ZIF-8 (zeolitic imidazolate framework-8) is a member of the growing family of metal-organic frameworks (MOFs). Homogeneous and crack-free ZIF-8 thin films of optical quality were crystallized on silicon and glass substrates. The refractive index of such ZIF-8 thin films in the complete visible light spectrum was directly determined for the first time. By incubating the porous films in different substances, the refractive index could be modulated over a wide range, two times larger than previously reported for MOF thin films. Reversible refractive index switching in ZIF-8 thin films was performed via the liquid and the gas phase. The ability to adjust the refractive index over a broad range enables the use of ZIF-8 films for applications in optical devices such as sensors, coatings for mirrors and lenses, or as an optical medium in more complex optical devices.
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