  Two-dimensional (2D) materials, such as graphene1, boron nitride2, and
transition metal dichalcogenides (TMDs)3-5, have sparked wide interest in both
device physics and technological applications at the atomic monolayer limit.
These 2D monolayers can be stacked together with precise control to form novel
van der Waals heterostructures for new functionalities2,6-9. One highly coveted
but yet to be realized heterostructure is that of differing monolayer TMDs with
type II band alignment10-12. Their application potential hinges on the
fabrication, understanding, and control of bonded monolayers, with bound
electrons and holes localized in individual monolayers, i.e. interlayer
excitons. Here, we report the first observation of interlayer excitons in
monolayer MoSe2-WSe2 heterostructures by both photoluminescence and
photoluminescence excitation spectroscopy. The energy and luminescence
intensity of interlayer excitons are highly tunable by an applied vertical gate
voltage, implying electrical control of the heterojunction band-alignment.
Using time resolved photoluminescence, we find that the interlayer exciton is
long-lived with a lifetime of about 1.8 ns, an order of magnitude longer than
intralayer excitons13-16. Our work demonstrates the ability to optically pump
interlayer electric polarization and provokes the immediate exploration of
interlayer excitons for condensation phenomena, as well as new applications in
2D light-emitting diodes, lasers, and photovoltaic devices.
