Feb. 19, 1998 NEW YORK, N.Y., and HAIFA, Israel, February 13, 1998 -- Researchers at the Technion-Israel Institute of Technology have used strands of DNA, the biological molecule that makes up genes, to assemble tiny particles of silver into a conductive wire 1,000 times thinner than a human hair. The research, reported in the February 19, 1998 issue of Nature, is an important step toward the next quantum leap in electronics miniaturization. Making that leap is the ultimate goal of researchers in the emerging field of nanoelectronics.
The nanocircuits of the future will consist of wires, transistors and other components with dimensions measured in billionths of a meter. (One nanometer or a billionth of a meter, is about the length of five carbon atoms laid end to end.) By packing many more components closer together, scientists could produce computer chips that are much faster than today's, and far more sophisticated.
Scientists have already used DNA to assemble minute nanoparticles of semiconductors and other electronic material into crystal-like lattices and other orderly structures. But nobody until now has made a working electronic component.
"Our wire actually passes a current. This is the first demonstration of self assembly of any functioning electronic component," said physicist Uri Sivan, who conducted the research with Erez Braun, another physicist, and chemist Yoav Eichen.
Wires are the foundation of any circuit because they link circuit components to each other and to the outside world. The Technion team constructed their prototype nanowire between two gold electrodes separated by a narrow gap of 12 microns, about one-tenth the width of a human hair. They synthesized strands of DNA that linked themselves together to form a kind of construction scaffolding between the electrodes. Since DNA by itself does not pass current, they finished the wire off by attaching grains of silver along the scaffold by a process similar to photographic developing, forming a silver wire connecting the electrodes.
The Technion wire is 100 nanometers wide, an almost threefold improvement over the existing technology used to make computer chips. The standard process, using photolithography, reaches the limits of miniaturization at about 250 nanometers. In principle we could have wires that are 100 times narrower than that, according to the researchers.
In electrical tests of the nanowire, the Technion team discovered a potentially useful property. Under certain conditions, sending current through the wire turns it into a kind of one-way switch called a diode. The wire remembers which way you ran current through it the first time. The polarity of the diodeDwhich direction it passes a currentDrepresents a piece of stored information. Perhaps we can use that to make computer memories, the team members speculate.
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The Technion-Israel Institute of Technology is the country's premier scientific and technological center for applied research and education. It commands a worldwide reputation for its pioneering work in communications, electronics, computer science, biotechnology, water-resource management, materials engineering, aerospace and medicine, among others. The majority of Israel's engineers are Technion graduates, as are most of the founders and managers of its high-tech industries. The university's 11,000 students and 700 faculty study and work in the Technion's 19 faculties and 30 research centers and institutes in Haifa.
The American Technion Society is the university's support organization in the United States. Based in New York City, it is the leading American organization supporting higher education in Israel. Technion societies are located in 24 countries around the world.
Technical Backgrounder: How to Build a Nanowire
Technion-Israel Institute of Technology scientists Erez Braun, Yoav Eichen and Uri Sivan used a bottom up approach to building their wire: They assembled small particles of matter into a wire, like masons cementing bricks into a wall. To do it, they harnessed the remarkable information-carrying properties of DNA, deoxyribonucleic acid.
DNA, the stuff of genes, stores the information cells need to live and reproduce. But to nanoengineers, DNA is the stuff of dreams because it has the power to bind molecules or tiny particles of matter to specific addresses spelled out in its chemical code.
The linear DNA molecule resembles a twisted ladder. Each rung of the long, stringy molecule represents a pair of chemical subunits called bases. There are only four kinds of bases in DNA: adenine, thymine, guanine and cytosine. The bases are complementary, meaning they bind to each other with lock-and-key specificity: Adenine only pairs up with thymine, and guanine with cytosine. Unzipping the DNA ladder leaves two complementary strands which will zip right back together in the same way under the right conditions.
The Technion scientists use short sequences of DNA called oligonucleotides and long DNA ladders to create a self-assembling scaffold to guide the construction of the silver nanowire. First they deposit two gold electrodes on a small glass plate and coat each with one of two types of 12-base oligonucleotides. Each type has a different chemical code, a unique chemical identity.
DNA by itself won't bind to gold. The scientists use a chemical subunit called a disulfide group to anchor the oligonucleotides to the gold electrodes. The disulfide groups bind strongly to gold, acting as a kind of glue between the DNA strands and the electrodes.
Then the scientists use lengths of double-stranded DNA to span the distance between the electrodes. A 12-base single-stranded tail hangs off each end of the spans. There are two kinds of tails, each complementary to the other oligonucleotide on the electrode. That enables the spans to straddle the distance between the electrodes, forming a scaffold for depositing silver.
To bridge the gap between the electrodes, the scientists deposit a droplet of water laced with the DNA spans across the two electrodes. Then they use a tiny glass tube to create suction on one end of the droplet. This induces a river-like flow that stretches the DNA spans across the gap, allowing the tails to bind to the complementary oligonucleotides on the electrodes.
DNA by itself is an insulator and won't work as a wire. So the scientists use a chemical process to deposit silver along the scaffold. The first step is immersing the glass plate in a solution of silver ions, made up of silver atoms with a net positive charge. The ions arrange themselves along the DNA, attracted there by the negatively charged DNA molecule. Then they use a chemical called a reducing agent to convert the silver ions into stable, electrically neutral grains of silver metal. The grains of silver are then used to catalyze the growth of metal silver to form a conducting pathway for electricity -- a wire.
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