The new hybrid, according to Rice chemist James Tour, could, when stacked in a few layers, make a cost-effective alternative for costly indium tin oxide (ITO). ITO is currently used in displays and solar cells.
Tour set nanotubes into graphene in a way that is analogous to how steel rebar is used in concrete. This method also conserves and even improves both materials’ electrical and mechanical qualities.
The finding is significant because it should make large, flexible, conductive and transparent sheets of graphene much easier to manipulate, which should be of interest to electronics manufacturers, Tour said.
Re-bar Graphene Benefits
A single-layer matrix of carbon atoms, graphene is one of the strongest materials on the planet. Nevertheless, it can be quite the challenge to lift the tiny sheets from the catalyst substrate they are grown on, typically by chemical vapor deposition, Tour said.
“Normally you grow graphene on a metal, but you can’t just dissolve away the metal,” Tour said. “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.”
To fabricate what they are calling rebar graphene, the researchers just spin-coat and then heat and cool functionalized single- or multiwalled carbon nanotubes on copper foils, using the nanotubes themselves as the carbon source.
When heated, the functional carbon groups decompose and form graphene, while the nanotubes partially split and form covalent junctions with the new graphene layer.
Covalently Bonded True Hybrid Material
“The nanotubes actually become one with the material in certain places,” Tour said. “It’s a true hybrid with in-plane nanotubes covalently bonded to graphene.”
The graphene is strengthened by the interconnected, embedded nanotubes.
“We can see in our images how well the nanotubes bear the load. When we stretch the material, the tubes get thinner,” said Tour.
Since the electron microscope images allow them to them determine the nanotubes’ chirality (the angles of the hexagons that make up the tube), the researchers were able to calculate the tubes’ diameters and know exactly how much thinner they get under tension.
Additionally, the networked nanotubes make the material a better conductor than standard chemical vapor deposition grown graphene, Tour said.
The Road to Flexible Displays
Graphene, as grown, is never a perfect matrix of hexagons; instead, it consists of crystals that grow separately and connect at grain boundaries that disrupt the flow of electrons. The nanotubes in rebar graphene effectively bridge those boundaries.
“The big thing for industry is to see if they can get graphene to substitute for ITO for transparent displays,” Tour said. “But ITO is rigid, and it breaks when you drop your smartphone, for example. Graphene and nanotubes, on the other hand, would afford flexible displays. We showed in our tests that rebar graphene has better conductivity than normal graphene at the same transparency, and with layering, it could be ITO-competitive.”
Zheng Yan, Zhiwei Peng, Gilberto Casillas, Jian Lin, Changsheng Xiang, Haiqing Zhou, Yang Yang, Gedeng Ruan, Abdul-Rahman O. Raji, Errol L. G. Samuel, Robert H. Hauge, Miguel Jose Yacaman, James M. Tour.
ACS Nano, 2014; 140402145529003 DOI: 10.1021/nn501132n#sthash.WppbV65B.dpuf
Images courtesy of Tour Group/Rice University