电子衍射断层成像术对纳米材料结构弛豫的研究

Electron diffraction tomography has been used to identify the crystal structures of known and unknown nanomaterials by acquiring tilt-series of diffraction pattern within a large tilting-angle and the following 3D reconstruction. However, currently it has not been seriously taken into consideration that the intensity oscillations around Bragg diffraction spots reflect the defect structures, as well as the structural relaxation of nanomaterials. To extract this infromation in 3D reciprocal space, this proposal has suggested to develop a reciprocal tomography method mapping 3D intensity of specific diffraction spots based on transmission electron microscope. First of all. It has been proposed to establish an automated experimental and data-analysis platform performing the precise mapping of 3D intensity of specific diffraction spots at a small regular angle-increment within a small tilting-range. Based on this platform, this proposal has planned to study the internal structural relaxation mechanism of five-fold twinned nanowires and reveal its relationship with the nanowire's intrinsic angular misfit and the diameter. Additionally, combining this reciprocal-space tomography method with relative theoretical simulations of electron diffraction, the atomic relaxation models at the surface of metal and metal oxide nanoparticles will be explored.

通过大角度范围内的系列衍射谱采集与重构，电子衍射断层成像术已用于确定已知和未知纳米材料的晶体结构。然而，目前该方法却忽略了能够反映纳米材料缺陷及结构弛豫特征的布拉格衍射点周围的强度振荡信息。因此，本项目提出基于透射电镜发展针对特定衍射点的强度分布进行倒空间三维重构的表征方法。首先，拟建立在小倾转角度范围内，以小倾转步长对纳米材料特定衍射点进行精细三维扫描及重构的自动化实验与分析平台。以此平台为基础，将特定衍射点的倒空间三维重构方法应用于五次孪晶纳米线的内部结构弛豫现象研究，揭示此类纳米结构的弛豫机制与其本征失配角度、纳米线直径的关系。将该方法与理论模拟相结合深入理解并阐述金属及金属氧化物纳米颗粒的表面原子弛豫模式。

本项目扩展了三维电子衍射断层成像技术对特定衍射点的强度信息的采集和提取方法，并将该方法应用于五次孪晶纳米材料的内部结构弛豫现象及缺陷特征的研究。本项目建立了在小倾转角度范围内，以小倾转步长对纳米材料特定衍射点进行精细三维扫描的半自动化实验方法，通过自主编程实现了从二维系列倾转电子衍射花样中重构特定衍射点的三维空间强度分布的数据处理平台，建立了一套有效的实验及数据处理方法。.我们将以上建立的实验方法及数据分析处理程序应用于硼化物及贵金属银五次孪晶纳米线的内部结构研究中，揭示了此类纳米材料的循环孪晶结构特征、截面形貌、及其非均匀结构弛豫现象和内部缺陷分布特征。将衍射模拟结果与实验得到的特定衍射点三维强度分布进行对比分析，揭示了碳化硼五次孪晶纳米线内部非均匀结构弛豫现象的存在，且某些单晶结构单元内部存在高密度微孪晶结构，这些微孪晶缺陷的存在有利于降低由于角度失配而引起的碳化硼五次孪晶结构内部的应变能。而直径约30纳米的贵金属银纳米线的(331)衍射点在垂直于纳米线轴线的二维平面衍射强度分布具有明显的非对称特征，理论计算揭示该特征源于角度失配引起的银五次孪晶纳米线单晶结构单元内部非均匀应力场，即早期关于五次孪晶结构所提出的向错（disclination）结构模型。另外，某些(331)衍射点出现沿着特定晶向拉长的现象，这预示着某些单晶结构单元内部可能存在少量的{111}面缺陷或小角度位错墙。我们希望能将本课题发展的方法推广到其他复杂纳米结构的研究中。