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05/15/2003
Carbon Nanostuff
Every so often I feel compelled to talk about carbon. In the past
decade or so, the element carbon has found to exist in some most
interesting forms such as the soccer ball-like buckyballs and as
teensy tubes known as nanotubes. We’ve discussed these new
structures at length in previous columns and they continue to be
the subject of intense interest. Why? Aside from the purely
scientific interest, “nanocarbon” structures have been proposed
for uses ranging from vessels for drug delivery, components for
strengthening structural materials, materials for metal hydride
batteries or fuel cells, elements in flat panel TV displays, tips in
ultrahigh power scanning probe microscopes and many other
applications. Possibly the most exciting application would be the
use of nanotubes as a replacement for silicon as the material for
making transistors in the next decade or so when silicon can be
miniaturized no further.
These carbon nanotubes are quite strong; hence the talk of using
them to strengthen other construction materials. We golfers
already have carbon-shafted (graphite) golf clubs. Could
nanocarbon shafts be down the pike? Could carbon nanotubes be
added to concrete or other construction materials? There’s a
problem in that in such applications the strengthening material
typically has to be incorporated in the form of long fibers or
strands. While nanotubes are often formed in fibers or bundles
of fibers, the lengths of these are typically of microscopic
dimensions or at most around a millimeter, about the size of that
C. elegans nematode we discussed last week.
A step forward in the length department was reported a year ago
in the May 3 2002 issue of Science by Zhu, Xu, Wu, Wei, Vajtai
and Ajayan of Tsinghua University in China and Rensselaer
Polytechnic Institute in the U.S. They heated the bejeebers out
of the organic solvent n-hexane, with some added ingredients
and special equipment, and managed to obtain strands of
nanotubes that were as long as 8 inches or so. These nanotube
“ropes” were thicker than a human hair and consisted of
individual strands of SWNTs, single wall nanotubes. SWNTs
are nanotubes in which the walls are as thin as possible, only one
atom layer thick. Fabrication of such long ropes of SWNTs is
one step forward toward future structural applications.
Since the discovery of carbon nanotubes, investigators have been
finding other materials that form nanotubes. Boron nitride, BN,
with one atom of nitrogen and one of boron was a logical choice.
Boron precedes carbon in the Periodic Table while nitrogen
follows carbon. This means that when you put boron and
nitrogen together, they have the same number of electrons that
form bonds as carbon. If you’re a chemist, you won’t be too
surprised if BN can be made in the same forms as carbon. Sure
enough, BN exists in a soft, easily machined form similar to
graphite. I’ve used this material to make crucibles when I was at
Bell Labs. It’s white, not black like graphite. I’ve also used so-
called pyrolytic BN, which is like glassy carbon.
As anticipated, BN nanotubes (BNNTs) have been made.
BNNTs have very low electrical conductivities, that is, they’re
electrical insulators. Carbon nanotubes and buckyballs can be
good electrical conductors, if treated properly. Why worry about
such things? If some day you’re going to have carbon nanotube
transistors on a chip, you’re going to have to connect the
transistors electrically. How are you going to do that? With
nanowires, of course. However, with nanowires so close to each
other on the chip, you might have to insulate them, just as you do
with the rubber insulation around the wires in your lamp cord.
So, how about packing buckyballs inside boron nitride
nanotubes? The buckyballs packed together might form a
nanowire, insulated by being encased in the boron nitride
nanotube. But can you put BBs (buckyballs) inside the BNNTs?
Researchers Mickelson, Aloni, Han, Cumings and Zettl at the
University of California at Berkeley answer affirmatively in a
paper in the April 18 2003 issue of Science. The problem is like
stacking peas in a pod, in this case a round pod of BNNT a little
more than a nanometer or so in diameter. By varying the
diameter of the BNNTs, they obtained different stacking
arrangements of the buckyballs. They then went a step further
and heated the composite structures. The buckyballs joined
together and form a carbon single wall nanotube, ending up with
a carbon SWNT inside a BNNT, one nanotube inside another. It
sure beats trying to pick up an SWNT and place it inside a
BNNT manually!
If you follow Doonesbury, last week’s strip focused on the two
college “students” somehow latching onto one of the ancient
scrolls looted from the Baghdad Museum. Why not carbon
nanoscrolls? In the February 28 2003 issue of Science, Lisa
Viculis, Julia Mack and Richard Kaner of UCLA describe a neat
procedure they used to make just such objects. As a lithium
battery man, I was especially intrigued by their use of graphite to
make these carbon nanoscrolls.
To set the stage, remember that graphite is slippery and is used in
your “lead” pencils because it consists of layers of carbon held
together by weak so-called Van der Waal’s forces. These weak
forces allow the layers to slip onto your paper when you press
down and slide your pencil on the paper. In the lithium-ion
battery in your laptop or cell phone, one electrode is probably
graphite. As your battery charges and discharges, lithium atoms
slide in and out between the carbon layers. Potassium is larger,
but in the same family as lithium. What the UCLA team did was
to heat graphite and potassium together. The potassium, like
lithium, goes in between the layers to form a pretty gold colored
compound, KC8, one atom of potassium for every eight carbons.
When they stick this compound into ethyl alcohol, the kind you
drink, the potassium reacts with the alcohol, which gives off
hydrogen in the process. With all that hydrogen bubbling and
shaking going on, the sheets of carbon are jumbled askew and
aren’t lined up properly to form graphite again. The potassium
compound is washed away and the resulting mixture of messed
up graphite and alcohol is “sonicated” using an ultrasonic probe.
When they looked at the stuff, most of the carbon sheets had
curled up into nanoscrolls! Some of the nanoscrolls had up 60
complete turns. Could these little scrolls be used to transmit
secret communications if nanowriting could be accomplished?
The authors of the paper had other, more sensible ideas. One
possibility is to store hydrogen. Carbon nanotubes have been
proposed for this application. However nanotubes are closed at
both ends so you’re wasting that inside surface for picking up
hydrogen. With both sides of the sheet available in a nanoscroll,
there’s a much larger surface area available. As mentioned
earlier, the hydrogen would be stored for use in either nickel-
metal hydride batteries or fuel cells. The authors also suggest
that nanoscrolls might be used in structural applications.
We now have nanotubes, buckyballs and nanoscrolls. What
about nanocones? The April 18 2003 issue of Science obliges
with an article by Zhang, Jiang and Wang of the Fraunhofer
Institute for Surface Engineering and Thin Films in Germany and
the Institute of Physics, Chinese Academy of Sciences in Beijing.
Actually, they call their objects “tubular graphite cones” and the
dimensions range into the thousands of nanometers. However,
I’m willing to go out on a limb and call them nanocones, if only
because their tips are just a handful of nanometers in diameter.
It’s the nanotip that’s important in the application they suggest,
namely, using the cones as tips in scanning probe microscopes.
In case you’ve forgotten, scanning probe microscopes operate by
dragging a very fine point on or close to a surface. At high
resolutions, single atoms can be “seen”. When carbon nanotubes
are used as tips in these instruments, it’s not an easy job to
handle them and mount them properly. With a cone, there’s
more to grab onto and manipulate. In addition, the cone should
be more rigid and less likely to break than a slender nanotube.
To sum up, things are really humming in the carbon nanoworld.
Except perhaps for the probe tip application, however, there’s
much to be done before nanotubes and nanoscrolls come into
widespread use. It currently costs too much money and time to
make and handle large quantities. Meanwhile, I’ll just admire
the beauty of these nanoobjects and marvel at how clever the
experimenters have been in handling and transforming them.
Allen F. Bortrum
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