Band engineering using the van der Waals heterostructure of two-dimensional materials allows for the realization of high-performance optoelectronic devices by providing an ultrathin and uniform PN junction with sharp band edges. In this study, a highly sensitive photodetector based on the van der Waals heterostructure of WSe 2 and MoS 2 was developed. The MoS 2 was utilized as the channel for a phototransistor, whereas the WSe 2 −MoS 2 PN junction in the out-of-plane orientation was utilized as a charge transfer layer. The vertical built-in electric field in the PN junction separated the photogenerated carriers, thus leading to a high photoconductive gain of 10 6 . The proposed phototransistor exhibited an excellent performance, namely, a high photoresponsivity of 2700 A/W, specific detectivity of 5 × 10 11 Jones, and response time of 17 ms. The proposed scheme in conjunction with the largearea synthesis technology of two-dimensional materials contributes significantly to practical photodetector applications.

Low-power, nonvolatile memory is an essential electronic component to store and process the unprecedented data flood arising from the oncoming Internet of Things era. Molybdenum disulfide (MoS 2 ) is a 2D material that is increasingly regarded as a promising semiconductor material in electronic device applications because of its unique physical characteristics. However, dielectric formation of an ultrathin low-k tunneling on the dangling bond-free surface of MoS 2 is a challenging task. Here, MoS 2 -based low-power nonvolatile charge storage memory devices are reported with a poly(1,3,5-trimethyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) tunneling dielectric layer formed via a solvent-free initiated chemical vapor deposition (iCVD) process. The surface-growing polymerization and low-temperature nature of the iCVD process enable the conformal growing of low-k (≈2.2) pV3D3 insulating films on MoS 2 . The fabricated memory devices exhibit a tunable memory window with high on/off ratio (≈10 6 ), excellent retention times of 10 5 s with an extrapolated time of possibly years, and an excellent cycling endurance of more than 10 3 cycles, which are much higher than those reported previously for MoS 2based memory devices. By leveraging the inherent flexibility of both MoS 2 and polymer dielectric films, this research presents an important milestone in the development of low-power flexible nonvolatile memory devices.

In this study, we propose the fabrication of a photodetector based on the heterostructure of p-type Si and n-type MoS 2 . Mechanically exfoliated MoS 2 flakes are transferred onto a Si layer; the resulting Si−MoS 2 p−n photodiode shows excellent performance with a responsivity (R) and detectivity (D*) of 76.1 A/ W and 10 12 Jones, respectively. In addition, the effect of the thickness of the depletion layer of the Si−MoS 2 heterojunction on performance is investigated using the depletion layer model; based on the obtained results, we optimize the photoresponse of the device by varying the MoS 2 thickness. Furthermore, low-frequency noise measurement is performed for the fabricated devices. The optimized device shows a low noise equivalent power (NEP) of 7.82 × 10 −15 W Hz −1/2 . Therefore, our proposed device could be utilized for various optoelectronic devices for low-light detection.

An increasing demand for nonvolatile memory has driven extensive research on resistive switching memory because it uses simple structures with high density, fast switching speed, and low power consumption. To improve the storage density, the application of multilevel cells is among the most promising solutions, including three-dimensional cross-point array architectures. Two-dimensional nanomaterials have several advantages as resistive switching media, including flexibility, low cost, and simple fabrication processes. However, few reports exist on multilevel nonvolatile memory and its switching mechanism. We herein present a multilevel resistive switching memory based on graphene oxide (GO) and MoS 2 fabricated by a simple spin-coating process. Metallic 1T-MoS 2 nanosheets, chemically exfoliated by Li intercalation, were successfully embedded between two GO layers as charge-trapping sites. The resulting stacks of GO/MoS 2 /GO exhibited excellent nonvolatile memory performance with at least four resistance states, >10 2 endurance cycles, and >10 4 s retention time. Furthermore, the charge transport mechanism was systematically investigated through the analysis of low-frequency 1/f noise in various resistance states, which could be modulated by the input voltage bias in the negative differential resistance region. Accordingly, we propose a strategy to achieve multilevel nonvolatile memory in which the stacked layers of two-dimensional nanosheets are utilized as resistive and charge-storage materials.