Electron-beam damage mechanisms and strategies in MOFs: Comparison
Please note this is a comparison between Version 1 by Kuibo Yin and Version 2 by Wendy Huang.

Metal–organic frameworks (MOFs), composed of metal nodes and inorganic linkers, are promising for a wide range of applications due to their unique periodic frameworks. Understanding structure–activity relationships can facilitate the development of new MOFs. Transmission electron microscopy (TEM) is a powerful technique to characterize the microstructures of MOFs at the atomic scale. In addition, it is possible to directly visualize the microstructural evolution of MOFs in real time under working conditions via in situ TEM setups. Although MOFs are sensitive to high-energy electron beams, much progress has been made due to the development of advanced TEM.




  • metal–organic frameworks
  • transmission electron microscopy
  • in situ TEM
  • structural characterization
  • structure–activity
  • dynamics visualization

1. Introduction

Metal–organic frameworks (MOFs) are porous crystalline materials composed of inorganic metal nodes and organic ligands [1]. As a unique class of materials featuring tunable topologies, large specific surface areas, and adjustable chemical compositions, MOFs are promising candidates for catalysis [2], gas storage and separation [3][4][3,4], energy storage and conversion [5], chemical sensing [6], water adsorption [7], and lithium-ion storage [8]. Structure–activity relationships can guide the rational design and applications of flexible and functional MOFs. Atomic-scale determination of the crystal structures is a well-founded prerequisite for understanding the relationships. Surfaces, interfaces, defects, and host–guest interactions are the main microstructures, which directly affect the properties of MOFs. Surfaces influence the surface-related properties and growth processes [9][10][9,10]. Interfaces are of great significance for plasma crystals, composites, MOF-based devices, among other applications [11][12][13][11,12,13]. Defects provide a method to tune the porosity locally and to create active open metal sites for MOFs [14][15][14,15], which allows for defect engineering of the specific function [16][17][18][16,17,18]. Guest species, including ions, particles, clusters, and molecules, accommodate in ideal frameworks of MOFs crystals with distinctive porosity and long-range orderly structures [19][20][21][19,20,21]. In addition to the intrinsic properties of the static structure, a thorough understanding of the growth and transformation mechanisms, as well as of the evolution pathways of MOFs under working conditions is crucial to improve the applications of MOFs.
Transmission electron microscopy (TEM) is an undoubtedly unique and powerful technique to characterize atomic structures and dynamics of nanomaterials. Apart from crystal structural analysis by electron diffraction, both TEM mode and scanning transmission electron microscopy (STEM) mode permit direct imaging of regions of interest in the specimen, including its periodic, non-periodic, local, and porous details inside and on the surface of MOFs at the atomic scale [22][23][24][22,23,24]. Meanwhile, TEM is capable of being combined with spectroscopic techniques to examine the chemical elements [25]. Moreover, by using novel in situ sample holders, TEM can introduce external fields and conditions such as low temperatures, heating, biasing, liquids, and gases in real-time, which allows for on-demand scenarios of the sample in practical applications [26].
However, using TEM for characterization or as a research platform for MOFs is challenging due to the extreme sensitivity of MOFs to electron-beam irradiation. This fundamental hinderance of MOFs originates from their organic components and the coordination bonds that link organic parts to the metals. It results in either structural decomposition before the detection completion or in a lack of intrinsic characteristics during acquisition [22][27][22,27]. Therefore, the feasibility and utility of research using TEM is limited due to the nature of MOFs. Interestingly, several TEM-based methods and techniques have been developed to make them suitable for beam-sensitive materials. The idea of low electron dose and low temperature was established under the consideration of damage mechanisms in MOFs [28][29][28,29]. A variety of advanced TEM techniques have been developed, mainly including three-dimensional electron diffraction (3DED), imaging using direct-detection electron-counting (DDEC) cameras, and integrated differential phase-contrast scanning transmission electron microscopy (iDPC-STEM) [30][31][32][30,31,32].
Structural characterization of MOFs via TEM is challenging because MOFs are so unstable and sensitive to the high-energy electron beam. This requires an understanding of electron-beam damage mechanisms in MOFs.

2. Damage Mechanisms

The radiation damage mechanisms of MOFs under the high-energy incident electron-beam mainly include radiolysis, knock-on damage, and thermal effects [22]. In practical situations, identifying the predominant mechanism can help determine the proper method to minimize damage [27]. Radiolysis (or ionization damage) is the ionization of specimen atoms by electron–electron interactions via inelastic scattering, resulting in chemical bond weakening or breakage. It is the main cause of the reported degradation of MOFs, especially at lower voltages. High-energy incident electrons may mitigate this beam damage by decreasing inelastic scattering events. However, the signal-to-noise ratio of the image and spectrum is not improved because both the inelastic and the elastic cross-sections are inversely proportional to the incident energy [33][39]. Low temperature effectively improves the beam stability of the specimen and reduces radiolysis. This can be achieved by cooling the sample areas in TEM to cryogenic temperatures using liquid nitrogen or helium (cryo-TEM) [27][29][27,29]. Knock-on damage results from direct electron–nucleus interactions, specifically atomic displacements or sputtering in the original specimen caused by high-energy electrons. Low-energy incident electrons prevent knock-on damage, but this comes at the cost of weak beam penetration depth and poor signal resolution [22]. This problem can be alleviated by the reduction of the TEM accelerating voltage below the sample-specific threshold value without loss of resolution (low-voltage TEM). Thermal effects (or beam heating) result from collective crystal lattice vibration caused by electron–atom interactions, and can be mitigated by lowering the incident-beam current [33][39]. Cryogenic temperatures can partially alleviate this damage. In addition to structural disintegration [34][40], materials with poor electrical conductivity are charged by electron beams, causing image blurring due to image drift and vibration [35][41].

3. Strategies for Minimizing Damages

Low-dose TEM and cryo-TEM could be used for probing the microstructure of MOFs by TEM without damaging their intrinsic properties.

3.1. Low-Dose TEM

Reducing the electron dose (low-dose TEM) is a general solution applied to MOFs regardless of the damage mechanism, given that all these electron beam-induced irradiation damages in MOFs are dose dependent. A preliminary assessment is required to determine whether the low-dose conditions are within the acceptable range to maintain the crystallinity stability. Electron diffraction (ED) is an effective and feasible way to determine the electron dose that the MOF can withstand. The ED pattern varies with the increasing of electron dose, indicating the changes in the structure and the appearance of disordered phases. Shorter exposure time and lower intensity can achieve low electron doses [28]. The maximum electron dose that MOFs can withstand depends on the materials and the TEM operating conditions. Taking typical MOFs in TEM mode under 300 kV accelerated electron beam as examples, the electron dose that MIL-101(Cr) can withstand is ~16 e Å−2 [36][42]. The onset of beam damage for UiO-66(Zr) was from 10 to 20 e Å−2 [22], and ZIF-8(Zn) was about 25 e Å−2 [37][43]. Compared to the static characterization, in situ TEM observations require a longer irradiation time, which necessitates an electron dose well below the damage threshold to ensure the integrity of the sample region of interest [38][44].

3.2. Cryo-TEM

Cryo-TEM contributes to probe structural details of MOFs by preserving their stability over prolonged irradiation times [22][29][22,29]. This is because cryogenic temperatures diminish radiolysis and compensate for thermal effects to a certain extent. In the absence of advanced imaging and signal acquisition methods, high-resolution TEM (HRTEM) at liquid nitrogen temperature imaged the complete pore structure and the crystal lattice periodicity of MOF-5(Zn) nanocrystals [29]. Cryogenic temperatures also allowed the elucidation of the ordered internal architecture of large-area conductive 2D Cu2(TCPP) (TCPP = meso-tetra(4-carboxyphenyl)porphine) MOF films on dielectric substrates by HRTEM and ED [39][45]. Furthermore, ED determined the structures of the highly porous CAU-7(Bi) at 120 K [40][46]. Apart from minimizing structural damage and improving electron tolerance during characterization, cryo-TEM is capable of freezing specimens in the state of interest during an in situ TEM observation. Examples include in situ studies and corresponding ex situ examinations of the crystallinity of MOFs in the liquid phase at specific reaction stages [41][42][47,48] and the construction of MOFs after interaction with gas species [43][49].  In addition, cryogenic conditions are well suited for the biological field, including biomacromolecule-metal–organic frameworks (biomacromolecule-MOFs). The amorphous precursor phase in the nucleation of protein-ZIF-8(Zn) was directly observed. This revealed a non-classical nucleation approach of dissolution-recrystallization and protein-rich amorphous solid phase transformation [44][50]. Furthermore, cryogenic temperatures allowed for an understanding of their nanoarchitectures at the atomic level. The structural difference resulted from different crystallization pathways in synthetic scenarios and significantly affected the bioactivity.




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