Building the foundations for nano-sized phosphors that convert light energy

Описание: Light has many familiar applications, including displays and illumination.
Recently, LEDs have become topical as a means of energy-efficient lighting. LEDs use materials called phosphors, which emit light.
At Keio University, the Isobe Group, in the Department of Applied Chemistry, is doing R&D on such phosphors.

Q. We're studying these phosphors, which are usually made by mixing powders and heating them to over 1,000 degrees. But in our research, we make phosphors in solutions. The reason we use solutions is that, with the usual method, the particles created are micro-sized, but we make nano-sized particles. In this way, we're steadily building the foundations of globally pioneering research.

To achieve the luminescence efficiency of micro-sized particles with nano-sized particles, the Isobe Group has developed nano-sized crystals, where atoms are arranged regularly, as in single crystals.
The researchers also utilize a unique processing method, to further increase the luminescence efficiency.

Q. Even nanoparticles with regular arrangements, like ordinary single crystals, don't emit light very well. That's because bonds on the surface of the particles are broken. So, a big issue is how to overcome that. To achieve this, we cause the particles to adsorb molecules, in order to eliminate surrounding surface defects. Alternatively, we form a core and shell that have the same crystal structure, to eliminate surface defects. So even with nano-sized particles, what we're doing is chemical design, to increase the fluorescence intensity of the phosphor.

One method suggested by the Isobe Group is to use micelles, which are clusters of surfactant molecules, as nano-reactors. Another method is to use a sealed reaction vessel, like a pressure cooker, to make nano-phosphors below 300 degrees C.
In particular, YAG:Ce phosphors produced by the pressure-cooker method have attracted attention worldwide.

It's also possible to make transparent phosphors, which don't scatter light, by keeping the particle size below 50 nm, which is one-tenth the wavelength of visible light. This could enable various applications. These include security for forgery prevention, bio-imaging using bonding with biological molecules, and coating the surface of solar cells to convert UV into visible light.

Q. In the phosphor field, practical use is extremely important, so attention tends to be on luminescence efficiency and applications. People in industry have worked very hard to develop phosphors and produce usable ones. Meanwhile, in universities, an important task is to think about why some phosphors aren't very bright. In that sense, we'd like researchers to focus harder on the foundations, rather than just aiming for applications.
In particular, our field combines optical physics and the chemistry required to make substances. The challenge for us is to do new things in the phosphor field. By focusing on how to combine physics and chemistry, we'd like to continue opening up new areas of research.