Synthesis And Stability Research Of InP/ZnS Core/Shell Quantum Dots

InP quantum dots(QDs)are regarded as the most desirable candidate to replace the role of Cd Se QDs in the applications of bio-labeling,LEDs,solar cells,etc,because InP is more environmentally friendly compared to Cd based QDs,and could also offer a tunable emission from blue to near-infrared.Nevertheless,the studies and applications of InP QDs are rather sparse in comparison with Cd Se QDs,which are principally caused by significant difficulties in its synthesis.In this report,we developed a novel method for the synthesis of InPZnS/ZnS QDs by using zinc phosphide as phosphorus precursor,and the zinc and sulfur precursors were also added at the start of reaction,which allows the continuous injection of phosphine gas into the reaction,resulting in high quality InPZnS/ZnS quantum dots with emission up to 680 nm.The core synthesis and shell coating were separated by controlling the reaction temperature.During the first 30minutes,the temperature of reaction solution was kept at 250°C to grow the InPZnS core QDs.Then,the coating of ZnS shell was happened and kept about 1 hour to guarantee the complete decomposition of DDT after the reaction temperature was increased to 300°C.The biggest advantage of this synthetic method is the tunable emission region from blue to near-infrared.The effects of reaction parameters were systematically investigated.We observed that the molar ratio of In:myristic acid(MA)and that of In:Zn(S)had significant influences on the size of the InP QDs.The structure of InPZnS/ZnS QDs was confirmed by transmission electron microscope(TEM),X-ray powder diffraction(XRD),and Energy Dispersive X-ray analyzer(EDX).TEM characterization indicated the final core/shell InPZnS/ZnS QDs were good monodispersity with an average size of 7 nm.Furthermore,we investigated the versatility of this method by using other phosphorus precursor.The injection pump leaded to a continuous supply of phosphorus precursor on a timescale and reacted with indium precursor to form InP QDs.The final sample showed an emission at 710 nm.The present method gives access to larger sized InP QDs,making it prosperous for applications in biological labeling.As an effort to improve photostability of InP quantum dots(QDs),we synthesized InP QDs overcoating with ZnS shell via continuous injection method,followed by a photostability test.It was shown that the overgrowth of wide band gap ZnS shell could improve the photostability of InP QDs against light radiation.However,ZnS shell as a sulfide was intrinsically vulnerable to the presence of moisture and oxygen,which should be further protected.Then,a sol-gel process was developed using TMOS as precursor to further enhance the stability performance of InP/ZnS QDs.A significant improvement in photostability as well as thermal stability for InP/ZnS QDs-Si O₂ was observed in comparison with the InP/ZnS QDs,resulting from the formed Si O₂ protection layer.The InP/ZnS QDs-Si O₂ could maintain over 80%of its initial intensity after120 h of operation on the blue LED chip.Lastly,we adopted a convenient method to improve the photostability of InP/ZnS core/shell QDs by doping of Al into the ZnS shell,and the resultant QDs was labeled as InP/ZnS:Al QDs.According to the stability performance test,it was observed that the photostability of InP/ZnS:Al QDs obviously increased against intense illumination under blue LED module compared with the InP/ZnS QDs.The doped Al in the ZnS shell can be oxidized to Al₂O₃,which worked as a self-passivation layer and accounted for the enhanced photostability performance.