Single-phase Modified Zinc Sulfide Quantum Dots For Fabrication Of White LEDs

People are searching for substitutes to fossil fuel to decrease carbon dioxide pollution as they become more conscious of climate change and environmental issues.White light quantum dot diodes（WQLEDs）can significantly reduce global power demand and fossil fuel consumption due to their benefits of extended life,low power usage,and great luminous efficacy.In order to prevent the reabsorption between polyphase phosphors and conserve the use of rare earth materials in the production of diodes,this paper aims to synthesize a single phase non-rare earth quantum dots phosphors for the preparation of white emitting diodes.Zinc sulfide quantum dots（ZnS-QDs）are a kind of luminescent materials with excellent properties.It has the advantages of wide band gap,low toxicity（compared to cadmium and lead sulfides）,and low cost.In this paper,ZnS-QDs,Copper doped with zinc sulfide（ZnS:Cu-QDs）and Silicon dioxide coated copper doped zinc sulfide（ZnS:Cu@Si O₂-QDs）are synthesized.The fluorescence intensity and transmittance of ZnS-QDs were improved by doping Cu²⁺and coating Silicon dioxide（Si O₂）on the surface of quantum dots.The emission band of quantum dots is adjusted by changing the concentration of doped Cu²⁺.The main work is as follows:ZnS-QDs were synthesized by hydrothermal technique,the best reaction conditions were discovered through experimental tuning using zinc acetate as the zinc source and thiourea as the sulfur source.ZnS-QDs exhibit great crystallinity and a quasi-spherical shape..It belongs to the cubic sphalerite structure.The particle size is between 2.8±0.4 nm,and the thermal decomposition temperature is 680℃.The tricolor ratio（R,G,B）of the WQLEDs fabricated with ZnS-QDs were（R=14.6%,G=83.5%,B=1.8%）,color development index（CRI）was 76.6,color temperature（CCT）was 4500 K and chromaticity coordinate were（X=0.3725,Y=0.4006）.It appears white light in the CIE system.In order to enhance fluorescence intensity of ZnS-QDs and realize the advantage of adjustable emission band,Cu²⁺was introduced into the synthesized ZnS-QDs.ZnS:Cu-QDs with different fluorescence intensity and emission bands were obtained by adjusting the doped concentration of Cu²⁺.The fluorescence intensity of ZnS:Cu0.2%-QDs was the strongest,and the thermal decomposition temperature could reach 610℃.The luminescence mechanism of ZnS:Cu0.2%-QDs can be explained as electron emission from V_Znlevel to t₂ level of Cu²⁺.The color temperature range of the WQLEDs produced by ZnS:Cu-QDs with different Cu²⁺contents is 3851-4764 K,belonging to the range of warm white light to white light.In order to solve the problem of light transmittance of ZnS:Cu-QDs phosphor.ZnS:Cu0.2%-QDs shows that it is coated with a layer of Si O₂ shell.The optimal synthesis conditions were found by fluorescence emission spectrum.The results of XPS show that Si O₂ and ZnS:Cu-QDs are chemically bonded.It can be seen that the light transmittance of ZnS:Cu@Si O₂-QDs is significantly improved through the comparison experiment.The three-color ratio（R,G,B）of the ZnS:Cu@Si O₂-QDs is（R=19.1%,G=80.2%,B=0.7%）,the CRI is 71,the CCT is 3155 K and the color coordinates are（0.3570,0.3677）,which are located in the white light region of the CEI system.