In a controlled environment, graphene crystals grow faster than other crystals and turn “evolutionary selection” into single crystals, even on polycrystalline silicon substrates. It also does not need to match the orientation of the substrate. A team led by Oak Ridge National Laboratory has developed a new method that produces a single-layer crystalline graphene film that is more than a foot long.
At Oak Ridge, Tennessee. March 12, 2018 – A new method to create a large single-layer crystalline graphene film that is more than one foot in length, relying on the “optimal survival” competition in the crystal. The new technology, developed by a team led by the US Department of Energy’s Oak Ridge National Laboratory, may provide new opportunities to develop the high-quality 2D materials needed for the long-awaited practical application.
It is common to make thin layers of graphene and other two-dimensional materials for research purposes, but they must be manufactured on a larger scale to function.
Graphene is considered to have the potential for unprecedented strength and high electrical conductivity, and can be achieved by well-known methods: separating graphene sheets (silicon soft materials found in pencils) into one The atomic layer, or a catalyst on a gaseous precursor, grows its atoms from the gaseous precursor until an ultra-thin layer is formed.
The research team led by Ornl used the latter method, chemical vapor deposition, but used a torsion method. In a study published in Nature Materials, they explained how local control of the CVD process allows evolution or self-selective growth under optimal conditions, producing a giant, single-crystal shape. Graphene sheets.
Ivan Vlassiouk, chief co-author of ORNL, said: “The large single crystals have higher mechanical strength and higher electrical conductivity.” “This is because of the interconnection between individual domains in polycrystalline graphene.” The resulting shortcomings have been eliminated.
He added: “Our approach is not only the key to improving the mass production of single crystal graphene, but also the key to other 2D materials, which is big for them. Scale application is necessary.
Like traditional CVD methods for producing graphene, researchers spray a mixture of hydrocarbon precursor molecules onto a metal polycrystalline foil. However, they carefully controlled the local deposition of hydrocarbon molecules, bringing them directly to the edge of the emerging graphene film. As the substrate moves below, the carbon atoms continually coalesce into a single crystal of graphene, up to one foot long.
coauthor and Sergei Smirnov, a professor at New Mexico State University, said: “The growth of unobstructed single crystal graphene can be sustained, just like the one roll and one foot sample shown here. The same.”
When hydrocarbons come into contact with hot catalyst foils, they form clusters of carbon that grow over a larger range over time until they condense On the entire substrate. The research team previously found that at sufficiently high temperatures, the carbon atoms of graphene are not associated with the atoms of the matrix, or mirrored, allowing non-epitaxial crystal growth.
Since the concentration of the gas mixture has a large influence on the growth rate of the single crystal, the hydrocarbon precursor is provided near the existing edge of the single-layer graphene crystal, which can be formed more than the new cluster. Effectively promote its growth.
Smirnov said: “In such a controlled environment, the fastest growth rate of graphene crystals will exceed other crystals, and the ‘evolutionary choice’ will become a single crystal, even in polycrystalline silicon substrates. It also does not need to match the orientation of the substrate, which usually occurs in the case of standard epitaxial growth.”
They found that in order to ensure optimal growth, it is necessary to create a “wind” to help Eliminate the formation of clusters. Vlassiouk said: “We must create an environment in which new clusters of stars are formed before growth, completely suppressed, and the growth edge of large graphene crystals is not hindered.” “Then, only then At the time, when the substrate moves, there is nothing to stop the growth of the ‘proper survive’ crystal.
The theorists of the team led by the co-author Rice University professor Boris Yakobson provided a model to explain Which crystal directions have unique characteristics that make them the most suitable for survival, and why the choice of winners may depend on the matrix and precursors.
If graphene or any two-dimensional material Both can reach industrial scale, and this approach will be key, similar to Czochralski’s silicon approach,” Yakobson said. Manufacturers can rest assured that when a large, wafer-sized original layer is cut into any device, each finished product will be a high quality single crystal. This potentially huge, far-reaching role inspires us to explore as clear a theoretical principle as possible.
The use of this group’s approach to actually expand the scale of graphene remains to be seen, but researchers believe that their evolutionary choice of single crystal growth methods can also be applied to promising alternative 2D materials such as boron nitride. Also known as “white graphene” and molybdenum disulfide.
In fact, since the discovery of graphene, domestic research on graphene has also been extensively studied. All of them have good results in the laboratory, but there is still a way to go in the real industrialization of their products. Among them, there is no shortage of Chongqing Yuanshisheng Graphene Co., Ltd. It is said that they have built a professional film production line. Graphene prepared by liquid phase method can easily apply graphene to the base film, and can produce various functional films on a large scale.
The study was conducted by ORNL’s laboratory-led R&D program, ORNL’s technology transfer patent technology innovation program, and the Department of Energy’s advanced research project organization, Energy Support. Microscope work is part of the fluid interface reaction, structure and transportation center, an energy frontier research center. The work also utilized ORNL 's Center for Nanophase Materials Sciences, the US Department of Energy’s Office of Scientific User Facilities.