Moiré superlattices in van der Waals heterostructures have given rise to a number of emergent electronic phenomena due to the interplay between atomic structure and electron correlations. Indeed, electrons in these structures have been recently found to exhibit a number of emergent properties that the individual layers themselves do not exhibit. This includes superconductivity^1,2, magnetism³, topological edge states^4,5, exciton trapping⁶ and correlated insulator phases⁷. However, the lack of a straightforward technique to characterize the local structure of moiré superlattices has thus far impeded progress in the field. In this work we describe a simple, room-temperature, ambient method to visualize real-space moiré superlattices with sub-5-nm spatial resolution in a variety of twisted van der Waals heterostructures including, but not limited to, conducting graphene, insulating boron nitride and semiconducting transition metal dichalcogenides. Our method uses piezoresponse force microscopy, an atomic force microscope modality that locally measures electromechanical surface deformation. We find that all moiré superlattices, regardless of whether the constituent layers have inversion symmetry, exhibit a mechanical response to out-of-plane electric fields. This response is closely tied to flexoelectricity wherein electric polarization and electromechanical response is induced through strain gradients present within moiré superlattices. Therefore, moiré superlattices of two-dimensional materials manifest themselves as an interlinked network of polarized domain walls in a non-polar background matrix.

由于原子结构和电子相关性之间的相互作用，范德华异质结构中的莫尔超晶格引起了许多新兴的电子现象。实际上，最近发现这些结构中的电子表现出许多单独层本身不表现出的新兴特性。这包括超导^1,2，磁性³，拓扑边缘状态^4,5，激子俘获⁶和相关的绝缘体相⁷。然而，迄今为止，缺乏一种简单的技术来表征波纹超晶格的局部结构已经阻碍了该领域的进展。在这项工作中，我们描述了一种简单的室温环境方法，以可视化方式在各种扭曲的范德华异质结构中（包括但不限于传导石墨烯，绝缘硼）以低于5nm的空间分辨率可视化具有3nm以下空间分辨率的莫尔超晶格氮化物和半导体过渡金属二卤化物。我们的方法使用压电响应力显微镜，一种原子力显微镜模式，可局部测量机电表面变形。我们发现，所有莫尔超晶格，无论构成层是否具有反对称性，都表现出对平面外电场的机械响应。该响应与柔电紧密相关，在柔电中，莫尔超晶格内存在的应变梯度引起了极化和机电响应。因此，二维材料的莫尔超晶格表现为非极性背景矩阵中极化畴壁的互连网络。