Regulation of Monomer Allocation and Surface Passivation via Dual Ligands for the Synthesis of NIR-IIc Ag2Se 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.