Monolayer transition metal dichalcogenides (TMDs) and their van der Waals heterostructures have been experimentally and theoretically demonstrated as potential candidates for photovoltaic and optoelectronic devices due to the suitable bandgap and excellent light absorption. In this work, we report the observation of photodiode behavior in (both n-and p-type) silicon/monolayer MoS 2 vertical heterostructures. The photocurrent and photoresponsivity of heterostructures photodiodes were dependent both on the incident light wavelength and power density, and the highest photoresponsivity of 7.2 A/W was achieved in n-Si/ monolayer MoS 2 vertical heterostructures photodiodes. Compared with n-Si/MoS 2 heterostructures, the photoresponsivity of p-Si/MoS 2 heterostructure was much lower. Kelvin probe microscope (KFM) results demonstrated the more efficient separation of photogenerated excitons in n-Si/MoS 2 than that in p-Si/ MoS 2 . Coupling KFM results with band alignments of (p-, n-) Si/MoS 2 heterostructures, the origins of photodiode-like phenomena of p-Si/MoS 2 and n-Si/MoS 2 have been unveiled, that is intrinsic built-in electric field in p-n junction, and modulated barrier height and width at the interface in n-n junction. Our work may benefit to the deep understanding of the integration of two-dimensional materials with more conventional three-dimensional semiconductors, and then contribute to the developments in the area of van der Waals heterostructures.T wo-dimensional transition metal dichalcogenides (TMDs) have been experiencing significant progress because of the wide range of physical and chemical properties from the ultrathin planar structures at the atomic scale, such as strong electron-hole confinement, and excellent mechanical and thermal stability 1-4 . Besides, due to the relatively small bandgap, TMDs are also considered as excellent light absorbers, stimulating the development of TMDs-based photovoltaic and optoelectronic devices recently 5,6 . Molybdenum disulfide (MoS 2 ), a typical member of TMDs, is demonstrated as a promising candidate that goes beyond graphene for next generation of optoelectronics and spintronics due to the intrinsic and tunable bandgap, relatively high carrier mobility, and strong coupling of spin and valley degrees 7-9 . In the past few years, more researches concentrated on pursuing better photoresponsivity and broad wavelength response of MoS 2 -based photodetectors 10-14 . For example, Yin et al. first fabricated monolayer MoS 2 phototransistor with higher responsivity (7.5 mA/W) than that of graphene 10 . The wavelength-dependent photosensivity of monolayer to trilayer MoS 2 field-effect transistors was investigated by Im et al., and the results revealed that monolayer and bilayer MoS 2 exhibited excellent photodetection capabilities for green light, while tri-layer MoS 2 was efficient for red light detection 12 . By optimizing the carrier mobility, electrical contact quality and positioning technique, the maximum external photoresponsivity of 880 A/W was achieved for m...

The mechanical properties of black phosphorus (BP) nanosheets suspended over circular holes were measured by an atomic force microscope nanoindentation method. The continuum mechanic model was introduced to calculate the elastic modulus and pretension of BP nanosheets with thicknesses ranging from 14.3 to 34 nm. Elastic modulus of BP nanosheets declines with thickness, and the maximum value is 276 ± 32.4 GPa. Besides, the effective strain of BP ranges from 8 to 17% with a breaking strength of 25 GPa. Our results show that BP nanosheets serve as a promising candidate for flexible electronic applications.

The behavior of excitons in van der Waals (vdWs) heterostructures depends on electron–electron interactions and charge transfer at the hetero‐interface. However, what still remains to be unraveled is to which extent the carrier densities of both counterparts and the band alignment in the vdWs heterostructures determine the photoluminescence properties. Here, we systematically study the photoluminescence properties of monolayer MoS2/graphene heterostructures by modulating the carrier densities and contact barrier at the interface via electrochemical gating. It is shown that the PL intensities of excitons can be tuned by more than two orders of magnitude, and a blue‐shift of the exciton peak of up to 40 meV is observed. By extracting the carrier density of MoS2 using an electric potential distribution model, and the Schottky barrier using first‐principle calculations, we find that the controllable carrier density in MoS2 plays a dominant role in the PL tuning at negative gate bias, whereas the interlayer relaxation of excitons induced by the Schottky barrier has a major contribution at positive gate bias. This is further verified by controlling the tunneling barrier and screening field across MoS2 by inserting self‐assembled monolayers (SAMs) at the interface. These findings will benefit to better understand the effect of many‐body interactions and hetero‐interfaces on the optical and optoelectronic properties in vdWs heterostructures.

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Hybrid graphene (Gr)–quantum dot (QD) photodetectors have shown ultrahigh photoresponsivity combining the strong light absorption of QDs with the high mobility of Gr. QDs absorb light and generate photocarriers that are efficiently transported by Gr. Typically, hybrid PbS–QD/graphene photodetectors operate by transferring photogenerated holes from the QDs to Gr while photoelectrons stay in the QDs inducing a photogating mechanism that achieves a responsivity of 6 × 107 A W−1. However, despite such high gain, these systems have poor charge collection with quantum efficiency below 25%. Herein, a ZnO intermediate layer (PbS‐QD/ZnO/Gr) is introduced to improve charge collection by forming an effective p‐n PbS‐ZnO junction driving the electrons to the ZnO layer and then to Gr. This improves the photoresponsivity of the devices by nearly an order of magnitude with respect to devices without ZnO. Charge transfer to Gr is demonstrated by monitoring the change in Fermi level under illumination for conventional PbS‐QD/Gr and for ZnO intermediate PbS‐QD/ZnO/Gr devices. These results improve the capabilities of hybrid QD/Gr configurations for optoelectronic devices.