# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 4 Light-Emitters  
Electroluminescence (EL)--the emission of light from an electronic device in response to an electric current--results from radiative recombination of charge carriers (electron-hole pairs or excitons) in a semiconductor. It forms the basis of light emitting optoelectronic devices like light-emitting diodes (LEDs) or semiconductor lasers. Monolayer TMDs are usually used for EL-devices because of their direct band gap. Light emission from multi-layer TMDs was reported by carrier redistribution from the indirect to the direct valleys in a high electric field [64], by filling the indirect valleys in the conduction and valence bands in an electric-double-layer transistor [65], or by the injection of hot electrons from metal/TMD junction [66]. [PERSON] _et al._[67] exploited hot carrier processes by driving a high unipolar current through a monolayer MoS\({}_{2}\) field-effect transistor to achieve electroluminescence. A high drain-source bias leads to strong band bending in the vicinity of the metal contacts of the device. Electrons, which are injected from the metal to the semiconductor channel, are accelerated by the electric field and can eventually gain enough kinetic energy to create electron-hole pairs by impact excitation. Due to a large exciton binding energy in 2D materials (which also prevents the bound electron-hole pairs from being dissociated by the electric field), impact excitation is very efficient in low-dimensional systems [68; 69]. The characteristic features of impact excitation include a threshold behavior and an exponential increase of the emission above threshold [68]. The localization of the light emission near the metal contact, where the electric field is the highest, was confirmed by spatial mapping. An electroluminescence efficiency of \(10^{-5}\) at wavelengths in accordance with the A-excitonic peak of MoS\({}_{2}\) was achieved. [PERSON] _et al._[70] demonstrated electroluminescence emission from MoS\({}_{2}\) grown by chemical vapor deposition (CVD). Transport in the space-charge limited regime and the existence of a large number of deep traps that act as localization centers, led to emission from an area close to the injecting contact at low source-drain bias in their device. Light emitting diodes usually rely on the recombination of electrons and holes in a p-n junction. Both types of carriers are injected into a channel where they recombine radiatively. This emission process does not have a threshold and is more efficient than electron-hole pair generation by impact excitation. Typically, two regions of a semiconductor are treated with different doping atoms. [PERSON] _et al._[71] showed chemical p-type doping of intrinsically \(n\)-type few-layer MoS\({}_{2}\) using AuCl\({}_{3}\) and demonstrated current rectification in their device, but light emission from chemically doped p-n junctions has not yet been demonstrated. Figure 4: (**a**) Schematic drawing of a graphene-TMD-graphene photodetector. Reproduced with permission from [59], Copyright Nature Publishing Group, 2013. (**b**) Electrical characteristics with and without optical illumination. Reproduced with permission from [60], Copyright The American Association for the Advancement of Science, 2013. (**c**) Schematic band diagrams for symmetric graphene doping (upper image; no photoresponse without bias) and asymmetric doping (lower image; charge separation due to built-in electric field). Reproduced with permission from [60]. [PERSON] _et al._[34], [PERSON] _et al._[35] and [PERSON] _et al._[72] demonstrated the first monolayer TMD p-n junctions, formed by electrostatic doping. In their devices two gate electrodes couple to different regions of a TMD crystal, as illustrated in Figure 5a. Biasing of one gate electrode with a positive voltage and the other gate electrode with a negative voltage leads to injection of electrons and holes into the channel. Efficient electron and hole transport was accomplished by two different metal contacts (Ti/Au, Pd/Au) fabricated on a monolayer of WSe\({}_{2}\).