Synthesis Of ZnSe/ZnS Core/shell Quantum Dots And SiO₂ Coated Namoparticles

With the development of Fluorescence Micro-Imaging Technique, labelling technique of biological material has been one of the most important key technologies in Clinical Test Systems. Semiconductor quantum dots (QDs) have attracted much interest as novel bright phosphors for biological and medicine applications. Comparing to the organic dyes, QDs exhibit quantum confinement effects and unique optical properties, for example broad excitation by all wavelengths smaller than the emission wavelength, narrow and symmetric emission with tunable colors, large Stokes shift and excellent photostability. The cadmium chalcogenide (CdS, CdTe and CdSe) with high brightness have been extensively studied at present. However, the toxicity of cadmium is a concern that will limit the use of these visible emitting QDs. ZnSe, a wide band gap semiconductor, can replace the cadmium chalcogenide because of its less biotoxicity. However, ZnSe QDs have low quantum efficiency. Therefore, our work is to synthesize the High quality ZnSe/ZnS (core/shell) QDs to improve the luminescence quality of the bare ZnSe QDs. Silica encapsulated ZnSe/ZnS QDs are prepared via a modified St?ber method by hydrolyzing of the tetraethyl orthosilicate (TEOS) on the surface of QDs.High quality ZnSe/ZnS (core/shell) QDs were synthesized via a epitaxial growth procedure under the lower temperature than the ZnS crystallization (120℃). The core/shell structure was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Photoluminescence (PL) spectra and absorbance spectra analysis were taken to prove that the formation of the core/shell structure can improve the luminescence quality of the bare ZnSe quantum dots. By adapting the reaction temperature, reaction time and the amount of the reactants, the ZnSe/ZnS QDs with different shell thickness can be achieved. Some reaction factors (such as oil bathing temperature, reaction time, the amount of FeS usage, and pH value), which influence on the luminescence properties of ZnSe/ZnS QDs, were investigated by the fluorescence spectra. The results indicated that the high quality ZnSe/ZnS QDs were obtained under these conditions. The provider of S2- is FeS; the mol ratio of Se2+ to S2- is 1:3; the pH value is 8.5; the reaction time is 1 h under 100℃. The growth of the ZnS shell on ZnSe nanocrystals, that is, the formation of ZnSe/ZnS core/shell semiconductor nanocrystals, resulted in the enhancement in the PL intensity in comparison with that of bare ZnSe nanocrystals. That is mostly attributed to the elimination of the surface defects.Silica encapsulated ZnSe/ZnS QDs are prepared via a modified St?ber method by hydrolyzing TEOS on the surface of QDs. By partial interchange of capping ligand from thioglycolic acid (TGA) to 3-mercaptopropyltri-methoxysilane (MPS) on QDs, initial photoluminescence (PL) efficiency of the QDs in water was retained. The factors such as reaction time of partial interchange of capping ligand from TGA to MPS on QDs, molar ratio of MPS to QDs, the pH value of reaction solution, the concentration of TEOS and the hydrolyzation time of TEOS were studied by experiments. The synthesized products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Environmental scanning electron microscopy (ESEM) and infrared spectrometric analyzer.Photoluminescence (PL) spectra and absorbance spectra analysis were taken to analysis the differences of PL property between the two kinds of nanoparticles. The test results demonstrated that the MPS can maintain the PL property of QDs to a great extent, the mole ratio of MPS to QDs can affect the number of the QDs pregnanted in the composite nanoparticles. Composite nanoparticles with multi QDs can be got by adjusting the mole ration of MPS: QDs. The pH value and the concentration of TEOS can influence the hydrolysis rate of silane. As the concentration of TEOS enlarging the grain size of ZnSe/ZnS@SiO₂ composite nanoparticles became bigger and PL performance of the composite nanoparticles became weaker. The average size and morphology of the multicore-shell structured ZnSe/ZnS@SiO₂ can be controlled by varying the MPS and TEOS concentration. When the reaction temperature is 35℃, molar ratio of MPS to QDs is 60, volume ratio of ethanol to distilled water is 3, the concentration of TEOS is 0.64 mmol/L, pH value is 5, the uniform size distribution of ZnSe/ZnS@SiO₂ nanoparticles with regular shape and smooth surface can be got.