Ballistic conductance observed in carbon nanotubes

09/01/1998

Ballistic conductance observed in carbon nanotubes

A team of scientists from Georgia Institute of Technology has observed ballistic conductance - a phenomenon in which electrons pass through a conductor without heating it. The observation was made at room temperature in multiwalled carbon nanotubes up to 5 ?m long. Walter de Heer, a professor in Georgia Tech`s School of Physics, said, "This is the first time that ballistic conductance has been seen at any temperature in a three-dimensional system of this scale."

While far from existence today, one of the ultimate applications of this fundamental research into electron wave effects is extremely small electronic devices; conducting relatively large currents without resistance heating would allow use of the very small conductors. Basically it introduces a new stage of electronics in which the wave nature of electrons becomes important.

The phenomenon behind the observation is quantum conductance where electron resistance is independent of conductor length or diameter. This occurs because 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," explained de Heer. "It`s more like optics than electronics."

Briefly described, the experiment involved attaching a tiny electrode to a bundle of nanotubes that had a single long tube protruding from one end. With a battery connected to the electrode, the bundle acted as a probe in an atomic force microscope. The researchers used the microscope`s controls to raise and lower the single protruding nanotube into and out of a pool of mercury (and alternately molten gallium and Cerrolow-117) to complete the circuit.

With the nanotubes, energy dissipates only at 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 is considered "quite surprising," de Heer said. "Until now, these effects were considered to be exotic and seen only under very special conditions. Now we are seeing them abundantly and easily at room temperature with very simple devices."

So far de Heer`s group of researchers, including professor Z. L. Wang, Stefan Frank, and Philippe Poncharal, have measured current densities greater than 10 million amperes/cm2. Normal resistance heating would have generated temperatures of 20,000 K in the nanotubes, well beyond their combustion temperature of 700 K. The caveat is that these effects were measured only in nanotubes of less than 5 ?m; at greater lengths de Heer believes electron scattering may defeat the ballistic conductance effect. "This will certainly not be a way to transport current over large distances," he noted.

The tubes averaged 15 nm wide and 4 ?m long, but ranged from 1-5 ?m in length, with diameters from 1.4-50 nm. 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 silicon, the basis of current ICs. 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." - P.B.