Observation of ultra-low thermal conductivity in ultra-thin single-crystalline PbI2 nanosheets

Low thermal conductivity materials are key for next-generation thermal management. However, non-destructive characterization of ultra-thin single crystals remains challenging. Here, we report a room-temperature thermal conductivity of 0.24 W/(m·K) in 7.9 nm single-crystalline PbI2 nanosheets, representing an experimentally realized ultra-low value in the ultra-thin single-crystal regime. This observation is enabled by an integrated in‑situ growth‑location‑measurement strategy that couples aqueous‑solution nucleation with gas‑flow‑assisted alignment, enabling the non‑destructive transfer of nanosheets onto a suspended thermal bridge. Combining first‑principles density functional theory calculations and Raman spectroscopy, we trace the suppressed thermal transport in PbI2 to flattened flexural acoustic phonons with markedly low group velocities and short lifetimes. Furthermore, this suppression is exacerbated by thickness-induced vibrational softening, while in-situ irradiation allows for dynamic thermal modulation. This work establishes a robust experimental framework for characterizing ultra-thin single crystals with ultra-low thermal conductivity, bridging theoretical prediction with experimental realization for advanced flexible thermal management applications. The authors report low room-temperature thermal conductivity (0.24 W m⁻¹ K⁻¹) in 7.9 nm single-crystalline PbI₂ nanosheets. Non-destructive suspended-device measurements and theory reveal flexural-phonon suppression, while irradiation enables dynamic thermal tuning.