Low-Dose and In-Painting Methods for (Near) Atomic Resolution STEM Imaging of Metal Organic Frameworks (MOFs)

Abstract

Metal-organic Frameworks (MOFs) are a group of crystalline and highly porous materials consisting of inorganic metal ions/clusters (nodes) that are coordinated by organic linkers. The ability to create a wide range of porous structures, where the pore size can be easily changed in size and shape offers the potential for many applications in gas storage/separation and catalysis [1]. The presence of the organic linkers or "struts" in the sample creates challenges for high resolution microscopy as the sample itself is very sensitive to beam damage. A key challenge for understanding the structures of MOFs and how the applications can be modified by doping the nodes and changing the nature of the organic linkers, is therefore to be able to image the samples on the sub-nm length scale (the nodes are ~1 nm) [2].The study of organics, where large single crystals with long-range order cannot be synthesized, is usually performed by either electron crystallography or direct imaging in the (scanning) transmission electron microscope (S/TEM). In the (S)TEM, large single crystals are not needed as the electron beam can be focused to a very small area (sub-nm if needed). The downside to this ability to see small areas is that because the electron beam has a strong interaction with the sample, it can cause significant levels of electron beam damage. However, the last 40 years of protein crystallography and more recently the use of in-situ liquid stages to study chemical reactions in the (S)TEM [3], have shown that this beam damage effect can in most cases be mitigated by the use of extremely low-dose imaging (a dose rate of less than 0.1 electrons/angstrom 2 /s and a cumulative dose of less than 10 electrons/angstrom 2 ). In addition to simply lowering the dose through conventional means (changing the emission current and probe dwell time), more recent use of compressive sensing/in-painting methods for STEM has also been shown to lower the effective dose and dose rate [4]. Figure 1 shows a Z-contrast image obtained from an NU-1000 MOF [2] obtained from a probe corrected FEI 80-300kV Titan operating in the low-dose mode described above. The sample can clearly survive under these conditions long enough to obtain a full scan with a probe size of ~0.1 nm. Results obtained from doped NU-1000 MOF structures (not shown) indicate that the atomic structure of the nodes themselves can be resolved under certain imaging conditions (including the use of in-painting methods). In this presentation the results from these samples will be described in more detail, along with the experimental strategy to obtain higher resolution images in the future [5] [6].

Atomically Dispersed Metals on Well-Defined Supports including Zeolites and Metal–Organic Frameworks: Structure, Bonding, Reactivity, and Catalysis

“…Several investigators have shown HAADF STEM images of pristine MOFs and metals in MOFs, with the MOFs including MIL-100, MIL-101 (Figure ), , MOF-5, UiO-66, ZIF-8, NU-1000, , NU-1301 (uranium) and NU-901 (bismuth) . However, the MOF structures typically degraded after exposure to only a few doses of the electron beam, leading to low-resolution images and to imaging of only primary channels.…”

“…Recently, new techniques such as HR TEM (not STEM) coupled with highly sensitive electron counting have enabled experiments with lower beam energies and, when combined with fast imaging, they have resulted in minimized sample degradation and clear images of crystalline samples with determination of local structural information (e.g., surfaces, interfaces, defects, and disorder). ,,, For example, isolated platinum atoms coordinated to oxygen atoms in MIL-100­(Cr) were observed by HR TEM, with images (Figure ) showing that the MOF structure is not highly crystalline (in contrast to many zeolites; see the amorphous structure of the MOF surface). Isolated platinum, iridium, and ruthenium atoms supported on or trapped in MOFs, or anchored on MOF nanotubes, or hosted on materials formed by pyrolysis of MOFs have been characterized by images demonstrating nearly uniform distributions of the metal.…”

“…78,79 MOFs have porous and defective structures, and the components are connected by coordinative bonds that are susceptible to radiation-induced damage (including bond breaking) that may correspond to structure degradation. 38,80 Several investigators have shown HAADF STEM images of pristine MOFs and metals in MOFs, with the MOFs including MIL-100, 44 MIL-101 (Figure 5), 81,82 MOF-5, 83 UiO-66, 84 ZIF-8, 80 NU-1000, 85,86 NU-1301 (uranium) 87 and NU-901 (bismuth). 88 However, the MOF structures typically degraded after exposure to only a few doses of the electron beam, leading to low-resolution images and to imaging of only primary channels.…”

Imaging defects and their evolution in a metal–organic framework at sub-unit-cell resolution

“…However, conventional HRTEM is unsuitable for studying MOFs because the electron beam can easily damage their structures. High-angle annular dark-field scanning TEM has been used to observe metal clusters [28][29][30][31] and lattice distortion 28 in MOFs, but it did not disclose more structural details due to the limited resolution and weak contrast of organic components. Here, we have combined the recently developed low-dose HRTEM technique 32,33 with electron crystallography to investigate defects in MOFs by real-space direct imaging.…”

Bulk and local structures of metal–organic frameworks unravelled by high-resolution electron microscopy

“…We also note that mesoporous MOFs with large unit cells, such as NU-1000 and MIL-101, undergo structural shrinkage at very low doses (<10 e − Å −2 ), thereby quickly losing atomic-scale structural ordering; meanwhile, such MOFs can retain the ordered arrangement of the pores even at high doses up to more than 100 e − Å −2 . Therefore, for these MOFs, it is relatively easy to obtain images with nanometre resolution but difficult to achieve atomicresolution imaging 40,46,48,92 . The change of the diffraction pattern is commonly used as a criterion of beam damage.…”