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.