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A Credit: Tavazza, NIST |
How hard do you have
to pull on a single atom of—let’s say—gold to detach it from the end of a
chain of like atoms?* It’s a measure of the astonishing progress in
nanotechnology that questions that once would have interested only
physicists or chemists are now being asked by engineers. To help with
the answers, a research team at the National Institute of Standards and
Technology (NIST) has built an ultra-stable instrument for tugging on
chains of atoms, an instrument that can maneuver and hold the position
of an atomic probe to within 5 picometers, or 0.000 000 000 5
centimeters.**
The basic experiment uses a NIST-designed
instrument inspired by the scanning tunneling microscope (STM). The NIST
instrument uses as a probe a fine, pure gold wire drawn out to a sharp
tip. The probe is touched to a flat gold surface, causing the tip and
surface atoms to bond, and gradually pulled away until a single-atom
chain (see figure) is formed and then breaks. The trick is to do this
with such exquisite positional control that you can tell when the last
two atoms are about to separate, and hold everything steady; you can at
that point measure the stiffness and electrical conductance of the
single-atom chain, before breaking it to measure its strength.
The NIST team used a combination of clever
design and obsessive attention to sources of error to achieve results
that otherwise would require heroic efforts at vibration isolation,
according to engineer Jon Pratt. A fiber-optic system mounted just next
to the probe uses the same gold surface touched by the probe as one
mirror in a classic optical interferometer capable of detecting changes
in movement far smaller than the wavelength of light. The signal from
the interferometer is used to control the gap between surface and probe.
Simultaneously, a tiny electric current flowing between the surface and
probe is measured to determine when the junction has narrowed to the
last two atoms in contact. Because there are so few atoms involved,
electronics can register, with single-atom sensitivity, the distinct
jumps in conductivity as the junction between probe and surface narrows.
The new instrument can be paired with a
parallel research effort at NIST to create an accurate atomic-scale
force sensor—for example, a microscopic diving-board-like cantilever
whose stiffness has been calibrated on NIST’s Electrostatic Force
Balance. Physicist Douglas Smith says the combination should make
possible the direct measurement of force between two gold atoms in a way
traceable to national measurement standards. And because any two gold
atoms are essentially identical, that would give other researchers a
direct method of calibrating their equipment. “We’re after something
that people that do this kind of measurement could use as a benchmark to
calibrate their instruments without having to go to all the trouble we
do, " Smith says. "What if the experiment you’re performing calibrates itself
because the measurement you’re making has intrinsic values? You can
make an electrical measurement that’s fairly easy and by observing
conductance you can tell when you’ve gotten to this single-atom chain.
Then you can make your mechanical measurements knowing what those forces
should be and recalibrate your instrument accordingly.”
In addition to its application to nanoscale
mechanics, say the NIST team, their system’s long-term stability at the
picometer scale has promise for studying the movement of electrons in
one-dimensional systems and single-molecule spectroscopy.
* The answer, calculated from atomic models,
should be something under 2 nanonewtons, or less than 0.000 000 007
ounces of force.
** D.T. Smith, J.R. Pratt, F. Tavazza, L.E.
Levine and A.M. Chaka. An ultra-stable platform for the study of
single-atom chains. J. Appl. Phys., in press, March, 2010.
Media Contact: Michael Baum, michael.baum @nist.gov, (301)
975-2763
