Preparation And Luminescence Properties Of Rare Earth Doped Silicate Phosphors For White Light Emitting Diodes

Decent lighting source are the main desire of human being from the ages. The application of the incandescent lamp is from very beginning to the near past, where the linear tube and compact florescent lamp replaced the conventional incandescent bulb due to the resent awareness regarding energy consumption and environmental issue. The fundamental characteristics of lighting technology that is the energetic efficiency has been solved with the development of compact florescent lamp but the emission of mercury vapor during continuous use created a new environmental problem. Material Science involved in the challenge to search the alternate lighting source. Solid state lighting ?SSL? has many advantages over incandescent bulb and compact florescent lamp in terms of outstanding efficiency, mercury free emission,long life time.Nowadays, most white light is achieved by the combination of blue LED chip and yellow emitting YAG: Ce³⁺ phosphor. But the light obtained from this combination cannot fulfill the requirements of home and office lighting, due to the lack of red component. To obtain WLEDs with a high CRI, highly efficient blue-emitting,green-emitting, and red-emitting phosphors, which can be excited by blue or near-ultraviolet ?NUV? LEDs, must be produced. However, re-absorption phenomena in such tri-phosphor LEDs may also occur and cause a decrease in the device efficiency. Thus high stable and high quantum efficient single-phase phosphors,which emit a broader wavelength ?cover the full region of visible light?, might promote the solid state lighting technology to the general market. So the search of new efficient single-phase phosphors and suitable synthesis methods attract great attention worldwide. Rare earth (Eu²⁺/Ce³⁺/Mn²⁺/Tb³⁺) singly doped and co - doped silicate phosphors are the most attractive materials in this context because the 5d of Ce³⁺/Eu²⁺ are sensitive to the surrounding environment and the silicate offer various surrounding environment to the activators due to the high variety of crystal structure. Moreover, the silicate can be synthesized under atmospheric conditions,resulting in a lower cost than nitridosilicate and oxynitridosilicate.The present research was dedicated to the synthesis and characterizations of Ce³⁺/Eu²⁺/Mn²⁺/Tb³⁺ singly doped and co - doped silicate phosphors prepared by high temperature solid state reaction routes. The structure and luminescence characteristics along with the energy transfer mechanism among the activators are investigated in details.The present research work is divided in the following sections1. The first chapter of the thesis has been divided into two sections; First part of chapter 1st shows the comparison of different technologies used for the generations of artificial white light, the advantages of the solid state lighting technology over other lighting technology were highlight in details. The second part belongs to the synthesis and characterizations of the prepared phosphors;solid state reaction method is used for the preparation of phosphors. The structure analysis is confirmed by using XRD diffractogram analysis; room and high temperature measurement are performed. Also the energy transfer process among different activators is calculated.2. Chapter 2 reports the synthesis and characterizations of a novel full color emitting Ba9Lu2Si6024: Ce³⁺/Mn²⁺/Tb³⁺ (BLSO: Ce³⁺/Mn²⁺/Tb³⁺) phosphors by high temperature solid state reaction. XRD analysis shows that all the doped activators preferably occupy the Ba²⁺ sites in the host lattice. Efficient energy transfer was observed in double-activated and triple activated BLSO phosphors. The triple-doped phosphors with specific amount of Ce³⁺/Mn²⁺/Tb³⁺ ions concentrations, generated a tunable ?peaks at 424 nm,552 nm and 600 nm? white light, which had good CRI ?color rendering index? values. The remaining intensity of tri-activated Ba9Lu2Si6O24 phosphors was 77% when the temperature reached 150 0C.3. Chapter 3 exhibits the efficient energy transfer from Ce³⁺ to Tb³⁺ in CaLa4Si3O13 ?CLSO? phosphors. We synthesized the CaLa4Si3O13: Ce³⁺,Tb³⁺ phosphors by the high temperature solid state reactions. The doping of Ce³⁺ in the CLSO: Tb³⁺ phosphors considerably increased the green emission of Tb³⁺ ions. Efficient energy transfer from the sensitizer Ce³⁺ to the activators Tb³⁺ ions occurred and the energy transfer efficiency of the presented phosphors reached 93%. According to the Blasse and Dexter, the energy transfer from Ce³⁺ to Tb³⁺ is governed by a dipole-dipole interaction mechanism. The presented phosphor showed better thermal quenching property, as compared to the commercial green emitting Ba2SiO4: Eu²⁺phosphors.4. Chapter 4 illustrates the synthesis and characterizations of Ba9Lu2Si6O24(abbreviated BLSO: Eu²⁺) phosphors. The emission of the BLSO: Eu²⁺phosphors are efficiently tuned from dark blue ?463 nm? to efficient green?514 nm? by simply increasing the Eu²⁺ concentration. The red shifts in case of excitation and emission are discussed on the base of the occupation of Eu²⁺ in BLSO host. The issue of the decrease in the intensity on the base of concentration quenching could be solved by introducing Sr²⁺ and Ge4+ in the BLSO phosphors host. The room and high temperature photoluminescence were discussed.5. Chapter 5 reports the study of CaxBa?9-x?Lu2Si6O24: Eu²⁺/Mn²⁺ (CBLSO:Eu²⁺/Mn²⁺) solid solutions and their respective photo luminescence properties. We successfully synthesized pure CaxBa?9-x?Lu2Si6O24: Eu²⁺/Mn²⁺phosphors under the NH3 atmosphere by solid state reactions methods. The analysis of the XRD diffractograms confirmed the efficient solid solutions of CaxBa?9-x?Lu2Si6O24. With the increase of the Ca²⁺ contents, the emission became broad to cover both the blue and green regions The efficient introduction of Mn²⁺ in the optimized Ca1.5Ba7.5Lu2Si6O24 phosphors was performed, whose respective photoluminescence along with the transfer of energy among the activators were discussed in detailed.6. Chapter 6 shows the conclusion of the present dissertation and research prospect.