Reversible superdense ordering of lithium between two graphene sheets

Material characterization methods for investigating charge storage processes in 2D and layered materials-based batteries and supercapacitors

Two-dimensional (2D) materials offer distinct advantages for electrochemical energy storage (EES) compared to bulk materials, including a high surface-to-volume ratio, tunable interlayer spacing, and excellent in-plane conductivity, making them highly attractive for applications in batteries and supercapacitors. Gaining a fundamental understanding of the energy storage processes in 2D material-based EES devices is essential for optimizing their chemical composition, surface chemistry, morphology, and interlayer structure to enhance ion transport, promote redox reactions, suppress side reactions, and ultimately improve overall performance. This review provides a comprehensive overview of the characterization techniques employed to probe charge storage mechanisms in 2D and thin-layered material-based EES systems, covering optical spectroscopy, imaging techniques, X-ray and neutron-based methods, mechanical probing, and nuclear magnetic resonance spectroscopy. We specifically highlight the application of these techniques in elucidating ion transport dynamics, tracking redox processes, identifying degradation pathways, and detecting interphase formation. Furthermore, we discuss the limitations, challenges, and potential pitfalls associated with each method, as well as future directions for advancing characterization techniques to better understand and optimize 2D material-based electrodes.

Photoconductivity Switching in Semiconducting Two-Dimensional Crystals via Molecular Tetris

Two-dimensional organic materials are mainly constructed by using orthogonal anisotropic connectivity of covalent bonding and π-π stacking. The noncovalent connectivity between building blocks is presumed to be too delicate to stabilize the two-dimensional (2D) layers. Contrary to this assumption, we constructed graphite-like 2D layered material by utilizing pure noncovalent connectivity, i.e., weak intermolecular and π-π interaction via a molecular Tetris strategy. We produce X-ray mountable single crystals comprising polycyclic aromatic heterocycles by employing a single-crystal-to-dissolution-to-single-crystal transformation methodology. The macromechanical analysis of this layered crystal shows shearing behavior, which is quantified using nanoindentation experiments. The 2D lattice's layer space allows reversible intercalation-deintercalation of iodine, which enhances the photoconductivity by 17 folds. Combined efforts of X-ray diffraction, solid-state spectroscopy, and electrochemical studies established the mechanism of intercalation and resulting photoconductivity enhancement.

Interlayer reconstruction phase transition in van der Waals materials

Van der Waals materials display rich structural polymorphs with distinct physical properties. An atomistic understanding of the phase-transition dynamics, propagation pathway and associated evolution of physical properties is essential for capturing their potential in practical technologies. However, direct visualization of the rapid phase-transition process is fundamentally challenging due to the inherent trade-offs among atomic resolution, field of view and imaging frame rate. Here we exploit a controllable current-driven phase transition and utilize in situ scanning transmission electron microscopy to visualize dynamic atomic rearrangements during the 2H-α to 2H-β transition in layered In2Se3. We identify a unique intralayer-splitting (unzipping) and interlayer-reconstruction (zipping) pathway, driven by an energy-cascading mechanism through which bond formation across the van der Waals gap facilitates bond cleavage in the covalent layers. We also observe current-direction-dependent asymmetric phase-transition propagation and attribute it to a temperature profile induced by the Peltier effect at the heterophase interface. These findings provide insights that are essential for designing tailored structural phase transitions in advanced technologies.

In Situ TEM Characterization of Battery Materials

Transmission electron microscopy (TEM) is an indispensable analytical technique in materials research as it probes material information down to the atomic level and can be utilized to examine dynamic phenomena during material transformations. In situ TEM resolves transient metastable states via direct observation of material dynamics under external stimuli. With innovative sample designs developed over the past decades, advanced in situ TEM has enabled emulation of battery operation conditions to unveil nanoscale changes within electrodes, at interfaces, and in electrolytes, rendering it a unique tool to offer unequivocal insights of battery materials that are beam-sensitive, air-sensitive, or that contain light elements, etc. In this review, we first briefly outline the history of advanced electron microscopy along with battery research, followed by an introduction to various in situ TEM sample cell configurations. We provide a comprehensive review on in situ TEM studies of battery materials for lithium batteries and beyond (e.g., sodium batteries and other battery chemistries) via open-cell and closed-cell in situ TEM approaches. At the end, we raise several unresolved points regarding sample preparation protocol, imaging conditions, etc., for in situ TEM experiments. We also provide an outlook on the next-stage development of in situ TEM for battery material study, aiming to foster closer collaboration between in situ TEM and battery research communities for mutual progress.

Defect Migration and Phase Transformations in Two-Dimensional Iron Chloride inside Bilayer Graphene

The intercalation of metal chlorides, and particularly iron chlorides, into graphitic carbon structures has recently received lots of attention, as it can not only protect this two-dimensional (2D) magnetic system from the effects of the environment but also substantially alter the magnetic, electronic, and optical properties of both the intercalant and host material. At the same time, intercalation can result in the formation of structural defects or defects can appear under external stimuli, which can affect materials performance. These aspects have received so far little attention in dedicated experiments. In this study, we investigate the behavior of atomic-scale defects in iron chlorides intercalated into bilayer graphene by using scanning transmission electron microscopy and first-principles calculations. We observe transformations between the FeCl2 and FeCl3 phases and elucidate the role of defects in the transformations. Specifically, three types of defects are identified: Fe vacancies in FeCl2 domains and Fe adatoms and interstitials in FeCl3 domains, each exhibiting distinct dynamic behaviors. We also observed a crystalline phase with an unusual stoichiometry of Fe5Cl18 that has not been reported before. Our findings not only advance the understanding of intercalation mechanism of 2D materials but also highlight the profound impact of atomic-scale defects on their properties and potential technological applications.

Electrochemical Control of the Ultrafast Lattice Response of a Layered Semimetal

The unique layer-stacking in two-dimensional (2D) van der Waals materials facilitates the formation of nearly degenerate phases of matter and opens novel routes for the design of low-power, reconfigurable functional materials. Electrochemical ion intercalation between stacked layers offers a promising approach to stabilize bulk metastable phases and to explore the effects of extreme carrier doping and strain. However, in situ characterization methods to study the structural evolution and dynamical functional properties of these intercalated materials remains limited. Here a novel experimental platform is presented capable of simultaneously performing electrochemical lithium-ion intercalation and multimodal ultrafast characterization of the lattice using both electron diffraction and nonlinear optical techniques. Using the layered semimetal WTe2 as a model system, the interlayer shear phonon mode that modulates stacking between 2Dlayers is probed, showing that small amounts of lithiation enhance the amplitude and lifetime of the phonon, contrary to expectations. This results from the dynamically fluctuating and anharmonic structure between nearly degenerate phases at room temperature, which can be stabilized by electronic carriers accompanying the inserted lithium ions. At high lithiation, the Td' structure emerges and quenches the phonon response. This work defines new approaches for using electrochemistry to engineer the dynamic structure of 2D materials.

In situ transmission electron microscopy insights into nanoscale deformation mechanisms of body-centered cubic metals

Nanostructured body-centered cubic (BCC) metals exhibit remarkable mechanical properties under various stress fields, making them promising candidates for novel micro/nanoelectromechanical systems (M/NEMS). A deep understanding of their mechanical behaviors, particularly at the atomic scale, is essential for optimizing their properties and expanding their applications at the nanoscale. Newly developed nanomechanical testing techniques within transmission electron microscopy (TEM) provide powerful tools for uncovering the atomic-scale microstructural evolution of nanostructured BCC materials under external forces. This article reviews recent progress in the experimental methods used in in situ TEM nanomechanical testing and the achievements of these techniques in understanding the deformation mechanisms of BCC nanomaterials. By outlining the current challenges and future research directions, this review aims to inspire continued exploration in the nanomechanics of BCC metals, contributing to the development of advanced BCC nanomaterials with tailored mechanical properties.

Synthesis and properties of phosphorus nanostructures on Au(110)

Phosphorene, a semiconducting two-dimensional material, has recently attracted huge interest due to its potential applications in opto-electronics. The first attempts to synthesize phosphorene were based mainly on mechanical and chemical exfoliations. A few years later, different groups reported the synthesis of phosphorene using the molecular beam epitaxy process, which opened the way for research on physical properties. In this article, we report phosphorus growth on an Au(110) substrate via molecular beam epitaxy. The atomically resolved scanning tunneling microscopy images exhibit a self-assembled phosphorus-dimer structure, which is converted into phosphorus chains when increasing the phosphorus coverage. The chemical composition of the obtained structures is determined via Auger electron and X-ray photoelectron spectroscopies. Density functional theory calculations support the experimental findings.

Nanoscale Operando Imaging of Electrically Driven Charge-Density Wave Phase Transitions

Structural transformations in strongly correlated materials promise efficient and fast control of materials' properties via electrical or optical stimulation. The desired functionality of devices operating based on phase transitions, however, will also be influenced by nanoscale heterogeneity. Experimentally characterizing the relationship between microstructure and phase switching remains challenging, as nanometer resolution and high sensitivity to subtle structural modifications are required. Here, we demonstrate nanoimaging of a current-induced phase transformation in the charge-density wave (CDW) material 1T-TaS2. Combining electrical characterizations with tailored contrast enhancement, we correlate macroscopic resistance changes with the nanoscale nucleation and growth of CDW phase domains. In particular, we locally determine the transformation barrier in the presence of dislocations and strain, underlining their non-negligible impact on future functional devices. Thereby, our results demonstrate the merit of tailored contrast enhancement and beam shaping for advanced operando microscopy of quantum materials and devices.

Graphene Bilayer as a Template for Manufacturing Novel Encapsulated 2D Materials

Bilayer graphene (BLG) has recently been used as a tool to stabilize encapsulated single sheets of various layered materials and tune their properties. It was also discovered that the protecting action of graphene sheets makes it possible to synthesize completely new two-dimensional materials (2DMs) inside the BLG by intercalating various atoms and molecules. In comparison to the bulk graphite, BLG allows for easier intercalation and a much larger increase in the interlayer separation of the sheets. Moreover, it enables studying the atomic structure of the intercalated 2DM by using high-resolution transmission electron microscopy. In this review, we summarize the recent progress in this area, with a special focus on new materials created inside BLG. We compare the experimental findings with the theoretical predictions, pay special attention to the discrepancies, and outline the challenges in the field. Finally, we discuss unique opportunities offered by intercalation into 2DMs beyond graphene and their heterostructures.

Unveiling Cu Nanoparticles Formed During Li Deposition in Anode-Free Batteries

Developing high-energy-density Li metal batteries is essential for sustainable progress, necessitating in-depth studies of complex battery reactions. The presence of metallic Cu impurities detrimentally impacts battery performance, leading to issues such as self-discharging and internal soft short-circuit. Nevertheless, their formation mechanism and structural characteristics have not been revealed clearly. Here the formation of single-crystalline Cu nanoparticles during the Li deposition process in anode-free cells was identified by transmission electron microscopy. Through investigation of the chemical state of Cu before and after Li deposition, the formation of Cu NPs was attributed to the reduction of the oxide layers formed on the surface of Cu current collectors. Additionally, it was observed that Cu nanoparticles can be formed inside of deposited Li metal. This work reveals the formation pathway and microstructural characteristics of Cu nanoparticles appearing during Li deposition, underscoring the importance of nanoscale investigations into the underlying battery reactions.

Gate-Switchable Molecular Diffusion on a Graphene Field-Effect Transistor

Controlling the surface diffusion of particles on 2D devices creates opportunities for advancing microscopic processes such as nanoassembly, thin-film growth, and catalysis. Here, we demonstrate the ability to control the diffusion of F4TCNQ molecules at the surface of clean graphene field-effect transistors (FETs) via electrostatic gating. Tuning the back-gate voltage (VG) of a graphene FET switches molecular adsorbates between negative and neutral charge states, leading to dramatic changes in their diffusion properties. Scanning tunneling microscopy measurements reveal that the diffusivity of neutral molecules decreases rapidly with a decreasing VG and involves rotational diffusion processes. The molecular diffusivity of negatively charged molecules, on the other hand, remains nearly constant over a wide range of applied VG values and is dominated by purely translational processes. First-principles density functional theory calculations confirm that the energy landscapes experienced by neutral vs charged molecules lead to diffusion behavior consistent with experiment. Gate-tunability of the diffusion barrier for F4TCNQ molecules on graphene enables graphene FETs to act as diffusion switches.

In-situ imaging of heat-induced phase transition in a two-dimensional conjugated metal-organic framework

Atomically-resolved in-situ high-resolution transmission electron microscopy (HRTEM) imaging of the structural dynamics in organic materials remains a major challenge. This difficulty persists even with aberration-corrected instruments, as HRTEM images necessitate a high electron dose that is generally intolerable for organic materials. In this study, we report the in-situ HRTEM imaging of heat-induced structural dynamics in a benzenehexathiol-based two-dimensional conjugated metal-organic framework (2D c-MOF, i.e., Cu3(BHT)). Leveraging its hydrogen-free structure and high electrical conductivity, Cu3(BHT) exhibits high electron beam resistance. We demonstrate atomic resolution imaging at an 80 kV electron accelerating voltage using our Cc/Cs-corrected SALVE instrument. However, continuous electron irradiation eventually leads to its amorphization. Intriguingly, under heating in a MEMS holder, the Cu3(BHT) undergoes a phase transition to a new crystalline phase and its phase transition, occurring within the temperature range of 480 °C to 620 °C in dependence on the electron beam illumination. Using HRTEM and energy-dispersive X-ray mapping, we identify this new phase as CuS. Our findings provide insights into the mechanisms governing structural transitions in purposefully engineered structures, potentially pivotal for future endeavours involving the production of metal oxide/sulfide nanoparticles from MOF precursors.

In-plane staging in lithium-ion intercalation of bilayer graphene

The ongoing efforts to optimize rechargeable Li-ion batteries led to the interest in intercalation of nanoscale layered compounds, including bilayer graphene. Its lithium intercalation has been demonstrated recently but the mechanisms underpinning the storage capacity remain poorly understood. Here, using magnetotransport measurements, we report in-operando intercalation dynamics of bilayer graphene. Unexpectedly, we find four distinct intercalation stages that correspond to well-defined Li-ion densities. Transitions between the stages occur rapidly (within 1 sec) over the entire device area. We refer to these stages as 'in-plane', with no in-plane analogues in bulk graphite. The fully intercalated bilayers represent a stoichiometric compound C14LiC14 with a Li density of ∼2.7·1014 cm-2, notably lower than fully intercalated graphite. Combining the experimental findings and DFT calculations, we show that the critical step in bilayer intercalation is a transition from AB to AA stacking which occurs at a density of ∼0.9·1014 cm-2. Our findings reveal the mechanism and limits for electrochemical intercalation of bilayer graphene and suggest possible avenues for increasing the Li storage capacity.

Investigation of the Single-Particle Scale Structure-Activity Relationship Providing New Insights for the Development of High-Performance Batteries

As electric vehicles, portable electronic devices, and tools have increasingly high requirements for battery energy density and power density, constantly improving battery performance is a research focus. Accurate measurement of the structure-activity relationship of active materials is key to advancing the research of high-performance batteries. However, conventional performance tests of active materials are based on the electrochemical measurement of porous composite electrodes containing active materials, polymer binders, and conductive carbon additives, which cannot establish an accurate structure-activity relationship with the physical characterization of microregions. In this review, in order to promote the accurate measurement and understanding of the structure-activity relationship of materials, the electrochemical measurement and physical characterization of energy storage materials at single-particle scale are reviewed. The potential problems and possible improvement schemes of the single particle electrochemical measurement and physical characterization are proposed. Their potential applications in single particle electrochemical simulation and machine learning are prospected. This review aims to promote the further application of single particle electrochemical measurement and physical characterization in energy storage materials, hoping to achieve 3D unified evaluation of physical characterization, electrochemical measurement, and theoretical simulation at the single particle scale to provide new inspiration for the development of high-performance batteries.

Intercalation in 2D materials and in situ studies

Intercalation of atoms, ions and molecules is a powerful tool for altering or tuning the properties - interlayer interactions, in-plane bonding configurations, Fermi-level energies, electronic band structures and spin-orbit coupling - of 2D materials. Intercalation can induce property changes in materials related to photonics, electronics, optoelectronics, thermoelectricity, magnetism, catalysis and energy storage, unlocking or improving the potential of 2D materials in present and future applications. In situ imaging and spectroscopy technologies are used to visualize and trace intercalation processes. These techniques provide the opportunity for deciphering important and often elusive intercalation dynamics, chemomechanics and mechanisms, such as the intercalation pathways, reversibility, uniformity and speed. In this Review, we discuss intercalation in 2D materials, beginning with a brief introduction of the intercalation strategies, then we look into the atomic and intrinsic effects of intercalation, followed by an overview of their in situ studies, and finally provide our outlook.

Mechanism of WS2 Nanotube Formation Revealed by in Situ/ex Situ Imaging

Multiwall WS2 nanotubes have been synthesized from W18O49 nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the "surface-inward" mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS2 nanotubes are obtained. In the present work, an upgraded in situ SEM μReactor with H2 and H2S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named "receding oxide core", which complements the "surface-inward" model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS2 layers via the surface-inward reaction. At this point, the diffusion of H2S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H2S gas, forming fresh WS2 layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 °C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with contemporary insight.

Strain Engineering of Twisted Bilayer Graphene: The Rise of Strain-Twistronics

The layer-by-layer stacked van der Waals structures (termed vdW hetero/homostructures) offer a new paradigm for materials design-their physical properties can be tuned by the vertical stacking sequence as well as by adding a mechanical twist, stretch, and hydrostatic pressure to the atomic structure. In particular, simple twisting and stacking of two layers of graphene can form a uniform and ordered Moiré superlattice, which can effectively modulate the electrons of graphene layers and lead to the discovery of unconventional superconductivity and strong correlations. However, the twist angle of twisted bilayer graphene (tBLG) is almost unchangeable once the interlayer stacking is determined, while applying mechanical elastic strain provides an alternative way to deeply regulate the electronic structure by controlling the lattice spacing and symmetry. In this review, diverse experimental advances are introduced in straining tBLG by in-plane and out-of-plane modes, followed by the characterizations and calculations toward quantitatively tuning the strain-engineered electronic structures. It is further discussed that the structural relaxation in strained Moiré superlattice and its influence on electronic structures. Finally, the conclusion entails prospects for opportunities of strained twisted 2D materials, discussions on existing challenges, and an outlook on the intriguing emerging field, namely "strain-twistronics".

Mass Inversion at the Lifshitz Transition in Monolayer Graphene by Diffusive, High-Density, On-Chip Doping

Experimental setups for charge transport measurements are typically not compatible with the ultrahigh vacuum conditions for chemical doping, limiting the charge carrier density that can be investigated by transport methods. Field-effect methods, including dielectric gating and ionic liquid gating, achieve too low a carrier density to induce electronic phase transitions. To bridge this gap, we developed an integrated flip-chip method to dope graphene by alkali vapor in the diffusive regime, suitable for charge transport measurements at ultrahigh charge carrier density. We introduce a cesium droplet into a sealed cavity filled with inert gas to dope a monolayer graphene sample by the process of cesium atom diffusion, adsorption, and ionization at the graphene surface, with doping beyond an electron density of 4.7 × 1014 cm-2 monitored by operando Hall measurement. The sealed assembly is stable against oxidation, enabling measurement of charge transport versus temperature and magnetic field. Cyclotron mass inversion is observed via the Hall effect, indicative of the change in Fermi surface geometry associated with the Liftshitz transition at the hyperbolic M point of monolayer graphene. The transparent quartz substrate also functions as an optical window, enabling nonresonant Raman scattering. Our findings show that chemical doping, hitherto restricted to ultrahigh vacuum, can be applied in a diffusive regime at ambient pressure in an inert gas environment and thus enable charge transport studies in standard cryogenic environments.

Enabling Stable and Low-Strain Lithium Plating/Stripping with 2D Layered Transition Metal Carbides by Forming Li-Zipped MXenes and a Li Halide-Rich Solid Electrolyte Interphase

Two-dimensional (2D) layered materials demonstrate prominent advantage in regulating lithium plating/stripping behavior by confining lithium diffusion/plating within interlayer gaps. However, achieving effective interlayer confined lithium diffusion/plating without compromising the stability of bulk-structural and the solid electrolyte interphase (SEI) remains a considerable challenge. This paper presents an electrochemical scissor and lithium zipper-driven protocol for realizing interlayer confined lithium plating with pretty-low strain and volume change. In this protocol, lithium serves as a "zipper" to reunite the adjacent MXene back to MAX-like phase to markedly enhance the structural stability, and a lithium halide-rich SEI is formed by electrochemically removing the terminals of halogenated MXenes to maintain the stability and rapid lithium ions diffusion of SEI. When the Ti3 C2 I2 serves as the host for lithium plating, the average coulomb efficiency exceeds 97.0 % after 320 lithium plating/stripping cycles in conventional ester electrolyte. Furthermore, a full cell comprising of LiNi0.8 Mn0.1 Co0.1 O2 and Ti3 C2 I2 @Li exhibits a capacity retention rate of 73.4 % after 200 cycles even under high cathode mass-loading (20 mg cm-2 ) and a low negative/positive capacity ratio of 1.4. Our findings advance the understanding of interlayer confined lithium plating in 2D layered materials and provide a new direction in regulating lithium and other metal plating/stripping behaviors.

Direct visualization of stacking-selective self-intercalation in epitaxial Nb1+xSe2 films

Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb1+xSe2. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.

Interfacial Assembly of 2D Graphene-Derived Ion Channels for Water-Based Green Energy Conversion

The utilization of sustained and green energy is believed to alleviate increasing menace of global environmental concerns and energy dilemma. Interfacial assembly of 2D graphene-derived ion channels (2D-GDICs) with tunable ion/fluid transport behavior enables efficient harvesting of renewable green energy from ubiquitous water, especially for osmotic energy harvesting. In this review, various interfacial assembly strategies for fabricating diverse 2D-GDICs are summarized and their ion transport properties are discussed. This review analyzes how particular structure and charge density/distribution of 2D-GDIC can be modulated to minimize internal resistance of ion/fluid transport and enhance energy conversion efficiency, and highlights stimuli-responsive functions and stability of 2D-GDIC and further examines the possibility of integrating 2D-GDIC with other energy conversion systems. Notably, the presented preparation and applications of 2D-GDIC also inspire and guide other 2D materials to fabricate sophisticated ion channels for targeted applications. Finally, potential challenges in this field is analyzed and a prospect to future developments toward high-performance or large-scale real-word applications is offered.

Replica exchange molecular dynamics for Li-intercalation in graphite: a new solution for an old problem

Li intercalation and graphite stacking have been extensively studied because of the importance of graphite in commercial Li-ion batteries. Despite this attention, there are still questions about the atomistic structures of the intermediate states that exist during lithiation, especially when phase dynamics cause a disordered Li distribution. The Li migration event (diffusion coefficient of 10-5 nm2 ns-1) makes it difficult to explore the various Li-intercalation configurations in conventional molecular dynamics (MD) simulations with an affordable simulation timescale. To overcome these limitations, we conducted a comprehensive study using replica-exchange molecular dynamics (REMD) in combination with the ReaxFF force field. This approach allowed us to study the behavior of Li-intercalated graphite from any starting arrangement of Li at any value of x in LixC6. Our focus was on analyzing the energetic favorability differences between the relaxed structures. We rationalized the trends in formation energy on the basis of observed structural features, identifying three main structural features that cooperatively control Li rearrangement in graphite: Li distribution, graphite stacking mode and gallery height (graphene layer spacing). We also observed a tendency for clustering of Li, which could lead to dynamic local structures that approximate the staging models used to explain intercalation into graphite.

Lithium intercalation in two-dimensional penta-NiN2: insights from NiN2/NiN2 homostructure and G/NiN2 heterostructure

High-energy-density lithium-ion batteries (LIBs) are essential to meet the requirements of emerging technologies for advanced power storage and enhanced device performance. The next generation of LIBs will require high-capacity anode materials that move beyond the lithium intercalation chemistry of conventional graphite electrodes. The use of two-dimensional (2D) bilayer structures offers immediate advantages in the development of LIBs. Herein, motivated by the recently synthesized 2D Cairo pentagon nickel diazenide (NiN2) material, we conduct a scrutiny of the intercalation process of lithium atoms in the interlayer gap of NiN2/NiN2 homostructure. Based on density functional theory (DFT), we demonstrate that the diffusion energy barrier of lithium move across the NiN2/NiN2 anode is relatively low, ranging from 0.058 to 0.52 eV, and the corresponding reversible capacity reaches a remarkable value of 499.0927 mA h g-1 per formula unit, surpassing that of graphite (372 mA h g-1). Furthermore, we investigate a 2D van der Waals (vdW) heterostructure composed of pre-strained structures of graphene and NiN2 for use as an anode material in LIBs. It is found that the introduction of graphene leads to improvements in both electrochemical activity and deformation characteristics. The presented results provide theoretical support for the potential of bilayer structures combining NiN2, suggesting them as promising candidates for the development of high-performance anode materials.

Alkali metal bilayer intercalation in graphene

Alkali metal (AM) intercalation between graphene layers holds promise for electronic manipulation and energy storage, yet the underlying mechanism remains challenging to fully comprehend despite extensive research. In this study, we employ low-voltage scanning transmission electron microscopy (LV-STEM) to visualize the atomic structure of intercalated AMs (potassium, rubidium, and cesium) in bilayer graphene (BLG). Our findings reveal that the intercalated AMs adopt bilayer structures with hcp stacking, and specifically a C6M2C6 composition. These structures closely resemble the bilayer form of fcc (111) structure observed in AMs under high-pressure conditions. A negative charge transferred from bilayer AMs to graphene layers of approximately 1~1.5×1014 e-/cm-2 was determined by electron energy loss spectroscopy (EELS), Raman, and electrical transport. The bilayer AM is stable in BLG and graphite superficial layers but absent in the graphite interior, primarily dominated by single-layer AM intercalation. This hints at enhancing AM intercalation capacity by thinning the graphite material.

Molybdenum Chloride Nanostructures with Giant Lattice Distortions Intercalated into Bilayer Graphene

The nanospace of the van der Waals (vdW) gap between structural units of two-dimensional (2D) materials serves as a platform for growing unusual 2D systems through intercalation and studying their properties. Various kinds of metal chlorides have previously been intercalated for tuning the properties of host layered materials, but the atomic structure of the intercalants remains still unidentified. In this study, we investigate the atomic structural transformation of molybdenum(V) chloride (MoCl5) after intercalation into bilayer graphene (BLG). Using scanning transmission electron microscopy, we found that the intercalated material represents MoCl3 networks, MoCl2 chains, and Mo5Cl10 rings. Giant lattice distortions and frequent structural transitions occur in the 2D MoClx that have never been observed in metal chloride systems. The trend of symmetric to nonsymmetric structural transformations can cause additional charge transfer from BLG to the intercalated MoClx, as suggested by our density functional theory calculations. Our study deepens the understanding of the behavior of matter in the confined space of the vdW gap in BLG and provides hints at a more efficient tuning of material properties by intercalation for potential applications, including transparent conductive films, optoelectronics, and energy storage.

Imaging Field-Driven Melting of a Molecular Solid at the Atomic Scale

Solid-liquid phase transitions are basic physical processes, but atomically resolved microscopy has yet to capture their full dynamics. A new technique is developed for controlling the melting and freezing of self-assembled molecular structures on a graphene field-effect transistor (FET) that allows phase-transition behavior to be imaged using atomically resolved scanning tunneling microscopy. This is achieved by applying electric fields to 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-decorated FETs to induce reversible transitions between molecular solid and liquid phases at the FET surface. Nonequilibrium melting dynamics are visualized by rapidly heating the graphene substrate with an electrical current and imaging the resulting evolution toward new 2D equilibrium states. An analytical model is developed that explains observed mixed-state phases based on spectroscopic measurement of solid and liquid molecular energy levels. The observed nonequilibrium melting dynamics are consistent with Monte Carlo simulations.

Superfast Mass Transport of Na/K Via Mesochannels for Dendrite-Free Metal Batteries

Fast ion diffusion in anode hosts enabling uniform distribution of Li/Na/K is essential for achieving dendrite-free alkali-metal batteries. Common strategies, e.g. expanding the interlayer spacing of anode materials, can enhance bulk diffusion of Li but are less efficient for Na and K due to their larger ionic radius. Herein, a universal strategy to drastically improve the mass-transport efficiency of Na/K by introducing open mesochannels in carbon hosts is proposed. Such pore engineering can increase the accessible surface area by one order of magnitude, thus remarkably accelerating surface diffusion, as visualized by in situ transmission electron microscopy. In particular, once the mesochannels are filled by the Na/K metals, they become the superfast channels for mass transport via the mechanism of interfacial diffusion. Thus-modified carbon hosts enable Na/K filling in their inner cavities and uniform deposition across the whole electrodes with fast kinetics. The resulting Na-metal anodes can exhibit stable dendrite-free cycling with outstanding rate performance at a high current density of up to 30 mA cm^-2 . This work presents an inspiring attempt to address the sluggish transport issue of Na/K, as well as valuable insights into the mass-transport mechanism in porous anodes for high-performance alkali-metal storage.

Electron-Beam- and Thermal-Annealing-Induced Structural Transformations in Few-Layer MnPS3

Quasi-two-dimensional (2D) manganese phosphorus trisulfide, MnPS₃, which exhibits antiferromagnetic ordering, is a particularly interesting material in the context of magnetism in a system with reduced dimensionality and its potential technological applications. Here, we present an experimental and theoretical study on modifying the properties of freestanding MnPS₃ by local structural transformations via electron irradiation in a transmission electron microscope and by thermal annealing under vacuum. In both cases we find that MnS_1-xP_x phases (0 ≤ x < 1) form in a crystal structure different from that of the host material, namely that of the α- or γ-MnS type. These phase transformations can both be locally controlled by the size of the electron beam as well as by the total applied electron dose and simultaneously imaged at the atomic scale. For the MnS structures generated in this process, our ab initio calculations indicate that their electronic and magnetic properties strongly depend on both in-plane crystallite orientation and thickness. Moreover, the electronic properties of the MnS phases can be further tuned by alloying with phosphorus. Therefore, our results show that electron beam irradiation and thermal annealing can be utilized to grow phases with distinct properties starting from freestanding quasi-2D MnPS₃.

Fabrication of liquid cell for in situ transmission electron microscopy of electrochemical processes

Fundamentally understanding the complex electrochemical reactions that are associated with energy devices (e.g., rechargeable batteries, fuel cells and electrolyzers) has attracted worldwide attention. In situ liquid cell transmission electron microscopy (TEM) offers opportunities to directly observe and analyze in-liquid specimens without the need for freezing or drying, which opens up a door for visualizing these complex electrochemical reactions at the nano scale in real time. The key to the success of this technique lies in the design and fabrication of electrochemical liquid cells with thin but strong imaging windows. This protocol describes the detailed procedures of our established technique for the fabrication of such electrochemical liquid cells (~110 h). In addition, the protocol for the in situ TEM observation of electrochemical reactions by using the nanofabricated electrochemical liquid cell is also presented (2 h). We also show and analyze experimental results relating to the electrochemical reactions captured. We believe that this protocol will shed light on strategies for fabricating high-quality TEM liquid cells for probing dynamic electrochemical reactions in high resolution, providing a powerful research tool. This protocol requires access to a clean room equipped with specialized nanofabrication setups as well as TEM characterization equipment.

Effect of 3d Transition Metal Atom Intercalation Concentration on the Electronic and Magnetic Properties of Graphene/MoS2 Heterostructure: A First-Principles Study

The electronic and magnetic properties of graphene/MoS2 heterostructures intercalated with 3d transition metal (TM) atoms at different concentrations have been systematically investigated by first principles calculations. The results showed that all the studied systems are thermodynamically stable with large binding energies of about 3.72 eV-6.86 eV. Interestingly, all the TM-intercalated graphene/MoS2 heterostructures are ferromagnetic and their total magnetic moments increase with TM concentration. Furthermore, TM concentration-dependent spin polarization is obtained for the graphene layer and MoS2 layer due to the charge transfer between TM atoms and the layers. A significant band gap is opened for graphene in these TM-intercalated graphene/MoS2 heterostructures (around 0.094 eV-0.37 eV). With the TM concentration increasing, the band gap of graphene is reduced due to the enhanced spin polarization of graphene. Our study suggests a research direction for the manipulation of the properties of 2D materials through control of the intercalation concentration of TM atoms.

Formation and impact of nanoscopic oriented phase domains in electrochemical crystalline electrodes

Electrochemical phase transformation in ion-insertion crystalline electrodes is accompanied by compositional and structural changes, including the microstructural development of oriented phase domains. Previous studies have identified prevailingly transformation heterogeneities associated with diffusion- or reaction-limited mechanisms. In comparison, transformation-induced domains and their microstructure resulting from the loss of symmetry elements remain unexplored, despite their general importance in alloys and ceramics. Here, we map the formation of oriented phase domains and the development of strain gradient quantitatively during the electrochemical ion-insertion process. A collocated four-dimensional scanning transmission electron microscopy and electron energy loss spectroscopy approach, coupled with data mining, enables the study. Results show that in our model system of cubic spinel MnO₂ nanoparticles their phase transformation upon Mg²⁺ insertion leads to the formation of domains of similar chemical identity but different orientations at nanometre length scale, following the nucleation, growth and coalescence process. Electrolytes have a substantial impact on the transformation microstructure ('island' versus 'archipelago'). Further, large strain gradients build up from the development of phase domains across their boundaries with high impact on the chemical diffusion coefficient by a factor of ten or more. Our findings thus provide critical insights into the microstructure formation mechanism and its impact on the ion-insertion process, suggesting new rules of transformation structure control for energy storage materials.

Deep-Learning Pipeline for Statistical Quantification of Amorphous Two-Dimensional Materials

Aberration-corrected transmission electron microscopy enables imaging of two-dimensional (2D) materials with atomic resolution. However, dissecting the short-range-ordered structures in radiation-sensitive and amorphous 2D materials remains a significant challenge due to low atomic contrast and laborious manual evaluation. Here, we imaged carbon-based 2D materials with strong contrast, which is enabled by chromatic and spherical aberration correction at a low acceleration voltage. By constructing a deep-learning pipeline, atomic registry in amorphous 2D materials can be precisely determined, providing access to a full spectrum of quantitative data sets, including bond length/angle distribution, pair distribution function, and real-space polygon mapping. Accurate segmentation of micropores and surface contamination, together with robustness against background inhomogeneity, guaranteed the quantification validity in complex experimental images. The automated image analysis provides quantitative metrics with high efficiency and throughput, which may shed light on the structural understanding of short-range-ordered structures. In addition, the convolutional neural network can be readily generalized to crystalline materials, allowing for automatic defect identification and strain mapping.

Dynamic Investigation of Battery Materials via Advanced Visualization: From Particle, Electrode to Cell Level

Li-ion batteries, the most-popular secondary battery, are typically electrochemical systems controlled by ion-insertion dynamics. The battery dynamics involve mass transport, charge transfer, ion-electron coupled reactions, electrolyte penetration, ion solvation, and interfacial evolution. However, it is difficult for the traditional electrochemical methods to capture the accurate and individual details of the dynamic processes in "black box" batteries; instead, only the net result of multi-factors on the whole scale. Recently, different advanced visualization techniques have been developed, which provide powerful tools to track and monitor the internal real-time dynamic processes, giving intuitive details and fine information at various scales from crystal lattice, single particle, electrode to cell level. Here, the recent progress on the investigation of electrochemical dynamics in battery materials are reviewed, via developed techniques across wide timescales and space-scales, including the dynamic process inside the active particle, kinetics issues at the electrode/electrolyte interface, dynamic inhomogeneity in the electrode, and dynamic transportation at the cell level. Finally, the fundamental principles to improve the battery dynamics are summarized and new technologies for future more stringent conditions are highlighted. In prospect, this review opens sight on the battery interior for a clearer, deeper, and more thorough understanding of the dynamics.

Homogeneous Lateral Lithium Intercalation into Transition Metal Dichalcogenides via Ion Backgating

Lithium intercalation has become a versatile tool for realizing emergent quantum phenomena in two-dimensional (2D) materials. However, the insertion of lithium ions may be accompanied by the creation of wrinkles and cracks, which prevents the material from manifesting its intrinsic properties under substantial charge injection. By using the recently developed ion backgating technique, we successfully realize lateral intercalation in 1T-TiSe₂ and 2H-NbSe₂, which shows substantially improved sample homogeneity. The homogeneity at high lithium doping is not only demonstrated via low-temperature transport measurements but also directly visualized by topographical imaging through in situ atomic force microscopy (AFM). The application of lateral intercalation to a broad spectrum of 2D materials can greatly facilitate the search for exotic quantum phenomena.

Twist Angle-Dependent Molecular Intercalation and Sheet Resistance in Bilayer Graphene

Bilayer graphene (BLG) has a two-dimensional (2D) interlayer nanospace that can be used to intercalate molecules and ions, resulting in a significant change of its electronic and magnetic properties. Intercalation of BLG with different materials, such as FeCl₃, MoCl₅, Li ions, and Ca ions, has been demonstrated. However, little is known about how the twist angle of the BLG host affects intercalation. Here, by using artificially stacked BLG with controlled twist angles, we systematically investigated the twist angle dependence of intercalation of metal chlorides. We discovered that BLG with high twist angles of >15° is more favorable for intercalation than BLG with low twist angles. Density functional theory calculations suggested that the weaker interlayer coupling in high twist angle BLG is the key for effective intercalation. Scanning transmission electron microscope observations revealed that co-intercalation of AlCl₃ and CuCl₂ molecules into BLG gives various 2D structures in the confined interlayer nanospace. Moreover, before intercalation we observed a significantly lower sheet resistance in BLG with high twist angles (281 ± 98 Ω/□) than that in AB stacked BLG (580 ± 124 Ω/□). Intercalation further decreased the sheet resistance, reaching values as low as 48 Ω/□, which is the lowest value reported so far for BLG. This work provides a twist angle-dependent phenomenon, in which enhanced intercalation and drastic changes of the electrical properties can be realized by controlling the stacking angle of adjacent graphene layers.

Graphene protection improves the stability of two-dimensional halide perovskites under the electron irradiation

The crystal structure of two-dimensional (2D) organic-inorganic halide perovskites undergoes fast structural collapse under the electron beam irradiation, hindering high-resolution transmission electron microscopy imaging. Graphene protection is an effective solution to mitigate the damage of electron-beam irradiation and has been applied in 2D materials such as MoS₂ . However, the effectivity of graphene protection has not been demonstrated in 2D halide perovskites yet, as traditional wet-transfer of graphene with aqueous solution would cause serious degradation for moisture-sensitive halide perovskites. Here, we verified that graphene protection plays a protection role and developed a method using nonpolar solvent to transfer the graphene layer atop the perovskite nanosheets. With this method, the perovskite nanosheets might be well protected by graphene encapsulation. HIGHLIGHTS: Transfer method of graphene on moisture-sensitive 2D halide perovskites using nonpolar solvents was developed. Graphene substrate is proven to be able to mitigate electron-beam damage to 2D halide perovskites. Encapsulation structure of graphene/halide perovskite/graphene was demonstrated.

Lithiophilic onion-like carbon spheres as lithium metal uniform deposition host

Lithium metal is considered as a promising anode material for next-generation secondary batteries, owing to its high theoretical specific capacity (3860 mA h g^-1). Nevertheless, the practical application of Li in lithium metal batteries (LMBs) is hampered by inhomogeneous Li deposition and irreversible "dead Li", which lead to low coulombic efficiency (CE) and safe hazards. Designing unique lithiophilic structure is an efficient strategy to control Li uniformly plating /stripping. Here, we report the silver (Ag) nanoparticles coated with nitrogen-doped onion-like carbon microspheres (Ag@NCS) as a host to reduce the nucleation overpotential of Li for dendrite-free LBMs. The Ag@NCS were prepared by a simple one-step injection pyrolysis. The lithiophilic Ag is demonstrated to be priority selective deposition of Li in the carbon cage. Meanwhile, the onion-like structure benefits to uniform lithium nucleation and dendrite-free lithium during cycling. Impressively, we successfully captured lithium metal on different hosts at atomic scale, further proving that Ag@NCS can effectively and uniformly deposit Li. Besides, Ag@NCS show a superiorly electrochemical performance with a low nucleation overpotential (∼1 mV), high CE and stable cycling performance (over 400 cycles at 0.5 mA cm^-2) compared to the Ag-free onion-like carbon in LMBs. Even under harsh conditions (1 mA cm^-2, 4 mA h cm^-2), Ag@NCS still present superior cycling stability for more than 150 cycles. Furthermore, a full cell composed of LiFePO₄ cathode exhibits significantly improved voltage hysteresis with low voltage polarization. This work provides a new choice and route for the design and preparation of lithiophilic host materials.

Microreactor-Based TG-TEM Synchronous Analysis

It is challenging to measure the heating-induced mass change of the material during its morphological/structural evolution process. Herein, a silicon microreactor is developed for thermogravimetric-transmission electron microscopy synchronous analysis (TG-TEM synchronous analysis). A self-heating resonant microcantilever for mass-change detection and a dummy microcantilever with electron-beam transparent SiN_x windows for in situ TEM imaging are integrated back-to-back inside the microreactor. The TEM resolution of the microreactor reaches the Ångström level in an air atmosphere of 100 mbar, and the TGA function is realized by the heatable resonant microcantilever. The TG-TEM synchronous analysis has been successfully used to characterize two Ni(OH)₂ samples. During the in situ TEM observing process, the desorption of the intercalated H₂O molecules and dehydration of the lattice OH groups in the amorphous Ni(OH)₂·xH₂O nanosheets can be clearly distinguished from the microcantilever-based TGA. For single-crystal Ni(OH)₂ nanosheets, the TG-TEM synchronous analysis can distinguish the desorption of physiosorbed water, the condensation of surface OH groups, and the dehydration of lattice OH groups. The amorphous Ni(OH)₂ nanosheets transformed to polycrystalline NiO completed at 290 °C, and the decomposition from the single-crystalline Ni(OH)₂ nanoplates to NiO at 315 °C is also clearly recognized. The proposed microreactor-based TG-TEM synchronous analysis provides an interrelated characterization platform to obtain more comprehensive information on materials.

Toward Exotic Layered Materials: 2D Cuprous Iodide

Heterostructures composed of 2D materials are already opening many new possibilities in such fields of technology as electronics and magnonics, but far more could be achieved if the number and diversity of 2D materials were increased. So far, only a few dozen 2D crystals have been extracted from materials that exhibit a layered phase in ambient conditions, omitting entirely the large number of layered materials that may exist at other temperatures and pressures. This work demonstrates how such structures can be stabilized in 2D van der Waals (vdw) stacks under room temperature via growing them directly in graphene encapsulation by using graphene oxide as the template material. Specifically, an ambient stable 2D structure of copper and iodine, a material that normally only occurs in layered form at elevated temperatures between 645 and 675 K, is produced. The results establish a simple route to the production of more exotic phases of materials that would otherwise be difficult or impossible to stabilize for experiments in ambient.

Site-Specific Reduction-Induced Hydrogenation of a Helical Bilayer Nanographene with K and Rb Metals: Electron Multiaddition and Selective Rb+ Complexation

The chemical reduction of π-conjugated bilayer nanographene 1 (C138 H120 ) with K and Rb in the presence of 18-crown-6 affords [K+ (18-crown-6)(THF)2 ][{K+ (18-crown-6)}2 (THF)0.5 ][C138 H122 3- ] (2) and [Rb+ (18-crown-6)2 ][{Rb+ (18-crown-6)}2 (C138 H122 3- )] (3). Whereas K+ cations are fully solvent-separated from the trianionic core thus affording a "naked" 1.3 - anion, Rb+ cations are coordinated to the negatively charged layers of 1.3 - . According to DFT calculations, the localization of the first two electrons in the helicene moiety leads to an unprecedented site-specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon reduction-induced site-specific hydrogenation provokes dramatic changes in the (electronic) structure of 1 as the helicene backbone becomes more compressed and twisted upon chemical reduction, which results in a clear slippage of the bilayers.

Microstructure of Lithium Dendrites Revealed by Room-Temperature Electron Microscopy

The uncontrolled deposition/dissolution process of lithium dendrites during electrochemical cycling in batteries limits the large-scale application of Li metal anodes. Investigating the microstructure of Li dendrites is a focal point. Currently, the only way to protect and observe sensitive Li dendrites is through low-temperature transmission electron microscopy (LT-TEM), whereas room-temperature characterization is still lacking. In this work, the room-temperature microstructure of Li dendrites was obtained by TEM using both vacuum- and inert-gas-transfer methods. Detailed comparison between LT- and room-temperature (RT-)TEM characterizations was provided to show the pros and cons of each method. Especially, RT-TEM shows the advantage of flexible incorporation with multifunctional characterizations, such as 3D tomography. By using RT-TEM, microstructural evolution of Li dendrites during the electrodeposition/dissolution process, including increase of the quantity of inorganic Li₂O compounds in the solid electrolyte interphase, lateral growth behavior, and two types of inactive Li, has been revealed, enriching the understanding of the structure-property relationship of Li dendrites.

Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals

Crystalline films offer various physical properties based on the modulation of their thicknesses and atomic structures. The layer-by-layer assembly of atomically thin crystals provides a powerful means to arbitrarily design films at the atomic level, which are unattainable with existing growth technologies. However, atomically clean assembly of the materials with high scalability and reproducibility remains challenging. We report programmed crystal assembly of graphene and monolayer hexagonal boron nitride, assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield. The atomic configurations of the films are tailored with layer-resolved compositions and in-plane crystalline orientations. We demonstrate batch-fabricated tunnel device arrays with modulation of the resistance over orders of magnitude by thickness control of the hexagonal boron nitride barrier with single-atom precision and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. Our results constitute an important development in the artificial design of large-scale films.

Strategies and challenges of carbon materials in the practical applications of lithium metal anode: a review

Lithium (Li) metal is strongly considered to be the ultimate anode for next-generation high-energy-density rechargeable batteries. Carbon materials and their composites with excellent structure tunability and properties have shown great potential applications in Li metal anodes.

Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation

The past decades of materials science discoveries are the basis of our present society - from the foundation of semiconductor devices to the recent development of internet of things (IoT) technologies. These materials science developments have depended mainly on control of rigid chemical bonds, such as covalent and ionic bonds, in organic molecules and polymers, inorganic crystals and thin films. The recent discovery of graphene and other two-dimensional (2D) materials offers a novel approach to synthesizing materials by controlling their weak out-of-plane van der Waals (vdW) interactions. Artificial stacks of different types of 2D materials are a novel concept in materials synthesis, with the stacks not limited by rigid chemical bonds nor by lattice constants. This offers plenty of opportunities to explore new physics, chemistry, and engineering. An often-overlooked characteristic of vdW stacks is the well-defined 2D nanospace between the layers, which provides unique physical phenomena and a rich field for synthesis of novel materials. Applying the science of intercalation compounds to 2D materials provides new insights and expectations about the use of the vdW nanospace. We call this nascent field of science '2.5 dimensional (2.5D) materials,' to acknowledge the important extra degree of freedom beyond 2D materials. 2.5D materials not only offer a new field of scientific research, but also contribute to the development of practical applications, and will lead to future social innovation. In this paper, we introduce the new scientific concept of this science of '2.5D materials' and review recent research developments based on this new scientific concept.

Recent Advances in Fluorinated Graphene from Synthesis to Applications: Critical Review on Functional Chemistry and Structure Engineering

Fluorinated graphene (FG), as an emerging member of the graphene derivatives family, has attracted wide attention on account of its excellent performances and underlying applications. The introduction of a fluorine atom, with the strongest electronegativity (3.98), greatly changes the electron distribution of graphene, resulting in a series of unique variations in optical, electronic, magnetic, interfacial properties and so on. Herein, recent advances in the study of FG from synthesis to applications are introduced, and the relationship between its structure and properties is summarized in detail. Especially, the functional chemistry of FG has been thoroughly analyzed in recent years, which has opened a universal route for the functionalization and even multifunctionalization of FG toward various graphene derivatives, which further broadens its applications. Moreover, from a particular angle, the structure engineering of FG such as the distribution pattern of fluorine atoms and the regulation of interlayer structure when advanced nanotechnology gets involved is summarized. Notably, the elaborated structure engineering of FG is the key factor to optimize the corresponding properties for potential applications, and is also an up-to-date research hotspot and future development direction. Finally, perspectives and prospects for the problems and challenges in the study of FG are put forward.

Mechanistic Probing of Encapsulation and Confined Growth of Lithium Crystals in Carbonaceous Nanotubes

Encapsulation of lithium in the confined spaces within individual nanocapsules is intriguing and highly desirable for developing high-performance Li metal anodes. This work aims for a mechanistic understanding of Li encapsulation and its confined growth kinetics inside 1D enclosed spaces. To achieve this, amorphous carbon nanotubes are employed as a model host using in situ transmission electron microscopy. The carbon shells have dual roles, providing geometric/mechanical constraints and electron/ion transport channels, which profoundly alter the Li growth patterns. Li growth/dissolution takes place via atom addition/removal at the free surfaces through Li⁺ diffusion along the shells in the electric field direction, resulting in the formation of unusual Li structures, such as poly-crystalline nanowires and free-standing 2D ultrathin (1-2 nm) Li membranes. Such confined front-growth processes are dominated by Li {110} or {200} growing faces, distinct from the root growth of single-crystal Li dendrites outside the nanotubes. Controlled experiments show that high lithiophilicity/permeability, enabled by sufficient nitrogen/oxygen doping or pre-lithiation, is critical for the stable encapsulation of lithium inside carbonaceous nanocapsules. First-principles-based calculations reveal that N/O doping can reduce the diffusion barrier for Li⁺ penetration, and facilitate Li filling driven by energy minimization associated with the formation of low-energy Li/C interfaces.

Recent Progress in the Transfer of Graphene Films and Nanostructures

The one-atom-thick graphene has excellent electronic, optical, thermal, and mechanical properties. Currently, chemical vapor deposition (CVD) graphene has received a great deal of attention because it provides access to large-area and uniform films with high-quality. This allows the fabrication of graphene based-electronics, sensors, photonics, and optoelectronics for practical applications. Zero bandgap, however, limits the application of a graphene film as electronic transistor. The most commonly used bottom-up approaches have achieved efficient tuning of the electronic bandgap by customizing well-defined graphene nanostructures. The postgrowth transfer of graphene films/nanostructures to a certain substrate is crucial in utilizing graphene in applicable devices. In this review, the basic growth mechanism of CVD graphene is first introduced. Then, recent advances in various transfer methods of as-grown graphene to target substrates are presented. The fabrication and transfer methods of graphene nanostructures are also provided, and then the transfer-related applications are summarized. At last, the challenging issues and the potential transfer-free approaches are discussed.