Rare Earth Ions Doped Phosphors

Decent lighting source are the main desire of human being from the ages.The application of the incandescent lamp is from the beginning to the near past,whereas the linear tube and compact florescent lamp replaced the conventional incandescent bulb due to the energy consumption and environmental issues.The fundamental characteristic of lighting is the energetic efficiency,has been partly solved with the development of compact florescent lamp.On the other hand,environmental problems arise due to the continuous emission of mercury.Researchers accepted this challenge and gave alternative approach to solve these problems.Solid state lighting(SSL)has many advantages over incandescent and compact florescent lamps,such as high luminescence efficiency,low energy consumption,long lifetime,high safety coefficients,and are environment friendly.Nowadays,commercial white light emitting diodes are usually fabricated by combining blue chip with yellow Y3Al5O12:Ce3+phosphors.This type of white light emitting diode can provide high conversion efficiency close to the theoretical maximum,but low color rendering index and high correlated color temperature restrict its applications due to lack of red color emission compared with sunlight.Therefore,red phosphors are needed to solve this problem.The alternative approach to obtain warm white light with high color rendering index,near-ultraviolet chip is combined with tricolor(red,blue and green)phosphors have been proposed.These two techniques require red phosphors;however,the unstable nature of commercially available sulfide-based red phosphors restrict device performance for long-time operation.Therefore,it is necessary and challenging to find novel red-emitting phosphor.Moreover,multi-color phosphors are not easy to make because phosphors are prepared individually,the particle size of the individual phosphor must be adapted to one another to avoid agglomeration or sedimentation,and the final product must be mixed homogeneously in exact ratios.To solve this problem,another way to obtain warm white light is single-phased phosphors in view of their higher luminescence efficiency,lower manufacturing costs,easier fabrication processes,higher color rendering index and better reproducibility.The ultraviolet-light emitting diodes coated with single-phased phosphors are very promising candidates to make white light emitting diodes as they have many advantages over multiphase phosphors.The present research work can be divided in the following sections.1.The first chapter of this thesis gives the importance of title selected for research.It also compares artificial lighting technologies with solid state lighting.The advantages of solid state lighting technology are elaborated.Luminescence phenomena in phosphors,characteristic of luminescence including effect of crystal field,influence of activator contents and configurational coordinate model are discussed in detail.Moreover,luminescence phenomenon in rare earth activated phosphors and properties of ideal phosphors are explained.Furthermore,this chapter covers the characterizations and optical properties of prepared phosphors using X-ray diffraction,scanning electron microscopy,photoluminescence spectroscopy and high temperature photoluminescence.2.In second chapter,pure phase Tm3+,Dy3+,Sm3+and Eu3+co-activated Y2WO6 single crystal phosphors are synthesized through a hydrothermal method-Upon the excitation of 311 nm,Tm3+,Dy3+,Sm3+and Eu3+show tremendous emission properties in their corresponding regions.The optimum concentration of Tm3+for Y2WO6 phosphors is measured to be around 4%.The energy transfer efficiency from Tm3+to Dy3+ increases gradually with increasing concentration of Dy3+,and reaches up to 72%at 10%Dy3+.The critical distance between Tm3+ and Dy3+ is calculated to be 16.77 A.Hence,dipole-dipole interaction dominates the mechanism of energy transfer.The color tone of Y2WO6:Tm3+,Dy3+ phosphors was effectively tuned to warm white light by co-doping Sm3+or Eu3+.Furthermore,the energy transfer phenomenon from Tm3+,Dy3+ to Sm3+ and Eu3+ is investigated.The photoluminescence properties of Y2WO6(Tm3+,Dy3+,Sm3+or Eu3+)phosphors renders them potential candidates for ultra-violet excited white light emitting diodes.3.In third chapter,to develop potential phosphors used in UV based WLEDs,a novel red Na3.6Y1.8(PO4)3:Eu3+(NYP:Eu3+)phosphor was successfully prepared by a solid-state method.Rietveld refinement analysis determined that NYP has a cubic unit cell with space group Pm-3m.The structural properties were analyzed in detail by employing X-ray diffraction(XRD),transmission electron microscopy(TEM)and energy dispersive spectrometry.Scanning electron microscope(SEM)image revealed the irregular morphology of NYP:Eu3+ particles and elemental mapping verified that Eu3+ ions were uniformly distributed on Y3+sites.PL spectra exhibited strong red emission under the excitation of 394 nm and optimal Eu3+doping concentration in the NYP host was determined to be 30 mol%.The critical distance was calculated to be about 13.954 A and dipole-dipole interaction was responsible for the concentration quenching mechanism.NYP:Eu3+phosphor exhibited high thermal stability with emission intensity at 424 K was 97.4%of that at 298 K.Moreover,by employing the title red,commercial green(Ba,Sr)2SiO4:Eu2+,blue BaMgAl10O17:Eu2+phosphors and near UV light-emitting diode chip,a WLED device was fabricated,which possessed excellent CIE chromaticity coordinates of(0.3351,0.3499)and high color rendering index(Ra=87.5).4.The fourth chapter gives the conclusions of research projects.