Enhancement of Photoluminescence from Organic and Inorganic Surface Passivated ZnS Quantum Dots
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ENHANCEMENT OF PHOTOLUMINESCENCE FROM ORGANIC AND INORGANIC SURFACE PASSIVATED ZnS QUANTUM DOTS Hatim Mohamed El-Khair, Ling XU, Minghai LI, Yi MA , Xinfan HUANG and Kunji CHEN National Laboratory of Solid State Microstructures and Department of Physics Nanjing University, Nanjing 210093, China ABSTRACT ZnS quantum dots (QDs) chemically synthesized in PVP stabilizing medium have been coated with Zn(OH)2, SiO2 and polystyrene (PS) shells as inorganic and organic passivation agents. to synthesize ZnS/Zn(OH)2, ZnS/SiO2 and ZnS/PS QDs. PL properties of inorganically passivated ZnS/Zn(OH)2 and ZnS/SiO2 had reported band edge enhancement of 8-10 times, while organically passivated ZnS/PS QDs exhibit tremendous enhancement of band edge emission as much as 10-15 times,. Therefore inorganic and organic coating can passivate trap states of different energies on the surface of ZnS QDs. INTRODUCTION Quantum confinement phenomena have been observed in II-VI compound semiconductor quantum dots whose sizes are comparable to Bohr radii.[1] By adopting various preparatory and passivation techniques monodispersed QDs with high quantum yields (>50%) can be obtained. Organic surfactants such as gelatin,[2] polyvinyl pyrrolidone-K30 (PVP),[3] sodium dioctyl sulfosuccinate (AOT),[4] were used as stabilizing agents for QDs chemically grown in different solvents. Large surface to volume ratio in semiconductor QDs made their optical properties to be affected by physical properties of their surfaces.[1] Hence detectable surface trap states of different energies have been observed in luminescence properties from different bare QDs such as CdS[5] and CdSe.[6] Recently new synthesis techniques based on organo metallic interface such as emulsifier,[7] molecular adhesion[8] and zeolite architecture[9] can not only improve the structure and properties of semiconductor QDs but can also give rise to new material with photonic band gap phenomena.[10] Semiconductor QDs coated with inorganic shell such as CdSe/CdS[11] and CdSe/ZnSe[12] and organic shell (TOPO) have exhibited enhanced band edge emission of high quantum yield .[1] In this work we are enabled to synthesize ZnS QDs in PVP/ethanol and PVP/aqueous stabilizing medium. Zn(OH)2, SiO2 and polystyrene (PS)were used as inorganic and organic surface passivation agents to chemically synthesized ZnS/Zn(OH)2, ZnS/SiO2 and ZnS/PS QDs, respectively. The band edge emission has been enhanced as much as 8-10 times for inorganic coating and 10-15 times for organic coating. EXPERIMENT G3.1.1
Synthesis of ZnS QDs We described in detail the synthesis and capping of monodispersed ZnS QDs using polyvinyl pyrrolidone-k30 (PVP) stabilizer in our previous paper [13]. 200 ml PVP/ethanol (5%) solution was prepared as QDs stabilizing agent. Zn(NO3)2 and Na2S dispersed in water and ethanol solvent (5%) were used as precursors for Zn++ and S-- ions, which reacted according to the following equation:
Zn( NO3 )2
+ Na2 S → ZnS + 2NaNO3
The reactions were carried out at room temperature and in the air. By varying of th
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