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Nanotube Radio
Did Marconi invent radio the wrong way?
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Making a mockery of the complex architecture of wireless
transmission, a single molecule of carbon nanotube performs
all the functions of a wireless Radio! Being only a few
billionths of a meter in size, the nanotube radio is so small it
can easily fit inside a living cell or float about in your
bloodstream. In fact, it is about the size of a virus. Nanotubes
are a better conductor of electricity than copper, silver or even
superconductors! The science of miniaturization is indeed
venturing into the eerie vista of infinitesimally small
Looks like we have been making radios the wrong way, piecing together
complex circuitry with its hundreds of components, while nature had already
configured a few atoms of carbon to do the job most brilliantly. The situation is
akin to a supercomputer which, with all its complexity, size and tremendous
energy consumption, compares pitiably with the size and efficiency of human
brain.
The conventional radio essentially comprises four parts: antenna, tuner,
amplifier, and demodulator as shown in the block diagram. The antenna
collects radio signals from the surrounding and together with a tuner selects
the desired frequency. The weak signal so selected is then handed over to an
amplifier which amplifies the signal. Next comes demodulation; the process of
extracting information from the career signal. This information, the audio
program in this case, is further amplified and fed to a speaker which converts
electrical pulses into sound waves; speech, song, music, etc. It involves
thousands of components and, as compared to a carbon nanotube, is housed
in a huge box. And of course the traditional radio consumes thousands of
times more energy than its nano counterpart.
The Nanotube Radio
Carbon Nanotubes are tubular structures of molecular scale and are among
the stiffest and strongest fibers known. The force that binds the atoms of
carbon together in a nanotube is the strongest in nature. They also have
remarkable electronic properties and many other unique characteristics.
Nanotubes are a better conductor of electricity than copper, silver or even
superconductors. The stable and strong structure of carbon in graphite and
diamond led to speculations that this element could be molded into even more
stronger and stable microstructures. The research in this area resulted in the
discovery of nanotubes in 1999. These tubes can be single layered or
multilayered. The outer diameter of a multilayer nanotube ranges from 3
nanometer to 30 nanometer but the diameter of a typical single layer nanotube
has a diameter of 1-2 nanometer s. A single layer nanotube may be
considered as a sheet of graphite rolled into a hollow tube, without any
overlapping where the two edges meet.
A team of researchers at UC Berkeley lead by physicist Alex Zettl have
invented a radio made of a single carbon nanotube. Zettl was interested in
making tiny sensors capable of communicating with one another wirelessly.
During the course of investigation, it was found that a nanotube fixed to some
base at one end could be made to vibrate if hit by molecules at its free end.
The frequency of vibration depended on the mass of hitting molecules. Zettl
further noticed that the nanotube could be made to vibrate at frequencies
used by commercial radio transmissions. It was here that the idea of creating a
radio at nanoscale germinated. The radio that resulted was small enough to fit
inside a living cell and was capable of performing all the functions of a
conventional radio with astonishing simplicity.
The Mysterious Realm of Nano-world
A conventional radio picks signals through its antenna tuned to the frequency
to be selected. Due to the phenomenon of resonance, a small current is
induced in the antenna by the selected frequency. The process is electronic.
By contrast, the process of capturing the signal is physical in the nanotube
radio. The nanotube is so small that when it is hit by the incoming
electromagnetic signal in a suitable environment, it starts vibrating physically in
tune with the electromagnetic wave.
In the original experiment by Zettl and his team, a multi-walled nanotube was
grown on a tiny electrode. Some distance apart from nanotube was a counter-
electrode. A small DC voltage applied across the electrodes caused electrons
to flow from the tip of the nanotube to the counter-electrode. When hit by an
electromagnetic signal, the nanotube starts vibrating physically causing a
change in the current flowing between the tiny electrodes. This is called a field
emission current. In this phenomenon of quantum mechanics, a small applied
voltage produces a large flow of current from the tip of an object. In this case,
the flow of current is in sync with the electromagnetic waves hitting the
nanotube. The nanotube thus functions both as an antenna and as an
amplifier for the detected signal. You would, however, need speakers or
earphones to hear the transmission.
But how do you tune a nanotube for different frequencies? As mentioned
earlier, unlike a conventional antenna that resonates electronically in
response to an electromagnetic wave, a nanotube vibrates physically under
the direct physical impact of the electromagnetic waves hitting it. It was only
natural to expect that the frequency a nanotube could be changed physically
which is indeed the case. The frequency of a nanotube can be altered exactly
as you physically change the frequency of a string of guitar. There are two
ways of doing it. By increasing or decreasing the length of the guitar string or
by increasing or decreasing the tension in the string. In the case of a
nanotube however, the process of changing the frequency by changing its
length is irreversible and is thus not a useful option. Tempering with the
tension in the nanotube is easier. The tension in a nanotube is increased or
decreased by varying the strength of the electric field in which the nanotube is
placed. This change in tension also changes its frequency. When the
frequency of nanotube is fixed by tweaking the strength of the applied electric
field, the nanotube will vibrate only if electromagnetic waves of exactly the
same frequency are hitting the nanotube. By adjusting the frequency of the
nanotube, one can make it vibrate in sync with the desired electromagnetic
signal (radio transmission).
Next comes demodulation; the process of extracting information (program)
from the career wave. In conventional AM radio, this is accomplished by
rectification and filtering the signal whereby the career wave is not allowed to
pass and only the variations in amplitude (information) is passed on. By a
stroke of luck, this is also achieved naturally by the nanotube. When it vibrates
in response to an electromagnetic signal, the current coming out of the
nanotube varies with the coded (informational) signal embedded within the
career. By a string of highly favorable attributes of a nanotube, all the vital
functions of a radio are performed by it single handedly. A nanotube:
• can be tuned to radio frequencies
• detects the electromagnetic signal
• amplifies the signal
• separates information from the career signal
In the words of Zettl,
“In electronics, often you have a trade off. If you optimize this, you lose
something else. Here [in the case of nanotube radio] everything seems to just
work for you, which is a little unusual. You don’t see that often in science. It is
one of those rare opportunities to see Murphy’s Law not rearing its ugly head.
Here everything that can go right is going right.”
Zettl and his colleagues believe that there will be many applications for the
radio. It could be used in medical devices that swim through your body,
responding to radio commands. Or it could be put inside tiny wireless devices.
It could even be put inside a human ear.
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