|
The STM uses quantum tunneling, or the ability of electrons to "tunnel"
across a barrier, to detect changes in the distance between a
needlelike probe and a conducting surface. Researchers apply a tiny
voltage to the sample and move the probe - a simple platinum-iridium
wire snipped to end in a point just one atom wide - just a few
angstroms (10ths of a nanometer) over the sample's surface. By
measuring changes in current as electrons tunnel between the sample
and the probe, they can reconstruct a map of the surface topology down
to the atomic level.
Since its invention in the 1980s, the STM has enabled major
discoveries in fields from semiconductor technology to
nano-electronics.
But while current can change in a nanosecond, measurements with the
STM are painfully slow. And the limiting factor is not in the signal
itself - it's in the basic electronics involved in analyzing it. A
theoretical STM could collect data as fast as electrons can tunnel -
at a rate of one gigahertz, or 1 billion cycles per second of
bandwidth. But a typical STM is slowed down by the capacitance, or
energy storage, in the cables that make up its readout circuitry -
to about one kilohertz (1,000 cycles per second) or less.
Researchers have tried a variety of complex remedies. But in the end,
said Schwab, an associate professor of physics at Cornell, the
solution was surprisingly simple. By adding an external source of
radio frequency (RF) waves and sending a wave into the STM through a
simple network, the researchers showed that it's possible to detect
the resistance at the tunneling junction - and hence the distance
between the probe and sample surface - based on the characteristics of
the wave that reflects back to the source.
The technique, called reflectometry, uses the standard cables as paths
for high-frequency waves, which aren't slowed down by the cables'
capacitance.
"There are six orders of magnitude between the fundamental limit in
frequency and where people are operating," said Schwab. With the RF
adaptation, speeds increase by a factor of between 100 and 1,000. "Our
hope is that we can produce more or less video images, as opposed to a
scan that takes forever."
The setup also offers potential for atomic resolution thermometry -
precise measurements of temperature at any particular atom on a
surface - and for motion detection so sensitive it could measure
movement of a distance 30,000 times smaller than the size of an atom.
"This STM will be used for a lot of good physics experiments," said
Schwab. "Once you open up this new parameter, all this bandwidth,
people will figure out ways to use it. I firmly believe 10 years from
now there will be a lot of RF-STMs around, and people will do all
kinds of great experiments with them."
The research was supported by the National Science Foundation.
|
