Regulation of Monomer Allocation and Surface Passivation via Dual Ligands for the Synthesis of NIR-IIc Ag ₂ Se Quantum Dots

Narrow bandgap silver selenide (Ag₂Se) is a promising candidate for noninvasive in vivo imaging in the long-wavelength region of the second near-infrared window (NIR-IIc, 1700-2000 nm), yet precise control over its size remains challenging due to ineffective regulation of nucleation and growth, largely constrained by its extremely low solubility product. Here, we reveal a growth bottleneck in the Ag₂Se quantum dots (QDs) synthesis arising from rapid monomer depletion during nucleation. The sole use of strongly binding thiol ligands further confines the system to a thermodynamically metastable size regime, hindering the continuous growth required for precise spectral tuning. By employing a dual-ligand strategy combining strongly binding thiol ligands with labile oleate ligands, we modulate monomer supersaturation and the dynamic adsorption-desorption equilibrium on the nanocrystal surface, thereby suppressing nucleation to preserve sufficient monomer for subsequent monomer diffusion-limited growth. This approach yields size-tunable Ag₂Se QDs with emission peaks extending into the NIR-IIc window. Furthermore, the synergistic passivation by zinc oleate ligands significantly improves the size dispersity and colloidal stability of large-sized Ag₂Se QDs. Our in-depth investigation not only addresses challenges in size control of QDs exhibiting nonclassical growth but also highlights the transformative potential of low-toxicity, NIR-IIc-emissive Ag₂Se QDs for optoelectronics and noninvasive deep-tissue imaging.