# Optoelectronic Devices Based on Atomically Thin Transition Metal Dichalcogenides
## 5 Single Photon Emitters  
In quantum technologies (_i.e._, quantum communications and quantum computing), single-photon sources are a basic building block. Single molecules, quantum dots or color centers in diamond are traditionally used for the generation of single-photons. In TMDs, bright and narrow lines in the PL spectrum are observed at crystal imperfections, mostly at the flake-edge or the transition from mono- to few-layer flakes (compare Figure 7a,b). These structural defects and/or local strain are possibly  
Figure 6: **(a)** Photonic crystal cavity WSe\({}_{2}\) nanolaser. The color profile shows the fundamental cavity mode before a WSe\({}_{2}\) monolayer is transferred on the structure. The on-cavity excitation photoluminescence spectrum (red) shows a narrow emission line compared to off-cavity excitation (green). Reproduced with permission from [81], Copyright Nature Publishing Group, 2015. **(b)** Schematic image of a WS\({}_{2}\) microdisc laser. Sandwiching the WS\({}_{2}\) monolayer between silicon nitride and hydrogen silsesquioxane (HSQ) ensures a higher confinement factor. Reproduced with permission from [83], Copyright Nature Publishing Group, 2015.  
the origin of the narrow-line emission. Studies performed by several groups [84; 85; 86; 87; 88] confirmed single photon emission from defect centers by photon antibunching measurements (Figure 7c). Second-order autocorreleation factors \(g^{(2)}(0)\) in the range between 0.14 and 0.36 were determined. Due to charge confinement and strong coupling between charge carriers and phonons, quantum emission was observed only at low temperatures. The emission linewidth was determined to be about 100-120 \(\upmu\)eV for freestanding [84] and supported samples [85; 86; 87; 88]. Luminescence decay times were measured to be in the range of a few ns, comparable to the decay time of the free exciton at room temperature (-4 ns) [89].  
All studies confirm long-term stability of the defect centers and robustness against temperature cycling as well as exposure to ambient (air) conditions. Temporal stability of the spectral position (jittering) was found to be on the order of the linewidth (\(\sim\)1 meV) on a millisecond timescale and larger jumps occurred on a longer timescale [87]. Stability improved for low excitation power and freshly exfoliated samples. Jittering may be reduced by encapsulation of the TMD sheets in dielectric coatings like thin flakes of hBN.  
[PERSON] _et al._[84] found that single photon emission centers can be introduced artificially by mechanical scratching. An external electric field can be used to control the emission of localized defect centers, while a magnetic field allows for spectral manipulation [87; 88]. Electrically driven single-photon emission, which would allow for convenient and compact sources, is yet to be demonstrated.  
Quantum emission from mono- and multi-layer hexagonal boron nitride (hBN) with high emission rates at room temperature was demonstrated by [PERSON] _et al._[90]. Calculations confirmed that anti-site nitrogen vacancies are responsible for single-photon emission with an energy within the bandgap of hBN.