Anti-Stokes Photoluminescence of van der Waals Layered Semiconductor PbI₂

Recently, organic–inorganic lead iodide perovskite has become one of the most promising emergent materials, which exhibits great potential in photovoltaics, lasing, laser cooling, etc. The building block of such great material is PbI₂, a van der Waals (vdW) layered semiconductor material, which arouses increased interest also owing to its potential applications for X-ray and γ-ray detection, lasing, etc. Similar and even superior to some vdW layered materials such as thin layers of transition metal dichalcogenides like MoS₂, WS₂, MoSe₂, or WSe₂, PbI₂ layers possess a direct bandgap of visible frequency with a wide range of thicknesses (>3 layers). This study reports the anti-Stokes photoluminescence (ASPL) of PbI₂ layers, which is very rarely investigated at present. Universe and robust ASPL in both 4H- and 2H-PbI₂ layers are observed and a phonon-assisted and multiphoton absorption up-conversion mechanisms are proposed through in situ temperature-dependent, incident laser-power-dependent measurements of both Stokes photoluminescence and ASPL. The successful observation and explanation of the universal anti-Stokes emission of PbI₂ layers, will certainly enrich fundamental understandings of vdW layered semiconductors and perovskite, then further benefit to developing applications based on such emerging materials.