Atomic‐Thin 2D Copper Sulfide Nanocrystals with over 94% Photothermal Conversion Efficiency as Superior NIR‐II Photoacoustic Agents

Abstract Exploring photothermal nanomaterials is essential for new energy and biomedical applications; however, preparing materials with intense absorption, highly efficient light‐to‐heat conversion, and enhanced photostability still faces the enduring challenge. Herein, the study synthesizes atomic‐thin (≈1.6 nm) 2D copper sulfide (AT‐CuS) plasmonic nanocrystals and find its extraordinary photothermal conversion efficiency (PCE) reaching up to 94.3% at the second near‐infrared (NIR‐II) window. Photophysical mechanism studies reveal that the strong localized surface plasmon resonance (LSPR) and out‐of‐plane size effect of AT‐CuS induce strong optical absorption and non‐equilibrium carrier scattering, resulting in a significant carrier‐phonon coupling (7.18 × 10 17 J K −1 s −1 m −3 ), ultimately enhancing the heat generation. Such a photothermal nanomaterial demonstrates at leastmes stronger NIR‐II photoacoustic (PA) signal intensity than that of most commonly used miniature gold nanorods, together with greater biocompatibility and photo‐/thermal‐stability, enabling noninvasive PA imaging of brain microvascular in living animals. This work provides an insight into the rational exploration of superb NIR‐II photothermal and photoacoustic agents for future practical utilizations.