Effect of Rare-Earth Doping on Structural and Luminescent Properties of Screen-Printed ZnO Films
L. KhomenkovaВ. И. КушниренкоN.M. OsipyonokA.F. SingaеvskyG. S. PekarK. A. AvramenkoV. V. StrelchukL. Borkovska
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Over the past decades, zinc oxide has attracted considerable attention for its possible application in optoelectronics due to simultaneous observation of intense ultraviolet and visible emission offering the development of white light-emitting devices. However, in the most cases the native defects responsible for visible emission are not stable upon materials processing. Moreover, for white phosphors, efficient and controllable emission in specific spectral range is often required. This can be achieved in particular via materials doping with rare-earth (RE) ions. In the present study the results on effect of doping of screen-printed ZnO films with Sm 3+ and/or Ho 3+ ions are presented. Photoluminescence (PL) and Raman scattering spectra as well as the X-ray diffraction patterns of undoped and doped films are examined in details versus sintering conditions and doping level. In the PL spectra of undoped ZnO films sintered at 400−700°C, the excitonic emission was observed only, whereas sintering at higher temperatures (up to 1200°C) resulted in the appearance of visible defect-related PL bands peaked at 540−600 nm. The most intensive defect-related emission was found in the films sintered at 1000°C and these latter were doped with rare-earth ions of different concentration in the range of 1·10 19 − 4·10 20 cm -3 . In the PL spectra of the RE-doped films, the corresponding RE emission was observed at low temperatures, but not at room temperature. Since the defect-related PL band caused by intrinsic defects in ZnO overlapped essentially with corresponding Ho and Sm PL bands, the PL bands peaked at about 700 nm due to 4 G 5/2 → 6 H 11/2 transitions in Sm 3+ ions were observed only. At the same time, simultaneous codoping with both Sm 3+ and Ho 3+ ions produced significant decrease in the intensity of the UV and visible defect-related PL bands as well as the increase in the intensity of Sm 3+ and Ho 3+ PL components. In the PL excitation spectra of the PL band peaked at about 720 nm in addition to excitation caused by ZnO band-to-band absorption and absorption involving intrinsic defects of zinc oxide, a new characteristic absorption band at about 410 nm appeared. The effect of RE doping on the PL and PL excitation spectra is discussed in terms of the formation of RE ions complexes as well as energy transfer from ZnO host to RE ions.Keywords:
Ultraviolet
High-luminous green phosphors and a red phosphor have been newly developed for white LEDs. The developed green phosphors are Ca3(Sc,Mg)2Si3O12:Ce made by replacing with Mg a part of Sc of green phosphor Ca3Sc2Si3O12:Ce, a new green phosphor CaSc2O4:Ce, and a new host material doped with a rare-earth element, Ba3Si6O12N12:Eu. The new red phosphor is (Sr,Ca)AlSiN3:Eu made by replacing a part of Ca of CaAlSiN3:Eu with Sr.
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In the present investigation, Dy3+, Tb3+ activated/co-activated CaTiO3 phosphor prepared by Sol-gel pechini technique. XRD pattern of the reported phosphor demonstrates crystalline nature, and prepared phosphor’s XRD synchronized with the ICSD record. Phase identification and crystal structure of prepared phosphor is examined by Rietveld refinement. Vibrational features of proposed phosphors were verified via Fourier Transform Infrared Spectroscopy (FT-IR). Morphological study of the proposed phosphor is investigated by SEM analysis. Photoluminescence (PL) investigation of phosphor sample was confirmed by employing a SHIMADZU Spectrofluorophotometer RF-5301 PC. Photoluminescence study of the proposed phosphor shows three visible emission peaks that occur simultaneously gives white light emission. These all results confirm the suggested phosphor is a possible competitor for WLEDs and solid state lighting applications.
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At present,the commercial tri-color phosphors for PDP application are(Y,Gd)BO3∶ Eu3+(red phosphor),Zn2SiO4∶Mn2+,BaAl12O19∶Mn2+ or(Gd,Y)BO3∶Tb3+(green phosphors) and BaMgAl10O17∶Eu2+(blue phosphor).However,there exists a common problem in the above tri-color phosphors,that is,their luminescence efficiency is low.Moreover,they also present the disadvantages as follows which restrict their luminescence properties: the color purity of(Y,Gd)BO3∶Eu3+ is poor;the decay lifetimes of Zn2SiO4∶Mn2+ and BaAl12O19∶Mn2+ are long;the luminescence intensity of(Gd,Y)BO3∶Tb3+ is insufficiency,while BaMgAl10O17∶Eu2+ presents poor stability when treated by heating or VUV irradiation.Aiming at these problems,this paper summarizes the recent research progresses on the VUV luminescence mechanism and the improvement of luminescence properties for the tri-color phosphors.Meanwhile,the research progress on the exploring of novel VUV phosphors is also involved.
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In this paper, we present a color tunable long persistent phosphor, prepared by using a remote phosphor. The blue light emission of Sr2MgSi2O7: Eu2+, Dy3+ long persistent phosphor was tuned from blue to near white region with the help of Y3Al5O12: Ce3+ as remote phosphor. To achieve the multicolor tunability, the two phosphors, Sr2MgSi2O7: Eu2+, Dy3+ and Y3Al5O12: Ce3+ were mixed in a different weight ratio(S: Y). The cause behind such kind of color tunability is the persistent radiative energy transfer from long persistent luminescence (LPL) of Sr2MgSi2O7: Eu2+, Dy3+ phosphor to non-LPL Y3Al5O12: Ce3+ phosphor. The composite Sr2MgSi2O7: Eu2+, Dy3+/Y3Al5O12: Ce3+ has a long afterglow with steady color during decay. To excite Y3Al5O12: Ce3+ phosphor, here we used the blue light energy of the Sr2MgSi2O7: Eu2+, Dy3+ as a phosphorescent light source. Our principal objective to achieve color tunable long persistent phosphors was obtained by combining these two, blue and yellow, light emissions. For detailed investigations, the phosphors were further characterized by XRD, Photoluminescence and afterglow decay. So our approach may be a way to convert the color of other long persistent phosphor by using remote phosphor.
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Flame‐excited luminescence (candoluminescence) has been measured for transition‐ion‐activated phosphors , , and . Willemite and the Cr‐activated phosphors exhibit candoluminescent spectra similar to emission spectra obtained by other excitations. The emission spectra of is profoundly different. Certain other common phosphors , , Pb, and do not luminesce under flame excitation. All emitting phosphors require a temperature of several hundred degrees centrigrade for most efficient emission.
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The thermal and optical characteristics of phosphor converted white light-emitting diodes (LEDs) with different phosphor concentrations ranging from 4 wt % to 13 wt % are investigated. The light output of LEDs with higher phosphor concentration is found to have larger degradation in constant current compared with pulse current than that with lower phosphor concentration. In addition, the junction temperatures of phosphor converted white LEDs raise with increasing phosphor concentration, so that the decreased phosphor conversion efficiency is observed both in pulse and constant current modes. The physical mechanisms for these observations are discussed. This study elucidates the phosphor dependent optical and thermal behavior of phosphor converted white LEDs.
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The strongest lines have been measured in 73 quasar spectra from the archives of the International Ultraviolet Explorer Satellite. The optically bright quasars observed with IUE were typically discovered by powerful radio emission or ultraviolet excess. They therefore should not be biased directly by observational selection with respect to ultraviolet line strength.
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The photoluminescence properties and quantum efficiency of potential red, green, and blue (RGB) phosphors are measured under near-ultraviolet excitation (380–420 nm). The suitability of several phosphors is discussed for their application in phosphor-liquid crystal displays (LCDs). K5Eu2.5(WO4)6.25, SrGa2S4:Eu, and BaMgAl10O17:Eu phosphors are chosen as RGB phosphors for making a phosphor-LCD prototype. The color coordinates and optical properties of phosphor-LCDs are compared to those of conventional LCDs.
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