Tillotson Evan, McHugh James G, Howarth James, Hashimoto Teruo, Clark Nicholas J, Weston Astrid, Enaldiev Vladimir, Sullivan-Allsop Sam, Thornley William, Wang Wendong, Lindley Matthew, Pollard Andrew J, Fal'ko Vladimir I, Gorbachev Roman V, Haigh Sarah J
Department of Materials, University of Manchester, Manchester M13 9PL, U.K.
National Graphene Institute, University of Manchester, Manchester M13 9PL, U.K.
ACS Nano. 2024 Dec 17;18(50):34023-34033. doi: 10.1021/acsnano.4c09364. Epub 2024 Dec 6.
Twisted two-dimensional (2D) material heterostructures provide an exciting platform for investigating fundamental physical phenomena. Many of the most interesting behaviors emerge at small twist angles, where the materials reconstruct to form areas of perfectly stacked crystals separated by partial dislocations. However, understanding the properties of these systems is often impossible without correlative imaging of their local reconstructed domain configuration, which exhibits random variations due to disorder and contamination. In particular, visualization of the local domain configuration allows determination of the local twist angle and, hence, the local lattice strain. Here, we demonstrate a simple and widely accessible route to visualize domains in the as-produced twisted transition metal dichalcogenide (TMD) heterostructures using electron channeling contrast imaging (ECCI) in scanning electron microscopy (SEM). This nondestructive approach is compatible with conventional substrates and allows domains to be visualized even when sealed beneath an encapsulation layer. Complementary theoretical calculations reveal how a combination of elastic and inelastic scattering leads to contrast inversions at the specified detector scattering angles and sample tilts. We demonstrate that optimal domain contrast is therefore achieved by maximizing signal collection while avoiding contrast inversion conditions.
扭曲的二维（2D）材料异质结构为研究基本物理现象提供了一个令人兴奋的平台。许多最有趣的行为出现在小扭曲角处，此时材料会重构形成由部分位错分隔的完美堆叠晶体区域。然而，如果没有对其局部重构畴结构进行相关成像，通常就无法理解这些系统的特性，因为局部重构畴结构会因无序和污染而呈现出随机变化。特别是，局部畴结构的可视化能够确定局部扭曲角，进而确定局部晶格应变。在此，我们展示了一种简单且广泛适用的方法，即利用扫描电子显微镜（SEM）中的电子通道对比度成像（ECCI）来可视化所制备的扭曲过渡金属二硫属化物（TMD）异质结构中的畴。这种无损方法与传统衬底兼容，即使在封装层之下密封时也能使畴可视化。补充性的理论计算揭示了弹性散射和非弹性散射的组合如何在指定的探测器散射角和样品倾斜度下导致对比度反转。我们证明，通过最大化信号收集同时避免对比度反转条件，能够实现最佳的畴对比度。
