Luo Xiaonan, Varambhia Aakash, Song Weixin, Ozkaya Dogan, Lozano-Perez Sergio, Nellist Peter D
Department of Materials, University of Oxford, Parks Road, OX1 3PH, Oxford, United Kingdom.
Johnson Matthey Technology Centre, Sonning Common, RG4 9NH, Reading, United Kingdom.
Ultramicroscopy. 2022 Sep;239:113561. doi: 10.1016/j.ultramic.2022.113561. Epub 2022 May 22.
Strain is a crucial factor that influences the physicochemical properties of nanoparticles. Being able to precisely measure strain is important in understanding the intrinsic mechanism of the enhanced performance of nanoparticles. Techniques that have been developed for strain analysis using scanning transmission electron microscopy (STEM) images can be categorized into diffraction-based method and imaging-based method. Here, using image simulation techniques, it is found that the measured two-dimensional (2D) displacements from annular dark field (ADF) STEM images of a nanoparticle are a good approximation to a projection of the actual three-dimensional (3D) displacements. A methodology for deformation analysis is presented which is based on the detection of atomic columns from atomic-resolution STEM images in real space. Elastic deformation parameters such as strain are usually defined on the basis of a continuum of deformation. The appropriateness of various deformation parameters for atomic-scale investigation on STEM images is explored and a method for determining these is presented. We found that the local lattice parameter and principal strain components are the most physically meaningful parameters to express the materials distortion behaviour. Apart from the local lattice parameter, the other deformation parameters such as normal strains, shear strains and displacements, heavily rely on the choice of reference lattice. It is also found that different reference grids add a series of uniform offsets to these strain variations. Finally, this approach is applied to a PtCo bimetallic nanoparticle to quantify its deformation behaviour.
应变是影响纳米颗粒物理化学性质的关键因素。能够精确测量应变对于理解纳米颗粒性能增强的内在机制很重要。利用扫描透射电子显微镜（STEM）图像进行应变分析所开发的技术可分为基于衍射的方法和基于成像的方法。在此，通过图像模拟技术发现，从纳米颗粒的环形暗场（ADF）STEM图像测量的二维（2D）位移很好地近似于实际三维（3D）位移的投影。提出了一种基于在实空间中从原子分辨率STEM图像检测原子列的变形分析方法。诸如应变等弹性变形参数通常基于连续变形来定义。探讨了各种变形参数在STEM图像原子尺度研究中的适用性，并提出了确定这些参数的方法。我们发现局部晶格参数和主应变分量是表达材料畸变行为最具物理意义的参数。除了局部晶格参数外，其他变形参数如法向应变、剪应变和位移，在很大程度上依赖于参考晶格的选择。还发现不同的参考网格会给这些应变变化添加一系列均匀的偏移量。最后，将该方法应用于PtCo双金属纳米颗粒以量化其变形行为。
