Investigation On Synthesis And Characterization Of Metal Chalcogenide Semiconductor Nanocrystals

Recently,metal chalcogenide semiconductor nanomaterials have become an active field among physics,chemistry and materials,because they have unique physical and chemical properties and their wide applications in optoelectronic devices and biological medicine.It is interesting to study and develop new metal chalcogenide semiconductor nanocrystals in combination with their optical,electrical and magnetic properties.In this dissertation,this work is mainly focused on CdSe nanocrystals,Cu₂S-In₂S₃ heterostructure nanocrystals and cuprous ions doped CdS nanocrystals,and the study of their applications in light-emitting diodes,photovoltaic devices,self-assembled into superlattices and their optical properties.Firstly,CdSe nanocrystals were synthesized in aqueous solution by using mercapto molecules as the stabilizer.The nanocrystals were transferred from aqueous solution into organic phase through static effect by using a surfactant with positive charge.The multilayered light-emitting diodes were fabricated using the blends of CdSe and PVK with different mass ratios of CdSe to PVK.The effects of applied voltages,the mass ratios of CdSe to PVK and the particle sizes of CdSe nanocrystals on the electroluminescence(EL) from the devices were studied,and the photoluminescence (PL) and EL from CdSe:PVK blends were compared.The luminescence mechanism of the light-emitting diodes based on CdSe:PVK nanocomposites were proposed based on the above results.It can be found that the energy transfer from PVK to CdSe and the cartier injection play different roles in the luminescence from the devices when the different applied voltages were applied and the different-size CdSe nanocrystals were used in the devices.Secondly,spherical Cu₂S nanocrystals were synthesized through one-pot synthetic method under high temperature,and matchstick-like Cu₂S-In₂S₃ heterostructure nanocrystals and In₂S₃ nanorods were also constructed by multi-injection of precursors. The photovoltaic devices were fabricated by using the blends of the nanocrystals and poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene](MEH-PPV) as active layers.As compared to the photovoltaic performance of the devices based on the blends of Cu₂S or In₂S₃ nanocrystals alone and MEH-PPV,the power conversion efficiency of the photovoltaic device based on the blends of Cu₂S-In₂S₃ and MEH-PPV was enhanced by～3-5 times.This improvement is consistent with the improved exciton dissociation and better charge transport abilities in the semiconducting heterostructure nanocrystals. This method may supply a class of new materials to design high-efficiency nanoerystal/ polymer hybrid photovoltaic devices.Finally,undoped CdS and Cu(â… ) ions doped CdS nanocrystals were successfully prepared in dedocanethiol using one-pot synthetic method under high temperature.The luminescent properties of doped CdS nanocrystals with different concentrations of Cu(â… ) ions.It is found that the emission from surface-related luminescence to the red emission of Cu(â… ) ions in CdS nanocrystals was dominant in the PL spectra with different dopant concentrations.The maximum fluorescence quantum yields of the doped CdS nanoerystals can reach 15.8%.On the other hand,the doped CdS:Cu(â… ) nanocrystals can be self-assembled into highly-ordered 2D or 3D supedattices when the dopant concentration is appropriate.The size-distribution,synthetic condition and morphology of CdS:Cu(â… ) nanocrystals with different dopant concentrations were studied,and we preferred to attribute the superlattiees formation to a polar system created on the nanoerystal surface originating from electric dipole interaction between the adjacent particles,which promoted rapid and spontaneous superlattice formation.Meanwhile, monodispersed Cu₂S nanocrystals were also formed because excess Cu(â… ) ions can react with dodeeanethiol.This method can be extended to prepare other metal chalcogenide semiconductor nanomaterials.