Ultra‐High Photoresponsivity in Freestanding PbI ₂ Nanosheets via In Situ Uniaxial Strain Regulation

ABSTRACT Mechanical strain provides an effective route to modulate the properties of two‐dimensional(2D) materials, yet its intrinsic coupling to optical and electronic responses remains poorly quantified due to the difficulty of the experimental platform. Here, an in situ growth‐location‐measurement approach based on a flexible selection rod is developed, enabling precise uniaxial straining of freestanding 2D nanosheets together with correlated mechanical, optical, and electrical measurements on the same specimen. We identify an intrinsically ultralow in‐plane Young's modulus in PbI 2 nanosheets (≈20 GPa), establishing an experimental benchmark for mechanically flexible 2D materials. Correlated in situ strained Raman spectroscopy, photoluminescence, and electrical transport measurements reveal a pronounced mechano‐optoelectronic coupling, in which lattice vibrations, electronic structure, and carrier transport are coherently modulated by tensile strain. The optoelectronic response becomes ultra‐highly strain‐tunable, with the photoresponsivity enhanced by nearly 20‐fold to values exceeding 3 A/W under moderate strain. This work establishes a general experimental paradigm for investigating strain‐engineered optoelectronic phenomena in low‐dimensional materials.