An innovative technique to make large, monolayer single-crystal-like graphene films measuring more than one foot in length is dependent on making productive a “survival of the fittest” competition among crystals. The new method was devised by a research group headed by the Department of Energy’s Oak Ridge National Laboratory and might open the door for innovative opportunities for developing high-quality, two-dimensional (2D) materials needed for highly anticipated practical applications.
Producing thin layers of graphene and other 2D materials on a scale needed for performing research is customary; however, they have to be produced on a considerably larger scale to be functional.
Graphene is endorsed for its attributes of higher electrical conductivity and unparalleled strength. It can be produced using familiar strategies: isolating graphite flakes (graphite is the silvery soft material that can be seen in pencils) into single-atom-thick layers, or developing it one atom at a time on a catalyst by using a gaseous precursor till ultrathin layers are developed.
The ORNL-headed study group used the second technique, called chemical vapor deposition (CVD), however with a twist. In research reported in the Nature Materials journal, the researchers have described the way localized control of the CVD process enables evolutionary (or self-selecting) growth under most favorable conditions, producing a large, single-crystal-like graphene sheet.
Large single crystals are more mechanically robust and may have higher conductivity. This is because weaknesses arising from interconnections between individual domains in polycrystalline graphene are eliminated.
Our method could be the key not only to improving large-scale production of single-crystal graphene but to other 2D materials as well, which is necessary for their large-scale applications.
Ivan Vlassiouk, Lead Co-Author - ORNL
Very similar to conventional CVD strategies to synthesize graphene, the scientists sprayed a gaseous mixture including hydrocarbon precursor molecules onto a metallic, polycrystalline foil. They cautiously regulated the local deposition of the hydrocarbon molecules, making them be directly deposited at the edge of the developing graphene film. When the substrate moved at the bottom, the carbon atoms continuously gathered as a single graphene crystal up to a length of one foot.
“The unencumbered single-crystal-like graphene growth can go almost continuously, as a roll-to-roll and beyond the foot-long samples demonstrated here,” stated Sergei Smirnov, coauthor of the study who is a professor at New Mexico State University.
Upon getting into contact with the hot catalyst foil, the hydrocarbons form carbon atom clusters that gradually grow into larger domains before combining to cover the entire substrate. Earlier, the researchers discovered that at adequately higher temperatures, the carbon atoms in graphene did not interlink, or mirror, the atoms in the substrate, thereby enabling non-epitaxial crystalline growth.
As the speed of growth of the single crystal is highly influenced by the concentration of the gas mixture, depositing the hydrocarbon precursor close to the existing edge of single graphene crystal can effectively boost its growth than the evolution of new clusters.
In such a controlled environment, the fastest-growing orientation of graphene crystals overwhelms the others and gets ‘evolutionarily selected’ into a single crystal, even on a polycrystalline substrate, without having to match the substrate’s orientation, which usually happens with standard epitaxial growth.
Sergei Smirnov, Co-Author
The researchers discovered that to confirm the highest growth, it is mandatory to form a “wind” that assists in getting rid of the cluster formations. “It was imperative that we create an environment where the formation of new clusters ahead of the growth front was totally suppressed, and enlargement of just the growing edge of the large graphene crystal was not hindered,” stated Vlassiouk. “Then, and only then, nothing stands in the way of the ‘fittest’ crystalline growth when the substrate is moving.”
The theoreticians in the group, headed by Boris Yakobson - co-author of the study, who is a Rice University professor—developed a model elucidating the crystal orientations that had the distinctive characteristics that make them suitable, and why the selection of the fittest might rely on the precursors and the substrate.
If graphene or any 2D material ever advances to industrial scale, this approach will be pivotal, similar to Czochralski’s method for silicon. Manufacturers can rest assured that when a large, wafer-size raw layer is cut for any device fabrication, each resulting piece will be a quality monocrystal. This potentially huge, impactful role motivates us to explore theoretical principles to be as clear as possible.
Boris Yakobson, Co-Author
Workable advancement of graphene by adopting the technique developed by the researchers is still to be tested; however, the team is confident that their survival of the fittest single-crystal growth technique could also be used for propitious substitutive 2D materials such as molybdenum disulfide and boron nitride, which is also called as “white graphene.”
Coauthors of the study titled “Evolutionary selection growth of two-dimensional materials on polycrystalline substrates” are Ivan Vlassiouk, Yijing Stehle, Raymond R. Unocic, Arthur P. Baddorf, Ilia N. Ivanov, Nickolay V. Lavrik, and Frederick List from Oak Ridge National Laboratory; Philip D. Rack from ORNL and University of Tennessee; Pushpa Raj Pudasaini from University of Texas; Nitant Gupta, Ksenia Bets, and Boris I. Yakobson from Rice University; and Sergei Smirnov from New Mexico State University.
ORNL’s Laboratory Directed Research and Development program, ORNL’s technology transfer royalty funded Technology Innovation Program and DOE’s Advanced Research Projects Agency-Energy supported the study. Microscopy study was supported as part of the Fluid Interface Reactions, Structures, and Transport Center, an Energy Frontier Research Center. This study also leveraged ORNL’s Center for Nanophase Materials Sciences, a DOE Office of Science User Facility.