Gas-induced changes in the optical parameters of ZIF-8 thin films under atmospheric pressure and vacuum studied by spectroscopic ellipsometry

Zeolitic imidazolate framework-8 (ZIF-8) is a widely studied metal–organic framework whose porous structure and gas adsorption behavior strongly influence its optical properties. In particular, variations in the refractive index caused by gas loading into the metal–organic frameworks (MOF) pores represent a viable signal-generating mechanism for optical gas sensors. In this study, we investigate the gas-dependent optical properties of ZIF-8 thin films using spectroscopic ellipsometry. ZIF-8 thin films were prepared via a liquid-phase deposition process comprising surface functionalization with triethoxy-3-(2-imidazolin-1-yl)propylsilane and successive growth cycles from aqueous solutions. The influence of the number of deposition cycles and the gas environment on the optical properties was systematically studied with a total of 108 individual ellipsometric measurements. The structural properties of the films were analyzed by grazing-incidence x-ray diffraction. The refractive index of the ZIF-8 films exhibited statistically significant, gas-dependent changes, with average values of 1.4242, 1.4246, and 1.4253 measured under vacuum, atmospheric-pressure nitrogen, and atmospheric-pressure methane, respectively. Film thickness increased from ∼630 nm after one deposition cycle to up to 1400 nm after three cycles, while no significant increase in surface roughness was observed with increasing thickness. This indicates homogeneous, densely packed film growth without the formation of voids or discontinuities, as further supported by scanning electron microscopy. This work provides a transparent and reproducible ellipsometric modeling strategy for MOF thin films that is applicable beyond the specific case of ZIF-8, addressing a key limitation in the current literature and improving inter-study comparability of optical constants under varying environmental conditions.