快猫短视频

SCIENTISTS often talk of 鈥渒icking around ideas鈥�, as if deciphering nature
were a friendly game of soccer, and insight might come as easily as a lucky
bounce of the ball. However, the efforts of Hongkun Park and Paul McEuen give
the clich茅 new meaning. They and their colleagues at the University of
California, Berkeley, spent months trying to explain the weird behaviour of the
world鈥檚 smallest transistor. It was a tiny cousin of ones that power everything
from wristwatches to supercomputers, but it didn鈥檛 perform in quite the same
way. And then they remembered a simple truth usually more relevant to football
than to physics. Kick a ball and it will bounce.

Their ball was a buckyball, a single molecule measuring less than a billionth
of a metre across, made of 60 carbon atoms and shaped like a tiny soccer ball.
However, the thing that set it in motion was a million times smaller still. A
single electron landing on the ball was enough to make it bounce. These tiny,
jumping circuits are something of a surprise, McEuen admits. 鈥淚n electronics,鈥�
he says, 鈥渨e鈥檙e not used to our devices moving.鈥�

But the experiment demonstrates something even more significant than the
electron鈥檚 heavy footfall. The mechanical movement was not a random wobble, but
a direct, predictable consequence of the quantum mechanics of the electron and
the buckyball. Every aspect of these electronic devices鈥攅ven the way they
move鈥攊s shaped by the same rules that hold sway over the behaviour of
atoms and molecules. Researchers are making devices so small, pushing into the
realm where current is measured in individual electrons, and mechanical parts
can consist of single molecules, that these rules now govern everything. 鈥淚
think there鈥檚 a new field emerging,鈥� says Keith Schwab, a physicist at the
Laboratory for Physical Sciences in College Park, Maryland, 鈥渁nd that is QEM:
quantum electro-mechanics.鈥�

Researchers believe that the key feature of QEM鈥攖he link between
electronic and mechanical motion鈥攎ight lead to wild new technologies many
times smaller and faster than today鈥檚 best electronics. It also shows how
technologists might mimic the way biology works, perhaps making devices that
behave less like lifeless tools and more like living creatures. Nature routinely
exploits the connection between electronic and mechanical motion. A couple of
electrons鈥� worth of charge, for instance, is all it takes to open and close ion
channels, the trapdoor-like molecules that let substances such as calcium and
potassium flow through cell membranes.

The behaviour of devices at these scales could eventually mean fundamental
changes in the way we build things, forcing us to abandon old ideas. 鈥淢aybe
there will be entirely new types of nanomechanical devices that will do things
we haven鈥檛 even thought about,鈥� McEuen says.

Their experiment may demonstrate the essential aspect of QEM, but Park,
McEuen and their colleagues set out with more modest aims. They simply wanted to
see how individual electrons would rattle through the 0.7-nanometre-wide
buckyball. 鈥淎lthough a lot of people talk about it, not much is known about how
electrons pass through such structures,鈥� says Park, who has since moved to
Harvard University.

They made their buckyball into the central part of a transistor, a switch for
controlling the current flow in a circuit. Starting with tiny gold wires drawn
on a wafer of silicon dioxide, they painted the wafer with a solution of toluene
and buckyballs. They ran a strong current through each wire which burned a
1-nanometre-wide gap where the wire was weakest. Occasionally a buckyball would
fall into this gap. Electrons moving along the wire could cross the gap by
hopping on and off the buckyball.

Park and McEuen intended to control this flow of electrons by applying a
voltage to a layer of silicon buried inside the silicon dioxide chip. Called the
鈥済ate electrode,鈥� this was the key to making the device into a transistor. When
there is no gate voltage, electrons would hop onto the buckyball from the gold
鈥渟ource鈥� electrode, and then hop off, onto the gold 鈥渄rain鈥� electrode. But a
gate voltage impedes this flow of charge by changing the electric field
arrangement. To overcome the gate, the voltage between the source and
drain鈥攃alled the bias鈥攎ust be raised beyond a certain threshold.

McEuen and Park figured that the way its current flow rose with increasing
bias would give them some clues about how the buckyball reacted to passing
electrons. When they powered up the device, the researchers found that the
transistor initially behaved as they had expected. As soon as the bias rose past
the threshold, the current began to climb swiftly. But it didn鈥檛 rise smoothly
as the bias increased. Instead it jumped in a series of even steps. Every 5
millivolts, the current through the transistor suddenly stepped up a notch.

These discrete steps clearly showed that the electrons and the buckyball were
interacting in some quantum-mechanical way, although it wasn鈥檛 clear just
how.

To get onto the buckyball, an electron had to have a specific amount of
energy: enough to hop across the gap, and then the right amount to fit somewhere
in the discrete electron energy levels of the ball. McEuen and Park first
suspected that the passing electrons in the current were using their energy to
kick the buckyball鈥檚 own electrons into higher levels, and then jumping into the
gap that was left. The rising steps in the current, they surmised, somehow
reflected the discrete quantum states of the electrons bound inside the
ball.

But this scenario didn鈥檛 quite work. The researchers knew that the energy
levels for the electrons in the buckyball were unevenly spaced. So passing
electrons from the current couldn鈥檛 use these levels to create the evenly spaced
steps the researchers observed.

Ringing bells

The researchers鈥� next guess was that the interaction between electron and
buckyball was in some way mechanical. The passing electrons, they reasoned,
might somehow make the buckyball deform and vibrate, ringing it like a bell.
This would also make the current rise in steps because, at the molecular scale,
quantum mechanics dictates that a molecule can only twist, contort and vibrate
in limited number of ways or 鈥渕odes鈥�, each of which has a fixed amount of
energy.

At low bias, the researchers reasoned, passing electrons would not have
enough energy to make the buckyball reach its first vibrational mode, and could
only hop into the lowest available energy level. But at a particular higher
value of the bias, the electron鈥檚 extra energy would be exactly that needed to
trigger the buckyball鈥檚 lowest energy vibration. Suddenly, there would be two
ways to cross the gap鈥攐ne triggering the vibration mode and one not. This
would step up the current. At still higher bias, the electrons could nudge the
buckyball into the next vibrational mode, giving three ways across the gap and
creating another current step.

But this explanation also hit a problem. The lowest energy vibration should
appear at 35 millivolts, but the first step happened at a measly 5 millivolts.
At this setback, McEuen and Park ran out of inspiration. 鈥淔or three to five
months we were struggling to understand what was happening,鈥� says Park.

And then they remembered what balls do best. 鈥淲e were stumped until we had
the idea that the ball was actually bouncing up and down on the surface,鈥� McEuen
says.

The insight came when they began thinking about how the ball was held in
place. The buckyball hovered roughly half a nanometre above the surface of the
silicon dioxide wafer, in the gap between the gold electrodes and, by chance,
closer to one or the other. A combination of forces squeezed the molecule. The
electrons in the buckyball repelled those in the nearby gold electrode, pushing
the ball away from the surface. At the same time, the electrical charges within
the molecule and the electrode both rearranged themselves in a way that created
an attraction known as the van der Waals force, which kept the molecule from
floating away entirely.

When an extra electron landed on the ball, the shifting charge pulled the ball closer to one electrode
(see Diagram). The increased attraction would last
only as long as the extra electron lingered. When it hopped off, the ball would
rebound, released of the extra pull. It would then bounce up and down, like a
compressed spring when it is suddenly released.

And this bouncing motion is, of course, under the sway of quantum mechanics.
That meant it too could only have discrete energy levels. Only when the
electrons had exactly the right amount of energy could they trigger a bounce,
which stepped up the current by opening a new path onto the ball. McEuen and
colleagues calculated the energy levels of the bouncing molecule. They would be
reached with every 5-millivolt increase in the bias voltage, just what was
required to explain their data (Nature, vol 407, p 57).

The buckyball didn鈥檛 move far, only a few thousandths of a nanometre. But it
rattled back and forth at a furious rate, more than a trillion times a second.
鈥淚t鈥檚 somewhat amazing,鈥� says Park, 鈥渢hat an individual molecule is kicked
around by a single electron.鈥�

Though unexpected, the bouncing is an opportunity, not a problem, McEuen
says. The rattling buckyball could find its way into a variety of
ultra-sensitive detectors.

For example, a photon could jostle the buckyball, but only if the frequency
of the photon matches the quantised frequency at which the buckyball bounces
back and forth. So you could use the transistor as an extremely discerning
radiation detector. Stick it onto an object and it might make an exquisitely
sensitive force detector. If anything knocks the object, the movement of the
ball within the transistor will produce a measurable change in the current. The
device should also be able to detect tiny amounts of nearby electric charge
because an electric field would pull on the molecule, affecting its springy
connection to the electrode and changing the frequency at which the ball
moves.

Perhaps most ambitiously, the transistor might make a uniquely sensitive
chemical detector, says Charles Lieber, a chemist at Harvard University. The
chemistry of the buckyball can be precisely controlled and altered, he says, so
you might be able to build a chemical receptor that would latch onto a
particular target. 鈥淚 could imagine making some sort of chemical sensor that may
be sensitive to the single molecule level,鈥� Lieber says.

But it won鈥檛 be easy getting the buckyball transistor and molecular devices
like it out of the lab and into your living room, Schwab warns. First,
researchers must overcome the daunting challenge of putting molecules too small
to be seen or grasped precisely in the right place. So far, the only solutions
to this problem are luck and large numbers. 鈥淚t鈥檚 really a shotgun approach
where you make a gazillion of these things and hope a few of them work,鈥� Schwab
says. 鈥淗ow are you going to put down a whole circuit of this stuff?鈥�

Overheating may also be a problem for the molecule-sized machines. Last year,
Schwab and Michael Roukes, a physicist at the California Institute of
Technology, showed that quantum mechanics sets a limit on how fast heat can move
out of a nanometre-sized device (Nature, vol 404, p 974). Heat energy
is lost through mechanical vibrations, but these devices are so small that they
can support only a handful of vibration modes. 鈥淗ow many modes couple the thing
to the outside world sets a speed limit to how fast you can get energy out,鈥�
Roukes says.

That in turn sets a limit on how fast a molecule-sized device can run. The
devices themselves are intrinsically very fast, Roukes says, but you might not
be able to run them flat out all the time. Indeed, with Park and McEuen鈥檚
bouncing, and Roukes and Schwab鈥檚 heat flow problem, it may be difficult to get
them working at all, says Leo Kouwenhoven, a physicist at the Delft University
of Technology. 鈥淭hey could start vibrating and melting before they even start to
work,鈥� he says.

Changing shape

Nanometre-sized devices may even tend to change shape all by themselves, says
Ellen Williams, a physicist at the University of Maryland in College Park.
Williams has shown that atoms spontaneously move along the edges of tiny silicon
structures, gradually reshaping the pieces. The problem could be even worse for
molecular-size parts, she says. 鈥淭he smaller the structure the worse it鈥檚 going
to be, and the warmer your operating temperature the worse it鈥檚 going to
产别.鈥�

Whether the buckyball transistor proves practical or not, it will certainly
help answer a fundamental scientific question, says Wilson Ho, a physicist at
the University of California, Irvine. Most chemical reactions occur when a few
electrons move and cause molecules to contort, move, and merge, he says, but no
one knows precisely how a tiny electron can push around objects millions of
times more massive. 鈥淭hat鈥檚 a central problem of chemistry,鈥� Ho says. 鈥淗ow does
the electron cause the motion of something much heavier?鈥�

McEuen and Park will continue exploring their bouncing transistor. They would
like to answer several more questions, such as how long the ball will keep
oscillating and what eventually brings it to a halt. McEuen wonders whether it
would be possible to relax the pull holding the ball to the surface, which would
let the molecule move farther and slower. The ball might then work like a tiny
ferry, carrying one electron on each trip across the gap. Different molecules
might sit between the electrodes, Park says, and could behave in other unusual
ways.

And it鈥檚 the lure of the unexpected that keeps the researchers coming back
for more, even if the work promises more long stretches of confusion and
guarantees no practical pay-off. 鈥淚鈥檓 a physicist,鈥� McEuen says. 鈥淚鈥檓 just in it
to see how things work.鈥� Park shares that sentiment. 鈥淲e don鈥檛 know which way
the ball will roll,鈥� he says, 鈥渂ut we are constantly learning new things about
nature.鈥� And in science, that鈥檚 the goal of the entire game.