Dynamical theories of dark‐field imaging using diffusely scattered electrons in STEM and TEM
Dynamical theories of dark‐field imaging using diffusely scattered electrons in STEM and TEM
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在 STEM 和 TEM 中使用漫散射电子的暗场成像的动力学理论
DOI：
10.1107/s0108767395002066
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发表时间：
1995
期刊：
影响因子：
--
通讯作者：
Z. L. Wang
中科院分区：
文献类型：
--
作者：
Z. L. Wang
基于第一性原理，提出了热漫散射（TDS）电子形成原子序数敏感像（或Z衬度像）的动力学理论。“精确”的理论推导出用于模拟在扫描透射电子显微镜（STEM）中使用环形暗场检测器或在透射电子显微镜（TEM）中使用轴上物镜孔径下空心锥束照明下获得的图像。原子的热振动用晶格动力学描述，考虑相位关联。该理论中全面涵盖的效应包括：TDS前后光束的动态衍射，依赖于厚度的光束展宽或沟道，缺陷区域的黄散射，相干长度内原子层包裹产生的热漫散射电子的相干性，多声子和多声子激发，以及探测器的几何形状。在各种近似下，从这种统一的方法导出了简化的理论。结果表明，非相干成像理论是实际成像条件的一种简化情况，仅适用于图像的定性解释。z方向的相干长度随柱中原子质量的变化而变化。因此，重元素的z相干性可能消失。最后，对黄散射在大角度暗场TEM成像中的理论进行了阐述，并在实验上观察到了理论预期的结果。
Dynamical theories of atomic number sensitive image (or Z-contrast image) formed by thermal diffusely scattered (TDS) electrons are proposed based on first-principles considerations. 'Exact' theories are derived for simulating images obtained either in scanning transmission electron microscopy (STEM) using an annular dark-field detector or in transmission electron microscopy (TEM) using an on-axis objective aperture under hollow-cone beam illumination. The atom thermal vibrations are described using lattice dynamics with consideration of phase correlations. The effects that are comprehensively covered in the theory include: dynamical diffraction of the beam before and after TDS, thickness-dependent beam broadening or channelling, Huang scattering from defect regions, coherence of the thermal diffusely scattered electrons generated from the atomic layers packed within the coherent length, multiphonon and multiple phonon excitations, and the detector geometry. Simplified theories have been derived from this unified approach under various approximations. It has been shown that the incoherent imaging theory is a much simplified case of the practical imaging condition, and can be applied only for qualitative image interpretation. The coherent length in the z direction varies with the change of atomic mass in the column. It is thus possible that the z coherence may disappear for heavy elements. Finally, the theory of Huang scattering in high-angle dark-field TEM imaging has been illustrated, and the theoretically expected results have been observed experimentally.