  Real-space imaging of three-dimensional atomic structures is a critical yet
challenging task in materials science. Although scanning transmission electron
microscopy has achieved sub-angstrom lateral resolution through techniques like
electron ptychography1,2, depth resolution remains limited to only 2 to 3
nanometers with a single projection setup3,4. Attaining better depth resolution
typically necessitates large sample tilt angles and many projections, as seen
in atomic electron tomography5,6. Here, we develop a new algorithm based on
multislice electron ptychography which couples only a few projections at small
tilt angles, but is sufficient to improve the depth resolution by more than
threefold to the sub-nanometer scale, and potentially to the atomic level. This
technique maintains high resolving power for both light and heavy atoms, and
significantly improves the visibility of single dopants. We are thus able to
experimentally detect dilute substitutional praseodymium dopants in a
brownmillerite oxide, Ca2Co2O5, in three dimensions and observe the
accompanying lattice distortion. This technique requires only a moderate level
of data acquisition or processing, and can be seamlessly integrated into
electron microscopes equipped with conventional components.
