| Nanosized optical materials, such as rare-earth doped Y3Al5O12 and YBO3 as well as the two dimensional (2D) layered transition metal dichalcogenides (TMDs) (MX2; M= W, Mo; X= S, Se, Te) which are similar to graphene layered material, are essential substances for appliances on light emitting diode (LED), plasma display planar (PDP), solar cell and flexible electronic device and so on. The YAG has the cubic phase structure, YBO3 possesses the hexagonal vaterite structure, and the 2D layered MoSe2 has the hexagonal symmetry. These materials have great potential applications in the field of industry technology due to their desirable thermal stability, photonic and photoelectronics properties, which have significantly promoted the development of illumination, displayer and flexible photoelectronics devices.Micro-sized or nano-sized optical materials exhibit the novel properties that are different from their bulk counterparts, and their luminescent properties can be tuned by the processing conditions. In this thesis, YAG:Ce and YBO3:Eu3+ were synthesized by a hydrothermal method and MoSe2 was synthesized by a chemical vapor deposition (CVD) method. The effects of the pH value of precursor, post-treatment and composition variation as well as the different substrates on the luminescent properties were investigated. The main research contents and the obtained results are as follows:Chapter 1:We introduce the application background, structure and luminescent properties of nano-sized YAG:Ce, YBO3:Eu3+ as well as 2D atomic layered TMDs material. Then the current research states of these materials and the motivation of this thesis are also introduced.Chapter 2:We introduce the synthesis methods, the used raw materials, characterization methods and apparatuses.Chapter 3:YAG:Ce(5%) phosphors were synthesized by a supercritical hydrothermal method at 360 ℃ in a series of reaction time tr of 6,10,14,18,22 and 26 h, respectively. Then the sample with tr= 22 h was subsequently treated by spark plasma sintering (SPS) and annealing in vacuum and in air, at the same time, the effects of post-treatment on the luminescent properties were investigated. As a contrast, the sample prepared by a conventional solid state method was also studied. The results indicated that:1) The single-phase YAG:Ce (5%) can be obtained at much lower temperature of 360 ℃ than the conventional solid-state method at 1350 ℃.2) The luminescent intensity can be significantly enhanced by SPS and annealing in vacuum because of the valence state varying from Ce4+ to Ce3+ and the better crystallization after annealing. However, annealing in air decreases the luminescent intensity because Ce3+ changes to Ce4+ without optical activity due to empty 4f and 5d orbit.3) Compared with the solid-state reacted sample, the emission peak of hydrothermally prepared YAG:Ce (5%) red-shifted from 540 nm to 557 nm, which is useful in white light-emitting diode lighting sources. Red-shift can be attributed to the variation of 5d level due to the surface and interface effects of nanoparticles.Chapter 4:A series of YBO3:Eu3+ samples were synthesized by a hydrothermal method at 260 ℃ with additive of multi-walled carbon nano-tubes (MWCNT) in the precursor solution. The pH value of the precursor solution was changed from 5 to 10. The effect of pH on the morphology and luminescent properties were investigated. The main results are as follows:1) The single phase YBO3:Eu3+ can be directly obtained at low temperature of 260 ℃.2) Rich morphologies are observed, including drum-, flake-, hexahedron-, hexagonal-and flower-like particles. The possible reasons for the different morphologies are suggested.3) All samples exhibit the characteristic emissions of Eu3+ due to the 5D0-7FJ (J= 1,2 and 3) transitions. The Red/Orange ratios are enhanced for the samples with pH= 9 and 10 which are 1.07 and 1.22, respectively, indicating better color purity. The underlying origins are attributed to the lower local symmetry of the Eu3+ ions at the grain boundaries or particles’ surface.Chapter 5:The 2D MoSe2 nano-sheets were grown on 300 nm SiO2/Si and sapphire substrates by CVD process, and the effect of substrate type on luminescent properties were investigated. The measurement of the photoluminescence (PL) at different temperature and laser power showed that the PL peak of MoSe2 grown on sapphire blue-shifted and the PL intensity was enhanced. In other word, MoSe2 grown on SiO2/Si showed the red-shift and the weakened PL intensity. The mismatch of the lattices between MoSe2 and substrate leads to this consequence, on the one hand, more defects in MoSe2 result in the red-shift and the decrease in PL intensity, on the other hand, the worse crystallization of MoSe2 which is another possible reason for the decrease in PL intensity. Additionally, the PL intensity at the boundary of MoSe2 is stronger than that inside the MoSe2 due to the strong activity of boundary states.Chapter 6:We make the conclusion and outlook of this thesis. |
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