How To Grow a Super Material or Troubleshoot a Classic One

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2 How To Grow a Super Material or Troubleshoot a Classic One David A. Muller APPLIED AND ENGINEERING PHYSICS Being able to see each atom of a material has allowed us to troubleshoot, discover, and create materials of all kinds. The SuperSTEM and Us Material failures can cause devastating accidents or take down entire systems because a few atoms of the material are in the wrong place. Figuring out what caused a failure or problem and rectifying it is the suitable solution. The answer may also be to create a new, better material. But first, we have to see the atoms that make up a material. Using our electron microscopes that we perfected here at Cornell, I can see a material s makeup, atom by atom. I can work out what each atom is and what each atom is doing. Developing techniques for our instruments and using them in materials research has led my lab to substantive discoveries as we create, characterize, and control materials at this very, very small length scale. To give you an idea of the scale at which we re working, imagine that a wafer of computer chips is the size of the United States; a single transistor on a single chip is the size of a car parked somewhere in the United States, and that car has a pin in it. We can find that pin somewhere in a car parked somewhere in the United States, and tell you the color of the pin. This is the scale of magnification we have with our electron microscopes the Nion SuperSTEM (scanning transmission electron microscope) and the F-20. So the challenge is, if we are looking at trillions and trillions of atoms in a piece of material, where do we start? We start with something you can handle with your hands, perhaps 30 centimeters across, and chop out a piece of a few microns. We use micromanipulators and focused ion beams to carve out little sections of the material that are just a few atoms thick. We transfer that to our electron microscopes and look at the individual atoms in it.

3 An Electron Microscope Our two electron microscopes do slightly different processes. We need both in order to do our work. The Nion SuperSTEM takes color pictures, where every color is not just a different atomic species but also a different electronic state of each species. It gives us color maps. The SuperSTEM is a great tool for discovery. Cornell has the first of this new generation of electron microscopes designed for chemical analysis and imaging at the atomic scale. Only four exist in the world, but others will follow. Fascinating! t TROUBLESHOOTING, DISCOVERING, AND CREATING MATERIALS: What we see the structure of materials, atom by atom What we use to see them Aberrationcorrected and monochromated electron microscopes, used with multiple Cornelldeveloped techniques To what benefit to see what has not been seen before, in order to make new and better materials, including those that do not exist in nature To see one atom at a time, we have perfected these machines and developed many techniques for them at Cornell. If we have a collection of computer chips, and one transistor on one chip is not working properly, we would take our focused ion beam which is like a little trench digger or excavator and cut out just that one transistor. We pick it up with a microscopic needle and put it on a grid. Then put that in the microscope and shine the electron beam on it. The electron beam goes through the sample and forms an image on the screen underneath, and we can see every atom. Just a few atoms in the wrong place will stop a computer chip from working, cause a turbine blade to crack, or poison an industrial catalyst. It s a bit like CSI forensics, except with single atom sensitivity. Growing a Super Material My students and I are working with Paul McEuen, Physics, and Jiwoong Park, Chemistry and Chemical Biology, and their groups to learn more about the properties of a new supermaterial, graphene. When we zoom in very close with our electron microscopes, we see that graphene is a layer of common carbon atoms whose structure looks like chicken wire lots of little hexagons, but it s only one atom thick. It consists of many patches that have grown together, stitched at the boundaries like pieces of a quilt. Each piece of material is a little scrap that cannot grow big enough by itself, but the pieces stitch themselves together reasonably strongly. Each patch in the quilt is a different grain with a different orientation. When we color it by orientation, it looks like a patchwork quilt. Until we took these pictures, no one really knew exactly how these pieces join up or how big they are. The really cool thing about graphene is that this one atom thick material can be grown into sheets that are almost a meter across. We have been able to image all the different patches to show that graphene, even with the patches, is still a good conductor. The first applications for graphene might be things like flatpanel displays for your cell phone or large screen TV and other similar devices. It would make an excellent electrode for solar cells. Discovering Soil Facts My lab is having fun looking at soils from the Amazon and other tropical regions with Johannes Lehmann, Crop and Soil Sciences. Adding biochar to these soils can greatly improve their fertility, which is surprisingly not very good the nutrients just keep getting washed away. Learning how a soil is put together at the atomic scale is intriguing. 38

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5 Why this Research? I was going to do electrical engineering and build faster computers. I realized that I could not take a bunch of chips and string them together in parallel, because things were too slow. I thought about how to make faster transistors and recognized I needed new materials to do that. And those new materials had to be designed at the atomic scale because that s how small some parts of transistors are now. When you get down to things that small, you cannot look up the properties of materials in a table the way device simulation programs do. Materials take on new physical properties just by virtue of being small. Understanding the physics is a must. And it is imperative to have some way of knowing what is happening at that small scale. The most effective way to do this is with electron microscopy.

6 Research in Progress Graphene has lots of little patches that have grown together. Grains rotated in one direction are green, and grains rotated in another direction are red. The little black dots are the stitching at the grain boundaries. This image shows about 250,000 atoms across, thinner than a piece of paper. Muller Lab Traditionally, scientists think of soil as big blobs of this and that we know what each blob is, because we can look it up in a table. But when we look at soil with an optical microscope or any x-ray synchrotron, it s messily intermixed and looks like a blur. It s confusing. Muller Lab Silica Glass In collaboration with the University of Ulm, we accidentally discovered a two-dimensional silica glass just one structural unit thick making it the world s thinnest pane of glass. For the first time, we were able to see the location of all the atoms in an amorphous material and directly confirm a model first proposed 80 years ago. When we look at it with our electron microscopes, we realize the soil is made up of nanoparticles that are just 5 to 10 nanometers across, and some of them are only 2 nanometers across. The nanoparticles have very well-defined shapes, structures, and chemistry. Now that we realize what the nanoparticles are, when we look at soils from Australia, we see that they are composed of the same building blocks as the soils in the Amazon, but the building blocks are so small that scientists missed them previously. At previous lower resolutions, what looked like iron, carbon, aluminum, and silicon stacked on top of each other, is actually in little separate pieces. The result is we may get a new way to understand, classify, and predict the properties and behavior of soils, so that we can improve their fertility. Troubleshooting GM s Fuel Cell The last several years, we have worked with General Motors to help make a fuel cell vehicle more durable. A fuel cell vehicle can have a 300-mile range you can drive it from Rochester, New York to New York City. My students have had fun driving the vehicle, powered by fuel cell, at high speeds on backcountry roads, as well as around the Cornell campus. It works just fine. The problem is that the fuel cell does not last as long as it needs to in order to be an economically viable vehicle, because it contains very expensive catalysts with platinum. And the catalysts degrade over time as you use them. The big question for GM was how do the catalysts degrade at the atomic scale? We were able to look at the catalyst material with our electron microscopes at different stages of the fuel cell s lifetime. From that observation, we identified two distinct degradation mechanisms. The debate over which of these mechanisms was the culprit had previously divided scientists into two camps, arguing for decades. No one really knew the answer. With the ability to look at this material directly at different stages of the degradation, we identified the actual mechanism by which the catalysts degrade. It is a combination of the two methods, where each by itself does a little bit of damage, but the two methods together greatly accelerate the damage. If you slow one mechanism, you ll slow the other as well. Scientists in the field had not considered this. We helped GM build a better fuel cell. We figured out the problem, and they changed their process. ` people.ccmr.cornell.edu/~davidm 41