Studies On The Synthesis And Luminescence Behaviors Of Red Nitride Phosphors

Facing fossil fuel depletion and global climate changes, energy saving and environmental protection has become the realistic requirements for sustainable development for current economy and society. Owing to its high energy efficiency, duration, reliability, economical environment and small size, the w-LEDs based on solid state lighting is expected to replace the incandescent lamp and fluorescent lamp for the third generation lighting source. In recent years, nitride phosphors, like the representative ones M2Si5N8:Eu2+(M =Ca, Sr, Ba) and Ca Al Si N3 :Eu2+, have drawn wide attentions in phosphor-converted light emitting diodes(pc-LEDs) due to their excellent photoluminescence properties and stabilities. Usually, the synthesis of nitride phosphors uses metal nitrides as raw materials, which requires high temperature, high pressure, anhydrous and oxygen free environments, resulting in tedious preparation procedure and high cost. Though the carbothermal reduction and nitriding method can simplify the preparation conditions, the carbon residues seriously attenuate the luminescence properties of the products. Then further operation of carbon removal is needed.Afterglow materials occupy an important position in the field of rare earth luminescent materials and possess great application value in safety signal, emergency lighting, paint, in vivo imaging and information storage. C urrently, researches about afterglow materials mainly focus on blue and green light-emitting materials, such as Sr Al2O4: Eu2+,Dy3+(green) and Ca Al2O4:Eu2+,Nd3+(blue), which been put into industrial production and practical application. Although red afterglow materials have obvious advantages for in vivo imaging, the research progress about the red afterglow materials is quite slow and the available material is relatively little. Common red afterglow materials, like Ca S:Eu2+ and Y2O2S:Eu3+, still suffer from toxicity and poor stability.In this paper, rare-earth-doped nitride phosphors, which own high luminous efficiency, good chemical and thermal stability, high thermal quenching temperature and tunable emission wavelength, were chosen as research objects. Specifically, we try to obtain high-quality nitride phosphors for LED applications at atmospheric pressure after selecting suitable raw materials, reducing agent and optimizing preparation conditions. Also, we IV investigated the afterglow properties of nitr ide phosphors by introducing appropriate traps(oxygen vacancies and trivalent rare-earth ions). In addition, new test methods were used to further explore the luminescence properties of nitride phosphors and probe the defect luminescence mechanisms. The main research results are as follows:(1) Red-emitting nitridoaluminosilicate phosphor Ca Al Si N3 :Eu2+ was prepared by high temperature solid-state reaction method using calcium hydride(Ca H2) as the reducing agent and calcium source. Influence of synthesis temperature on the crystalline structure, morphology and photoluminescence properties of Ca Si Al N 3 :Eu2+ was examined in detail. Meantime, the relevance of oxygen content and photoluminescence properties was discussed. After that, dependence of photoluminesce nce and diffuse reflection spectra on Eu2+ ion concentration was investigated. Surprisingly, red persistent luminescence was observed in Ca Al Si N3 : Eu2+ with a longer duration in low Eu2+ concentration regions(x ? 0.01).(2) Based on previous work, this experiment investigated the persistent luminescence in Ca Al Si N3:Eu2+. The electron traps was introduced by the partial substitutions of O in N sites to form positive trap levels at low Eu2+ concentration. The broad thermoluminescence emission bands indicated multiple trap levels: the co-existence of shallow traps and deep traps. Under UV-NIR simultaneous excitation, the phosphors performed enhanced photoluminescence and phosphorescence. It is expected that the UV-NIR simultaneous excitation may be a promising technique to investigate the trap distributions in optical energy storage materials.(3) In this part, optical energy storage properties of(Ca1-x Srx)2Si5N8:Eu2+,Tm3+(x = 0-1) solid solutions were tuned by cation substitution. The solid solutions exhibited similar persistent- and photo-stimulated luminescence behaviors with a maximum intensity at about x = 0.2. Such optical energy storage characters of the samples were attributed to the more appropriate trap depths(322-333 K) and higher density of energy level traps indicated by the thermoluminescence analysis.(4) In the present work, temperature-dependent photoluminescence, afterglow decay curves and photo-stimulated persistent luminescence of Ba2Si5N8:Eu2+ phosphor were investigated in detail. Evaluations of energy transfer at two emitting sites and luminescence quenching were based on careful investigations of photoluminescence at various temperatures. Furthermore, we brought up an integrated temperature-dependent afterglow decay curves method to estimate the trap density distribution in Ba2Si5N8:Eu2+ phosphor, accompanied with thermo- luminescence and photo-stimulated persistent luminescence analysis. Results indicate integrating afterglow decay curves of various temperatures can be a useful attempt to detect trap distribution in charge carrier trapped materials.