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2 Handbook on the Physics and Chemistry of Rare Earths 1 INTRODUCTION Two decisive advances in semiconductor electroluminescence have led to the production of visible light for illumination by light-emitting diodes (LEDs). First, red light emission was developed in 1962 (Holonyak and Bevacqua, 1962). In the decades that followed, LEDs were used only in numerical dis- plays and signaling applications, but not in major lighting devices. The second advance was the demonstration of a bright blue LED by Nakamura in 1993 (Nakamura et al., 1994), along with earlier materials advances by Akasaki and Amano group (Amano et al., 1986, 1989), to whom the Nobel Prizes in Physics 2014 were awarded, for honoring their significant invention of the blue LEDs. Importantly, since blue is at the short wavelength side of the visi- ble spectrum, this LED has significant potential to “down-convert” its blue light into green, yellow, and even red light using passive phosphorescent and fluorescent materials (Shimizu et al., 2003). Traditional incandescent and fluorescent lamps rely on either heat or dis- charge of gases, associated with large energy consumption that occurs due to high temperatures and large Stokes shifts (Xie et al., 2007). On the other hand, LEDs have numerous advantages over these lamps, such as small size, long lifetime, robustness, fast switching, and high efficiency. Indeed, as illu- strated in Fig. 1, the rapid increase in the luminous efficacy of LEDs is quite remarkable. There is now virtually no question that solid-state lighting (SSL) will eventually displace all conventional technologies in general illumination applications, leading to a considerable reduction of worldwide electricity consumption. Phosphor materials in LED technology play a definite role for achieving high color quality white light. They are photon conversion materials which Sodium-vapor lamp 1965 Fluorescent lamp 1938 Incandescent lamp 1879 Halogen lamp 1959 White LED 1996 FIG. 1 150 100 50 0 1900 1950 2000 Year Luminous efficacy improvements of various light sources. Redrawn from Nakamura, S., Mukai. T., Senoh, M., 1994. Candela-class high-brightness InGaN/AlGaN double-heterostructure blue-light-emitting diodes. Appl. Phys. Lett. 64, 1687. Luminous efficacy (Im/W)PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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