Light sources and photodetectors enabled by 2D semiconductors

The successful preparation of 2D metallic, semiconducting, and insulating crystals enables many opportunities for fundamental studies and device applications. [1–3] Since the experimental exploration of direct-bandgap monolayer (1L) MoS2, 2D semiconductors have rapidly drawn great attention from the global research communities of materials, physics, engineering, and chemistry. [6–8] Optical bandgaps of emerging 2D semiconductors such as 1L transitionmetal dichalcogenides (TMDs),[9–14] 1L black phosphorous (BP),[15] ultrathin group III and IV chalcogenides (e.g., GaS and GeS),[16,17] and 1L hexagonal boron nitride (hBN),[18] cover the broad spectral range from ultraviolet to infrared wavelengths (Figure 1), which promotes the new development of photonic and optoelectronic applications. [17,19] These 2D semiconductors have exhibited many interesting properties like robust excitonic effects,[20] significant quantum confinement,[21] strong spin–orbit coupling,[22] and many-body interactions. [23] In particular, 1Ls of 2H-type MoS2, MoSe2, WS2, and WSe2 are direct-bandgap semiconductors, which are intrinsically suitable for light-emitting and photodetecting applications. [4,24–26] Meanwhile, atomically thin TMDs and their heterostructures have shown intriguing optical responses such as unique valley polarization,[27–29] room-temperature excitonic emission,[4] a wealth of excitonic states (e.g., trion, biexciton, dark exciton),[10,11,13,30,31] valley Zeeman splitting,[32–34] single-photon emission,[35–38] valleyselective optical Stark effect,[39] formation of exciton–polaritons,[40–42] and exciton–plasmon coupling. [43] Understanding of these effects is fundamentally important for developing highperformance optoelectronic devices with desirable and/or new features. Structurally, 2D semiconductors can be ultrathin, flexible, and transparent, which make them suitable for fabricating wearable optoelectronic devices. Moreover, controllable growth[44] and transfer techniques[45–47] of large-area 2D semiconductors make scalable and low-cost device fabrication practical. Technically, semiconductor-based light sources and photodetectors are two key elements for the realization of many prevailing optoelectronic applications, such as general illumination devices, various light indicators, optical storage, light sensing, optical interconnects, optical isolators, and optical The emerging 2D semiconductors have aroused increasing attention due to their fascinating fundamental properties and application prospects. Technical investigation of 2D semiconductor–based electronics and optoelectronics is paving the way to realizing practical applications, which opens up new opportunities to reshape the current semiconductor industry. Particularly, 2D semiconductor–based optoelectronics can be extensively utilized in the promising semiconductor and information industries, such as solid-state lighting, on-chip optical interconnects, quantum computing, and communication. Here, the research progress regarding the fabrication and characterization of rapidly growing light-emitting devices and photodetectors enabled by 2D semiconductors is reviewed. According to different emission mechanisms, 2D semiconductor–activated light sources are classified into four types: excitonic light-emitting diodes (LEDs), quantum LEDs, valley LEDs, and lasers. Moreover, photodetecting devices based on atomically thin MoS2, other 2D semiconductors, and van der Waals heterostructures are discussed, where diverse device structures, performance parameters, and working principles are compared. Furthermore, the remaining challenges in the realization of practical devices with desirable features are outlined and new research opportunities for 2D semiconductor optoelectronics are proposed. 2D Semiconductors