  We report on large exciton tuning in WSe$_2$ monolayers via substrate induced
non-degenerate doping. We observe a redshift of $\sim$62 meV for the $A$
exciton together with a 1-2 orders of magnitude photoluminescence (PL)
quenching when the monolayer WSe$_2$ is brought in contact with highly oriented
pyrolytic graphite (HOPG) compared to the dielectric substrates such as hBN and
SiO$_2$. As the evidence of doping from HOPG to WSe$_2$, a drastic increase of
the trion emission intensity was observed. Using a systematic PL and Kelvin
probe force microscopy (KPFM) investigation on WSe$_2$/HOPG, WSe$_2$/hBN, and
WSe$_2$/graphene, we conclude that this unique excitonic behavior is induced by
electron doping from the substrate. Our results propose a simple yet efficient
way for exciton tuning in monolayer WSe$_2$, which plays a central role in the
fundamental understanding and further device development.

Materials Science