Preparation And Luminescence Properties Of Sulfur/oxide Upconversion Luminescent Materials

Up conversion luminescent?UCL?materials have great potential applications in many fields such as infrared detection,molecular recognition,biological fluorescence label and three-dimensional display technology.For application,many properties of the UCL materials need to be considered,for example,the physical and chemical stability,color purity and luminescence efficiency and so on.In addition,though the UCL process caused by?980 and?800 nm excitations have been researched extensively,studies on UCL properties of powder materials excited by 1550 nm LD are still scarce.In view of the problems mentioned above,oxide,multiple oxides and oxysulfide UCL materials doped with different kinds of rare earth ions were successfully synthesized by corecipitation and solid-state methods.The UCL properties of the materials were optimized and the transition mechanisms of the UCL processes were discussed in detail.The main works of this paper are as follows:The spherical Y₂O₃:Er³⁺,Yb³⁺ nanocrystals with average particle size of 30 nm were prepared by coprecipitation method.High calcination temperature could obviously improve the crystallization degree of the sample and leaded to an enhancement on UCL intensity.Under the 980 nm excitation,the typical emission peaks of Er³⁺ ion locates at about 530,550 and 660 nm,which are derived from 2H11/2?⁴?15/2,4S3/2?⁴?15/2 and 4F9/2?⁴?15/2 transitions,respectively.The down conversion emission observed at 1530 nm corresponds to ⁴?13/2?⁴?15/2 transition.The quenching concentration of Er³⁺ in Y₂O₃:Er³⁺,Yb³⁺ nanocrystalline is only 1.0 mol%,which is associated with the phonon condision and smaller spatial scale of the nanoparticles.A relatively high Yb³⁺ content?6mol%?can support high efficient Yb³⁺?Er³⁺energy transfer.However,the reverse energy transfer from 4S3/2(Er³⁺)to 2F7/2(Yb³⁺)can also be improved,which can result in the luminescence quenching of green emission and purify the red emission?IR/IG=19.55?.Accordingt to the intensity variation trend of 1530nm infrared emission,the transition mechanism of Er³⁺-Yb³⁺ system in Y₂O₃ nanocrystalline was supplemented.Near-spherical Y₂O₂S:Ho³⁺,Yb³⁺ nanocrystals?NCs?were prepared by coprecipitation method followed by a solid-gas sulfuration technic.The nanocrystals are in hexagonal structure,and the particle size is 40 nm on average.Under the excitation of 980 nm laser diode?LD?,three different emissions located at 545 nm,655 nm,1180 nm can be observed,which are derived from 5F4/5S2?5?8,5F5?5?8,5?6?5?8 transitions,respectively.The UCL efficiency of Ho³⁺ in the nanociystals can be improved obviously by co-doping Yb³⁺.However,because of the high-energy vibration quantum produced by surface adsorption,the red emission could be held up hardly.The intensity ratio of green to red emission??G/?R?is only 3.75,which is much smaller than that of micron Y2O2S?46.6?.When Yb³⁺ content is higher than 6 mol%,?5F4/5S2,2F7/2???5?6,2F5/2?inverse energy transfer make the green and red emission intensities decrease sharply,but this process is beneficial to the population of 5?6 energy level and leads to an enhancement on IR emission intensity.The spherical Y₂O₃:Tm³⁺,Yb³⁺ nanocrystalline wes also prepared by coprecipitation method.The average particle size is about 30 nm.The typical emission peaks of Tm³⁺ ion locates at about 475 and 650 nm under 980 nm exitation,which are derived from 1G4?3H6 and 1G4?3F4 transitions,respectively.High calcination temperature is benefit to improving the crystallization degree of the nanoparticles and obtaining a high UCL intensity.According to the analysis on transition mechanism,the UPL process of the nanocrystals is realized through a three step energy transfer of Yb?Tm.The highest luminescence intensity can be obtained when the content of Yb³⁺ was raised to 3 mol%.The red emission of Tm³⁺ was suppressed well,which provided a high purity blue emission.Finally,the prepared Y₂O₃:Er³⁺,Yb³⁺,Y2O2S:Ho³⁺,Yb³⁺ and Y₂O₃:Tm³⁺,Yb³⁺ nanocrystals composed of the nanosized UCL materials with tricolor luminescence property.Solid-state method was employed to synthesize a novel Ba5Zn4Y8O21:Er³⁺,Yb³⁺upconversion phosphor.The phosphor crystalized well and presented granular appearance with average particle size of 1?5?m.Under the excitation of 980 nm LD,typical emissions and transition properties of Er³⁺ can be obtained.The quenching concentration of Er³⁺ is as high as 3 mol%,and the highest UCL intensity appeared when the content of the codoped Yb³⁺ was raised to 7mol%.Because of the high doping concentration of Er³⁺ and Yb³⁺,cross relaxation of ⁴?11/2(Er³⁺)+ 4F7/2(Er³⁺)?4F9/2(Er³⁺)+ 4F9/2?Er34?and reverse energy transfer of 43/2(Er³⁺)+ 2F7,2(Yb³⁺)?4113/2?Er34?+ 2F5/2(Yb³⁺)become more effective,so that the intensity of red emission is significantly stronger than that of green emission??R/?G=12.0?.Improving the excitation power density can not only enhance the UCL,but also improve the color purity of red light.Under high power excitation,the blue and blue-green emisions caused by three photon absorption can also be observed.Ba₅Zn₄Y₈O₂₁ UCL phosphor mono-doped with Er³⁺ was prepared and used for the systematically study of UCL properties excited by 1550 nm LD.The results show that the peak locations of red and green characteristic emisions of Er³⁺ ion have no changed.The highest UCL intensity was obtained when the doping content was 7 mol%.The curve of emission intensities as a function of pump intensity show that both red and green emissions are generated by three photon absorption process.According to the transition mechanism,the lifetime of ⁴?11/2 level in Er³⁺ ions is much higher than that of ⁴?_9/2 level.This result makes the red emission intensity is always 4-6 times larger than that of green emission.High excitation power density can improve the efficiency of excited state absorption of ⁴?g/2 energy level,and improve the proportion of green light to a certain extent.With the help of Er³⁺?Yb³⁺ reverse energy transfer,the intensity of green emission can be effectively reduced.When the content of Yb³⁺ is greater than 15mol%,the red emission with high purity can be achieved and the value of ?r/?g can up to 62.28.When the excitation power density is higher than 700mW/cm2,the blue?405?420nm?,blue-green?460nm?and IR?810nm?emission can also be observed,which are correspond to the transitions of 2H9/2?⁴?15/2,4F5/2?⁴?15/2 and ⁴?_9/2?⁴?_15/2,respectively.