semiconductors are thought to belong to the most promising candidates for future nanoelectronic applications, due to their unique advantages and capability in continuing the downscaling of complementary metal-oxide-semiconductor (CMOS) devices while retaining decent mobility. Recently, optoelectronic devices based on novel synthetic 2D semiconductors have been reported, exhibiting comparable performance to the traditional solid-state devices. This review briefly describes the development of the

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... growth of 2D crystals for applications in optoelectronics, including photodetectors, light-emitting diodes (LEDs), and solar cells. Such atomically thin materials with promising optoelectronic properties are very attractive for future advanced transparent optoelectronics as well as flexible and wearable/portable electronic devices. Crystals 2018, 8, 252 2 of 26 exfoliated samples, these CMOS-compatible fabrication techniques are essential in realizing the practical application of the 2D materials in future electronic devices. Therefore, to achieve stable, high-quality, and large-area growth of 2D materials and to establish integrated circuits based on the implementation of homogeneous device units, they have been widely pursued in recent years. Optoelectronic devices that can generate or sense light, or produce electric signals are important components in future optical-electrical, sensing, and energy-storage systems, such as light emitting diodes (LED), lasers, photodetectors, and solar cells. 2D semiconductors are intrinsically attractive in such applications due to their proper electronic band structures and ultra-thin body with scaling capabilities. Graphene with a single layer of carbon atoms in a honeycomb structure (schematically shown in Figure 1a ), has impressively high electron mobility owing to its distinctive bands [19] . However, the lack of intrinsic band gap limits the further application in electrical devices. Up to now, there has been no mature or well-controlled method to open up a gap in graphene and maintain the electronic mobility at the same time [20] . However, graphene has been identified as a promising candidate in optoelectronics, specifically in high-frequency optoelectronic application with the reported photodetector bandwidth up to GHz [11, 21] . On the other hand, TMDCs which typically have a general formula of MX 2 (M = Mo, W, Re, etc.; X = S, Se, Te) have thickness-dependent band gaps from 0.5 eV to 2 eV. Figure 1d and 1e illustrate the band structures of several typical TMDC semiconductors. For most TMDC materials like MoX 2 and WX 2 , the band gap shows a similar indirect-to-direct transformation with decreasing thickness. One exception is ReS 2 which possesses a direct band gap for both monolayer and multilayer/bulk (Figure 1c ) [22] . Therefore, TMDCs can be expected to exhibit good optoelectronic response in the near-infrared to visible spectral region. BP is a newly-discovered and stable semiconducting allotrope of phosphorus (structure shown in Figure 1b ) whose intrinsic hole mobility can be as high as 300-1000 cm 2 V −1 s −1 at room temperature [23, 24] , and the on-state current density can reach 850 µA/µm [25] . However, the device performance of BP field-effect transistor (FET) is still far below the standards of practical application owing to the fact that BP can be easily oxidized by O 2 and H 2 O at ambient [26, 27] . Monolayer BP has a direct band gap of 2 eV at the gamma point of the first Brillouin zone. The band gap reduces with increasing thickness, and finally reaches 0.35 eV for bulk BP [28] [29] [30] , resulting in attractive optoelectronic applications from the infrared to the visible region. In this paper, we review the development of approaches to synthesize thin films of 2D semiconductors, while presenting further strategies to extend their applications in future high-performance optoelectronic devices. Crystals 2018, 8, x FOR PEER REVIEW 2 of 26