Engineering Multilayered Nanocrystal Solids with Enhanced Optical Properties Using Metal Oxides for Photonic Applications

Managing deposition of multilayered nanocrystal quantum dot (NQD) thin films is crucial for future photonic devices to maximize solar energy extraction efficiency. Solution based NQD deposition methods require additional protection to achieve a discrete layered structure and to prevent optical degradation during processing. An attractive method to passivate and protect NQD films is overcoating with metal oxides, usually grown using atomic layer deposition (ALD). However, a significant quenching of NQD photoluminescence (PL) is typically observed after encapsulation, hindering performance and applicability. Here, we demonstrate a modified gas-phase deposition technique that fully passivates NQD assemblies and, in contrast to standard ALD, maintains PL properties. Combined in situ FTIR and ex situ XPS measurements reveal that upon Al2O3 deposition by ALD, the metal precursor trimethylaluminum (TMA) interacts with oleic acid-capped CdSe-CdS-ZnS core-shell-shell NQDs by reorganizing the ligands and replacing Zn atoms with Al. This modification leads to PL quenching, particularly severe at elevated temperatures (similar to 100 degrees C). In contrast, simultaneous exposures of both precursors (TMA and water) lead to metal oxide deposition from gas-phase reactions taking place in the immediate vicinity of the NQD surface, without affecting the chemical nature of the NQDs. Contrary to ALD, this technique retains and even improves NQDs' photoluminescence, observed as increased PL intensities and longer lifetimes.