Phosphors, Up Conversion Nano Particles, Quantum Dots and Their Applications: Volume 1
Climate change affects people's lives by altering temperature, water supply, and rainfall and by influencing the frequency of natural disasters. Researchers are actively looking for alternative energy sources that do not emit carbon dioxide. White light emitting diodes (WLEDs) have been considered one of the most promising next generation lighting technologies because they significantly reduce global power requirements and the use of fossil fuels. WLEDs have attracted considerable attention because of their significant luminous efficiency, low power consumption, reliability, and environmental friendliness. This chapter discusses the fundamental principles and optical properties of phosphors and quantum dots (QDs) for light emitting diodes (LEDs) The discussions are mainly focused on the luminescent mechanisms, phosphor and QD components, and the corresponding effects on their optical properties and prospect. We also tackle a number of concepts involved in the nephelauxetic effect, crystal field splitting, energy transfer, thermal effect, and quantum confinement effect, which leads to luminescence. Illustrative examples from luminescent materials applied in lighting are used. Phosphors for ultraviolet (UV) LEDs and blue LEDs are treated separately because the processes leading to excitation, emission, color, bandwidth, and thermal stability are comparable with each other in fluorescent lamps. The production of semiconducting C.C. Lin Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands C.C. Lin R.S. Liu Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan e mail: rsliu@ntu.edu.tw R.S. Liu Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, No. 1, Sec. 3, Zhongxiao East Road, Taipei 10608, Taiwan C.C. Lin Institute of Organic and Polymeric Materials, National Taipei University of Technology, No. 1, Sec. 3, Zhongxiao East Road, Taipei 10608, Taiwan (c) Springer Verlag Berlin Heidelberg 2017 R.S. Liu (ed. Phosphors, Up Conversion Nano Particles, Quantum Dots and Their Applications, DOI 10.1007/978 3 662 52771 9_1 1 QDs will be discussed with the concept of "the separation of nucleation and growth." The exploration of QD emission colors emphasizes the core material and nanocrystal size for tuning from near UV to near infrared spectra. 1.1 History and Classification of LEDs An important challenging application, namely, the replacement of fluorescent and incandescent lamps, has recently attracted considerable attention because of their energy savings and positive environmental issues. These conventional lighting devices rely on either discharge or incandescent in gases, which usually accompanies large energy loss because of the simultaneous occurrence of large Stokes shift and high temperature. Therefore, researchers have focused on the use of light emitting diodes (LEDs) which offer an alternative approach for illumination, as a novel approach for the generation of solid state lighting sources with high efficiency to significantly reduce worldwide energy consumption [1 6] The low electrical power consumption of LEDs is ascribed to the recombination of electrons and holes produced from current injection with small energy losses [3] In 1907, Round [7] developed the first LED, which is an SiC junction diode. Subsequently, Losev [8] independently discovered an emission phenomenon from an SiC and ZnO diode in 1927. However, during these two decades, the potential of LED applications was not maximized, and inventions remained unclear. In 1961, the first inorganic semiconductor LED was fabricated by Baird and Pitman at Texas Instruments. This device emitted near infrared (NIR) radiation because the material is InGaP with a band gap of 1.37 eV. After 1 year, Holonyak, who worked as a consulting scientist at General Electric Company in New York, invented the first practically useful visible LED device on the basis of GaAsP material in red light; he is called "the father of LEDs." The main yellow phosphor, cerium doped Y3Al5O12 (YAG:Ce) [9] which is now used in phosphor converted LEDs (pc LEDs) was reported in 1967 [10] YAG:Ce was primarily used in cathode ray tubes during that Fig. 1.1 History of luminous efficacy for incandescent, halogen, fluorescent, sodium vapor lamps, and white LED. Adapted from Ref. [12] (c) IOP Publishing. Reproduced with permission. All rights reserved 2 C.C. Lin and R.S. Liu