June 18, 1998 A report to be published in the June 12 issue of the journal Science moves researchers one step closer to a practical application for electron wave effects in extremely small-scale circuits.
In the paper, a team of scientists from the Georgia Institute of Technology reports observing ballistic conductance -- a phenomenon in which electrons pass through a conductor without heating it -- at room temperature in multi-walled carbon nanotubes up to five microns long. (A micron is a millionth of a meter).
Structures of that size operating under those conditions could one day be useful for fabricating ever-smaller electronic devices. Their ability to conduct relatively large currents without harmful resistance heating would allow use of the very small conductors.
"This is the first time that ballistic conductance has been seen at any temperature in a three dimensional system of this scale," said Dr. Walt de Heer, a professor in Georgia Tech's School of Physics. "There would be interest in this for ultra-small electronics, because it shows that you can constrain current flows to narrow areas without heating up the electronics. It also introduces a new stage of electronics in which the wave nature of electrons becomes important."
In a simple experimental design using the positioning equipment of an atomic force microscope, the researchers found that the electrical resistance of the multi walled carbon nanotubes remained constant -- regardless of their length or width. This quantum conductance is not seen in larger structures.
"In classical physics, the resistance of a metal bar is proportional to its length," said Dr. Z.L. Wang, a professor in Georgia Tech's School of Materials Science and Engineering. "If you make it twice as long, you will have twice as much resistance. But for these nanotubes, it makes no difference whether they are long or short because the resistance is independent of the length or the diameter."
That's possible, explained de Heer, because the electrons act more like waves than particles in structures whose size approaches that of the wavelength of electrons. "The electrons are passing through these nanotubes as if they were light waves passing through an optical waveguide," he said. "It's more like optics than electronics."
In normal wires, the electrical energy they carry dissipates in the conductor, but in the nanotubes, energy dissipates only in the leads used to connect the tubes. Such effects had previously been seen only in structures a thousand times smaller, and finding them in the comparatively large nanotubes was "quite surprising," de Heer said.
"Until now, these effects were considered to be exotic and seen only under very special conditions," he said. "Now we are seeing them abundantly and easily at room temperature with very simple devices."
The absence of heating allows extremely large current densities to flow through the nanotubes. Wang and de Heer measured current densities greater than ten million amperes per square centimeter. Normal resistance heating would have generated temperatures of 20,000 K in the nanotubes, well beyond their combustion temperature of 700 K.
Though they these effects were measured only in nanotubes of less than five microns, such current densities are far greater than could be handled by any other conductor, Wang noted. At lengths of more than five microns, however, de Heer believes electron scattering may defeat the ballistic conductance effect.
"We can only guarantee that we can carry that kind of current over five microns," he said. "We don't know what will happen if you try to conduct for longer distances. This will certainly not be a way to transport current over large distances."
In their laboratory, de Heer, Wang and collaborators Stefan Frank and Philippe Poncharal attached a tiny electrode to a bundle of nanotubes that had a single long tube protruding from one end. They mounted the bundle in place of the probe normally used in an atomic force microscope and connected a battery to the electrode.
They then used the microscope controls to raise and lower the single protruding nanotube into and out of a pool of mercury that served to complete the circuit back to the battery. The resistance they measured as the nanotube was raised and lowered into the mercury remained constant, changing only when a shorter tube protruding from the bundle -- which resembles a handful of straw -- made contact with the liquid metal.
The researchers measured the resistance of 20 nanotubes of different lengths and diameters through as many as 1,000 cycles that consisted of dipping them in and out of mercury and two other molten metals -- gallium and Cerrolow-117. The tubes averaged 15 nanometers wide and four microns long, but ranged from one to five microns in length, with diameters from 1.4 nanometers to 50 nanometers. The quantum of resistance remained 12.9 kiliohms.
Despite the importance of the discovery, de Heer cautions that electronic devices using nanotube conductors are perhaps decades away. One fundamental issue is that carbon materials are incompatible with the silicon that is the basis of current integrated circuits. Solving that challenge will require a revolution in electronic design.
"It would be like introducing silicon transistors during the age of vacuum tubes," he said. "You couldn't combine the two because they are from different worlds. This just opens the door, it doesn't tell you how to build a better world. This should be seen as the proof of principle showing that we can do ballistic conductance at room temperature."
The researchers hope to follow up their work with measurements of other predicted device properties of the nanotubes. The research is sponsored by the U.S. Army Research Office and the Georgia Tech Foundation.
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