| Quantum dots (QDs) have attracted great interest in biomedicine and light-emitting device because of different properties from bulk materials. In this paper, II-VI QDs were synthesized in aqueous phase from the application requirements. The aqueous synthesis of semiconductor QDs has many advantages. Firstly, it is convenience for preparation and under lower reaction temperatures. Secondly, it can provide the functionalization of QDs capped by various ligands which can be applied in various areas. Finally, QDs prepared in aqueous phase are monodisperse, size-controlled and high fluorescent efficiency.Owing to a narrow fullwidthathalfmaximum, high fluorescent efficiency, size-dependent emission wavelength and so on, Ⅱ-Ⅵ semiconductor QDs have attracted the attention of many researchers. In this paper, CdTe QDs were synthesized in aqueous phase, which were purified before CdS shell epitaxial overgrowth. And then CdTe/CdS core shell QDs were successfully prepared, which efficiently improved the fluorescent efficiency of QDs. This method can avoid the nanoalloy formation of CdTeS for the shell directly growth in CdTe precursor solution. There are two other important methods to enhance the PL intensity of QDs besides the passivation of the QDs surface. One is the use of the interaction of surface plasmon (SP) with QDs. The other is the use of surface-textured substrates with nano-structure (nanocones, nanowires or nanoholes).The main results are listed as follows:1.3-Mercaptopropionic acid (MPA)-capped, uniform-sized and monodisperse CdTe QDs were synthesized in aqueous solution. When the reaction time is 15 min, the first excitonic absorption peak at 469nm and strong near band-edge emission peak at 485nm appeared. PL spectra exhibited that the band-edge emission peaks showed red-shifts from 467 to 570 nm after re fluxing from 15 min to 5 h, indicating that the average size of CdTe QDs gradually increases. The strongest PL intensity of CdTe QDs was obtained when the reaction time increased to 2.5h. To improve the fluorescence efficiency and photostability of QDs, CdTe/CdS core-shell QDs were synthesized by the epitaxial growth of CdS shell on the CdTe core. The PL intensity of core-shell QDs increases firstly with the increase of shell thickness. When the shell thickness comes to 2-3 mono layers (ML), PL intensity of core-shell QDs shows a two-fold increase compared with that of the core QDs. Then the PL intensity of core-shell QDs decreases gradually when the shell thickness continues to grow. Moreover, the emission peaks of the core-shell QDs showed red-shift of 50nmwith the shell thickness increasing from OML to 5ML. The reason for the red-shift was the size of QDs increased, which was consistent with the data from the transmission electron microscopy (TEM) images of QDs. In addition, through the growth of CdS shell on the CdTe core QDs, the photo-oxidation and photobleach of the samples was slowed down, which enhanced the photostability of QDs and unproved PL quantum yield of QDs.2. An effective approach by using the interaction of surface plasmon (SP) with QDs has been raised to enhance the photoluminescence (PL) emission of QDs. CdTe/CdS QDs/Au NPs nanocomposite films were fabricated. The fluorescent properties of nanocomposite films were influenced by the thickness of spacer layer between the metal nanoparticles and QDs. The PL intensity of nanocomposite films increased firstly with the increase of spacer layer thickness. When the spacer layer was six PDDA/PSS bilayers, the PL intensity of nanocomposite films showed 16-fold increase compared with that of QDs film without Au NPs. Further increase in the spacer thickness led to the decrease of PL intensity of QDs. These results can be attributed to the competition between the energy transfer quenching and local electromagnetic field induced PL enhancement. The size of Au NPs was another important factor that controlled the plasmonic interaction and therefore the PL emission. The Au NPs with the average sizes of 100nm and 300nm were prepared by magnetron sputtering and rapid high temperature annealing process. The intensity of absorbance and emission of nanocomposite films increased with the average size of Au NPs increase. These might attribute to the stronger local electric field surrounding the 300nm Au NPs compared with that of 100nm Au NPs. The enhanced local electric field surrounding the Au NPs was confirmed by the finite-difference time-domain simulation. The electric field surrounding 100nm Au NPs was enhanced as high as 3.7-fold compared with the incident light, however, that near the 300nm Au NPs was enhanced eight-fold. These results indicated that the increased size of Au NPs resulted in stronger localization of electric field, which induced the enhancement of PL intensity of nanocomposite films. The shape of metal nanoparticles, particularly Au nanorods, has influenced the interaction between MNPs and the incident light. CdTe/CdS QDs/Au NPs (including nanospheres and nanorods) nanocomposite films were fabricated and the optical properties of all samples were investigated. The absorbance and emission intensity of sample with Au nanorods was higher compared with that of samples with and without Au nanospheres, which can be attributed to the stronger local electric field surrounding the Au NRs. The local electric field in the vicinity of Au nanorods and nanospheres were modeled by the finite-difference time-domain. The electric field surrounding Au nanospheres was enhanced as high as 4.6-fold compared with the incident light. However, the electric field parallel to the longitudinal direction of Au nanorods was enhanced as high as 25-fold.These results indicated that Au nanorods resulted in stronger localization of electric field, which results in increasing PL intensity of QDs film. The decay curves of samples were obtained under the excitation of picosecond laser pulse durations (λ=375nm) at room temperature. The PL lifetime at the emission peaks of CdTe/CdS QDs was 5.2 ns. Otherwise, the PL lifetimes of QDs films decreased to 3.3ns and 2.8 ns after the incorporation of AuNSs and NRs, respectively. The reduced lifetimes might indicate the energy transfer between the Au NPs and QDs because of the surface plasmon resonance.3. Patterning the surface of substrates with nanostructure was another available method to improve the PL intensity of QDs films. SiNWs were performed by wet etching process on Si substrates. Au NPs were formed on Si NWs by ion sputtering in a vacuum chamber and then high temperature rapid thermal annealing process. SEM images of Si NWs without and with Au NPs showed straight growth of nanowires vertical to the substrate and the formation of Au NPs on SiNWs. CdTe/CdS QDs/AuNPs nanocomposite films were prepared by the spin-coating of MPA capped CdTe/CdS QDs on Si NWs substrates. The optical properties of samples showed that CdTe/CdS QDs films on planar Si substrates reflected about 45% of incident radiation whereas the reflectance of QDs films on Si NWs substrates decreased to 10%. The PL intensity of QDs films on Si NWs substrates was lower than that of QDs films on planar Si. In addition, it was found that the PL intensity of QDs films was enhanced as high as 8-fold after the incorporation of Au NPs into Si NW s, which exceeded that of CdTe/CdS QDs/Au NPs nanocomposite films on planar Si. The decrease in PL intensity of QDs films might be that most of the incident light was absorbed by Si NWs, which reduced the absorption of QDs films. There are may be two reasons for the enhancement of PL intensity. On the one hand, the light extraction rate was improved due to the low reflectance of Si NWs substrates, which enhanced the PL efficiency. On the other hand, the enhanced local electric field surrounding the Au NPs improved the emission rate of nanocomposite films.
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