  Two-dimensional (2d) nano-electronics, plasmonics, and emergent phases
require clean and local charge control, calling for layered, crystalline
acceptors or donors. Our Raman, photovoltage, and electrical conductance
measurements combined with \textit{ab initio} calculations establish the large
work function and narrow bands of $\alpha$-RuCl$_3$ enable modulation doping of
exfoliated, chemical vapor deposition (CVD), and molecular beam epitaxy (MBE)
materials. Short-ranged lateral doping (${\leq}65\ \text{nm}$) and high
homogeneity are achieved in proximate materials with a single layer of \arucl.
This leads to the highest monolayer graphene (mlg) mobilities ($4,900\
\text{cm}^2/ \text{Vs}$) at these high hole densities ($3\times10^{13}\
\text{cm}^{-2}$); and yields larger charge transfer to bilayer graphene (blg)
($6\times10^{13}\ \text{cm}^{-2}$). We further demonstrate proof of principle
optical sensing, control via twist angle, and charge transfer through hexagonal
boron nitride (hBN).
