President, Japan Advanced Institute of Science and Technology
Seeing a single hydrogen atom
—Analysis of nano-structures through STM light emission spectroscopy—
Prof. Ushioda says, “It may be hard to imagine putting labels on individual nano-structures.” But he has realized such labeling with STM light emission spectroscopy. “You can see atoms of nano-structure surfaces with a conventional scanning tunneling microscope. But you see only the exterior shapes of these atoms. With STM light emission spectroscopy, you can understand the characteristics of each atom.”
In STM light emission spectroscopy, it is necessary first to check the shape of the surface of a sample material through measurements with STM to decide which area of the nano-structure should be investigated. The next step is fixing the STM tip on the nano-structure, and injecting electrons into the structure through the tip. Then, light with energy characteristic to the structure is emitted. Through spectral analysis of the emitted light, the electronic properties of the structure can be understood and, as a result, one can identify what materials the structure is composed of. A research group led by Prof. Ushioda has recently identified single hydrogen atoms whose image cannot be observed through conventional STM measurements. The diameter of a hydrogen atom adsorbed on the surface of nickel is 0.1 nm -- that is, a picometer-scale object.
Light emitted by nano-structures is faint, and only several photons come out from such structure per second. Prof. Ushioda's group has been devising various ideas to catch this feeble light. The group believed the properties of the STM tip to be the key to improving the luminous efficiency of nano-structures. It developed STM tips of various shapes and materials. As a result, the group has developed silver tips with STM luminous efficiency several dozen times higher than that of probes used in conventional STM measurements. It has also found that a sharp tip with an apex angle of some 90 degrees can generate the strongest STM light emission. The group has also developed a unique method to fix the probe on a specific point of a sample material accurately for collecting light strong enough for spectral analyses. This method allows the probe to return to its original position by making an STM image identical to the image at its original position, if its position changes.
Prof. Ushioda says he specializes in characterizing nano-structures. He says, “Even if you develop a new nano-structure, you cannot understand what it is nor what applications it could be used for, unless you can characterize it.” He says there are only four or five groups worldwide that can characterize the electronic properties of individual nano-structures by analyzing the faint light from them. He is sometimes asked by other researchers to characterize their nano-structures. He says, “I usually provide characterization data to researchers who ask me to do so, without showing strenuous effort. But, much of my characterization data were actually obtained through a year’s worth of hard work.” Data obtained through such work tends to contain a lot of new information. His efforts to analyze the faint light are leading to major research projects in the nanotechnology field for characterizing and searching for new nano-structures and nano-devices.
To promote intellectual curiosity among students, Prof. Ushioda stresses the need for education that provides students with a broad outlook. He is also the research supervisor for “Creation of Innovative Technology by Integration of Nanotechnology with Information, Biological and Environmental Technologies,” the Nano Virtual Lab (Virtual Laboratory in Nanotechnology Areas) of the Japan Science and Technology Agency. He is helping to create an environment in which young researchers from various fields can stimulate their intellectual curiosity by collaborating together. He expects new unique technologies to be developed via interaction among young researchers.
