Rice technique points toward 2-D devices
Oak Ridge National Laboratories/Rice University |
Rice introduced a technique to stitch the identically structured
materials together nearly three years ago. Since then, the idea has received a
lot of attention from researchers interested in the prospect of building 2-D,
atomic-layer circuits, said Rice materials scientist Pulickel Ajayan. He is one
of the authors of the new work that appears this week in Nature Nanotechnology. In particular, Ajayan
noted that Cornell University scientists reported an advance late last year on
the art of making atomic-layer heterostructures through sequential growth
schemes.
This
week's contribution by Rice offers manufacturers the possibility of shrinking
electronic devices into even smaller packages. While Rice's technical
capabilities limited features to a resolution of about 100 nanometers, the only
real limits are those defined by modern lithographic techniques, according to
the researchers. (A nanometer is one-billionth of a meter.)
"It
should be possible to make fully functional devices with circuits 30, even 20
nanometers wide, all in two dimensions," said Rice researcher Jun Lou, a
co-author of the new paper. That would make circuits on about the same scale as
in current semiconductor fabrication, he said.
Graphene
has been touted as a wonder material since its discovery in the last decade.
Even at one atom thick, the hexagonal array of carbon atoms has proven its
potential as a fascinating electronic material. But to build a working device,
conductors alone will not do. Graphene-based electronics require similar,
compatible 2-D materials for other components, and researchers have found
hexagonal boron nitride (h-BN) works nicely as an insulator.
H-BN
looks like graphene, with the same chicken-wire atomic array. The earlier work
at Rice showed that merging graphene and h-BN via chemical vapor deposition
(CVD) created sheets with pools of the two that afforded some control of the
material's electronic properties. Ajayan said at the time that the creation
offered "a great playground for materials scientists."
Zheng Liu/Rice University |
He has
since concluded that the area of two-dimensional materials beyond graphene
"has grown significantly and will play out as one of the key exciting
materials in the near future."
His
prediction bears fruit in the new work, in which finely detailed patterns of
graphene are laced into gaps created in sheets of h-BN. Combs, bars, concentric
rings and even microscopic Rice Owls were laid down through a lithographic
process. The interface between elements, seen clearly in scanning transmission
electron microscope images taken at Oak Ridge National Laboratories, shows a
razor-sharp transition from graphene to h-BN along a subnanometer line.
"This
is not a simple quilt," Lou said. "It's very precisely engineered. We
can control the domain sizes and the domain shapes, both of which are necessary
to make electronic devices."
The new
technique also began with CVD. Lead author Zheng Liu, a Rice research
scientist, and his colleagues first laid down a sheet of h-BN. Laser-cut photoresistant
masks were placed over the h-BN, and exposed material was etched away with
argon gas. (A focused ion beam system was later used to create even finer
patterns, down to 100-nanometer resolution, without masks.) After the masks
were washed away, graphene was grown via CVD in the open spaces, where it
bonded edge-to-edge with the h-BN. The hybrid layer could then be picked up and
placed on any substrate.
While
there's much work ahead to characterize the atomic bonds where graphene and
h-BN domains meet and to analyze potential defects along the boundaries, Liu's
electrical measurements proved the components' qualities remain intact.
"One
important thing Zheng showed is that even by doing all kinds of growth, then
etching, then regrowth, the intrinsic properties of these two materials are not
affected," Lou said. "Insulators stay insulators; they're not doped
by the carbon. And the graphene still looks very good. That's important,
because we want to be sure what we're growing is exactly what we want."
Liu
said the next step is to place a third element, a semiconductor, into the 2-D
fabric. "We're trying very hard to integrate this into the platform,"
he said. "If we can do that, we can build truly integrated in-plane
devices." That would give new options to manufacturers toying with the
idea of flexible electronics, he said.
"The
contribution of this paper is to demonstrate the general process," Lou
added. "It's robust, it's repeatable and it creates materials with very
nice properties and with dimensions that are at the limit of what is
possible."
Source: Rice
University
Posted by Unknown
on Monday, January 28, 2013.
Filed under
Chemistry and Physics
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