银基金属纳米材料的各向异性生长机理、可控合成和光学性能调控研究

It is well-established that the shape of silver (Ag) nanocrystals plays an important role in determining their physical and chemical properties, including localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS). However, the challenge to synthetically and systematically tailor and fine-tune the symmetry/topology of Ag-based nanocrystals, and thus precisely control their physical properties, has met with limited success. This difficulty can be attributed primarily to the limited understanding of the anisotropic growth mechanism involved in the synthesis. For example, it remains unclear how the symmetry could be broken in a controllable fashion. One of the effective ways to address this challenge is to separate the formation of seeds from the growth of seeds by employing a seed-mediated approach. It is this belief that motivates us to put together this proposal, with an ultimate goal to develop the fundamental science and knowledge concerning the anisotropic growth mechanism of Ag, which are expected to be used to predictably and reproducibly generate Ag-based nanocrystals with controlled structures and thus LSPR and SERS properties. The proposed research will be organized into three thrusts: (1) Systematical study of the anisotropic growth mechanism of Ag nanocrystals. This work will focus on the growth pathways of Ag on hybrid seeds. Specifically, we will seek to connect the mode of growth with both thermodynamic and kinetic conditions. We will also study the migration of Ag atoms on the surface of nanocrystals and identify specific reaction conditions capable of controlling the symmetry and thus shape of Ag-based nanocrystals. (2) Controlled synthesis of Ag-based nanocrystals with tunable LSPR properties ranging from 400-2500 nm. This work will focus on the design, screening, and synthesis of Ag-based nanocrystas with specific topological structures or symmetries. It is expected that the major LSPR peak of these Ag-based nanocrsytasl could be tuned in the range of 400-2500 nm. We will also calculate their extinction/absorption/scattering coefficients and the relative cross sections. (3) Controlled synthesis of Ag "gap" nanocrystals with tunable gap structures and enhanced SERS properties. This work will focus on the design and synthesis of Ag-based nanocrystas with tunable gap structures. It is expected that hot spots could be formed in the gap. We will also try to control the width and length of the gaps, aiming to manipulate the SERS enhancement factors of such kinds of nanocrsytals.

各向异性生长机制普遍存在于银纳米晶的制备过程中，其从根本上控制了纳米晶的三维拓扑结构、对称性、金属间界面以及因此导致的不同寻常的光学效应。在本项目中，我们拟采用液相晶种介导生长的合成路径，结合球差校正的高分辨透射电镜等手段，深入地研究银基纳米结构各向异性生长机理中的关键问题，包括分析各种热力学和动力学参数与纳米结构可控生长之间的关系、探讨银原子在纳米结构表面的迁移过程、观察银基纳米材料制备过程中的各种对称破缺和对称保持现象。通过设计和构筑特定金属间界面、三维拓扑结构、对称破缺结构等方式，实现在400-2500 nm近全波段的范围内对银基纳米结构的局域表面等离子共振光谱进行连续调控。设计并可控合成颗粒内部自带裂缝的银基纳米结构，进一步控制裂缝的宽度和长度，实现对缝隙处"热点"结构表面增强拉曼散射活性的精细调控。

尽管对于纳米晶的控制合成已经取得了较为丰硕的成果，对其形貌依赖的表面等离子共振光学特征和表面增强拉曼散射活性的研究也逐渐清晰，但当前的研究仍然存在一些关键科学问题，如对纳米晶的各向异性生长机理还很不清楚，纳米晶的局域表面等离子共振光谱的调控范围还有待进一步提高，缺乏简易且有效构造表面增强拉曼散射“热点”结构的方法等，针对以上关键科学问题，项目负责人在本项目实施期内深入研究多种纳米结构的各向异性生长机理，包括分析和总结各种热力学和动力学参数与纳米结构可控生长之间的关系、观察纳米材料制备过程中的各种对称破缺和对称保持现象、探讨原子在纳米结构表面的迁移过程，对模板诱导生长，外延诱导生长，还原动力学调控，氧化刻蚀调控，共沉淀调控表界面结构，多级结构的生长机制等方面进行了详细研究；在对上述机理研究的基础上，通过设计和构筑特定金属间界面、三维拓扑结构、对称破缺结构的方式，实现在特定波段的范围内对纳米结构的局域表面等离子共振光谱进行调控，例如，构筑了具有独特光热效应的五角星形纳米晶，研究了水溶液中氯离子对银纳米片SPR的影响，设计并构造出优异非线性光学特性的Au/AgAu/Ag2S/PbS金属半导体核壳结构纳米棒等；通过可控合成技术直接构筑“热点”结构，并对“热点”结构表面增强拉曼散射活性的精细调控，构筑的平面四角叉纳米结构不仅具有独特的SPR性质，其增强因子更是达到近10000，在分子的超灵敏度检测方面表现出一定的应用潜力。在项目执行期间，在Nat. Energy，Nat. Commun.，JACS等期刊共发表文章34篇，申请专利15项。