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Scanning transmission electron microscopy (STEM), which uses a focused electron beam (probe) to form images, has also been used to study MOFs38,39,40. Depending on the scattering angles of the electrons used, STEM can produce bright-field (BF), annular BF, annular dark-field (ADF), and high-angle ADF (HAADF) images41,42. STEM has been proven to be more destructive than TEM when imaging low-Z materials at an identical electron dose due to the high instantaneous dose rate of STEM, thereby suggesting that it is more challenging to image MOFs with STEM than with TEM43,44. This is particularly true for HAADF-STEM, which utilizes only a rather small fraction of the incident electrons (those scattered to high angles), thereby requiring a higher electron dose to produce sufficient signals in the image compared with other imaging modes45. On the other hand, HAADF-STEM is sensitive to the variation of atomic number (Z) in the specimen and its image contrast is approximately proportional to Z2; the higher image contrast makes HAADF more tolerant to noise than other imaging methods, and particularly useful for identifying heavy elements in a matrix of material composed of light elements. Early work has shown that although the image resolution is limited due to beam damage, HAADF-STEM can be used to identify the distribution of metal nodes and heavy dopants in MOFs (Fig. 1)46,47,48,49,50.
Figure 1 presents a series of electron microscopy images of MIL-101 acquired in different years23,36,46,55, clearly illustrating that with the advancement of imaging techniques, the continuously improved image resolution leads to the revelation of increasingly more structural details: it was difficult to identify the mesoporous cage in the initial study, but currently it is already easy to resolve the smaller super-tetrahedral building units. In this article, we review the progress in high-resolution (scanning) transmission electron microscopy ((S)TEM) imaging of MOF materials in recent years, discuss new understanding and insights on MOF structures (particularly their local structures) brought about by these technological advances, and also provide our perspectives and outlook to this subject.
Identifying the surface structure of a MOF is critical to determine its affinity, reactivity, and accessibility of internal pores61,62. Hmadeh et al. reported the first example of using HR-TEM to observe the surface of a MOF (activated Ni-CAT-1), but the obtained image did not match well with the simulated image and the resolution was not sufficiently high to provide much structural information57. Before the ultralow dose HR-TEM was available, scanning probe microscopy (SPM), such as atomic force microscopy (AFM) or scanning tunnelling microscopy, was the only high-resolution tool to characterize the surface structures of MOFs63,64. However, there are few reports of using SPM to study MOFs, because SPM is more suitable for studying flat and clean surfaces rather than discrete crystals65.
When used in various applications, MOF often forms interfaces between crystals or with other materials. Metal/MOF and oxide/MOF composites are designed to combine the sensitivity of the metal (oxide) and the selectivity of the MOF in gas sensors69,70; MOF crystals are often used as fillers in polymeric membranes for improved gas separation71,72; the attachment, assembly, and agglomeration of MOF crystals during the synthesis produces grain boundaries or MOF/MOF interfaces that influence mass transfer and diffusion properties73,74.
In principle, HR-(S)TEM can be utilized to investigate these interfacial structures, but there is little research on this subject. One possible reason is that many interfaces involving MOF are not compatible with the imaging or specimen preparation conditions. For example, performing atomic-resolution imaging of a noble metal/MOF interface is difficult because the electron dose required to retain the MOF structure is too low to produce a lattice image for the metal, while increasing the dose to image the metal causes damage to the MOF structure. Similarly, observing the inherent interface between the MOF filler and polymer matrix in membranes is difficult, because the TEM specimen preparation process can dislocate or damage the filler; in addition, the amorphous nature of the polymer matrix is not conducive to probing the interfacial structure with atomic precision. Currently, only MOF/MOF interfaces have been studied with HR-(S)TEM.
a HAADF-STEM image of MIL-100(Fe) along the [110] direction, with a twin plane marked by a white arrow. Inset is the enlarged review of the twin boundary (adapted with permission from ref. 76. 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). b Low-dose HR-TEM image of the interface of two interconnected ZIF-8 crystals along the [111] zone axis. Purple lines are used to show the coherence between the lattice fringes in the two crystals that are separated by an interface represented by the yellow line. c CTF-corrected image of the green square area in b. Yellow circles highlight Zn triplets with distortions. d Structure model of the interface between two ZIF-8 crystals based on the HR-TEM image (adapted with permission from ref. 22. 2017 Springer Nature).
Like other crystalline materials, MOFs contain structural defects. Defect engineering of MOFs has attracted considerable research attention, as it offers a means to create open-metal sites, modulate surface properties, and locally tune porosity, thereby having important implications for various applications of MOFs78,79.
The porous, crystalline structures of MOFs provide well-defined microenvironments for various guest species (molecules, clusters, and particles), and the ability to probe confined guest species is crucial for understanding the interaction and synergy between the host and guest components88,89,90. In most cases, guest species are not periodically distributed in the framework of the host material, which makes it difficult to precisely locate them using diffraction data; real-space imaging is therefore a better choice in this regard. However, the precise positioning of guest species requires the structure of the host material to be well preserved during imaging, which was not possible before the development of suitable low-dose electron microscopy techniques.
a, b HAADF-STEM image and schematic illustration of perovskite CsPbI3 quantum dots confined in MIL-101 (adapted with permission from ref. 90. 2019 American Chemical Society). c HR-TEM and d HAADF-STEM images of single-molecular magnet, Mn12Ac, isolated by the porous matrix of NU-1000. White arrows indicated the encapsulated Mn12Ac clusters selectively residing in the hexagonal channels. The insets show the schematic illustration and the enlarged images of the highlighted square areas (adapted with permission from ref. 34. 2019 American Chemical Society). e Cryo-TEM image and simulated structure of ZIF-8 with adsorbed CO2 molecules. The arrow indicates the adsorbed CO2 molecule (adapted with permission from ref. 91. 2019 Elsevier).
In the previous sections, we demonstrated that the development of novel low-dose electron microscopy techniques has enabled atomic-resolution imaging of MOFs, with a number of bulk and local MOF structures unravelled by HR-(S)TEM. In this section, we share some of our experiences, perspectives, and outlook on this subject.
The change of the diffraction pattern is commonly used as a criterion of beam damage. However, it is worth noting that this method is in principle only suitable for evaluating the beam tolerance of the bulk structure, whereas the local, non-periodic structures may be more beam sensitive. For example, previous studies indicate that the electron beam damage at crystal surfaces is more severe than at the bulk14. Unfortunately, there is no better way to determine the precise beam tolerance of individual local structural feature. Therefore, we recommend the use of lowest possible (lower than the threshold determined for the bulk structure) electron dose for imaging local structures; meanwhile, it is better to acquire multiple images from different specimens to confirm that the observed local structural feature is inherent and not randomly generated due to beam damage.
Compared with commonly used dark-field STEM (ADF-STEM and HAADF-STEM), iDPC-STEM utilizes the incident electrons more efficiently, which combined with the integration progress produces better SNR under the low-dose conditions required for materials that are sensitive to electron beams. On the other hand, iDPC-STEM images are easier to interpret than HR-TEM images. We successfully imaged various guest molecules, including volatile organic compounds and MoO3 clusters, in the micropores of zeolites using iDPC-STEM, showing that iDPC-STEM images are almost directly interpretable without the need for image processing53.
Overall, iDPC-STEM offers a good balance between the structural preservation of the specimen (resolution), the SNR of the image, and the contrast of organic species. Therefore, we predict that iDPC-STEM will be increasingly used in MOF imaging. 152ee80cbc
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