| Semiconductor quantum dots(SQDs) is a kind of important of nanomaterials. It has unique optical properties of different from those of the bulk materials and has been widely applied in fields of biological marker, solar cells, LED display technology, laser, SQDs spectrometer. Because of the large specific surface area of the quantum dot, it is easy to be oxidized and reunited, which brings inconvenience to the storage and application. In order to solve this problem, it is usually used for wrap SQDs to keep them stable. Although the package of organic matter can make SQDs uniformly distributed in the organic solvent, the form of the existence of SQDs and the characteristics of light emission will be greatly changed(for example, the collective effect of multiple SQDs will become obvious, also known as superradiance of quantum dots), which provides a new idea for the application of SQDs. In this thesis, the quantum dots speckle system composed of multiple SQDs(also known as quantum dot clusters) is considered as the research object, using laser spots with nano scale to study fluorescent properties of the single SQDs speckle system and its compound membrane on scanning near-field optical microscopy and several methods of regulating their surface enhanced fluorescence are also explored. The main research work and important results of this paper are also explored as:1. In the oleic acid- paraffin system, the colloidal CdSe and the colloidal ZnSe core /ZnS shell structures quantum dot clusters packaged oleic acid were prepared. The results showed that the size of single colloidal CdSe and ZnSe core / ZnS shell structure quantum dot clusters were 120 nm and 88 nm and single SQDs were uniformly distributed, as well as single crystal SQDs with zinc blender structure was clearly observed. At the same time, compared with the bulk materials, their absorption spectra were obviously the blue shift.2. The principle and experimental methods of surface enhanced fluorescence were studied by using Au nanoparticles controlling the fluorescence intensity of composite films composed of PMMA and QDs. Surface enhanced fluorescence and frequency shift effect were experimentally observed by the interaction of the single quantum dot clusters and its films excited by nanometer aperture tip with Au nanoparticles on a scanning near-field optical microscope platform. Thes results showed:(1) Due to the limitation of nano aperture, conical nano aperture tip emissive the evanescent wave near field to increase SQDs fluorescent flux and to improve the resolution, which increased the interaction cross sections of quantum dots with gold nanoparticles and promoted their coupling.(2) For ZnSe/ZnS SQDs, the fluorescence can be enhanced by 2.5 times when Au nanoparticles were considered as the interlayer between the quantum dot clusters and the porous titanium dioxide. When Au nanoparticles were considered as the surface enhancement agent, owing to the reflection effect of quantum dots film, the fluorescence would be increased to 10 times.(3)For CdSe SQDs, the fluorescent was increased to 11.7 times when Au nanoparticles were considered as the interlayer between the porous titanium dioxide and SQDs film. However, the fluorescence was increased to 1.7 times when Au nanoparticles were made as the surface reinforcing agent. Comparing with ZnSe/ZnS SQDs, the differences between CdSe SQDs and ZnSe/ZnS SQDs were dued to the big overlap area from the absorption spectra and fluorescence spectra of CdSe SQDs, which resulted in increasing the absorption of CdSe SQDs again, and increased fluorescence enhancement rate.(4) The Mapping results of ZnSe/ZnS SQDs film demonstrated that, after Au nanoparticles absorbing the fluorescence of SQDs, which produced localized surface Plasmon resonance effect and would effectively enhance the fluorescence of quantum dots. Moreover, Au nanoparticles aggregates could make the ground state exaction in quantum dots had a large frequency shift, but fluorescent intensity was not the biggest. It was to say, the fluorescence of SQDs was enhanced with increasing the number of Au nanoparticles. But, too much Au nanoparticles would only lead to a larger frequency shift and could not produce more fluorescence. The Mapping results of CdSe SQDs film showed that, the interaction between the absorption spectrum with the overlaps of fluorescent spectrum of the quantum dots, which modified the energy level structure of the quantum dots and induced the phenomenon of superradiance. Moreover, the evanescent wave from the aperture tip enhanced the collective effect of quantum dots, and lead to a big frequency shift. In addition, the use of Au nanoparticles as fluorescence enhancement agent further increased the fluorescence of SQDs. What is more, the efficiency of fluorescence enhancement was lower, when Au nanoparticles were considered as fluorescence enhancement agent on the surface of quantum dots comparing with the interlayer. And the reasons were that Au nanoparticles in the interlayer could obtain more efficient surface Plasmon energy.(5) With changing the polarization direction of incident light, the peak of fluorescence of ground state exciton was shifted and emerged the sine oscillation.3. The method of controlling fluorescent spectra profile were experimentally studied and theoretically explained by using the interaction between conical layered nano waveguide and ZnSe/ZnS quantum dots. And the results indicated that through varying the distance of conical nano waveguide tip and ZnSe/ZnS quantum dots, the far field fluorescence was impulse type to decrease. Surface plasmon resonance effect was produced by the interaction nano waveguide surface metal film and quantum dot fluorescence which resulted in multiple absorption peak of in the far field fluorescent spectrum, and decreased the full width half width of the spectrum.In order to control selectively the fluorescent emissive ratio of non-ground state excitons and to induce novel the peak of fluorescence and to increase the fluorescence of SQDs, N type polishing silicon was used for the substrate of sample to enhance the reflectivity of fluorescence, which increased the interaction of the surface metal of conical layered nano waveguide with the fluorescence. The results showed that the single quantum dot fluorescence radiation of the uniformly dispersed single layer quantum dot film was remarkable, and it reduced the coupling between the quantum dot and the super radiation phenomenon, and also make the full width half width of the quantum dot reduced by 53nmIn addition, due to the change of the quantum dot surface parcel layer, making the peak wavelength of quantum dots ground state exciton had blue shift 13 nm. Meanwhile, a new enhanced fluorescence peak was emergence at 534 nm, and a balance relationship was showed the fluorescence peak intensity between quantum dots ground state exciton and surface enhanced fluorescence. Moreover, many times repeated the experiment results had also confirmed the balance relationship. That was to say, the more higher the intensity of ground state exciton would result in more lower the surface enhanced fluorescence peak intensity, and vice versa, and it around a certain balance curve to fluctuation.4. On the condition of vacuum Low temperature annealing, the periodic array of nano-wires was prepared on porous titanium dioxide substrate by using the method of self-assembled colloidal CdSe quantum dots. The fluorescent spectra of nano-wire was characterized by the scanning near-field optical microscopy imaging method. Measurement of nano-wires Spectra showed that the more smaller quantum confine structure was produced. The peak of ground exciton was blue shift and the novel fluorescent peak arose at 420 nm and 640 nm. The research results for the physical method by preparation of self-assembled SQDS nano-wires provided a novel technical idea.The innovation of this thesis lies in:1. Considering the synergistic interaction between multiple quantum dots, taking single quantum dot clusters as the research object, a new experimental method that increased surface enhanced fluorescence of quantum dots was presented by boosting the coupling effect of Au nanoparticles and SQDs with the evanescent wave near-field from nanometer aperture tip. The physical mechanism of frequency shift and enhancing the ratio of fluorescence were obtained in different composite membrane structure of single quantum dots clusters.2. In oleic acid-paraffin system, the preparation of quantum dots was stable, good oxidation resistance, evenly dispersed colloidal quantum dot clusters.3. A theoretical mode and its experimental method which controlled the fluorescent spectra were presented by interaction an integrated cone layered nano waveguide with SQDs. And it was verified on experiment.4. The experimental process and the near-field measuring method of supper diffraction limit that prepared the periodic arrays of nano-wires were presented by taking colloidal quantum dots clusters as an raw materials on the condition of vacuum and low temperature annealing. |
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