Engineering Polarons for Circumventing Energy Gap Law: Nd-doped CdTe Quantum Dots With High-efficiency Emission at 1730 nm for NIR-IIb/c Bioimaging.

Abstract

Despite widespread attention in near-infrared IIb/c (NIR-IIb/c, 1500-2000 nm) fluorescence for deep-tissue bioimaging, the development of NIR-IIb/c emissive materials with high photoluminescence quantum yield (PLQY) remains hindered by the energy gap law (EG law). Herein, we propose a polaron engineering strategy to circumvent the EG law via accelerating the radiative recombination rate by 150-fold through increased charge localization. By incorporating Nd³⁺ into CdTe quantum dot lattice, we achieved broadband NIR-IIb/c emission peak at 1730 nm, with PLQY of 11.34 ± 0.79% in aqueous solution and a Stokes shift exceeding 900 nm. We demonstrate Nd³⁺ substitution introduces beneficial defects that perturb electronic structure and local symmetry. Upon 808 nm excitation, these defects trap photoexcited carriers, inducing lattice distortions and strong electron-phonon coupling, which leads to the formation of polarons with high NIR-II emission efficiency. The resulting QDs enable clinical-grade NIR-II imaging via dual modes, including low-power (10 mW cm^- ²) real-time deep-tissue imaging and white-light-activatable fluorescence-guided surgery navigation. This study provides a promising approach to bypass the EG law, offering a pathway toward highly emissive emitters in the NIR regime.