PbI-MoS Heterojunction: van der Waals Epitaxial Growth and Energy Band Alignment.

van der Waals (vdW) epitaxy offers a promising strategy without lattice and processing constraints to prepare atomically clean and electronically sharp interfaces for fundamental studies and electronic device demonstrations. Herein, PbI was thermally deposited at high-vacuum conditions onto CVD-grown monolayer MoS flakes in a vdW epitaxial manner to form 3D-2D heterojunctions, which are promising for vdW epitaxial growth of perovskite films. X-ray diffraction, X-ray photoemission spectroscopy, Raman, and atomic force microscopy measurements reveal the structural properties of the high-quality heterojunctions. Photoluminescence (PL) measurements reveal that the PL emissions from the bottom MoS flakes are greatly quenched compared to their as-grown counterparts, which can be ascribed to the band alignment-induced distinct interfacial charge-transfer behaviors. Strong interlayer excitons can be detected at the PbI/MoS interface, indicating an effective type II band alignment, which can be further confirmed by ultraviolet photoemission spectroscopy measurements. The results provide a new material platform for the application of the vdW heterojunctions in electronic and optoelectronic devices.