# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 4 Light-Emitters
By driving a forward current through the p-n junction, electroluminescence emission was achieved at comparable wavelengths as the emission of the neutral exciton (\(\sim\)750 nm). This suggests that electrons and holes form excitons before radiative recombination. The electroluminescence efficiency in such devices is \(\sim\)0.1%, limited by resistive losses at the contacts and non-radiative recombination. Electroluminescence from WS\({}_{2}\) was achieved by using ionic liquid gating, yielding visible light emission at \(\sim\)630 nm [73]. Vertical structures--with current flowing perpendicular to the layers--overcome several problems that arise in their lateral counterparts. The contact resistance is reduced due to a larger contact area and the emission is enhanced due to higher current density. Moreover, the whole device area can contribute to light emission, which allows for easier scalability. Two types of vertical structures can be distinguished: First, a 2D material can be placed on top of a 3D semiconductor (2D/3D heterostack), and second, several different 2D materials can be stacked on top of each other (2D/2D heterostack). In both cases, van der Waals forces between the layers are high enough to keep the stack together. Three-dimensional/two-dimensional heterojunctions (illustrated in Figure 5b) have been shown by [PERSON] _et al._[76] and [PERSON] _et al._[74]. They placed a monolayer MoS\({}_{2}\) sheet on a silicon wafer, where the p-doped silicon acts as hole-injector, while electrons are injected from the intrinsically n-doped MoS\({}_{2}\) sheet. Light emission occurred from the entire device area [74]. Emission spectra showed contributions from neutral, charged and bound excitonic species, depending on current density and temperature. [PERSON] _et al._[64] demonstrated light emission from a few-layer MoS\({}_{2}\)/GaN heterostructures. Figure 5: Light emitting devices. (**a**) Lateral device design (top). Split-gate electrodes couple to two different regions of a WSe\({}_{2}\) monolayer to efficiently inject electrons and holes into the channel. The bottom panel shows electroluminescence response under a forward bias current of 200 nA. Reproduced with permission from [34], Copyright Nature Publishing Group, 2014. (**b**) 3D/2D heterojunction. A p-n junction is formed between p-doped bulk silicon and n-doped monolayer MoS\({}_{2}\). Light emission occurs at the whole junction area. Reproduced with permission from [74], Copyright American Chemical Society, 2014. (**c**) Schematic of a Si/SiO2/BN/Gr/BN/MoS2/BN/Gr/BN heterostack light-emitting diode (Gr, graphene; BN, hexagonal boron nitride) and corresponding band diagram under forward bias. Reproduced with permission from [75], Copyright Nature Publishing Group, 2015. (**d**) Optical microscope image of the emission of the device described in (**c**). Reproduced with permission from [75]. Two-dimensional/two-dimensional heterostructures can be composed of mono- or few-layer sheets of metallic (e.g., graphene), insulating (e.g., hexagonal boron nitride, hBN) or semiconducting (e.g., MoS\({}_{2}\) or WSe\({}_{2}\)) crystals. Different composition allows for different emission wavelengths, tailored to the need of the application. Graphene is widely used as a transparent electrode on top and bottom to inject electrons and holes into the TMD layer (Figure 5c). Thin hBN layers between the electrode and the semiconductor prevent carriers from direct tunneling between the graphene sheets and allow for accumulation of carriers in the semiconductor. Electroluminescence sets in above a threshold that is associated with the alignment of the top and bottom graphene Fermi levels with the TMD conduction and valence band, respectively. [PERSON] _et al._[75] could achieve external quantum efficiencies (EQEs) higher than 1% at room temperature, a tenfold increase compared to the lateral p-n junctions. By using multiple quantum well structures, EQEs of up to 8.4% were obtained at low temperatures.