Professor Hiromi Nakano has been observing and analyzing materials by transmission electron microscopy (TEM) for many years at companies and universities. She is a TEM professional who is now attracting attention from domestic and foreign researchers and enterprises for her tenacious efforts in areas such as discovering unknown material properties and developing new materials with controlled crystal structure and texture at the atomic level. Recently, she succeeded in developing innovative new oxide phosphors used for white LEDs together with students in her lab. We talked with her about the contents of her research and the beliefs that guide it.
Interview and report by Madoka Tainaka
"My specialty is to do material analysis using a transmission electron microscope (TEM)," Professor Hiromi Nakano says.
A TEM is an electron microscope that irradiates an observation specimen with an electron beam and searches the microstructure of the surface or interior of the observation object with the transmitted electron beam. With TEM, it is possible to analyze the texture and crystal structure of materials at the atomic level, and the technique is widely used for analyzing everything from inorganic materials such as ceramics and metals to organic materials and biomaterials. TEM is useful for material property control and new material design, and is an indispensable device for nanotechnology development. It is also used in various fields such as materials engineering, biology and medicine because researchers can observe samples while heating or cooling them, making it possible to capture images of shape change and behavior of nanoparticles due to temperature change.
"However, you must be very persistent to make observations with TEM. To be able to transmit the electron beam, the thickness of the sample must be 0.1 μm or less with a diameter within 3 mm. To achieve this one punctures the center of the sample with an argon ion beam, but if it is a thin laminated material, important parts may be peeled apart in the course of fabricating the thin film samples. If this happens one must start over again from the beginning. With difficult samples, it sometimes takes more than two months to make one thin film sample," says Nakano.
Nakano was once requested to observe aluminum nitride, which has the world's highest level of high thermal conductivity, by TEM. The material was produced by adding yttrium oxide to aluminum nitride as a sintering aid and applying heat treatment in a reducing atmosphere. Its high thermal conductivity was believed to be a result of the loss of the grain-boundary phase from that process. However, Professor Nakano was able to demonstrate with TEM photography that the grain-boundary phase remained even after heat treatment for 100 hours.
"When I presented a TEM image showing a thin layer containing 0.6 nm of yttrium including at the grain boundary at a conference, the place erupted. I was an unknown researcher at that time, and researchers didn’t believe me even though I showed them the TEM picture as evidence. The researchers who were present exclaimed, ‘We have taken images many times, but could not observe the grain-boundary phase, so why is it only you that found it?’ I replied, ‘It’s perseverance.’ After that, I became called ‘The person with perseverance’ for a while," says Nakano, laughing.
It was an accomplishment that was achieved as a result of Professor Nakano’s observation technique that makes grain boundaries precisely vertical to the electron beam. To that extent, Nakano says that TEM research requires a certain amount of skill and tenacity, and, above all, beliefs which are firmly rooted in theory.
However, Nakano also feels that it is difficult for students with a limited research period to be forced to only do TEM research. "Especially recently, there is a strong tendency to seek results as soon as possible," says Nakano. Therefore, in recent years, she has been working in the laboratory on the development of oxide phosphor materials used in white LEDs.
For white LEDs used in applications such as lighting and LCD monitors, methods such as combining yellow phosphor on a blue LED chip or combining red, green and blue phosphors on a near ultraviolet LED chip are used. While the former has the merit that it can be realized at a low cost, there is the demerit that the combination of only blue and yellow has poor color rendering properties and only a cold white can be reproduced.
"For this reason, I started thinking about red oxide phosphor. It can be sintered at a lower temperature than nitride phosphor that is currently used at normal pressure, and because it is an oxide, it is highly stable, and is also easy to procure. It is also non-toxic, of course." Specifically, red phosphor is prepared with a Li-Ta-Ti-O system solid solution as a matrix then adding a rare earth or the like as an activator.
"The students are more motivated because we can confirm light emission only by shining light on a sample with phosphor. Of course, it is difficult to find good synthesis conditions, and there may be repeated failures, but in fact, there is no such thing as a failure. Sometimes, unexpected results are born from failure. Our mission as researchers is to derive mechanisms from there. I hope that as many students as possible can experience this real pleasure."
Dr. Hiromi Nakano
Dr. Hiromi Nakano received her B.S., M.S., and Dr. (Eng.) from Toyohashi University of Technology, Japan in 1981, 1983 and 2000, respectively. She joined Murata Manufacturing Co., Ltd. in 1983 then transferred to Ryukoku University in 1989. She joined the Cooperative Research Facility Center in Toyohashi University of Technology as an associate professor in 2009 and presently works as a professor and presidential advisor (Gender Equality). Her current interest is in the synthesis of new phosphors and characterization of ceramic materials using a transmission electron microscope to control of the material properties and the design of new materials. - See more at: http://www.crfc.tut.ac.jp/nakano/index.html
Madoka Tainaka is a freelance editor, writer and interpreter. She graduated in Law from Chuo University, Japan. She served as a chief editor of “Nature Interface” magazine, a committee for the promotion of Information and Science Technology at MEXT (Ministry of Education, Culture, Sports, Science and Technology).