  Functional properties of nanomaterials strongly depend on their surface
atomic structure, but they often become largely different from their bulk
structure, exhibiting surface reconstructions and relaxations. However, most of
the surface characterization methods are either limited to 2-dimensional
measurements or not reaching to true 3D atomic-scale resolution, and
single-atom level determination of the 3D surface atomic structure for general
3D nanomaterials still remains elusive. Here we show the measurement of 3D
atomic structure of a Pt nanoparticle at 15 pm precision, aided by a deep
learning-based missing data retrieval. The surface atomic structure was
reliably measured, and we find that <100> and <111> facets contribute
differently to the surface strain, resulting in anisotropic strain distribution
as well as compressive support boundary effect. The capability of single-atom
level surface characterization will not only deepen our understanding of the
functional properties of nanomaterials but also open a new door for fine
tailoring of their performance.
