Water‐Soluble Alumina‐Coated Indium Phosphide Core–Shell Quantum Dots with Efficient Deep‐Red Emission Beyond 700 nm

Solution‐processed colloidal III‐V semiconductor‐based quantum dots (QDs) represent promising and environmentally‐friendly alternatives to Cd‐based QDs in the realms of optoelectronics and biological applications. While InP‐based core–shell QDs have demonstrated efficient light‐emitting diode (LED) performance in the visible region, achieving deep‐red emission (above 700 nm) with a narrow linewidth has proven challenging. Herein, the study presents a novel strategy for synthesizing InP/ZnSe/ZnS core–shell–shell QDs tailored for emission in the first biological transparency window. The resulting QDs exhibit an emission wavelength up to 725 nm with a narrow peak full width at half maximum (FWHM) down to 107 meV (45 nm). To enhance the biocompatibility and chemical stability of the QDs, their surface is further capped with a layer of amorphous alumina resulting in an InP/ZnSe/ZnS/Al2O3 heterostructure. This surface passivation not only ensures environmental‐ and photostability but also enhances the photoluminescence quantum yield (PLQY). The alumina capping enables the aqueous phase transfer via surface ligand exchange using mercaptopropionic acid (MPA) while maintaining the initial quantum yield. The resulting QDs demonstrate a significant potential for advancing next‐generation optoelectronic technologies and bio‐applications. Large‐sized InP/ZnSe/ZnS QDs emitting in the near‐infrared range are overcoated with an alumina shell increasing their photoluminescence quantum yield to 42%. The enhanced environmental stability enables the aqueous phase via surface ligand exchange without decreasing the emission efficiency providing efficient heavy metal‐free emitters for biological imaging and LED applications.