Department of Electrophysics, National Chiao Tung University , Hsinchu 30010, Taiwan.
ACS Nano. 2014 Mar 25;8(3):2951-8. doi: 10.1021/nn500228r. Epub 2014 Feb 27.
Optical second harmonic generation (SHG) is known as a sensitive probe to the crystalline symmetry of few-layer transition metal dichalcogenides (TMDs). Layer-number dependent and polarization resolved SHG have been observed for the special case of Bernal stacked few-layer TMDs, but it remains largely unexplored for structures deviated from this ideal stacking order. Here we report on the SHG from homo- and heterostructural TMD bilayers formed by artificial stacking with an arbitrary stacking angle. The SHG from the twisted bilayers is a coherent superposition of the SH fields from the individual layers, with a phase difference depending on the stacking angle. Such an interference effect is insensitive to the constituent layered materials and thus applicable to hetero-stacked bilayers. A proof-of-concept demonstration of using the SHG to probe the domain boundary and crystal polarity of mirror twins formed in chemically grown TMDs is also presented. We show here that the SHG is an efficient, sensitive, and nondestructive characterization for the stacking orientation, crystal polarity, and domain boundary of van der Waals heterostructures made of noncentrosymmetric layered materials.
光学二次谐波产生（SHG）被认为是探测少层过渡金属二硫属化物（TMDs）晶体对称性的灵敏探针。对于特殊的 Bernal 堆叠少层 TMDs，已经观察到了依赖于层数和偏振分辨的 SHG，但对于偏离这种理想堆叠顺序的结构，它在很大程度上仍未被探索。在这里，我们报告了由具有任意堆叠角的人工堆叠形成的同构和异构 TMD 双层的 SHG。扭曲双层的 SHG 是来自各个层的 SH 场的相干叠加，其相位差取决于堆叠角。这种干涉效应对组成层状材料不敏感，因此适用于异质堆叠双层。我们还展示了使用 SHG 探测在化学生长的 TMD 中形成的镜像孪晶的畴界和晶体极性的概念验证演示。我们在这里表明，SHG 是一种有效的、灵敏的、非破坏性的方法，可以对由非中心对称层状材料制成的范德华异质结构的堆叠方向、晶体极性和畴界进行表征。
