 So first up is Mark Blungesmaf. He's a professor of material science and engineering, worked with us for many, many years, and has a very interesting project with Shan-Wi Fan. And we look forward to hearing from him. So Mark, the stage is yours. Well, thank you so much. It's a great pleasure to be here and discuss a little bit work, a project that we're doing together with Shan-Wi Fan's group. Let's say here over here. He's the theoretician of the team. I'm the experimentalist. And we're trying to see whether we can bridge the length scales of the atomic scale all the way to the length scales of the universe. And we're hoping to use atomically thin materials, things like graphene or molybdenum disulfide, hexagonal boronitride, to achieve passive cooling of buildings, ultimately, and sort of intriguing that you could do this with atomically thin materials. Why we're doing that? Well, right now in the United States, we're currently using about 15% of our electricity for air conditioning purposes. And that puts an total amount of CO2 emissions in the atmosphere. And wouldn't it be wonderful if we could reduce energy consumption by pulling cooling roofs onto all of our buildings and reduce our electricity consumption? So the way it goes is as follows. We're very used to harvesting energy from the sun, for example, using photovoltaics. And we're ultimately, from an energy perspective, using the energy differential between the sun, which is hot 6,000 Kelvin and the much cooler Earth at 300 Kelvin to harvest energy. And this can be done in efficiencies that were long time ago, already predicted by Carnot in terms of efficiency limits that depend on the temperature of the hot and the cold body. Now, intriguingly, one may not fully realize, but one can also harvest energy from another temperature differential, which is the Earth hot as seen as a hot body as compared to the cold universe. So there's a notable, about 300 Kelvin differential and wouldn't it be wonderful if we could use all that space around it to harvest energy? And sort of interesting to note that the energy flux towards, from the sun towards the Earth is actually balanced by this energy flux out, otherwise the Earth would notably heat up over time. So there's notable energy to be fluxes to be tapped into. So we do have very good radiative access, the way we can connect to the universe is by radiating thermal radiation. There's good radiative access, but we need very careful engineering. And this is in part why we're doing this project with Shenhui's group. If you think about a simple experiment where we might have a thermal emitter that we would like to cool down, maybe put it in a thermally insulating environment from the Earth surface, it could radiate day and night out to the universe, to the cold space. But there's a blanket here, the atmosphere, that actually acts as a filter that filters out certain frequencies of light or electromagnetic radiation. And this is, this filter characteristic of the atmosphere is shown here, is the transmission through the sky as you're looking at the universe. At some wavelength in the visible, we can nicely see the universe, but at longer wavelength, where actually the Earth emits a lot of radiation, there are strong absorptions, for example, by CO2 or water. Their electromagnetic radiation can excite vibrations of such rotations of such molecules. So if I plot on top of this wavelength, here's the visible, here's the mid infrared, the thermal radiation, the black body emission of the Earth, it looks something like that. There's a peak here around maybe eight, nine, 10 micron, where we would love to have a high thermal emissivity in the region, a spectral region here, maybe from eight to 13 micron, where there's a good transmissivity. So if we let a body radiate all of its energy at those wavelengths, then it can get good space access and it should cool down ultimately if we have good thermal insulation here to the temperatures of the universe. So how do we design such a thing? We use a fundamental law called Kirchhoff's law that says for an object in thermodynamic equilibrium, the emissivity is equal to the absorptivity in terms of its frequency and angular dependence. So it basically says if something absorbs well at a certain wavelength, and if I shine light at it from a certain angle, then it will also thermally emit well at that frequency and that angle. So it becomes a very simple problem. We just need to find things that very well absorb in the spectral range at these wavelengths, then they could also thermally emit well. So Shen Hui, my dear collaborator here, you see him here in the center with his students, started doing some of this work already long time ago in 2014 building layered structures of dielectrics and insulators, but also some materials such as silicon and silver absorbing materials and made these stacks engineered their thicknesses carefully to be highly reflective in the visible. So it should look mirror like in the visible then such a material could not absorb the heat of the sun in the visible and therefore not heat up tremendously because there's a lot of, I guess, energy coming in the visible spectrum. Here you see the solar emissivity here, AM1 solar spectrum and here you can see the low reflectivity of their sample that allows them to see themself. However, these layers were designed such that there is strong absorption and therefore thermal emission also in the mid infrared. Of course, this is a quite complicated stack that maybe is expensive to make using deposition techniques. So would it be wonderful that we could ultimately use a single or a few atomic layers of material? That's sort of the concept of this project. Here you see some of the experiments that they did. Here's their little wafer that looks at the sky. Here they tested on the roof of the building across the cloud here and here they look at the temperature versus the time of day morning to mid afternoon. Here you see the temperature, the ambient air temperature and here at this time, 10 o'clock, good time for students to start experiments after coffee. You can see they looked at the temperature with a little temperature sensor of this wafer and they see the temperature rapidly dropped below ambient air temperature in daylight and then started following, but at a lower temperature, the ambient temperature in the air. So they showed daytime cooling and this work attracted a lot of attention. You can, as a brief intermatt, so you can not only just heat or, sorry, cool by thermally radiating, you can actually directly use this to make energy and drive an LED. So here was an example, which was sort of a proof of concept that got featured in the New York Times where there's a radiative cooler that was designed to thermally radiate, connected to a thermoelectric to a room temperature area and using this temperature differential, the thermoelectric can produce a little electricity that drives an LED. So it's a very simple system where there's no energy storage with batteries needed that directly can provide a little bit of light. For example, in a home, maybe in a distant desert, somewhere far away from electricity nets, one can, in this initial proof of concept, derive a little bit of power per square meter enough to drive an LED that allows you to read a book very far away from the net. Okay, so Shanhui and his students didn't sit still. They started designing more advanced and more perfectly engineered the spectral absorption there for spectral emissivity properties and things came out like this, multiple stacks of different materials, silicon, carbide, titanium oxide, things that have interesting lattice or phenolic resonances in the mid IR, structures on top to try and approximate this emissivity spectrum. And they found that ultimately they can very closely match the transmissivity window and get sub-freezing temperatures right underneath the sun and cooling that could exceed here in theory a hundred watts per square meter. That's sort of a goal. So it's something like a 10th of what you may harvest from a sun with solar energy. So how do we make this simpler? Maybe we don't want this very complex stack that's nanostructure, et cetera. And to understand our approach, I want to go back to a little bit of history. Here, Michael Faraday showed that very small and that's the key point, very small nanostructures can be extremely strong tunable absorbers. So he made colloidal suspensions of gold nanoparticles that were later analyzed in the transmission electromicroscope that give this liquid that has many gold particles in it, this ruby red color. You can take a transmission spectrum through this. Here's the, for gold particles, the absorption versus wavelength across the visible. And you can see that the gold particles show a peaked, look like a resonant response in the absorption right here in the yellow part of the spectrum. You see the blues and the yellows get absorbed but in the red, there's not much absorption. That's why if you illuminated with white light from the back, it gets this beautiful ruby red color. The key point is that a single layer of such gold particles on a glass substrate can already absorb about 50% of the light, very strong absorption. So this has caused, and some of the nice work was here, funded by GSEP, a lot of people to think about how do we understand the light absorption properties of metallic and other nanostructures? Can we optimize them? And the interesting thing is here, if you look at the length scale, that goes from millimeters to nanometers and somewhere here, the wavelength scale of light, that's critical in optics, the optical properties of a little mirror, a continuous metal sheet, completely change as you go to the nanoscale. And here's examples of vials, now engineered, gold particles of different size and shape to cause absorption at any wavelength across the spectrum, even into the mid infrared, where we want to have strong absorption. And the key piece of physics here is that in a metal, when you shine light, the electric fields that oscillate back and forth very quickly can drive oscillating currents. But if you have a finite sized particle in your little sphere, then these currents run into walls, and then when a current runs in the wall, you have charge buildup, electrons run into the bottom, putting negative charge, but of charge neutrality, they're positive charges, and these two charges pull at each other through the Coulombic force. There's a restoring force on the displaced electrons that are driven by the electric fields of the light. And this works much akin to a mass spring system. If I have a little mass attached to a spring and I let it go, the mass will oscillate. Here I have the masses of the electrons being pulled back by the Coulomb force, giving a resonance like a mass spring. And this resonance here determines at what wavelength I get strong absorption. So sort of nice that Faraday already did work to that now is starting to have an impact. And here's sort of an evolution. Here's 20 years of history, quite recent history from 2007, where people tried to make metallic structures of different shape. These are electron microscopy images of gold particles. And you can see that the different shapes and different metals all produce absorption and scattering at different wavelengths. The resonances can be moved, then people started putting two together, then multiple particles. And now we can make entire paintings that are generated. If you would look at this painting in the electron microscope, you would see metal particles of all different sizes and shapes. The key point here is that we can control light absorption at will now over large areas using current nanopatterning techniques. So what can we do with all of this when we start controlling building? Well, if we have control over absorption at every frequency, we could make something that looks like a beautiful color, maybe a red Stanford roof, but has the ideal thermal emission spectrum in the mid infrared. And that's here an experiment shown here temperature versus time of day, similar measurements on the roof across the street, different pieces. I don't know whether you can see that, but one little piece looks black, one looks pink. And it turns out that although these have different colors, they may have similar blackness in the eight to 13 micron range. So we can independently make black paints or pink paints or other paints. You can see that things that have the same color here could have very different temperatures as a function of the daytime because we manipulate differently the mid IR thermal emission. So things can look cooler than black paint or things can look measure hotter than black paint. So we can independently tune. Another interesting question is, could we dynamically tune thermal emission? Because maybe clouds roll out over us in the sky and we would want to manipulate the thermal emissivity spectrally throughout the day. All sorts of interesting questions. So to solve this, we're proposing, and that's the key of our current project, to see whether we can achieve light absorption control with tunable, spectrally tunable, atomically thin layers. And some of this work has recently gotten a lot of attention that if you have a single atomic layer, for example, here a little experiment of tungsten disulfide, one of the known atomically thin semiconductors. There you can make transitions from a valence to a conduction band and make electron hole pairs or excitons, and that causes strong absorption. And this absorption actually, characterized by the real and imaginary part of the refractive index, the imaginary part giving the strong absorption as a function of energy is highly tunable. So it's interesting, you have a thin, atomically thin material that very strongly absorbs, but it's also tunable if I apply electric fields. Here's another, this is a low temperature experiment, but people are pushing this to high temperatures now where there's an atomically thin semiconductor from a little disulfide selenide. If you look at it, here's the little flakes seen in white light reflection. And if you inject charges into it, you change the electronic properties and it becomes completely black. So they show that low temperature should get unity reflection or absorption changes. So this inspired a lot of the work we're doing, some of the work here in our own group at Stanford, we made little tungsten disulfide islands that we can electrically inject charges into. You can see in this case, there's not a resonance of the sloshing electrons, but there's an electron hole pair resonance that shows up in the reflectivity versus wavelengths. You see peaks and the absorption. These peaks note are pronounced, but quite small here. The reflectivity changes are on the order of percent. So we need to do something better to get to a stronger reflectivity or absorption changes, but note that for example, if we inject charges into this, these excitons, electron hole pairs completely disappear. So the key here is that we can tune spectral reflectivity. One of the early works in our groups and it's quite reproducible. Here you go, inject the current, take the charges out, put them back in. We can get these resonances on demand. So what can we do with these atomically thin materials for radiative cooling? Here's a simulation that we did with Shenhui based on things we believe we can make. We make carpets of atomically thin material and these, for example, graphene, hexagonal boronitride. We can get these now on four inch or six inch wafers commercially. When you pattern this into little strips, you get very similar to these metallic particles that Faraday studied, so-called plasmonic or collective electron resonances, meaning that if I shine light, the light has an electric field that rapidly oscillates. This electric field can induce current oscillations, displace the charges in a doped piece of graphene, which acts in this case metallic-like and there will again be these restoring forces that can give resonances. If I make the width of a certain size, what I'll see is if I calculate absorption and thereby the emissivity as a function of wavelength that this monolayer of graphene sitting above a metallic mirror can achieve an emissivity that reaches almost unity, which it means it's equal in its emissivity as a black body thermal emitter. So that's pretty impressive for a single atomic layer. Moreover, in contrast to most black bodies, here we can easily tune it by changing the width of this little strip. So here you see the wavelength at which I get strong thermal emission. Here's the emissivity plotted in a color scale, so yellow means very strong thermal emissivity. Here's the ribbon width and you can see that we can tune it. So we can start making ribbons of different width or different shapes and that's the goal of this project to start having multiple resonances fit the entire absorption spectrum. So we can start tuning and engineering the thermal emissivity and it's quite nice because note that the width of these strips is only 40 nanometers, that gives you the strong absorption so we can fit many of these little resonators of different size and shape within about the diffraction limit of light. There are other materials that we want to use and that's hexagonal boron nitride, here written often as HBN. And HBN doesn't have a good electrical conductivity but it has a very good phononic resonance as you can drive lattice vibrations and thereby transfer light energy to directly to the lattice and note here's the emissivity versus wavelength versus in this case we checked out the number of layers and you can see interestingly enough that the number of layers tunes the spectral emissivity but also by tuning it we can get to very high emissivities again for a limited number of layers. So there are all of these interesting materials maybe we'll use combinations of these to get unity absorption in the window that we want. Experimentally we wanted to boost some of our light absorption in these materials and to do that we're exploring the use of corrugated mirrors and we're using right now tungsten disulfide as a model system and we're seeing whether if we can shine light in, reflected of this interesting corrugated mirror that I'll explain more, can we modulate the reflected light indicating that we're tuning and modulating maybe the absorption inside this structure. So we also have an electrode here that can inject the charge in the tungsten disulfide thereby changes optical properties. So if you, and this is interesting physics, if you shine light on a corrugated mirror, so here's a cross section, here's a metal film that we dig in little groove using the nano patterning tools we have available at Stanford, then light takes a very interesting path. Here we simulate the flow of light as given by the pointing vector and in free space over here where we radiate that normal instance, light goes in straight lines, luckily that's what we know, but near these metallic structures, interesting the flow of light gets redirected and goes into the groove. That's pretty interesting the way that works actually is that the light comes in, the electric field drives current oscillations and the current oscillations note, for example if the negative electrons move to the left in each of the teeth, that we get opposite charges across the gap and therefore a very high field concentration in this gap. That's one way of thinking about this field redirection. So we've made such structures, here's the schematic design, tungsten disulfide, thin oxide, grooves in the metal, this is one unit cell. Here you see a little corrugation in a silver film and here's an optical simulation that shows the field intensity. So using these corrugations we can get about 15 times more field intensity and therefore notably more absorption which kills with the field strength squared. So we want to use these to enhance absorption. Here's a, I guess this project is in progress since maybe November or so. As some of the simulations we've been doing here is the reflection versus wavelength. We can tune the electron hole pairs on and off or change the optical properties and we get about here now 20% absorption change. Here's a, I guess reflectance ratio if you take the ratio of these curves. So we've been able to start seeing these things in experiment, here's reflectance changes when we dope the material with electrons or holes and we can start seeing the characteristic reflectance changes and they are a very reproducible in time. So what I guess want to conclude here is that atomically thin materials are exhibiting very strong light matter interaction. They're electrically tunable, they're engineerable in their absorption spectrum and hopefully by the end of this project we'll show that we can do this over large area and get notable cooling throughout daytime. So thank you so much for your attention. Anyone has any questions? I was gonna say this is, in my mind, beautiful work because it incorporates so many things in the Andes, in India and in the highlands, people can radiate energy to make ice or protect their crops from plants. Professor Winston at Chicago made the non-optical devices, but what you have here is like almost that electromagnetic lens thing that's what you have. That's right, we're trying to use the tricks of nanophidomics to enhance light matter interaction. Yeah. Definitely, I'll be here. Yeah, so it's a very good question. So we believe one, that the developments on generating these materials over large area is happening very rapidly. So we can get commercial, I guess, wafers of single layer graphene or multi-layer is a little bit easier over large area. So four or six inches is quite common. Same for hexagonal boronitride. Another nice thing is that for the quality that we need for these absorption systems is not as high as people would like to have in the semiconductor electronics industry where sort of the push right now is to develop better and better materials over a large area. So there's, I guess, to believe if we do the research now on that front, the fabrication or the synthesis of these materials will be there. The other part is nano-patterning and I think techniques such as nano-imprint lithography or there are now new types of optical rolling lithography techniques that are getting relatively inexpensive sort of at the few dollars per square meter cost level. These techniques can now make sort of nanostructures on the sort of the 40, 50 nanometer scale that we hope to use. So I guess we're excited about all of this progress. We couldn't do it yet, maybe tomorrow, but I think by the time we start six new companies, no. But by the time this has developed, if we can show it works, I think there will be a path. So, again, really cool. Another question on scale, which is I know over the last several years, people have been thinking about if you could do this at large scale, what's the impact on whatever energy demand and so on. I think there's been some people kind of trying to estimate that. Is that, is anything changed? Like are some of the techniques you're developing, could that change the total impact on kind of energy demand? Well, yeah, I think we're trying to solve a little bit of a different challenge, for example, as with or compliant, maybe that's a better way to say it, for example, solar energy harvesting or other techniques that can harvest energy. This is something that can work day and night, could have solutions where maybe in some cases we don't need energy storage. I think what we're targeting is sort of twofold. One, can we reduce the electricity consumption for air conditioning? So this could sit maybe on buildings, maybe we believe these things could be lighter maybe then solar modules, although that's to be determined. So that's one area where we're probably not going to compete with solar utility because solar get more energy per unit area. So there's some specific applications and then there's, I think applications, can we impact the life of people living in a little village somewhere far away from the grid where we can in nighttime provide energy storage in a very robust, I think, fashion without the need for batteries and other things infrastructure that could go wrong. So yeah, I think many of the things in scaling still hold it's just one maybe extra component that can help.