On the Electron Dose-rate Dependency of Radiation Effects and Total Dose Criteria in ZIF-8 Metal-organic-framework (MOF)

The characterization of materials at the atomic level is crucial for understanding advantageous structure–property correlations at the nm scale. In general, this can be achieved using high-resolution scanning/transmission electron microscopy (HR-S/TEM), but by using high-energy electrons. When studying the extremely sensitive soft matter, using an electron beam as a probe to characterize the structure could be rather invasive [1]. Therefore, we need to investigate and understand when and how the studied original structure could be altered in these atomic-scale experiments. Thereby we gain control over electron-sample interactions in our experiment, e.g., not only by adjusting HT, but also by controlling total electron dose as well as electron dose rate, and we can reduce (or at least manage) effects on the structure during our experiments. Zeolitic imidazolate frameworks-8 (ZIF-8) are a sub-class of metal-organic-frameworks (MOF), hierarchically porous materials, where in ZIF-8 zinc (Zn) atoms are linked to 2-methylimidazolate linkers. ZIF-8 has a cubic crystal structure (space group I4¯3m) with a lattice parameter of 16.99 Å. The spacing between Zn triplets in unit cell along [111] direction is 3.4 Å with a pore size is 11.6 Å [2], which makes it suitable for the adsorption of small guest aggregates as well as guest gas molecules [3]. This porous but chemically stable crystalline structure opens up a wide range of applications, e.g. in catalysis and gas separation. However, this requires that we are able to characterize the structure with atom precision [3]. Here, we study the stability of ZIF-8 in HR-S/TEM atomic scale imaging experiments, specifically the critical total electron dose tolerated. In the literature [4–6], values of total dose are reported, however not all affecting parameters have been considered. One method of assessing structural changes in a crystalline sample, e.g. caused by exposure to high-energy electrons, is by quantifying the drop in the intensity of specific Bragg spots in an electron diffraction (ED) pattern [1]. Here, we have examined the dose-rate dependence of the stability of ZIF-8 MOF nanoparticles (average size ∼ 160 nm) with a varying dose rate of 0.1, 1, and 6.6 e- A-2 s-1, respectively. Time series of ED pattern has been recorded in FEI Tecnai G2 microscope (LaB6, 200 keV) with an exposure time of 1 sec until all the Bragg diffraction spots have completely faded out. The bright field TEM image of the ZIF-8 nanoparticles and the corresponding ED pattern, taken with a dose rate of 1 e- A-2 s-1, are shown in fig. 1a and 1b, respectively. The radial average of the intensity of the Bragg rings (shown in fig. 1b) is plotted in fig. 1c. The time dependence of the relative intensity of the {431} Bragg spot, corresponding to an interplanar spacing of 3.3 Å and an indication of resolving Zn triplets in the HR-S/TEM imaging, is plotted in fig. 1d. We use this approach to estimate the effect of dose rate on the critical dose tolerance limit in our ZIF-8 material [7]. (a) Bright field image of ZIF-8 MOF nanoparticles taken with an electron dose rate of 1 e- A-2 s-1. (b) Ring diffraction pattern showing the {431} Bragg reflection at t =1sec (first acquisition). (c) Radial average of the Bragg reflections. Inset showing schematic of ZIF-8 unit cell along [111] direction (d) Relative intensity plot of 431 Bragg reflection with time (I0 = intensity at t = 1sec, I = intensity at t sec).