ORNL computing, microscopy expertise aid study of carbon 'it' child
When Manchester University scientists Andre Geim and Konstantin Novoselov famously used cellophane tape and a pencil to create a one-atom-thick layer of carbon in 2003, they gave birth to the scientific wunderkind of the past decade—graphene.
The seemingly simple material, made of a single sheet of carbon atoms in a chicken-wire-like lattice, landed the scientists the 2010 Nobel Prize for Physics and spawned a flurry of research into its amazing properties. Graphene's high strength, light weight, optical transparency and ability to conduct electrons make for potentially exciting applications in flexible electronics, sensors and fuel cells, among others. But, like a lot of children, graphene has some growing up to do.
Current manufacturing methods can produce graphene in bulk but often result in defects or impurities that are by-products of the growth process. Defects can include other kinds of atoms such as silicon that sneak into the otherwise purely carbon lattice. Carbon itself can be an impurity, forming uneven lumps on graphene's two-dimensional surface.
Although the word "defect" automatically implies a problem, graphene defects are not necessarily undesirable. They may even be beneficial for certain applications. But their very existence means a lack of control over the production process, so scientists are trying to learn how defects form and how to control them for different applications.
"It's still difficult to fabricate a sheet of graphene without defects," said Bobby Sumpter, a staff scientist at ORNL's Center for Nanophase Materials Sciences. "Defects are nature's way, and you have to either develop a good understanding of what they do to the material or figure out a way to minimize them."
For Sumpter, getting to know a material such as graphene and its defects comes down to a mathematical equation. "The equations we solve are the same.
It doesn't matter if it's graphene or copper or gold or a hybrid," Sumpter said. "There are specific sets of equations we need to solve; it's just not always easy. Oak Ridge has a core capability in mathematics and the infrastructure in high-performance computing. That allows us to solve a number of those equations very accurately."
Sumpter uses computational simulation, in collaboration with experimental researchers, to explain and ultimately predict how materials such as graphene behave in different circumstances. One recent study focused on developing a new way to make graphene nanoribbons, where the material is constrained in width. Controlling the width of a graphene piece is important for electronic applications because the width determines graphene's semiconducting properties.
The research team realized multiwalled carbon nanotubes, or carbon cylinders rolled up inside one another, could be unzipped to form graphene nanoribbons with reasonably clean edges, another critical property. The researchers first filled the nanotubes with liquid nitrogen and then dumped boiling water on top to turn the liquid nitrogen into a gas. The resulting high pressure caused the nanotubes to rip apart along a neat line and form graphene nanoribbons. The team's simulations helped explain the mechanics behind the process.
"It's like taking a hot dog and putting it in boiling water," Sumpter said. "It always splits down the middle along a seam. Here we have a 'nano hot dog'!"
Another study took advantage of graphene's defects. Sumpter and his collaborators made a graphene-based supercapacitor, a device that can store electrical energy. In this case defects can be a benefit because they may serve as local reaction sites that contribute to the supercapacitor's energy storage capability.
"For lightweight, strong materials, you don't necessarily need defect-free graphene," Sumpter said. "As a matter of fact, it could be better to have some defects for certain applications because that's where stronger interactions between one material and another material typically happen—at the interface. A plane of graphene is inert and fairly stable, so the defects are where things go to oxidize or functionalize."
Under the microscope
While Sumpter studies graphene with equations and simulations, fellow lab researchers are using complementary experimental techniques to examine the material in real time. With the help of some of the most powerful microscopes in the world, ORNL's Juan-Carlos Idrobo and his colleagues are looking at graphene at a scale that has never been reached—down to the level of individual atoms. "Our goal is to poke around in materials to see if we can find new physical phenomena or understand, for instance, why specific defects behave in the way they do or how they change the material properties on a larger scale," Idrobo said. "Defects are the way you control the performance of a material and therefore the device.
"The electron microscopes available at ORNL through DOE's Shared Equipment Research User Facility and the lab's Scanning Transmission Electron Microscopy group are ideal for studying defects in two-dimensional materials such as graphene. Scientists can use the microscopes to precisely pinpoint individual atoms and understand how those atoms bond to the surrounding structure. Such information is useful for predicting or improving different material properties and performance.
Aberration-corrected scanning transmission electron microscopy, which ORNL researchers have helped perfect in recent years, is well suited for imaging carbon-based materials such as graphene because of the technique's unique ability to maintain high spatial resolution at low voltages.
"To study these materials, you need two things: you need a very good resolution, but you also need to have the microscope working at low voltages," Idrobo said. "The problem is that when you work at low voltages, your spatial resolution decrease. Aberration correction allows you to go to lower voltages without losing spatial resolution. There is nothing comparable to these microscopes, if you want to study materials on the atomic scale."
In a proof-of-concept experiment published in Nature Nanotechnology, Idrobo and his coauthors used the high-powered microscopes to show how silicon defects in graphene could potentially transfer data at the atomic level.
"We showed that a tiny wire made up of a pair of single silicon atoms in graphene, in principle, can be used to convert light into an electronic signal, transmit the signal and then convert it back into light," Idrobo said.
The team's imaging analysis found that the silicon atoms act like atomic-sized antennae, enhancing the optical-like signals of graphene and creating what's known as a plasmonic device.
"The idea with plasmonic devices is that they can convert optical signals into electronic signals," Idrobo said. "So you could make really tiny wires, put light in one side of the wire, and that signal will be transformed into collective electron excitations known as plasmons. The plasmons will transmit the signal through the wire, come out the other side and be converted back to light."
In an ongoing project, Idrobo and his colleagues are putting ORNL's powerful microscopes to work to understand what happens at the atomic level when current is applied to a sheet of graphene. "We can heat the material up under the microscope and see how the atoms move," he said. "Because our microscopes have the best atomic resolution and sensitivity, we will be able to see what nobody else can."
As they continue to study graphene and its defects, Sumpter and Idrobo are also interested in combining graphene with other two-dimensional materials to make hybrids that could yield even more interesting characteristics.
"Graphene has all these wonderful properties that make it suitable for applications in a number of areas ranging from simple electronics to photonics to energy storage to arrays for solar energy," Sumpter said. "The problem is going from the little bitty thing up to something that's macroscopic and then to the device scale. That's not easy, and that's where composites come into play."
Idrobo added that graphene has to overcome substantial barriers to entry into today's electronics industry, which has invested billions of dollars and decades of research in silicon-based devices.
"Graphene by itself is not going to replace silicon in the electronics industry," Idrobo said. "But the combination of graphene with other materials is something that could be used to make novel devices. In theory you could make a flexible cellphone by imprinting a whole electronic circuit into a graphene substrate.
"Developing the quality that the electronics industry requires for mass production is not going to happen over night," Idrobo added. "You need to understand the material's surface, chemistry, physics and more. That's what we're working to understand."
It may take a while before graphene, by itself or in a composite, matures into technologies like bendable iPhones or automotive fuel cells, but the basic research done by Sumpter, Idrobo and others at ORNL is helping the material come of age, one baby step at a time. óMorgan McCorkle