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.     

Surface dislocation nucleation mediated deformation and ultrahigh strength in sub-10-nm gold nanowires

The plastic deformation and the ultrahigh strength of metals at the nanoscale have been predicted to be controlled by surface dislocation nucleation. In situ quantitative tensile tests on individual 〈111〉 single crystalline ultrathin gold nanowires have been performed and significant load drops observed in stress-strain curves suggest the occurrence of such dislocation nucleation. High-resolution transmission electron microscopy (HRTEM) imaging and molecular dynamics simulations demonstrated that plastic deformation was indeed initiated and dominated by surface dislocation nucleation, mediating ultrahigh yield and fracture strength in sub-10-nm gold nanowires.      

Transmission electron microscopy characterization of TiN/SiNx multilayered coatings plastically deformed by nanoindentation

Plastic deformation of TiN_5 nm/SiN_0.5 nm multilayers by nanoindentation was investigated by transmission electron microscopy in order to identify deformation mechanisms involved in film failure resulting from severe plastic deformation. The TiN layers exhibited a crystalline fcc structure with a [002] preferential orientation; further crystal growth was interrupted by the amorphous SiN_x layers. After severe plastic deformation collective vertical displacement of slabs of several TiN/SiN_x-bilayers, which resulted from shear sliding at TiN/TiN grain boundaries, was observed. They are, together with horizontal fractures along the SiN_x layers, vertical cracks under the indenter tip following the TiN grain boundaries and delamination from the substrate, the predominant failure mechanisms of these coatings. The deformation behaviour of these films provides an experimental support for the absence of dislocation activity in grains of 5 nm size.     

Influence of surface oxidation on transmission electron microscopy characterization of Fe–Ga alloys

Fe–Ga alloys are rapidly oxidized when exposed in air, forming both amorphous and crystalline surface oxides. These oxides hinder the observation of the ordered phases of B2 and D0₃ in Fe–Ga alloys by dark-field imaging and high-resolution imaging of transmission electron microscopy (TEM) techniques. Proper imaging techniques and reduction of surface oxides are necessary to obtain representative microstructural features to the bulk alloys by TEM.     

Investigation of the Correlation between Texture and Microstrucure on a Submicrometer Scale in the TEM

In recent years the investigation of local texture and microstructure by analysis of electron backscatter diffraction patterns (EBSP) in the SEM has become a very powerful and popular method. With the introduction of SEM with field emission guns (FEG) the spatial resolution of EBSP measurements could be enhanced from 500 nm with a tungsten emitter to better than 50 nm. This evolution of SEM techniques raises the question whether transmission electron microscopy (TEM) still has fields of application in texture research. The present article answers this question with a clear “yes” and presents three examples of investigations where TEM is indispensable. The three examples comprise the investigation of the correlation between dislocation structure and deformation texture, a study on nucleation mechanisms of recrystallization in highly deformed metals and the investigation of microtexture and microstructure in nanocrystalline materials. Together with the presentation of these cases some of the necessary measurement techniques are described briefly. It is shown that TEM has to be applied when highest spatial resolution of orientation determination and imaging and high accuracy of orientation determination are to be reached, when the three‐dimensional and quantitative characterization of lattice defects is required or when materials with a high density of lattice defects are to be investigated.     

Investigation of the Correlation between Texture and Microstructure on a Submicrometer Scale in the TEM

In recent years the investigation of local texture and microstructure by analysis of electron backscatter diffraction patterns (EBSP) in the SEM has become a very powerful and popular method. With the introduction of SEM with field emission guns (FEG) the spatial resolution of EBSP measurements could be enhanced from 500 nm with a tungsten emitter to better than 50 nm. This evolution of SEM techniques raises the question whether transmission electron microscopy (TEM) still has fields of application in texture research. The present article answers this question with a clear “yes” and presents three examples of investigations where TEM is indispensable. The three examples comprise the investigation of the correlation between dislocation structure and deformation texture, a study on nucleation mechanisms of recrystallization in highly deformed metals and the investigation of microtexture and microstructure in nanocrystalline materials. Together with the presentation of these cases some of the necessary measurement techniques are described briefly. It is shown that TEM has to be applied when highest spatial resolution of orientation determination and imaging and high accuracy of orientation determination are to be reached, when the three‐dimensional and quantitative characterization of lattice defects is required or when materials with a high density of lattice defects are to be investigated.     

Electron tomography in soft materials

This review summarizes the recent advances in electron tomography (ET) and its application to polymer nanostructures. Truly quantitative three-dimensional (3D) images of polymer nanostructures can now be obtained by reducing or eliminating the missing tilt range in ET experiments. The reduction of the resulting missing wedge provides sub-nanometer resolution, which is sufficiently small for soft materials. Because soft materials often exhibit hierarchical structures, observation of a large volume with edges several micrometers in length is important to capture the structural elements on a scale larger than tens of nanometers. The introduction of scanning optics to ET has made it possible to obtain 3D data from micrometer-thick polymer specimens by using conventional electron microscopes at a relatively low accelerating voltage of 200 kV. We present some examples of the structural analysis of soft materials, such as nanostructures of self-assembled block copolymers and fuel cell electrodes.     

Emerging X-ray imaging technologies for energy materials

With the growing need for sustainable energy technologies, advanced characterization methods become more and more critical for optimizing energy materials and understanding their operation mechanisms. In this review, we focus on the synchrotron-based X-ray imaging technologies and the associated applications in gaining fundamental insights into the physical/chemical properties and reaction mechanisms of energy materials. We will discuss a few major X-ray imaging technologies, including X-ray projection imaging, transmission X-ray microscopy, scanning transmission X-ray microscopy, tender and soft X-ray imaging, and coherent diffraction imaging. Researchers can choose from various X-ray imaging techniques with different working principles based on research goals and sample specifications. With the X-ray imaging techniques, we can obtain the morphology, phase, lattice and strain information of energy materials in both 2D and 3D in an intuitive way. In addition, with the high-penetration X-rays and the high-brilliance synchrotron sources, operando/in-situ experiments can be designed to track the qualitative and quantitative changes of the samples during operation. We expect this review can broaden readers’ view on X-ray imaging techniques and inspire new ideas and possibilities in energy materials research.     

Integrating in situ TEM experiments and atomistic simulations for defect mechanics

With recent advances in computational modeling and in situ transmission electron microscopy (TEM) technologies, there have been increased efforts to apply these approaches to understand defect-based mechanisms dictating deformation mechanics. In situ TEM experiments and atomistic simulations each have their own unique limitations, including observable length and time scales and accessibility of information, motivating approaches that combine the two approaches. In this paper, we review recent studies that combine atomistic simulations and in situ TEM experiments to understand defect mechanisms associated with deformation of metals and alloys. In addition, we discuss ongoing developments in characterization and simulation capabilities that are expected to significantly advance the field of defect mechanics and allow greater integration between atomistic simulations and in situ TEM experiments.     

Synthesis and characterization of poly (o-phenylenediamine) hollow multi-angular microrods by interfacial method

Poly (o-phenylenediamine) (PoPD) hollow multi-angular microrods have been synthesized at 10 °C via an interfacial process using ferric chloride and o-phenylenediamine (oPD) as starting materials. The chemical structure of the one dimensional (1D) superstructure is proved to be phenazine-like, and contains the benzenoid and quinoid imine units doped partly with Cl. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations indicate that the resulting superstructure of PoPD is built from hollow nanofibers aligned parallel to the fiber axis by van der Waals' force. The possible formation mechanism of the structure has been proposed.     

Microstructure and fatigue strength of high-strength Cu–Fe and Cu–V in-situ nanocomposite wires

The results of the quantitative analysis of the microstructure of the Cu–Fe and Cu–V in-situ nanocomposite wires with diameter of 0.44–0.80 mm by transmission electron microscopy are presented. Comparative fatigue tests of Cu–Fe and Cu–V in-situ nanocomposite wires and pure copper samples have been carried out using a dynamic mechanical analyzer (DMA). The in-situ nanocomposites have significantly higher characteristics of low-cycle fatigue failure resistance as compared to that of pure copper. The fatigue crack propagation areas for the nanocomposite conductors and pure copper are characterized by fatigue striations and secondary cracking.     

Preparation of sheet like polycrystalline NiFe2O4 nanostructure with PVA matrices and their properties

Sheet-like nickel ferrite (NiFe₂O₄) has been synthesised with PVA matrices using co-precipitation technique. The sheet is formed from the oriented aggregation of single crystalline NiFe₂O₄ nanoparticles with PVA as the structure directing template. The synthesised materials are characterised based on X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), high-resolution scanning electron microscopy (HRSEM), Fourier transform infrared spectroscopy (FTIR) and vibrating sample magnetometry (VSM). The XRD results show that the nanocrystal contains single phase spinel structure of Fd3m space group. The existence of PVA with nanoparticles has been confirmed by FTIR spectra. The room temperature ferromagnetic property is exhibited by the as synthesised sample with high saturation magnetisation.     

Effect of microstructure on non-linear behavior of ultrasound during low cycle fatigue of pearlitic steels

Influence of microstructural changes on the second harmonics of sinusoidal ultrasonic wave during low cycle fatigue (LCF) deformation in pearlitic steel was studied. Fatigue tests were interrupted and at every interruption, non-linear ultrasonic (NLU) parameter (β) was determined. Microstructures of cyclically deformed specimens at various cycles were examined by transmission electron microscopy (TEM). The variation of β with fatigue cycles was correlated with the microstructural changes and the results were explained through the variation in dislocation morphology and carbon content of the steel.     

Untersuchungen zur Wechselwirkung von Versetzungen und Ausscheidungen in einer Nickelbasis‐Superlegierung im VHCF‐Bereich

Over the past years the fatigue behaviour of structural materials at very high number of cycles (VHCF) has become a widely acclaimed topic of scientific and technical interest. One of the most important application fields for nickel‐base superalloys are turbines, which demand fatigue lives exceeding 10⁷ cycles. Hence, in this study the interrelationship between dislocation slip behaviour, precipitation condition (peak‐aged and overaged) and fatigue behaviour of the nickel‐base superalloy Nimonic 80A was investigated in the VHCF range focusing on the influence of the predominant microstructural feature. Surprisingly, the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime as compared to the peak‐aged condition. Accordingly, the VHCF‐behaviour does neither correspond to the static strength nor to the fatigue behaviour in the conventional range. Microstructural features evoked by cyclic deformation (e. g. development of slip markings at surface grains and dislocation/precipitate interaction) were characterized by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) providing preliminary explanations for the unexpected fatigue behaviour. A comparison of cyclic deformation curves both for the conventional and the VHCF range illustrates the pronounced difference in global and local strain range and its influence on the damage evolution during fatigue testing.     

D-penicillamine assisted hydrothermal synthesis of Bi2S3 nanoflowers and their electrochemical application

Single crystalline flower-like Bi₂S₃ nanostructures were successfully synthesized via a simple, facile and green hydrothermal method, with the assistance of D-penicillamine. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and found their morphologies mainly depend on the ratios of Bi^3 + to D-penicillamine, as well as the reaction temperature and time. And the possible growth mechanism has been discussed in some detail. In addition, the as-prepared Bi₂S₃ nanoflowers show good hydrogen storage ability. This strategy can be potentially expanded to prepare other metal chalcogenides materials.     

High-resolution and high-voltage electron microscopy at the University of California,Berkeley

After 20 years of exciting growth in our theoretical and experimental expertise in transmission electron microscopy, the development of new generations of instruments over the past few years has opened up new opportunities for sophisticated studies of microstructure under well-controlled conditions and at near-atomic levels. High-voltage electron microscopy now provides unique capabilities for dynamic studies of bulk specific behaviour during exposure to ‘ real world ’ conditions, e.g., varying environments, deformation, irradiation, or combinations of these, while developments in lattice-imaging techniques afford new insights into structures and mechanisms operating at a near-atomic level. Both these approaches are being explored and exploited at Berkeley in a variety of applications to practical problems in materials science. Several examples are presented in this paper which illustrate the power of high resolution lattice-imaging in studies of interfacial structures, and in its combination with laser optical microdiffraction for studying lattice parameters and compositional changes over small (10 Å) regions. Other examples will underline the importance of imaging techniques and environmental control duringin-situ studies of ceramic materials in the high-voltage electron microscope. Finally, future directions and anticipated developments are briefly discussed.     

Metal oxide nano-particles for improved electrochromic and lithium-ion battery technologies

Hot-wire chemical vapor deposition (HWCVD) has been employed as an economically scalable method for the deposition of crystalline tungsten oxide nano-rods and nano-particles. Under optimal synthesis conditions, only crystalline WO₃ nano-structures with a smallest dimension of ∼10-50 nm are observed with extensive transmission electron microscopy (TEM) analyses. The incorporation of these particles into porous films led to profound advancement in state-of-the-art electrochromic (EC) technologies. HWCVD has also been employed to produce crystalline molybdenum oxide nano-rods, particles and tubes at high density. TEM analyses show that the smallest dimension of these nano-structures is ∼5-30 nm. XRD and Raman analyses reveal that the materials are highly crystalline and consist of Mo, MoO₂ and MoO₃ phases. It is also possible to fabricate large-area porous films containing these MoO_x nano-structures. Furthermore, these films have been tested as the negative electrode in lithium-ion batteries, and a surprisingly high, reversible capacity has been observed.     

An analysis of evolution of grain size-lattice parameters dependence in nanocrystalline TiO2 anatase

Due to the specific properties of nanocrystalline materials in comparison with crystalline materials, it is essential to investigate their structural parameters like lattice parameters and grain sizes. We used the Rietveld method and refined electron powder data (recorded with selected area electron diffraction—SAED) on nanocrystalline (nc) TiO₂-anatase prepared by sol–gel route. We correlated refined lattice parameters with average grain size obtained from transmission electron microscopy (TEM) images. We give preliminary results on relative changes of lattice parameters a and c vs. the mean grain size in nc TiO₂-anatase.     

Microstructural and electrochemical properties of vertically aligned few layered graphene (FLG) nanoflakes and their application in methanol oxidation

Vertically aligned few layered graphene (FLG) nanoflakes were synthesised on silicon substrates by microwave plasma enhanced chemical vapour deposition (MPECVD) method. Transmission electron microscopy (TEM) shows that the structures have highly graphitized terminal planes of 1–3 layers of graphene. Raman spectroscopy revealed a narrow G band with a FWHM of ∼23 cm⁻¹ accompanied by a strong G′ (2D) band, with a FWHM of ∼43 cm⁻¹ and an I_G/I_G′ ratio of 1, which are all the characteristics of highly crystallized few layered graphene. The FLG electrodes demonstrate fast electron transfer (ET) kinetics for Fe(CN)₆^3−/4− redox system with an electron transfer rate, ΔE_p, of 60 mV. Platinum (Pt) nanoparticles of ∼6 nm diameter were deposited on as grown FLGs using magnetron DC sputtering for methanol oxidation studies. When used as electrodes for methanol oxidation, a mass specific peak current density of ∼62 mA mg⁻¹ cm⁻² of Pt is obtained with a high resistance to carbon monoxide (CO) poisoning as evident by a high value of 2.2 for the ratio of forward to backward anodic peak currents (I_f/I_b).     

Multi scale hard x-ray microscopy

The structure of crystalline materials is typically organised hierarchically on several length scales. Hard x-ray microscopy is presented as a collection of modalities that allows to zoom into a mm-sized sample to acquire 3D maps of any embedded region and at essentially all relevant length scales. For coarse mapping of grains, their orientations and average stress state diffraction based tomography methods can sample thousands of grains with a resolution of 2 µm. At the 100 nm scale, domains and dislocations and their associated strain fields can be visualised by diffraction microscopy. Similar to dark field electron microscopy, diffraction and imaging can be combined in several ways. For the ultimate resolution, a bulk version of coherent diffraction imaging is introduced. Hard x-ray microscopy is optimised for acquisition of 3D movies: directly visualising the structural changes during nucleation and growth, deformation or damage. The state of art is provided along with examples of use. I discuss how hard x-ray microscopy studies can enable the formulation and validation of improved multiscale models that account for the entire heterogeneity of materials.