Large band curvature in semimetal-semiconductor NbSe₂/WSe₂ heterojunctions for high-speed imaging

High-speed imaging relies on tunable interfacial band bending for carrier separation and collection almost instantaneously, which can be severely affected by finite barrier heights and Fermi-level pinning. Here, we report a large-curvature van der Waals heterojunction (LCVH) formed by the two-dimensional (2D) semimetal 2H-NbSe₂ and 2D semiconductor WSe₂, featuring a steep band structure. The interfacial barrier is increased by the semimetal's high work function, while Fermi-level pinning is weakened by its low interface-state density, enabling effective control of band bending. With this interfacial modulation, the LCVH exhibits an ultralow dark current of 8.62 × 10^-15 A, a light-to-dark current ratio of 10⁵, and fast photoresponse times of 6.12/4.69 μs. Furthermore, a high-speed visible-light imager is realized using the LCVH detector. Compared to conventional metal-semiconductor junctions, the semimetal-semiconductor heterojunction achieves a fourfold increase in detection bandwidth over the conventional metal-semiconductor structure without sacrificing signal-to-noise ratio. This work provides an effective interfacial band-engineering strategy for high-speed imaging in 2D optoelectronic devices.