New Phosphor Exploration by the Single Particle Diagnosis Approach

摘要

New phosphors are required for the advancement of lighting and display technologies. One of the most effective ways for new phosphors is to employ new materials for host materials. It takes much time and labor to develop new materials from powder synthesis or single crystal growth. However, even if the powder product is a mixture phase, each particle is a single phase and a single crystal. The single particle diagnosis approach focuses on the tiny single crystal particle. Here we show the concept of this approach and some phosphors discovered by this approach. Phosphor material is a key component for governing the color characteristics of white LEDs. Although the conventional white LED phosphor YAG:Ce has a high luminescence intensity, the luminescence spectrum is not suitable for high color rendering white LEDs in the lighting. Alternative phosphors have been searched, and some Eu~(2+)-doped Si, Al containing nitride and oxynitride phosphors were found to have excellent luminescence properties for better color rendering, and they have been commercialized. New phosphors are still required for 1) the wide variation of emission spectra (peak position, peak width) to produce various types of white LEDs, 2) the coming change of emission wavelength of LEDs (near-UV LED) and 3) the high power LEDs where the thermal quenching of luminescence is more predominant. The luminescence property of Eu~(2+) or Ce~(3+) doped phosphor is so affected by the coordination environment of luminescent center. The usage of different host material leads to new phosphor. The search for new phosphor is roughly classified to two methods. One is to employ a known crystal structure that is suitable for luminescent center doping and is not studied for host crystal. However, the promising candidates are already studied in most cases. The other is to find new host material by analyzing single crystals or powder product. In the single crystal analysis, a large size crystal (typically larger than 50～100 μm in all dimensions) is necessary to determine the crystal structure. In the powder process, at least it is necessary to synthesize the new material as single phase to solve the crystal structure from powder XRD data. Both normally require much time and effort. Therefore, it is desirable to find a way that allows the high-speed discovery of new phosphors. We have developed a new efficient method to find new phosphors from a powder synthesized by a standard process. We have named the new method "Single Particle Diagnosis Approach". Even if the powder product is not a single phase, by considering each (isolated) particle, it can be treated as a single phase and a single crystal. We choose the single particle from the product mixture and analyze its crystal structure and the luminescence properties. Figure 1(a) shows the powder samples of various compositions synthesized under standard process and they appear various types of emission by UV-LED excitation depending on the starting compositions. A microscope image of the powder in one crucible is shown in Figure 1(b). Although the product seems to have a uniform orange luminescence, it consists of different types of luminescence (yellow, orange, green, cyan, etc), size and form in a micro-scale observation. Single crystal particles are picked and mounted on the glass capillary for the screening by single crystal XRD measurement (Figure 1(c)). The lattice parameters and Bravais lattice are determined and compared to the database, and the candidates of new phosphor particle are selected. The crystal structure of the new phosphor is then explored in detail with the EDS analysis (Figure 1(d)). Due to the recent development of commercially available single crystal X-ray diffractometer, we can determine the crystal structure of a tiny microcrystal down to 5-10 μm. This microcrystal corresponds to the size synthesized by standard synthesis conditions. The special experiment for crystal growth is not necessary.