Controlled Structure And Luminescent Properties Of Mn²⁺-Doped Perovskite Trifluoride Micro/nanomaterials

Recently, more and more attentions have been paid to up-conversion（UC） luminescence nanoparticles（UCNPs） due to their potential applications in a broad range of fields, i.e. bioimaging, diagnositic and therapeutic systems, drug delivery systems, photovoltaic devices, photocatalysis. Practical applications of UCNPs set demanding requirements on the regulation of the structure and luminescence properties, which becomes one of the current research focus areas of the inorganic phosphors. Due to its special electronic and energy structure, the transition metal Mn²⁺ ion plays an extremely important role in the rare-earth UC single peak emitting. However, it has rarely reported that Mn²⁺ ions can also serve as research centers and regulatory converted UC emitting nanostructures as a single peak. Although many Mn²⁺-doped UCNPs have been obtained, the controlled growth of Mn²⁺-doped UCNPs with specific morphologies and improved UC luminescent properties is still remaining as a great challenge. Under this background, this dissertation is concerning the controllable synthesis, self-assembly behaviors and UC luminescent properties of Mn²⁺ doped perovskite trifluoride micro/nano particles. These monodisperse and controllable morphological particles are synthesized via several “green” and environmentally friendly methods. Their phase composition, their size and shape, their element distribution, their surface chemistry, as well as their optical properties are carefully investigated. Furthermore, possible formation mechanisms and the inner correlation between their performance and structure are proposed based on the experimental data. In addition, preliminary potential applications of these materials are explored.The dissertation is composed of seven chapters. Chapter 1 describes the basic concepts and theories on UC luminescence, the recent progress in design and biomedicine applications of UCNPs, and the controllable synthesis of UCNPs by soft chemical routes in detail, then the research contents of this dissertation is proposed. Chapter 2 introduces the samples preparation and characterizations. Chapters 3- 6 present a detail investigation on controllable synthesis, self-assembly behaviors and UC luminescent properties of Mn²⁺ doped perovskite trifluoride. Chapter 7 summarizes the conclusion and prospect. The main achievements are shown as follows:（1） A series of monodisperse NaMg（1-x）F3: 0.5%Yb3+, xMn²⁺（x=5%⁶0%） nanocrystals are synthesized by a facile solvothermal method. The nanoparticles have an average size in the range of 20⁴0 nm. In addition to the visible UC emission centered at 600 nm, the near-infrared（NIR） UC emission band at approximately 760 nm has been revealed in orthorhombic-phase NaMgF3: Yb3+, Mn²⁺ nanocrystals upon excitation of 980 nm laser diode（LD）. The emission intensity of NIR UC luminescence reaches a maximum as the doping concentration of Mn²⁺ at x=50%. Compared with the cubic-phase system（KZnF3, KMgF3）, the results can confirm that the NIR UC emission can be ascribed to the aggregation of Mn²⁺-Mn²⁺ effective dimers via super-exchange interactions, which are connected with the Mn²⁺-F--Mn²⁺ geometrical angle.（2） Uniform NaMnF3@NaMgF3 core-shell nanocubes（CSNCs） and double-shelled nanoboxes（DSNBs） have been successfully synthesized via a facile one-pot coprecipitation method at room temperature by tuning the ratio of Mn to Mg in the raw materials. The unique nanostructures of NaMn F3@NaMgF3 CSNCs were revealed by XRD, SEM, TEM, STEM and EDX elemental mapping. The possible self-assembly formation mechanisms consisting of separate nucleation, surface crystallization and self-phase separation have been elucidated based on the experimental data.（3） The phase structural and compositional features of Yb3+-doped NaMnF3@NaMgF3 core-shell nanocomposites are characterized in detail. Based on the experimental results with the UC luminescence of CSNCs and DSNBs, compared with the corresponding solid-solution nanoparticles and physically mixed samples, the conclusion can further comfirm that the enhanced NIR to NIR UC luminescence generated by Mn²⁺-Mn²⁺ activator aggregation-dependent effects. The results also provide a direct evidence for Mn²⁺-Mn²⁺ aggregation induced NIR emission at the single particle level.（4） Solid-solution phase K（Mn,Zn）F3 mesoporous microspheres have been successfully prepared via a simple one-pot hydrothermal method by using citrate ions as chelating agent. Effects of the reaction time on the phase transformation and morphological evolution of K（Mn,Zn）F3 microspheres have been carefully investigated. The results revealed that the hierarchical mesoporous spherical structure with a solid-solution phase was converted from a core-shell structure with mixed phases via the nanoscale Kirkendall effect. It is noted that Yb3+/Er3+-codoped K（Mn,Zn）F3 microspheres exhibit intense single-band red up-conversion emission upon excitation of 980 nm LD. Furthermore, the drug release test demonstrated the potential application of the K（Mn,Zn）F3:Yb3+/Er3+ mesoporous microspheres in the field of drug-delivery.