Preparation And Stability Of Alloy Quantum Dots With Silica Coating

Semiconductor quantum dots(QDs) have many advantages such as tunable emission spectrum, narrow and symmetric fluorescence emission peak, good resistance to photo bleaching and outstanding stability. These advantages make QDs have promising application in many fields including biological medicine, analysis and detection, clinical diagnosis, optoelectronic devices. Besides excellent fluorescent properties like other QDs, alloy QDs have better stability due to their gradient structure. In addition, because of their multiple components they no longer strictly depend on size. We can obtain alloy QDs with different emission wavelengths through component adjustment. Coating silica on the surface of QDs is a common way to acquire modified QDs. Not only can it make the oil-soluble QDs water-soluble but also it improves the stability of QDs. In this paper, alloy QDs with different emission wavelengths were synthesized. Then we chose different modifiers to coat silica onto the surface of alloy QDs and studied the photoluminescence properties.We used “one-pot” method in high temperature, anhydrous and anaerobic environment to prepare oil-soluble CdSe/CdxZn1-xS core/alloy shell quantum dots which can emit indigo(473 nm), green(537 nm), yellow(578 nm), and orange(586 nm) fluorescence respectively. Blue(447 nm) emitting CdxZn1-xS alloy quantum dots were also prepared. These quantum dots had symmetry fluorescence emission peak, narrow half-height peak width(<40 nm) and good stability. After 20 months storage they still had excellent fluorescent properties.Using sodium silicate(Na2SiO3) as modifier to cover the surface of green emitting CdSe/Cdx Zn1-x S QDs with an amorphous silica layer, we can obtain mondisperse water-soluble QDs under different pH values. The water-soluble QDs kept the favorable fluorescence properties of original QDs. Take CdSe/CdS/ZnS core-shell QDs as matrix to study the relationship between the fluorescence intensity of silica coated QDs and pH values during reaction. The fluorescence intensity was the highest when pH value was 9.0. Research showed that the silica coated QDs had good heat resistance, alkali resistance and light resistance properties.Using ammonia as catalyst, sixteen sodium alkyl sulfonate(CTAB) as template, silica shell layer was coated on the surface of aggregated carboxyl-QDs(QDs-COOH) and monodispersed amino-QDs(QDs-NH2) by hydrolyzing tetramethoxysilane(TMOS). Each of the silica spheres contained several QDs-COOH. The particles sizes ranged from 30 nm to 50 nm and the thickness of silica layer was about 8 nm. The fluorescence properties of silica coated QDs were stable, thus they can be preserved for a long time. The QDs-NH2@SiO2 had good monodispersity in aqueous phase. The mean particles diameter was about 25 nm and shell thickness was about 7 nm. Fluorescence intensity was enhanced significantly compared with QDs-NH2.