Ultrasensitive Photoresponsive Devices Based on Graphene/BiI₃ van der Waals Epitaxial Heterostructures

Abstract In recent years, bismuth iodide (BiI₃), a layered metal halide semiconducting light absorber with a wide bandgap of ≈1.8 eV and strong optical absorption in the visible region, has received greater attention for photovoltaic applications. In this study, ultrasensitive visible‐light photodetectors with graphene/BiI₃ vertical heterostructures are achieved by van der Waals epitaxies. The BiI₃ films deposited on graphene show flatter morphologies and significantly better crystallinities than that of BiI₃ films on SiO₂ substrates, mainly due to weak van der Waals interactions at the graphene/BiI₃ interface. Hybrid photodetectors with highly crystalline graphene/BiI₃ heterostructures demonstrate an ultrahigh responsivity of 6 × 10⁶ A W⁻¹, shot‐noise‐limited detectivity of 7 × 10¹⁴ Jones, and a relatively short response time of ≈8 ms. Compared to most previously reported graphene‐based hybrid photodetectors, these devices have comparable photosensitivities but a faster response speed and lower operation voltage, which is quite promising for ultralow intensity visible‐light sensors. Moreover, the electronic structure and interfacial chemistry at the graphene/BiI₃ heterojunctions are investigated using photoemission spectroscopy. The results give clear evidence that no chemical interactions occur between graphene and BiI₃, resulting in the van der Waals epitaxial growth, and the measured band bending consistently illustrates that a photoinduced charge transfer occurs at the graphene/BiI₃ interface.