIENCE: Ionization. 00:06:27.035 --> 00:06:28.146 MICHAEL SHORT: Ionization. 00:06:33.010 --> 00:06:35.470 Which we'll call inelastic collisions. 00:06:38.260 --> 00:06:39.187 And? 00:06:39.187 --> 00:06:40.520 AUDIENCE: Rutherford scattering. 00:06:40.520 --> 00:06:42.644 MICHAEL SHORT: Yep, Rutherford scattering. 00:06:46.520 --> 00:06:50.220 Which are kind of elastic or hard sphere collisions. 00:06:50.220 --> 00:06:53.970 And if we had to make kind of a table of when 00:06:53.970 --> 00:06:56.340 do we care about which effect, let's 00:06:56.340 --> 00:07:01.680 say this was an ion or electron, scattering off 00:07:01.680 --> 00:07:08.590 of either electrons or nuclei, in either elastic or inelastic 00:07:08.590 --> 00:07:09.090 ways. 00:07:12.940 --> 00:07:14.640 First of all, when do we actually 00:07:14.640 --> 00:07:17.550 care about elastic scattering off 00:07:17.550 --> 00:07:20.820 of electrons, which would be hard sphere collisions off 00:07:20.820 --> 00:07:21.570 of electrons? 00:07:26.080 --> 00:07:30.900 To help get you going, in an elastic collision, 00:07:30.900 --> 00:07:35.310 the maximum energy transfer can be this formula gamma 00:07:35.310 --> 00:07:38.370 times the incoming energy, where gamma 00:07:38.370 --> 00:07:42.780 is 4 times the incoming mass times the mass of whatever 00:07:42.780 --> 00:07:46.725 you're hitting over n plus big m squared. 00:07:49.270 --> 00:07:55.930 Let's say if one of these masses was mass of an electron. 00:07:55.930 --> 00:07:58.635 What is gamma approximately equal for most cases? 00:08:06.500 --> 00:08:08.750 Well, let's say this was like electrons scattering off 00:08:08.750 --> 00:08:11.540 of protons or vice versa. 00:08:11.540 --> 00:08:13.130 How much energy could an electron 00:08:13.130 --> 00:08:17.540 transfer to a proton in an elastic collision? 00:08:17.540 --> 00:08:19.610 Basically zero. 00:08:19.610 --> 00:08:23.600 The only time which this actually matters 00:08:23.600 --> 00:08:28.220 is if it's an electron hitting another electron, in which case 00:08:28.220 --> 00:08:30.860 you can have pretty significant energy transfer. 00:08:30.860 --> 00:08:35.270 So I'd say for elastic collisions off of electrons, 00:08:35.270 --> 00:08:39.830 you only care about those for other electrons. 00:08:42.400 --> 00:08:45.290 And I'm going to put in low energy electrons. 00:08:45.290 --> 00:08:47.930 Why do we only care about them for low energy electrons? 00:08:47.930 --> 00:08:51.530 Or in other words, what are the other methods of stopping power 00:08:51.530 --> 00:08:52.850 or interaction-- yeah, Chris. 00:08:52.850 --> 00:08:55.562 AUDIENCE: [INAUDIBLE] 00:08:55.562 --> 00:08:56.520 MICHAEL SHORT: Exactly. 00:08:56.520 --> 00:08:57.270 Yep. 00:08:57.270 --> 00:08:58.920 We already saw that Bremsstrahlung 00:08:58.920 --> 00:09:01.700 the radiated power scales with something like z 00:09:01.700 --> 00:09:04.120 squared over m squared. 00:09:04.120 --> 00:09:06.270 So with a really small mass and a really high z 00:09:06.270 --> 00:09:09.870 and also a higher energy, you end up 00:09:09.870 --> 00:09:12.240 radiating most of that power away as Bremsstrahlung. 00:09:12.240 --> 00:09:15.810 And there's not much of a chance of elastic collision. 00:09:15.810 --> 00:09:18.120 So we only care about low energy electrons 00:09:18.120 --> 00:09:21.660 when it comes to elastic collisions with electrons. 00:09:21.660 --> 00:09:25.410 For inelastic collisions with electrons, 00:09:25.410 --> 00:09:30.210 well, that's the hollow cylinder derivation 00:09:30.210 --> 00:09:32.280 that we had done from before where 00:09:32.280 --> 00:09:38.010 you have some particle with a mass m and a charge little ze, 00:09:38.010 --> 00:09:42.990 getting slightly deflected by feeling the pull-- 00:09:42.990 --> 00:09:46.020 depending on what charge it is, it could be towards or away-- 00:09:46.020 --> 00:09:50.340 of that electron away from some impact parameter B. 00:09:50.340 --> 00:09:54.090 So we care about this pretty much all the time. 00:09:54.090 --> 00:09:59.040 Electrons and ions or stripped bare nuclei 00:09:59.040 --> 00:10:01.290 actually matter in this case. 00:10:01.290 --> 00:10:04.287 For elastic collisions off of nuclei, 00:10:04.287 --> 00:10:05.870 this is what Rutherford scattering is. 00:10:05.870 --> 00:10:08.810 It's a simple hard simple hard sphere collisions, 00:10:08.810 --> 00:10:12.530 so this matters pretty much all the time. 00:10:12.530 --> 00:10:15.030 What about inelastic collisions with nuclei? 00:10:15.030 --> 00:10:18.545 What does an inelastic collision actually mean with a nucleus? 00:10:22.070 --> 00:10:26.360 So fusion could be one of them, but let's go more generally. 00:10:26.360 --> 00:10:30.290 We have some nuclear reaction, where it's the old thing 00:10:30.290 --> 00:10:34.880 that I keep drawing all the time of some little nucleus striking 00:10:34.880 --> 00:10:36.950 a large nucleus. 00:10:36.950 --> 00:10:38.630 In an inelastic collision, this is 00:10:38.630 --> 00:10:40.170 the case we haven't considered yet, 00:10:40.170 --> 00:10:43.130 but I want to show you what actually happens. 00:10:43.130 --> 00:10:46.280 In an inelastic collision, these two nuclei 00:10:46.280 --> 00:10:49.580 join together to form what's called 00:10:49.580 --> 00:10:53.450 a compound nucleus or CN, at which point 00:10:53.450 --> 00:10:57.420 it breaks apart in some other way. 00:10:57.420 --> 00:10:59.990 So there might be some different small particle 00:10:59.990 --> 00:11:03.770 and some different large particle coming off. 00:11:03.770 --> 00:11:05.300 But in an inelastic collision, it's 00:11:05.300 --> 00:11:08.360 almost like the incoming particle is absorbed 00:11:08.360 --> 00:11:10.820 and something else is readmitted. 00:11:10.820 --> 00:11:13.580 It could be that same particle at a different energy, 00:11:13.580 --> 00:11:17.500 and it could be a different energy altogether. 00:11:17.500 --> 00:11:19.360 So yeah, I'd say fusion is an example. 00:11:19.360 --> 00:11:21.417 It's kicked off by an inelastic collision, 00:11:21.417 --> 00:11:23.500 because you've got to have some sort of absorption 00:11:23.500 --> 00:11:26.560 event of the small nucleus by the big nucleus. 00:11:26.560 --> 00:11:29.480 And then, maybe if it fuses and just stays that way, 00:11:29.480 --> 00:11:31.960 it releases a ton of its binding energy, 00:11:31.960 --> 00:11:33.700 well, that's pretty cool. 00:11:33.700 --> 00:11:37.900 So these actually do matter, but not 00:11:37.900 --> 00:11:40.330 for all energies in all cases. 00:11:40.330 --> 00:11:44.920 So let's go back to the Janis database of cross-sections 00:11:44.920 --> 00:11:50.170 to see when inelastic scattering actually matters. 00:11:50.170 --> 00:11:51.895 Bring us back to normal size. 00:11:55.367 --> 00:11:57.200 And we'll look at some of the cross-sections 00:11:57.200 --> 00:12:01.490 to see when do we actually care about inelastic scattering? 00:12:01.490 --> 00:12:03.620 So we haven't selected a database yet. 00:12:03.620 --> 00:12:07.310 Let's say we're firing protons at things. 00:12:07.310 --> 00:12:10.190 And pick a database that actually 00:12:10.190 --> 00:12:12.710 has some elements listed. 00:12:12.710 --> 00:12:13.340 Not a lot. 00:12:13.340 --> 00:12:18.310 But iron, that works. 00:12:18.310 --> 00:12:21.490 So we can look at the difference between the elastic scattering 00:12:21.490 --> 00:12:26.410 cross-section and the anything cross-section. 00:12:26.410 --> 00:12:28.260 So the red curve here-- 00:12:28.260 --> 00:12:30.950 can I make it thicker easily? 00:12:30.950 --> 00:12:31.630 Probably. 00:12:31.630 --> 00:12:33.380 Yeah, I can make it thicker pretty easily. 00:12:33.380 --> 00:12:34.160 Easier to see. 00:12:41.095 --> 00:12:41.595 Plots. 00:12:44.930 --> 00:12:47.588 Wait. 00:12:47.588 --> 00:12:48.630 That's not what I wanted. 00:12:48.630 --> 00:12:50.588 I'm not going to mess around with this anymore. 00:12:50.588 --> 00:12:52.070 Do you guys see the two lines? 00:12:52.070 --> 00:12:55.310 OK, so this is the elastic scattering cross-section. 00:12:55.310 --> 00:12:57.198 Kind of funny to see it negative. 00:12:57.198 --> 00:12:59.240 But then there's the anything cross-section which 00:12:59.240 --> 00:13:01.930 picks up at around 3 MeV or so. 00:13:01.930 --> 00:13:05.540 And it usually takes somewhere between 1 and 10 MeV 00:13:05.540 --> 00:13:09.050 for inelastic scattering to quote unquote turn on, 00:13:09.050 --> 00:13:10.460 and that's because you have to be 00:13:10.460 --> 00:13:14.450 able to excite the nucleus to some next energy level. 00:13:14.450 --> 00:13:17.900 So sending in a proton at like 0.01 MeV 00:13:17.900 --> 00:13:20.900 is not going to excite any of the internal particles 00:13:20.900 --> 00:13:23.030 to a higher energy level. 00:13:23.030 --> 00:13:26.800 So if you want to see some pretty interesting cases, 00:13:26.800 --> 00:13:28.280 let's go to incident neutron data 00:13:28.280 --> 00:13:29.690 where we have a ton of this data. 00:13:32.377 --> 00:13:33.710 And I'll show you some examples. 00:13:37.550 --> 00:13:39.350 We've got lots more data for neutrons. 00:13:42.930 --> 00:13:45.320 So now we can look at some of these cross-sections. 00:13:49.600 --> 00:13:52.390 Like this z n prime. 00:13:52.390 --> 00:13:54.400 Let's take a look at what that looks like. 00:13:54.400 --> 00:13:56.680 That means a neutron comes in. 00:13:56.680 --> 00:13:58.030 Different neutron comes out. 00:13:58.030 --> 00:14:00.970 Notice that the scale only starts at 862 keV. 00:14:00.970 --> 00:14:05.030 So let's make it something else. 00:14:05.030 --> 00:14:05.540 Oh my. 00:14:05.540 --> 00:14:07.340 Look at that. 00:14:07.340 --> 00:14:11.720 Nothing going on until you reach almost 1 MeV, which means, 00:14:11.720 --> 00:14:13.400 hey, inelastic scattering doesn't really 00:14:13.400 --> 00:14:15.330 turn on until that. 00:14:15.330 --> 00:14:18.140 So I would say that this can matter, 00:14:18.140 --> 00:14:23.680 but for higher energy collisions. 00:14:23.680 --> 00:14:26.170 So yeah, it matters pretty much all the time. 00:14:26.170 --> 00:14:28.750 But higher energy collisions. 00:14:28.750 --> 00:14:29.950 And there's actually-- yeah. 00:14:29.950 --> 00:14:31.908 AUDIENCE: What does it say in the top left box? 00:14:31.908 --> 00:14:33.970 MICHAEL SHORT: Only for low energy electrons. 00:14:37.390 --> 00:14:40.060 That's the sort of compound reason that I and Chris said, 00:14:40.060 --> 00:14:42.790 one, is that you can't transfer much mass 00:14:42.790 --> 00:14:44.560 in an elastic collision, or I'm sorry, 00:14:44.560 --> 00:14:46.540 much energy in an elastic collision 00:14:46.540 --> 00:14:49.300 unless the masses are close enough to each other. 00:14:49.300 --> 00:14:52.330 And two, at higher energies, the electron 00:14:52.330 --> 00:14:55.240 radiates Bremsstrahlung much, much, much faster. 00:14:55.240 --> 00:14:58.900 As we saw at around 10 MeV, Bremsstrahlung 00:14:58.900 --> 00:15:01.720 and inelastic scattering give about equal contributions 00:15:01.720 --> 00:15:06.220 to the stopping power for high z materials like lead. 00:15:06.220 --> 00:15:09.340 So once you're down and let's say like the keV range, 00:15:09.340 --> 00:15:11.980 yeah, electron elastic collisions might matter. 00:15:14.510 --> 00:15:17.230 So we talked about those three. 00:15:17.230 --> 00:15:20.243 Now I think we can launch into the analytical technique. 00:15:20.243 --> 00:15:22.660 So for the rest of the lecture today, it's all going to be 00:15:22.660 --> 00:15:24.577 what can you do with the stuff that we've been 00:15:24.577 --> 00:15:26.620 learning since the first exam. 00:15:26.620 --> 00:15:28.000 I know it hasn't been long since, 00:15:28.000 --> 00:15:29.550 but we've actually learned a ton. 00:15:29.550 --> 00:15:31.973 And I want to show you what's actually possible. 00:15:31.973 --> 00:15:33.640 And this is not going to be with slides. 00:15:33.640 --> 00:15:35.650 It's all live from websites that I'd 00:15:35.650 --> 00:15:38.290 love for you guys to be able to follow along with or check out 00:15:38.290 --> 00:15:39.430 at home. 00:15:39.430 --> 00:15:42.160 So I'm going to show you an awesome resource 00:15:42.160 --> 00:15:45.550 through the MIT libraries and how to get there. 00:15:45.550 --> 00:15:49.300 If you go to vera.mit.edu, there's 00:15:49.300 --> 00:15:51.502 a great tool called the ASM Handbook. 00:15:51.502 --> 00:15:52.960 You can see I've been there before. 00:15:56.760 --> 00:15:58.890 There's the ASM handbooks online, 00:15:58.890 --> 00:16:00.660 and this is kind of that's everything 00:16:00.660 --> 00:16:03.180 to know about material science, metallurgy, 00:16:03.180 --> 00:16:05.520 and analytical techniques, absolutely everything 00:16:05.520 --> 00:16:08.700 from corrosion to fractography, to characterization, 00:16:08.700 --> 00:16:11.340 to structure of materials to where you can find 00:16:11.340 --> 00:16:14.490 every single alloy, to binary phase diagrams of how 00:16:14.490 --> 00:16:18.980 things mix, and we're going to head to one of these handbooks. 00:16:18.980 --> 00:16:22.280 Number nine or 10, materials characterization, 00:16:22.280 --> 00:16:24.920 because with the stuff that's on this board, 00:16:24.920 --> 00:16:27.980 you can understand how most materials characterization 00:16:27.980 --> 00:16:29.450 techniques work. 00:16:29.450 --> 00:16:31.290 And I want to show you a few of them. 00:16:31.290 --> 00:16:34.190 One of which-- no, two of which, we're going to demo 00:16:34.190 --> 00:16:36.850 out next Friday's recitation. 00:16:36.850 --> 00:16:38.600 So I think I told you guys in the syllabus 00:16:38.600 --> 00:16:40.310 and probably in person that we're 00:16:40.310 --> 00:16:41.990 going to try out some scanning electron 00:16:41.990 --> 00:16:45.950 microscopy and some energy dispersive X-ray or EDC 00:16:45.950 --> 00:16:47.280 analysis. 00:16:47.280 --> 00:16:49.640 So with the X-ray transition stuff you've learned, 00:16:49.640 --> 00:16:52.400 you actually know how to elementally analyze 00:16:52.400 --> 00:16:53.690 different materials. 00:16:53.690 --> 00:16:55.430 And with scanning electron microscope, 00:16:55.430 --> 00:16:59.360 you can get some idea about how electrons can make images much 00:16:59.360 --> 00:17:01.880 better than optical images. 00:17:01.880 --> 00:17:08.630 So let's head to electron optical methods, scanning 00:17:08.630 --> 00:17:15.310 electron microscopy, and show you what one of these things 00:17:15.310 --> 00:17:16.420 actually looks like. 00:17:16.420 --> 00:17:22.609 Let's take a look at an SEM, or scanning electron microscope. 00:17:22.609 --> 00:17:26.160 So up at the top, there is a device called the electron gun. 00:17:26.160 --> 00:17:28.500 For now, just imagine it's a source of electrons, 00:17:28.500 --> 00:17:30.000 but in a few minutes, we'll actually 00:17:30.000 --> 00:17:32.460 explain how it works using the principle 00:17:32.460 --> 00:17:35.790 of thermionic emission, which we talked about last Friday. 00:17:35.790 --> 00:17:38.110 You've got some electronic lenses, 00:17:38.110 --> 00:17:40.740 some focusing coils, that caused this beam to get 00:17:40.740 --> 00:17:43.840 focused further and further. 00:17:43.840 --> 00:17:46.800 So let's say you had this electron filament giving off 00:17:46.800 --> 00:17:49.050 electrons in all directions. 00:17:51.610 --> 00:17:54.400 When you see boxes with x's like this on an electron optics 00:17:54.400 --> 00:17:57.250 diagram, it usually means this is like a focusing coil 00:17:57.250 --> 00:17:58.510 of some sort. 00:17:58.510 --> 00:18:03.910 So that will cause the electrons to get bent and focused. 00:18:03.910 --> 00:18:09.430 There'll be another set of coils that focuses them further 00:18:09.430 --> 00:18:14.140 and some scanning coils that actually raster or xy 00:18:14.140 --> 00:18:18.430 scan this beam across the surface of a material. 00:18:18.430 --> 00:18:20.440 And so in this way, what you're actually 00:18:20.440 --> 00:18:22.870 doing is putting the electron beam at one 00:18:22.870 --> 00:18:26.270 part of your material and then with another detector, 00:18:26.270 --> 00:18:29.110 let's call it a secondary electron detector. 00:18:29.110 --> 00:18:32.290 Looking at the electrons produced from collisions 00:18:32.290 --> 00:18:36.190 with those other electrons that then get detected here, 00:18:36.190 --> 00:18:38.680 and the number of electrons produced at a point 00:18:38.680 --> 00:18:41.020 gives you the brightness of the image. 00:18:41.020 --> 00:18:43.540 That's kind of as simple as it is despite how 00:18:43.540 --> 00:18:45.670 complicated this diagram looks. 00:18:45.670 --> 00:18:47.500 There's an electron source. 00:18:47.500 --> 00:18:49.570 There's coils that scan it back and forth. 00:18:49.570 --> 00:18:52.660 Like has anyone ever seen the old cathode ray tube, 00:18:52.660 --> 00:18:55.620 CRT televisions? 00:18:55.620 --> 00:18:57.880 There's going to come a day when that answer is no. 00:18:57.880 --> 00:18:59.255 And I'm kind of worried for that, 00:18:59.255 --> 00:19:01.800 because that's the day I'll officially become old. 00:19:01.800 --> 00:19:04.853 But for now, everyone's seen a CRT, 00:19:04.853 --> 00:19:06.270 and the way that actually works is 00:19:06.270 --> 00:19:09.600 there's an electron gun that fires and scans left to right 00:19:09.600 --> 00:19:13.920 and up to down our rasters and produces that electron image. 00:19:13.920 --> 00:19:18.480 In an SEM, you use an electron gun, kind of similar, and then 00:19:18.480 --> 00:19:21.930 collect the electrons generated in the specimen, what's 00:19:21.930 --> 00:19:23.880 called secondary electrons. 00:19:23.880 --> 00:19:25.860 And the number that you see gives you 00:19:25.860 --> 00:19:27.810 the brightness of the image. 00:19:27.810 --> 00:19:29.940 The cool thing is this actually allows 00:19:29.940 --> 00:19:34.560 you to look at both secondary electron contrast 00:19:34.560 --> 00:19:38.680 and topology of a sample. 00:19:38.680 --> 00:19:44.530 So let's say this was your secondary electron detector. 00:19:44.530 --> 00:19:49.680 And you had an electron beam scanning across your sample 00:19:49.680 --> 00:19:51.710 to some of those peaks and valleys. 00:19:51.710 --> 00:19:54.810 And I'll probably draw one right here for a good reason. 00:19:54.810 --> 00:19:58.050 Let's say the electrons hit right here, 00:19:58.050 --> 00:20:03.330 and you send out a wave of secondary electrons. 00:20:03.330 --> 00:20:06.450 The material partly determines how many electrons come off, 00:20:06.450 --> 00:20:08.550 but also, so does the geometry. 00:20:08.550 --> 00:20:11.160 There will usually be a little cage 00:20:11.160 --> 00:20:14.220 with some sort of a positive voltage on it 00:20:14.220 --> 00:20:16.980 to attract those secondary electrons. 00:20:16.980 --> 00:20:19.740 And some of them will curve into the detector and become part 00:20:19.740 --> 00:20:24.000 of your signal, but some of them won't. 00:20:24.000 --> 00:20:27.390 Meanwhile, if you have this beam right here producing 00:20:27.390 --> 00:20:30.450 secondary electrons, pretty much all of them 00:20:30.450 --> 00:20:32.640 go slamming into your detector. 00:20:32.640 --> 00:20:34.620 And that's what actually allows the electron 00:20:34.620 --> 00:20:36.960 microscope to get topology. 00:20:36.960 --> 00:20:40.470 That's why images in the SEM look fairly 3D. 00:20:40.470 --> 00:20:45.490 So I want to show you a few examples from my own boredom 00:20:45.490 --> 00:20:49.450 when I was doing a lot of science. 00:20:49.450 --> 00:20:50.950 There we go. 00:20:50.950 --> 00:20:53.510 I have a whole gallery of SEM images 00:20:53.510 --> 00:20:55.730 when I was supposed to be doing something better. 00:20:55.730 --> 00:20:56.300 Oh no. 00:21:00.610 --> 00:21:05.490 404. 00:21:05.490 --> 00:21:07.450 My website's broken. 00:21:07.450 --> 00:21:07.950 Oh yeah. 00:21:07.950 --> 00:21:10.033 This is also what you do when you're bored, right? 00:21:10.033 --> 00:21:13.690 Make your own 404 page. 00:21:13.690 --> 00:21:15.610 My SEM galleries are dead. 00:21:15.610 --> 00:21:16.510 Well, that's OK. 00:21:16.510 --> 00:21:18.370 I have other images ready to show you guys. 00:21:22.210 --> 00:21:25.380 So this is a neat-- this is a paper that I published out 00:21:25.380 --> 00:21:28.750 of my PhD work that shows the real difference 00:21:28.750 --> 00:21:32.530 between optical and electron microscopy. 00:21:32.530 --> 00:21:36.190 Part of it is the limit of your resolution 00:21:36.190 --> 00:21:39.610 depends on the wavelength or de Broglie wavelength of the thing 00:21:39.610 --> 00:21:41.470 you're using to make the image. 00:21:41.470 --> 00:21:43.960 So an optical microscope, in this case, 00:21:43.960 --> 00:21:47.290 you can't get better resolution than about half a micron, 00:21:47.290 --> 00:21:49.150 because even the blue wavelengths of light 00:21:49.150 --> 00:21:52.510 are getting down into about the 450 nanometer regime. 00:21:52.510 --> 00:21:55.210 And it's very difficult without interference techniques 00:21:55.210 --> 00:21:58.720 or other fancy things to beat that diffraction limit, 00:21:58.720 --> 00:22:02.570 to beat the sort of wavelength limit of optical microscopy. 00:22:02.570 --> 00:22:05.650 So this is a 500x optical microscope image, 00:22:05.650 --> 00:22:07.690 and you can see these little fingers-- 00:22:07.690 --> 00:22:09.610 in this case, it's liquid lead bismuth 00:22:09.610 --> 00:22:11.920 penetrating into a stainless steel 00:22:11.920 --> 00:22:14.170 that we were doing corrosion experiments on. 00:22:14.170 --> 00:22:15.700 And that's as good as the image can 00:22:15.700 --> 00:22:19.220 get in an optical microscope. 00:22:19.220 --> 00:22:22.040 Switch down to an SEM, and then all of a sudden 00:22:22.040 --> 00:22:24.740 the picture becomes much, much, much more clear. 00:22:24.740 --> 00:22:27.920 You can start to see things-- the best SEM we have in our lab 00:22:27.920 --> 00:22:32.600 has an ultimate resolution of about 1 nanometer. 00:22:32.600 --> 00:22:35.510 Now, resolution is kind of a funny thing. 00:22:35.510 --> 00:22:37.760 It's neat to tell you what that means. 00:22:37.760 --> 00:22:42.530 It doesn't mean that if you have a pattern of lines 00:22:42.530 --> 00:22:47.540 that are exactly one nanometer thick, that you will see them 00:22:47.540 --> 00:22:50.060 as lines 1 nanometer thick. 00:22:50.060 --> 00:22:51.950 It means that if you then plot, let's 00:22:51.950 --> 00:22:58.710 say, your signal or your brightness versus x, 00:22:58.710 --> 00:23:02.820 you'll have some barely distinguishable and fuzzy 00:23:02.820 --> 00:23:06.600 lines, just enough for you to say those are two optically 00:23:06.600 --> 00:23:07.980 distinct features. 00:23:07.980 --> 00:23:14.340 So what you'll actually see in a 1 nanometer microscope 00:23:14.340 --> 00:23:17.850 is maybe something like this. 00:23:17.850 --> 00:23:20.790 That's technically resolved at the level of 1 nanometer. 00:23:20.790 --> 00:23:23.250 So the best you can do for crisp objects in this thing 00:23:23.250 --> 00:23:25.260 is about 20 nanometers. 00:23:25.260 --> 00:23:26.430 Not bad. 00:23:26.430 --> 00:23:30.250 It's like something that's a few thousand atoms on a side. 00:23:30.250 --> 00:23:32.420 Pretty cool. 00:23:32.420 --> 00:23:36.260 And so what you can see in here is liquid lead bismuth 00:23:36.260 --> 00:23:38.900 penetrating into this stainless steel, 00:23:38.900 --> 00:23:40.970 and you notice a few different things. 00:23:40.970 --> 00:23:45.770 This image was taken in backscatter electron mode. 00:23:45.770 --> 00:23:49.160 Back scattering is-- we've talked about this before. 00:23:49.160 --> 00:23:54.140 When you have a scattering event where theta equals pi, 00:23:54.140 --> 00:23:55.820 we call that backscatter. 00:24:04.950 --> 00:24:10.410 Let's kind of split this into regular and backscatter. 00:24:14.230 --> 00:24:17.560 For a backscattering, the cross-section for this 00:24:17.560 --> 00:24:21.580 is proportional to z squared, another one 00:24:21.580 --> 00:24:25.000 of those extremely z dependent cross-sections, which 00:24:25.000 --> 00:24:28.540 means that the larger the z, the higher the atomic number 00:24:28.540 --> 00:24:31.090 the more backscatter contrast you get, 00:24:31.090 --> 00:24:34.240 so if you want to figure out where the little lead whiskers 00:24:34.240 --> 00:24:36.520 are penetrating into the stainless steel, 00:24:36.520 --> 00:24:41.650 since lead has a z of like 82, and iron has a z of like 26, 00:24:41.650 --> 00:24:43.435 it shows up like night and day. 00:24:43.435 --> 00:24:45.340 Do you have a question, Julia? 00:24:45.340 --> 00:24:46.347 OK. 00:24:46.347 --> 00:24:48.680 Yeah, so this is something we'll actually be able to do. 00:24:48.680 --> 00:24:50.840 So for the two folks I asked to bring in samples, 00:24:50.840 --> 00:24:53.270 if you want to bring in something with very 00:24:53.270 --> 00:24:55.310 different elements in it, we should 00:24:55.310 --> 00:24:59.350 be able to see it in backscatter contrast very, very clearly. 00:24:59.350 --> 00:25:03.950 And in the image of the SEM, I'll go back to that-- 00:25:03.950 --> 00:25:07.190 which one of these pages is it? 00:25:07.190 --> 00:25:10.520 Notice here that there is a backscatter detector. 00:25:10.520 --> 00:25:12.740 So it will detect which of those electrons 00:25:12.740 --> 00:25:16.110 scatter back at almost 180 degrees. 00:25:16.110 --> 00:25:20.090 And that's at about z squared proportionality, super useful 00:25:20.090 --> 00:25:23.110 tool, because if you want to see, for example, where 00:25:23.110 --> 00:25:24.860 the circuit board traces are, and you want 00:25:24.860 --> 00:25:27.420 to look at aluminum versus oxygen contrast, 00:25:27.420 --> 00:25:29.210 that'll help you really well. 00:25:29.210 --> 00:25:31.160 If you want to see where is lead penetrating 00:25:31.160 --> 00:25:35.572 into stainless steel, it shines up clear as day, 00:25:35.572 --> 00:25:38.020 which is pretty fun. 00:25:38.020 --> 00:25:39.700 The other thing the electrons will 00:25:39.700 --> 00:25:41.410 do when they enter into a material 00:25:41.410 --> 00:25:44.260 is excite lots of things. 00:25:44.260 --> 00:25:47.470 So anything from X-rays to Auger electrons. 00:25:47.470 --> 00:25:50.440 So now I'd like to bring up Auger electron spectroscopy. 00:25:52.960 --> 00:25:56.380 Electron or X-ray spectroscopic methods. 00:25:56.380 --> 00:25:58.030 Auger electron spectroscopy, it's 00:25:58.030 --> 00:26:00.220 not just a thing to trip you up on the exam 00:26:00.220 --> 00:26:04.360 or a little minutia from radioactive decay. 00:26:04.360 --> 00:26:09.020 It's actually incredibly useful, because 00:26:09.020 --> 00:26:12.080 of where the Auger electrons are generated 00:26:12.080 --> 00:26:15.650 and what they tell you about the material. 00:26:15.650 --> 00:26:18.790 So as a quick refresher, normally you could have, 00:26:18.790 --> 00:26:23.140 let's say, if a photon comes in and injects a photo electron 00:26:23.140 --> 00:26:26.620 as another electron comes to fill that hole, 00:26:26.620 --> 00:26:30.160 either an X-ray will be emitted or an Auger electron 00:26:30.160 --> 00:26:31.270 will be emitted. 00:26:31.270 --> 00:26:32.920 And it's those Auger electrons, they're 00:26:32.920 --> 00:26:34.540 outer binding energy electrons. 00:26:34.540 --> 00:26:38.770 They have very low binding energy, which means-- 00:26:38.770 --> 00:26:39.280 let's see. 00:26:39.280 --> 00:26:41.752 I keep running out of room. 00:26:41.752 --> 00:26:43.210 You know, I'm not going to draw it. 00:26:43.210 --> 00:26:46.090 I'm going to show you, because I know there is a diagram of what 00:26:46.090 --> 00:26:47.890 I want to show you here. 00:26:47.890 --> 00:26:50.800 If you want to see where the Auger electrons are actually 00:26:50.800 --> 00:26:52.630 produced in the material-- 00:26:52.630 --> 00:26:55.090 here we go. 00:26:55.090 --> 00:26:58.210 Since they're such low energy, the only Auger electrons 00:26:58.210 --> 00:27:02.620 that actually get out would be in this outer few mono layers. 00:27:02.620 --> 00:27:04.510 In fact, there's some Auger electron energies 00:27:04.510 --> 00:27:07.180 that can only get out one or two atomic mono 00:27:07.180 --> 00:27:08.780 layers from a material. 00:27:08.780 --> 00:27:10.840 So it's one of the best surface analysis 00:27:10.840 --> 00:27:12.460 techniques that we have. 00:27:12.460 --> 00:27:15.730 You can both use Auger electrons to make an electron image, 00:27:15.730 --> 00:27:18.100 like any other SEM. 00:27:18.100 --> 00:27:22.930 And you can collect them and measure their energy 00:27:22.930 --> 00:27:26.490 to figure out which elements they came from. 00:27:26.490 --> 00:27:29.070 And this kind of teardrop shape is a-- 00:27:29.070 --> 00:27:31.740 one, it's a great synthesis of all the information 00:27:31.740 --> 00:27:34.560 you need to know in the SEM that we'll see on Friday. 00:27:34.560 --> 00:27:39.060 And two, its why people screw up SEM analysis a lot. 00:27:39.060 --> 00:27:43.660 A lot of the X-ray excitation happens down here. 00:27:43.660 --> 00:27:47.100 Why do you think that the X-ray exaltation would happen 00:27:47.100 --> 00:27:49.873 near the end of the path of the electron beam 00:27:49.873 --> 00:27:51.540 from what you know about stopping power? 00:27:58.290 --> 00:28:03.150 Or, if I asked you to draw a graph of let's say 00:28:03.150 --> 00:28:10.800 energy versus stopping power for ionization, what would 00:28:10.800 --> 00:28:11.400 it look like? 00:28:16.110 --> 00:28:16.910 Yeah. 00:28:16.910 --> 00:28:19.779 AUDIENCE: It comes up like a peak at low energy. 00:28:19.779 --> 00:28:20.612 MICHAEL SHORT: Yeah. 00:28:20.612 --> 00:28:22.450 AUDIENCE: And then drops back down. 00:28:22.450 --> 00:28:23.283 MICHAEL SHORT: Yeah. 00:28:23.283 --> 00:28:26.322 AUDIENCE: And as the energy goes out, it sort of flattens out. 00:28:26.322 --> 00:28:28.030 MICHAEL SHORT: Yep, sort of flattens out, 00:28:28.030 --> 00:28:30.940 and then eventually starts picking up again but not very 00:28:30.940 --> 00:28:31.970 much. 00:28:31.970 --> 00:28:35.632 So as the energy of whatever you're going into-- 00:28:35.632 --> 00:28:37.090 I'm sorry, whatever particle you're 00:28:37.090 --> 00:28:40.120 sending in it gets lower, it's stopping power increases, 00:28:40.120 --> 00:28:42.310 and you have a much higher chance of this ionization 00:28:42.310 --> 00:28:44.740 happening, especially in the case of electrons. 00:28:44.740 --> 00:28:48.923 They usually come in at between 10 and 40 kV. 00:28:48.923 --> 00:28:50.340 And so near the end of their range 00:28:50.340 --> 00:28:52.870 is where they produce a lot of the X-rays. 00:28:52.870 --> 00:28:54.520 Now there's a lot of other nuances 00:28:54.520 --> 00:28:57.790 to say, well, which X-rays were produced here 00:28:57.790 --> 00:29:01.930 and what elements are they from. 00:29:01.930 --> 00:29:06.350 Let's say you had the same material here or here. 00:29:06.350 --> 00:29:08.600 Fewer X-rays will get out of the bottom region 00:29:08.600 --> 00:29:10.330 than they will from the top region. 00:29:10.330 --> 00:29:12.080 So if you happen to be analyzing something 00:29:12.080 --> 00:29:15.080 that has a gradient in composition 00:29:15.080 --> 00:29:18.110 or a change in composition from the top to the bottom, 00:29:18.110 --> 00:29:21.830 you might be like, oh, well, I have a few nanometers 00:29:21.830 --> 00:29:24.050 of oxygen on silicon. 00:29:24.050 --> 00:29:26.480 Why aren't I seeing any oxygen X-rays? 00:29:26.480 --> 00:29:30.380 Because you're probably generating them down here. 00:29:30.380 --> 00:29:32.150 That's one of those things to note. 00:29:32.150 --> 00:29:34.040 So sometimes you'll see and elect 00:29:34.040 --> 00:29:38.362 an elemental map of things that shows X-rays 00:29:38.362 --> 00:29:40.070 of certain element coming from somewhere, 00:29:40.070 --> 00:29:41.930 and you can't see it at all in the image. 00:29:41.930 --> 00:29:43.460 That's because they might be underneath what 00:29:43.460 --> 00:29:44.600 you can see in the image. 00:29:44.600 --> 00:29:46.945 It's kind of tricky like that. 00:29:46.945 --> 00:29:49.070 I'll show you some examples of what those maps look 00:29:49.070 --> 00:29:53.850 like also from this paper. 00:29:53.850 --> 00:29:56.820 So from the electron image, we sort of 00:29:56.820 --> 00:29:59.310 concluded, all right, lead is probably penetrating 00:29:59.310 --> 00:30:01.050 into the stainless steel. 00:30:01.050 --> 00:30:02.700 How do we know for sure? 00:30:02.700 --> 00:30:05.970 You can make EDX or elemental dispersive-- 00:30:05.970 --> 00:30:08.670 I'm sorry, energy dispersive X-ray maps 00:30:08.670 --> 00:30:11.310 by focusing the electron beam at one point, 00:30:11.310 --> 00:30:12.810 collecting all the different X-rays 00:30:12.810 --> 00:30:15.870 and then moving from one point to another 00:30:15.870 --> 00:30:18.630 to see when do you get characteristic X-rays from each 00:30:18.630 --> 00:30:19.870 of those elements. 00:30:19.870 --> 00:30:21.360 So you can actually prove to say, 00:30:21.360 --> 00:30:26.550 yes, those little fingers are indeed bismuth and lead, 00:30:26.550 --> 00:30:29.220 and you can see that, in this case, where the lead in bismuth 00:30:29.220 --> 00:30:31.140 is, the iron is not. 00:30:31.140 --> 00:30:32.520 But the curious thing we found is 00:30:32.520 --> 00:30:35.580 that in this whole band right here, most of the chromium 00:30:35.580 --> 00:30:36.870 disappeared. 00:30:36.870 --> 00:30:39.540 So it turns out that the corrosion mechanism 00:30:39.540 --> 00:30:41.340 was chromium dissolution. 00:30:41.340 --> 00:30:44.730 And we would not have been able to know that without this EDX 00:30:44.730 --> 00:30:47.370 mapping, and without understanding how the EDX 00:30:47.370 --> 00:30:50.550 maps are made from the electrons interacting with matter 00:30:50.550 --> 00:30:52.820 and producing characteristic X-rays, 00:30:52.820 --> 00:30:55.190 wouldn't have been able to prove this. 00:30:55.190 --> 00:30:56.920 Yet another example where the basic stuff 00:30:56.920 --> 00:31:01.090 you're learning in 22.01 is the theoretical underpinning 00:31:01.090 --> 00:31:04.190 of the techniques that we use all the time 00:31:04.190 --> 00:31:09.106 in material science, which I thought was pretty cool. 00:31:09.106 --> 00:31:11.080 I've got more examples of that that 00:31:11.080 --> 00:31:13.570 are even more striking, because I let 00:31:13.570 --> 00:31:16.840 it collect for a little longer. 00:31:16.840 --> 00:31:20.200 You can actually see right here that where the bismuth is, 00:31:20.200 --> 00:31:23.340 the iron isn't, but the iron's not dissolving. 00:31:23.340 --> 00:31:24.120 The chromium is. 00:31:24.120 --> 00:31:25.590 It's just the lead and bismuth are 00:31:25.590 --> 00:31:28.080 kind of sucking the chromium out of the metal right there, 00:31:28.080 --> 00:31:31.860 and that's what making the stainless steel less stainless. 00:31:31.860 --> 00:31:33.700 It's pretty neat. 00:31:33.700 --> 00:31:37.540 Then on to EDX analysis, what sort of information 00:31:37.540 --> 00:31:41.960 are we looking at every one of these pixels? 00:31:41.960 --> 00:31:45.400 I have a couple of other example X-ray spectra. 00:31:45.400 --> 00:31:48.090 So now we're in a position to understand 00:31:48.090 --> 00:31:51.420 why one of these X-ray spectra looks the way it does. 00:31:51.420 --> 00:31:54.580 In this case, we're firing electrons at a material. 00:31:54.580 --> 00:31:55.260 Let's see. 00:31:55.260 --> 00:31:57.600 Where is our material we're firing at? 00:31:57.600 --> 00:31:59.250 Right here. 00:31:59.250 --> 00:32:02.010 So we're firing in electrons. 00:32:02.010 --> 00:32:05.280 And in some cases, let's say we had an iron atom. 00:32:08.190 --> 00:32:13.998 That electron can eject another electron. 00:32:13.998 --> 00:32:15.540 And then one of those other electrons 00:32:15.540 --> 00:32:18.270 will fall down in that shell, giving off 00:32:18.270 --> 00:32:20.100 a characteristic X-ray. 00:32:20.100 --> 00:32:23.250 In this case, since it's from the third to the second shell, 00:32:23.250 --> 00:32:26.550 that would be what we call an L X-ray or a something 00:32:26.550 --> 00:32:29.310 to level two transition. 00:32:29.310 --> 00:32:32.010 And every element has got its characteristic X-ray 00:32:32.010 --> 00:32:36.840 transitions, like we saw on the NIST X-ray transition database. 00:32:36.840 --> 00:32:38.820 And since we know what all of those are, 00:32:38.820 --> 00:32:40.200 we know where to expect them. 00:32:40.200 --> 00:32:43.320 So we know where we expect to see chromium's X-rays 00:32:43.320 --> 00:32:45.420 and iron's X-rays. 00:32:45.420 --> 00:32:48.190 Gold's kind of an interesting one. 00:32:48.190 --> 00:32:52.150 There's two things about doing analysis with gold. 00:32:52.150 --> 00:32:54.400 A lot of times you have to coat your materials in gold 00:32:54.400 --> 00:32:57.830 to boost their secondary electron contrast. 00:32:57.830 --> 00:33:01.820 But also gold, I think it's its L line or M line, 00:33:01.820 --> 00:33:06.643 I forget which one, is the same as argon's K line. 00:33:06.643 --> 00:33:08.810 And we have an expression in the electron microscopy 00:33:08.810 --> 00:33:11.960 world, the probability of finding argon in your sample 00:33:11.960 --> 00:33:15.420 decreases with experience. 00:33:15.420 --> 00:33:17.135 Takes a second to parse that. 00:33:17.135 --> 00:33:19.260 Chances are, if you're looking at a solid material. 00:33:19.260 --> 00:33:20.790 You don't have argon in it. 00:33:20.790 --> 00:33:23.940 But there are extra lines that overlap with each other, 00:33:23.940 --> 00:33:27.330 like the L line for gold and the K line for argon 00:33:27.330 --> 00:33:31.110 are at pretty much the same energy, certainly 00:33:31.110 --> 00:33:34.920 similar enough that it's within the resolution 00:33:34.920 --> 00:33:38.908 or like full width of half maximum of these two peaks. 00:33:38.908 --> 00:33:40.950 So remember how we were analyzing the uncertainty 00:33:40.950 --> 00:33:44.190 of our banana spectra with the FWHM or full width 00:33:44.190 --> 00:33:45.432 at half maximum? 00:33:45.432 --> 00:33:46.890 Same thing here, and you can really 00:33:46.890 --> 00:33:49.710 see that the energy resolution of this detector 00:33:49.710 --> 00:33:51.740 is not the best. 00:33:51.740 --> 00:33:53.450 So if you see a peak, it might be 00:33:53.450 --> 00:33:58.672 due to two or more peaks crowding in that right there. 00:33:58.672 --> 00:34:01.130 And with a lot of correction factors that I won't get into, 00:34:01.130 --> 00:34:02.930 you can then use this information 00:34:02.930 --> 00:34:04.760 to integrate the area under these peaks 00:34:04.760 --> 00:34:07.920 and get elemental analysis. 00:34:07.920 --> 00:34:11.300 You can say how much chromium or how much iron and silicon is 00:34:11.300 --> 00:34:14.400 in one of these samples. 00:34:14.400 --> 00:34:16.545 What's this stuff here on the bottom? 00:34:16.545 --> 00:34:17.170 Anyone tell me? 00:34:21.530 --> 00:34:24.484 That continuum of observed X-ray energies? 00:34:27.906 --> 00:34:28.810 AUDIENCE: Compton. 00:34:28.810 --> 00:34:31.360 MICHAEL SHORT: Compton scattering is a photon effect, 00:34:31.360 --> 00:34:34.250 so that would be-- 00:34:34.250 --> 00:34:38.500 if this were a photon analysis spectrum, then 00:34:38.500 --> 00:34:41.170 you would see something like this but of a different shape. 00:34:41.170 --> 00:34:43.719 You'd have that Compton bowl with an edge. 00:34:43.719 --> 00:34:45.429 But this is, in this case, electrons 00:34:45.429 --> 00:34:47.630 interacting with material. 00:34:47.630 --> 00:34:49.896 What do you think is causing that broad background? 00:34:53.030 --> 00:34:55.880 Well, what are the different ways in which electrons 00:34:55.880 --> 00:34:58.460 can interact with matter? 00:34:58.460 --> 00:35:00.923 You're seeing the ionizations here. 00:35:00.923 --> 00:35:02.840 We're not really seeing Rutherford scattering. 00:35:02.840 --> 00:35:05.725 What's left? 00:35:05.725 --> 00:35:07.120 AUDIENCE: Bremsstrahlung? 00:35:07.120 --> 00:35:08.370 MICHAEL SHORT: Bremsstrahlung. 00:35:08.370 --> 00:35:09.920 Yep, that's exactly it. 00:35:09.920 --> 00:35:12.610 So the observed Bremsstrahlung spectrum 00:35:12.610 --> 00:35:15.550 follows this sort of characteristic peak 00:35:15.550 --> 00:35:18.350 early and then tail off curve. 00:35:18.350 --> 00:35:20.530 What's the actual Bremsstrahlung spectrum 00:35:20.530 --> 00:35:21.595 that we're not sensing? 00:35:25.560 --> 00:35:26.790 What would it look like? 00:35:29.610 --> 00:35:33.000 Always running out of room. 00:35:33.000 --> 00:35:35.040 If this is what we're actually observing, 00:35:35.040 --> 00:35:40.730 let's say we have a few peeks, that would be intensity, 00:35:40.730 --> 00:35:45.183 and that would be energy, what's really going on physically 00:35:45.183 --> 00:35:46.100 that we're not seeing? 00:35:50.180 --> 00:35:51.254 Yeah. 00:35:51.254 --> 00:35:56.670 AUDIENCE: Isn't it sort of like almost like exponential decay. 00:35:56.670 --> 00:36:00.260 So it starts out with very high intensity and goes down. 00:36:00.260 --> 00:36:02.330 MICHAEL SHORT: That's right. 00:36:02.330 --> 00:36:07.144 You actually should get more low energy Bremsstrahlung. 00:36:09.750 --> 00:36:11.610 One of some of the reasons you don't is 00:36:11.610 --> 00:36:14.940 that the lower energy, those X-rays come out, 00:36:14.940 --> 00:36:19.350 the more they get self-absorbed in the material in the few gas 00:36:19.350 --> 00:36:22.710 molecules in the SEM and in the window of the detector. 00:36:22.710 --> 00:36:24.210 So just because this is what you see 00:36:24.210 --> 00:36:25.710 doesn't mean this is what's actually 00:36:25.710 --> 00:36:28.560 going on in your material. 00:36:28.560 --> 00:36:32.840 If we think back then to where the electrons and X-rays are 00:36:32.840 --> 00:36:36.253 generated, the X-rays that are generated down here, 00:36:36.253 --> 00:36:38.420 the lower energy ones are going to be shielded more. 00:36:38.420 --> 00:36:41.300 And this kind of messes up your elemental analysis, 00:36:41.300 --> 00:36:45.500 because if the X-rays produced here, 00:36:45.500 --> 00:36:47.690 proportionally more of the low energy 00:36:47.690 --> 00:36:50.300 ones will get out than the ones produced here. 00:36:50.300 --> 00:36:53.030 So as you change your-- 00:36:53.030 --> 00:36:55.070 as you change your electron beam energy, 00:36:55.070 --> 00:36:58.040 you might see your elemental composition appear to change 00:36:58.040 --> 00:36:59.858 when you know it's really not. 00:36:59.858 --> 00:37:02.150 And that's because where the X-rays are being generated 00:37:02.150 --> 00:37:05.180 change, and proportionately, more of the low energy 00:37:05.180 --> 00:37:08.120 ones get self shielded by your material. 00:37:08.120 --> 00:37:10.010 So you actually have to correct for that 00:37:10.010 --> 00:37:13.070 and input your beam energy into the EDX analyzer 00:37:13.070 --> 00:37:16.660 so it knows how to correct for this. 00:37:16.660 --> 00:37:18.790 But with the understanding I've been giving you 00:37:18.790 --> 00:37:21.400 guys in this class you can understand like well, 00:37:21.400 --> 00:37:22.700 why can you get screwed up? 00:37:22.700 --> 00:37:26.820 Why do we have to have all these correction factors? 00:37:26.820 --> 00:37:29.090 I think it's pretty neat. 00:37:29.090 --> 00:37:33.600 Then let's get on to some of the other methods, like X-ray photo 00:37:33.600 --> 00:37:36.020 electron spectroscopy or XPS. 00:37:36.020 --> 00:37:37.770 This is something I hinted to a little bit 00:37:37.770 --> 00:37:41.430 earlier that actually uses the photoelectric effect, 00:37:41.430 --> 00:37:44.130 because it's a photo electron spectroscopy method. 00:37:47.410 --> 00:37:49.900 This one's incredibly useful because not only does it 00:37:49.900 --> 00:37:51.670 tell you what elements are there, 00:37:51.670 --> 00:37:53.650 but in what binding state they are, 00:37:53.650 --> 00:37:55.420 because photo electron spectrometers 00:37:55.420 --> 00:37:57.760 can be incredibly precise. 00:37:57.760 --> 00:38:01.810 The energy equation should look pretty familiar to you 00:38:01.810 --> 00:38:03.700 it's whatever photo electron you get 00:38:03.700 --> 00:38:05.290 is the gamma ray energy that comes 00:38:05.290 --> 00:38:08.980 in minus the binding energy of that electron 00:38:08.980 --> 00:38:10.600 and the work function. 00:38:10.600 --> 00:38:13.210 And so you can very, very simply figure out 00:38:13.210 --> 00:38:15.580 for a given element and a given electron 00:38:15.580 --> 00:38:19.720 shell what photo electron energy's do you expect. 00:38:19.720 --> 00:38:22.030 So you can collect them. 00:38:22.030 --> 00:38:25.100 So I'll show you another example from this paper, where 00:38:25.100 --> 00:38:26.700 we started to do that. 00:38:26.700 --> 00:38:28.970 We wanted to answer the question, what 00:38:28.970 --> 00:38:32.390 are the oxides forming on the stainless steel 00:38:32.390 --> 00:38:34.130 when lead corrodes it? 00:38:34.130 --> 00:38:36.140 And just telling you which elements 00:38:36.140 --> 00:38:38.150 are there and in what proportion doesn't give 00:38:38.150 --> 00:38:39.980 the answer, because what if there's 00:38:39.980 --> 00:38:41.810 multiple phases of each oxide? 00:38:41.810 --> 00:38:53.490 Like, for example, iron can take forms like FeO, Fe2O3, Fe3O4, 00:38:53.490 --> 00:38:57.108 and this FeO can actually have a range of stoichiometries. 00:38:57.108 --> 00:38:57.900 So how do you know? 00:38:57.900 --> 00:38:58.650 You don't know. 00:38:58.650 --> 00:39:02.237 There could be like scores of phases of this iron oxide. 00:39:02.237 --> 00:39:04.320 The question is how you know which ones are there. 00:39:04.320 --> 00:39:07.350 The photo electrons will tell you. 00:39:07.350 --> 00:39:12.870 So what you can do first is fire monochromatic X-rays, so single 00:39:12.870 --> 00:39:17.520 energy X-rays, in this case from aluminum at your material 00:39:17.520 --> 00:39:21.330 and see which photo electrons of which energy come off. 00:39:21.330 --> 00:39:24.090 And you can tell which atomic shell 00:39:24.090 --> 00:39:26.460 they're from and which elements they should 00:39:26.460 --> 00:39:28.530 be to a very high precision. 00:39:28.530 --> 00:39:33.390 In this case, this is done to 100 milli MeV or 0.1 MeV 00:39:33.390 --> 00:39:36.360 precision, 0.1 eV precision. 00:39:36.360 --> 00:39:38.790 So we can tell not only what elements are there, 00:39:38.790 --> 00:39:41.430 but what shells they came from. 00:39:41.430 --> 00:39:43.170 Then you can get even crazier. 00:39:43.170 --> 00:39:45.330 You can scan very slowly over one 00:39:45.330 --> 00:39:50.670 of these peaks with 0.001 eV precision 00:39:50.670 --> 00:39:52.680 and start to see something pretty cool. 00:39:52.680 --> 00:39:56.008 If you look at the carbon 1s electrons, 00:39:56.008 --> 00:39:58.050 you can see that there are actually three of them 00:39:58.050 --> 00:40:01.470 only a couple eV apart, and this corresponds 00:40:01.470 --> 00:40:06.220 to different binding states of molecules with that carbon. 00:40:06.220 --> 00:40:08.410 For an even more subtle example, but ended up 00:40:08.410 --> 00:40:11.170 being incredibly important for us, 00:40:11.170 --> 00:40:13.750 here's one of the chromium 2p shell peaks. 00:40:13.750 --> 00:40:15.340 You can actually see there's three 00:40:15.340 --> 00:40:18.400 of them superimposed give that funny 00:40:18.400 --> 00:40:20.148 looking peak shape right there. 00:40:20.148 --> 00:40:21.940 What that actually tells us is that there's 00:40:21.940 --> 00:40:26.290 chromium in three different binding states in that oxide. 00:40:26.290 --> 00:40:29.080 And the ones we figured out must be there, 00:40:29.080 --> 00:40:35.840 we saw the ones corresponding to Cr2O3, FeCr2O4, 00:40:35.840 --> 00:40:38.810 and I forget which other one, but we have them tabulated. 00:40:41.440 --> 00:40:43.570 There we go. 00:40:43.570 --> 00:40:44.270 Oh wow. 00:40:44.270 --> 00:40:49.270 Fe 2.4 Cr 0.64, known crystallographic phases 00:40:49.270 --> 00:40:51.130 of these oxides. 00:40:51.130 --> 00:40:53.350 So you can look at the peaks found 00:40:53.350 --> 00:40:56.560 to have resolution of like 100th of an electron volt, 00:40:56.560 --> 00:40:59.560 compared to reference values taken on pure compounds 00:40:59.560 --> 00:41:03.850 and materials to figure out what actual oxides do you have. 00:41:03.850 --> 00:41:06.415 That can help tell you things like how protective are they, 00:41:06.415 --> 00:41:08.290 how fast are they going to grow, and are they 00:41:08.290 --> 00:41:09.820 going to be a problem if you want 00:41:09.820 --> 00:41:12.430 to use this new stainless steel that we developed 00:41:12.430 --> 00:41:14.763 in a lead bismuth reactor. 00:41:14.763 --> 00:41:16.680 The biggest problem with lead bismuth reactors 00:41:16.680 --> 00:41:18.675 is lead corrodes like everything. 00:41:18.675 --> 00:41:20.550 And so the whole point of my graduate studies 00:41:20.550 --> 00:41:23.430 was design an alloy that doesn't corrode and lead 00:41:23.430 --> 00:41:25.780 and make ab alloy composite out of it. 00:41:25.780 --> 00:41:28.500 But you can't prove that it works unless you not only know 00:41:28.500 --> 00:41:32.670 how fast it corrodes, but how it corrodes, which oxides form, 00:41:32.670 --> 00:41:34.852 and in what order. 00:41:34.852 --> 00:41:36.310 That's the last part I haven't told 00:41:36.310 --> 00:41:38.650 you about yet is what order. 00:41:38.650 --> 00:41:44.910 So I want to switch to another technique called secondary ion 00:41:44.910 --> 00:41:47.430 mass spectroscopy or SIMS. 00:41:47.430 --> 00:41:50.760 In this case, you start off with firing ions 00:41:50.760 --> 00:41:54.840 at a material, which will then eject or sputter away 00:41:54.840 --> 00:41:57.130 secondary ions. 00:41:57.130 --> 00:42:00.570 In this case, this process of sputtering-- 00:42:00.570 --> 00:42:02.350 let's say this is your material here. 00:42:05.010 --> 00:42:08.950 You send in something like oxygen ions, which 00:42:08.950 --> 00:42:12.370 might be like O2 minus with a mass of 32, 00:42:12.370 --> 00:42:17.300 and then you blast off or sputter away. 00:42:19.880 --> 00:42:22.640 A few atoms at a time from that surface, 00:42:22.640 --> 00:42:25.287 and they'll come off the various masses and charges, 00:42:25.287 --> 00:42:26.870 and in this case, the sputtering could 00:42:26.870 --> 00:42:29.720 be due to Rutherford scattering, because you might directly 00:42:29.720 --> 00:42:32.510 ballistically slam and ion out of the surface. 00:42:32.510 --> 00:42:36.080 Then every one of these ions has a different mass 00:42:36.080 --> 00:42:38.070 and a different charge. 00:42:38.070 --> 00:42:40.100 And by sending it through a mass spectrometer, 00:42:40.100 --> 00:42:42.680 something that separates these materials by their mass 00:42:42.680 --> 00:42:45.290 to charge ratio, because the higher the charge, 00:42:45.290 --> 00:42:48.330 the more deflected an ion will be. 00:42:48.330 --> 00:42:51.588 But the higher the mass, the less deflected it will be. 00:42:51.588 --> 00:42:53.005 That should sound really familiar. 00:42:55.830 --> 00:43:00.710 In our idea here where how these ionization collisions happen, 00:43:00.710 --> 00:43:02.750 if you remember the higher the charge, 00:43:02.750 --> 00:43:05.270 the stronger the Coulomb force-- 00:43:05.270 --> 00:43:11.590 that q1, q2 over r squared. 00:43:11.590 --> 00:43:14.330 I think there was a constant in there. 00:43:14.330 --> 00:43:17.210 So the higher the charge, the higher those q's, 00:43:17.210 --> 00:43:19.340 and the stronger the Coulomb forces. 00:43:19.340 --> 00:43:23.480 But the larger the masses, the less momentum it can impart. 00:43:23.480 --> 00:43:26.000 And so the deflection will be weaker. 00:43:26.000 --> 00:43:28.340 Exact same thing's happening here. 00:43:28.340 --> 00:43:30.890 And you can separate out atoms not only 00:43:30.890 --> 00:43:33.560 by their charge and their mass, but specifically 00:43:33.560 --> 00:43:35.078 by their isotope. 00:43:35.078 --> 00:43:36.620 So this is one of those ways that you 00:43:36.620 --> 00:43:40.640 can figure out and make an isotopic map of a material 00:43:40.640 --> 00:43:42.140 in three dimensions. 00:43:42.140 --> 00:43:44.900 You can scan your ion beam across the material 00:43:44.900 --> 00:43:47.180 and collect the ions at every point. 00:43:47.180 --> 00:43:52.160 And as you sputter, you slowly wear away 00:43:52.160 --> 00:43:54.400 layers of this material. 00:43:54.400 --> 00:43:56.900 And so you can actually reconstruct a 3D map 00:43:56.900 --> 00:44:01.070 with almost nanometer precision of every single isotope that 00:44:01.070 --> 00:44:05.280 was at every location, which is quite cool. 00:44:05.280 --> 00:44:07.560 As you can see, these master charge ratios 00:44:07.560 --> 00:44:10.830 can depend on which isotope of silicon you have, 00:44:10.830 --> 00:44:14.230 what sort of cluster, what molecule, what charge you have. 00:44:14.230 --> 00:44:16.530 And you can do some pretty crazy analysis of things 00:44:16.530 --> 00:44:18.420 to even figure out what sort of compounds 00:44:18.420 --> 00:44:21.870 exist on surfaces, because sometimes you sputter off 00:44:21.870 --> 00:44:23.220 whole molecules. 00:44:23.220 --> 00:44:26.170 They're going to have their own mass to charge ratio. 00:44:26.170 --> 00:44:31.260 And that's what we did for this lead bismuth work. 00:44:31.260 --> 00:44:33.750 Lots of XPS spectra to jump through. 00:44:33.750 --> 00:44:37.170 We wanted to find out which oxides were forming 00:44:37.170 --> 00:44:40.110 and in what order. 00:44:40.110 --> 00:44:41.820 Which one's the best one I want to show? 00:44:44.277 --> 00:44:45.110 Think it's this one. 00:44:48.260 --> 00:44:52.330 So in this case, we used ion sputtering 00:44:52.330 --> 00:44:55.810 to sputter away surface layers to a depth of a few hundred 00:44:55.810 --> 00:44:59.800 nanometers, and we're actually able to show that the chromium 00:44:59.800 --> 00:45:03.640 oxide, right here, was on the outside of the sample, 00:45:03.640 --> 00:45:07.540 followed by silicon oxide, followed by iron metal. 00:45:07.540 --> 00:45:10.330 So in this way, we were able to figure out 00:45:10.330 --> 00:45:12.280 using XPS, the nature of the oxides 00:45:12.280 --> 00:45:15.160 and using SIMS, the order of the oxides, 00:45:15.160 --> 00:45:18.100 so not only how fast were they growing to nanometer precision, 00:45:18.100 --> 00:45:20.920 but in what order did they form. 00:45:20.920 --> 00:45:22.610 And that helped us figure out this sort 00:45:22.610 --> 00:45:27.412 of synergistic chromium and silicon oxidation mechanism 00:45:27.412 --> 00:45:29.870 that helps really protect the layers of the stainless steel 00:45:29.870 --> 00:45:34.350 and explain why it's corrosion resistant lead bismuth, 00:45:34.350 --> 00:45:38.970 all using principles from 22.01. 00:45:38.970 --> 00:45:41.607 So it's about two of five of. 00:45:41.607 --> 00:45:43.440 So I wanted to stop and see if you guys have 00:45:43.440 --> 00:45:45.500 any questions on these analytical techniques, 00:45:45.500 --> 00:45:47.250 knowing that we're actually going to go do 00:45:47.250 --> 00:45:50.690 a couple of these next Friday. 00:45:50.690 --> 00:45:53.508 Has anyone used any of these before? 00:45:53.508 --> 00:45:54.800 Yeah, which ones have you used? 00:45:54.800 --> 00:46:01.260 AUDIENCE: SEM and XPS, XPS [INAUDIBLE] 00:46:01.260 --> 00:46:03.110 MICHAEL SHORT: SEM and XPS? 00:46:03.110 --> 00:46:04.108 OK, cool. 00:46:04.108 --> 00:46:05.650 Yeah, we've got all these instruments 00:46:05.650 --> 00:46:07.630 I think except for SIMS here at MIT. 00:46:11.960 --> 00:46:12.470 Yeah. 00:46:12.470 --> 00:46:12.970 Yeah. 00:46:12.970 --> 00:46:14.845 AUDIENCE: Sorry, what is that second equation 00:46:14.845 --> 00:46:18.585 on the energy for Compton scattering? 00:46:18.585 --> 00:46:21.210 MICHAEL SHORT: This would be the energy of the Compton electron 00:46:21.210 --> 00:46:25.440 that comes out when a photon scatters off of it. 00:46:25.440 --> 00:46:28.020 So the photon will end up losing some energy, 00:46:28.020 --> 00:46:32.165 and the Compton electron will pick up that energy. 00:46:32.165 --> 00:46:33.040 AUDIENCE: [INAUDIBLE] 00:46:33.040 --> 00:46:33.370 MICHAEL SHORT: Sorry? 00:46:33.370 --> 00:46:35.530 AUDIENCE: What's the denominator of that? 00:46:35.530 --> 00:46:37.810 MICHAEL SHORT: It's a 1 plus alpha times 1 00:46:37.810 --> 00:46:40.030 minus cosine theta, where alpha-- 00:46:42.745 --> 00:46:45.330 I'll mention what alpha is. 00:46:45.330 --> 00:46:50.230 It's a ratio of the photon energy to the electron rest 00:46:50.230 --> 00:46:51.070 mass energy. 00:46:57.400 --> 00:46:59.740 This is kind of a nice-- on these two boards right here, 00:46:59.740 --> 00:47:01.960 it's kind of a nice summary of the stuff we've 00:47:01.960 --> 00:47:03.912 been doing over the last three weeks or so, 00:47:03.912 --> 00:47:05.620 and then all the stuff I showed you today 00:47:05.620 --> 00:47:07.890 is what you can do with it.