If the revolution comes, it will have started with Scotch tape, pencil lead, and cosmetic powder.
Physicists first produced graphene—a one-atom-thick sheet of carbon atoms— from humble graphite in 2004. It was the sort of accidental discovery that scientific legends are built on: Andrei Geim and Kostya Novoselov stuck plain old Scotch tape onto chunks of graphite, a substance found in pencil lead, and peeled it off. Voila, graphene. And for Geim and Noveselov, a Nobel prize.
The technique was so simple that it was soon adapted for high school classrooms, but it never scaled well. In fact, graphene has been maddeningly difficult to manufacture in mass quantities, holding back an entire class of revolutionarily fast, flexible, and tiny electronic devices based on the material.
But graphene dreams just moved one step closer to reality. A recent experiment shows how to make new and unique graphene alloys, which could form the building blocks of miniscule circuits that could power svelte consumer electronics. These kinds of alloys may even form a new class of magnetic materials for sensors. Another group discovered how to produce sheets of pure graphene in large quantities, and peel the finished product without resorting to Scotch tape. Together, these studies, published recently in the journal Science Advances, hint at the possibility of using graphene in new ways on an unprecedented scale.
In the lab of Swastik Kar, lead author of one of the recent papers and a professor of physics at Northeastern University, I find myself staring at a piece of graphene that his group has meticulously deposited on a postage-stamp sized piece of copper foil. It looks like a postage-stamp sized piece of copper foil; with two-dimensional materials, you’ve got to get up close and personal to see what the big deal is about.
Across Kar’s lab, graduate student Anthony Vargas is using a microscope to examine another two-dimensional material called molybdenum disulfide. Magnified 1,000 times, it’s a Picasso-like frenzy of intersecting pink triangles against a magenta background. The triangle theme is neither accident nor artistry—it’s physics, resulting from how the atoms form up in groups and tessellate across the surface. Molybdenum disulfide crystals form triangles, which happen to be neatly compatible with graphene’s hexagons.
Another, more common substance also forms sheets of tessellated hexagons. Known as hexagonal boron nitride, or hBN, the powdered form is found in myriad products from lubricants to cosmetics. The hexagons in a sheet of hBN are close enough in size to the hexagonal arrangements of carbon in graphene that several research groups have already managed to substitute hBN for some of the carbon to create a three-element alloy. It’s a step beyond the previous state of the art, which had researchers applying layers of electrically-insulating hBN on top of graphene sheets to manipulate its conductivity.
Kar is part of a group of researchers who recently discovered how to incorporate a fourth element: oxygen. In what is becoming a common thread in stories about graphene, Kar and his collaborators didn’t set out to see what happens when you add oxygen into a graphene alloy. Rather, they stumbled into the question while doing what scientists spend a lot of time doing: troubleshooting. The team was originally interested in finding new materials for thermal imaging—cameras that create images from temperature rather than light. (Night vision goggles work this way). They were investigating graphene, but were having trouble replicating results published by another lab.
By introducing oxygen in the reaction chamber, they found they could see all four elements existing in a single sheet. That oxygen could fit into the structure at all was a surprise. “From a quantum mechanical point of view,” Kar says, “it’s very difficult to conceive of how oxygen could form a purely two-dimensional structure because of how far it is away from carbon.”
Kar compares graphene alloy production to cooking: “You know what the recipe is, but every time you do it, it comes out a little bit differently. So if you’re a chef, you’ll know by the color, or texture, or maybe you’ll taste it to know how far it is from being complete.” In this case, the undesired “flavor” came from oxygen, which had snuck into the reaction chamber. They realized that no one had studied what the oxygen contamination does—an important question, because some amount of oxygen is inevitable. They veered off to track down the answer.
Not only did they find that oxygen can be worked into the two-dimensional lattice, they also discovered that they could use the concentration of oxygen to control the final result. Suddenly, scientists could expertly tweak the composition of graphene alloys, allowing them to alter its properties.
That opens up a world of possibilities. Kar relishes the prospect of cooking up new graphene-based materials, which could potentially be applied to make all kinds of tiny components for miniaturized electronics. Furthermore, the collaboration’s theoretical work suggests that the four-element alloy can have magnetic properties, opening up the possibility of creating one-atom thick magnets. In addition to their small size, such devices would have the advantage of not requiring mined or imported rare earth metals.
“It’s very interesting and inspiring,” says Gong Gu, a professor of electrical engineering at University of Tennessee who is unaffiliated with the study. “But it’s initial work.” Gu points out that researchers haven’t yet experimentally verified the atomic structure of their sample.
Beyond Scotch Tape
Figuring out how to mix graphene with other elements to imbue it with new properties is just one hurdle that’s been delaying efforts to create more mainstream applications of graphene. To get the good stuff—graphene with high electron mobility, which is ideal for electronics applications—the “Scotch tape method” is pretty much the current state of the art, according to Christoph Stampfer of the University of Aachen, co-author on another recent study. Needless to say, you probably couldn’t retool a factory to produce it using the method. “There will never be technology built on using Scotch tape,” he says.
Stampfer works on a technique called chemical vapor deposition, also known as CVD, a process that’s widely used to produce computer chips, solar cells, and other devices where thin films of materials are useful. While CVD is widely studied, materials scientists have struggled to successfully create swaths of graphene with it. Typically, researchers heat gases to over 1,800˚ F. The gases then react with the copper foil and leave behind a single layer of carbon atoms on the surface. Large, table-top-wide expanses of graphene can be made this way. But removing the graphene from the copper is difficult, and the final product is not as good as its Scotch tape counterpart.
Stampfer’s new method changes that. In a recent paper, he and his colleagues describe a way to peel the CVD graphene off the copper without sacrificing quality. For an added bonus, the copper itself can then be reused over and over again, increasing the method’s manufacturing appeal.
The key is the very same hBN used to alloy graphene. Stampfer and colleagues created a “stamp” consisting of layered polymers topped with hBN. The layer of hBN is attracted to graphene by van der Waals forces—the same forces that attract a gecko’s foot to a wall—which are more powerful than the forces that hold graphene and copper together. Stampfer’s group carefully configured the stamp to pick up the graphene from the foil without ripping it apart, or degrading its electronic properties. The team observed that electrons move as easily in their samples as they do in Scotch-tape flakes of graphene, just in far greater quantities. Stampfer says the Scotch tape method “is like making a handwritten copy of a book, and CVD is like printing. It’s a completely different level of technology.”
Scientists have long hoped that graphene’s high electron mobility—how easily electrons travel through—will lead to development of extremely fast transistors, explains Jeewhan Kim, a materials science researcher at IBM Watson. But such technology remains a dream because no one has figured out how to engineer an on/off switch called a band gap that controls the flow of current.
Without such a switch, there’s no way to transmit the ones and zeros encoding the emails, cat videos, and other information that computers shuffle around. An-Ping Li, a materials scientist at Oak Ridge National Laboratory, explains that researchers have been hoping an alloy approach—like combing hBN with graphene—could help produce a band gap. But so far, they’ve been stymied. “It’s easy to think what we should do: combine these two materials, you might think you get something in between. Thermodynamically, it doesn’t work that way. It’s like mixing oil with water,” Li says.
Though ultrafast graphene transistors are still the stuff of fantasies, Li points out that graphene electrodes and screens are on the horizon, and thinks that a technique to produce lots of pure graphene is a big step forward. Kim echoes the enthusiasm. “Really high mobility in really large graphene samples—this is terrific.”