(color online) (a) Schematic of monolayer WSe₂ p–n junction devices with palladium back gates (V_g1 and V_g2) and source (S) and drain (D) contacts (left). Photoluminescence and electroluminescence of the LEDs (middle). Electroluminescence intensity as a function of bias current and photon energy (right). From left to right, the arrows indicate the impurity-bound exciton, the negative and positive charged excitons and the neutral exciton.^[92] (b) Schematic illustrations of an ionic-liquid gated device in the ambipolar injection regime (top). Optical microscope images of the monolayer device with clearly visible spot of emitted light due to electroluminescence (bottom left). Photoluminescence and electroluminescence spectra of the LEDs (bottom right).^[93] (c) Circularly polarized electroluminescence spectra from a polarized WSe₂-based LED for two opposite current directions (top). The contribution to electroluminescence from two valleys (bottom).^[94]