Ultrasensitive Photoresponsive Devices Based on Graphene/BiI 3 van der Waals Epitaxial Heterostructures

In recent years, bismuth iodide (BiI 3 ), 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/BiI 3 vertical heterostructures are achieved by van der Waals epitaxies. The BiI 3 films deposited on graphene show flatter morphologies and significantly better crystallinities than that of BiI 3 films on SiO 2 substrates, mainly due to weak van der Waals interactions at the graphene/BiI 3 interface. Hybrid photodetectors with highly crystalline graphene/BiI 3 heterostructures demonstrate an ultrahigh responsivity of 6 × 10 6 A W −1 , shot‐noise‐limited detectivity of 7 × 10 14 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/BiI 3 heterojunctions are investigated using photoemission spectroscopy. The results give clear evidence that no chemical interactions occur between graphene and BiI 3 , resulting in the van der Waals epitaxial growth, and the measured band bending consistently illustrates that a photoinduced charge transfer occurs at the graphene/BiI 3 interface.