New Haven, CT -- Researchers at Yale have succeeded for the first timein measuring an electric current flowing through a single organic moleculesandwiched between metal electrodes. The feat could pave the way for aradically new generation of transistors so small that a beaker full wouldcontain more transistors than exist in the world today, according to Yaleelectrical engineer Mark A. Reed, team leader.
The accomplishment, announced in the Oct. 10 issue of the journalScience, is a fundamental step toward creating computers and sensors that aresmaller, faster and cheaper than today's silicon-based computers, Professor Reedsaid. The next step is to design computer chips whose wires are made ofself-assembling strings of organic molecules that grow in a beaker, since thewires would be far too small to produce any other way. The organic wires wouldadhere to metal electrodes, a revolutionary strategy for fabricating electronicdevices for which Professor Reed and Yale hold a joint patent.
"Scientists have gone from one transistor on a single chip to tens ofmillions. Now we are ready to go to billions of transistors on a single chip,"said Professor Reed, a nanotechnology expert who works with electricalcomponents only about one-billionth of a meter wide (one nanometer), or thewidth of about three atoms. He and his colleagues have been studying quantummechanical effects that become crucial at such small scales.
He warned, however, not to expect to see organic circuits next year at alocal electronics store. "Just as it took a decade from the discovery of thefirst transistor until the first integrated circuit was made, it could take adecade for us to learn to make useful devices out of quantum components madefrom organic compounds," said Professor Reed, chairman of the electricalengineering department at Yale. "But success would mean not just anevolutionary change but a revolutionary jump in computer technology."
To capture the historic measurement of current across a single organicmolecule, the researchers made a mechanically controllable break junction bygluing a notched gold wire to a flexible substrate, then fracturing the wire tomake an adjustable gap. Next, they sandwiched a single molecule of benzene (ahexagonal ring made up of six carbon and six hydrogen atoms) flanked by twosticky sulfur atoms between the two gold electrodes. The process requiredself-assembly of benzene molecules onto the electrodes.
Collaborators were graduate student Chong Wo-Zhou and formerpostdoctoral 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, whichare rapidly reaching a point where further miniaturization is too costly. "Thousands of silicon transistors can be produced now for less than a penny, butthe dramatic decrease in cost per transistor that we've enjoyed over the lasttwo decades will start to slow down soon," Professor Reed said. "Nanotechnologycould become the solution, if we can surmount the hurdles."
Perhaps the greatest obstacle Professor Reed must overcome in order tofabricate useful quantum devices is to find better, faster ways to make largequantities. Quantum devices are made individually by a process called electronbeam lithography, but making billions of transistors that way "would be likewhittling all the books in the Library of Congress from a single block of woodor carving a bridge from a block of steel," he said.
The answer is to find materials that will assemble themselves intoquantum components. "When you cook a sauce, billions of butter and flourcomponents self-assemble," he said. "Our goal is to find organic chemicals thatwill combine to form a substrate of conducting molecules -- a goal we have beenworking toward for the last five years."
Among the imaginative uses scientists have suggested for quantum devicesare "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 transistorfunctions like a neuron in the brain. It might even be possible to blendelectronics with biological systems by forcing damaged nerves to regeneratethrough porous quantum computer chips so the human brain can be connected toartificial limbs.
Quantum components also could be formed into materials capable ofabsorbing and emitting light at whatever wavelengths their designers specify and"could become the basis for semiconductor lasers more efficient and moreprecisely tuned than any now in existence," said Professor Reed, adding thatlaser diodes found in compact-disc players and sensitive microwave receivers insatellite dishes are relatively simple applications of quantum technologydeveloped 20 years ago.
More like waves than particles
"When working with quantum components, we must deal with special laws ofphysics that can be ignored when working with larger components. For example,electrons behave more like waves than particles at quantum scales and can dounexpected things like tunnel through barriers," Professor Reed said. "Thanksto powerful scanning tunneling microscopes, we have observed behavior thattremendously surprised us and made us realize how little we understand quantummechanics in extremely small electronic devices."
Understanding quantum mechanics is crucial in Professor Reed's specialty-- "low-dimension" electronics. His devices prevent electrons from moving insome or all of nature's three dimensions. For example, electrons confined in aplane 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 anextremely thin quantum wire are free to move in only one dimension whileelectrons trapped in a quantum "dot" can't move at all -- they are free to movein zero dimensions.
Professor Reed, who invented the first quantum dot in 1988 by carving apillar in a semiconductor substrate using lithography, said the smallestconceivable 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 physicistRichard 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 theDefense Advanced Research Projects Agency (DARPA). "Feynman's talk was titled'There's Plenty of Room at the Bottom' -- room for more growth, morebreakthroughs. In electronics, we are finally reaching the bottom Feynmanpredicted."
The above story is based on materials provided by Yale University--Office of Public Affairs. Note: Materials may be edited for content and length.
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