Department of Materials Science and Mechanical Engineering, Swanson School of Engineering, University of Pittsburgh, 636 Benedum Hall, 3700 O Hara Street, Pittsburgh PA 15261, USA.
Ultramicroscopy. 2013 Mar;126:48-59. doi: 10.1016/j.ultramic.2012.12.016. Epub 2012 Dec 23.
The measurement of accurate and precise structure factors and Debye Waller (DW) factors by quantitative convergent beam electron diffraction (QCBED) permits experimental determination of the electron density distribution and probing of interatomic bonding in crystals. The three QCBED methods used successfully for high precision measurements of low order structure factors to date, namely the zone axis pattern (ZAP) method, the excited row ER method and the multi-beam off-zone axis (MBOZA) technique, differ from each other regarding the crystal orientation relative to the incident electron beam. Consequently, the details of their respective dispersion surface representations differ regarding the number, relative amplitudes and phases of excited Bloch wave branches. Under the same experimental setup conditions, the factors most important to the degree of accuracy and precision achievable in electron density determination for crystals with QCBED methods ultimately depend on the sensitivity of the excited Bloch wave branches and the resultant contrast in the respective CBED patterns to changes in both structure and DW factors. In general, a QCBED pattern will be more sensitive to changes in both structure and DW factor, if it contains more and stronger excited Bloch wave branches, as dynamic interactions of the Bloch waves increase the sensitivity of the pattern. In this work we analyzed Bloch wave excitation and dispersion surfaces for the three most popular QCBED methods. The analysis indicates, that the QCBED patterns obtained using the MBOZA orientation generally contain more and stronger excited Bloch wave branches. Hence, MBOZA diffraction patterns are more sensitive than the ZAP and the ER patterns to changes in both DW and structure factors and therefore allow in differences to the other two methods simultaneous refinements effectively and robustly.
通过定量会聚束电子衍射 (QCBED) 测量准确和精确的结构因子和德拜-沃勒（DW）因子，可以实验确定晶体中的电子密度分布并探测原子间键合。迄今为止，成功用于高精度测量低阶结构因子的三种 QCBED 方法，即晶带轴花样（ZAP）法、激发行 ER 法和多束离轴（MBOZA）技术，在晶体相对于入射电子束的取向方面彼此不同。因此，它们各自的色散面表示在激发布洛赫波分支的数量、相对幅度和相位方面存在差异。在相同的实验设置条件下，对于使用 QCBED 方法的晶体，电子密度确定的精度和精度达到的程度最重要的因素最终取决于激发布洛赫波分支的灵敏度以及各自的 CBED 图案中结构和 DW 因子变化的对比度。一般来说，如果 QCBED 图案包含更多和更强的激发布洛赫波分支，则对结构和 DW 因子的变化更敏感，因为布洛赫波的动态相互作用会增加图案的灵敏度。在这项工作中，我们分析了三种最流行的 QCBED 方法的布洛赫波激发和色散面。分析表明，使用 MBOZA 取向获得的 QCBED 图案通常包含更多和更强的激发布洛赫波分支。因此，MBOZA 衍射图案对 DW 和结构因子的变化比 ZAP 和 ER 图案更敏感，因此允许同时有效地和稳健地对其他两种方法进行细化。
