Synthesis And Modification Study Of Cd-doped ZnO Quantum Dots

Quantum dots (QDs), also called Semiconductor nanocrystals, are semiconductor materials with a particle size close to that of its exciton Bohr radius or de Broglie wavelength. Because these QDs possess good optical properties and unique optical and electric properties with narrow emission spectrum, broad excitation spectrum and tunable photoluminescence, and so on, they have drawn considerable attention from researchers. And have become a bright spot in nanomaterials research. This article mainly studied the affection on optical property of ZnO QDs after adulteration with Cd and modification with other semiconductor materials. The main work as follows:(1) The Cd-doped ZnO QDs were first prepared using the typical sol-gel method with a stabilizer of polyvinylpyrrolidone (PVP K-30). After purification, zinc acetate and TAA were added sequentially for the deposition of a ZnS layer and form Cd-doped ZnO/ZnS core/shell QDs. The morphologies and structures of the product were verified using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy-dispersive Spectrum (EDS) and X-ray photoelectron spectroscopy (XPS). The optical properties of the product were researched by the UV-Vis absorption spectra (UV) and photoluminescence spectra (PL). The affections on luminescent properties of these QDs with various amounts of Cd and TAA (for control the thickness of ZnS) and the mechanism of photoluminescence have been investigated. The results show that the core/shell QDs is bigger 1.37nm then that core QDs with about 7.37 nm, core Cd-doped ZnO QDs have hexagonal wurtzite and the shell ZnS have cubic sphalerite structures. The luminescent intensity of the Cd-doped ZnO QDs enhanced significantly after coated with ZnS result from the passivation by ZnS and the introduction of oxygen vacancy when Cd ions displace some Zn ions in the ZnO lattice which will lead to a change of the kation coordination number. At proper Cd doping with 20% and TAA content with a molar ratio of 5:1.5 for the Cd-doped ZnO to TAA, maximal luminous intensity can be achieved, which was approximately 6 times higher than that of Cd-doped ZnO QDs without TAA and approximately 12 times higher than that of undoped ZnO/ZnS QDs, the absolute quantum yield can reach 21%for Cd-doped ZnO/ZnS QDs from 3% and 8%for undoped ZnO/ZnS and Cd-doped ZnO QDs respectively.(2) Cd-doped ZnO QDs were functionalized with 3-aminopropyltriethoxysilane (APTES) to be water-soluble. The morphologies and structures and optical properties of the product have been characterized by XRD, TEM, EDS, FTIR and UV, PL respectively. The results indicated that the fluorescence intensity of the Cd-doped ZnO QDs enhanced significantly and blue shift in the PL after modification with APTES. Compare the fluorescent photographs of the QDs, we can found that the Cd-doped ZnO QDs have excellent water solubility after modification with APTES, while the aqueous solution of unmodified QDs is muddy due to the agglomeration of QDs in the solution. Under 356nm UV light, the samples unmodified exhibited yellow luminescence while the modified with APTES exhibited green which indicated that that the QDs can still keep a good optical property in the aqueous solution after modification with APTES. These water-soluble QDs have a wide application prospect in the biological fluorescence label due to simple preparation, the high fluorescence intensity and stability.