Dense Silica Encapsulation of Perovskite Quantum Dots via Decylphosphonic Acid Self‐Catalysis for Robust X‐ray Scintillators

Perovskite quantum dots (PQDs) offer exceptional optoelectronic properties but suffer from poor stability, limiting their practical use. Silica (SiO 2 ) encapsulation can improve the thermal and photostability of CsPbBr 3 PQDs, yet conventional methods relying on slow tetraethyl orthosilicate (TEOS) condensation under ambient humidity yield low‐density shells that permit moisture penetration and rapid degradation under harsh conditions. Here, we present a ligand‐assisted reprecipitation (LARP) strategy in which decylphosphonic acid (DPA) replaces oleic acid (OA) as the surface ligand to enable in situ SiO 2 encapsulation. The intrinsic acidity of DPA self‐catalyzes TEOS hydrolysis, driving the formation of a cross‐linked Si‐O‐Si network and producing dense, uniform SiO 2 shells directly on the PQD surface. The resulting DPA‐CsPbBr 3 QDs@SiO 2 retain 91.8% of their initial photoluminescence intensity after 18 min of ultrasonic treatment in water, far exceeding the 12.7% retention of OA‐CsPbBr 3 QDs@SiO 2 . They also exhibit excellent photostability and X‐ray stability. Embedding these PQDs in hydroxyl‐terminated polysiloxane enables the fabrication of flexible scintillator films with high stability and spatial resolution for X‐ray imaging. This simple, low‐cost, and scalable approach offers a versatile route to robust PQDs for advanced optoelectronic applications.