  Doping inhomogeneities in solids are not uncommon, but their microscopic
observation and understanding are limited due to the lack of bulk-sensitive
experimental techniques with high-enough spatial and spectral resolution. Here,
we demonstrate nanoscale imaging of both dopants and free charge carriers in
La-doped BaSnO3 (BLSO) using high-resolution electron energy-loss spectroscopy
(EELS). By analyzing both high- and low-energy excitations in EELS, we reveal
chemical and electronic inhomogeneities within a single BLSO nanocrystal. The
inhomogeneous doping leads to distinctive localized infrared surface plasmons,
including a novel plasmon mode that is highly confined between high- and
low-doping regions. We further quantify the carrier density, effective mass,
and dopant activation percentage from EELS data and transport measurements on
the bulk single crystals of BLSO. These results represent a unique way of
studying heterogeneities in solids, understanding structure-property
relationships at the nanoscale, and opening the way to leveraging nanoscale
doping texture in the design of nanophotonic devices.
