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Knowledge Co-Creation - Profiles of researchers

Seeking to create revolutionary materials technology through a long-term vision and daily effort

Taishi Takenobu
Associate Professor, Faculty of Science and Engineering, Waseda University

Awakening to the appeal of applied research

In order to produce advance results in the field of material physics, I place emphasis on two points. The first point is to do things differently from past conventions. The second point is to skillfully use the outstanding qualities possessed by materials. The field of materials research is segmented and researchers normally work almost exclusively within their particular area of expertise. Although in-depth research is conducted for materials themselves, not many researchers conduct research to expand application of that research. However, my approach is slightly different.

To begin with, when I was a student, I researched material made from the carbon atom fullerene. Broadly speaking, this material could be classified as an organic material. However, when being more exact, it could also be classified as an inorganic material. The experience of starting research with such a borderline material may be the roots of my philosophy for conducting materials research without being restricted to a certain field.

Afterwards, I entered a corporation and conducted research in carbon nanotubes. This was a major impetus for me to change my research approach. Specifically, I was engaged in research for the application of carbon nanotubes as hydrogen adsorbents for fuel cells. I found research which combined carbon nanotubes and organic materials to be a very interesting type of materials research. At the same time, I imagined a wide variety of possible applications for the research.

The very first idea which occurred to me was application to transistors. I transferred from my corporation to the Tohoku University Institute for Materials Research and began earnest research from around 2005. I published essays which emphasized the possibility of developing highly functional transistors from carbon nanotubes. However, it is not possible to change past convention using only statements from basic research. In order to transform my new ideas into revolutionary technology, it was necessary for me to undertake applied research.

With this need in mind, I applied for the Young Researcher grant sponsored by NEDO (New Energy and Industrial Technology Development Organization). This grant is a project to support research in industrial technology by young researchers. Beginning from 2006, I spent 5 years conducting development research to create carbon nanotube thin-film transistors using the ink jet method and to expand that technology to translucent flexible transistors.

Precise control of carbon nanotube film

Figure 1: Image of carbon nanotube containing organic molecules.

Carbon nanotubes are a material that possesses a nano-level hollow architecture (Figure 1). There are great expectations for the application of carbon nanotubes as next-generation nanotech materials. Therefore, applied research has focused exclusively on the nano-level. However, the heterogeneous material properties of carbon nanotubes present a difficult barrier to nano-level application.

When examining the structure of carbon nanotubes, they are a collection of tubes which differ wildly in terms of diameter, length and shape. Due to this heterogeneity, material researchers even go as far as to describe carbon nanotubes as a "dirty" material. I believe that carbon nanotubes are a very difficult substance for developing materials which exert uniform capability on the nano-level. Therefore, my development strategy is to convert the nanotubes into a film which can be handled as a highly stable material. I also had intuitive faith that carbon nanotubes would enable the production of transistors using the ink jet method instead of the conventional evaporation method.

When performing conventional microfabrication technology using the evaporation method, the material is first applied to the entire surface of the board. Afterwards, the material is scraped away so that the shape of the circuit remains. This method wastes nearly 80% of the material. Conversely, the ink jet method eliminates wasted material by initially spraying material in accordance with the circuit shape. This method also results in significant conservation of energy and resources. However, practical application was difficult and I was forced to wait for the development of a material which is appropriate for the ink jet method. Research was being conducted for organic EL material, which had exhibited high capability using the evaporation method. However, difficulty was encountered during adaptation of this material to the ink jet method.

The 5 years that I spent researching as part of the NEDO project resulted in the successful development of a production method for carbon nanotube thin-film transistors. My research solved the issue of enabling precise control for film density, and the carbon nanotubes possessed overwhelmingly superior properties when compared to conventional nanotubes. (News release published March 2010)

Figure 2: Production diagram for carbon nanotube thin-film transistors using the ink jet method
The number of drips was used to control the density of the nanotube. High density film was used for the electrode and low density film was used for the active layer of the transistor. The bottom-right photograph shows an actually produced device viewed through a microscope.

Flexible devices possess outstanding flexibility

Another keyword of my development strategy is "flexible transistor." This means that transistors are flexible, soft and bendable. By forming transistor boards out of soft, plastic-like material, I hope to realize lightweight devices which are soft like paper. Plastic is susceptible to heat and is only able to resist temperatures of about 200 degrees Celsius. Conversely, current silicon boards require heat-treatment processing at high temperatures which reach several hundred degrees Celsius. The NEDO project also enabled the production of devices for plastic boards by switching the device material to carbon nanotubes for which low-temperature processing is possible.

The range of application for items such as solar panels will increase by switching to flexible devices. Current solar panels are very heavy because they use silicon. However, if panels are changed to soft and lightweight material, then it will become possible to hang the panels on walls or to wrap the entire body of a car in solar panels. I envision these kinds of possibilities for carbon nanotubes.

Attempting to create the world's first organic laser device

Another project which I have advanced as the application of organic transistors is the research and development of organic laser devices. Practical application of transistors is limited exclusively to the property of on-off switch functions. However, application as a light-emitting device may be possible by utilizing a special structure. Lasers which use conventional inorganic material are capable of realizing red light and blue light, but the development of green light is difficult. The variety of color is limited. Regarding this point, organic semiconductors offer an extremely high degree of freedom for material design. Organic semiconductors are also capable of emitting many different colors of light and of highly efficient emissions. Although application is expected for a wide variety of fields, organic laser devices have yet to be realized.

Organic laser device capable of emitting a various colors of light

As an innovative approach which is completely different from the conventional diode structure, I am working to develop the world's first organic laser device. This device consists of an organic monocrystal transistor which uses highly efficient light-emitting materials. Through polarization of such a monocrystal transistor, I seek to achieve both highly efficient emission of light and high current density. From 2008, I have participated in the Photon on Soft Materials Project of PRESTO, which is the research grant project sponsored by JST. Within the project, I have conducted research and development under the theme of "Creating an organic laser transistor."

Since assuming my position at Waseda University in 2010, I have participated in priority research promoted by the Opto-Science Laboratory, which is a project research center of Organization for University Research Initiatives, Waseda University. The theme of this priority research is "Interaction of light and substances-from fundamental physics to device application." I am one of 6 research members participating in the research. The research has just started, and I plan to begin conducting joint research with other researchers inside of our university. I feel that Waseda University features researchers in an extremely broad variety of fields, and I feel that the university offers an environment where daily discussion can be held under the common denominator of physics.

Seeking research for the next new material

I have conducted two types of research for organic transistors; namely, research for the production of electronic devices using the ink jet method and research for laser devices. Both types of research have produced a certain level of results, and I feel that the two research projects are combining into a single course. Although these two projects were originally completely different types of research, I have always felt that the research projects were proceeding smoothly as two wheels of the same car. The two types of research have gradually grown closer over the course of 5 years, and I will finally advance them as a single project from 2011.

My research was selected for the Funding Program for Next Generation World-Leading Researchers, a research grant project established by the Japan Cabinet Office with the goal of promoting green innovation and life innovation. Within the project, I plan to spend 4 years working under the theme of developing ultra-high capability electronics printed by ink jets. This theme will enable the conservation of energy and resources while also realizing high efficiency. It is truly a theme that can contribute to the promotion of green innovation.

When working in such pioneering projects, it is necessary to set a long-term goal which will be of certain benefit to society. Conversely, it is also necessary to set a goal which will be achieved in a relatively short period of time and to announce results which have academic value. Once I achieve one goal, I set my next goal. This process is repeated in a straightforward manner. I have been able to find the path to long-term goals because I have calmly worked to achieve short-term goals during the past 5 years, resulting in a steady accumulation of success.

Actually, I recently feel an anxious need to begin work on something new. From a broad perspective, I have continued to conduct the carbon material research of my university days and the organic material research from when I was a corporate employee. I suppose that I will someday begin to work concurrently on a third and new topic.

Advanced experiment facilities

Taishi Takenobu
Associate Professor, Faculty of Science and Engineering, Waseda University

In 2001, completed the Doctoral Program at the Japan Advanced Institute of Science and Technology. Holds a PhD (materials science). Employed at Sony Corporation Frontier Science Laboratories. Served as a Research Associate, Assistant Professor and Associate Professor at the Institute for Materials Research, Tohoku University. Assumed his current position in 2010. During his employment at Waseda University, has concurrently served as a Visiting Researcher at the Delft University of Technology and a Visiting Researcher at Nanyang Technological University. Has received a large number of awards including the Intelligent Cosmos Prize and the Physical Society of Japan Incentive Prize for Young Researchers.