Cryogenic Focused Ion Beam Enables Atomic-Resolution Imaging of Local Structures in Highly Sensitive Bulk Crystals and Devices

Atomic-Level Electron Crystallography of Metal-Organic Frameworks Via Resin-Peeled Ultramicrotomy

Electron crystallography offers the potential for ultra-high-resolution diffraction and imaging of materials. However, challenges remain, particularly regarding beam/mechanical sensitivity, severe electron multiple scattering, and the low contrast of light elements in fragile porous organic crystals that often cannot be thinned without compromising their crystallinity. Herein, we develop a method, named resin-peeled ultramicrotomy, to prepare high-crystalline lamella free of embedding resin. By applying crystalline lamellas for 3D electron diffraction, we improve data quality and achieve resolutions up to 0.39 Å. This improvement allows for single-crystal structure determinations of metal-organic frameworks (MOFs) with large unit cells and giant pores─previously unsolved by X-ray and electron diffraction─and resolves ordered/disordered positions of (hydro)oxyl bridging atoms. Moreover, resin-peeled ultramicrotomy enables thinning of 3D MOFs along multiple crystallographic orientations. The further combination with electron ptychography reveals the local structures of 3D MOFs with high resolution and contrast. This advancement opens new avenues for resolving atomic and local structures in soft functional materials.

Exploring guest species in zeolites using transmission electron microscopy: a review and outlook

Zeolites, with their well-defined microporous frameworks, accommodate diverse guest species, including metal ions, atoms, clusters, complexes, and organic molecules. Direct imaging of these species and their interactions with the framework is crucial for understanding their structural and functional roles. Transmission electron microscopy (TEM), particularly aberration-corrected scanning TEM (STEM), has become an indispensable tool, offering atomic-resolution real-space insights. This review summarizes key (S)TEM techniques for probing guest species in zeolites, with a focus on low-dose strategies to minimize beam damage. We discuss the principles, applications, and limitations of various imaging modalities and highlight recent advances in visualizing metallic and organic species. Finally, we explore future directions for (S)TEM in zeolite research, emphasizing the opportunities and challenges of in situ, three-dimensional, and cryogenic imaging for resolving host-guest interactions with greater precision.

FIB-SEM: Emerging Multimodal/Multiscale Characterization Techniques for Advanced Battery Development

The advancement of battery technology necessitates a profound understanding of the physical, chemical, and electrochemical processes at various scales. Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) has emerged as an indispensable tool for battery research, enabling high-resolution imaging and multiscale analysis from macroscopic structures to nanoscale features at multiple dimensions. This review starts with introducing the fundamentals of focused beam and matter interaction under the framework of FIB-SEM instrumentation and then explores the application of FIB-SEM characterization on rechargeable batteries (lithium-ion batteries and beyond), with a focus on cathode and anode materials, as well as solid-state batteries. Analytical techniques such as Energy Dispersive X-ray Spectroscopy, Electron Backscatter Diffraction, and Secondary Ion Mass Spectrometry are discussed in the context of their ability to provide detailed morphological, crystallographic, and chemical insights. The review also highlights several emerging applications in FIB-SEM including workflow to maintain sample integrity, in-operando characterization, and correlative microscopy. The integration of Artificial Intelligence for enhanced data analysis and predictive modeling, which significantly improves the accuracy and efficiency of material characterization, is also discussed. Through comprehensive multimodal and multiscale analysis, FIB-SEM is poised to significantly advance the understanding and development of high-performance battery materials, paving the way for future innovations in battery technology.

Activated Corrosion and Recovery in Lead Mixed-Halide Perovskites Revealed by Dynamic Near-Ambient Pressure X-ray Photoelectron Spectroscopy

Herein, we quantify rates of O2-photoactivated corrosion and recovery processes within triple cation CsFAMAPb(IBr)3 perovskite active layers using dynamic near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS). Activated corrosion is described as iodide oxidation and lead reduction, which occurs only in the presence of both O2 and light through photoinduced electron transfer. We observe electron density reorganization from the Pb-I bonds consistent with ligand exchange, evident from the nonstoichiometric redox change (i.e., <1 e-). Approximately half of the Pb centers are reduced to weakly coordinated Pb-higher oxidation number than metallic Pb-with a rate coefficient of ∼3 (±0.3) × 10-4 atomic percent/s. Hole capture by I- yields I3- and is accompanied by increased concentrations of near-surface bromides, hypothesized to be due to anion vacancies and/or oxidation of mobile iodide resulting from ion demixing. Activated corrosion is found to be quasi-reversible; initial perovskite stoichiometry slowly recovers when the O2/light catalyst is removed, postulated to be due to mobile halide species present within the film below XPS sampling depth. Small deviations in near-surface composition (<2%) of the perovskite are used to connect reaction rates to quantified, near-band edge donor and acceptor defect concentrations, demonstrating two energetically distinct sites are responsible for the redox process. Collectively, environmental flux and rate quantification are deemed critical for the future elucidation of chemical degradation processes in perovskites, where rate-dependent reaction pathways are expected to be very system dependent (environment and material).

Low-dose electron microscopy imaging for beam-sensitive metal-organic frameworks

Metal-organic frameworks (MOFs) have garnered significant attention in recent years owing to their exceptional properties. Understanding the intricate relationship between the structure of a material and its properties is crucial for guiding the synthesis and application of these materials. (Scanning) Transmission electron microscopy (S)TEM imaging stands out as a powerful tool for structural characterization at the nanoscale, capable of detailing both periodic and aperiodic local structures. However, the high electron-beam sensitivity of MOFs presents substantial challenges in their structural characterization using (S)TEM. This paper summarizes the latest advancements in low-dose high-resolution (S)TEM imaging technology and its application in MOF material characterization. It covers aspects such as framework structure, defects, and surface and interface analysis, along with the distribution of guest molecules within MOFs. This review also discusses emerging technologies like electron ptychography and outlines several prospective research directions in this field.

Real-space imaging for discovering a rotated node structure in metal-organic framework

Resolving the detailed structures of metal organic frameworks is of great significance for understanding their structure-property relation. Real-space imaging methods could exhibit superiority in revealing not only the local structure but also the bulk symmetry of these complex porous materials, compared to reciprocal-space diffraction methods, despite the technical challenges. Here we apply a low-dose imaging technique to clearly resolve the atomic structures of building units in a metal-organic framework, MIL-125. An unexpected node structure is discovered by directly imaging the rotation of Ti-O nodes, different from the unrotated structure predicted by previous X-ray diffraction. The imaged structure and symmetry can be confirmed by the structural simulations and energy calculations. Then, the distribution of node rotation from the edge to the center of a MIL-125 particle is revealed by the image analysis of Ti-O rotation. The related defects and surface terminations in MIL-125 are also investigated in the real-space images. These results not only unraveled the node symmetry in MIL-125 with atomic resolution but also inspired further studies on discovering more unpredicted structural changes in other porous materials by real-space imaging methods.

Atomic-level imaging of beam-sensitive COFs and MOFs by low-dose electron microscopy

Electron microscopy, an important technique that allows for the precise determination of structural information with high spatiotemporal resolution, has become indispensable in unravelling the complex relationships between material structure and properties ranging from mesoscale morphology to atomic arrangement. However, beam-sensitive materials, particularly those comprising organic components such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), would suffer catastrophic damage from the high energy electrons, hindering the determination of atomic structures. A low-dose approach has arisen as a possible solution to this problem based on the integration of advancements in several aspects: electron optical system, detector, image processing, and specimen preservation. This article summarizes the transmission electron microscopy characterization of MOFs and COFs, including local structures, host-guest interactions, and interfaces at the atomic level. Revolutions in advanced direct electron detectors, algorithms in image acquisition and processing, and emerging methodology for high quality low-dose imaging are also reviewed. Finally, perspectives on the future development of electron microscopy methodology with the support of computer science are presented.

Spin coating epitaxial heterodimensional tin perovskites for light-emitting diodes

Environmentally friendly tin (Sn) perovskites have received considerable attention due to their great potential for replacing their toxic lead counterparts in applications of photovoltaics and light-emitting diodes (LEDs). However, the device performance of Sn perovskites lags far behind that of lead perovskites, and the highest reported external quantum efficiencies of near-infrared Sn perovskite LEDs are below 10%. The poor performance stems mainly from the numerous defects within Sn perovskite crystallites and grain boundaries, leading to serious non-radiative recombination. Various epitaxy methods have been introduced to obtain high-quality perovskites, although their sophisticated processes limit the scalable fabrication of functional devices. Here we demonstrate that epitaxial heterodimensional Sn perovskite films can be fabricated using a spin-coating process, and efficient LEDs with an external quantum efficiency of 11.6% can be achieved based on these films. The film is composed of a two-dimensional perovskite layer and a three-dimensional perovskite layer, which is highly ordered and has a well-defined interface with minimal interfacial areas between the different dimensional perovskites. This unique nanostructure is formed through direct spin coating of the perovskite precursor solution with tryptophan and SnF2 additives onto indium tin oxide glass. We believe that our approach will provide new opportunities for further developing high-performance optoelectronic devices based on heterodimensional perovskites.

Focused ion beam milling and MicroED structure determination of metal-organic framework crystals

We report new advancements in the determination and high-resolution structural analysis of beam-sensitive metal organic frameworks (MOFs) using microcrystal electron diffraction (MicroED) coupled with focused ion beam milling at cryogenic temperatures (cryo-FIB). A microcrystal of the beam-sensitive MOF, ZIF-8, was ion-beam milled in a thin lamella approximately 150 nm thick. MicroED data were collected from this thin lamella using an energy filter and a direct electron detector operating in counting mode. Using this approach, we achieved a greatly improved resolution of 0.59 Å with a minimal total exposure of only 0.64 e-/A2. These innovations not only improve model statistics but also further demonstrate that ion-beam milling is compatible with beam-sensitive materials, augmenting the capabilities of electron diffraction in MOF research.

A Customized Hydrophobic Porous Shell for MOF-5

The susceptibility to moisture of metal-organic frameworks (MOFs) is a critical bottleneck for their wider practical application. Constructing core-shell composites has been postulated as an effective strategy for enhancing moisture resistance, but for fragile MOFs this has rarely been accomplished. We report herein, for the first time, the construction of a customized hydrophobic porous shell, NTU-COF, on the particularly fragile MOF-5 by a "Plug-Socket Anchoring" strategy. Notably, the pore structure of MOF-5 was well maintained, and it could still achieve complete CO2/N2 separation under humid conditions. The homogeneous interface between MOF-5 and NTU-COF has been inspected at atomic resolution by a combination of cryogenic focused ion beam (cryo-FIB) and ultralow-dose (scanning) transmission electron microscope giving profound insight into the mechanism of assembly of the core-shell structure. This work presents a facile strategy for the fabrication of a hydrophobic porous shell for labile MOFs, and provides a general approach for solving the problem of moisture instability of porous materials for practical applications.

Unveiling the Intrinsic Structure and Intragrain Defects of Organic-Inorganic Hybrid Perovskites by Ultralow Dose Transmission Electron Microscopy

Transmission electron microscopy (TEM) is a powerful tool for unveiling the structural, compositional, and electronic properties of organic-inorganic hybrid perovskites (OIHPs) at the atomic to micrometer length scales. However, the structural and compositional instability of OIHPs under electron beam radiation results in misunderstandings of the microscopic structure-property-performance relationship in OIHP devices. Here, ultralow dose TEM is utilized to identify the mechanism of the electron-beam-induced changes in OHIPs and clarify the cumulative electron dose thresholds (critical dose) of different commercially interesting state-of-the-art OIHPs, including methylammonium lead iodide (MAPbI₃ ), formamidinium lead iodide (FAPbI₃ ), FA_0.83 Cs_0.17 PbI₃ , FA_0.15 Cs_0.85 PbI₃ , and MAPb_0.5 Sn_0.5 I₃ . The critical dose is related to the composition of the OIHPs, with FA_0.15 Cs_0.85 PbI₃ having the highest critical dose of ≈84 e Å^-2 and FA_0.83 Cs_0.17 PbI₃ having the lowest critical dose of ≈4.2 e Å^-2 . The electron beam irradiation results in the formation of a superstructure with ordered I and FA vacancies along <110>_c , as identified from the three major crystal axes in cubic FAPbI₃ , <100>_c , <110>_c , and <111>_c . The intragrain planar defects in FAPbI₃ are stable, while an obvious modification is observed in FA_0.83 Cs_0.17 PbI₃ under continuous electron beam exposure. This information can serve as a guide for ensuring a reliable understanding of the microstructure of OIHP optoelectronic devices by TEM.

Ground-State Spin Dynamics in d1 Kagome-Lattice Titanium Fluorides

Many quantum magnetic materials suffer from structural imperfections. The effects of structural disorder on bulk properties are difficult to assess systematically from a chemical perspective due to the complexities of chemical synthesis. The recently reported S = 1/2 kagome lattice antiferromagnet, (CH₃NH₃)₂NaTi₃F₁₂, 1-Ti, with highly symmetric kagome layers and disordered interlayer methylammonium cations, shows no magnetic ordering down to 0.1 K. To study the impact of structural disorder in the titanium fluoride kagome compounds, (CH₃NH₃)₂KTi₃F₁₂, 2-Ti, was prepared. It presents no detectable structural disorder and only a small degree of distortion of the kagome lattice. The methylammonium disorder model of 1-Ti and order in 2-Ti were confirmed by atomic-resolution transmission electron microscopy. The antiferromagnetic interactions and band structures of both compounds were calculated based on spin-polarized density functional theory and support the magnetic structure analysis. Three spin-glass-like (SGL) transitions were observed in 2-Ti at 0.5, 1.4, and 2.3 K, while a single SGL transition can be observed in 1-Ti at 0.8 K. The absolute values of the Curie-Weiss temperatures of both 1-Ti (-139.5(7) K) and 2-Ti (-83.5(7) K) are larger than the SGL transition temperatures, which is indicative of geometrically frustrated spin glass (GFSG) states. All the SGL transitions are quenched with an applied field >0.1 T, which indicates novel magnetic phases emerge under small applied magnetic fields. The well-defined structure and the lack of structural disorder in 2-Ti suggest that 2-Ti is an ideal model compound for studying GFSG states and the potential transitions between spin liquid and GFSG states.

4D-STEM Ptychography for Electron-Beam-Sensitive Materials

Recent advances in high-speed pixelated electron detectors have substantially facilitated the implementation of four-dimensional scanning transmission electron microscopy (4D-STEM). A critical application of 4D-STEM is electron ptychography, which reveals the atomic structure of a specimen by reconstructing its transmission function from redundant convergent-beam electron diffraction patterns. Although 4D-STEM ptychography offers many advantages over conventional imaging modes, this emerging technique has not been fully applied to materials highly sensitive to electron beams. In this Outlook, we introduce the fundamentals of 4D-STEM ptychography, focusing on data collection and processing methods, and present the current applications of 4D-STEM ptychography in various materials. Next, we discuss the potential advantages of imaging electron-beam-sensitive materials using 4D-STEM ptychography and explore its feasibility by performing simulations and experiments on a zeolite material. The preliminary results demonstrate that, at the low electron dose required to preserve the zeolite structure, 4D-STEM ptychography can reliably provide higher resolution and greater tolerance to the specimen thickness and probe defocus as compared to existing imaging techniques. In the final section, we discuss the challenges and possible strategies to further reduce the electron dose for 4D-STEM ptychography. If successful, it will be a game-changer for imaging extremely sensitive materials, such as metal-organic frameworks, hybrid halide perovskites, and supramolecular crystals.

Defined metal atom aggregates precisely incorporated into metal-organic frameworks

Nanosized metal aggregates (MAs), including metal nanoparticles (NPs) and nanoclusters (NCs), are often the active species in numerous applications. In order to maintain the active form of MAs in "use", they need to be anchored and stabilised, preventing agglomeration. In this context, metal-organic frameworks (MOFs), which exhibit a unique combination of properties, are of particular interest as a tunable and porous matrix to host MAs. A high degree of control in the synthesis towards atom-efficient and application-oriented MA@MOF composites is required to derive specific structure-property relationships and in turn to enable design of functions on the molecular level. Due to the versatility of MA@MOF (derived) materials, their applications are not limited to the obvious field of catalysis, but increasingly include 'out of the box' applications, for example medical diagnostics and theranostics, as well as specialised (bio-)sensoring techniques. This review focuses on recent advances in the controlled synthesis of MA@MOF materials en route to atom-precise MAs. The main synthetic strategies, namely 'ship-in-bottle', 'bottle-around-ship', and approaches to achieve novel hierarchical MA@MOF structures are highlighted and discussed while identifying their potential as well as their limitations. Hereby, an overview of standard characterisation methods that enable a systematic analysis procedure and state-of-art techniques that localise MA within MOF cavities are provided. While the perspectives of MA@MOF materials in general have been reviewed various times in the recent past, few atom-precise MAs inside MOFs have been reported so far, opening opportunities for future investigation.