Graphene has become a major research topic in physical sciences in recent years and has many potential applications. Graphene, carbon sheets are one atom thick and their properties have caused researchers and companies to consider using this material in several fields. One of the methodologies used to prepare graphene sheets is the chemical exfoliation of graphite in aqueous medium, which produces oxygen functionalized graphene sheets and the other method is to grow using copper. The quality of graphene depends on the crystal structure of the copper substrate it grows on. Graphene is a very important material and the future of electronics may depend on it but the quality of its production is one of the key unsolved problems.
Production
A very cost-effective, straightforward way to make graphene on a large scale is to grow on copper substrate. To produce large sheets of graphene, methane gas is piped into a furnace containing a sheet of copper foil. When the methane strikes the copper, the carbon-hydrogen bonds crack. Hydrogen escapes as gas, while the carbon sticks to the copper surface. The carbon atoms move around until they find each other and bond to make graphene. Copper is an appealing substrate because it is relatively cheap and promotes single-layer graphene growth, which is important for electronics applications.
While graphene grown on copper tends to be better than graphene grown on other substrates, it remains riddled with defects and multi-layer sections, precluding high-performance applications. Graphene grows in a single layer on the (111) copper surface and in islands and multilayer elsewhere. But researchers have speculated that the roughness of the copper surface may affect graphene growth.
Copper crystal structures
In fact copper foils are a patchwork of different crystal structures. As the methane falls onto the foil surface, the shapes of the copper crystals it encounters affect how well the carbon atoms form graphene. For making high-quality, high-performance electronics the copper's crystal structure is more important.
Different crystal shapes are assigned index numbers. Using several advanced imaging techniques, the researchers found that patches of copper with higher index numbers tend to have lower-quality graphene growth. Researchers also found that two common crystal structures, numbered (100) and (111), have the worst and the best growth, respectively. The (100) crystals have a cubic shape, with wide gaps between atoms. Meanwhile, (111) has a densely packed hexagonal structure.
In the (100) configuration the carbon atoms are more likely to stick in the holes in the copper on the atomic level, and then they stack vertically rather than diffusing out and growing laterally. The (111) surface is hexagonal, and graphene is also hexagonal. It is not a perfect match, but that there is a preferred match between the surfaces.
Cost
Researchers now are faced with balancing the cost of all (111) copper against the value of high-quality, defect-free graphene. It is possible to produce single-crystal copper, but it is difficult and prohibitively expensive. It may be possible to improve copper foil manufacturing so that it has a higher percentage of (111) crystals. Graphene grown on such foil would not be ideal, but may be good enough for most applications.
Future research
Researchers hope to use their methodology to study the growth of other two-dimensional materials, including insulators to improve graphene device performance. They also plan to follow up on their observations by growing graphene on single-crystal copper. The fact that there is a clear observational difference between these different growth indices helps steer the research and will probably lead to more quantitative experiments as well as better modeling.