Plasma-Treated CsPbBr₃ Nanocrystal Films for Anticounterfeiting Applications

Lead halide perovskite (LHP) nanocrystals (NCs) are considered propitious materials due to their extraordinary optoelectronic properties. Their poor ambient stability hinders their practical applications. Among the several approaches taken to enhance the ambient stability, plasma treatment is considered one of the best approaches because it does not hinder charge transport or reduce relative NC content while allowing easy and scalable processing. The plasma treatment increases the overall ambient stability of LHPs but at the cost of photoluminescence quantum yield (PLQY). We found that a short duration of the plasma treatment enhances the PL intensity by 30%, along with enhanced moisture stability. However, longer duration of plasma treatment decreases the photoluminescence (PL), and the NCs become hydrophilic. In this work, we report the underlying chemistry of stability enhancement during plasma treatment and how it affects the PL intensity. We performed Ar–O₂ plasma treatment on the CsPbBr₃ NCs thin films, which induces the cross-linking of the passivating ligand oleylamine that creates a stronger network of ligands, providing better encapsulation and higher PL intensity. A longer duration of plasma treatment results in oxidation of the passivating ligands in the presence of oxygen that eventually degrades the NCs. We created double-layer fluorescent security tags using the PL-stabilized NCs and as-synthesized NCs, having the same emission profile. The security pattern was created using the stabilized perovskite and masked with the as-synthesized perovskite, which is relatively unstable and can be washed off under certain treatments.