| Researchers at Arizona State University have developed a relatively straightforward method for measuring the electrical resistance of single molecules. The advance, a technical achievement in terms of its precision and repeatability, promises to have a huge impact on the burgeoning field of molecular electronics. The researchers, Nongjian Tao, an ASU electrical engineering professor, and his student Bingqian Xu, said their method overcomes three thorny issues in the electrical resistance measurements of a single molecule. “What we have is a technique that guarantees one molecule is attached between two electrodes every time; we can identify how many molecules are present; and we can do thousands of measurements in a matter of minutes,” Tao said. Tao and Xu published their research in the Aug. 29 issue of Science magazine. The paper is titled “Measurement of single molecule resistance by repeated formation of molecular junctions.” Demands for faster electronic devices are pushing scientists to consider new types of electronic circuits as engineers reach the physical limits of circuits built of silicon. One promising alternative is molecular electronics, where individual molecules would be the basis for electronic circuits. Advances in molecular electronics have been steadily made in recent years, Tao said, but basic questions remain, one of which is what is the resistance of a single molecule? Making measurements on a molecular level presents several problems related to the size of the materials being tested, Tao said. “There are techniques that can handle some of these problems, but not all of them,” Tao said. “They allow you to determine the resistance of a single molecule, but some won’t tell you how many molecules are there (which could range from a few to thousands), some don’t always have a proper contact to the molecule to make the measurement and still others don’t have the statistics there. Ours does.” Tao and Xu make the measurement of single molecule resistance by repeatedly forming thousands of molecular junctions in which the molecules are directly connected to two electrodes. They performed these tests on various molecules with two ends that can strongly attach to gold electrodes. The ASU researchers create the molecular junctions by repeatedly moving a gold scanning tunneling microscope tip into and out of contact with a gold substrate in a solution containing the sample molecule to form a molecular junction. During the initial stage of pulling the tip electrode out of contact with the substrate electrode, the conductance decreases in a stepwise fashion with each step occurring at an integer multiple of conductance quantum (1 over 12,900 ohms). The conductance quantum steps signal that two electrodes are connected by merely a few gold atoms and molecules. Further pulling breaks the last few gold atoms and leaves the two electrodes connected by a few molecules. This later stage is associated with the appearance of a new series of conductance steps that are many orders of magnitude lower than the conductance quantum and vary from molecule to molecule. “Our idea is fairly straightforward,” Tao explained. “Because of its simplicity, it can be done repeatedly and provide a quality of data that has been missing in many other experiments.” While the measurements made are minute, their importance could be huge, he said. “Now you can start to test and understand a molecule before you build a device out of it,” Tao said. “This technique provides a basic test platform that is necessary towards the effort of building molecular electronic devices.” Arizona State has several other researchers who have made important contributions to solving basic electron transport problems in molecular electronics, including professors Stuart Lindsay and Otto Sankey in physics and astronomy; Devens Gust, Thomas Moore and Anna Moore in chemistry and biochemistry; and David Ferry in electrical engineering. |