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Ultrathin Destiny

ORNL materials scientists Kai Xiao and Akinola Oyedele explore the practical applications of 2–D nanomaterials. Image credit: Jason Richards, ORNL

Exploring the 2-D nanomaterials frontier

Materials are identified by their properties, desirable or otherwise. Iron is strong, but it rusts. Plastics are versatile and useful but end up polluting the environment because they don’t easily decay.

Scientists and engineers improve materials by manipulating them to get just the right properties, a process that leads to improvements such as longer-lasting automobiles made with non rusting steel and packaging and consumer products made with biodegradable materials.

The latest frontier in materials research is the nanoscale. Scientists now design materials atom by atom, determining the best arrangement of elements for the job at hand. At ORNL, researchers are focusing on ultrathin layered materials where each layer is no more than several atoms thick—in other words, materials so thin they are considered two-dimensional.

At such a small scale, the materials are transformed. Researchers can permeate their nanostructures with exotic properties: nearly unbelievable combinations of light weight and strength, or efficient, long-lasting energy storage, or resistance to extreme temperatures, or even superconductivity.

Imagine electronic devices that run for days on a single charge or transparent solar cells that can efficiently harvest energy from every window. High-powered ultrathin transistors could be woven into clothing and perform smartphone-like communications or monitor personal biometrics.

“What’s fascinating is that age-old layered materials like graphite and molybdenum disulfide are suddenly being rediscovered as atomically thin 2-D layers with radical new properties,” ORNL group leader David Geohegan said. “A single layer behaves much differently from a bilayer, and so on, up to a few layers thick when the material begins to behave more or less like the bulk crystal, or starter material.”

The study of 2-D materials has become a hot field. Geohegan and his group at the lab’s Center for Nanophase Materials Sciences can synthesize and process a variety of ultrathin layered crystals and characterize where each atom is with sophisticated microscopes. Using the power of ORNL's Titan supercomputer, theorists can also model these materials like never before to understand the origins of their functionality.

By concentrating on growing and assembling 2-D layers that absorb and emit light, Geohegan’s group not only develops ultrathin electronics but also reveals the quality of the crystals. “Right now a grand challenge is to grow atomically thin materials as well as they grow in nature,” he said.

Kai Xiao, a materials scientist in Geohegan’s group, leads the effort at the CNMS to understand the practical application of these atomically thin 2-D crystals by finding ways to wire up actual prototype devices with novel structures. Indeed, semiconducting 2-D materials may well be the future of electronics.

Semiconductors—materials that meticulously control the flow of electricity—are the key to microprocessors and modern technology. Labeled either n-type or p-type—depending on whether extra electrons are available to be conducted—they can be combined to create the p-n junction that is the building block of a transistor. The power of electronics depends on packing more and more transistors into devices like smartphones; as a result, we need smaller transistors.

Enter 2-D materials. While the features on today’s silicon-based semiconductors continue to shrink, manufacturers could greatly benefit from new materials created from the thinnest layer up. Xiao’s team is already there. “By stacking atomically thin 2-D layers like ultrathin Lego blocks, we can develop a prototype using new 2-D nanomaterials with custom properties ideal for semiconductors,” Xiao said.

Moving beyond Scotch tape

Before you can experiment with a 2-D material, however, you must produce it. Two-dimensional materials burst onto the science scene in 2004 with the simple production of graphene, which is graphite in a single-atom sheet form. The silvery metal like graphite commonly found in pencils flakes off easily into thin layers using the clear adhesive tape you can buy at a grocery store. By repeating this process, layers of graphite can be exfoliated, or peeled and separated from the bulk material, again and again until reaching a layer just one atom thick.

As arcane as that may sound, the adhesive technique is effective as a top-down method for studying 2-D materials. But that’s just scratching the surface.

Xiao uses this approach to extract the ultrathin layers exfoliated from beautiful semiconductor crystals grown at ORNL such as molybdenum disulfide and gallium selenide. When the crystals are reduced to flakes, they are considered 2-D and can be stacked, rotated, doped (with another element inserted into its crystal structure), and otherwise manipulated to produce new materials with different properties.

“Although using the top-down Scotch tape method is an easy way to make high-quality 2-D nanomaterials for fundamental research, it is hard to control the layer number and size,” Xiao said. “Also, it is nearly impossible to align the atoms precisely, so we need to try and grow the layers epitaxially one on top of another.”

As a result, Geohegan and Xiao’s team explores a variety of high-tech methods such as chemical vapor deposition or pulsed laser deposition to directly grow the 2-D crystals in a controlled manner. This bottom-up approach ensures materials with consistent thickness, comparable to the way conventional semiconductors are grown. These scalable techniques promise to grow large-area 2-D crystals for future research and, eventually, in

commercial applications.

Using a suite of powerful scanning tools, ultrathin layers are “read” and mapped as sensors scan the surface to reveal the individual atoms in the layers like stars in a honeycomb galaxy, the heavier atoms a bit brighter than the others. The resulting latticework of molecules takes on shapes and structures unique to the materials’ particular properties.

ORNL microscopists on the CNMS team, including JuanCarlos Idrobo Tapia and An-Ping Li, measure the electronic properties of the layers with atomic resolution, which provides CNMS team theorists including Bobby Sumpter, Liangbo Liang and Mina Yoon a roadmap of the electronic interactions between the atoms in the 2-D galaxy to guide their computations.

By bridging the gap from atoms to real devices, the team is able to explain why different 2-D material combinations would be successful or not for new technology and to predict the next step.

“How these layers react and change when subjected to various stimulations—like light or electrons—tells us about a material's characteristics and behavior,” Xiao said. “It leads us to question what’s there and what’s missing and how can we use that information to our advantage when creating new materials.”

ORNL’s work in 2-D nanomaterials has been described in many high-impact science journals, garnering recognition across the global physics community. Hopefully, as these materials find application, the pathway to commercialization will happen at the speed of nanotechnology.

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