  In this work we demonstrate that a free-standing van der Waals
heterostructure, usually regarded as a flat object, can exhibit an intrinsic
buckled atomic structure resulting from the interaction between two layers with
a small lattice mismatch. We studied a freely suspended membrane of well
aligned graphene on a hexagonal boron nitride (hBN) monolayer by transmission
electron microscopy (TEM) and scanning TEM (STEM). We developed a detection
method in the STEM that is capable of recording the direction of the scattered
electron beam and that is extremely sensitive to the local stacking of atoms.
Comparison between experimental data and simulated models shows that the
heterostructure effectively bends in the out-of-plane direction, producing an
undulated structure having a periodicity that matches the moir\'e wavelength.
We attribute this rippling to the interlayer interaction and also show how this
affects the intralayer strain in each layer.
