  In photon upconverting core-shell nanoparticles, structure strongly dictates
performance. Conventional imaging in scanning transmission electron microscopy
has sufficient resolution to probe the atomic structure of these nanoparticles,
but contrast, dose, and projection limitations make conventional imaging modes
insufficient for fully characterizing these structures. Phase retrieval methods
provide a promising alternative imaging mode, and in particular, multislice
electron ptychography can recover depth-dependent information. Here, we study
beam-sensitive photon upconverting core-shell nanoparticles with a multislice
ptychography approach using a low electron dose to avoid damage. Large strain
fields arise in these heterostructures due to the mismatch in lattice parameter
between the core and the shell. We reconstruct both a nanoparticle that appears
defect-free and one that has a large break in the side and map the distribution
of strain in 3D by computing distortion fields from high-resolution potential
images of each slice. In the defect-free nanoparticle, we observe twisting of
the shell, while in the broken nanoparticle we measure the 3D position of the
crack, the core, and dislocations. These results highlight the advantage of
multislice electron ptychography to recover 3D information from a single scan,
even under strict electron dose requirements from beam-sensitive samples.
