  Van der Waals heterostructures, which explore the synergetic properties of
two-dimensional (2D) materials when assembled into three-dimensional stacks,
have already brought to life a number of exciting new phenomena and novel
electronic devices. Still, the interaction between the layers in such assembly,
possible surface reconstruction, intrinsic and extrinsic defects are very
difficult to characterise 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, out-of-plane corrugation of the layers with atomic precision. Being
holographic in nature, our approach allows extraction of quantitative
information about the three-dimensional structure of the typical defects from a
single image covering thousands of square nanometres. 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.
