低维镉系半导体纳米材料的结构调控及光学特性研究

低维半导体纳米材料是一种新型的具有优异光学性质的半导体材料，这种由人工合成的尺寸小于百纳米量级的特殊材料在量子限制效应的影响下具有其体材料所不具备的独特物理性质。目前研究最为广泛的低维半导体纳米材料是胶体半导体量子点，其发光波长可调、发光效率高、可溶液处理、低成本制备等优点已经使其在显示领域得到了重要应用。为了进一步实现对低维半导体纳米材料光学性质的调控，设计与优化材料的结构及形貌是两个主要的技术手段。然而目前这些因素对于低维纳米材料光学性质的影响情况的研究还很有限。 本论文以具有不同结构及形貌的镉系半导体纳米材料为研究对象，通过多种激光光谱研究方法详细讨论其光学性质的变化，如发光效率、发光稳定性、俄歇复合、光学增益、表面缺陷等。通过研究结果掌握对低维半导体纳米材料光学性质的调控方法，实现对其光学性质的提升并拓展其应用，并为材料的结构设计提供理论与技术支持。根据低维半导体纳米材料的结构与形貌，本论文的具体研究内容可以归纳为以下四个方面： （1）制备了发光效率以及稳定性较好的CdSe/CdS单层核壳量子点，初步研究了其光学特性。随后将其与高分子聚合物混合，通过提拉法实现了量子点掺杂的有源微纳光波导结构。利用单点激发末端接收光致发光信号的方法探讨了偏振激发、纤维直径、衬底折射率对这种结构的波导光学性质的影响效果。通过蓝光发光二极管直接激发量子点掺杂的聚合物微纤维，实现了小体积、结构紧凑的有源光波导结构，拓展了量子点在微型化光电子集成器件中的应用。 （2）利用了宽带隙的ZnS材料包覆CdSe/CdS核壳量子点形成了多壳层核壳量子点结构。通过稳态光谱以及变温光致发光光谱研究了ZnS的包覆对量子点光学性质的影响。得益于ZnS的包覆，量子点的发光效率以及稳定性均得到了提升，内部的缺陷数量明显减少。随后通过这种多壳层核壳量子点结合回音壁模式的谐振微腔实现了低阈值的激光发射，探索了量子点在微纳激光器领域的应用。 （3）进一步改进量子点的核壳结构，合成了梯度合金壳层的蓝光量子点材料。利用飞秒瞬态吸收光谱对其双激子动力学过程进行研究与分析，通过变功率密度激发测试拟合计算出双激子俄歇复合寿命。由于梯度合金的结构使得界面势垒从突变的情况变为平滑过渡，得到了1268.2±67.5 ps 的长双激子俄歇复合寿命。得益于明显抑制的俄歇复合过程，实现了蓝光量子点中超低阈值(6.9 μJ/cm2)的放大自发辐射。壳层合金结构对俄歇复合过程具有明显的调控作用，进一步改善了量子点光学增益特性，使其更加适合应用在激光领域。 （4）以四个单分子层厚度的二维CdSe纳米片为研究对象，研究了纳米材料的形状对光学性质的影响。通过稳态光谱以及瞬态吸收光谱研究了纳米片的特殊光学性质，确认了其二维片状结构。利用变温光致发光光谱对其低温下双发射峰的起源进行了探究，分析了内部的发光机制，认为低能峰起源于表面态相关的发光。通过高分子聚合物聚二甲基硅氧烷（PDMS）对纳米片的包覆实现了对表面态相关发光峰的抑制。实验结果显示二维的片状结构使得纳米材料表面对光学性质的影响变得更加明显。 本论文利用多种光谱测试手段研究了核壳结构以及形状对低维半导体纳米材料的光学性质影响，研究结果对后续进一步改善优化材料的光学性质具有重要意义，并且拓展了低维半导体纳米材料在光电子集成器件以及光学增益介质方面的应用。

Low-dimensional semiconductor nanomaterials are a new type of semiconductor material with excellent optical properties. Due to the influence of the quantum confinement effect，these artificially synthesized special materials with a size smaller than one hundred nanometers have some special physical properties that bulk materials do not have. At present, the most widely studied low-dimensional semiconductor nanomaterials are colloidal semiconductor quantum dots (QDs). Its advantages such as adjustable wavelength, high efficiency, solution processability, and low-cost preparation have made it important in the display field. In order to further realize the manipulation of the optical properties of low-dimensional semiconductor nanomaterials, designing and optimizing the structure and shape are two important technological means. However, the current research on the influence of these factors on the optical properties of low-dimensional nanomaterials is still limited. This thesis focuses on the group cadmium-based semiconductor nanomaterials with different core-shell structures and shapes as the research object. A variety of laser spectroscopy research methods were used to study the changes in optical properties, such as luminous efficiency, luminescence stability, Auger recombination，optical gain, and surface defects, etc. By further mastering the method of adjusting the optical properties of low-dimensional semiconductor nanomaterials, the improvement of their optical properties and the expansion of their application prospects can be achieved, and the theoretical and technical support for the structural design of materials can be provided. According to the structure and shape of low dimensional semiconductor nanomaterials, the specific research contents of this thesis are as follows: (1) CdSe/CdS single-layer core-shell QDs with good luminescence efficiency and stability are fabricated and their optical properties are studied. Mixed with polymers, the active micro optical waveguide structure doped with QDs is realized by the pulling method. The photoluminescence signal is received in the end of microfiber after single-point excitation to explore the effects of polarization excitation, fiber diameter, and substrate refractive index on the optical properties of the waveguide of this structure. The blue light emitting diode directly excite the QDs-doped polymer microfiber, realizing a small-sized and compact active optical waveguide structure, and expanding the application of QDs in miniaturized optoelectronic integrated devices. (2) Through coating the CdSe/CdS core-shell QDs with wide band gap ZnS material to form a multi-shell core-shell QDs structure. The effect of ZnS coating on the optical properties of QDs is studied by steady-state spectroscopy and temperature dependent photoluminescence spectroscopy. Benefit from the ZnS shell, the luminescence efficiency and stability of QDs are improved, and the internal defects are significantly reduced. Subsequently, a low-threshold laser action achieve through these multi-shell core-shell QDs combined with a whispering gallery mode resonant microcavity, the application of QDs in the field of micro-nano lasers is explored. (3) The core-shell structure of QDs is further improved with gradient alloyed shell. The femtosecond transient absorption spectroscopy is used to study the biexciton dynamic process, and the biexciton Auger recombination lifetimes are calculated through variable excitation power density measurement. Due to the structure of the gradient alloyed shell, the interface barrier changes from sharp to smooth, and a long biexciton Auger recombination lifetime of 1268.2±67.5 ps is obtained. Due to the significantly suppressed Auger recombination, the ultra-low threshold (6.9 μJ/cm2) amplified spontaneous emission in blue QDs is achieved. The gradient alloyed shell structure has an obvious regulatory effect on the Auger recombination process, and further improves the optical gain of the QDs, making it more suitable for applications in the laser field. (4) Taking two-dimensional CdSe nanoplatelets with a thickness of 4 monolayers as the research object, the influence of the shape on the optical properties is studied. The special optical properties of nanoplatelets are studied through steady-state spectroscopy and transient absorption spectroscopy, and their two-dimensional sheet-like structure is confirmed. Using temperature dependent photoluminescence spectroscopy, the origin of its dual emission peaks at low temperatures is explored, and the mechanism is analyzed. It is believed that the low-energy peak originates from surface state-related emission. The coating of the nanoplatelets by the polydimethylsiloxane (PDMS) achieves the suppression of the surface state-related emission. The experimental results show that the nanoplatelets structure makes the influence of the surface on the optical properties more obvious. In this thesis, a variety of spectroscopic methods are used to study the influence of core-shell structure and shape on the optical properties of low-dimensional semiconductor nanomaterials. The research results are of great significance for the subsequent improvement and optimization of the optical properties of materials, and expand the application of low-dimensional semiconductor nanomaterials on the optoelectronic integrated devices and optical gain media.