高压下低维金属卤化物激子发光性质调控研究

低维金属卤化物具有高效的激子发光特性，在高性能显示、人造光源、辐射探测等领域都展现出极大的应用前景与研究价值。低维金属卤化物材料的激子发光特性与其结构维度紧密相关，主要存在两种本征的激子发光类型：窄带的自由激子发射和宽带的自陷态激子发射。其中，自陷态激子发射又可以根据发射能量的相对高低，进一步分为高能量的单重态发射和低能量的三重态发射。目前，虽然可以通过化学合成手段对低维金属卤化物的结构和激子发光性质进行一定程度的调控，但尚且无法全面系统地揭示其激子发光调控的内在机制并实现不同发光机制之间的相互转化。基于金刚石对顶砧的静态高压技术可以在不改变原有材料组分的情况下，连续地对材料的结构和性质进行精准的调控。因此，高压技术作为一种理想的研究手段，可以对低维金属卤化物的结构和激子发光性质实现可控调节，并深入揭示其晶体结构变化及对激子发光性质的影响机制。

本论文围绕低维金属卤化物材料，分别选取具有自陷态激子发射及自由激子发射和自陷态激子发射共存的材料体系，综合利用高压研究手段，实现对其结构和激子发光特性的调控，并对其内在光物理机制和结构-性质关系进行分析。对于仅存在自陷态激子三重态发射的低维金属卤化物体系，本文主要对其激发依赖发射特性开展研究。零维金属卤化物Cs2InBr5(H2O)虽然在常压下表现出不依赖于激发能量的自陷态激子发射，但在压力引发相变后，其无机发光单元 InBr5O 产生相当大的结构扭曲并生成新的自陷态激子发射，导致Cs2InBr5(H2O)在高压下表现出反激发依赖的特性。类似地，在混合卤素的零维金属卤化物(C4N2H14Br)4SnBr3I3中，具有不同配位环境的SnX6 (X = Br, I)发光单元为其在低温和高压下产生激发依赖发射响应提供了结构基础。这些发光单元在高压下不同程度的结构扭曲进一步引起了其中可以通过压力调控的激发依赖发射现象。

基于对自陷态激子发射特性的深入理解，进一步探索了低维金属卤化物中自陷态激子发射与自由激子发射之间相互转变的光物理机制及内在的结构-性质关系。在具有一定无机带宽度的准一维金属卤化物(C2H10N2)8[Pb4Br18]∙6Br中，通过压力提高了其电子维度并引发激子的离域，使其从常压下的自陷态激子发光完全转变为高压下的自由激子发光。压力一方面提高了自陷态子的能量，促进了其脱陷过程；另一方面减小了沿无机带方向载流子的有效质量，并改善了构成带边的电子轨道的连接性，有利于脱陷后的激子在晶格内发生迁移并在带边附近发生辐射复合，形成自由激子发射。这一过程清晰地展示了低维金属卤化物中不同激子发射类型之间相互转变的光物理机制。此外，在对自由激子发射和自陷态激子发射共存的二维金属卤化物(C4H12NO)2PbBr4的高压研究中，实现了其中自由激子发射和自陷态激子发射之间多次的相互转变，并结合高压结构表征，揭示了其产生不同类型激子发射的结构基础。压力引起(C4H12NO)2PbBr4的结构相变，减小了其无机层内近邻Pb构成的四边形的内角差。在此过程中，(C4H12NO)2PbBr4的自陷态激子发射强度显著降低，显示出明显的结构相关性。最后通过(C4H12NO)2PbBr4在低温下激子发光特性和结构特征关系的对比研究，再次验证了上述结构-性质关系在二维金属卤化物中的适用性。

Low-dimensional metal halides demonstrate efficient exciton emission properties, showing great potential for applications in high-performance displays, artificial light emitting sources, radiation detection, and other fields. Therein, the excitonic emission properties of low-dimensional metal halide materials are closely correlated to their structural dimensionalities and there are mainly two intrinsic types: narrow-band free exciton emission and broad-band self-trapped exciton emission. In particular, the self-trapped exciton emission can be further divided into singlet emission and triplet emission according to distinct emission energy. Although the structure and excitonic emission properties of low-dimensional metal halides can be regulated through chemical synthesis, it is still not enough to fully and systematically decipher the intrinsic mechanisms of exciton emission regulation or enable mutual transformation between different emission types. Employing static high-pressure techniques with diamond anvil cells enables precise control over the structure and properties of materials without changing their composition, Therefore, high-pressure techniques prove to be an ideal research method for regulating the structure and excitonic emission properties of low-dimensional metal halides, thereby extensively unveil crystalline structural changes and the mechanisms influencing their excitonic emission properties.

In this thesis, several low-dimensional metal halide materials with self-trapped exciton emission and free exciton emission are selected for high-pressure studies to realize excitonic emission properties regulating and accordingly reveal the underlying photophysical mechanisms and structure-property relationships. This study primarily investigates the excitation-dependent emission characteristics of low-dimensional metal halide systems with triplet self-trapped exciton emission. While the zero-dimensional metal halide Cs2InBr5(H2O) exhibits excitation-independent self-trapped exciton emission at ambient conditions, its inorganic emissive unit InBr5O octahedra experience significant structural distortion during the pressure-induced phase transition, leading to the generation of new self-trapped exciton emission to exhibit the rare inverse excitation-dependent emission response under high pressure. Similarly, in zero-dimensional metal halide (C4N2H14Br)4SnBr3I3 with mixed halogen, the emissive SnX6 (X = Br, I) octahedra with distinct coordination environments offer the structural foundation for the excitation-dependent emission response under low temperature and high pressure. The pressure-dependent structural distortion of these emissive units under high pressure gives rise to the adjustable excitationdependent emission phenomenon in (C4N2H14Br)4SnBr3I3.

Based on the understanding of the relationship between structure and self-trapped exciton emission, further explorations are then conducted on exploring the photophysical mechanisms and intrinsic structure-property relationships of the transition between free exciton emission and self-trapped exciton emission in low-dimensional metal halides. In the quasi-one-dimensional metal halide (C2H10N2)8[Pb4Br18]∙6Br, pressure promotes the electronic dimensionality and induces excitonic delocalization, leading to a complete transition from self-trapped exciton emission to free exciton emission. Pressure not only enhances the energy of self-trapped excitons, promoting their detrapping process but also reduces the effective mass of carriers in the inorganic ribbons and improves the connectivity of the electronic orbitals constituting the band edges. This facilitates the migration of detrapped excitons within the lattice and their radiative recombination near the band edges, leading to the formation of intense free exciton emission. This process demonstrates the photophysical mechanisms governing the transitions between different types of excitonic emissions. Furthermore, in the high-pressure study of two-dimensional metal halide (C4H12NO)2PbBr4, where free exciton emission coexists with self-trapped exciton emission, multiple mutual transformations between free exciton emission and self-trapped exciton emission are achieved. By conducting high-pressure structural characterization, the structural basis for generating different types of excitonic emissions is accordingly revealed. Pressure-induced phase transitions in (C4H12NO)2PbBr4 decreased the difference in interior angles of the quadrilaterals formed by neighboring Pb atoms in the inorganic layers. During this process, the intensity of self-trapped exciton emission significantly decreased, showing a clear correlation with structure. Finally, through the comparative study of the relationship between excitonic emission properties and structural features of (C4H12NO)2PbBr4 at low temperature, the applicability of the above structure-property relationships in two-dimensional metal halides is once again verified.

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