UPDATE
'Small is Different'
ELECTRICAL, MECHANICAL AND other properties of microscopic wires change significantly as their width narrows to the nanoscale -- less than ten atoms -- reports a paper published in the March 24, 1995, issue of the journal Science.
The paper describes results of experimental and supercomputer-based simulation research done by scientists at the Georgia Institute of Technology and the Universidad Autonoma de Madrid in Spain.
Simulation above shows the formation of a "nanowire" after the probe of a scanning tunneling microscope is pressed into a gold surface and then pulled away.
Information about materials systems of greatly-reduced dimensions is relevant to the production of new types of electronic devices, because the need for miniaturization is driving engineers to design smaller and smaller components. The studies also reveal new information about the fundamental behavior of materials in the "nanorealm."
These combined experimental and theoretical investigations of electronic transport and mechanical elongation in ultra-thin metallic wires may be the first to measure in three-dimensional wires at room temperature a localization phenomena previously seen only in one-dimensional "whiskers" at cryogenic temperatures.
"Small is different," says Dr. Uzi Landman, director of Georgia Tech's Center for Computational Materials Science. "In the desire to reduce the size of electronic devices, there are certain effects pertaining to size and dimensions that must be considered. If you go below a certain limit of size, you should beware because the behavior of the system may not be what you would expect on the macroscopic level."
The researchers found that under certain conditions, the ability of the nanowires to conduct electricity declines to the point that they resemble insulators. Conductance of such atomic-scale gold wires depends on their length, lateral dimensions, the state of atomic order and disorder and the elongation mechanism of the wires.
To study the phenomena, researchers at the Universidad Autonoma de Madrid created tiny nanowires by bringing the tip of a scanning tunneling microscope (STM) near a gold surface and applying an electrical voltage, or by simply pushing the tip into the surface. As predicted by the molecular dynamics simluations, the retracting tip carried gold atoms with it when pulled back from the surface, producing tiny wires. Depending on the preparation conditions, the nanowires extended between 50 and 400 angstroms in length. (An angstrom is one ten-billionth of a meter, approximately the diameter of a hydrogen atom.)
Using very precise measurements for electrical conductance, the researchers studied what happened as they slowly pulled the tip away from the surface, elongating and eventually breaking the nanowire. The experimental measurements were correlated with predictions obtained from molecular dynamics simulations performed by the Georgia Tech research group.
"The simulated elongation process reveals an interesting mechanism exhibiting periodic oscillations of the pulling force," Landman explained. "These oscillations reflect the atomic-scale mechanism of elongation, which proceeds layer-by-layer as stress accumulates up to a certain limiting value. After that point, relief of the stress occurs through the formation of a new layer and a corresponding reduction in the diameter of the wire."
The elongation process is also reflected in the conductance of the nanowires.
"The current through the wire varies in a stepwise manner as a function of the tip displacement away from the substrate, which translates into elongation of the wire," he added. "We saw that the jumps in the conductance occurred with a periodicity of about the interlayer spacing in the wire. Moreover, the conductance changed in each stage in steps whose height was one or two times a unit known as the conductance quantum, a measure of electrical resistance in very small conductors."
The conductance measurements revealed a repeating pattern in which the conductance exhibited dips corresponding to enhanced degrees of disorder in the wires during elongation. The increases in the conductance subsequent to each of these dips are correlated with a restoration of a higher degree of order in the elongated wire. Such a repeating pattern manifested itself as the thickness of the wire dropped below five to ten atoms.
In wires between 50 and 400 angstroms in length, the researchers observed nonlinear dependence of the electrical resistance as a function of the voltage across the wire. As the length of the narrow wires increased, the conductance decreased and ultimately the wires behaved more like insulators than metallic conductors.
"While at a fixed point of elongation, wires that are short (less than 50 angstroms) showed linear and regular Ohmic characteristics, but the nature of the resistance trace changed as the wire became longer and more narrow, going through a stage in which the wire looked very much like a semiconductor," Landman adds. "Eventually, the very long wires -- up to 150 or 200 Angstroms -- began to appear insulating."
Based on their observations, the group developed an explicit formula for the nonlinear behavior of the resistance of the long wires and related it to a phenomenon known as the Anderson localization. Landman believes this study was the first to observe the Anderson localization in a three-dimensional system at room temperature.
Besides their value to fundamental physicists, the results should also be of interest to the designers of future microelectronics equipment.
"If we are to reduce the size of microelectronics systems, connecting wires between elements of such devices must be reduced in size and therefore such quantization patterns of conductance could start to appear," Landman concludes. "These phenomena are expected to occur when the physical dimensions of the systems approach that of the electronic wavelength."
The research team producing the Science paper included J.I. Pascual, J. Mendez, J. Gomez-Herrero and A.M. Baro from the Departamento de Fisica de la Materia Condensada and N. Garcia from the Fisica de Sistemas Pequenos at the Universidad Autonoma de Madrid, and W.D. Luedtke, E.N. Bogachek and H.-P. Cheng from the School of Physics and the Center for Computational Materials Science at the Georgia Institute of Technology.
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