Study On The Structure And Luminescent Properties Of Carbon Modified Nitride Phosphor With High Thermal Stability

As the fourth-generation of green solid-state lighting sources,white-light-emitting diodes(wLEDs)have become the mainstream of general lighting market due to their advantages of high luminescence efficiency,small size,long lifetime and environmental friendly.Phosphors,as one of the core component of wLEDs,is the key factor to determine the color quality of wLEDs.High power white-light-emitting diodes(HP-wLEDs)develops rapidly and have important applications in automobile headlights,landscape lighting,outdoor large screen,traffic signal lights,special lighting and other fields.Compared with ordinary wLEDs,HP-wLEDs have higher working current density,which put forward higher requirements on the thermal stability of phosphor materials.Nevertheless,current commercial nitride phosphors still suffer from lacking thermal stability for using in HP-wLEDs.Hence,enhancing thermal stability of Eu2+-activated nitride phosphor is an essential challenge.In this paper,three carbon modification methods are proposed based on the strategies of host doping and superficial modification,with carbon as the core modification material,so as to effectively improve the thermal stability of Eu2+-activated nitride phosphor,and the effects on the structure and luminescent properties of the phosphor are analyzed mechanically.According to the host doping strategy,a series of red-emitting carbidonitride phosphors Ca0.992AlSiN3-4/3xCx:0.008Eu2+(x=0,0.06,0.12,0.18,0.24)are designed and synthesized by carbon doping into the lattice of CaAlSiN3:Eu2+.The carbon-doped sample exhibits an obviously enhanced thermal quenching performance which is improved by 8.9%at 300?.The preferential crystallographic site of carbon in CaAlSiN3 lattice is validated by the first-principles DFT calculations and the Rietveld refinement.According to crystal field theory,the slightly blue-shifted emission caused by the changes of local structure of Eu2+ due to carbon doping is discussed.According to the thermal ionization model,the enhancement of thermal quenching performance is ascribed to the enlargement of thermal ionization energy barriers for the nitride phosphors.Also,a wLED fabricated by the combination of a blue chip with the as-synthesized red phosphor and LuAG:Ce3+green phosphor verifies its promising application for HP-wLEDs.According to the superficial modification strategy,a superficial carbon modification method consisting of carbon coating and thermal post-treatment to enhance thermal stability of red-emitting CaAlSiN3:Eu2+phosphor is proposed and achieved.Based on the principle of minimizing the loss of brightness,the optimal technological parameters of superficial carbon modification are determined.During the whole processing,the route of carbon and its effects on the crystal structure and luminescence properties are investigated in detail.After superficial carbon modification,the surface local structure of CaAlSiN3:Eu2+phosphor is changed,but the host structure is not changed.The modifications on the crystal surface include(1)the introduction of carbon,(2)the elimination of amorphous phase and(3)the prevention of the oxidation of Eu2+.The introduction of carbon strengthens the rigid of structure and causes a tiny blue-shift of emission.The whole processing reduce the damage for both activator ions and nitride host on the surface of phosphor during use,so that the superficial modified CaAlSiN3:Eu2+samples still maintain good luminescence performance for long term use.According to the superficial modification strategy,a superficial organic carbon modification method consisting of solution mixing and thermal post-treatment is proposed and achieved for red-emitting(Sr,Ca)AlSiN3:Eu2+phosphor,which could not only enhance thermal stability,but also improve luminescent brightness.As for the superficial organic carbon modified sample with the best comprehensive performance,the relative brightness is strengthened by 2.15%,the thermal quenching performance is improved by 8.9%at 300?,and the relative brightness remains 99.1%with respect to that measured before the 200 h aging test.Under the double reduction effect of organic carbon and N2-H2 atmosphere,the superficial organic carbon modification could not only reduce the oxidation of Eu2+,but also prevent the formation of impurity phases on the surface of phosphor during use,which are the mechanisms of the enhancement of luminescent brightness and thermal stability.