  The ability to engineer atomically thin nanoscale lateral heterojunctions
(HJs) is critical to lay the foundation for future two-dimensional (2D) device
technology. However, the traditional approach to creating a heterojunction by
direct growth of a heterostructure of two different materials constrains the
available band offsets, and it is still unclear if large built-in potentials
are attainable for 2D materials. The electronic properties of atomically thin
semiconducting transition metal dichalcogenides (TMDs) are not static, and
their exciton binding energy and quasiparticle band gap depend strongly on the
proximal environment. Recent studies have shown that this effect can be
harnessed to engineer the lateral band profile of monolayer TMDs to create a
heterojunction. Here we demonstrate the synthesis of a nanoscale lateral
heterojunction in monolayer MoSe2 by intercalating Se at the interface of a
hBN/Ru(0001) substrate. The Se intercalation creates a spatially abrupt
modulation of the local hBN/Ru work function, which is imprinted directly onto
an overlying MoSe2 monolayer to create a large built-in potential of 0.83 eV.
We spatially resolve the MoSe2 band profile and work function using scanning
tunneling spectroscopy to map out the nanoscale depletion region. The Se
intercalation also modifies the dielectric environment, influencing the local
band gap renormalization and increasing the MoSe2 band gap by ~0.26 eV. This
work illustrates that environmental proximity engineering provides a robust
method to indirectly manipulate the band profile of 2D materials outside the
limits of their intrinsic properties, providing avenues for future device
design.
