March 23, 1999
Simple method will help test theories about nanotubesWEST LAFAYETTE, Ind. -- Scientists at Purdue University are the first to develop a simple method for accurately measuring the electrical properties of a single carbon nanotube, a step that is essential if the tiny structures are to one day realize their promise in new generations of electronics and computers.
The technique, presented March 23 during a meeting of the American Physical Society in Atlanta, already has shown that nanotubes may conduct electricity without heating up -- a result that confirms their potential value in making extremely small circuits and computer chips.
Carbon nanotubes, elongated molecules that are as small as a few atoms in diameter, were discovered less than 10 years ago. In theory, they might be used to build devices to replace much larger silicon-based devices in electronic circuits.
A major obstacle in producing ever-smaller circuits and electronic components is the self-destructive heat they generate. The smaller the electronic circuit, the more susceptible it is to heat damage. Materials that conduct electrons with little resistance -- producing little heat -- are needed for the job, says Ronald Reifenberger, professor of physics at Purdue.
Measurements made by the Purdue team suggest that nanotubes do, in fact, possess this low-heat characteristic.
Preliminary data from the Purdue research also confirms a theoretical characteristic of nanotubes -- that they conduct electrons by "ballistic transport" -- meaning the electrons collide with few impurities as they flow from one end of the nanotube to the other. Lack of collisions means low resistance and low heat production.
"It is difficult to test theories about nanotubes because of their small size," Reifenberger says. "The best way to reliably analyze the electrical characteristics of a nanotube is to connect it on both ends with metallic contacts and then run a current through it. But, until now, scientists have been unable to manipulate an individual nanotube and make reliable contacts."
The Purdue team devised a method using an atomic force microscope -- an instrument designed to make three-dimensional images and surface measurements with extremely high resolution -- to precisely position a nanotube so that both ends are bridging a circuit.
They then coated the ends with gold, using a process known as shadow masking, creating strong contacts at both ends. Having solid contacts at both ends ensures that passing a current through the nanotube will yield accurate data about its electrical characteristics, such as resistance, Reifenberger says.
"Now we can measure standard things that electrical engineers are interested in," he says, noting that the method is especially attractive to researchers because, unlike other techniques, it does not require elaborate facilities.
"The thing that's appealing is the simplicity of the idea," he says. "A small college can do these measurements now, because all you need is a steady hand. You can make a half dozen samples in a week and get data on them."
Working with Reifenberger on the research project were engineering Professors Ronald Andres and Supriyo Datta, and graduate students Pedro de Pablo, Elton Graugnard and Brian Walsh.
The research was funded by National Science Foundation.
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E. Graugnard, B. Walsh (Purdue University), P.J. de Pablo (Universidad Autonoma de Madrid), R.P. Andres, S. Datta, R. Reifenberger (Purdue University)
A simple method of making reliable electrical contact to multi-walled carbon nanotubes is described. With these contacts, current in the mA range can be routinely passed through individual multi-walled nanotubes without adverse consequences, thus allowing their resistance to be measured using a common multimeter. The contacts are robust enough to withstand temperature excursions between room temperature and 77 K. Data from the different multi-walled nanotubes are found to exhibit three different I(V) characteristics which can be classified as: i) highly ohmic, ii) diode-like, or iii) non-linear. The I(V) data will be discussed and analyzed.