Physicists at the University of California, Berkeley, have built the
smallest radio yet — a single carbon nanotube one ten-thousandth the
diameter of a human hair that requires only a battery and earphones to
tune in to your favorite station.


Over
the past century, radio has shrunk dramatically from the wooden
"cathedral" style radios of the 1930s to the pocket-sized transistor
radios of the 1950s and more recently to the single-chip radios found
in cell phones and wireless sensors. Continuing this trend, researchers
at UC Berkeley have further miniaturized the radio by cleverly
implementing multiple radio functions with a single component, the
carbon nanotube. This nanotube radio is over 19 orders of magnitude
smaller than the Philco vacuum tube radio from the 1930s!

The scientists successfully received their first FM broadcast last year
— Derek & The Dominos’ "Layla" and the Beach Boys’ "Good
Vibrations" transmitted from across the room. In homage to last year’s
100th anniversary of the first voice and music radio transmission, they
also transmitted and successfully tuned in to the first music piece
broadcast in 1906, the "Largo" from George Frederic Handel’s opera
"Xerxes."

"We were just in ecstasy when this worked," says team leader Alex Zettl, UC Berkeley professor of physics. "It was fantastic."

The
nanoradio, which is currently configured as a receiver but could also
work as a transmitter, is 100 billion times smaller than the first
commercial radios, and could be used in any number of applications —
from cell phones to microscopic devices that sense the environment and
relay information via radio signals, Zettl says. Because it is
extremely energy efficient, it would integrate well with
microelectronic circuits.

Nanotubes are rolled-up sheets
of interlocked carbon atoms that form a tube so strong that some
scientists have suggested using a nanotube wire to tether satellites in
a fixed position above Earth. The nanotubes also exhibit unusual
electronic properties because of their size, which, for the nanotubes
used in the radio receiver, are about 10 nanometers in diameter and
several hundred nanometers long. A nanometer is one billionth of a
meter; a human hair is about 50,000 to 100,000 nanometers in diameter.

"The
nanotube radio may lead to radical new applications, such as
radio-controlled devices small enough to exist in a human’s
bloodstream," the authors wrote in a paper published online (Oct. 31)
by the journal Nano Letters. The paper appeared in the print edition of
Nano Letters in November.

The authors of the nanoradio
paper are Zettl, graduate student Kenneth Jensen, and their colleagues
in UC Berkeley’s Center of Integrated Nanomechanical Systems (COINS)
and in the Materials Sciences Division at Lawrence Berkeley National
Laboratory (LBNL). COINS is a Nanoscale Science and Engineering
Research Center supported by the National Science Foundation (NSF).

In
the nanoradio, a single carbon nanotube works as an all-in-one antenna,
tuner, amplifier, and demodulator for both AM and FM. These are
separate components in a standard radio. A demodulator removes the AM
or FM carrier frequency, which is in the kiloHertz and megaHertz range
respectively, to retrieve the lower frequency broadcast information.

The
nanoradio detects radio signals in a radically new way — it vibrates
thousands to millions of times per second in tune with the radio wave.
This makes it a true nanoelectromechanical device, dubbed NEMS, that
integrates the mechanical and electrical properties of nanoscale
materials.

In a normal radio, ambient radio waves from
different transmitting stations generate small currents at different
frequencies in the antenna, while a tuner selects one of these
frequencies to amplify. In the nanoradio, the nanotube, as the antenna,
detects radio waves mechanically by vibrating at radio frequencies. The
nanotube is placed in a vacuum and hooked to a battery, which covers
its tip with negatively charged electrons, and the electric field of
the radio wave pushes and pulls the tip thousands to millions of times
per second.

While large objects, like a stiff wire or a
wooden ruler pinned at one end, vibrate at low frequencies — between
tens and hundreds of times per second — the tiny nanotubes vibrate at
high frequencies ranging from kiloHertz (thousands of times per second)
to hundreds of megaHertz (100 million times per second). Thus, a single
nanotube naturally selects only one frequency.

Although
it might seem that the vibrating nanotube yields a "one station" radio,
the tension on the nanotube also influences its natural vibration
frequency, just as the tension on a guitar string fine tunes its pitch.
As a result, the physicists can tune in a desired frequency or station
by "pulling" on the free tip of the nanotube with a positively charged
electrode. This electrode also turns the nanotube into an amplifier.
The voltage is high enough to pull electrons off the tip of the
nanotube and, because the nanotube is simultaneously vibrating, the
electron current from the tip is an amplified version of the incoming
radio signal. This is similar to the field-emission amplification of
old vacuum tube amplifiers used in early radios and televisions, Zettl
says. The amplified output of this simple nanotube device is enough to
drive a very sensitive earphone.

Finally, the field-emission and vibration together also demodulate the signal.

"I
hate to sound like I’m selling a Ginsu knife — But wait, there’s more!
It also slices and dices! — but this one nanotube does everything; it
performs all radio functions simultaneously and extremely efficiently,"
Zettl says. "It’s ridiculously simple — that’s the beauty of it."

Zettl’s
team assembles the nanoradios very simply, too. From nanotubes
copiously produced in a carbon arc, they glue several to a fixed
electrode. In a vacuum, they then bring the electrode within a few
microns of a second electrode, close enough for electrons to jump to it
from the closest nanotube and create an electrical circuit. To achieve
the desired length of the active nanotube, the team first runs a large
current through the nanotube to the second electrode, which makes
carbon atoms jump off the tip, trimming it down to size for operation
within a particular frequency band. Connect a battery and earphones,
and voila!

Reception by the initial radios is scratchy,
which Zettl attributes in part to an insufficient vacuum. In future
nanoradios, a better vacuum can be obtained by insuring a cleaner
environment, or perhaps by encasing the single nanotube inside a
second, larger, non-conducting nanotube, thereby retaining the
nanoscale.

Zettl won’t only be tuning in to oldies
stations with his nanoradio. Because the radio static is actually the
sound of atoms jumping on and off the tip of the nanotube, he hopes to
use the nanoradio to sense the identity of atoms or even measure their
masses, which is done today by cumbersome large mass spectrometers.

Via Designfax