Opportunities for Cryogenic Electron Microscopy in Materials Science and Nanoscience

The hierarchical structure of organic mixed ionic-electronic conductors and its evolution in water

Polymeric organic mixed ionic-electronic conductors underpin several technologies in which their electrochemical properties are desirable. These properties, however, depend on the microstructure that develops in their aqueous operational environment. We investigated the structure of a model organic mixed ionic-electronic conductor across multiple length scales using cryogenic four-dimensional scanning transmission electron microscopy in both its dry and hydrated states. Four-dimensional scanning transmission electron microscopy allows us to identify the prevalent defects in the polymer crystalline regions and to analyse the liquid crystalline nature of the polymer. The orientation maps of the dry and hydrated polymers show that swelling-induced disorder is mostly localized in discrete regions, thereby largely preserving the liquid crystalline order. Therefore, the liquid crystalline mesostructure makes electronic transport robust to electrolyte ingress. This study demonstrates that cryogenic four-dimensional scanning transmission electron microscopy provides multiscale structural insights into complex, hierarchical structures such as polymeric organic mixed ionic-electronic conductors, even in their hydrated operating state.

Electrified Operando-Freezing of Electrocatalytic CO2 Reduction Cells for Cryogenic Electron Microscopy

The ability to freeze and stabilize reaction intermediates in their metastable states and obtain their structural and chemical information with high spatial resolution is critical to advance materials technologies such as catalysis and batteries. Here, we develop an electrified operando-freezing methodology to preserve these metastable states under electrochemical reaction conditions for cryogenic electron microscopy (cryo-EM) imaging and spectroscopy. Using Cu catalysts for CO2 reduction as a model system, we observe restructuring of the Cu catalyst in a CO2 atmosphere while the same catalyst remains intact in air at the nanometer scale. Furthermore, we discover the existence of a single valence Cu (1+) state and C-O bonding at the electrified liquid-solid interface of the operando-frozen samples, which are key reaction intermediates that traditional ex situ measurements fail to detect. This work highlights our novel technique to study the local structure and chemistry of electrified liquid-solid interfaces, with broad impact beyond catalysis.

Visualization of Nanocrystallites in Organic Semiconducting Blends Using Cryo-Electron Microscopy

The efficiency of an organic solar cell is highly dependent on the complex, interpenetrating morphology, and molecular order within the composite phases of the bulk heterojunction (BHJ) blend. Both these microstructural aspects are strongly influenced by the processing conditions and chemical design of donor/acceptor materials. To establish improved structure-function relationships, it is vital to visualize the local microstructural order to provide specific local information about donor/acceptor interfaces and crystalline texture in BHJ blend films. The visualization of nanocrystallites, however, is difficult due to the complex semi-crystalline structure with few characterization techniques capable of visualizing the molecular ordering of soft materials at the nanoscale. Here, it is demonstrated how cryo-electron microscopy can be utilized to visualize local nanoscale order. This method is used to understand the distribution/orientation of crystallites in a BHJ blend. Long-range (>300 nm) texturing of IEICO-4F crystallites oriented in an edge-on fashion is observed, which has not previously been observed for spin-coated materials. This approach provides a wealth of quantitative information about the texture and size of nanocrystallites, which can be utilized to understand charge generation and transport in organic film. This study guides tailoring the material design and processing conditions for high-performance organic optoelectronic devices.

Single crystals of purely organic free-standing two-dimensional woven polymer networks

The aesthetic and practicality of macroscopic fabrics continue to encourage chemists to weave molecules into interlaced patterns with the aim of providing emergent physical and chemical properties when compared with their starting materials. Weaving purely organic molecular threads into flawless two-dimensional patterns remains a formidable challenge, even though its feasibility has been proposed on several occasions. Herein we describe the synthesis of a flawless, purely organic, free-standing two-dimensional woven polymer network driven by dative B-N bonds. Single crystals of this woven polymer network were obtained and its well-defined woven topology was revealed by X-ray diffraction analysis. Free-standing two-dimensional monolayer nanosheets of the woven polymer network were exfoliated from the layered crystals using Scotch Magic Tape. The surface features of the nanosheets were investigated by integrated low-dose and cryogenic electron microscopy imaging techniques. These findings demonstrate the precise construction of purely organic woven polymer networks and highlight the unique opportunities for the application of woven topologies in two-dimensional organic materials.

Structural Complexities in Sodium Ion Conductive Antiperovskite Revealed by Cryogenic Transmission Electron Microscopy

We use low-dose cryogenic transmission electron microscopy (cryo-TEM) to investigate the atomic-scale structure of antiperovskite Na2NH2BH4 crystals by preserving the room-temperature cubic phase and carefully monitoring the electron dose. Via quantitative analysis of electron beam damage using selected area electron diffraction, we find cryogenic imaging provides 6-fold improvement in beam stability for this solid electrolyte. Cryo-TEM images obtained from flat crystals revealed the presence of a new, long-range-ordered supercell with a cubic phase. The supercell exhibits doubled unit cell dimensions of 9.4 Å × 9.4 Å as compared to the cubic lattice structure revealed by X-ray crystallography of 4.7 Å × 4.7 Å. The comparison between the experimental image and simulated potential map indicates the origin of the supercell is a vacancy ordering of sodium atoms. This work demonstrates the potential of using cryo-TEM imaging to study the atomic-scale structure of air- and electron-beam-sensitive antiperovskite-type solid electrolytes.

Three-Dimensional Crystalline Organic Framework Stabilized by Molecular Mortise-and-Tenon Jointing

Three-dimensional (3D) crystalline organic frameworks with complex topologies, high surface area, and low densities afford a variety of application prospects. However, the design and construction of these frameworks have been largely limited to systems containing polyhedron-shaped building blocks or those relying on component interpenetration. Here, we report the synthesis of a 3D crystalline organic framework based on molecular mortise-and-tenon jointing. This new material takes advantage of tetra(4-pyridylphenyl)ethylene and chlorinated bis(benzodioxaborole)benzene as building blocks and is driven by dative B-N bonds. A single-crystal X-ray diffraction analysis of the framework reveals the presence of two-dimensional (2D) layers with helical channels that are formed presumably during the boron-nitrogen coordination process. The protrusion of dichlorobenzene units from the upper and lower surfaces of the 2D layers facilitates the key mortise-and-tenon connections. These connections enable the interlocking of adjacent layers and the stabilization of an overall 3D framework. The resulting framework is endowed with high porosity and attractive mechanical properties, rendering it potentially suitable for the removal of impurities from acetylene.

Metal-Organic Framework (MOF)-Based Clean Energy Conversion: Recent Advances in Unlocking its Underlying Mechanisms

Carbon neutrality is an important goal for humanity . As an eco-friendly technology, electrocatalytic clean energy conversion technology has emerged in the 21st century. Currently, metal-organic framework (MOF)-based electrocatalysis, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), carbon dioxide reduction reaction (CO2RR), nitrogen reduction reaction (NRR), are the mainstream energy catalytic reactions, which are driven by electrocatalysis. In this paper, the current advanced characterizations for the analyses of MOF-based electrocatalytic energy reactions have been described in details, such as density function theory (DFT), machine learning, operando/in situ characterization, which provide in-depth analyses of the reaction mechanisms related to the above reactions reported in the past years. The practical applications that have been developed for some of the responses that are of application values, such as fuel cells, metal-air batteries, and water splitting have also been demonstrated. This paper aims to maximize the potential of MOF-based electrocatalysts in the field of energy catalysis, and to shed light on the development of current intense energy situations.

Solid Interfaces for the Garnet Electrolytes

Solid-state electrolytes (SSEs) have attracted extensive interests due to the advantages in developing secondary batteries with high energy density and outstanding safety. Possessing high ionic conductivity and the lowest reduction potential among the state-of-the-art SSEs, the garnet type SSE is one of the most promising candidates to achieve high performance solid-state lithium batteries (SSLBs). However, the elastic modulus of the garnet electrolyte leads to deteriorated interfacial contacts, and the increasing in electronic conduction at either anode/garnet interface or grain boundary results in Li dendrite growth. Here, recent developments of the solid interfaces for the garnet electrolytes, including the strategies of Li dendrite suppression and interfacial chemical/electrochemical/mechanical stabilizations are presented. A new viewpoint of the double edges of interfacial lithiophobicity is proposed, and the rational design of the interphases, as well as effective stacking methods of the garnet-based SSLBs are summarized. Moreover, practical roles of the garnet electrolyte in SSLB industry are also discussed. This work delivers insights into the solid interfaces for the garnet electrolytes, which provides not only the promotion of the garnet-based SSLBs, but also a comprehensive understanding of the interfacial stabilization for the whole SSE family.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

Advances in Nanoparticle-Enhanced Thermoelectric Materials from Synthesis to Energy Harvesting: A Review

This comprehensive review analysis examines the domain of composite thermoelectric materials that integrate nanoparticles, providing a critical assessment of their methods for improving thermoelectric properties and the procedures used for their fabrication. This study examines several approaches to enhance power factor and lattice thermal conductivity, emphasizing the influence of secondary phases and structural alterations. This study investigates the impact of synthesis methods on the electrical characteristics of materials, with a particular focus on novel techniques such as electrodeposition onto carbon nanotubes. The acquired insights provide useful guidance for the creation of new thermoelectric materials. The review also compares and contrasts organic and inorganic thermoelectric materials, with a particular focus on the potential of inorganic materials in the context of waste heat recovery and power production within industries. This analysis highlights the role of inorganic materials in improving energy efficiency and promoting environmental sustainability.

Elucidating Phase Transformation and Surface Amorphization of Li7 La3 Zr2 O12 by In Situ Heating TEM

Garnet-type Li7 La3 Zr2 O12 (LLZO) solid-state electrolytes hold great promise for the next-generation all-solid-state batteries. An in-depth understanding of the phase transformation during synthetic processes is required for better control of the crystallinity and improvement of the ionic conductivity of LLZO. Herein, the phase transformation pathways and the associated surface amorphization are comparatively investigated during the sol-gel and solid-state syntheses of LLZO using in situ heating transmission electron microscopy (TEM). The combined ex situ X-ray diffraction and in situ TEM techniques are used to reveal two distinct phase transformation pathways (precursors → La2 Zr2 O7  → LLZO and precursors → LLZO) and the subsequent layer-by-layer crystal growth of LLZO on the atomic scale. It is also demonstrated that the surface amorphization surrounding the LLZO crystals is sensitive to the postsynthesis cooling rate and significantly affects the ionic conductivity of pelletized LLZO. This work brings up a critical but often overlooked issue that may greatly exacerbate the Li-ion conductivity by undesired synthetic conditions, which can be leveraged to ameliorate the overall crystallinity to improve the electrochemical performance of LLZO. These findings also shed light on the significance of optimizing surface structure to ensure superior performance of Li-ion conductors.

Modeling Interfacial Dynamics on Single Atom Electrocatalysts: Explicit Solvation and Potential Dependence

ConspectusSingle atom electrocatalysts, with noble metal-free composition, maximal atom efficiency, and exceptional reactivity toward various energy and environmental applications, have become a research hot spot in the recent decade. Their simplicity and the isolated nature of the atomic structure of their active site have also made them an ideal model catalyst system for studying reaction mechanisms and activity trends. However, the state of the single atom active sites during electrochemical reactions may not be as simple as is usually assumed. To the contrary, the single atom electrocatalysts have been reported to be under greater influence from interfacial dynamics, with solvent and electrolyte ions perpetually interacting with the electrified active center under an applied electrode potential. These complexities render the activity trends and reaction mechanisms derived from simplistic models dubious.In this Account, with a few popular single atom electrocatalysis systems, we show how the change in electrochemical potential induces nontrivial variation in the free energy profile of elemental electrochemical reaction steps, demonstrate how the active centers with different electronic structure features can induce different solvation structures at the interface even for the same reaction intermediate of the simplest electrochemical reaction, and discuss the implication of the complexities on the kinetics and thermodynamics of the reaction system to better address the activity and selectivity trends. We also venture into more intriguing interfacial phenomena, such as alternative reaction pathways and intermediates that are favored and stabilized by solvation and polarization effects, long-range interfacial dynamics across the region far beyond the contact layer, and the dynamic activation or deactivation of single atom sites under operation conditions. We show the necessity of including realistic aspects (explicit solvent, electrolyte, and electrode potential) into the model to correctly capture the physics and chemistry at the electrochemical interface and to understand the reaction mechanisms and reactivity trends. We also demonstrate how the popular simplistic design principles fail and how they can be revised by including the kinetics and interfacial factors in the model. All of these rich dynamics and chemistry would remain hidden or overlooked otherwise. We believe that the complexity at an electrochemical interface is not a curse but a blessing in that it enables deeper understanding and finer control of the potential-dependent free energy landscape of electrochemical reactions, which opens up new dimensions for further design and optimization of single atom electrocatalysts and beyond. Limitations of current methods and challenges faced by the theoretical and experimental communities are discussed, along with the possible solutions awaiting development in the future.

Realistic Modeling of the Electrocatalytic Process at Complex Solid-Liquid Interface

The rational design of electrocatalysis has emerged as one of the most thriving means for mitigating energy and environmental crises. The key to this effort is the understanding of the complex electrochemical interface, wherein the electrode potential as well as various internal factors such as H-bond network, adsorbate coverage, and dynamic behavior of the interface collectively contribute to the electrocatalytic activity and selectivity. In this context, the authors have reviewed recent theoretical advances, and especially, the contributions to modeling the realistic electrocatalytic processes at complex electrochemical interfaces,  and illustrated the challenges and fundamental problems in this field. Specifically, the significance of the inclusion of explicit solvation and electrode potential as well as the strategies toward the design of highly efficient electrocatalysts are discussed. The structure-activity relationships and their dynamic responses to the environment and catalytic functionality under working conditions are illustrated to be crucial factors for understanding the complexed interface and the electrocatalytic activities. It is hoped that this review can help spark new research passion and ultimately bring a step closer to a realistic and systematic modeling method for electrocatalysis.

Visualizing the Interfacial Chemistry in Multivalent Metal Anodes by Transmission Electron Microscopy

Multivalent metal batteries (MMBs) have been considered potentially high-energy and low-cost alternatives to commercial Li-ion batteries, thus attracting tremendous research interest for energy-storage applications. However, the plating and stripping of multivalent metals (i.e., Zn, Ca, Mg) suffer from low Coulombic efficiencies and short cycle life, which are largely rooted in the unstable solid electrolyte interphase. Apart from exploring new electrolytes or artificial layers for robust interphases, fundamental works on deciphering interfacial chemistry have also been conducted. This work is dedicated to summarizing the state-of-the-art advances in understanding the interphases for multivalent metal anodes revealed by transmission electron microscopy (TEM) methods. Operando and cryogenic TEM with high spatial and temporal resolutions realize the dynamic visualization of the vulnerable chemical structures in interphase layers. Following a scrutinization of the interphases on different metal anodes, we elucidate their features for appealing multivalent metal anodes. Finally, perspectives are proposed for the remaining issues on analyzing and regulating interphases for practical MMBs.

Unlocking Liquid Sulfur Chemistry for Fast-Charging Lithium-Sulfur Batteries

A recent study of liquid sulfur produced in an electrochemical cell has prompted further investigation into regulating Li-S oxidation chemistry. In this research, we examined the liquid-to-solid sulfur transition dynamics by visually observing the electrochemical generation of sulfur on a graphene-based substrate. We investigated the charging of polysulfides at various current densities and discovered a quantitative correlation between the size and number density of liquid sulfur droplets and the applied current. However, the areal capacities exhibited less sensitivity. This observation offers valuable insights for designing fast-charging sulfur cathodes. By incorporating liquid sulfur into Li-S batteries with a high sulfur loading of 4.2 mg cm-2, the capacity retention can reach ∼100%, even when increasing the rate from 0.1 to 3 C. This study contributes to a better understanding of the kinetics involved in the liquid-solid sulfur growth in Li-S chemistry and presents viable strategies for optimizing fast-charging operations.

Aggregation-induced emission-active micelles: synthesis, characterization, and applications

Aggregation-induced emission (AIE)-active micelles are a type of fluorescent functional materials that exhibit enhanced emissions in the aggregated surfactant state. They have received significant interest due to their excellent fluorescence efficiency in the aggregated state, remarkable processability, and solubility. AIE-active micelles can be designed through the self-assembly of amphipathic AIE luminogens (AIEgens) and the encapsulation of non-emissive amphipathic molecules in AIEgens. Currently, a wide range of AIE-active micelles have been constructed, with a significant increase in research interest in this area. A series of advanced techniques has been used to characterize AIE-active micelles, such as cryogenic-electron microscopy (Cryo-EM) and confocal laser scanning microscopy (CLSM). This review provides an overview of the synthesis, characterization, and applications of AIE-active micelles, especially their applications in cell and in vivo imaging, biological and organic compound sensors, anticancer drugs, gene delivery, chemotherapy, photodynamic therapy, and photocatalytic reactions, with a focus on the most recent developments. Based on the synergistic effect of micelles and AIE, it is anticipated that this review will guide the development of innovative and fascinating AIE-active micelle materials with exciting architectures and functions in the future.

Nanoparticles for super-resolution microscopy: intracellular delivery and molecular targeting

Following an overview of the approaches and techniques used to acheive super-resolution microscopy, this review presents the advantages supplied by nanoparticle based probes for these applications. The various clases of nanoparticles that have been developed toward these goals are then critically described and these discussions are illustrated with a variety of examples from the recent literature.

High-Resolution Electron Tomography of Ultrathin Boerdijk-Coxeter-Bernal Nanowire Enabled by Superthin Metal Surface Coating

The rapid advancement of transmission electron microscopy has resulted in revolutions in a variety of fields, including physics, chemistry, and materials science. With single-atom resolution, 3D information of each atom in nanoparticles is revealed, while 4D electron tomography is shown to capture the atomic structural kinetics in metal nanoparticles after phase transformation. Quantitative measurements of physical and chemical properties such as chemical coordination, defects, dislocation, and local strain have been made. However, due to the incompatibility of high dose rate with other ultrathin morphologies, such as nanowires, atomic electron tomography has been primarily limited to quasi-spherical nanoparticles. Herein, the 3D atomic structure of a complex core-shell nanowire composed of an ultrathin Boerdijk-Coxeter-Bernal (BCB) core nanowire and a noble metal thin layer shell deposited on the BCB nanowire surface is discovered. Furthermore, it is demonstrated that a new superthin noble metal layer deposition on an ultrathin BCB nanowire could mitigate electron beam damage using an in situ transmission electron microscope and atomic resolution electron tomography. The colloidal coating method developed for electron tomography can be broadly applied to protect the ultrathin nanomaterials from electron beam damage, benefiting both the advanced material characterizations and enabling fundamental in situ mechanistic studies.

High-Resolution Cryo-Electron Microscopy Reveals the Unique Striated Hollow Structure of Photocatalytic Macrocyclic Polydiacetylene Nanotubes

High-resolution structures are crucial for understanding the functional properties of nanomaterials. We applied single-particle cryo-electron microscopy (cryo-EM), a method traditionally used for structure determination of biological macromolecules, to obtain high-resolution structures of synthetic non-biological filaments formed by photopolymerization of macrocyclic diacetylene (MDA) amphiphilic monomers. Tomographic analysis showed that the MDA monomers self-assemble into hollow nanotubes upon dispersion in water. Single-particle analysis revealed tubes consisting of six pairs of covalently bonded filaments held together by hydrophobic interactions, where each filament is composed of macrocyclic rings stacked in parallel "chair" conformations. The hollow MDA nanotube structures we found may account for the efficient scavenging of amphiphilic pollutants in water and subsequent photodegradation of the guest species.

Molecular Dynamics Simulation and Cryo-Electron Microscopy Investigation of AOT Surfactant Structure at the Hydrated Mica Surface

Structural properties of the anionic surfactant dioctyl sodium sulfosuccinate (AOT or Aerosol-OT) adsorbed on the mica surface were investigated by molecular dynamics simulation, including the effect of surface loading in the presence of monovalent and divalent cations. The simulations confirmed recent neutron reflectivity experiments that revealed the binding of anionic surfactant to the negatively charged surface via adsorbed cations. At low loading, cylindrical micelles formed on the surface, with sulfate head groups bound to the surface by water molecules or adsorbed cations. Cation bridging was observed in the presence of weakly hydrating monovalent cations, while sulfate groups interacted with strongly hydrating divalent cations through water bridges. The adsorbed micelle structure was confirmed experimentally with cryogenic electronic microscopy, which revealed micelles approximately 2 nm in diameter at the basal surface. At higher AOT loading, the simulations reveal adsorbed bilayers with similar surface binding mechanisms. Adsorbed micelles were slightly thicker (2.2–3.0 nm) than the corresponding bilayers (2.0–2.4 nm). Upon heating the low loading systems from 300 K to 350 K, the adsorbed micelles transformed to a more planar configuration resembling bilayers. The driving force for this transition is an increase in the number of sulfate head groups interacting directly with adsorbed cations.

Cryo-Electron Tomography of Highly Deformable and Adherent Solid-Electrolyte Interphase Exoskeleton in Li-Metal Batteries with Ether-Based Electrolyte

The 3D nanocomposite structure of plated lithium (Li_Metal ) and solid electrolyte interphases (SEI), including a polymer-rich surficial passivation layer (SEI exoskeleton) and inorganic SEI "fossils" buried inside amorphous Li matrix, is resolved using cryogenic transmission electron microscopy. With ether-based DOLDME-LiTFSI electrolyte, LiF and Li₂ O nanocrystals are formed and embedded in a thin but tough amorphous polymer in the SEI exoskeleton. The fast Li-stripping directions are along [ 1 ¯ 10 ] or [ 12 1 ¯ ] , which produces eight exposed {111} planes at halfway charging. Full Li stripping produces completely sagging, empty SEI husks that can sustain large bending and buckling, with the smallest bending radius of curvature observed approaching tens of nanometers without apparent damage. In the 2nd round of Li plating, a thin Li_BCC sheet first nucleates at the current collector, extends to the top end of the deflated SEI husk, and then expands its thickness. The apparent zero wetting angle between Li_BCC and the SEI interior means that the heterogeneous nucleation energy barrier is zero. Due to its complete-wetting property and chemo-mechanical stability, the SEI largely prevents further reactions between the Li metal and the electrolyte, which explains the superior performance of Li-metal batteries with ether-based electrolytes. However, uneven refilling of the SEI husks results in dendrite protrusions and some new SEI formation during the 2nd plating. A strategy to form bigger SEI capsules during the initial cycle with higher energy density than the following cycles enables further enhanced Coulombic efficiency to above 99%.

Graphitic-like Hexagonal Phase of Alkali Halides in Quasi-Two-Dimensional Confined Space under Ambient Conditions

The discovery of specific matter phases with abnormal physical properties in low-dimensional systems and/or on particular substrates, such as the hexagonal phase of ice and two-dimensional (2D) CaCl with an abnormal valence state, continuously reveals more fundamental mechanisms of the nature. Alkali halides, represented by NaCl, are one of the most common compounds and usually thought to be well-understood. In the past decades, many theoretical studies suggested the existence of one particular phase, that is, the graphitic-like hexagonal phase of alkali halides at high pressure or in low-dimension states, with the expectation of improved properties of this matter phase but lacking experimental evidence due to severe technical challenges. Here, by optimized cryo-electron microscopy, we report the direct atomic-resolution observation and in situ characterization of the prevalent and stable graphitic-like alkali halide hexagonal phases, which were spontaneously formed by unsaturated NaCl and LiCl solution, respectively, in the quasi-2D confined space between reduced graphene oxide layers under ambient conditions. Combined with a control experiment, density functional theory calculations, and previous theoretical studies, we believe that a delicate balance among the cation-π interaction of the solute and substrate, electrostatic interactions of anions and cations, solute-solvent interactions, and thermodynamics under confinement synergistically results in the formation of such hexagonal crystalline phases. These findings highlight the effects of the substrate and the confined space on the formation of specific matter phases and provide a universal scheme for the preparation of special graphitic-like hexagonal phases of alkali halides.

Cryo-EM for battery materials and interfaces: Workflow, achievements, and perspectives

The emerging cryogenic electron microscopy (cryo-EM) has demonstrated its power and essential role in probing the beam-sensitive battery materials and delivering new insights. With the increasing interest in cryo-EM for battery materials and interfaces, herein we provide the strategies of obtaining fresh and native structural information with minimal artifacts, including sample preparation, transferring, imaging, and data interpretation. We summarize the recent achievements enabled by cryo-EM and point out some unsolved/potential questions in terms of the bulk materials, solid-solid interface, and solid-liquid interfaces of batteries. Finally, we conclude with perspectives on the future developments and applications of cryo-EM in battery materials and interfaces.

Graphene Oxide-Supported Microwell Grids for Preparing Cryo-EM Samples with Controlled Ice Thickness

Cryogenic-electron microscopy (cryo-EM) is the preferred method to determine 3D structures of proteins and to study diverse material systems that intrinsically have radiation or air sensitivity. Current cryo-EM sample preparation methods provide limited control over the sample quality, which limits the efficiency and high throughput of 3D structure analysis. This is partly because it is difficult to control the thickness of the vitreous ice that embeds specimens, in the range of nanoscale, depending on the size and type of materials of interest. Thus, there is a need for fine regulation of the thickness of vitreous ice to deliver consistent high signal-to-noise ratios for low-contrast biological specimens. Herein, an advanced silicon-chip-based device is developed which has a regular array of micropatterned holes with a graphene oxide (GO) window on freestanding silicon nitride (Six Ny ). Accurately regulated depths of micropatterned holes enable precise control of vitreous ice thickness. Furthermore, GO window with affinity for biomolecules can facilitate concentration of the sample molecules at a higher level. Incorporation of micropatterned chips with a GO window enhances cryo-EM imaging for various nanoscale biological samples including human immunodeficiency viral particles, groEL tetradecamers, apoferritin octahedral, aldolase homotetramer complexes, and tau filaments, as well as inorganic materials.

Cryogenic Electron Microscopy for Energy Materials

The development of clean energy generation, transmission, and distribution technology, for example, high energy density batteries and high efficiency solar cells, is critical to the progress toward a sustainable future. Such advancement in both scientific understanding and technological innovations entail an atomic- and molecular-resolution understanding of the key materials and fundamental processes governing the operation and failure of the systems. These dynamic processes span multiple length and time scales bridging materials and interfaces involved across the entire device architecture. However, these key components are often highly sensitive to air, moisture, and electron-beam radiation and therefore remain resistant to conventional nanoscale interrogation by electron-optical methods, such as high-resolution (scanning) transmission electron microscopy and spectroscopy.Fortunately, the rapid progress in cryogenic electron microscopy (cryo-EM) for physical sciences starts to offer researchers new tools and methods to probe these otherwise inaccessible length scales of components and phenomena in energy science. Specifically, weakly bonded and reactive materials, interfaces and phases that typically degrade under high energy electron-beam irradiation and environmental exposure can potentially be protected and stabilized by cryogenic methods, bringing up thrilling opportunities to address many crucial yet unanswered questions in energy science, which can eventually lead to new scientific discoveries and technological breakthroughs.Thus, in this Account, we aim to highlight the significance of cryo-EM to energy related research and the impactful results that can be potentially spawned from there. Due to the limited space, we will mainly review representative examples of cryo-EM methodology for lithium (Li)-based batteries, hybrid perovskite solar cells, and metal-organic-frameworks, which have shown great promise in revealing atomic resolution of both structural and chemical information on the sensitive yet critical components in these systems. We will first emphasize the application of cryo-EM to resolve the nanostructure and chemistry of solid-electrolyte interphases, cathode-electrolyte interphase, and electrode materials in batteries to reflect how cryo-EM could inspire rational materials design and guide battery research toward practical applications. We then discuss how cryo-EM helped to reveal guest intercalation chemistry in weakly bonded metal-organic-frameworks to develop a complete picture of host-guest interaction. Next, we summarize efforts in hybrid perovskite materials for solar cells where cryo-EM preserved the volatile organic molecules and protected perovskites from any air or moisture contamination. Finally, we conclude with perspectives and brief discussion on future directions for cryo-EM in energy and materials science.

A Close Look at Molecular Self-Assembly with the Transmission Electron Microscope

Molecular self-assembly is pervasive in the formation of living and synthetic materials. Knowledge gained from research into the principles of molecular self-assembly drives innovation in the biological, chemical, and materials sciences. Self-assembly processes span a wide range of temporal and spatial domains and are often unintuitive and complex. Studying such complex processes requires an arsenal of analytical and computational tools. Within this arsenal, the transmission electron microscope stands out for its unique ability to visualize and quantify self-assembly structures and processes. This review describes the contribution that the transmission electron microscope has made to the field of molecular self-assembly. An emphasis is placed on which TEM methods are applicable to different structures and processes and how TEM can be used in combination with other experimental or computational methods. Finally, we provide an outlook on the current challenges to, and opportunities for, increasing the impact that the transmission electron microscope can have on molecular self-assembly.

Probing morphology and chemistry in complex soft materials within situresonant soft x-ray scattering

Small angle scattering methodologies have been evolving at fast pace over the past few decades due to the ever-increasing demands for more details on the complex nanostructures of multiphase and multicomponent soft materials like polymer assemblies and biomaterials. Currently, element-specific and contrast variation techniques such as resonant (elastic) soft/tender x-ray scattering, anomalous small angle x-ray scattering, and contrast-matching small angle neutron scattering, or combinations of above are routinely used to extract the chemical composition and spatial arrangement of constituent elements at multiple length scales and examine electronic ordering phenomena. Here we present some recent advances in selectively characterizing structural architectures of complex soft materials, which often contain multi-components with a wide range of length scales and multiple functionalities, where novel resonant scattering approaches have been demonstrated to decipher a higher level of structural complexity that correlates to functionality. With the advancement of machine learning and artificial intelligence assisted correlative analysis, high-throughput and autonomous experiments would open a new paradigm of material research. Further development of resonant x-ray scattering instrumentation with crossplatform sample environments will enable multimodalin situ/operando characterization of the system dynamics with much improved spatial and temporal resolution.

Extending Cryo-EM to Nonaqueous Liquid Systems

Cryogenic-temperature transmission electron microscopy (cryo-TEM) of aqueous systems has become a widely used methodology, especially in the study of biological systems and synthetic aqueous systems, such as amphiphile and polymer solutions. Cryogenic-temperature scanning electron microscopy (cryo-SEM), while not as widely used as cryo-TEM, is also found in many laboratories of basic and applied research. The application of these methodologies, referred to collectively as cryogenic-temperature electron microscopy (cryo-EM) for direct nanostructural studies of nonaqueous liquid systems is much more limited, although such systems are important in basic research and are found in a very large spectrum of commercial applications. The study of nonaqueous liquid systems by cryo-EM poses many technical challenges. Specimen preparation under controlled conditions of air saturation around the specimen cannot be performed by the currently available commercial system, and the most effective cryogen, freezing ethane, cannot be used for most such liquid systems. Imaging is often complicated by low micrograph contrast and high sensitivity of the specimens to the electron beam.At the beginning of this Account, we describe the basic principles of cryo-EM, emphasizing factors that are essential for successful direct imaging by cryo-TEM and cryo-SEM. We discuss the peculiarities of nonaqueous liquid nanostructured systems when studied with these methodologies and how the technical difficulties in imaging nonaqueous systems, from oil-based to strong acid-based liquids, have been overcome, and the applicability of cryo-TEM and cryo-SEM has been expanded in recent years. Modern cryo-EM has been advanced by a number of instrumental developments, which we describe. In the TEM, these include improved electron field emission guns (FEGs) and microscope optics, the Volta phase plate to enhance image contrast by converting phase differences to amplitude differences without the loss of resolution by an objective lens strong underfocus, and highly sensitive image cameras that allow the recording of TEM images with minimal electron exposure. In the SEM, we take advantage of improved FEGs that allow imaging at a low (around 1 kV) electron acceleration voltage that is essential for high-resolution imaging and for avoiding specimen charging of uncoated nonconductive specimens, better optics, and a variety of sensitive detectors that have considerably improved resolution and, under the proper conditions, give excellent contrast even between elements quite close on the periodic table of the elements, such as the most important oxygen and carbon atoms.Finally we present and analyze several examples from our recent studies, which illustrate the issues presented above, including the remarkable progress made in recent years in this field and the strength and applicability of cryo-EM methodologies.

Visualizing the Sensitive Lithium with Atomic Precision: Cryogenic Electron Microscopy for Batteries

Lithium (Li)-metal batteries are one of the most promising candidates for the next-generation energy storage devices due to their ultrahigh theoretical capacity. The realistic development of a Li metal battery is greatly impeded by the uncontrollable dendrite proliferation upon the chemically active metallic Li. To visualize the micromorphology or even the atomic structure of Li deposits is undoubtedly crucial, while imaging the sensitive Li still faces a huge challenge technically.Cryogenic electron microscopy (cryo-EM), an emerging imagery technology renowned for structural elucidation of biomaterials, is offering increased possibilities for analyzing sensitive battery materials reaching subangstrom resolution. Particularly for revealing metallic Li, cryo-EM exhibits remarkable superiority compared with the conventional electron imaging technique. On the one hand, cryo-EM could prevent the low melting-point Li metal from being damaged by the high electron dose induced thermal effect. On the other hand, the extremely low temperature immensely retards the rate of the side reaction where the Li reacts with the atmosphere or water vapor before the vacuum state. Consequently, the cryo-EM could acquire a high-resolution image of electron-beam sensitive Li in its native state at the nano- or even atomic scale, thus benefiting the fundamental perception and rational design of Li metal anodes.Thus, in this Account, we aim to highlight the significance of cryo-EM in analyzing metallic Li and developing a high-performance Li metal battery. We focus on how highly resolved cryo-EM realizes the breakthrough in detecting the crucial evolution during battery cycling, e.g., lattice ordering of Li, nanostructures of the solid electrolyte interphase (SEI), nucleation sites, and interface between the solid electrolyte and the Li anode. First, we briefly summarize the progress of Li metal imaging by cryo-EM in a timed sequence. In particular, the recent studies from our group are classified in order to systematically delineate the advantages that cryo-transmission electron microscopy (cryo-TEM) addressed on understanding and developing the Li metal battery. Second, the efforts of exhibiting the long-range ordering Li lattice are described to cognize the crystal orientation of both Li dendrites and uniform spheres. Subsequently, the nanostructures of SEI detected by cryo-TEM, maybe the most key information during Li plating/stripping, are systematically summarized. Benefitting from the subangstrom visualization on the newly formed and the particular inactive SEI after long-term cycling, we emphasize cryo-TEM's guidance in designing a robust, highly Li⁺ conductive, and Li-restoration facilitated SEI. We then propose the strategy of introducing a nucleation-site to enable uniform Li deposition by showing the evidence of Li nucleation atomically monitored through cryo-TEM. Moreover, the series of the work of atomic imagery and corresponding optimization of the interfaces between the polymer-based solid electrolyte and the Li anode are concluded. Finally, critical perspectives about the further step of cryo-TEM in the realistic development of high-energy density battery systems are also succinctly reviewed.

Exploring valence states of abnormal mineral deposits in biological tissues using correlative microscopy and spectroscopy techniques: A case study on ferritin and iron deposits from Alzheimer's disease patients

Abnormal accumulation of inorganic trace elements in a human brain, such as iron, zinc and aluminum, oftentimes manifested as deposits and accompanied by a chemical valence change, is pathologically relevant to various neurodegenerative diseases. In particular, Fe²⁺ has been hypothesized to produce free radicals that induce oxidative damage and eventually cause Alzheimer's disease (AD). However, traditional biomedical techniques, e.g. histology staining, are limited in studying the chemical composition and valence states of these inorganic deposits. We apply commonly used physical (phys-) science methods such as X-ray energy dispersive spectroscopy (EDS), focused-ion beam (FIB) and electron energy loss spectroscopy (EELS) in transmission electron microscopy in conjunction with magnetic resonance imaging (MRI), histology and optical microscopy (OM) to study the valence states of iron deposits in AD patients. Ferrous ions are found in all deposits in brain tissues from three AD patients, constituting 0.22-0.50 of the whole iron content in each specimen. Such phys-techniques are rarely used in medical science and have great potential to provide unique insight into biomedical problems.