On the mechanism of electron-beam-induced structural degradation in ZIF-8 and its electron dose tolerance

Zeolitic-imidazolate-frameworks (ZIFs) are crystalline microporous materials that have already shown promising application potential in areas such as gas adsorption and catalysis. Further research advances include studies on integrating ZIFs into nanodevice concepts. In detail for the application e.g. of electron-beam-assisted structural modifications or patterning, there is a need to understand potential structural degradation processes caused by such electron beams. Advanced transmission electron microscopy (TEM) has demonstrated its ability to study structures at the nanoscale. Here, we systematically investigated electron-beam-induced loss in crystallinity in ZIF-8 under various experimental conditions, using as measure the attenuation of the intensity and the relative displacement of electron diffraction Bragg spots with increasing cumulative electron dose. The intensity and the fading of the {110} Bragg spots indicate the overall stability of the ZIF-8 unit-cell structure, while the {431} Bragg spots are a measure of the stability of ZIF-8’s micropore structure. We considered a relative loss of Bragg spot intensity of 37% as the threshold for the critical total electron dose, which was found to be different for different Bragg planes, with 35.6 ± 8.4 e-Å-2 for {110} and 11.4 ± 3.0 e-Å-2 for {431}. However, the critical dose per breakage of Zn-N bonds in a ZnN4 tetrahedra per different Bragg plane was found to be ~ 3 e-Å-2. This indicates continuous, simultaneous breakage of Zn-N bonds throughout the crystal, confirming radiolysis as the dominant damage mechanism. In addition, we investigated the effects of TEM experiment parameters, including acceleration voltage, electron dose rate, cryogenic sample temperature, in-situ sample drying, as well as change in conductivity of the sample substrate (e.g., graphene). Our results unravel the degradation mechanisms in ZIF-8 and provide threshold parameters for maximizing resolution in electron-beam-assisted experiments and processes.