Tuning the optical properties of transparent ZIF-8 thin films by adjusting the crystal morphology

Optical properties are the most significant properties of optical thin films, which determine its application in optical and photoelectric devices. Designable optical properties are the most challenging goals in optical material engineering. Recently, Metal-organic framework (MOF)-based optical thin films have attracted increasing attention due to their novel optical properties, especially their modular design, which allows fine-tuning of the relevant properties. In this work, a strategy for tuning the optical properties of MOF-based thin films is developed, which is realized by adjusting MOF crystal morphology. A commonly used MOF, ZIF-8 (zeolitic imidazolate framework-8), is selected as the research model. The morphology of ZIF-8 nanocrystal is adjusted using a surfactant/end-capping agent modulation method. The surfactant/end-capping agents are absorbed onto different crystal planes of ZIF-8, slowing the growth of the crystal planes and causing the formation of ZIF-8 crystals with different morphologies. Six different morphologies of ZIF-8 crystals are successfully synthesized in the presence of different amounts of CTAB and/or TRIS: nano-spherical, rhombohedral dodecahedron, nano-cubic, rough octahedron, flake-shaped and hexapod-like morphology. Then, the corresponding ZIF-8 thin films with different crystal morphologies are prepared via spin-coating. The optical properties of the thin films are investigated using UV–vis spectroscopy and ellipsometry in detail. The results indicate that the film with rough octahedron morphology has the best transmittance, reaching 93.35 % at 581 nm. As a key optical parameter of the film, its refractive index can be adjusted between 1.27 and 1.54. The success of tuning of the optical properties of MOF optical thin films by adjusting the crystal morphology will make it meet the different required optical properties in various optical and photoelectric devices.