Convergent beam electron holography for analysis of van der Waals heterostructures

Significance Assembling 2D materials into vertically stacked heterostructures allows an unprecedented control over their properties. The interaction between the individual crystals plays the crucial role here; thus, the information about the local atomic stacking is of great importance. Still, there are no techniques which would allow investigation of the stacking between such crystals with any reasonable throughput. We present the use of convergent beam electron diffraction (CBED) to investigate the quality of the interface in such heterostructures. We demonstrate that defects such as misorientation, strain, ripples, and others can be visualized, and quantitative information about such structures can be easily extracted. Furthermore, CBED images can be treated as holograms; thus their reconstruction gives 3D profiles of the heterostructures over a large area. The van der Waals heterostructures, which explore the synergetic properties of 2D materials when assembled into 3D stacks, have already brought to life a number of exciting phenomena and electronic devices. Still, the interaction between the layers in such assembly, possible surface reconstruction, and intrinsic and extrinsic defects are very difficult to characterize by any method, because of the single-atomic nature of the crystals involved. Here we present a convergent beam electron holographic technique which allows imaging of the stacking order in such heterostructures. Based on the interference of electron waves scattered on different crystals in the stack, this approach allows one to reconstruct the relative rotation, stretching, and out-of-plane corrugation of the layers with atomic precision. Being holographic in nature, our approach allows extraction of quantitative information about the 3D structure of the typical defects from a single image covering thousands of square nanometers. Furthermore, qualitative information about the defects in the stack can be extracted from the convergent diffraction patterns even without reconstruction, simply by comparing the patterns in different diffraction spots. We expect that convergent beam electron holography will be widely used to study the properties of van der Waals heterostructures.