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Monolayers of molybdenum and tungsten dichalcogenides are direct bandgapsemiconductors, which makes them promising for opto-electronic applications. Inparticular, van der Waals heterostructures consisting of monolayers of MoS2sandwiched between atomically thin hexagonal boron nitride (hBN) and grapheneelectrodes allows one to obtain light emitting quantum wells (LEQWs) withlow-temperature external quantum efficiency (EQE) of 1%. However, the EQE ofMoS2 and MoSe2-based LEQWs shows behavior common for many other materials: itdecreases fast from cryogenic conditions to room temperature, undermining theirpractical applications. Here we compare MoSe2 and WSe2 LEQWs. We show that theEQE of WSe2 devices grows with temperature, with room temperature EQE reaching5%, which is 250x more than the previous best performance of MoS2 and MoSe2quantum wells in ambient conditions. We attribute such a different temperaturedependences to the inverted sign of spin-orbit splitting of conduction bandstates in tungsten and molybdenum dichalcogenides, which makes thelowest-energy exciton in WSe2 dark.

WSe2 light-emitting tunneling transistors with enhanced brightness at room temperature