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By heating metal to make graphene, Rice University
researchers may warm the hearts of high-tech electronics manufacturers.
The lab of Rice chemist James Tour published two papers that advance the
science of making high-quality, bilayer graphene. They show how to grow it on a
functional substrate by first having it diffuse into a layer of nickel.
Graphene is commonly grown on a metal catalyst, usually copper, and must be
transferred to an electrically insulating substrate like silicon dioxide before
it can be used in a circuit. The transfer process is cumbersome and
time-consuming and can be as frustrating as manipulating household plastic
wrap, Tour says.
The new processes outlined in two related ACS Nano papers show large-scale bilayer graphene can be grown
directly onto a variety of insulating substrates. They eliminate the transfer
process and facilitate the growth of large sheets of semiconducting graphene
ready for incorporation into patterned transistors, Tour says.
“The ability to grow bilayer graphene directly onto an insulator can permit
electronic device manufacturers to build transistors without the industrially
burdensome step of placing one sheet of graphene upon another,” says Tour,
Rice’s T.T. and W.F. Chao Chair in Chemistry as well as a professor of
mechanical engineering and materials science and of computer science.
Graphene, the single-atom-thick form of carbon, has been the subject of much
study since its discovery in 2004. Tour’s lab has become a major player in
graphene research by publishing in recent years papers on unzipping nanotubes
into graphene nanoribbons, characterizing its electrical properties through
lithography, creating transparent electrodes for touch screens, and making
graphene from a variety of cheap sources, even Girl Scout cookies. All aim to
cut the cost and complexity of making graphene and bring it into widespread
use.
A single layer of graphene, which at the atomic scale looks like chicken
wire, is a semimetal and has no bandgap; this makes it unsuitable for many
electronic applications. But bilayer graphene is a semiconductor. Its
properties depend upon the offset or rotation of the layers in relation to each
other and it is tunable using an electric field applied across the layers.
The new processes depend on the solubility of carbon atoms in hot nickel. In
one study, a group led by graduate student Zhiwei Peng evaporated a coat of
nickel onto silicon dioxide and placed a polymer film—the carbon source—on top.
Heating the sandwich to 1,000 C in the presence of flowing argon and
hydrogen gas allowed the polymer to diffuse into the metal; upon cooling,
graphene formed on the nickel and on the silicon dioxide surfaces. When the
nickel and incidental graphene that formed on top were etched away, bilayer
graphene was left attached to the silicon dioxide substrate.
In the other study, graduate student Zheng Yan shuffled the sandwich. He
topped a layer of silicon dioxide with a sliver of one of a variety of polymers
and then put the nickel on top. Again, under high temperature and low pressure,
bilayer graphene formed between the silicon dioxide and nickel. Experimentation
with other substances revealed that bilayer graphene would also form on hexagonal
boron nitride, silicon nitride, and sapphire.
“This type of process eliminates the need for roll-to-roll transfer of the
graphene to an electronic substrate, because bilayer graphene can now be grown
directly upon the substrate of interest,” Tour says.
