Encapsulating CdSe/CdS QDs in the MOF ZIF-8 Enhances Their Photoluminescence Quantum Yields in the Solid State

Colloidal semiconductor nanocrystals, or quantum dots (QDs), show great promise as light absorbers and emitters in next-generation optoelectronic devices, but deleterious charge and energy-transfer processes that occur in solid-state thin films of QDs hinder their wide-scale utilization. Several classes of materials have been used previously to encapsulate QDs in the solid state but have failed to fully prevent charge and energy transfer in densely packed thin films of QDs relevant for device applications, necessitating the exploration of other materials. This paper describes the use of a metal-organic framework (MOF), zeolitic imidazolate framework-8 (ZIF-8), as a novel matrix encapsulation material to regulate the inter-QD spacing of core-shell QDs in a crystalline host, thus enhancing the photoluminescence quantum yield (PL QY) and irradiance of the films. The brightest composite films have PL QYs of 7.4-9.8% (whereas the PL QY of the QD-only film is 2.1%) and volume-normalized irradiances that are at least a factor of 5 higher than that of the QD-only film. Spectrally resolved PL lifetime measurements indicate that ZIF-8 improves the emission of the QDs by inhibiting inter-QD energy transfer, primarily by preventing the formation of large aggregates of QDs that are present in the film of poly(vinylpyrrolidone) (PVP)-coated QDs without ZIF-8. This demonstration of control over QD spacing within the crystalline MOF matrix reveals new pathways toward mesoscale QD structures with controlled electronic coupling and dielectric environments. Furthermore, this study is one of the first investigations of the photophysics of QD@MOF composite materials.