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

Abstract

The periodic bulk structures of metal-organic frameworks (MOFs) can be solved by diffraction-based techniques; however, their non-periodic local structures-such as crystal surfaces, grain boundaries, defects, and guest molecules-have long been elusive due to a lack of suitable characterization tools. Recent advances in (scanning) transmission electron microscopy ((S)TEM) has made it possible to probe the local structures of MOFs at atomic resolution. In this article, we discuss why high-resolution (S)TEM of MOFs is challenging and how the new low-dose techniques overcome this challenge, and we review various MOF structural features observed by (S)TEM and important insights gained from these observations. Our discussions focus on real-space imaging, excluding other TEM-related characterization techniques (e.g. electron diffraction and spectroscopy). M etal-organic frameworks (MOFs) comprise metal centres/clusters with organic coordinating ligands and are characterized by their designable topologies, porosity, and functionalities 1,2. Studies on the structure-property relationship of MOFs are mainly based on their periodic crystallographic structures solved by diffraction-based techniques. In fact, the local non-periodic structures of MOFs-such as crystal surfaces, boundaries/interfaces, guest molecules, and point or extended defects-also have important effects on the properties of mass transport, sorption, and catalysis; for example, defects and interfaces severely affect how MOF membranes separate gases 3,4. However, probing local structures in MOF crystals with high spatial resolution is difficult, which poses a challenge to establishing a correlation between the properties of a MOF and its local structures with atomic precision. High-resolution transmission electron microscopy (HR-TEM) is the most widely used tool for imaging non-periodic local structures of crystalline materials, which are invisible in diffraction 5,6. Unfortunately, MOFs are extremely sensitive to the electron beam irradiation and their structures can be easily damaged during HR-TEM imaging 7-9. In fact, high-resolution

Mixed hierarchical local structure in a disordered metal–organic framework

“…With this nanocomposite structure in mind, high-resolution scanning transmission electron microscopy (HR-STEM) data were collected at ePSIC (Diamond Light Source, UK) to probe the structural differences between the phases in Fe-BTC and detect the presence of crystallinity. When operated at suitably low doses (electron fluence), HR-STEM enables the observation of structural periodicities in MOFs in the form of lattice fringes 33 . These give an indication of the length scale associated with periodicity in a material.…”

“…This study emphasises the necessity for multifaceted approaches towards characterising potentially disordered or amorphous systems, and particularly where, like here, both may coexist. In particular, local crystallinity probes such as HR-STEM are highly complementary to X-ray diffraction and PDF measurements 33 , 46 . From a computational perspective, polymerisation-based modelling to generate amorphous MOF structures has been demonstrated.…”

Probing Molecular Motions in Metal–Organic Frameworks by Three-Dimensional Electron Diffraction

“… 24 , 25 While TEM imaging is advantageous for analyzing local structures, it can only be applied to limited MOFs that can stand relatively high electron doses. In comparison to imaging, three-dimensional electron diffraction (3D ED) requires a 2 magnitudes lower electron dose, 24 with a dose rate of approximately 0.01 e – A –2 s –1 . The strong interaction between electrons and matter allows single-crystal 3D ED studies on nanometer-sized MOFs.…”

Single-particle cryo-EM structures from iDPC-STEM at near-atomic resolution

“…Using iDPC-STEM with a low-dose exposure of as little as 40 e -/Å 2 , a resolution of 1.8 Å has been successfully obtained from a single micrograph for such materials [19]. For these dose-sensitive low-contrast specimens, iDPC-STEM enables direct interpretation of the image without the need of defocusing and subsequent contrast-transfer function (CTF) correction of the images [20].…”