  Van der Waals heterostructures have been lately intensively studied because
they offer a large variety of properties that can be controlled by selecting 2D
materials and their sequence in the stack. The exact arrangement of the layers
as well as the exact arrangement of the atoms within the layers, both are
important for the properties of the resulting device. Recently it has been
demonstrated that convergent beam electron diffraction (CBED) allows
quantitative three-dimensional mapping of atomic positions in three-dimensional
materials from a single CBED pattern. In this study we investigate CBED in more
detail by simulating and performing various CBED regimes, with convergent and
divergent wavefronts, on a somewhat simplified system: a 2D monolayer crystal.
In CBED, each CBED spot is in fact an in-line hologram of the sample, where
in-line holography is known to exhibit high intensity contrast in detection of
weak phase objects that are not detectable in conventional in-focus imaging
mode. Adsorbates exhibit strong intensity contrast in zero and higher order
CBED spots, whereas lattice deformation such as strain or rippling cause
noticeable intensity contrast only in the first and higher order CBED spots.
The individual CBED spots can be reconstructed as typical in-line holograms,
and the resolution of 2.13 A can be in principle achieved in the
reconstructions. We provide simulated and experimental examples of CBED of a 2D
monolayer crystal. The simulations show that individual CBED spots can be
treated as in-line holograms and sample distributions such as adsorbates, can
be reconstructed. Individual atoms can be reconstructed from a single CBED
pattern provided the later exhibits high-order CBED spots. Examples of
reconstructions obtained from experimental CBED patterns, at a resolution of
2.7 A, are shown.
