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Graphene is a ‘wonder material’ that holds untold promise but handling and effectively manipulating it has proven to be a major stumbling block.
Researchers have looked to address this challenge by using carbon nanotubes to strengthen graphene in the same way that steel rebar is used to strengthen concrete. One ‘rebar graphene’ initiative by researchers at Rice University was able to produce a material that improves on both the mechanical and electrical qualities of graphene.
Graphene is a matrix of carbon atoms just one atom thick, and it is one of the strongest known materials. However, from the moment it is created, graphene is hard to hard. For instance, it can be difficult to extract small sheets of graphene from the catalyst substrate on which they’re grown. The substrate cannot just be dissolved away. Normally, a polymer is placed over the graphene to reinforce it, and then the substrate is digested and removed.
When the polymer is dissolved, however, it can leave behind residues and impurities that affect graphene’s performance potential in high-speed electronics and devices. By eliminating the polymer step through the use of carbon nanotubes, this factor alone can greatly boost graphene's potential.
Making Rebar Graphene
To create rebar graphene, Rice scientists spin-coated, heated and then cooled functionalized carbon nanotubes on copper foils, with the nanotubes acting as the carbon source for the graphene itself. When they were heated, the functional carbon groups broke down to form graphene, while the nanotubes partially unzipped and created covalent junctions with the new graphene layer. The nanotubes merged with the material in some places, the research team said.
Images captured with an electron microscope showed how interlocked, embedded nanotubes strengthen the graphene, increasing its load-bearing capabilities. When the rebar graphene was stretched, the nanotubes got thinner. With the images allowing the team to ascertain the nanotubes’ chirality or the angles of the hexagons that constitute the tube, the scientists could determine the tubes’ diameters and understand specifically how much slimmer they get under stress.
The networked nanotubes also made rebar graphene a superior conductor of electricity than conventional graphene., which has crystals connected at grain boundaries that can disturb a flow of electrons. The nanotubes in rebar graphene effectively bridge those borders.
Recent experiments with rebar graphene have revealed the addition of nanotubes can also help graphene to remain 'stretchy' and reduce the issues related to cracks. This is because nanotube rebar redirects and closes splits that would otherwise spread in unreinforced graphene.
Current techniques to make graphene rebar produce large, bendable, conductive and transparent sheets of graphene that are easy to manipulate, which could lead to a wider range of electronics applications for graphene. For instance, stacking a handful of layers could be a cost-effective alternative to costly indium tin oxide (ITO) that is currently used in displays and solar cells. ITO is stiff and breaks relatively easily, like when a smartphone is dropped. Rebar graphene would allow for sturdier, flexible displays that could reduce production costs.
Recent experiments showing rebar graphene can stay 'stretchy' could be useful not only for flexible electronics of the future but also for electrically active wearables or other devices where stress tolerance, flexibility, transparency, and mechanical stability are desired.
Furthermore, if additives could be introduced into the rebar graphene matrix during the annealing sequence, the doped material could have possible applications in next-generation fuel cells and batteries.
As the incremental improvement of nanomaterials like graphene continues thanks to atomically-precise production techniques, we can expect to see more cases of rare, costly materials being replaced by functionally comparable materials with precise atomic structures made from common elements like carbon.