The mid-infrared (MIR) spectral range is of immense use for civilian and military applications. The large number of vibrational absorption bands in this range can be used for gas sensing, process control and spectroscopy. In addition, there exists transparency windows in the atmosphere such as that between 3.6-3.8 µm, which are ideal for free-space optical communication, range finding and thermal imaging. A number of different semiconductor platforms have been used for MIR light-emission. This includes InAsSb/InAs quantum wells 1 , InSb/AlInSb 2 , GaInAsSbP pentanary alloys 3 , and intersubband transitions in group III-V compounds 4 . These approaches, however, are costly and lack the potential for integration on silicon and silicon-on-insulator platforms. In this respect, two-dimensional (2D) materials are particularly attractive due to the ease with which they can be heterointegrated. Weak interactions between neighbouring atomic layers in these materials allows for deposition on arbitrary substrates and van der Waals heterostructures enable the design of devices with targeted optoelectronic properties. In this Letter, we demonstrate a light-emitting diode (LED) based on the 2D semiconductor black phosphorus (BP). The device, which is composed of a BP/molybdenum disulfide (MoS 2 ) heterojunction emits polarized light at l = 3.68 μm with room-temperature internal and external quantum efficiencies (IQE and EQE) of ~1% and ~3×10 -2 %, respectively. The ability to tune the bandgap, and consequently emission wavelength of BP, with layer number, strain and electric field make it a particularly attractive platform for MIR emission.Electroluminescence (EL) from 2D transition metal dichalcogenides (TMDs) was observed shortly after monolayers from this class of materials were first isolated 5,6,7,8,9 . In monolayer TMD crystals, the formation of a direct bandgap allows for reasonable light-emission efficiencies to be achieved. The high degree of confinement in monolayers also ensures a large exciton binding

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