Schematic illustration of single-atom-thick films with patterned regions of conducting graphene (gray) and insulating boron nitride (purple-blue). Credit: Jiwoong Park

Integrated
circuits, which are in everything from coffeemakers to computers and
are patterned from perfectly crystalline silicon, are quite thin—but
Cornell researchers think they can push thin-film boundaries to the
single-atom level.

Their
materials of choice are graphene, single atom-thick sheets of repeating
carbon atoms, and hexagonal boron nitride, similarly thin sheets of
repeating boron and nitrogen atoms. Researchers led by Jiwoong Park,
assistant professor of chemistry and chemical biology, have invented a
way to pattern single atom films of graphene and boron nitride, an
insulator, without the use of a silicon substrate. The work is detailed
in an article in the journal Nature, published online Aug. 30.

The
technique, which they call patterned regrowth, could lead to
substrate-free, atomically thin circuits—so thin, they could float on
water or through air, but with tensile strength and top-notch electrical
performance.

“We
know how to grow graphene in single atom-thick films, and we know how
to grow boron nitride,” Park said. “But can we bring them together side
and side? And when you bring them together, what happens at their
junctions?”

As
it turns out, researchers’ patterned regrowth, which harnesses the same
basic photolithography technology used in silicon wafer processing,
allows graphene and boron nitride to grow in perfectly flat,
structurally smooth films—no creases or bumps, like a well-knitted scarf—which, if combined with the final, yet to be realized step of
introducing a semiconductor material, could lead to the first atomically
thin integrated circuit.

Simple
really is beautiful, especially in the case of thin films, because
photolithography is a well-established technique that forms the basis
for making integrated circuits by laying materials, one layer at a time,
on flat silicon.

Patterned
regrowth is a bit like stenciling, Park said. He and colleagues first
grew graphene on copper and used photolithography to expose graphene on
selected areas, depending on the desired pattern. They filled that
exposed copper surface with boron nitride, the insulator, which grows on
copper and “fills the gaps in very nicely.”

“In the end, it forms a very nice cloth you just peel off,” Park said.

The
research team, which includes David A. Muller, professor of applied and
engineering physics, is working to determine what material would best
work with graphene-boron nitride thin films to make up the final
semiconducting layer that could turn the films into actual devices.

The
team was helped by already being skilled at making graphene—still
relatively new in the materials world—as well as Muller’s expertise in
electron microscopy characterization at the nanoscale. Muller helped the
team confirm that the lateral junctions of the two materials were,
indeed, smooth and well connected.

The
paper’s co-first authors were chemistry graduate student Mark Levendorf
and postdoctoral associate Cheol-Joo Kim, who fabricated the graphene
and boron nitride samples and also performed the patterned regrowth at
the Cornell NanoScale Science and Technology Facility.

The
work was supported primarily by the Air Force Office of Scientific
Research, and the National Science Foundation through the Cornell Center
for Materials Research.

Source: Cornell University