Effect Of Zinc Sulfide Coating To The Properties Of Multivariate Sulfide Quantum Dots

Quantum dots are semiconductors with dimensions in nano-scale.Due to the quantum size effect,their band gap are connected with the particle size.When being excited by light or electric field,quantum dots can emit fluorescence of different colors.This phenomenon has drawn much attention in photovoltaic materials and other fields.However,quantum dots prepared from conventional methods are still unstable in air,since they have large surface areas,which can easily react with water and oxygen,leaving defects on the surface and thus quench fluorescence.Therefore,a layer of inorganic compounds with wide band gaps is generally applied to coat quantum dots,which helps isolate quantum dots from external environment and eliminate the defect on the surface.In this work,the CuInS₂ quantum dots coated with different amounts?10,30,and 50 times of the molar ratio of CuInS₂?of ZnS were synthesized using continuous injection method.The samples were mono-dispersed with uniform size distribution.The crystal structure of the quantum dots gradually swithed from chalcopyrite structure of CuInS₂ to Zinc blende structure of ZnS.TEM images showed the average size of quantum dot increased with increasing ZnS amount.Base on TEM images,we calculated the CuInS₂ quantum dots were coated with 1,3 and 7 monolayers of ZnS.The ZnS shell effectively enhanced the optical performance and stability of quantum dots.The quantum yield increased to 49%with increasing ZnS shell thickness.In photostability test,quantum dots coated with 7monolayers of ZnS maintained 20%of its original fluorescence intensity after 7 hours of continuous irradiation.The ZnS shell also improved the chemical stability of quantum dots with increasing shell thickness.Chemical stability test showed that quantum yield of CuInS₂ coated with 1monolayer of ZnS decreased from 67%to 42%while the quantum yield of CuInS₂ coated with 7 monolayers of ZnS remained alomost unaffected by changes to external chemical environment.CuInS₂ quantum dot coated with1 and 7 monolayers of ZnS were transferred into optical adhesive and applied to LED devices.After same irradiation time,the relative intensity of samples coated with 1 monolayer of ZnS dropped to 16.2%its original intensity,while samples coated with 7 monolayers of ZnS maintained 33.9%its original intensity.This proved CuInS₂ coated with multiple monolayers of ZnS possessed high stability in solid phase.Copper-tin co-doped CdS quantum dots with different doping concentrations were synthesized.The influence of reaction temperature and doping concentration on their properties were investigated.The result showed the absorption peak gradually red shifted with increasing reacting temperature.With increasing doping concentration,the fluorescence peak FWHW?full width at half maximum?gradually decreased,the UV absorption peak gradually red shifted and the stokes shift decreased.XRD test showed that Cu and Sn have high lattice compatibility with CdS crystal,thereby forming a solid solution structure.Finally,by coating the Cu/Sn co-doped CdS with ZnS shell,we effectively increased the fluorescence intensity of quantum dots.The absorption peak blue shifted at first,then red shifted when extending the coating time.This may be Zinc replacing the Cadium in quantum dots and forming narrow bandgap compounds and consequently leading to the red shift of absorption peak.The flourscence intensity of the coated samples gradually decreased with increasing the coating time.