Direct Evaluation about Structure and Optical Property of Individual Multi-Shell Quantum Dot By TEM

Abstract

Quantum dots (QDs) such CdSe nanocrystals attract much attention owing to their promising applications in light emitting devices and biomedical tags. They are usually coated by the wider band materials such CdS and ZnS in order to improve the photoluminescence quantum yield together with the chemical protection. However, it is not clarified the relationship between optical properties and core/shell particle structure, and we should optimize the coating treatment condition and the core/shell structure (shape and thickness), throughout many experiments. It is important to clarify the relationship between optical properties and particle structure. Direct evaluation about the optical properties of the individual particles is very effective. As we all know, TEM is a powerful tool for the shape structure evaluation of nanoparticle. In addition, even the optical properties from UV to NIR of individual single nanoparticles can be evaluated by the HR-EELS using the monochromator TEM. In this study, we have investigated the particle structure and optical property using advanced TEM technique. We have synthesized CdSe, CdSe/CdS and CdSe/CdS/ZnS multi-shell particles. The optical properties of individual single nanoparticles have been evaluated by HR-EELS using the monochromator TEM in addition to the usual particle structure evaluation by HR-STEM and EDX. All nanoparticles were synthesized in an organic solvent with some ligands such as amine, oleic acids and trioctylphoshine. The synthesized and refined core CdSe particles were re-heated together with CdS raw material in an organic solvent, and we have obtained the CdSe/CdS core/shell particles. The CdSe/CdS/ZnS core/shell/shell particles have been also obtained in the similar treatments. We have evaluated the particle size and shape by STEM. Their size and shape was homogeneous. CdSe core particles were spherical and ca. 5 nm. CdSe/CdS and CdSe/CdS/ZnS particles were cubic shape, and ca. 10 nm and 15 nm. In STEM-EDX analysis, we have been able to confirm the core/shell and core/shell/shell structure. We could obtain some spectral structures attributed to interband transitions in the CdSe core and CdS shell by EELS analysis. However, the spectral structure about ZnS outer shell could be hardly confirmed. This would be attributed the low crystallinity of ZnS. Through the detail investigation of spectra, dielectric property of multi-shell nanoparticle is clarified and relation with the photo luminescent property will be discussed. [Acknowledgement] The research was partially supported by JSPS KAKENHI Grant Number 26286026, Grant-in-Aid for Scientific Research (B). In addition, this STEM observation was partly supported by Advanced Characterization Platform of the Nanotechnology Platform Japan sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.