  The layered transition metal dichalcogenides (TMDs) have attracted
considerable interest due to their unique electronic and optical properties.
Here we report electric field induced strong electroluminescence in multi-layer
MoS2 and WSe2. We show that GaN-Al2O3-MoS2 and GaN-Al2O3-MoS2-Al2O3-graphene
vertical heterojunctions can be created with excellent rectification behaviour.
Electroluminescence studies demonstrate prominent direct bandgap excitonic
emission in multi-layer MoS2 over the entire vertical junction area.
Importantly, the electroluminescence efficiency observed in multi-layer MoS2 is
comparable to or even higher than that in monolayers, corresponding to a
relative electroluminescence enhancement factor of >1000 in multi-layer MoS2
when compared to its photoluminescence. This striking enhancement of
electroluminescence can be attributed to the high electric field induced
carrier redistribution from low energy points (indirect bandgap) to high energy
points (direct bandgap) of k-space, arising from the unique band structure of
MoS2 with a much higher density of states at high energy points. The electric
field induced electroluminescence is general for other TMDs including WSe2, and
can provide a fundamental platform to probe the carrier injection, population
and recombination in multi-layer TMDs and open up a new pathway toward TMD
based optoelectronic devices.
