So today, I'm, I'm really happy to welcome Tory Welsh to our, to our spring graduate student seminar series. So Tory, as a third-year doctoral student, working with, with Matt, matt Doty, who's an expert in quantum materials. And it never really occurred to me the irony of his last name and the fact that he's an expert in quantum materials. But, but he is. So anyway, Tory graduated with her bachelor's degree in chemistry from chemistry from Sydney gems CEO. And like many interesting people, she enjoy cycling, but she's also musically chat widget, which is great. And today she is going to speak to us about quantum dot nanostructures for photon up-conversion. Thank you very much for coming today, tori. Well, thank you for the invitation. Let me get my slides up here. Okay, So today I'm going to talk about quantum dot nanostructures for proton up conversion. That's a lot of words that you might not know what they mean besides maybe dot. So I'm going to go through and make sure that everyone's on the same page and can understand what kind of work we're doing in the dirty group. So first, why do we even care about this? Like what's the application? So the main thing that I'm interested in is helping improve the efficiency of solar cells. So what are these limitations of the solar cells? So you probably have some idea that you have your solar cells, you have the sun. You have some light from the sun coming in, and then you're getting electricity out. Now you might guess that this is not the full story here. There's actually a lot of other things that are happening. But one of the main limitations is that even on the brightest, sunniest day, not all this light, it's actually getting absorbed by the solar cell. And there's actually some amount that just passes right through does not get absorbed in if it's the best solar so that we can make, can't actually harness all the light from the sun. So this creates a huge limitation to how efficient the solar cells can be. So if you're more of a plot person, Here's spectrum of light from the sun, so graze all the light we get. And this red portion is actually the only part that we can use. So you can see that there's all of this gray section here that just passes through this others. Oh, we can't use it. And this is called below bandgap light. And I'll explain later what exactly that means. But for now you can just think of it as an intrinsic property of the material that limits what kind of light can be absorbed. When you look at different materials. And we can make solar cells out of, they each have their own band gap. And depending on that band gap, it creates a hard limit, which is represented by this red line as to what the efficiency can be. So traditional single junction silicon solar cell here you can see the hard limit is around 32, 33 percent. And even if we improve these as much as we can and made them like really robust, were still always going to hit this hard limit because of this energy loss. So this kind of is the overview of motivation behind the project. And I'll get into the background so we can understand what this means and then how we can actually overcome this lament. So first let's talk about how do these materials interact with light to begin with. So they interact through the excitation and then relax, relaxation of electrons. So every material has this ladder of allowed energy levels for electrons. So energy is going this way and this is the lowest energy state. So some of these states will be occupied and some are going to be empty and available. And the electron wants to be in the lowest energy state, goods no stable. Now, if we imagine a light shining onto this material, we can actually excite the electron, give it enough energy to get up into these higher energy states. Thing is, it doesn't really want to stay here. He wants to get back into the most stable state. So it's going to relax down to the low energy state and then emit that energy as a photon. So another pocket of light. Now you can imagine some other light coming in. Maybe it's lower energy light that doesn't exactly match up to this energy gap. So this is actually just going to pass right through and the electrons going to stay where it is. So this is kind of what happens when I'm talking about the light that's just passing through the solar cell because entered goes match, the energy doesn't match to the gaps of the allowed electron states. So there are a few other processes that can happen here. So the common one is photon down-conversion. We can make guesses opposite of what kind of conversion. So in this case, you can imagine a really high-energy photon that's exciting an electron up to this higher energy level. Now, it doesn't want to be here, wants to get more stable. So it might relax down to his intermediate level. But instead of emitting a photon, it might just relax and release energy as heat. So this is kind of a waste of energy here. And then it might relax back down and a photon. But this photon is going to be lower in energy than what was absorbed to get up to this higher energy level. So this is. One step up and then two steps down. So the opposite of that, the process that we're interested in is photon up conversion. So in this case, we can absorb one lower-energy photon to this intermediate level. And then if this electron stays here your long enough, it can actually absorb a second lowering Depot time and get up to this higher energy level. And if it stays here long enough again, it can actually drop all the way down back to the bottom and release a single high-energy photon. So this is two steps up and then one step down. So we're kind of adding these energies together and then getting one high-energy photon out. And as you can imagine, this is really difficult to achieve because it's a very special process that you have to make sure that each of these sub processes are happening in that it's staying in the right places for the right amount of time. And it's difficult to engineer this, but that's what we're trying to do. So back to the properties that we were talking about. Important one is the band gap. And this is simple if you understood what I was just explaining because it's the same as the energy levels we're just talking about just on a larger scale. So if you imagine here one atom and then two atoms, now we have that two or three energy level system that I was just showing. By 9th, you imagine adding more and more atoms. So going into molecules or maybe like a crystal structure, we're going to get more and more energy levels. The ones on the bottom are going to be occupied without an excitation and then the ones on the top are going to be empty. So if we keep adding these up and we could keep going into like an infinite amount, we're going to end up getting these bands. So instead of thinking about discrete levels, we're thinking about a band that's filled with many levels. So the bottom band is going to be filled with these electrons and then the top end is going to be empty. So it is analogous to a two or three level system is just on a larger nanoparticle scale rather than an atomic or molecular scale. And then instead of just thinking about the gap between the two levels, we're thinking about the gap between these two bands. And that's what's called the band gap. And this is unique, a unique property to each of the different materials. And those materials are called semiconductors, and they have those interesting electronic and optical properties that we can engineer. So back to the solar energy problem. All this below, bandgap light doesn't have the energy to excite the material and convert it to electricity. So how can we fix this with photon open region? So if we imagine a single junction solar cell here, we can imagine putting a back reflector layer that has these up converting materials in it. So then we can absorb that low energy light that just as passing through the solar cell. Do that two steps up, one step down process, where we're emitting one high-energy photon reflected back into the solar cell and then we can actually use it. Now we have a way that we could theoretically surpass that hard limit that we were talking about before. Where any question so far, because I don't mind stopping and making this more of a conversation if anyone has any questions. So what kind of materials are actually using for this? So the materials I'm using are called quantum dots, and these are semiconductor nanoparticles. And what's really important about them is that we can really tune where those energy levels are, which then changes how absorb intimately. So here we have the different materials and different band gaps that correspond to each of these quantum dots here. And these are really tiny, so they're like on the nanoscale. And the smaller, the smaller we go, the more large this bandgap becomes, and the more high-end energy they will emit light. So you can kind of think about it. Like if you take a water bottle and squeeze it, you're pushing the water level up because you're constricting it. So if you control the width of these particles and make them smaller, these energy levels are going to go higher and they're going to get more space. Now, hello corresponds to the color. So you might know that red is lower in energy than blue. So these larger particles that have a lower, smaller band gap are going to look red. And then the smaller ones with the wide bandgap are going to look blue. And what's special about them is that we can really carefully control where these levels are based on their size, their structure, and then also just what kind of material that is to begin with. And they're really useful for solar energy applications because they absorb really strongly, because they have these continuum, a continuum of the bands rather than these discrete states. And they can undergo transitions, would be electrons both across the band gap and then also within one band, which will allow us to undergo this photon up conversion process. And they're useful because they're pretty easy to process. They're just in solution. And you can imagine like inkjet printing them on to any kind of substrate. And they're really compatible with the processes that we already used to manufacture solar cells. So this is our model structure. This is a band diagram that's a lot more complicated than Van der I've just showing, but I'll walk through each step. So our structures start with a core quantum dot and then we grow and nanorod off of them. And then we grow another client. At the end. It says basically two of those quantum dots with this nanorod spacer layer between them. And the way that these can actually undergo foot on up-conversion is through multi-step process. So first, we're absorbing a smaller energy photon to excite an electron. Now the absence of an electron as a whole. And so down in this band we have this particle that's just the absence of an electron. Now this electron that's been excited, since there's not really a lot of offset between these bands, it's free to move here wherever it wants to be. So it becomes delocalized across this entire band and it can even move into this other quantum dot. Now a second lower-energy photon is absorbed. And this is where we do that too step ladder process. But instead of going for the upper, actually exciting an electron from here up to here, which is the same as exciting, uh, hold down. So now that this hole is down further into the filled band here, it can also become delocalized and then travel over to the other quantum dot. And then now, since both the carriers are in this emitter quantum dot, they can recombine and emit that one single higher energy photon. So energy of this photon is going to be a little less than the addition of these two, by its way, more useful than either of these alone. And we can actually see this happening in the lab. So this is a laser shining through a solution of these. And you can see that actually looks yellow, even though the particles themselves and Liza refining and it's red. So here's the photoluminescence intensity of these particles. So over here we have a peak that's coming from this lower energy transition. And then over here we have the peak that's coming from this higher energy transition. And when we shine a laser that's has energy higher than both of these, then we can get recombination and both of these, so we get the two peaks. But if we shine a laser that only has enough energy to excite these transitions, we still see this higher energy peak. And the only way that this could possibly happen is because we're actually undergoing this up conversion process. So there are a few issues with the nanorod base platform. Doesn't have the best performance and doesn't have the best synthesis because it's a complicated structure. But we were able to learn a few things from that structure and then we can apply it to new structures that might be easier to make. So what I'd been working on specifically is this spherical structure. And it's actually kind of easier to understand because instead of that rod dot is just core shell, shell, so it's a bunch of concentric spheres. But the process works exactly the same. It's analogous. It's just look symmetrical because it's a sphere would spheres over it. And this has some benefits over the other structure. So first of all, since it's spherical, it's easier to synthesize. The methods are a little bit more robust. We had better control over the size and the sheet because we don't have to worry so much about the shape. Whereas in the other structure we had to try and grow that nanorod with his kind difficult making use some of the things we've learned from the other structure. So some things we did was alloying, alloying these layers with other materials. And that helps optimize the conversion process. We can do that with this structure. Also. Another thing is that we can passivate the entire structure with another material which helps the electrons stay confined and structure a eliminates any kind of defects on the surface that can lead to non-radiative recombination that we lose energy as heat. So this is what it actually looks like in the lab when I make these. So one important thing is that all of these materials are air sensitive while they're being synthesized. So some of our materials we have to store in the glove box over here, which is filled with nitrogen. And then when we actually run the reaction, we do it under a syncline where we can pull vacuum or we can use argon. And then our class are not exposed to air at all during the reaction. So this is just a schematic of how the reaction works. The details aren't really important, but it basically is just that we have. A solution of some of our starting materials, and then we're injecting the other materials into it. So in this case for the core, it's a cadmium telluride quantum dot. So we have our cadmium in the one solution and then we're injecting our delirium into it. And this is going to form our quantum dots. So this is what it actually looks like in the lab. So in here we have cadmium with 12 different solvents. And then in this strange, I have delirium with other solvents. And when you inject it into it, you can actually see the quantum dots form in real time. So you'll, you'll be able to see you in a second. So as the particles get bigger, as they're growing, they're going to get redder and redder because of the side effects that I was talking about. So it ends up being really break and pretty. And then we can actually see what we made using electron microscopy. So it kind of works the same as a regular microscope, except that instead of shining light at the materials we're finding electrons. And this is actually what those dots look like. So the scale bar here is five nanometers, which is really tiny. I mean, you can see all these dots here. So this is really exciting for me because as a chemist, never get to see where you actually make like you get to see maybe like a peak on an NMR something. And that's the most exciting thing you're going to get. But with nano materials, you can actually image them and see them, which I really enjoy doing. So here we have those CAD telluride course, and this is the PL peak that we get from them. So the photoluminescence peak. And you can see it's really sharp and it's centered at around, I think it's like 630, which corresponds to red. And the reason why it sharp is because these are all relatively the same size. And it's just a very simple quantum dots. So it's the same as the first image I showed with the schematic about the different quantum dots. And this is what it actually looks like in real life. So this is a solution of them and I'm shining laser through it and you can see that it's glowing red. So then what I do is I use kind of the same process to add a shell. So in this case I ended a three nanometer thickness shell. And this is just a histogram of the different sizes and I measured. And you can see that this peel peak is shifting further into the red. So it's shifting to lower energy wavelengths. And this is because by adding the size to it, we're reducing the confinement and we're bringing those empty levels and down. So then finally I add the emitter shelled and you can see they get even bigger. So this green bar on the screen borrow the same. So now the center of my histogram here is larger. And so we still have the same peak. It gets red shifted a little bit. But then we get the emergence of this new peak from the cad selenide, which is where we're emitting from. So just some of the other things that had been working on. Here's some more band diagrams of things I'd like to be able to make. So here we're putting in these alloys into this spacer layer here. And that'll help bring the carriers into the emitter where they can recombine. And we can do a couple different alloys in the core and then also in the spacer layer here. And this is something we did with the rod base structures that I really like to be able to incorporate into these structures. And also I'm focused on is improving the synthesis of these. So I said like, Oh, they're really robust, isn't the size. They are compared to the rods. But in general, nanoparticles are really finicky. It's hard to get the same thing over and over again. So you can actually do like a repeatable study, said, something I'm working on. Another thing is that while we are focused on solar energy, the wavelengths are not exactly lined up to what we would want for solar energy. So while I'm able to shift more into the red using these structures versus the rods is still not really in the ideal region where that below bandgap light is being lost. So in that case, I'll probably be shifting the other material types. So probably like lead-based structures in the near future. But for now I'm working in these cadmium structures as kind of like a model. So we can honor and also avenues and computational modeling which helps us understand where exactly the holes and the electrons are within the structure. So you can actually get a picture of the wave functions of those and then understand the processes and how they're moving up the stroke. So with that, I'd like to thank the rest of my group, especially Matt Bruce and Jill Jones, other student who works on this project with me. And I'm happy to take any questions. I have a lot of backup slides with more details, so please ask away. If you have any questions, just unmute yourself. And you can speak that Laura. Hi Tory, thanks for a really interesting presentation. I wanted to find out from you. I mean, you started the presentation talking a bit about efficiency. So with these new materials that you're working on, have you been able to kind of compare their efficiency to those of the the traditional ones that we're used to saying. Yeah, so actually before I bring the group, they did a lot of modeling with like different kinetic, kinetic rate models to try to figure out what efficiency you can actually get by adding these onto a solar cell. And they actually found that you can increase the efficiency significantly. So we haven't been able to incorporate these into a device. Mainly because while they do up convert, they don't do the conversion process very efficiently. So then to multiply that small number by the number by efficiency of the solar cell, it's actually going to make that much of a difference. So once we're able to improve the up-conversion process in these structures, then we'll be able to actually build a device with them and compare it to the models that we have. So I guess I'd like to follow up on that. So why, why is it a conversion? I guess more efficient or why don't, why, why don't more, why does it produce more up converted photons or why is that? Yeah, so if I go back, I grab my head here. So this process I explained here, it's kinda like the ideal process. And there's a lot that can go wrong. So first of all I say like, Oh, this electron has to escape the emitter. Maybe he doesn't do that. Maybe it actually stays right here and then recombines with the hole and emits from here on. Maybe it actually, there's some kind of defect. And then it relaxes over here and doesn't emit a photon at all. Maybe this hole gets emitted. Meaning this hole gets excited only to here not to hear. So it can actually get over to the emitter and then it gets stuck. So we're getting a lot of recombination, not where we want to be. So perfect efficiency would be we get two photons in and we get one photon out every time. And that would be like a 100 percent photon up-conversion. But for this entire process to actually work, it's actually pretty rare. That's why we're doing this engineering to try to make the process more feasible. Thanks. Thank you. Are there any other questions for toy? Yes. Hi Joy. For the interesting talk. So I'd like to ask. So most of that time you'll work with cadmium telluride are seeing venture that you got me. Um, so in the treasure we can find that guy. Yeah, he said toxic, poor environment. So being preparing such a vast volume of get relate to that you can embed the environment in some area. Yeah, that's a great question. So actually, since these absorbed so strongly, the concentrations that I'm using are really tiny. So when I filled the violin, it's linked bright red. I'm talking about like nanomoles of material that's in that entire vital. So the concentrations are so low that the environmental effects actually pretty minimal, honestly, the most environmental effect for our research is the amount of solvents that I use. So not really the cadmium, but just all of the organic solvents that we have to use in terms of actually putting them in a solar cell. The process for like disposing of it would be similar to this, like decommissioning irregular solar. So when you think about like so kind and all the metal contacts and stuff that's on a regular solar cell. Um, it wouldn't really have a huge impact when you consider all of that together. Okay, Thank you. But there's certainly other application, so I will say talk about solar energy, but there's also a lot of biological applications for this. So like imaging cells and doing like a photo release drug delivery and for something like that, yeah, cadmium is probably going to be an issue. That's why we're trying to use this as kind of like a model. So we can figure out how to make this process work really efficiently. And then we can start to incorporate other materials that there may be a little bit healthier. Any other questions for toy? So just out of curiosity, I mean, it looks like right now you're mainly working with metals. I know Matt Dodi is also involved in charm E10, so they have a lot of polymer research going on. There are, there are other types of materials that are also being investigated for these types of applications that you're looking at? Yeah. So actually don't know a lot about his involvement in time, but there's actually like two sections, so there's like polymer and division and then there's like a hard material like semiconductor division. So I think his work in farm is more about like quantum computing and it is like solid-state devices. In terms of upward than there actually are other materials. So there's materials called lanthanide. So it's like the very bottom of the periodic table, that part That's kind of floating that no one ever thinks about. So those like atoms can actually put on to convert. It just really inefficient and it's not tunable. So since they're atoms, their energy levels are very discrete. You can't change them. Whereas with these we can engineer them. And there's a couple other like organic on there probably TTA like emitters and sensitizes that can also promote conversion. And a lot of people who are doing this work for biological applications are focused more on those. But again, since they're just molecules, they're not nano materials, they're not tunable. So those energy levels, like when I was showing the the band diagram, they're stuck with whatever levels these are and they can't change them. So with these materials, we can carefully tune what wavelengths we're working with over a pretty wide range. So that's the main thing that makes these beneficial. So I want just one last question. So for me anyway, they show just how how temperature sensitive because all of this. So you hit these materials up to they did the bat does the bandgap move around or anything or yes, or actually when I run the reaction, it's at about 350 Kelvin. I don't really, or at Celsius, sorry not helping. So I don't really know what the conversion is to Fahrenheit with that, but it's, it's at a really high temperature to begin with. So I don't think that there, but the temperature can really change the bandgap because I'm changing like the intrinsic properties of the material on yes, like we do heat them up really substantially and cool them down to room temperature and their stable. So yeah, I know like some materials for solar cells that are sensitive to air and he, and everything that you wouldn't really want a solar cell to be. So luckily, these don't have this issue because once they're synthesized there in open air and they're perfectly fine. Thanks. Thank you. Any other questions for toy? All right. In that case, it I'd like to thank you again for coming and spending an afternoon with us and, and helping me understand quantum dots or perhaps cool. I wish you luck with your research. Well, thank you. Okay. Alright, thank story. Take care. Thanks.
Tory Welsch: Quantum Dot Nanostructures for Photon Upconversion
From Cindy King September 30, 2021
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