van der Waals heterostructures (vdWHs) overcoming the lattice and processing limitations of conventional heterostructures provide an opportunity to develop high-performance 2D vdWH solar cells and photodiodes. However, it is challenging to improve the sensitivity and response speed of 2D vdWH photovoltaic devices due to the low light absorption efficiency and electron/hole traps in heterointerfaces. Here, we design a PbS/ MoS 2 /WSe 2 heterostructure photodiode in which a light-sensitive PbS quantum dot (QD) layer combined with a MoS 2 /WSe 2 heterostructure significantly enhances the photovoltaic response. The electron current in the heterostructure is increased by the effective collection of photogenerated electrons induced by PbS QDs. The device exhibits a broadband photovoltaic response from 405 to 1064 nm with a maximum responsivity of 0.76 A/W and a specific detectivity of 5.15 × 10 11 Jones. In particular, the response speed is not limited by multiple electron traps in the PbS QDs/2D material heterointerface, and a fast rising/ decaying time of 43/48 μs and a −3 dB cutoff frequency of over 10 kHz are achieved. The negative differential capacitance and frequency dependence of capacitance demonstrate the presence of interface states in the MoS 2 /WSe 2 heterointerface that hamper the improvement of the response speed. The scheme to enhance photovoltaic performance without sacrificing response speed provides opportunities for the development of high-performance 2D vdWH optoelectronic devices.

Two-dimensional van der Waals heterostructures can combine properties of individual materials to enable high-performance photodetection. Here, a novel ReS 2 /graphene/WSe 2 heterostructure, prepared by dry transfer, demonstrates air-stable, high-performance, polarization-sensitive, and broadband photodetection. Dark current can be strongly suppressed by the built-in electric field of the heterostructure. The specific detectivities are up to 10 10 Jones and 10 9 Jones under zero and reverse bias, respectively. Response time is on the order of a millisecond. The polarizationsensitive photodetection has been observed in the heterostructure due to the low lattice symmetry of ReS 2 . Broadband photoresponse from visible to infrared range has been demonstrated. A high photoresponsivity of 1.02 A W −1 is achieved for illumination at the wavelength of 785 nm. This work provides a viable approach toward future high-performance, air-stable, and polarizationsensitive broadband photodetectors.

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Surface-enhanced Raman scattering (SERS) has been widely investigated as a powerful trace analysis technique. Twodimensional (2D) materials are recognized as potential platforms for SERS. Herein, Raman enhancement on a 2D nitrogen-doped graphene (NG) substrate is reported. The 2D NG with different layers is synthesized on a SiO 2 /Si substrate by a microwave plasma heating (MPH) treatment using silk fibroin (SF) as the precursor. Bonding configurations of nitrogen dopants in graphene are revealed by X-ray photoelectron spectroscopy (XPS). Electrical properties of monolayer, bilayer, and trilayer NG exhibit p-type semiconductor behavior, while four-layer and thicker NG exhibit metallic behavior. The p-type semiconductor behavior can be attributed to O atoms in NG, which is demonstrated by XPS. Then, significant Raman enhancement of Rhodamine 6G (R6G) molecules is achieved by NG as the SERS substrate, which is based on the charge-transfer mechanism, and the Raman enhancement effect is thickness-dependent. For monolayer NG, the detection limit of R6G molecules on NG can be as low as 10 −8 M and the Raman enhancement factor reaches 10 6 . Moreover, the detection is stable for days under an ambient environment. This work shows NG to be a potential Raman enhancement platform for sensitive molecular detection.

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