Van der Waals bound heterostructures constructed with two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, have sparked wide interest in device physics and technologies at the two-dimensional limit. One highly coveted heterostructure is that of differing monolayer transition metal dichalcogenides with type-II band alignment, with bound electrons and holes localized in individual monolayers, that is, interlayer excitons. Here, we report the observation of interlayer excitons in monolayer MoSe 2 -WSe 2 heterostructures by photoluminescence and photoluminescence excitation spectroscopy. We find that their energy and luminescence intensity are highly tunable by an applied vertical gate voltage. Moreover, we measure an interlayer exciton lifetime of B1.8 ns, an order of magnitude longer than intralayer excitons in monolayers. Our work demonstrates optical pumping of interlayer electric polarization, which may provoke further exploration of interlayer exciton condensation, as well as new applications in two-dimensional lasers, light-emitting diodes and photovoltaic devices.

Light-emitting diodes are of importance for lighting, displays, optical interconnects, logic and sensors [1][2][3][4][5][6][7][8] . Hence the development of new systems that allow improvements in their efficiency, spectral properties, compactness and integrability could have significant ramifications. Monolayer transition metal dichalcogenides have recently emerged as interesting candidates for optoelectronic applications due to their unique optical properties [9][10][11][12][13][14][15][16] . Electroluminescence has already been observed from monolayer MoS2 devices 17,18 . However, the electroluminescence efficiency was low and the linewidth broad due both to the poor optical quality of MoS2 and to ineffective contacts. Here, we report electroluminescence from lateral p-n junctions in monolayer WSe2 induced electrostatically using a thin boron nitride support as a dielectric layer with multiple metal gates beneath.This structure allows effective injection of electrons and holes, and combined with the high optical quality of WSe2 it yields bright electroluminescence with 1000 times smaller injection current and 10 times smaller linewidth than in MoS2 17,18 . Furthermore, by increasing the injection bias we can tune the electroluminescence between regimes of impurity-bound, charged, and neutral excitons. This system has the required ingredients for new kinds of optoelectronic devices such as spin-and valley-polarized light-emitting diodes, on-chip lasers, and two-dimensional electro-optic modulators. Main TextFew-layer group-VIB transition metal dichalcogenides (TMDs) represent a class of semiconductors in the two-dimensional (2D) limit 9,10,19 . Due to their large carrier effective mass and the reduced screening in 2D, electron-hole interactions are much stronger than in conventional semiconductors. This leads to large binding energies for both charged and neutral excitons which as a result are spectrally sharp, robust, and amenable to electrical manipulation 16,20,21 . In addition, the large spin-orbit coupling 22 and the acentric structure of TMDs provides a connection between spin and valley degrees of freedom 14 , light polarization 11,13,15,16 , and magnetic and electric fields 23 that can be exploited for new kinds of device operation.Although in bulk TMDs the band gap is indirect, in the limit of a single monolayer it becomes direct 9,10 , fulfilling the most basic requirement for efficient light emission. Indeed, electroluminescence (EL) has already been reported from monolayer MoS2 field-effect transistors (FETs), occurring near the Schottky contact with a metal 17 or with highly doped silicon 18 .However, the efficiency and spectral quality was much lower than has been demonstrated for other nanoscale light emitters such as carbon nanotubes 7 , for two reasons. First, efficient EL requires effective injection of both electrons and holes into the active region, which should therefore be within a p-n junction. Second, MoS2 is known to have poorer optical quality than other group VIB TMDs, possi...