and unique band structures. [1] Semiconducting monolayers of the TMDs, such as MoS 2 and WSe 2 , possess a direct band gap and excellent photoresponsivity. [2,3] In addition, single-layer MoS 2 is moderately easy to attain Ohmic contact with various metals [4][5][6][7] and graphene, [8] and the singlelayer MoS 2 field-effect transistors (FETs) show decent performances on the mobility (tens of cm 2 V −1 s −1 ) [9,10] and the on-off current ratio (exceeding 10 8 ) [4,10] at room temperature, suggesting that the monolayer MoS 2 holds greater potential for flexible nanodevices and optoelectronic applications. The huge surface-to-volume ratio of the atomically thin TMDs makes their performances extremely sensitive to the surface and interface conditions. The low-frequency electrical noise of devices often discloses the influences from the surface [11] and the interface conditions and has been widely adopted as a nondestructive tool to study the interface conditions. The subthreshold slope of the FETs and the photo response time of the MoS 2 photodetectors are significantly determined by the interfacial traps. [12] Unlike the thermal noise and shot noise that possess a constant power spectral density, low-frequency noise typically has a 1/f spectrum, and its effect on the accuracy of a device cannot be reduced by increasing the averaging time. [13] Such noise manner causes a critical issue of applications of the TMDs in the signal sensing and processing, [14] and raises numerous interests in investigating the 1/f noise in the TMD-based devices for identifying its origin and for reducing its strength.The first study of low-frequency electrical noise in exfoliated monolayer MoS 2 [15] claimed that the noise has a 1/f spectrum and is mainly explained by fluctuation of the mobility. By contrast, the 1/f noise in the exfoliated MoTe 2[16] with the ambipolar transport behaviors and many other TMD FETs [17][18][19][20] is further described as fluctuation of the carrier density. Finding a clear picture of the possible factors contributing to the 1/f noise, including the external adsorbates on the devices, [15] the defect states at the metal/TMD contacts, [17] and the trappingdetrapping centers in the substrate near the TMD channel, [18,21] remains a challenging issue because of the lack of Ohmic contacts, ideal material quality, and an effective control of the carrier density. Recently, the high-quality chemical-vapor-deposited (CVD) MoS 2 monolayers show the comparable electrical