Optical Properties Of Rare-earth Ions Coating ZnO Quantum Dots

Znic oxide (ZnO) nanomaterials demonstrated a lot of interesting features in optical and electronic because it's quantum confinement effect. When the surface of ZnO quantum dots (QDs) were covered by rare-earth ions, both the surface of ZnO QDs were modified and the energy transfer between ZnO substrate and rare-earth ions were occurred. Consequently, more excellent luminescent properties were obtained. The rare earth ions coating ZnO QDs may have an important research significance in the development of semiconductor optoelectronic device.In this thesis, the rare-earth ions coating ZnO QDs were prepared by direct precipitation method and a series of measures such as scanning electron microscope (SEM), X-ray diffraction (XRD), photoluminescence spectroscopy (PL) were used to measure pure-ZnO and rare-earth coating ZnO QDs. There are two main works in this thesis, described as follow:(1). Eu/Dy coating ZnO QDs were prepared by direct precipitation method. The particles obtained were about 30-50nm with uniform grain size. Comparison of Eu/Dy coating ZnO QDs and pure ZnO with the same prepare process, the former has the characteristics of small size, uniform distribution and high degree of dispersion. The diffraction peaks of samples is the same as that of ZnO powder diffraction standard card, which shows that the addition of rare earth ions does not change the crystal structure of ZnO. The rare earth ions most likely formed compounds then covered on the surface of ZnO. The emission peaks of the sample mainly come from the ZnO band edge emission and the characteristic emission of rare earth ions, including 593 nm (5D0→7F0),612 nm (5D0→7F2) emission originating from intra-atomic 4f-4f transition of Eu3+ ions and 484nm (4F9/2→6H15/2),575nm (4F9/2→6H13/2) transition emission of the Dy3+, whose intensity affected by the concentration of rare-earth ions. The concentration too high or too low would reduce the intensity. For example, The optimum concentration of Eu3+ in Eu-coated ZnO QDs was 8%. In Eu/Dy co-coating ZnO QDs, both emission belonging to Eu3+ and Dy3+. The white light would be obtained by adjusting the ratio of Eu/Dy. From the luminescence spectra, energy transfer between ZnO substrate and rare-earth ions was found.(2)By comparing structures and luminescent properties of the acetylacetone(AA)-treated and untreated Eu coating ZnO QDs to study the role of AA in energy transfer between ZnO substrate and rare-earth ions. According to ZnO powder diffraction standard card, the samples were high degree similar to it, which shows that the addition of rare earth ions does not change the crystal structure of ZnO. After annealed at 600℃, Eu2O3 characteristic diffraction peaks were appeared at XRD pattern of the two kinds of samples, which demonstrate Eu content exist. Not only can see a relatively sharp and strong UV emission band centered at 377nm in the PL spectrum of acetyIacetone(AA)-treated Eu coating ZnO QDs, but also have the characteristic emission of Eu3+ ions, which the maximum emission peak was at 5Do→7F2 (612nm).On the contray, both UV emission and a broad visible emission band belonging to ZnO were found in the PL spectrum of untreated Eu coating ZnO QDs, but no the characteristic emission of Eu3+ ions, which suggest that AA have a key role in energy transfer between the rare earth ions and the ZnO substrate and surface modification of ZnO. After annealing at 600℃, the characteristic emission peak disappear. Since the AA decompose at high temperatures, the energy transfer between ZnO and Eu3+ ions disappeared or weakened, This show AA play a great important role in Eu coating ZnO QDs once again.