Research On Preparation And Fluorescence Properties Of Quantum Dots Contained Cadmium

Fluorescent nano-materials have many kinds of novel effects, such as the quantum size effect, the surface effect and the macroscopic quantum tunnel effect and so on. Fluorescent nano-materials become the research hotspot in recent years because of the outstanding performance in the optics, electricity, magnetism, thermology. Quantum dots (QDs or semiconductor nanometer crystals) have the narrow emission line, wide stimulation spectral line, stable property, continuously adjustable emission wave length along with particle size and much other excellent physical and chemical nature. It is hopeful to substitute present organic materials and obtains the widespread application in the domains of cell imaging, DNA sequencing, immunity examination, temperature sensing, white light LED and so on. At present, the synthesis of QDs has the mature technology route, but the oil phase synthesis needs to reduce the cost and the aqueous phase synthesis needs to raise the fluorescence efficiency. QDs have strong surface and interfacial effect. The physical mechanism of many optical characters is not completely clear, sucn as the metal surface enhancement fluorescence, up-coversion fluorescence, the interaction between QDs and other materials etc.We used the octadecene (ODE) to substitute three octyl oxidation phosphine (TOPO) as the resolver. It reduced the synthetic cost. The CdSe QDs with different color have been synthesized in the oil phase. The fluorescence performances of CdSe QDs are similar to the QDs obtained from the traditional oil phase synthetic method. In the chloroform solution, the polyaniline (PAni) were mixed with the different size CdSe QDs. Then CdSe QDs/PAni compound was obtained. The fluorescence intensity of CdSe QDs/PAni compound reduces when the size of CdSe QDs reduces. Simultaneously the polyaniline density increases can also reduce the fluorescence intensity of CdSe QDs/PAni compound effectively. The analysis indicates that the energy transfer and charge transfer occur between the CdSe QDs and the polyaniline. It causes the fluorescence quenching. On the one hand, resonance energy transmission has produced from CdSe QDs to the polyaniline. On the other hand the polyaniline may intercept the electric charge transmission in CdSe QDs effectively and cause the interruption of its radiative recombination process.The CdSe and CdTe QDs with different size were synthesized in the aqueous phase. The mercaptoacetic acid and thiohydracrylic acid were taken as the superficial ligands. Metal-Enhanced Fluorescence from CdSe and CdTe QDs were investigated. The spectrum change of the QDs on the metal surface is dependent on at least two factors: an enhanced local electromagnetic field and an increase in the intrinsic decay rate of the QDs. A significant enhancement (4-fold) in fluorescence intensity is observed from the CdSe QDs on the Au island thin film as compared to that on the glass. On the other hand, the fluorescence intensity decay of CdSe QDs on the Au island thin film shows a faster decay with nearly 2-fold decrease in average fluorescence lifetime. The energy band structure matches between the CdSe QDs and Au, therefore the excited electrons in the Au island thin film can pour into the CdSe nuclear area. It increases the excited state electron density in QDs, thus causes the fluorescence enhancement. The fluorescent lifetime reduction is a result of the free exciton number and the radiative transition efficiency increase.CdTe QDs were self-assembled on the surface of silver nanoparticles (NPs) capped with polyvinylpyrrolidone (PVP) through the ligand field effect of Cd—O. A significant 2-fold enhancement in the integrated fluorescence intensities, red shift of fluorescence peak and obvious decrease of lifetime were observed in the CdTe QDs assembled on the Ag NPs in comparison with the pure CdTe QDs. The fluorescence enhancement factor and red shift were found to depend on the Ag NPs concentration. Compared with previous reports, the occurrence of the self-assembly of CdTe QDs on the surface of PVP-capped Ag NPs is fairly simple and easy. From a practical point of view, the combination of CdTe QDs with Ag NPs may lead to the fluorescence enhancement, which could be utilized in a variety of biological, chemical and medical detection applications.Water-soluble CdTe/CdS core/shell quantum dots (QDs) with the different shell thickness capped were synthesized following the synthetic method of successive ion layer adsorption and reaction. The relationships of fluorescence quantum yield, the spectrum structure with CdS shell thickness were discussed. The CdTe/CdS core/shell QDs exhibite a significant red shift of emission peak, full width of half maximum (FWHM) increase and fluorescence lifetime lengthening which is different to CdSe/ZnS, CdTe/ZnS core/shell QDs. We hypothesize that CdTe QD will gradually evolve into a type-II core/shell QDs from a type-I CdTe core with increasing CdS shell thickness. For the type-II CdTe/CdS QDs, the fluorescence can come not only from the recombination between an electron in the conduction band of CdS shell and a hole in the valence band of CdTe core, resulting in a significant red shift and a long radiative lifetime, but also from the recombination from an electron in the conduction and valence bands of CdTe core. The two recombination processes will cause a broadening of the fluorescence band, as well as a significant red shift. The lack of fluorescence lifetime lengthening and quantum yield increasing was ascribed to the surface influence of the thick shell.We study the steady-state and time-resolved fluorescent properties of CdTe and CdTe/CdS core/shell QDs by one and two photons excitation with a femto-second laser under the low intensity. 800 nm laser excitation causes a blue shift of the emission peak compared with 400 nm laser excitation. The blue shift value is related to fluorescence quantum yield. The blue shift value is small when the fluorescence quantum yield is high. Near-quadratic laser power dependence of the up-conversion intensity and bi-exponential decay kinetics are observed. It is found that up-conversion fluorescence is composed of a photoinduced trapping and a band edge excitonic state. The blue shift of the emission peak is caused by the relative change in fluorescence intensity between excitonic and trapping states. Two-steps and two-photons absorption involving the surface as the intermediate states has been proposed for up-conversion fluorescence of CdTe/CdS quantum dots.