Why the quest of new rare earth doped phosphors deserves to go on (original) (raw)
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Rare-earth doped phosphors: oldies or goldies
Materials Science and Engineering B-advanced Functional Solid-state Materials, 2003
The scientific research on phosphors has a long history starting more than 100 years ago. But recently the appearance of new kinds of displays and lighting devices (plasma display, fluorescent lamp without mercury, etc.) induced an increase of the research of new phosphors with better luminous efficiency than those available up to now. It has been shown that the behavior of "classical" phosphors in a plasma display panel is quite different than in a cathode ray tube and that the vacuum ultraviolet (VUV) excitation process has to be studied with care in order to improve the phosphors efficiency. That is particularly true in PDPs. It is well established now that a good phosphor for electronic or ultraviolet excitation is not necessarily a good choice for excitation in VUV. This is probably due to the fact that the excitation process is very different in that case and also because the penetration depth of the VUV photons is extremely small inducing a large contribution of the surface of the phosphor. We will illustrate this with some examples. Methods to accelerate luminous intensity decrease under VUV excitation will be described. Low efficiency, fast aging process are both drawbacks that can be solved only in the framework of fundamental studies. Quantum cutting emission may be a solution for the first one but no satisfactory process was proposed for the moment to solve the second.
Insight the Luminescence Properties of AlON: Eu, Mg Phosphor under VUV Excitation
Materials (Basel, Switzerland), 2017
Owing to high quantum efficiency, adjustable composition and antioxidation properties of oxynitride phosphors, extensive investigations have focused on their photoluminescence properties under low-energy light excitation (UV or blue light). However, the vacuum ultraviolet (VUV) luminescence properties of oxynitride phosphors are rarely researched. Present work studies the structure and VUV luminescence properties of an oxynitride phosphor: AlON: Eu, Mg, which is synthesized by solid-state reaction. Under 147 nm excitation, it was found that AlON: Eu, Mg phosphor shows a blue emission band centered at about 470 nm. The first principle calculation is used to analyze the origin of the VUV absorption. Compared with BaMgAl10O17: Eu(2+) phosphor, AlON: Eu, Mg phosphor shows better thermal stability.
Host emission from BaMgAl10O17 and SrMgAl 10O17 phosphor: Effects of temperature and defect level
2010
Understanding the mechanism of blue-light emission in Eu-doped BAM phosphor as well as its sensitive degradation is required because this is a very important material in fluorescent lamps and plasma-display panels. In this study, both theoretical and experimental investigations on the host emissions in BaMgAl 10 O 17 and SrMgAl 10 O 17 were performed. Host emissions from BaMgAl 10 O 17 and SrMgAl 10 O 17 by photoluminescence and thermoluminescence spectra were observed. Photoluminescence spectra suggested that the host emission from SrMgAl 10 O 17 was easily quenched by thermal vibrations. The thermoluminescence spectra showed the existence of shallow and deep defect levels in BaMgAl 10 O 17 and SrMgAl 10 O 17 phosphors. It was shown that SrMgAl 10 O 17 and its conduction plane could undergo degradation during irradiation of vacuum-ultraviolet (VUV) lights based on the calculated energy of formation of an oxygen vacancy. Moreover, the structural defects, such as oxygen vacancies, would cause localizing levels in the upper level in the valence band and in the conduction band. The results suggest the contribution of the host emission to the energy transfer to the Eu atoms would not be significant and the oxygen vacancies would act as the traps for excited carriers.
Degradation mechanism of phosphors by vacuum ultraviolet excitation
Optical Materials, 2006
The mechanism of luminance decrease of phosphors excited by a Xe plasma discharge has been studied. It is shown experimentally that the aging process is mainly due to the vacuum ultraviolet excitation (VUV). It is demonstrated that the degradation mechanism can be accelerated by using 193 nm laser excitation. Based on excitation, reflectance, thermoluminescence spectra and ageing or bleaching processes by laser excitation, the main causes of the degradation are demonstrated. The aging process can be separated in two different processes depending on the temperature: a first one, at low temperature, corresponding to the autoionisation of luminescent centers; and a second one, at high temperature, linked to the formation of traps in the phosphor. These traps induce a perturbation of the energy migration in the phosphor. The relevant parameters of the creation of traps are highlighted: density of the VUV excitation, temperature, atmosphere and pressure surrounding the phosphor.
Research of green emitting rare-earth doped materials as potential quantum-cutter
Optical Materials, 2008
Because the energy of vacuum ultraviolet (VUV) photons emitted by xenon plasma discharge is more than twice that of visible photons, quantum cutting appears to be a promising process in rare-earth doped materials in order to obtain efficient phosphors for mercury free lighting devices as well as for plasma display panels. With an aim of application, it is important to take into account the emitting color of the developed new phosphors. Most of the time, this leads to use systems with at least two kinds of rare earth ions: one of them playing the role of energy sensitizer, and the other one being in charge of emitting the light of the suitable color. We focus our attention on green rare-earth doped materials. In order to get very efficient phosphors, it is not only necessary to get the highest possible quantum yield, but also to have a material characterized by a strong absorption in the VUV range. Borate and fluoride matrices doped with Dy 3+ / Tb 3+ couples of ions are selected according to the position of the 5d band of dysprosium as green emitters.
Luminescence in the Concentrated Phosphor
Radiation Measurements, 2007
Powder samples of EuAl 2 O 4 and (Eu, Sr)Al 2 O 4 with a quantum efficiency (QE) of the Eu 2+ emission more than 0.30 were synthesized. A modified method of QE determination is described and results of the QE measurements for EuAl 2 O 4 , (Eu 0.8 Sr 0.2 )Al 2 O 4 , (Eu 0.65 Sr 0.35 )Al 2 O 4 , and a diluted sample (Sr 0.98 Eu 0.02 )Al 2 O 4 are presented. Excitation and emission spectra as well as decay behaviour of the Eu 2+ luminescence in (Eu 1−x Sr x )Al 2 O 4 in the temperature region of 20-300 K are studied. It is concluded that the main reason for the high QE is a pronounced relaxation of the crystal lattice after excitation of the Eu 2+ ions resulting in an emission with a large Stokes shift and, therefore, in a restriction of energy migration. Eu ions occupying the crystallographic sites with the higher coordination number (Eu II ) are supposed to be the luminescence centres in (EuSr)Al 2 O 4 lattice.
The commercially available white-light-emitting diodes (WLEDs) are made with a combination of blue LEDs and yellow phosphors. These types of WLEDs lack certain properties which make them meagerly applicable for general illumination and flat panel displays. The solution for such problem is to use near-ultraviolet (NUV) chips as an excitation source because of their high excitation efficiency and good spectral distribution. Therefore, there is an active search for new phosphor materials which can be effectively excited within the NUV wavelength range (350–420 nm). In this work, novel rare-earth free self-luminescent Ca 2 KZn 2 (VO 4) 3 phosphors were synthesized by a citrate assisted sol-gel method at low calcination temperatures. Optical properties, internal quantum efficiency and thermal stability as well as morphology and crystal structure of Ca 2 KZn 2 (VO 4) 3 phosphors for their application to NUV-based WLEDs were studied. The crystal structure and phase formation were confirmed with XRD patterns and Rietveld refinement. The optical properties of these phosphor materials which can change the NUV excitation into visible yellow-green emissions were studied. The synthesized phosphors were then coated onto the surface of a NUV chip along with a blue phosphor (LiCaPO 4 :Eu 2+) to get brighter WLEDs with a color rendering index of 94.8 and a correlated color temperature of 8549 K. A light-emitting diode (LED) is in the limelight since its invention by Nick Holonyak Jr. of General Electric Company and its application in solid-state lighting industry has gained a prominent interest. In the contemporary situation, white LEDs (WLEDs) have drawn great attention due to their wide range of applications and salient features like high efficiency, compact size, eco-friendly feature, high thermal stability and long operational lifetime 1,2. The commercially available WLEDs are prepared by coating a yellow-emitting Y 3 Al 5 O 12 : Ce 3+ (YAG: Ce 3+) phosphor on blue-emitting GaN LEDs 3,4. Such WLEDs have several drawbacks including poor color repro-ducibility, low color-rendering index (CRI) value and less thermal stability at high temperature. For the sake of its usage in general display applications, there is a need of high CRI and appropriate correlated color temperature (CCT). Therefore, the WLEDs visualized with near-ultraviolet (NUV) LED chips have been turned into a topic of research interest because their excitation efficiency is almost similar to that of fluorescent light bulbs and less than that of blue LED chips 5,6. For blue LED chips, the electroluminescence (EL) intensity considerably increases in the long wavelength band and saturates at high driving current, and the CCT value readily changes. There is also a non-uniformity in the spectral distribution due to low CRI values. On the other hand, NUV LED chips cannot be used to make YAG-based WLEDs due to weak light absorption in the NUV or deep blue spectral region. To overcome this, the search for a novel phosphor which can be excited in the wavelength range of 350–420 nm is building up. Broadly speaking, rare-earth materials are widely used in preparing host materials and as luminescent centers in many phosphors to generate multi-color lights 7–9. There are several reports found on rare-earth doped single host materials for WLEDs 10–15. Since all the rare-earth materials have similar chemical properties, they require high-cost separation, refinement and purification techniques which make them mostly expensive 16. Due to this, the design of rare-earth free materials is majorly needed. The rare-earth free phosphors can be understood with three main strategies: (i) use of transition metals for luminescent centers 17–22 , (ii) use of defects such as oxygen vacancies 23–26 and (iii) use of materials such as tungstates and vanadates 27–30 .
Performance of YAG:Eu3+, YAG:Tb3+ and BAM:Eu2+ plasma display nanophosphors
Journal of Nanoparticle Research, 2012
The luminous efficiency and lifetime of plasma display panels (PDPs) are directly related to the performance of phosphors used in PDPs, thus higher efficiency, higher stability against high temperature processes and a long lifetime along with good color chromaticity against vacuum-ultraviolet (VUV) radiation are major concerns in selecting suitable phosphors for PDPs. In the same pursuit, we have developed the nano-sized (15-40 nm) BAM:Eu 2? , YAG:Tb 3? and YAG:Eu 3? as blue, green and red phosphors and studied their luminescence properties under VUV excitations. In BAM:Eu 2? , the 5d-excitation of Eu 2? ions are found strongly dependent on the crystal field strength and Eu 2? occupy lattice 'sites I' by substituting Ba 2? ions. Whereas, in YAG:Tb 3? , the observed green luminescence is assigned to 5 D 4 ? 7 F j transitions (j = 3-6) due to electric dipoledipole interaction, while, YAG:Eu 3? shows strong red luminescence corresponding to 5 D 0 ? 7 F 2 transition. Time evolution studies along with decay time calculations are further employed to verify the sustainable emission without quenching.