Director-General, Nanomaterials Laboratory, National Institute of Materials Science (NIMS)
Professor, Graduate School of Engineering, Osaka University
Pioneering a new field of technology
— “Atomic Electronics” based on the invention of “Atomic Switch” —
Nanoscale structures with unique properties are a treasure trove for developing novel nanoelectronic devices of the next generation. However, understanding the properties of such nanoscale structures is harder than creating the nanostructures. Prof. Aono, who has been creating various nanostructures of interest by manipulating atoms and molecules using a probe tip of a scanning tunneling microscope (STM), has expanded his research field to the property measurement of nanostructures.
In 1989, Prof. Aono launched the "Atomcraft" Project under the sponsorship of the ERATO Program of the Research Development Corporation of Japan (JRDC) (currently, Japan Science and Technology Agency (JST)) in 1989. In this project, he demonstrated through many examples that it was possible not only to extract single atoms from any designated positions on a sample surface but also to supply single atoms at any designated positions on the sample surface even at room temperature by using the probe tip of a scanning tunneling microscope. The method of manipulating atoms at room temperature developed in the project is the basis for today's bottom-up nanoscale structure fabrication.
“A nanowire made of a chain of atoms, for example,” Prof. Aono says, “cannot be used for nanoscale electronic circuits unless we know how electrically conductive they are.” He began to develop a double-probe STM, in which two probe tips could be operated independently. The two probe tips can be positioned at any two different points on a given nanostructure for electrical conductivity measurements. Although the development work took longer than expected, the first STM with two probe tips or the “nanotester” was finally completed in 1998. The “nanotester” enabled him to make a direct measurement of the electrical conductivity of a one-dimensional electrical conductor with a thickness of about 1 nm for the first time in the world. “We succeeded to measure the electrical conductivity of a nanowire of metallic ErSi2 as thin as about 1 nm by placing one probe tip of an STM at a designated position on the nanowire and changing the position of the other probe tip on the nanowire over a length of several hundred nanometers,” Prof. Aono says. “Nobody would have ever imagined this kind of sophisticated measurement before.” Although he stresses that the development of a multiple probe STM is still in the early stage, his group has already developed one with four probe tips. The group has also developed an atomic force microscope (AFM) with four probe tips designed to measure electrical conductivities of nanostructures created on insulator substrates. By making use of these microscopes, Prof. Aono's group has been trying to measure electrical conductivities of DNA, carbon nanotubes, polymer nanowires, and other materials of interest, in addition to metal nanowires.
As one of his remarkable researches in recent years, Prof. Aono discovered an unexpected phenomenon of importance in his research on building nanostructures with silver atoms supplied from a STM tip made of silver sulfide. “I found that when a tunneling current flowed from a STM probe tip made of silver sulfide (a solid electrolyte) to a sample, silver atoms were deposited at the apex of the probe tip,” he says. He also found that when the direction of the electric current was reversed, the deposited silver atoms were re-dissolved into the probe tip. “I thought intuitively that this phenomenon could be used for novel switches because this mechanism would work as a two-terminal switch that turns on when the deposited silver atoms contact the opposite electrode and turns off when the silver atoms re-dissolve into the probe.”
It was found later that these switches were capable of working repeatedly and that their switching speeds were high. Prof. Aono believes that he will be able to develop a new scientific field, “atomic electronics,” based on these atomic switches. An atomic switch is made at the crosspoint of a silver sulfide-coated silver wire and a platinum wire. His group is conducting joint research with NEC Corporation to commercialize a new computer architecture based on this type of crossbar structure.
Prof. Aono tells younger researchers not to follow others. “Researchers should do attractive and unique studies which other people want to follow. They should not aim to be the best but to be the only ones. If the results of your unique research are appraised highly by others, you will not be able to forget such an exciting feeling.” Now, he wants to study an unexplored area of biotechnology. Prof. Aono says, “I want to study thoroughly the slick exchange mechanisms of signals among biological materials using a STM with at least 1,000 probe-tips. My ultimate goal is to create a new paradigm in the computation field based on my research in such an area.”
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| Fig. 4 Large Image |
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| Direct Measurement of Conductivity of Nanowire Using Multiprobe Scanning Tunneling Microscope(Nanotester) |
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| Fig. 5 Large Image |
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| Development of Atomic Switch New switch controlled by behavior of an atom or a few atoms |
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| Fig. 6 Large Image |
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| Formation of memory cells by crossbar structures made of platinum wires and silver wires coated with silver sulfide |
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| Fig. 7 Large Image |
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| Nanobiological measurement using Multiprobe scanning tunneling microscope (Nanotester) |
