James Tour at Rice University has a history of finding links between carbon nanotubes and graphene, which are often regarded merely as rivals for a host of electronic applications. A few years back, Tour developed a process for “unzipping” carbon nanotubes so that they transformed into graphene. Now Tour and his colleagues have used that unzipping technique to develop a method in which carbon nanotubes are used as a kind of reinforcing rebar for graphene, protecting it during the manufacturing process.
Why is this useful? Because to produce high-quality graphene for electronic applications (a promising one is replacing indium tin oxide as a transparent conductor in displays for controlling pixels), a manufacturing process known as chemical vapor deposition (CVD) is used, and CVD has an Achilles heel: while it is possible to grow large sheets of graphene on a copper substrate in a furnace, when you try to remove the graphene sheets from the copper, you find that it is difficult to do so without breaking the graphene. A reinforcement polymer is usually laid over the graphene to keep it from breaking during its removal, but this polymer leaves impurities.
Solving this manufacturing issue has led to some intriguing new methods, including work late last year out of the National University of Singapore, where researchers developed a method in which the copper is sandwiched between a silicon layer and graphene layer so that when the copper is etched away the graphene and silicon remain attached to each other.
The Rice University researchers have taken another approach. First, they coat the copper with both single-walled and multi-walled carbon nanotubes and then heat and cool the material. These carbon nanotubes serve as carbon source without the need of adding any other carbon to the process. When heated, the carbon nanotubes both decompose into graphene and—voilà!—unzip to form covalent junctions with the new graphene layer. The researchers have dubbed the resulting material "rebar graphene."
In a paper published in the journal ACS Nano, the Rice team describes how the rebar graphene could be transferred onto target substrates without needing a polymer coating due to the reinforcement effect. The resulting rebar graphene also exhibits better electrical conductivity than graphene produced through other CVD processes.
"Normally you grow graphene on a metal, but you can’t just dissolve away the metal," Tour said in a press release. “You put a polymer on top of the graphene to reinforce it, and then dissolve the metal. Then you have polymer stuck to the graphene. When you dissolve the polymer, you’re left with residues, trace impurities that limit graphene’s effectiveness for high-speed electronics and biological devices. By taking away the polymer support step, we greatly expand the potential for this material."
Tour believes that this rebar graphene could be a competitive alternative for the replacement of indium tin oxide in displays, an application that could potentially add flexibility to them. Before that happens, however, the researchers will have to show that their new manufacturing process can scale up enough and lower production costs to make it truly competitive with ITO.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.