**1. Introduction**

Photodetectors are one of the most important devices in photonic chips, which show a great potential in optical communication, flame sensing, environmental monitoring, and astronomical studies [1]. A lot of semiconductors have been exploited for photodetection such as silicon, GaN, PbS, InGaAs and HgCdTe, operating from ultraviolet to far-infrared region [2]. It is significant to decrease the dimension of photodetectors down to the sub-nano scale for the next generation highly integrated photonic chips.

Two-dimensional (2D) materials such as graphene and layered transition metal dichalcogenides (TMDs) have attractive electronic and optical properties such as flexibility, transparency and metal-oxide-semiconductor (CMOS) compatibility [3–7]. Tungsten disulfide (WS2), a typical member of the TMDs group, has a higher carrier mobility than other TMD materials due to smaller electron effective mass [8]. Moreover, WS2 enjoys excellent thermal stability for extensive applications [9].

Hence, WS2-based high stable photodetectors can be used in many attractive applications such as extreme environment detection. Furthermore, Two-dimensional stacked molybdenum disulfide (MoS2) has attracted many research interests for applications in optoelectronic devices, due to its outstanding merits of electronic and optical properties, especially photodetectors. But the few layers 2D materialbased photodetectors suffer from low photoresponsivity mainly due to the poor optical absorption in the atomic layered materials.

A number of 2D material-based photodetectors have been enhanced using resonance microcavity, PbS quantum dots, tunneling effect, heterojunctions or perovskite [10–14]. The narrow band absorption or complex preparation process limits the application of these attractive methods. Recently, localized surface plasmon resonance (LSPR)-enhanced photodetector has been demonstrated, and we have also provided effective ways to enhance the efficiency of light-harvesting [15–17]. LSPR can be excited by Ag or Au nanoparticles (NPs) such as deep metallic grating, nanodisk array, bowtie array, hybrid antenna and fractal metasurface [18–20]. Furthermore, patterning 2D materials into periodic structure can also excite plasmon resonance [21]. The methods for preparing the nanostructure involve electron beam lithography, hydrothermal synthesis, and template-based electrochemical method, which are generally complicated, expensive and may deleterious to the device.

For the current existing problems, we have demonstrated two methods for easy-preparation and high-performance 2D material-based photodetectors [22, 23]. (i) Au NPs solution was directly spun coated onto WS2-based photodetectors. The performance has been enhanced by the LSPR of Au NPs, and reach quite high responsivity of 1050 A/W at the wavelength of 590 nm. The diameters and distance of Au NPs will affect the resonant wavelength and absorption of the device. (ii) We have demonstrated a MoS2 plasmonic photodetector by depositing Au NPs on MoS2 sheet using magnetron sputtering without need of template, which shows a significant improvement of photo-response in near-infrared region. The spectral response of pure MoS2 was in visible light and which was extended to near-infrared region (700–1600 nm). Furthermore, the responsivity reaches up to 64 mA/W when the incident light is 980 nm. Detailed preparation and discussion of the mechanism are performed in the chapter.
