  Recent developments in fabrication of van der Waals heterostructures enable
new type of devices assembled by stacking atomically thin layers of
two-dimensional materials. Using this approach, we fabricate light-emitting
devices based on a monolayer WSe$_2$, and also comprising boron nitride
tunnelling barriers and graphene electrodes, and observe sharp luminescence
spectra from individual defects in WSe$_2$ under both optical and electrical
excitation. This paves the way towards the realization of electrically-pumped
quantum emitters in atomically thin semiconductors. In addition we demonstrate
tuning by more than 1 meV of the emission energy of the defect luminescence by
applying a vertical electric field. This provides an estimate of the permanent
electric dipole created by the corresponding electron-hole pair. The
light-emitting devices investigated in our work can be assembled on a variety
of substrates enabling a route to integration of electrically pumped single
quantum emitters with existing technologies in nano-photonics and
optoelectronics.
