  Next-generation, atomically thin devices require in-plane, one-dimensional
heterojunctions to electrically connect different two-dimensional (2D)
materials. However, the lattice mismatch between most 2D materials leads to
unavoidable strain, dislocations, or ripples, which can strongly affect their
mechanical, optical, and electronic properties. We have developed an approach
to map 2D heterojunction lattice and strain profiles with sub-picometer
precision and to identify dislocations and out-of-plane ripples. We collected
diffraction patterns from a focused electron beam for each real-space scan
position with a high-speed, high dynamic range, momentum-resolved detector -
the electron microscope pixel array detector (EMPAD). The resulting
four-dimensional (4D) phase space datasets contain the full spatially resolved
lattice information of the sample. By using this technique on tungsten
disulfide (WS2) and tungsten diselenide (WSe2) lateral heterostructures, we
have mapped lattice distortions with 0.3 pm precision across multi-micron
fields of view and simultaneously observed the dislocations and ripples
responsible for strain relaxation in 2D laterally-epitaxial structures.
