Organic Nanoelectronics
A silver-tipped piece of DNA, whose diameter is only half the size of the smallest technical nanoconductor, can conduct electricity. This could be the first step towards even smaller microcircuits. Researchers at the Technion-Israel Institute of Technology used strands of DNA to join tiny particles of silver into a conductor 1,000 times thinner than a human hair (Nature 19 February 1998).
The nanocircuits of the future will consist of wires, transistors and other components whose dimensions are measured in billionths of a meter (a nanometer or a billionth of a meter is about the length of five carbon atoms in a row). By packing many more components closer together, scientists could produce computer chips that are much faster and far more sophisticated than today's.
In the past, researchers have used DNA to arrange tiny nanoparticles of semiconductors and other electronic materials into crystal-like lattices and other regular structures. But to date, no one had made a working electronic component.
"Our wire actually conducts a current. This is the first demonstration of self-assembly of a working electronic component," said physicist Uri Sivan, who led the research along with physicist Erez Braun and chemist Yoav Eichen.
Wires are the foundation of any circuit, connecting its components both to each other and to the outside world. The Technion team constructed their prototype nanowire between two gold electrodes separated by a tiny gap of 12 microns (about a tenth the width of a human hair).
The scientists used short DNA sequences, so-called oligonucleotides, to build a self-assembling scaffold, and long stretches of DNA to control the construction of the silver nanowire. First, they placed two gold electrodes on a small glass plate and coated both with an oligonucleotide, made up of 12 building blocks that determined which other pieces of DNA the oligonucleotide could attach to. This gave the short stretches of DNA a unique chemical identity that can be used to target them in further reactions.
DNA itself does not combine with gold. The researchers therefore used auxiliary molecules that acted as a kind of "glue" between the oligonucleotides and the electrodes with the help of a disulfide group.
A longer piece of DNA served as the link between the two electrodes, the two ends of which fitted exactly to the oligonucleotides on the gold contacts. The specificity of the binding ensures that the DNA bridge also finds the right contact points.
DNA itself is a non-conductor and cannot function as a wire. Therefore, the scientists used a chemical process to attach the silver along the framework. First they immersed the glass plate in a solution of positive silver ions. These attached themselves to the negatively charged DNA and were then reduced to electrically neutral silver atoms. Eventually, the silver grains were used to catalyze the growth of metallic silver, creating a pathway for electric current-a wire.
Technion wire is 100 nanometers wide: a nearly three-fold improvement over existing computer chip-making technology. The conventional method using photolithography reaches its limit in terms of miniaturization at around 250 nanometers. In principle, according to the researchers, it would be possible to produce wires that are 100 times thinner.
In electrical testing of the nanowire, the scientists discovered another potentially useful property. Under certain conditions, a type of diode is formed when current flows through the wire for the first time. The conductor "remembers" in which direction the current flowed through it. The polarity of the diode, i.e. in which direction it allows current to pass, represents stored information. Team members speculate that this fact may be used to produce computer memory.
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