In situ microscopy techniques for characterizing the mechanical properties and deformation behavior of two-dimensional (2D) materials

Due to the atomic thickness and planar characteristics, two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are considered to be excellent electronic materials, which endow them with great potential for future device applications. The robust and reliable application of their functional devices requires an in-depth understanding of their mechanical properties and deformation behavior, which is also of fundamental importance in nanomechanics. Considering their exceedingly small sizes and thicknesses, this is a very challenge task. In situ microscopy techniques show great superiority in this respect. This review focuses on the progress in in situ microscopy techniques (including atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)) in characterizing the mechanical properties and deformation behavior of 2D materials. The technical characteristics, advantages, disadvantages, and main research fields of various in situ AFM, SEM, and TEM techniques are analyzed in detail, and the corresponding mechanical scenarios from point to plane are realized, including local indentation, planar stretching, friction sliding between atomic layers and atomic movement mechanisms. By virtue of their complementary advantages, in situ integrated microscopy techniques enable the simultaneous study of various mechanical properties, nanomechanical behavior, and inherent atomic mechanisms of 2D materials. Based on the present research, we look forward to further optimized in situ integrated microscopy techniques with high spatiotemporal atomic resolution that can reveal the dynamic structure-performance correlations and corresponding atomic mechanisms between the physical properties, such as mechanical, electrical, optical, thermal, and magnetic properties of 2D materials and their crystal structures, electronic structures, atomic layers, defect densities and other influencing factors under multifield coupling conditions. This will provide beneficial predictions and guidance for the design, construction and application of 2D material-based mechanoelectronic, piezoelectric, photoelectric, thermoelectric, etc. nanoelectronic devices.     

Preparation of H-bar cross-sectional specimen for in situ TEM straining experiments: A FIB-based method applied to a nitrided Ti-6Al-4V alloy

The in situ tensile straining of cross-sectional specimens inside a TEM is intrinsically very difficult to perform despite its obvious interest to study interfaces of surface treated materials. We have combined a FIB-based method to produce H-bar specimens of a nitrided Ti-6Al-4V alloy and in situ TEM straining stage, to successfully study the plastic deformation mechanisms that are activated close to the nitrided surface in the Ti-based alloy.     

Extending the scope of ‘in silico experiments’: Theoretical approaches for the investigation of reaction mechanisms, nucleation events and phase transitions

The investigation of the atomistic mechanisms of processes in complex systems constitutes a major challenge to both theory and experiment. While experimental studies offer a wide variety of insights at the macroscopic scale, the atomistic level of detail often remains elusive. On the other hand, molecular simulation approaches may easily achieve microscopic resolution and hence appear particularly suited for detailed mechanistic analyses. However, the computational effort is typically quite considerable and in many cases special simulation strategies are needed to make simulations possible. This review is dedicated to special approaches for tackling the time/length-scale problem inherent to molecular dynamics simulations. Employing these techniques opened a series of new perspectives. The latter are illustrated with the example of recent simulation studies of the atomistic mechanisms involved in complex processes like crystal nucleation, phase transitions and reactions in solution. Along this line, we discuss the reaction mechanisms for He insertion into C₆₀ fullerenes, nucleation events and domain morphogenesis in pressure-induced phase transitions in solids and ion aggregation from solution.     

Achieve atomic resolution in in situ S/TEM experiments to examine complex interface structures in nanomaterials

Characterization methods utilizing Scanning / Transmission Electron Microscopes have become routine techniques to investigate interface structures in nanomaterials. High resolution imaging methods reveals atomic structure; while spectroscopy gives additional access to elemental distribution and chemical bonding. Focus behind these developments is the research on nanomaterial-based technologies.Current trends in S/TEM research focus on extending atomic scale characterization capabilities from static to dynamic studies to understand in more detail the link between structure and its evolution vs. unique properties directly on its characteristic length scale.Progress in recent research is briefly reviewed to highlight the potential when using latest S/TEM methodology optimized for atomic scale investigations and how this can be extended to in situ studies of interfacial effects, followed by comments on how to achieve and maintain highest possible resolution & sensitivity when keeping the effect of electron beam under control during these atomic-scale in situ experiments.     

Insights on slip transmission at grain boundaries from atomistic simulations

To fully understand the plastic deformation of metallic polycrystalline materials, the physical mechanisms by which a dislocation interacts with a grain boundary must be identified. Recent atomistic simulations have focused on the discrete atomic scale motions that lead to either dislocation obstruction, dislocation absorption into the grain boundary with subsequent emission at a different site along the grain boundary, or direct dislocation transmission through the grain boundary into the opposing lattice. These atomistic simulations, coupled with foundational experiments performed to study dislocation pile-ups and slip transfer through a grain boundary, have facilitated the development and refinement of a set of criteria for predicting if dislocation transmission will occur and which slip systems will be activated in the adjacent grain by the stress concentration resulting from the dislocation pile-up. This article provides a concise review of both experimental and atomistic simulation efforts focused on the details of slip transmission at grain boundaries in metallic materials and provides a discussion of outstanding challenges for atomistic simulations to advance this field.     

Understanding all solid-state lithium batteries through in situ transmission electron microscopy

Owing to the use of solid electrolytes instead of flammable and potentially toxic organic liquid electrolytes, all solid-state lithium batteries (ASSLBs) are considered to have substantial advantages over conventional liquid electrolyte based lithium ion batteries(LIBs) in terms of safety, energy density, battery packaging, and operable temperature range. However, the electrochemistry and the operation mechanism of ASSLBs differ considerably from conventional LIBs. Consequently, the failure mechanisms of ASSLBs, which are not well understood, require particular attention. To improve the performance and realize practical applications of ASSLBs, it is crucial to unravel the dynamic evolution of electrodes, solid electrolytes, and their interfaces and interphases during cycling of ASSLBs. In situ transmission electron microscopy (TEM) provides a powerful approach for the fundamental investigation of structural and chemical changes during operation of ASSLBs with high spatio-temporal resolution. Herein, recent progress in in situ TEM studies of ASSLBs are reviewed with a specific focus on real-time observations of reaction and degradation occurring in electrodes, solid electrolytes, and their interfaces. Novel electro-chemo-mechanical coupling phenomena are revealed and mechanistic insights are highlighted. This review covers a broad range of electrode and electrolyte materials applied in ASSLBs, demonstrates the general applicability of in situ TEM for elucidating the fundamental mechanisms and providing the design guidance for the development of high-performance ASSLBs. Finally, challenges and opportunities for in situ TEM studies of ASSLBs are discussed.     

Revealing the atomistic deformation mechanisms of face-centered cubic nanocrystalline metals with atomic-scale mechanical microscopy: A review

Nanocrystalline metals have many functional and structural applications due to their excellent mechanical properties compared to their coarse-grained counterparts. The atomic-scale understanding of the deformation mechanisms of nanocrystalline metals is important for designing new materials, novel structures and applications. The review presents recent developments in the methods and techniques for in situ deformation mechanism investigations on face-centered-cubic nanocrystalline metals. In the first part, we will briefly introduce some important techniques that have been used for investigating the deformation behaviors of nanomaterials. Then, the size effects and the plasticity behaviors in nanocrystalline metals are discussed as a basis for comparison with the plasticity in bulk materials. In the last part, we show the atomic-scale and time-resolved dynamic deformation processes of nanocrystalline metals using our in-lab developed deformation device.     

Continuum metrics for deformation and microrotation from atomistic simulations: Application to grain boundaries

Motivated by a desire to incorporate micro- and nanoscale deformation mechanisms into continuum mechanical models of material behavior, we apply recently developed volume-averaged metrics to the results of atomistic simulations to investigate deformation and microrotation in the vicinity of grain boundaries. Three-dimensional bicrystalline structures are employed to study the inelastic deformation behavior under uniaxial tension and simple shear at a temperature of 10 K. Each bicrystal is constructed by molecular statics followed by thermal equilibration under NPT using an embedded atom method potential for copper. Strain is imposed in each simulation cell at a constant 10⁹ s⁻¹ strain rate applied perpendicular and parallel to the grain boundary plane for tension and shear, respectively. A variety of grain boundary deformation mechanisms arise and the resulting deformation and microrotation fields are examined. We also include an analysis showing how microrotation varies as a function of distance from the grain boundary with increasing strain for different grain boundary deformation mechanisms. This work demonstrates that critical interface behavior can be extracted from atomistic simulations using volume-averaged metrics, offering a potential avenue for translating fundamental information to continuum theories of grain boundary deformation in polycrystalline materials.     

Multiscale simulation of nanostructures based on spatial secant model: a discrete hyperelastic approach

The main objective of this paper is to present a coarse-grained material model for the simulation of three-dimensional nanostructures. The developed model is motivated by the recent progress in establishing continuum models for nanomaterials and nanostructures. As there are conceptual differences between the continuum field defined in the classical sense and the nanomaterials consisting of discrete, space-filling atoms, existing continuum measures cannot be directly applied for mapping the nanostructures due to the discreteness at small length scale. In view of the fundamental difficulties associated with the direct application of the continuum approach, we introduce a unique discrete deformation measure called spatial secant and have developed a new hyperelastic model based on this measure. We show that the spatial secant-based model is consistently linked to the underlying atomistic model and provides a geometric exact mapping in the discrete sense. In addition, we outline the corresponding computational framework using the finite element and/or meshfree method. The implementation is within the context of finite deformation. Finally we illustrate the application of the model in studying the mechanics of low-dimensional carbon nanostructures such as carbon nanotubes (CNT). By comparing with full-scale molecular mechanics simulations, we show that the proposed coarse-grained model is robust in that it accurately captures the non-linear mechanical responses of the CNT structures.     

Atomistic insights into Li-ion diffusion in amorphous silicon

Silicon has been critically examined for its potential use as an electrode material for Li-ion batteries. Diffusive transport of Li-ions in the crystalline silicon anode is one of the key mechanisms that controls the deformation during lithiation, the rate of the charge–discharge cycle, and eventual mechanical failure. The use of amorphous silicon, instead of its crystalline counterpart, is considered to offer several advantages. The atomistic mechanisms underpinning diffusive transport of Li-ions in amorphous silicon are, however, poorly understood. Conventional molecular dynamics, if used to obtain atomistic insights into the Li-ion transport mechanism, suffers from several disadvantages: the relaxation times of Li ion diffusion in many of the diffusion pathways in amorphous Si are well beyond the short time scales of conventional molecular dynamics. In this work we utilize a sequence of approaches that involve the employment of a novel and recently developed potential energy surface sampling method, kinetic Monte Carlo, and the transition state theory to obtain a realistic evaluation of Li-ion diffusion pathways in amorphous Si. Diffusive pathways are not a priori set but rather emerge naturally as part of our computation. We elucidate the comparative differences between Li-ion diffusion in amorphous and crystalline Si as well as compare our results with past studies based on other methods.     

Coordinated grain boundary deformation governed nanograin annihilation in shear cycling

Grain growth and shrinkage are essential to the thermal and mechanical stability of nanocrystalline metals,which are assumed to be governed by the coordinated deformation between neighboring grain boundaries(GBs)in the nanosized grains.However,the dynamics of such coordination has rarely been reported,especially in experiments.In this work,we systematically investigate the atomistic mechanism of coordinated GB deformation during grain shrinkage in an Au nanocrystal film through combined state-of-the-art in situ shear testing and atomistic simulations.We demonstrate that an embedded nanograin experiences shrinkage and eventually annihilation during a typical shear loading cycle.The continu-ous grain shrinkage is accommodated by the coordinated evolution of the surrounding GB network via dislocation-mediated migration,while the final grain annihilation proceeds through the sequen-tial dislocation-annihilation-induced grain rotation and merging of opposite GBs.Both experiments and simulations show that stress distribution and GB structure play important roles in the coordinated defor-mation of different GBs and control the grain shrinkage/annihilation under shear loading.Our findings establish a mechanistic relation between coordinated GB deformation and grain shrinkage,which reveals a general deformation phenomenon in nanocrystalline metals and enriches our understanding on the atomistic origin of structural stability in nanocrystalline metals under mechanical loading.     

Unveiling the structural origin to control resistance drift in phase-change memory materials

The global demand for data storage and processing is increasing exponentially. To deal with this challenge, massive efforts have been devoted to the development of advanced memory and computing technologies. Chalcogenide phase-change materials (PCMs) are currently at the forefront of this endeavor. In this Review, we focus on the mechanisms of the spontaneous structural relaxation – aging – of amorphous PCMs, which causes the well-known resistance drift issue that significantly reduces the device accuracy needed for phase-change memory and computing applications. We review the recent breakthroughs in uncovering the structural origin, achieved through state-of-the-art experiments and ab initio atomistic simulations. Emphasis will be placed on the evolving atomic-level details during the relaxation of the complex amorphous structure. We also highlight emerging strategies to control aging, inspired by the in-depth structural understanding, from both materials science and device engineering standpoints, that offer effective solutions to reduce the resistance drift. In addition, we discuss an important new paradigm – machine learning – and the potential power it brings in interrogating amorphous PCMs as well as other disordered alloy systems. Finally, we present an outlook to comment on future research opportunities in amorphous PCMs, as well as on their reduced aging tendency in other advanced applications such as non-volatile photonics.     

Dislocation modelling at atomistic and mesoscopic scales

Several basic dislocation mechanisms have recently been examined in detail by atomistic simulations. In parallel, mesoscale approaches of dislocation behaviour are being developed for the study of single crystal plasticity. Linking atomistic and mesoscopic simulations is emerging as a new challenge for materials modelling.     

Order-disorder transformation in Fe-Pd alloy nanoparticles studied by in situ transmission electron microscopy

We have studied order-disorder transformation in Fe-Pd alloy nanoparticles by in situ transmission electron microscopy (TEM) and electron diffraction. The transformation is size-dependent, and the transformation temperatures are lower than those of the bulk alloys. The transformation proceeds continuously but rather steeply as the temperature increases, which differs from the first-order transformation observed in a bulk alloy or gradual transformation predicted by simulations for nanoparticles. Experimental results indicated that the continuous nature can be attributed to the distribution of the transformation temperature due to the distributions of both particle size and alloy composition. Quantitative intensity analyses of nanobeam electron diffraction (NBD) patterns indicated the existence of short-range order (SRO) inside disordered nanoparticles. The SRO as well as particle size distribution are responsible for the remaining weak superlattice reflections above the transformation temperature. In situ high-resolution TEM (HRTEM) observation revealed the existence of the SRO, which was consistent with the results obtained by NBD. We show that the disorder may not necessarily proceed continuously from the surface toward the center of the nanoparticle. Ordering from the disordered phase upon cooling was also observed by in situ HRTEM, which can be attributed to growth of the SRO.     

In situ and operando characterisation of Li metal – Solid electrolyte interfaces

The use of lithium metal as the negative electrode holds great promise for high energy density solid-state batteries (SSBs) of the future, but at the same time presents major technical challenges in their development. Li metal, with its high reactivity, soft and ductile nature, and propensity towards mechanical deformation during electrochemical cycling, is susceptible to the formation of various defects such as voids, cracks and filamentary deposits at the Li metal - solid electrolyte interface, that eventually cause rapid degradation of electrochemical cell performance. In order to gain insights into these interfacial processes and identify mechanisms for failure, in situ and operando characterisation approaches are essential. In this perspective, we present our opinions on the current state of such techniques, while highlighting the existing limitations and scope of these methods. We also endeavour to present opportunities for future research into developing and building on existing approaches to better evaluate the Li metal-solid electrolyte interface so as to guide the appropriate choice of materials to further enable efficient SSB architectures.     

Penetration scaling in atomistic simulations of hypervelocity impact

We present atomistic molecular dynamics simulations of the impact of copper nano particles at 5 km s⁻¹ on copper films ranging in thickness from from 0.5 to 4 times the projectile diameter. We access both penetration and cratering regimes with final cratering morphologies showing considerable similarity to experimental impacts on both micron and millimetre scales. Both craters and holes are formed from a molten region, with relatively low defect densities remaining after cooling and recrystallisation. Crater diameter and penetration limits are compared to analytical scaling models: in agreement with some models we find the onset of penetration occurs for 1.0 < f/d_p < 1.5, where f is the film thickness and d_p is the projectile diameter. However, our results for the hole size agree well with scaling laws based on macroscopic experiments providing enhanced strength of a nano-film that melts completely at the impact region is taken into account.     

Design and analysis of sandwiched fullerene-graphene composites using molecular dynamics simulations

Computational design of a novel carbon based hybrid material that is composed of fullerene units covalently sandwiched between parallel graphene sheets is presented. In this regard, atomistic models for the proposed novel material structure are generated via a systematic approach by employing different fullerene types (i.e. C₁₈₀, C₃₂₀, C₅₄₀ and C₇₂₀) as sandwich cores. Then, thermodynamic stability of the atomistic structures is checked by monitoring free energy profiles and junctional bond configurations which are obtained through classical molecular dynamics (MD) simulations. Thermodynamic feasibility of all atomistic specimens with different fullerene types is suggested by the energy profiles, because total configuration energies for all systems are minimized and remained stable over a long period of time. Furthermore, mechanical behavior of the nano-sandwiched material system is investigated by performing compression tests via MD simulations and basic deformation mechanisms underlying the compressive response are determined. By detailed examination, it is shown that proposed nano-sandwiched material can be identified as quasi-foam material due to comparable energy absorbing characteristics. Furthermore, regarding the effect of fullerene size on the compressive response, it is found that for a given stress level, specimens with larger fullerenes exhibit higher energy absorbing capacity.     

The Use of Data Mining Techniques in Rockburst Risk Assessment

Rockburst is an important phenomenon that has affected many deep underground mines around the world. An understanding of this phenomenon is relevant to the management of such events, which can lead to saving both costs and lives. Laboratory experiments are one way to obtain a deeper and better understanding of the mechanisms of rockburst. In a previous study by these authors, a database of rockburst laboratory tests was created; in addition, with the use of data mining (DM) techniques, models to predict rockburst maximum stress and rockburst risk indexes were developed. In this paper, we focus on the analysis of a database of in situ cases of rockburst in order to build influence diagrams, list the factors that interact in the occurrence of rockburst, and understand the relationships between these factors. The in situ rockburst database was further analyzed using different DM techniques ranging from artificial neural networks (ANNs) to naive Bayesian classifiers. The aim was to predict the type of rockburst—that is, the rockburst level—based on geologic and construction characteristics of the mine or tunnel. Conclusions are drawn at the end of the paper.     

New opportunities for quantitative tracking of polycrystal responses in three dimensions

An important advance in understanding the mechanics of solids over the last 50 years has been development of a suite of models that describe the performance of engineering materials while accounting for internal fluctuations and anisotropies (ex., anisotropic response of grains) over a hierarchy of length scales. Only limited engineering adoption of these tools has occurred, however, because of the lack of measured material responses at the length scales where the models are cast. Here, we demonstrate an integrated experimental capability utilizing high energy X-rays that provides an in situ, micrometer-scale probe for tracking evolving microstructure and intergranular stresses during quasi-static mechanical testing. We present first-of-a-kind results that show an unexpected evolution of the intergranular stresses in a titanium alloy undergoing creep deformation. We also discuss the expectation of new discoveries regarding the underlying mechanisms of strength and damage resistance afforded by this rapidly developing X-ray microscopy technique.