Publications

The layered transition metal dichalcogenides (TMDs) have attracted considerable interest for their unique layer-number dependent electronic and optical properties. The monolayer MoS2 and WSe2 exhibit a direct bandgap with strong photoluminescence and are of particular interest for new types of optoelectronic devices. However, the multi-layer MoS2 is an indirect bandgap semiconductor and generally believed to be optically inactive, with the photoluminescence typically more than 3 orders of magnitude weaker than that of the monolayers. 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 and large vertical electric field across the multi-layer MoS2. 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 larger 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.