Lead halide perovskite (LHP) nanocrystals (NCs) have emerged as promising light emitting materials and have recently been considered as photocatalysts or photoconductive material. [1] These applications would greatly benefit from surface accessibility and energy transfer capabilities. While the commonly employed long chain organic ligands provide surface passivation, they also act as an insulating barrier. Employing a thin and charged oxide-based inorganic coating would not only improve the accessibility of photoexcited carriers but also keep the colloidal stability and processibility. A vast majority of the current literature about coating LHP NCs is based on SiO₂ and related oxides. [2] A common denominator among the oxides is the water based hydrolytic sol-gel reaction used for synthesis. Unfortunately, water is also highly detrimental to lead halide perovskite NCs and the synthetic strategies are normally a balance between the rate of NC degradation and rate of NC coverage. Therefore, we looked towards non-hydrolytic sol-gel reactions as a less detrimental coating strategy for LHP NCs. We found that non-hydrolytic reactions are capable of wrapping individual NCs with a thin sol-gel coating. In our work we used an alumina based gel coating as a model system for a full analysis and showcase titania and zirconia coatings to demonstrate the versatility of this approach. The resulting NCs proved to be more stable than their organically capped analog and the coating provides colloidal stability in polar solvents like alcohols, which previously degraded the NCs within seconds, for over a year without measureable degradation. UV/VIS absorption, photoluminescence (PL) and quantum yield measurements show that the optical properties remain virtually unchanged during the coating process. Compact films of such NCs feature strong delocalization of the photoexcited carriers leading to PL lifetimes two orders of magnitude higher than their organically capped counterparts, opening an avenue for applications where strong electronic coupling of individual NCs is essential. Analyzing the amorphous and only few nanometer thick coating is not straight forward, but was achieved with TEM, ICP MS, ²⁷Al MQMAS NMR and zeta potential measurements.

[2] Duan, Y.; Wang, D.-Y.; Costa, R. D., Recent Progress on Synthesis, Characterization, and Applications of Metal Halide Perovskites@Metal Oxide. Adv Funct Mater 2021, 31 (49), 2104634.