  Strain engineering enables the direct modification of the atomic bonding and
is currently an active area of research aimed at improving the electrocatalytic
activity. However, directly measuring the lattice strain of individual catalyst
nanoparticles is challenging, especially at the scale of a single unit cell.
Here, we quantitatively map the strain present in rhodium@platinum (core@shell)
nanocube electrocatalysts using conventional aberration-corrected scanning
transmission electron microscopy (STEM) and the recently developed technique of
4D-STEM nanobeam electron diffraction. We demonstrate that 4D-STEM combined
with data pre-conditioning allows for quantitative lattice strain mapping with
sub-picometer precision without the influence of scan distortions. When
combined with multivariate curve resolution, 4D-STEM allows us to distinguish
the nanocube core from the shell and to quantify the unit cell size as a
function of distance from the core-shell interface. Our results demonstrate
that 4D-STEM has significant precision and accuracy advantages in strain
metrology of catalyst materials compared to aberration-corrected STEM imaging
and is beneficial for extracting information about the evolution of strain in
catalyst nanoparticles.
