Orientation-Controlled Large-Area Epitaxial PbI Thin Films with Tunable Optical Properties.

Lead iodide (PbI) as a layered material has emerged as an excellent candidate for optoelectronics in the visible and ultraviolet regime. Micrometer-sized flakes synthesized by mechanical exfoliation from bulk crystals or by physical vapor deposition have shown a plethora of applications from low-threshold lasing at room temperature to high-performance photodetectors with large responsivity and faster response. However, large-area centimeter-sized growth of epitaxial thin films of PbI with well-controlled orientation has been challenging. Additionally, the nature of grain boundaries in epitaxial thin films of PbI remains elusive. Here, we use mica as a model substrate to unravel the growth mechanism of large-area epitaxial PbI thin films. The partial growth leading to uncoalesced domains reveals the existence of inversion domain boundaries in epitaxial PbI thin films on mica. Combining the experimental results with first-principles calculations, we also develop an understanding of the thermodynamic and kinetic factors that govern the growth mechanism, which paves the way for the synthesis of high-quality large-area PbI on other substrates and heterostructures of PbI on single-crystalline graphene. The ability to reproducibly synthesize high-quality large-area thin films with precise control over orientation and tunable optical properties could open up unique and hitherto unavailable opportunities for the use of PbI and its heterostructures in optoelectronics, twistronics, substrate engineering, and strain engineering.