Today”s subject is nanotubes. We”ve used the term nanothis or
nanothat a few times in previous columns but it occurs to me that
the term “nano” may need further consideration for those
unfamiliar with its meaning. When we talk about lengths or
distances, the term nano just means one-billionth of whatever unit
of length we”re using. A nanomile would be a billionth of a mile
or, if my math is correct, about 6 hundred thousandths of an inch,
pretty small. I suspect you might not be able to see anything that
small with your naked eye. At any rate, this demonstrates that a
billionth of something is a very small fraction of that something.
In the world of atomic dimensions today”s commonly used unit of
measurement is the nanometer, about the same as a nanoyard. (A
meter is 39.37 inches, about 3 inches longer than a yard.) A
billionth of a meter turns out to be about the length that you”d
have if you lined up a half dozen to a dozen carbon atoms in row.
When people talk about nanotechnology or nanoparticles, they”re
generally talking about things that are in the range of hundreds of
nanometers down to one nanometer or less in size.
A few weeks ago, we talked about the radically new forms of
carbon known as fullerenes or buckyballs. We discussed the fact
that these new compounds had a structure precisely like that of a
soccer ball or a geodesic domed building. While the buckyball
materials are of great interest, there is a lot of attention now being
paid to an offspring of the buckyball called the carbon nanotube.
You may recall that the buckyball material was first isolated in
1990. Shortly thereafter, the existence of a nanotube was first
predicted theoretically by workers at the Naval Research
Laboratory (NRL). If you look at a soccer ball or geodesic
dome, you”ll note that the structure involves a mixture of
hexagons and pentagons. The NRL theorists predicted that the
mixing of such 5- and 6-sided shapes could also yield a tubular
structure and that the tubes would have electronic properties
more like metals than graphite, the common form of carbon.
Strangely, their paper was rejected for publication as being too
speculative. Sure enough, it was soon found in Japan that,
nestled among the buckyballs in sooty deposits produced in arc
discharges, were tiny little tubes of carbon closed at both ends by
pentagons. Sumio Iijima published this work in Nature in 1991.
Belatedly, the NRL paper predicting the existence of the tubes
was then accepted for publication in Physical Review Letters and
not published until 1992! I”ve tended to be a little flippant about
theorists in some of these columns but this time they certainly
deserve plaudits for their accurate prediction. I”m glad I wasn”t
the reviewer who originally rejected the paper.
Iijima”s first nanotubes turned out to be what are now known as
multi-walled nanotubes, or MWNTs for short. The MWNTs, I”ve
seen pictured are about 50 nanometers wide. My impression is
that these MWNTs are something like those Russian dolls in
which smaller dolls are stacked inside the larger dolls, etc. In this
case, the dolls are tubes of various sizes. Each tube is like a sheet
of graphite folded into a cylinder. The graphite layers are
composed of hexagons, as we”ve mentioned before. I”ve been
staring at a model of a nanotube and the walls are all hexagons
but as near as I can tell, at the rounded bottom or top of the tube
there is one pentagon that closes the tube. If you”d like to stare at
nanotubes, log on to the Rice University Web site
http://rice.edu/images/allotropes.jpg for pictures and models of all
kinds of interesting carbons.
In 1993, the same Mr. Iijima and a group of workers at IBM in
California independently carried out arc discharges to which they
added various metals. What both groups observed were bundles
or “ropes” of really thin nanotubes only one nanometer or so in
diameter. Now we”re down to atomic scales and these are called
single-walled nanotubes (SWNTs). There is another, very
important “financial” characteristic of the SWNTs. They aren”t as
difficult as the MWNTs or the buckyballs to prepare in significant
quantities, i.e., they”re much less expensive. Furthermore, it
seems that the SWNTs have been made up to a millimeter long,
an astounding length to width ratio. In fact, one French group
claims that in one gram of material they”ve gotten the equivalent
of over a hundred thousand miles of nanotubes.
All this is great from the standpoint of fundamental research but
you might well say “So what?” It turns out that some of the
properties of these new forms of carbon are quite intriguing.
These nanotubes (and the other fullerenes) are extremely strong
and stable forms of carbon. At the same time, the nanotubes can
be bent quite sharply without breaking. They are also very good
electrical conductors but at the same time twisting or bending can
turn them from metals into semiconductors. Within the past year,
a number of reports have appeared showing that the SWNTs can
store large quantities of hydrogen. This opens up the possibility
of using them in fuel cells or batteries. There is even a
demonstration of their use in display applications such as a flat
panel TV set.
And what about the recent report in the December 10, 1999 issue
of Science on nanotweezers made with nanotubes? Philip Kim
and Charles Lieber, a Harvard chemistry professor, developed
these nanotweezers. It”s hard to say whether or not these
tweezers will have practical applications but they certainly have
quite definite applications in studying the properties of materials
and devices down in the range of a ten thousandth of an inch or
so. The tweezers themselves are a neat trick, building on an
effect demonstrated by others in 1992. Two nanotubes are
attached to two electrodes deposited on opposite sides of a
micropipette. By varying the voltage across the two electrodes
the nanotubes will bend toward each other, the amount depending
on the voltage. When the voltage is removed at 8.5 volts, the
tweezers stay closed. By applying the same polarity voltage to
both arms the tweezers will open. Pictures in the Science article
show the tweezers picking up tiny polystyrene balls of less than a
micron (about 1/20,000th of an inch) in diameter.
The tweezers not only can pick up and move tiny objects but also,
because the nanotubes are electrically conducting, allow
measurements to be made on the electrical properties of small
objects. As noted in a previous column, when things get really
small the ordinary electrical properties of the transistors on your
good old silicon chip can change completely. You now
frequently see articles that say that Moore”s Law is in jeopardy
and the great ride we”ve had in increased computing power at
lower prices will be over in ten years or so. Maybe these
tweezers can help out in elucidating what goes on in truly
nanodevices. Another severe problem in making transistors
smaller and smaller is that the connections (wires) have to be
smaller also. Miniaturizing the connections is no simple problem.
Could the nanotubes, being good conductors be used as wires? I
have no idea!
If you have a laptop or notebook computer, you”re familiar with
liquid crystal displays (LCDs). You will know that although the
image quality can be quite good, the LCD can”t compare with the
ordinary cathode ray tube (CRT) on your typical TV or desktop
monitor. The CRT uses a single electron gun at the small end of
your monitor to shoot a beam of electrons across the phosphor-
coated screen, lighting up the various color phosphor dots. An
alternative is something called “field emission” in which there are
thousands of tiny pointy sources of electrons mounted directly in
back of the phosphor dots in a flat screen. The problem with this
field emission process has been to find a material that can be
formed into these points and still stand up to the high currents of
electrons. Since the carbon nanotubes are good conductors and
are quite sturdy materials, they are good candidates if they can be
formed in the precise patterns necessary for a practical TV
display. Now, workers at Samsung in Korea have done just that
and have demonstrated a four and one half-inch nanotube-based
flat screen display. Meanwhile, LCD prices continue to fall and
the LCD technology is getting better. Regardless of which
technology wins out, we”ll all be winners if we can replace those
bulky TV sets with an affordable alternative.
Meanwhile, other workers at IBM have used ultrasonic bubbles to
make the carbon nanotubes curl up into circles several hundred
nanometers in diameter. Apparently, the nanotubes don”t like
water and prefer to attach to a bubble to minimize their contact
with water. Then, as the bubble collapses the end of the
nanotubes follows along until it forms a circle. At this point, the
ends attract each other and the circle, actually a coil, is rather
stable. These circular nanotubes also appear to have interesting
electrical properties.
As you can see, there”s an awful lot going on in nanostuff and
we”ll no doubt have more to say about these titillating tiny tubes
in a future column.
Allen F. Bortrum
