Crystal and Electronic Structures of Epitaxial Atomic Thick Lead Iodide

Lead iodide (PbI2), a typical layered semiconductor material, holds a wide visible band gap and a thickness-dependent band structure, which can be widely used in optical devices and perovskite solar cells. Although lead iodide is one of the most extensively studied materials, there is still a lack of experimental investigation of the electrical structure. In this study, we successfully grow the PbI2 monolayer on the highly oriented pyrolytic graphite (HOPG) substrate by molecular beam epitaxy (MBE). Scanning tunneling microscopy (STM) characterization demonstrates the high-quality PbI2 monolayer with a distinct Moiré pattern. Spectroscopic measurements reveal that the PbI2 monolayer possesses a band gap of ∼2.98 eV in the visible purple range and a decreased band gap of ∼2.8 eV for the bilayer. Theoretical calculations corroborate the experiments and reveal an indirect-to-direct bandgap transition in the bilayer. This study has significant implications for the future field of optical devices.