| Introducing transition metal ions into the quantum dots (QDs) could bring intrinsic QDs some new properties such as a large stokes shift, the enhancement of thermal and chemical stabilities. Compared with doped bulk semiconductor materials, the studies of doped QDs are still in a very nascent stage, even though those special nano materials had been reported earlier. So far, it has not formed a consensus on the understandings about the tunability of PL emission, the possibility of involving d-states, and the exact location of dopant in crystals, etc. Therefore, it is quite meaningful to take further researches on the doping process, the fluorescence mechanism and their applications of doped QDs. In the dissertation, the main results are listed as follows:Firstly, Mn2+-doped CdS QDs were successfully synthesized with stearic acid as the stabilizer by a nucleation-doping method. The first excitonic absorption peak of the intrinsic CdS nanocrystals almost stabilized at～445nm. Generally, Mn2+-doped emission appears at～585nm, which is well-established due to6A1-4T1transition of Mn2+ions. A high-luminescence Mn2+-doped emission at～563nm was observed in our Mn2+-doped CdS nanocrystals. To our knowledge, this blue-shifted emission could be explained that Mn2+ions were deeply doped in nanoparticle cores in the doping method, leading to the less splitting of Mn2+energy level. The results of TEM images and XRD patterns also demonstrated the small particle size (-4.2nm), monodispersity and high crystallinity. The highest quantum yield of as-prepared nanocrystals was calculated～41%. Furthermore, the surface states of CdS:Mn QDs could be well passivated by ZnS shell.Secondly, Cu-doped Zn1-xCdxS QDs capped with stearic acid were successfully synthesized by a cation exchange method, which contained three steps:nucleation of ZnS cores at first, subsequently doping Cu in core, and Cd exchanging Zn at last. As shown in UV-vis absorption and PL spectra, the wide Cu-doped emission could vary from580nm to705nm with different Cd/Zn ratios and Cu concentrations. The possible fluorescence mechanism of the Cu-doped Zn1-xCdxS QDs indicates that the energy gap changes with different Cd/Zn ratios, leading to the continuous tunability of emission. The valence hole transfers to the filled higher potential Cu+d-state and changes it to Cu2+, which has two different energy states’t’ and’e’. Both two energy states have the possibility to recombine with the excited electron from the conduction band, which may broaden the corresponding emission spectra. The results of TEM images and XRD patterns also demonstrated the small particle size (-4.8nm), monodispersity and cubic structure. The measured quantum yields were20%-26%.Finally, the Mn2+-doped CdS QDs together with Cu-doped Zn1-xCdxS QDs were coated on blue InGaN-LED chips to fabricate WLEDs. Owing to a large stoke shift and the high luminescence of Mn2+-and Cu-doped QDs, the as-encapsulated WLEDs could generate a warm white light with less reabsorption and low color temperature. Additionally, the possibility of QDs combining phosphors in WLEDs was also discussed. The CdSe/ZnS core/shell QDs, served as red fluorescence enhancement materials, were added to improve the color rendering properties of common YAG-WLEDs. Due to high QYs and red emission at～623nm, the color rendering index (Ra) of the WLED could be increased over90. |
PDF Full Text Request