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−O 2 plasma treatment on the CsPbBr 3 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 assynthesized perovskite, which is relatively unstable and can be washed off under certain treatments.
Lead halide perovskites (LHP) are of great interest for their optoelectronic properties and photovoltaic applications. Various heterostructures are created in these materials to achieve favorable optical properties and improved stability at the interfaces during the fabrication of devices. Such heterostructures are often assumed to be formed based on the reactivity of precursors and are not directly probed. In this Feature Article, we report how various strategies have been employed in LHP thin films and nanocrystals (NCs) that generate heterostructures to boost their stability and photovoltaic (PV) efficiencies and how variable energy photoelectron spectroscopy (VEPES) can probe the chemical composition variation in heterostructured materials and interfaces. We specifically discussed the internal heterostructures of LHP NCs generated due to the surface chemistry and postsynthesis anion exchange followed by a detailed discussion of the heterostructures induced by the chemical composition (anion, cation, and degradation) of LHP thin films. The difficulties in determining heterostructures as well as the potential scope of the application of VEPES in unwrapping heterostructures in diverse materials are also discussed.
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