The Photonics Research Institute of the National Institute of Advanced Industrial Science and Technology (AIST) has achieved a world-first with its successful development of a new type of fluorescent glass with a brightness three times that of the glass bodies currently used for CRTs and fluorescent light tubes. It contains semiconductor nano-particles that are dispersed in it in a stable and uniform manner.

This study has been conducted as part of the Nano-Glass Technology Project of the New Energy and Industrial Technology Development Organization (NEDO). Until the present, fluorescent bodies have been almost exclusively made of oxides and sulfides with additions of rare-earth ions. This well-established technology has a many decades' long history of research that is borne out in a gradual improvement in both the brightness and the durability of the products. Since they have a long emission lifetime, however, they are not capable of converting the excitation light to fluorescent light efficiently even when a powerful excitation light is used. Expectations had thus been pinned on the development of a new type of fluorescent body representing a radical break with conventional research to serve as a light source for displays and energy- conservation type lighting, applications with ever-increasing demands for high brightness and miniaturization.

In recent years, however, attention has focused worldwide on research aiming at synthesizing semiconductor nano-particles (with a diameter of 10 nano-meters or less) that are capable of emitting light at high efficiency, using the solution technique for controlling the surface condition. The superior characteristics of semiconductor nano-particles include:

1. They have a emission lifetime a hundred thousand times shorter than rare earths and repeat the absorption and emission cycles at a much faster rate, accordingly. This accounts for their outstandingly high brightness.
2. They emit light in various colors according to their diameters. (Fig.1)
3. They exhibit much less deterioration than organic pigments. (The number of photons released as fluorescent light until deterioration sets in is about a hundred thousand times greater than in the case of dyes.)

The problem, however, is that in the liquid form they are unstable. After exposure for a few days they will thus cease to emit light and are therefore not suitable for technical use.

The AIST has been successful in developing a technique for stabilizing the semiconductor nano-particles in the glass. The glass has some very outstanding properties, including a superior transparency, mechanical strength, heat-resistance and chemical stability. It also exhibits little deterioration on exposure to ultraviolet radiation. The newly developed technology consists of the following three stages:

1. Synthesis of the semiconductor nano-particles in an aqueous solution.
2. Complete intermingling of alkoxides with a favorable affinity with the nano-particle surface.
3. Preparation of a glass with nano-particle dispersed by the sol/-gel method while preserving the surface condition of the nano-particles to prevent coagulation. (Fig. 2,3)

Nano-particle glass applied to various substrates such as glass or polymers has a promising application potential as a fluorescent body for lighting and high-definition displays. Because it maintains a bright luminosity for a long time it opens up some very promising prospects for a number of new fields: In medicine it may be of great value in investigating the chemical reactions taking place in the cell. Research is also under way to consider its use in tracing the location of viruses in the body.

Fig. 1: Light emission in various colors on irradiation with a single ultraviolet light, depending on the particle diameter.

Fig. 2: Initially, semiconductor ultra-fine particles capable of dispersing in water were prepared, and the alkoxides with a favorable affinity with the particle surfaces were completely mixed with the ultra-fine nano-particles in the nanometer order. Based on these conditions, the nano-composite glass shown in the photograph was prepared by performing a sol/-gel reaction to preserve the surface condition and prevent coagulation. (a) Appearance of the glass in room light. (b) Light emission on exposure to ultraviolet radiation. The emission efficiency is estimated to be a bit less than 10%.

Fig. 3: Model view of the ultra-fine nano-particles in the glass and their vicinity. This is the first time in the world that light-emitting ultra-fine nano-particles have been maintained stable in a transparent vitreous solid (glass) glass .