WHEN Moonis Ally blew up a charge of TNT in a US government laboratory last
October, no one came running to the scene. Not even when Ally set off a few more
bombs as the first explosion died away. Despite the bangs and fireballs, flashes
and heat, no one was injured, no equipment was damaged and nobody called
security to arrest him.
That鈥檚 because Ally was blowing up just a few molecules at a time, with
explosions so small that a hundred going off in the palm of your hand would only
feel like an itch. You couldn鈥檛 hear the bangs, and you鈥檇 need a microscope to
see the flashes. 鈥淲e watched as they exploded in little fireballs鈥攋ust
like a regular explosion, only on a nanoscale,鈥� Ally says.
Within a few years, Ally and his colleague Zhiyu Hu hope to be triggering
these 鈥渘anobombs鈥� at airports around the world, and the US Federal Aviation
Administration (FAA) is even paying him to do it. It鈥檚 a case of exploding a
bomb to find a bomb: Ally and Hu are part of a research team developing the
world鈥檚 smallest bomb detector. The team, led by Thomas Thundat and based at the
Department of Energy鈥檚 Oak Ridge National Laboratory in Tennessee, eventually
hope to produce a detector the size and shape of a cellphone and so sensitive
that it will react to six molecules of TNT in a million molecules of air.
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At present, detecting bombs is a cumbersome and costly affair. The only thing
that can match the sensitivity of the Oak Ridge device is a sniffer dog, but
dogs, like humans, have built-in drawbacks. To acquire and train a sniffer dog
costs about $9000, and then they need food, rest breaks and sick days,
not to mention handlers with the same requirements. The dogs tend to be reserved
for special situations鈥攏ot the kind of workaday bomb detecting that
airports need.
Existing bomb detectors use technology ranging from radioactive cathodes to
infrared light detectors. But they can cost as much as $150 000, and many
are simply too slow: it can take minutes for them to react to an explosive
hidden in a suitcase. If you鈥檙e checking thousands of bags flowing through an
airport terminal, you need an instant response.
The same is true when hunting for landmines, another potential application
for the Oak Ridge detector. Thundat thinks this is an even better use for his
detector than exposing the occasional mad bomber in an airport. 鈥淚t鈥檚 estimated
that there are 100 million landmines buried around the world,鈥� he says. At the
present rate, he explains, it would take 4000 years to remove all the mines
buried in Afghanistan alone. The Oak Ridge detector could improve significantly
on this rate of progress. And because the device will eventually be
mass-produced, even developing nations should be able to afford it. 鈥淎t the
moment, a mine can cost $3 to make and as much as $1000 to
remove,鈥� Thundat says. 鈥淲e want to make these detectors so cheap that everyone
in the world, even in Third World countries, can use them.鈥�
The device works because a few molecules of explosive, including 鈥渟tealth鈥�
explosives like Semtex, always escape from their container, even when bombs are
in sealed plastic bags. So TNT placed in a suitcase, for example, leaks out and
lingers at a concentration of up to six molecules of TNT per million air
molecules. Each molecule is a miniature version of the bigger bomb: trigger this
nanobomb in the right way and it will explode.
Think small
So how do you detonate a molecule of TNT? Quite simply, you heat it to above
315 掳C. If you can heat and detonate a few molecules at a time, the
researchers reasoned, it will reveal the presence of a nearby bomb. Except that
a couple of exploding molecules won鈥檛 exactly shake the room, so you need
something almost as small as the molecules themselves to detect the
explosion.
Enter the microcantilever. This microscopic sliver of silicon is fixed at one
end and free at the other鈥攔ather like a little diving board, says Thundat.
He began his work with microcantilevers by using them to detect tiny quantities
of chemicals like mercury. Put an adhesive coating on the microcantilevers and
mercury molecules stick to them, noticeably altering the cantilever鈥檚 resonant
frequency.
Then, in 1996, TWA flight 800 was blown out of the skies over New York鈥檚 Long
Island. The event made Thundat realise that the tiny cantilevers might also be
useful for detecting explosives. If the cantilever is small and flexible enough,
the shock of a few exploding TNT molecules will make it vibrate like the clapper
of an electric bell.
In the Oak Ridge nanobomb detector, the cantilever sits inside a small
chamber that also houses a tiny laser and an array of light-sensitive diodes
(see Diagram).
The laser beam shines onto the top of the cantilever鈥檚
free end, and reflects off it onto one of the diodes. When the cantilever is
motionless, the light beam hits a 鈥渃ontrol鈥� diode, which sends out a signal
indicating that all is peaceful.
But not for long鈥攈old the device within half a metre of a suitcase
containing explosives and all hell will break loose. A pump at the back of the
device sucks air, along with the free-floating TNT molecules, into the chamber.
The cantilever is kept at 315 掳C by passing about a millionth of a watt of
electrical power through it; because explosive molecules are sticky, some cling
to the heated surface of the cantilever, get hot and eventually detonate. When
the molecules blow up, the diving board jiggles under a tiny pico-newton
force鈥攅quivalent to the impact of a single red blood cell. The movement of
this tiny diving board deflects the laser beam onto the other diodes: these mini
alarms pass a warning signal to the operator that the suitcase holds
explosives.
The researchers found that increasing the temperature to about 575 掳C
speeds up the reaction and ignites TNT molecules in a fraction of a second,
making instant bomb detection a real possibility.
But detonating nanobombs in milliseconds is a piece of cake compared with
crafting the cantilever. Make the sliver of silicon too thick, and it will be
too stiff, needing a large number of molecules to explode at once before it
jiggles about. Make it poorly proportioned or too thin in the wrong places, and
repeated nanoblasts could pit or crack it.
The task of making the perfect cantilever fell to Oak Ridge physicist Panos
Datskos. He sculpted the cantilevers from silicon using a highly focused beam of
gallium ions. The ions shave material from the surface of microscopic objects
the same way a hand plane shaves wood from a board. 鈥淯sing this tool, we can try
out a particular geometry for the cantilever and gradually refine the shape,
instead of having to build a new one every time we want to try a different
geometry,鈥� Datskos says. Thanks to Datskos鈥檚 delicate hand, the resulting
cantilevers are small, but incredibly robust. They are 100 micrometres long,
around 20 micrometres wide, and just 0.7 micrometres thick. You鈥檇 need a stack
of 100 to match the diameter of a human hair. And after dozens of nanobangs,
these tiny structures show no signs of wear. 鈥淵ou can throw the cantilevers on
the floor or shoot them from guns and they鈥檒l be fine,鈥� he says.
The researchers are now refining their nanobomb detector to make it simpler
and more reliable. The next version may do without the laser and the diodes.
Instead, the silicon cantilever could be doped with boron to make it
piezoresistive, meaning that its electrical resistance will alter under
mechanical stress. The jiggle of a nanoblast will change the electrical
resistivity of the electrically charged cantilever. The change will only be a
tiny fraction of an ohm, but, Thundat says, that鈥檚 still big enough to measure.
To screen out external vibrations, a commercial version might sport dual
cantilevers in separate chambers. An exploding molecule would move only one
cantilever at a given moment, while an external jolt would result in
simultaneous signals: a false alarm.
Ally is concerned about potential problems arising from the explosives鈥�
inherent stickiness. The viscous molecules could stick to the walls of the
machine鈥檚 air intake system, only to break free some time later, explode and
create a false alarm. To get round the problem, the team is working on a design
that would somehow leave the cantilever exposed to open air.
The group is planning further refinements to the cantilevers: they will keep
experimenting with the shape to find one with the perfect balance of firmness
and flexibility. The team is thinking about coating them with transition-metal
oxides鈥攃atalysts such as vanadium oxide or platinum oxide鈥攚hich
would provide more free oxygen to trigger explosions at lower temperatures.
Eventually, Thundat sees the invention evolving into an all-purpose,
hand-held bomb detector with an array of cantilevers. Because every explosive
has a specific temperature at which it ignites, each cantilever will be heated
to a different temperature. Check the temperature and you鈥檝e nailed your
explosive.
The team expects to have a device ready for testing at Oak Ridge鈥檚 own
airport within three years. The detectors will probably be the size and shape of
a mobile phone, and run on ordinary AA batteries. 鈥淯ltimately, our idea is to
make a device that can be mass-produced,鈥� he says. 鈥淐omputer chips are amazingly
powerful and sophisticated, but you can micro-machine billions of them for a few
dollars apiece after you develop the manufacturing equipment.鈥�
The initial phase of research has proved so successful that the FAA has just
given the researchers another $300 000 to turn their bench-top device
into a prototype. The detectors could soon be in the hands of roaming airport
security guards, or installed on the X-ray machine to sniff bags as they pass
through. Detonating explosives may be a strange way to enhance your safety, but
if the person ahead of you is smuggling a bomb onto the plane, you could be very
glad of the nanoblasts going off in your face.