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Squashed nanotubes may be ripe with new possibilities for
scientists, according to a new study by Rice University.
Researchers at Rice’s Richard E. Smalley Institute for
Nanoscale Science and Technology have come up with a set of facts and figures
about carbon nanotubes that appear to collapse during the growth process; they
found that these unique configurations have properties of both nanotubes and graphene
nanoribbons.
What the researchers call “closed-edge graphene nanoribbons”
could kick-start research into their usefulness in electronics and materials
applications.
The pioneering work led by Robert Hauge, a distinguished
faculty fellow in chemistry at Rice, is detailed in a paper online in ACS Nano.
“A collapsed nanotube looks a lot like graphene in the
middle, but exactly like buckyballs (carbon-60 molecules, a Nobel Prize-winning
discovery at Rice) on the sides,” Hauge said. “That means you have the
chemistry of graphene in the middle and the chemistry of buckyballs on the
edges. And you can separate the two electronically by putting functional groups
on the sides to isolate the top and bottom layers.”
“If you do edge chemistry that turns the sides into
insulators, then the top doesn’t communicate with the bottom electronically,
except through some van der Waals-type or excited-state interaction,” he said. “That’s where the new physics and perhaps electronic properties will come
from.”
The finding may lead to grown-to-order, two- or four-layer
graphene nanoribbons with perfect edges, a product difficult to attain by unzipping
or otherwise slicing nanotubes. “The graphene world is searching for ways to
make well-defined ribbons,” Hauge said. “They always have to cut up graphene
and end up with ill-defined sides that affect their electronic properties.
These have the advantage of a much better-defined edge.”
Hauge’s awareness of earlier work on nanotube collapse led
him to study the phenomenon. “I’ve been interested in growing larger diameter
nanotubes, based on catalyst particle size, for some time now,” he said. “We
thought they could collapse, so we started looking for the evidence.”
The team found that folds, twists, and kinks in nanotubes
seen through a transmission electron microscope and measured through an atomic
force microscope were good indicators of collapsed nanotubes. These nanotubes
were about 0.7 nm in height along the middle and a little more at what the
researchers called the “highly strained bulbs” at the edges. But finding flattened
tubes didn’t indicate how they got that way.
Hauge approached Rice theoretical physicist Boris Yakobson
to see how the intrinsic energy of atoms in graphene—one of his specialties—would
allow such a collapse to happen. Yakobson put graduate student and co-author
Ksenia Bets on the case.
“Originally, we thought this would be a small and simple
problem, and it turned out to be simple—but not that small,” Bets said. Using
molecular dynamic simulation, she fit data from the experimentalists to
atomistic models of single-wall nanotubes. “And then, using the same
parameters, I produced results for double walls, and they also fit exactly with
the experimental data.”
The results gathered over six months confirmed the
probability that at growth temperature—750 C—flexible nanotubes fluttering in
the gas breeze inside a furnace could indeed be induced to collapse. If two
atoms on either side of the inner wall get close enough to each other, they can
start a van der Walls cascade that flattens the nanotube, Bets said.
“At first, it takes energy to press the nanotube, but you
reach a point where the two sides begin to feel each other, and they begin to
gain the energy of attraction,” Hauge said. “The van der Waals force takes
over, and the tubes then prefer to be collapsed.”
He said the energy required to collapse a nanotube decreases
as the tube’s diameter increases. “It’s like a straw,” he said. “For a
single-wall nanotube, the bigger it gets, the easier it is to distort.”
More significant were calculations that determined the
specific diameters at which nanotubes become prone to collapse. There’s a
point, Hauge said, at which a nanotube could go either way, so the dispersion
of nanotubes to nanoribbons in a batch of a particular diameter should be about
equal. As diameter increases, the balance shifts in the ribbons’ favor.
“It’s a playoff between the strain energy on the edges
versus the van der Walls interaction in the center,” he said. Specifically,
they found that freestanding single-wall tubes become amenable to collapse when
they are at least 2.6 nm in diameter—what the researchers called the “energy
equivalence point.” Theory dictates that diameter would drop to 1.9 nm for a
single-wall tube sitting on a graphene surface, he said, because of additional
atomic interaction with the substrate.
Double-wall nanotubes reach energy equivalence at 4 nm,
Hauge said, but nanotubes with more walls would take much more—probably too
much—energy to collapse.
Bets’ formulas agreed nicely with his group’s observations,
Hauge said. “What we measured in this paper for the first time is the point
where the energy of a collapsed tube is equal to that of an uncollapsed tube,”
he said. “That’s the tipping point. Anything above, energetically, prefers to
be collapsed rather than uncollapsed. It’s a fundamental property of nanotubes
that hadn’t been measured before.”
The discovery has implications for bundles of nanotubes beginning
to see use in fibers for electrical applications or as strengthening elements
in advanced materials. “The question is whether a layer of collapsed tubes in a
bundle is actually more energetically favorable than that same bundle of hexagonally
shaped tubes,” Hauge said. “That hasn’t been determined.”
Many basic questions remain, Hauge said. The researchers
don’t know whether a nanotube collapses along its entire length, nor whether
pressure from outside could start a chain reaction leading to collapse. “It’s
possible that you could apply pressure to force everything to collapse, and it
would stay that way because that’s what it wants to be,” he said. They would
also like to know whether a nanotube’s chirality—its internal arrangement of
atoms—influences collapsing.
But he believes nano researchers will have a field day with
the possibilities. “This should get people thinking about the whole area of
larger-diameter nanotubes and what they might offer,” he said. “It’s like what that
guy on the radio used to say: We’ve all heard the story of nanotubes—and now we
know the rest of the story.”
Source: Rice University
