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Luminescence Science and Display Materials

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Luminescence Science and Display Materials Lighting and Imaging Devices Photoluminescence. In the fluorescent lamp, the cathode at one end of the lamp emits electrons and the anode collects electrons at its other end. (In ac operated devices, the electrodes act both as anode and cathode, depending on the phase of the current.) The electrons excite and ionize mercury atoms in the vapor phase. UV radiation from the excited mercury atoms is absorbed by impurity ions in the powder phosphor coating on the lamp wall. The activator impurities in turn relax and may transfer energy to other ions, which then re-emit radiation at a lower energy in the visible region of the Luminescence Science spectrum or emit themselves. The development of the Sb3+ and Mn2+ activated halophosphate phosphor [Ca (PO ) (F,Cl):Sb,Mn] in the 1940s and Display Materials 5 4 3 was a significant breakthrough in by Cees Ronda and Alok Srivastava fluorescent lighting. The lighting industry underwent a revolution Luminescence is defi ned broadly as the generation of light in the 1970s when a blend of three rare-earth ion containing phosphors in excess of that radiated thermally. Man’s fascination with [activators: Eu2+ (blue), Tb3+ (green), luminescence stems from when an otherwise invisible power and Eu3+ (red)] was proposed for good is converted into visible light. The commercial importance of color rendering, higher efficiency, and better maintenance in fluorescent luminescence is ubiquitous, being manifest in lamps, displays, lamps. Prior to their discovery, three- X-ray machines, etc. In ECS, the Luminescence and Display band spectra were predicted to provide Materials (LDM) Division provides a forum for work on the good color rendition and maximum physics and chemistry of light generation from electrical energy luminous efficiency. These and newer rare-earth phosphors have revolutionized and the technology of the various light producing devices. fluorescent lamps, making high current Aspects of these areas are briefl y described in the following density compact lamps possible and sections. providing the best efficiency, even more than 100 lumens per watt, and good color rendition (mid-1980s) in Luminescence Science radiation from mercury discharge 36 W fluorescent lamps. In recent and Technology (photoluminescence) or electron years, research on new luminescent bombardment (cathodoluminescence). compositions for fluorescent lamps has Materials that generate luminescence In addition, luminescence may be come to a standstill. Important issues are called phosphors. Commercial generated directly from an electric field nowadays are Hg consumption by phosphors are mostly inorganic (electroluminescence) or from infrared fluorescent lamps and reduction of the compounds prepared as powders (with radiation (up-converting luminescence) or price of the phosphor layer. grain sizes usually in the order of 2- even mechanically (triboluminescence). 20 µm) or thin films. The phosphor Fluorescent lamps are used not only materials contain one or more impurity To have any technical importance, for interior lighting. Liquid crystal ions or activators (A), typically present a luminescent material must be easily display (LCD) screens have boosted in 0.01-100 mol % concentrations. The excited by the appropriate excitation and the demand for fluorescent lamps with actual emission is generated on these must have high quantum efficiency, the a small diameter. Generally, the color activator ions. Typical activators are ratio of the number of quanta absorbed temperature of the light emitted by such rare earth- or transition-metal ions, ions to the number emitted. Nonradiative lamps is high, some 9000 K. undergoing s-p transitions (like Bi3+), losses are commonly caused by The advent of efficient near UV and and molecular anions like the tungstate interactions with lattice vibrations and blue light-emitting diodes (LEDs) has or vanadate groups. Sensitizers (S) what are known as killer impurities. revived research on photoluminescent are useful if the activator ions cannot Further, the activator must convert the materials. The requirement of absorption be excited, e.g., because of forbidden energy absorbed to a useful frequency of in the near UV/blue region and emission transitions. In such cases, the exciting visible light. A suitable phosphor must in the visible part of the spectrum, energy is absorbed by the sensitizers and maintain well under the excitation mode coupled to the necessity of strong subsequently transferred to the activator and must be easily manufactured. The (optically allowed) absorption of the LED ions. Common to all these moieties is synthesis of efficient phosphors not only radiation by the phosphors has induced the not completely filled electron shell requires the best in high temperature a world-wide research activity for Eu2+ in at least one state (ground state, excited chemistry, but also in precursor and Ce3+ doped inorganic phosphors. state) involved. preparation and handling and purity of To tune the optical properties of these starting materials. Device manufacturing Activators or sensitizers may be activator ions as to their suitability in involves still other sciences, such as excited by high-energy photons, LEDs, apart from well-known yttrium- thin film technology and suspension such as X-rays or ultraviolet chemistry. (continued on next page) The Electrochemical Society Interface • Spring 2006 55 Luminescence Science... phosphors include materials with (continued from previous page) much greater X-ray stopping power and better conversion efficiency. This greatly reduces the patient exposure needed for a good quality picture. The luminescent materials applied are X-ray stimulated phosphors in the form of a single crystal, a sintered ceramic, or a glass, which converts the X-ray radiation to a wavelength detectable by a light detector that is placed on one side of the rod. These rods form the detectors for the computed tomography (CT) unit. The X-ray scintillation counters must be efficient in converting X-ray radiation into visible or near IR radiation, must be able to be fabricated so as not to scatter light, and must have little or no afterglow. The scintillators CsI: Tl and CdWO4 are currently used as single crystal detectors in commercial CT scanners. Examples of commercial ceramic scintillator materials are 3+ 3+ (Y,Gd)2O3:Eu and Gd2O2S:Pr . FIG. 1. Energy level scheme and quantum cutting in the Gd3+-Eu3+ ion pair. In step 1, energy is Scintillators are also used in high- transferred from Gd3+ to Eu3+ by cross relaxation, energy particle detection, such as γ-rays. leaving a still excited Gd3+ ion behind, which A total of 12,000 crystals of Bi4Ge3O12 transfers the remaining energy to a second Eu3+ ion (BGO) are used at CERN (Geneva) for in step 2. this purpose. In addition, the medical imaging modality positron emission tomography (PET) also relies on high- energy particle detection. For this application, very fast scintillators with a high density (in view of the high energy of the particles) are required. Examples are BGO, Gd2SiO5:Ce, and Lu2SiO5: Ce. A material which is less dense but extremely fast and bright is LaBr3:Ce. Developments in ceramic crystals are also driven by the laser industry. Ceramic crystals have distinct advantages FIG. 2. Evolution of light sources. over single crystal rods in terms of ease of production and costs. aluminum-garnet (YAG):Ce, many new the ultraviolet radiation is provided by a Electroluminescence compositions based on or containing xenon discharge (emitting between 147 Apart from inorganic LEDs (see N3- ions have been developed. This race is and 200 nm) instead of the conventional above), there is strong interest in still going on. In the meantime, efficacies mercury-based discharge (254 nm). electroluminescence originating from have reached those of fluorescent lamps, Quantum cutting based on energy organic or polymeric molecules (OLED). albeit at lower light levels. At this time, transfer in ion pairs (like Gd3+ -Eu3+, see Work on organic luminescence started in LEDs are used for backlighting in (small) Fig. 1) has been studied in detail after the late 1980s, after exploring work by displays, in automotive applications, pioneering work at Utrecht University Tang and van Slyke. Electrons and holes and for decorative and signing purposes. in The Netherlands. In principle, the recombine in organic layers, leading to General lighting is a next target. In quantum efficiency can be high, but is the emission of (visible) light. Issues here addition, solid-state light sources can be hampered by the weak absorption of are charge transport (toward the emitting solar powered, enabling light generation the Gd3+ ion. No suitable sensitization layer) and device stability. Emission can at virtually all places in the world. schemes have been identified thus be due to band-to-band like transitions Vacuum UV phosphors. Vacuum UV far. Until now, no systems suitable for but also to optical transitions on rare excited phosphors lead to the possibility practical applications have been found. earth- (like Eu3+) or transition-metal of developing a mercury-free fluorescent A third mechanism in which quantum ions (like Ir3+). Apart from the search lamp. Quantum efficiencies in excess of cutting proceeds via host lattice states for efficiently emitting compositions, unity (up to 140%) have been reported (multiplication over the bandgap) stability of these compositions and by GE and Philips research groups in has also been investigated. In this their machinability are issues.
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