  The ability to control the size of the electronic bandgap is an integral part
of solid-state technology. Atomically-thin two-dimensional crystals offer a new
approach for tuning the energies of the electronic states based on the
interplay between the environmental sensitivity and unusual strength of the
Coulomb interaction in these materials. By engineering the surrounding
dielectric environment, we are able to tune the electronic bandgap in
monolayers of WS2 and WSe2 by hundreds of meV. We exploit this behavior to
present an in-plane dielectric heterostructure with a spatially dependent
bandgap, illustrating the feasibility of our approach for the creation of
lateral junctions with nanoscale resolution. This successful demonstration of
bandgap engineering based on the non-invasive modification of the Coulomb
interaction should enable the design of a new class of atomically thin devices
to advance the limits of size and functionality for solid-state technologies.
