CVD method produces long single crystal graphene films (credit: Andy Sproles/Oak Ridge National Laboratory, US Dept. of Energy)A novel “self-selection” method for growing large monolayer single-crystal-like graphene films has been devised by a team led by the US DOE's Oak Ridge National Laboratory, in research that contributes to our understanding of the growth mechanism of 2D materials. The films, which are over a foot long, were produced by chemical vapor deposition (CVD), and depend on harnessing a Darwinistic “survival of the fittest” competition among the crystals.
For graphene to be used on a practical level it needs to be made into thin layers on a larger scale than currently possible. This is usually attempted by separating out flakes of graphite into atom-thick layers, or by growing it on an atom-by-atom basis on a catalyst from a gaseous precursor. It is this second approach that is being used here.
Single crystal graphene samples that were more mechanically stable than polycrystalline ones, and as reported in Nature Materials [Vlassiouk et al. Nat. Mater. (2018) DOI: 10.1038/s41563-018-0019-3], their production involved spraying a gaseous mixture of hydrocarbon precursor molecules onto metallic, polycrystalline foil. The local deposition of the hydrocarbon molecules on a moving substrate causes the carbon atoms to be continuously assembled as a single crystal of graphene of up to a foot in length.
As the hydrocarbons land on the hot catalyst foil, they can also form clusters of carbon atoms that gradually grow into larger domains instead of attaching to existing edge but the wind of buffer gas prevents it from happening. In this tightly controlled environment, the fastest-growing orientation of graphene crystals overwhelm the others and get “evolutionarily selected” into one crystal, even on a polycrystalline substrate. Large single crystals are more mechanically robust and possess higher conductivity by eliminating weaknesses arising from interconnections between individual domains in the graphene.
“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 application”Ivan Vlassiouk
As lead co-author Ivan Vlassiouk said, “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 application”. Co-author Sergei Smirnov adds, “The unencumbered single-crystal-like graphene growth can go almost continuously, as a roll-to-roll and beyond the foot-long samples demonstrated here”.
Although the scaling up will be difficult on a practical level, their approach to growing single crystals like monolayer sheets of graphene can be applied to various 2D materials, such as boron nitride and molybdenum disulphide, and heterostructures could also be grown in this way. They next want to test other engineering solutions to improve the availability of these atomically thin materials for large-scale experimentation.