The Hydrothermal Synthesis And Size Regulation Of CdS And CdSe Semiconductor Nanocrystals

Semiconductor quantum dots?QDs?have attracted more and more attention and have been well researched in recent years because of their promising application prospect in optical devices,solar cells,biosensors and bio-imaging.Semiconductor quantum dots have a lot of advantages,such as high fluorescence quantum yield,tunable emission wavelength and excellent stability.II-IV group semiconductor nanocrystals,especially CdS,CdSe and CdTe,have been widely studied.It is because not only the emission in the visible range can be changed only by the adjustment of particle sizes,but also their preparation methods are more advanced than other quantum dots.Organic metals synthesis routes which is also called hot injection method generally produce the best quality nanocrystals.However,subsequent ligand exchange procedure is essential to transfer nanocrystals to aqueous phase with respect to biological applications.Besides,the reagents used in the organometallic route are highly biotoxic and expensive.From the point of view of production and manufacturing capacity on large-scale in dustry,it is also very difficult.Thermal injection mode and high energy synthesis environment have greatly limited its mass production.In parallel with the success of organic synthetic routes,aqueous routes have also been developed to tackle problems related to phase transfer.Direct synthesis in aqueous medium can avoid the decline of optical properties,including fluorescence intensity,fluorescence quantum yield and optical stability.The successful synthesis of aqueous phase CdS NCs and CdSe NCs via one-pot non-injection method has been reported.The anions,cationic precursors and ligands mixed at room temperature and heated at the appropriate temperature can form nanocrystals.CdS and CdSe quantum dots with fluorescence properties can be synthesized via similar methods,and have high reproducibility.The obtained quantum dots have high crystallinity,wide size adjustable range,and the fluorescence yield can be increased by constructing the core-shell structure.The high temperature hydrothermal method used in this paper is different from the traditional refluxing method which reacts lower than 100?.It greatly increases the reaction rate at the temperature higher than 100?,and makes up the disadvantages that time consuming,energy waste and difficult to synthesize large size quantum dots of the conventional reflux method.And the quality of quantum dots is not worse even better than that of reflux method.Using CdCl₂ as cadmium source,thiourea as sulfur source and mercapto acid as ligand,water-soluble CdS NCs were synthesized by a simple and rapid non-injection high-temperature hydrothermal method.The effects of reaction temperature,pH and ligands/Cd on the growth of nanocrystals were monitored by the temporal evolution of CdS NCs absorption spectrum.using CdCl₂ as cadmium source,selenium urea as selenium source and sodium citrate as ligand,water-soluble CdSe NCs were synthesized by a simple and rapid high-temperature hydrothermal method.The effects of reaction temperature,concentration of ligands,pH and Cd/Se on the growth of nanocrystals were monitored by the temporal evolution of CdSe NCs absorption spectrum.Appropriate pH value and slower growth rate are the key factors affecting the quality of CdS nanocrystals.The CdS NCs of 1.9nm-5.3nm and CdSe NCs of1.6nm-6nm can be obtained by optimizing the reaction conditions.The prepared CdS QDs and CdSe QDs were further characterized by UV absorption spectra,fluorescence spectra,X-ray diffraction?XDR?and transmission electron microscope?TEM?.Although the quality is not comparable to organic phase quantum dots,the prepared CdS NCs and CdSe NCs have narrow size distribution,good crystallinity and dispersion,which is considered as high quality CdS NCs and CdSe nanocrystals in aqueous phase.