Upconversion Electroluminescence in Plasmonic Van der Waals Tunnel Diodes

Plasmonic tunnel junctions comprising van der Waals semiconductor are an attractive platform where the interplay between inelastically tunneling electrons, surface plasmons, and excitons is expected to give rise to novel light emission phenomena. Here, we report observation of peculiar upconversion electroluminescene in van der Waals tunnel diodes comprising a monolayer transition metal dichalcogenide (TMD) in the electron tunneling pathway. The device exhibits bimodal electroluminescence with a broad low energy band and a narrow high energy band. Interestingly, the high energy emission, which is attributed to the TMD ground exciton, is found to turn on at applied biases significantly lower than the threshold defined by its emission energy, whereas the low energy emission, which arises from plasmonic emission, strictly obeys the quantum cut-off. We examine several possible model and show that momentum-indirect excitation of high energy carriers enabled by inelastic electron tunneling is a key component enabling the apparent energy gain.