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07/10/2003
Of Planets and Nanotubes
In the past we’ve talked about things big and small, ranging from
universes to impossibly tiny strings. It’s too hot and humid to
worry ourselves with such overwhelming concepts, so let’s settle
for a couple of items somewhat smaller and somewhat larger,
respectively. I’ve written a number of times about two such
topics, specifically planets outside our solar system, so-called
exoplanets, and about tiny nanotubes of carbon that have been
touted as a replacement for the silicon chip when it’s been
miniaturized to its limit.
Our July 4th newspaper, The Star Ledger, contained an AP
dispatch that dealt with a conference in Paris titled “Extrasolar
Planets: Today and Tomorrow”. At the conference, Hugh Jones
of Liverpool John Moores University reported the discovery of a
Jupiter-like planet circling the star HD 70642 in the constellation
Puppis. I hadn’t heard of either HD 70642 or of Puppis and at
first thought this was old stuff since the Anglo-Australian team
of which Jones is a member has turned up Jupiter-like planets
before. Indeed, the number of planets discovered outside our
solar system now is over 100.
Typically, these planets have orbits very close to their star or
there’s another big planet stuck in between the outer planet and
the star. The latter planet would sweep out or in any earth-like
planet that was in the vicinity. Or the star isn’t much like our
sun. In general, these scenarios are not very conducive to an
earth being somewhere in an orbit that would be friendly to the
existence of life as we know it. This latest planet is roughly
twice as heavy as Jupiter and orbits its star in a circular orbit at a
bit over three times the distance from our earth to the sun. HD
70642 is comparable to our sun and there’s no intervening large
planet between the big planet and HD 70642. This is exciting
because it’s the closest we’ve come to finding a planetary system
that potentially could harbor an earth nestled in closer to its sun.
As it stands, we can hope that, when more powerful telescopes
are put in service, they might detect another planet like our own
out there in Puppis some 90 light-years away.
Now let’s turn to the other end of the size scale with the carbon
single-walled nanotube (SWNT). To review, think of chicken
wire with its wires twisted into a pattern of hexagons. Roll up a
piece of chicken wire into a tube and it’s like a carbon SWNT,
except that the SWNT will have closed ends with a few
pentagons sneaked into the structure. We have talked about
graphite a number of times over the years. Graphite is sort of
like layers of chicken wire with the layers of carbon atoms held
together weakly, allowing the layers to rub off on the paper when
you write with your graphite pencil. An SWNT is like rolling up
a layer of graphite.
One of the exciting features of SWNTs is that slight twists or
kinks in the structure of SWNTs can lead to quite different
properties. For example, certain SWNTs conduct electricity like
metals while other SWNTs are not good conductors but conduct
electricity like semiconductors. This excites circuit designers
who can imagine transistors made of semiconducting SWNTs
and/or electrical connections also made of metallic SWNTs.
Utopia might be a chip with semiconductor SWNT transistors
connected by “wires” of metallic SWNTs. We would be in the
Carbon Age after being in the Silicon Age the past 50 years or
so.
The path to realizing this dream is fraught with obstacles and
challenges. One is simply the tiny size of SWNTs and the
difficulty in handling them. For example, SWNTs are usually
made by some sort of electrical or laser zapping of carbon in
some form, be it graphite or a carbon compound. What typically
results are “ropes” of nanotubes bunched together like handfuls
of soda straws or worse. The individual nanotubes have this
attraction for each other that causes them to bunch up. In these
bunches of SWNTs there are both forms, the metallic and the
semiconducting types. Separation of the mix into piles of
metallic and piles of semiconducting SWNTs has been pretty
much impossible. Robert Service, in the June 27 issue of
Science, reports that there may be a breakthrough in the
separation and handling of these SWNTs.
Some German physicists, Ralph Krupke and Hilbert von
Lohneysen, combined with chemists Frank Hennrich and
Manfred Kappes to come up with a simple technique. As usual
in science, their scheme was spurred by earlier work reported in
Science in July last year by Richard Smalley and his group at
Rice University. (Smalley is one of the a founding father of the
field of carbon fullerenes, of which SWNTs are an example.)
His group found they could use ultrasound to blast a solvent
containing the SWNT bundles and unbundle them to create a
suspension of individual SWNTs. This at least separates the
SWNTs but the semiconducting and metallic SWNTs still are all
mixed up.
Here’s where the German workers come in. They decided to put
a couple of electrodes in the solution. If you hook up a battery,
one of the electrodes is positively and the other negatively
charged. The electrons in the nanotubes will be attracted to the
positive electrode, setting up positive charges at the other end of
the tube attracted to the negative electrode. This separation of
charges is called a “dipole”. These “induced” dipoles result in
the nanotubes near either one of the electrodes to be attracted to
it. If it’s close enough the nanotube will latch on to the
electrode. You rightly say, “Big deal. You’ve got the SWNTs
on the electrodes but they’re still mixed up.”
But Krupe and his co-workers weren’t stupid. They knew that
electrons in metals fairly fly through the material, whereas
electrons in semiconductors are relatively sluggish in moving
about. So, instead of a battery, let’s put an alternating current
across the electrodes. And let’s not use the ordinary 60 cycles
per second household AC. Instead let’s cycle the current back
and forth millions of times a second. Well, those poor electrons
in the semiconductor barely get going one way when they’re
pulled back the other way. The electrons in the metals, however,
think it’s a great ride, zipping back and forth like crazy, the
induced dipoles flipping accordingly. If the metallic nanotube
happens to be near an electrode it’s attracted to it and sticks. The
semiconducting nanotubes remain in the solvent, wandering
about aimlessly.
Kappes says that they’ve separated mixtures in minutes and
Smalley hails the new technique as long overdue and predicts it
will be quite useful. Lest you think the Carbon Age is now just
around the corner, hold your enthusiasm. So far the technique
has only been used to separate picograms of material. I hesitate
to remind you of what a picogram is. It’s a mere trillionth of a
gram, like a speck of dust. To put it mildly, there’s a ways to go
before the method is scaled up to a commercially viable process,
As I said, there are still many obstacles to be overcome and
challenges to be met before we enter the Carbon Age but hey, the
longest journey begins with…………
Allen F. Bortrum
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