For the first time, Northwestern University engineers have created a double
layer of atomically flat borophene, a feat that defies the natural tendency
of boron to form non-planar clusters beyond the single-atomic-layer limit.
Although known for its promising electronic properties, borophene—a
single-atom-layer-thick sheet of boron—is challenging to synthesize. Unlike
its analog two-dimensional material graphene, which can be peeled away from
innately layered graphite using something as simple as scotch tape,
borophene cannot merely be peeled away from bulk boron. Instead, borophene
must be grown directly onto a substrate.
And if growing one layer was difficult, growing multiple layers of
atomically flat borophene seemed impossible. Because bulk boron is not
layered like graphite, growing boron beyond single atomic layers leads to
clustering rather than planar films.
"When you try to grow a thicker layer, the boron wants to adopt its bulk
structure," said Northwestern's Mark C. Hersam, co-senior author of the
study. "Rather than remaining atomically flat, thicker boron films form
particles and clusters. The key was to find growth conditions that prevented
the clusters from forming. Until now, we didn't think you could go beyond
one layer. Now we have moved into unexplored territory between the single
atomic layer and the bulk, resulting in a new playground for discovery."
The research will be published Aug. 26 in the journal Nature Materials.
Hersam is the Walter P. Murphy Professor of Materials Science and
Engineering at the McCormick School of Engineeringand director of the
Materials Research Science and Engineering Center. He also is a member of
Northwestern's International Institute for Nanotechnologyand the Simpson
Querrey Institute. Hersam co-led the work with Boris Yakobson, the Karl F.
Hasselmann Chair in Engineering at Rice University.
Five years ago, Hersam and his collaborators created borophene for the first
time. Stronger, lighter and more flexible than graphene, borophene has the
potential to revolutionize batteries, electronics, sensors, solar cells and
quantum computing. Although theoretical research predicted that a double
layer of borophene was possible, many researchers, including Hersam, were
not convinced.
"It is challenging to make a new material, even when theoretical work
predicts its existence," Hersam said. "Theory rarely tells you the synthetic
conditions needed to achieve that new structure."
The key to the correct conditions, Hersam's team discovered, was the
substrate used for growing the material. In the study, Hersam and his
colleagues grew borophene on a flat, silver substrate. When exposed to very
high temperatures, the silver bunched to form exceptionally flat, large
terraces between bunches of atomic-scale steps.
"When we grew borophene on these large, flat terraces, we saw a second layer
forming," Hersam said. "Following that serendipitous observation, we
intentionally focused our effort in that direction. We weren't looking for
the second layer when we found it. Many materials discoveries occur in this
manner, but you have to realize the opportunity when you stumble upon
something unexpected."
The double-layered material maintained all of borophene's desirable
electronic properties, while offering new advantages. For example, the
material comprises two atomic-layer-thick sheets bonded together with space
between, which could be used for energy or chemical storage.
"There have been theoretical predictions that bilayer borophene is a
promising material for batteries," Hersam said. "Having space between the
layers provides a place to hold lithium ions."
Hersam's team hopes other researchers now are inspired to keep growing even
thicker layers of borophene or create double layers with different atomic
geometries.
"Diamonds, graphite, graphene and carbon nanotubes are all based on one
element (carbon) with different geometries," Hersam said. "Boron appears to
be just as rich in its possibilities, if not more so, than carbon. We
believe that we are still in the early chapters of the two-dimensional boron
saga."
Reference:
Borophene synthesis beyond the single-atomic-layer limit, Nature Materials
(2021).
DOI: 10.1038/s41563-021-01084-2
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