  The ability to create high-quality lateral p-n junctions at nanometer length
scales is essential for the next generation of two-dimensional (2D) electronic
and plasmonic devices. Using a charge-transfer heterostructure consisting of
graphene on $\alpha$-RuCl$_3$, we conduct a proof-of-concept study
demonstrating the existence of intrinsic nanoscale lateral p-n junctions in the
vicinity of graphene nanobubbles. Our multi-pronged experimental approach
incorporates scanning tunneling microscopy (STM) and spectroscopy (STS) and
scattering-type scanning near-field optical microscopy ($\textit{s}$-SNOM) in
order to simultaneously probe both the electronic and optical responses of
nanobubble p-n junctions. Our STM and STS results reveal that p-n junctions
with a band offset of more than 0.6 eV can be achieved over lateral length
scale of less than 3 nm, giving rise to a staggering effective in-plane field
in excess of 10$^8$ V/m. Concurrent $\textit{s}$-SNOM measurements confirm the
utility of these nano-junctions in plasmonically-active media, and validate the
use of a point-scatterer formalism for modeling surface plasmon polaritons
(SPPs). Model $\textit{ab initio}$ density functional theory (DFT) calculations
corroborate our experimental data and reveal a combination of sub-angstrom and
few-angstrom decay processes dictating the dependence of charge transfer on
layer separation. Our study provides experimental and conceptual foundations
for the use of charge-transfer interfaces such as graphene/$\alpha$-RuCl$_3$ to
generate p-n nano-junctions.
