Hot nickel nudges graphene

Rice University lab simplifies manufacture of semiconducting bilayer graphene
BY MIKE WILLIAMS
Rice News staff

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 this month 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.

	TOUR LAB/RICE UNIVERSITY
	This graphic shows the process of creating bilayer graphene on an insulating substrate to skip the need to transfer graphene from a metal catalyst. The final image, captured with an electron microscope, clearly shows two layers of graphene produced via the process.

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 said.

The new processes outlined in two related ACS Nano papers (here and here) 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 said.

”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,” said 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 degrees Celsius 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 said.

Authors of the first paper, “Growth of Bilayer Graphene on
Insulating Substrates,” are Yan, Peng, graduate student Zhengzong Sun,
former graduate student Jun Yao, postdoctoral research associates Yu Zhu and
Zheng Liu, Tour and Pulickel Ajayan, the Benjamin M. and Mary Greenwood
Anderson Professor in Mechanical Engineering and Materials Science and of
chemistry.

The Office of Naval Research MURI program, Lockheed Martin and the Air
Force Office of Scientific Research supported the research.

Authors of the second paper, “Direct Growth of Bilayer Graphene on
SiO₂ Substrates by Carbon Diffusion Through Nickel,” are Peng,
Yan, Sun and Tour.

The Office of Naval Research MURI program, the Air Force Research
Laboratory through United Technology Corp., the Air Force Office of Scientific
Research and M-I SWACO supported the research.