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Tools // NIST Researchers Holding Steady In An Atomic-scale Tug-of-war
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 instru- ment 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 brea- king 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
research team at the National Institute of Stan- dards and Technology (NIST) has built an ultra-
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 interfe- rometer 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 bet- ween two gold atoms in a way traceable to natio- nal measurement standards. And because any two gold atoms are essentially identical, that would give other researchers a direct method of calibrating their equipment.
10-05/06 :: May/June 2010