Oct. 11, 1997 New Haven, CT -- Researchers at Yale have succeeded for the first time in measuring an electric current flowing through a single organic molecule sandwiched between metal electrodes. The feat could pave the way for a radically new generation of transistors so small that a beaker full would contain more transistors than exist in the world today, according to Yale electrical engineer Mark A. Reed, team leader.
The accomplishment, announced in the Oct. 10 issue of the journal Science, is a fundamental step toward creating computers and sensors that are smaller, faster and cheaper than today's silicon-based computers, Professor Reed said. The next step is to design computer chips whose wires are made of self-assembling strings of organic molecules that grow in a beaker, since the wires would be far too small to produce any other way. The organic wires would adhere to metal electrodes, a revolutionary strategy for fabricating electronic devices for which Professor Reed and Yale hold a joint patent.
"Scientists have gone from one transistor on a single chip to tens of millions. Now we are ready to go to billions of transistors on a single chip," said Professor Reed, a nanotechnology expert who works with electrical components only about one-billionth of a meter wide (one nanometer), or the width of about three atoms. He and his colleagues have been studying quantum mechanical effects that become crucial at such small scales.
He warned, however, not to expect to see organic circuits next year at a local electronics store. "Just as it took a decade from the discovery of the first transistor until the first integrated circuit was made, it could take a decade for us to learn to make useful devices out of quantum components made from organic compounds," said Professor Reed, chairman of the electrical engineering department at Yale. "But success would mean not just an evolutionary change but a revolutionary jump in computer technology."
To capture the historic measurement of current across a single organic molecule, the researchers made a mechanically controllable break junction by gluing a notched gold wire to a flexible substrate, then fracturing the wire to make an adjustable gap. Next, they sandwiched a single molecule of benzene (a hexagonal ring made up of six carbon and six hydrogen atoms) flanked by two sticky sulfur atoms between the two gold electrodes. The process required self-assembly of benzene molecules onto the electrodes.
Collaborators were graduate student Chong Wo-Zhou and former postdoctoral fellow C.J. Muller, both of Yale; and chemistry professor James M. Tour and graduate student Timothy P. Burgin of the University of South Carolina.
Overcoming Cost of Miniaturization
Organic transistors could replace today's silicon semiconductors, which are rapidly reaching a point where further miniaturization is too costly. "Thousands of silicon transistors can be produced now for less than a penny, but the dramatic decrease in cost per transistor that we've enjoyed over the last two decades will start to slow down soon," Professor Reed said. "Nanotechnology could become the solution, if we can surmount the hurdles."
Perhaps the greatest obstacle Professor Reed must overcome in order to fabricate useful quantum devices is to find better, faster ways to make large quantities. Quantum devices are made individually by a process called electron beam lithography, but making billions of transistors that way "would be like whittling all the books in the Library of Congress from a single block of wood or carving a bridge from a block of steel," he said.
The answer is to find materials that will assemble themselves into quantum components. "When you cook a sauce, billions of butter and flour components self-assemble," he said. "Our goal is to find organic chemicals that will combine to form a substrate of conducting molecules -- a goal we have been working toward for the last five years."
Among the imaginative uses scientists have suggested for quantum devices are "intelligent" computers -- computers that can learn and reason like humans. They would be built from billions of quantum transistors linked together with reconfigurable interconnections so that each transistor functions like a neuron in the brain. It might even be possible to blend electronics with biological systems by forcing damaged nerves to regenerate through porous quantum computer chips so the human brain can be connected to artificial limbs.
Quantum components also could be formed into materials capable of absorbing and emitting light at whatever wavelengths their designers specify and "could become the basis for semiconductor lasers more efficient and more precisely tuned than any now in existence," said Professor Reed, adding that laser diodes found in compact-disc players and sensitive microwave receivers in satellite dishes are relatively simple applications of quantum technology developed 20 years ago.
More like waves than particles
"When working with quantum components, we must deal with special laws of physics that can be ignored when working with larger components. For example, electrons behave more like waves than particles at quantum scales and can do unexpected things like tunnel through barriers," Professor Reed said. "Thanks to powerful scanning tunneling microscopes, we have observed behavior that tremendously surprised us and made us realize how little we understand quantum mechanics in extremely small electronic devices."
Understanding quantum mechanics is crucial in Professor Reed's specialty -- "low-dimension" electronics. His devices prevent electrons from moving in some or all of nature's three dimensions. For example, electrons confined in a plane made of an extremely thin film are free to move in only two dimensions, since they can't move perpendicular to the plane. Those confined in an extremely thin quantum wire are free to move in only one dimension while electrons trapped in a quantum "dot" can't move at all -- they are free to move in zero dimensions.
Professor Reed, who invented the first quantum dot in 1988 by carving a pillar in a semiconductor substrate using lithography, said the smallest conceivable transistors would be made up of quantum dots linked in a circuit, with each dot holding one electron.
"More than 35 years ago, the Nobel Prize-winning American physicist Richard Feynman first described the fascinating possibilities that would arise when we achieved the ability to manipulate matter at the atomic scale," said Professor Reed, whose research is funded by a four-year grant from the Defense Advanced Research Projects Agency (DARPA). "Feynman's talk was titled 'There's Plenty of Room at the Bottom' -- room for more growth, more breakthroughs. In electronics, we are finally reaching the bottom Feynman predicted."
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