Situ Scanning Electron Microscopy Research Articles

Enhanced adhesion of ZnO nanowires during in situ scanning electron microscope peeling.

The interfacial adhesion behaviour of a ZnO nanowire-Si substrate system is investigated using an in situ scanning electron microscope (SEM) mechanical peeling technique. The peel front of a nanowire advances via stick-slip events, and an equilibrium between the driving and resistant force to separation occurs immediately prior to a slip event. The interfacial adhesion energy is one order higher than that predicted theoretically by van der Waals interactions. The enhanced adhesion is primarily attributed to chemical and electrostatic interfacial interactions induced by electron irradiation. This work demonstrates that the operating environment of a nanoscale system could dramatically influence its adhesion behaviour. These findings are expected to have significant implications for interpreting the adhesion behaviour exhibited by a 1D nanostructure-substrate system when applying different testing methodologies, and for the fabrication of future NEMS devices.

Evolution of Inclusions During the 1473 K (1200 °C) Heating Process of EH36 Shipbuilding Steel

Evolution behaviors of inclusions of EH36 shipbuilding steel during 1473 K (1200 °C) heating have been studied in conjunction with ex situ scanning electron microscope (SEM) examination and in situ confocal scanning laser microscopy (CSLM) observations. It has been found that Al-Ca-O-S complex inclusions dominate the particles in the cast billet. However, TiN inclusions are profusely populated after heating. Moreover, possible strategies governing austenite growth are offered here.

High frequency in situ fatigue response of Ni-base superalloy René-N5 microcrystals

A novel in situ scanning electron microscope (SEM), high frequency fatigue testing methodology is developed using a combination of laser milling, focused ion beam fabrication and nanoindentation. This methodology is used to investigate crack initiation, propagation, fracture, fatigue life, and the mechanical response of microcantilever samples of a Ni-based superalloy (René-N5) under different cyclic strain amplitudes. The crack initiation and propagation in the microcantilever is monitored by observing changes in the beam's dynamic stiffness and continuous SEM imaging. The dynamic stiffness response of the micro-beams exhibits a transition from softening to hardening at a critical strain amplitude of 7×10−3. Theoretical analysis indicates that this transition corresponds to the stress required to shear γ′ precipitates. SEM imaging reveals the evolution of significant extrusions, intrusions, and slip traces during cyclic loading above this critical strain amplitude. Below this strain amplitude, very little surface roughening is observed. In addition, the measured dynamic stiffness is observed to exhibit two regimes of decrease after crack initiation. These two regimes correspond to short and large crack propagation. Finally, an overall increase in fatigue life is observed when comparing to bulk scale experiments on nickel-base superalloys. It is proposed that this is an inherent size effect in the small-volume, single crystal specimens tested.

In situ SEM study of tensile deformation of a near-β titanium alloy

In this study, the deformation behavior of a near-β Ti-17 titanium alloy under uniaxial tension was investigated using electron backscatter diffraction (EBSD) and tensile tests with in situ scanning electron microscope (SEM) observation. It is found slip mode is the main deformation mechanism in primary α grains of Ti-17 during the tensile test. Slip systems activated were identified by performing calculations on EBSD orientation data. The results show that all the three slip systems with a-type Burgers vector, i.e. basal, prism and 1st-order pyramidal slip, could be activated in the primary α grains of Ti-17, but no a+c-type slip (2nd-order pyramidal slip) activated is observed. Analysis reveals that basal and prism slips are the dominating slip mode, while 1st-order pyramidal slip acts as a subsidiary or deviated slipping mode. For the equivalent slip systems, Schmid factor dominates the slip behavior, while for the non-equivalent slip systems, critical resolved shear stress (CRSS) must be taken into account. It is proved that CRSS for a+c-type slip is much larger than that for a-type slips (basal and prism slips) in Ti-17 alloy (at least 2.5 times that for basal slip). a-type slip remains the easiest slip to be activated even in the condition favoring a+c-type slip and suppressing a-type slip (an angle of ~10° between the tensile axis and c-axis).

High‐Entropy Alloy (HEA)‐Coated Nanolattice Structures and Their Mechanical Properties

Nanolattice structure fabricated by two‐photon lithography (TPL) is a coupling of size‐dependent mechanical properties at micro/nano‐scale with structural geometry responses in wide applications of scalable micro/nano‐manufacturing. In this work, three‐dimensional (3D) polymeric nanolattices are initially fabricated using TPL, then conformably coated with an 80 nm thick high‐entropy alloy (HEA) thin film (CoCrFeNiAl0.3) via physical vapor deposition (PVD). 3D atomic‐probe tomography (APT) reveals the homogeneous element distribution in the synthesized HEA film deposited on the substrate. Mechanical properties of the obtained composite architectures are investigated via in situ scanning electron microscope (SEM) compression test, as well as finite element method (FEM) at the relevant length scales. The presented HEA‐coated nanolattice encouragingly not only exhibits superior compressive specific strength of ≈0.032 MPa kg−1 m3 with density well below 1000 kg m−3, but also shows good compression ductility due to its composite nature. This concept of combining HEA with polymer lattice structures demonstrates the potential of fabricating novel architected metamaterials with tunable mechanical properties.

Surface sensitivity of four-probe STM resistivity measurements of bulk ZnO correlated to XPS

Multi-probe instruments based on scanning tunnelling microscopy (STM) are becoming increasingly common for their ability to perform nano- to atomic-scale investigations of nanostructures, surfaces and in situ reactions. A common configuration is the four-probe STM often coupled with in situ scanning electron microscopy (SEM) that allows precise positioning of the probes onto surfaces and nanostructures enabling electrical and scanning experiments to be performed on highly localised regions of the sample. In this paper, we assess the sensitivity of four-probe STM for in-line resistivity measurements of the bulk ZnO surface. The measurements allow comparisons to established models that are used to relate light plasma treatments (O and H) of the surfaces to the resistivity measurements. The results are correlated to x-ray photoelectron spectroscopy (XPS) and show that four-probe STM can detect changes in surface and bulk conduction mechanisms that are beyond conventional monochromatic XPS.

Reliability of Single Crystal Silver Nanowire-Based Systems: Stress Assisted Instabilities.

Time-dependent mechanical characterization of nanowires is critical to understand their long-term reliability in applications, such as flexible-electronics and touch screens. It is also of great importance to develop a theoretical framework for experimentation and analysis on the mechanics of nanowires under time-dependent loading conditions, such as stress-relaxation and fatigue. Here, we combine in situ scanning electron microscope (SEM)/transmission electron microscope (TEM) tests with atomistic and phase-field simulations to understand the deformation mechanisms of single crystal silver nanowires held under constant strain. We observe that the nanowires initially undergo stress-relaxation, where the stress reduces with time and saturates after some time period. The stress-relaxation process occurs due to the formation of few dislocations and stacking faults. Remarkably, after a few hours the nanowires rupture suddenly. The reason for this abrupt failure of the nanowire was identified as stress-assisted diffusion, using phase-field simulations. Under a large applied strain, diffusion leads to the amplification of nanowire surface perturbation at long wavelengths and the nanowire fails at the stress-concentrated thin cross-sectional regions. An analytical analysis on the competition between the elastic energy and the surface energy predicts a longer time to failure for thicker nanowires than thinner ones, consistent with our experimental observations. The measured time to failure of nanowires under cyclic loading conditions can also be explained in terms of this mechanism.

Fracture behavior of a composite composed by Ti-aluminide multi-layered and continuous-SiCf-reinforced Ti-matrix

In the present work, a novel hybrid composite composed of Ti-aluminide multi-layered part and continuous-SiCf-reinforced Ti-matrix part using a vacuum hot pressing/hot isostatic pressure (VHP/HIP) sintering process was synthesized. Then, the damage tolerance behavior of this hybrid composite was studied by tensile tests and in situ scanning electron microscopy (SEM) three-point bending tests. The results indicate that the mechanical properties and crack propagation of the hybrid composite are strongly influenced by the extrinsic toughness mechanism. In detail, the crack deflection of multi-layered composite and strengthening effect of SiCf-reinforced matrix composite are observed in the hybrid composite. Owing to the strengthening effect of continuous-SiCf-reinforced Ti-matrix part, the tensile strength of hybrid composite is much higher than that of neat Ti-aluminide multi-layered composite. Furthermore, it is found that the optimal damage tolerance ability is optimized when the notch position is located at the Ti-aluminide multi-layered part due to the crack deflection of multi-layered structure.

Origin of high Coulombic loss during sodiation in Na-Sn battery

Electrochemical sodiation is performed in crystalline Sn foil using in situ scanning electron microscopy (SEM) to simultaneously measure the changes in the electrical resistivity and volume of the Sn anode in a Na-Sn battery. We observe that sodiation causes an increase in the Sn anode resistivity by six orders of magnitude. Ab initio molecular dynamics simulations of the Na-Sn alloy system demonstrate that the increased resistivity of the anode is caused by the formation of an electrically resistive amorphous NaSn phase (a-NaSn) with a pseudogap. It is also observed that the formation of a-NaSn is always accompanied by a large volume expansion of ∼200%, causing the development of residual tensile stress. The residual stress in turn alters the electronic structure of the a-NaSn phase, further increasing the resistivity of a-NaSn and thus decreasing the energy efficiency of the Na-Sn battery.

Recent Advances on In Situ SEM Mechanical and Electrical Characterization of Low-Dimensional Nanomaterials.

In the past decades, in situ scanning electron microscopy (SEM) has become a powerful technique for the experimental study of low-dimensional (1D/2D) nanomaterials, since it can provide unprecedented details for individual nanostructures upon mechanical and electrical stimulus and thus uncover the fundamental deformation and failure mechanisms for their device applications. In this overview, we summarized recent developments on in situ SEM-based mechanical and electrical characterization techniques including tensile, compression, bending, and electrical property probing on individual nanostructures, as well as the state-of-the-art electromechanical coupling analysis. In addition, the advantages and disadvantages of in situ SEM tests were also discussed with some possible solutions to address the challenges. Furthermore, critical challenges were also discussed for the development and design of robust in situ SEM characterization platform with higher resolution and wider range of samples. These experimental efforts have offered in-depth understanding on the mechanical and electrical properties of low-dimensional nanomaterial components and given guidelines for their further structural and functional applications.

ALD TiO2-Coated Flower-like MoS2 Nanosheets on Carbon Cloth as Sodium Ion Battery Anode with Enhanced Cycling Stability and Rate Capability

We report the fabrication of 3D flower-like MoS2 nanosheets arrays on carbon cloth as a binder-free anode for sodium ion battery. Ultrathin and conformal TiO2 layers are used to modify the surface of MoS2 by atomic layer deposition. The electrochemical performance measurements demonstrate that the ALD TiO2 layer can improve the cycling stability and rate capability of MoS2. The MoS2 nanosheets with 0.5-nm TiO2 coating electrode show the highest initial discharge capacity of 1392 mA h g-1 at 200 mA g-1, which is increased by 53% compared with that of bare MoS2. After 150 cycles, the capacity retention rate of the TiO2-coated MoS2 achieves 75.8% of its second cycle's capacity at 200 mA h g-1 in contrast to that of 59% of pure MoS2. Furthermore, the mechanism behind the experimental results is revealed by ex situ scanning electron microscope (SEM), X-ray powder diffraction (XRD), and electrochemical impedance spectroscopy (EIS) characterizations, which confirms that the ultrathin TiO2 modifications can prevent the structural degradation and the formation of SEI film of MoS2 electrode.

Development of an electrochemically integrated SR-GIXRD flow cell to study FeCO3 formation kinetics.

An electrochemically integrated Synchrotron Radiation-Grazing Incidence X-Ray Diffraction (SR-GIXRD) flow cell for studying corrosion product formation on carbon steel in carbon dioxide (CO2)-containing brines typical of oil and gas production has been developed. The system is capable of generating flow velocities of up to 2 m/s at temperatures in excess of 80 °C during SR-GIXRD measurements of the steel surface, enabling flow to be maintained over the course of the experiment while diffraction patterns are being collected. The design of the flow cell is presented, along with electrochemical and diffraction pattern transients collected from an initial experiment which examined the precipitation of FeCO3 onto X65 carbon steel in a CO2-saturated 3.5 wt. % NaCl brine at 80 °C and 0.1 m/s. The flow cell is used to follow the nucleation and growth kinetics of FeCO3 using SR-GIXRD linked to the simultaneous electrochemical response of the steel surface which were collected in the form of linear polarisation resistance measurements to decipher in situ corrosion rates. The results show that FeCO3 nucleation could be detected consistently and well before its inhibitive effect on the general corrosion rate of the system. In situ measurements are compared with ex situ scanning electron microscopy (SEM) observations showing the development of an FeCO3 layer on the corroding steel surface over time confirming the in situ interpretations. The results presented demonstrate that under the specific conditions evaluated, FeCO3 was the only crystalline phase to form in the system, with no crystalline precursors being apparent. The numerous capabilities of the flow cell are highlighted and presented in this paper.

Increasing Fracture Toughness for 3D Multiscale Carbon Nanotube and Carbon Fiber Reinforced Composites

This paper presents a new concept of forming 3D hierarchical carbon nanotube (CNT) and carbon fiber (CF) reinforced composites by using a low temperature grafting method that connects CNTs to CF surface by covalent bonding. By using amorphous carbon to join the free ends of two cantilever CNTs, it is shown via in situ scanning electron microscope (SEM) pulling out experiments that the strengths at the joint between CF-CNT and CNT-CNT junction are high and breakage occurs in CNT itself. This offers evidence that the maximum load in a typical bridging load-displacement curve could be increased, which in turn can potentially increase intralaminar and interlaminar fracture toughness of 3D multiscale CNT-CF reinforced composites.

Ex situ characterization of metallurgical inclusions in X100 pipeline steel before and after immersion in a neutral pH bicarbonate solution

The initiation of corrosion pits in pipeline steels plays an important role in the development of stress corrosion cracking. In order to reveal the effect of inclusions on corrosion initiation sites and also to clarify contradictory results from previous literature, we proposed an ex situ characterization method that is allowed to characterize exactly the same inclusion or location of the surface of steel before and after corrosion tests. The time-dependent corrosion behaviour of the inclusions and the surrounding X100 steel matrix at the same area before and after early stage immersion in a near-neutral pH bicarbonate solution was investigated by ex situ scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and localized electrochemical impedance spectroscopy (LEIS). The sizes of most inclusions in X100 steel are below 3 μm. The results challenge the long-held opinion of previous work that corrosion pit initiations are related with the inclusions. It has been found that most of the inclusions remain stable (intact) during the whole testing time although severe corrosion occurs on the matrix of the steel. The chemical composition of the inclusion greatly affects the chemical stability of the inclusion. SiO2 inclusions and complex inclusions with a high SiO2 content remain intact although obvious general corrosion occurs on the steel matrix under the investigated immersion period. Inclusions with little Si, such as Al–Mg–Ca–O enriched inclusions, totally disappear after certain immersion time. During the immersion, the corrosion product tends to deposit at the interstice between the inclusion and steel matrix.

Formation mechanism of superconducting phase and its three-dimensional architecture in pseudo-single-crystalKxFe2−ySe2

We report how the superconducting phase forms in pseudo-single-crystal ${\mathrm{K}}_{x}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{2}$. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an ``imperfect'' order-disorder transition. It should be emphasized that the phase separation in pseudo-single-crystal ${\mathrm{K}}_{x}\mathrm{F}{\mathrm{e}}_{2\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{2}$ is caused by the iron-vacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Z. Wang et al., Phys. Rev. B 91, 064513 (2015)]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction or melt growth. By focused ion beam scanning electron microscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase.

XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O2 Batteries during the 1st Cycle

The surface chemistry of the commonly employed positive electrode substrate Super C65 carbon was investigated during the 1st cycle of a Li-O2 battery with a typical ether electrolyte (0.2 M LiTFSI in Diglyme) by performing in situ online electrochemical mass spectrometry (OEMS), ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). During discharge, a nanometer-thin (< 5 nm) layer of Li2O2 forms homogeneously throughout the electrode before it is passivated at a final specific charge of ∼365 mAh/g. Higher discharge charge rates lead to thinner and more densely packed layers of Li2O2. Electrolyte decomposition also occurs in parallel, as evidenced by the continuous increase of LiF and sulfur containing species of the degraded LiTFSI salt. On charge, O2 evolves initially according to a 2e−/O2 rate, but as the cell over-potentials increase, O2 evolution rate approaches 4e−/O2, thus demonstrating a significant extent of parasitic side-reactions. Unlike the discharge, the charge leads to inhomogeneous electrode reactions causing a continuous removal of Li2O2 with no indication of LiO2 or Li2O intermediates. The decomposition side-products formed during discharge are also removed and the spectra of the pristine electrode are nearly retained at a full charge. Our results show that the analysis of Li-O2 battery chemistry requires broad approach based on the combination of various electrode surface sensitive techniques, such as XPS and OEMS, in order to provide further fundamental understanding of the mechanisms behind the complex electrochemistry involving the Li and O2 as well as several other predominant degradation products of the electrode and electrolyte.

Study on microscopic ductile fracture of in-situ TiB2/2024 Al matrix composite

The microscopic fracture behavior, or in detail, damage initiation, growth and coalescence of in-situ TiB2 particle reinforced 2024-T4 Al matrix composite (in-situ PRAMC) synthesized through an exothermic reaction process is investigated via in situ scanning electron microscopy (SEM) test with tensile load. Brittle fracture of large particles and clusters of small particles are identified in the early stage. The number of micro cracks accumulates with the increase of tension load. The debonding at the interface of large particle or cluster of small particles with Al matrix is scarcely found, which ensure an effective load transfer from particles into ductile matrix. The coalescence of micro cracks leads to final fracture. The fracture morphology observation confirms the feature of ductile fracture.

Electron Microscopy of Living Cells During in Situ Fluorescence Microscopy.

We present an approach toward dynamic nanoimaging: live fluorescence of cells encapsulated in a bionanoreactor is complemented with in situ scanning electron microscopy (SEM) on an integrated microscope. This allows us to take SEM snapshots on-demand, that is, at a specific location in time, at a desired region of interest, guided by the dynamic fluorescence imaging. We show that this approach enables direct visualization, with EM resolution, of the distribution of bioconjugated quantum dots on cellular extensions during uptake and internalization.

High Strain Rate Tensile Testing of Silver Nanowires: Rate-Dependent Brittle-to-Ductile Transition.

The characterization of nanomaterials under high strain rates is critical to understand their suitability for dynamic applications such as nanoresonators and nanoswitches. It is also of great theoretical importance to explore nanomechanics with dynamic and rate effects. Here, we report in situ scanning electron microscope (SEM) tensile testing of bicrystalline silver nanowires at strain rates up to 2/s, which is 2 orders of magnitude higher than previously reported in the literature. The experiments are enabled by a microelectromechanical system (MEMS) with fast response time. It was identified that the nanowire plastic deformation has a small activation volume (<10b(3)), suggesting dislocation nucleation as the rate controlling mechanism. Also, a remarkable brittle-to-ductile failure mode transition was observed at a threshold strain rate of 0.2/s. Transmission electron microscopy (TEM) revealed that along the nanowire, dislocation density and spatial distribution of plastic regions increase with increasing strain rate. Furthermore, molecular dynamic (MD) simulations show that deformation mechanisms such as grain boundary migration and dislocation interactions are responsible for such ductility. Finally, the MD and experimental results were interpreted using dislocation nucleation theory. The predicted yield stress values are in agreement with the experimental results for strain rates above 0.2/s when ductility is pronounced. At low strain rates, random imperfections on the nanowire surface trigger localized plasticity, leading to a brittle-like failure.

Ordered Structures of Functionalized Silica Nanoparticles on Gold Surfaces: Correlation of Quartz Crystal Microbalance with Structural Characterization.

Quartz crystal microbalance (QCM) is frequently used to investigate adsorption of nanometer-sized objects such as proteins, viruses, or organic as well as inorganic nanoparticles from solution. The interpretation of the data obtained for heterogeneous adsorbate layers is not straightforward in particular if the systems exhibit sizable amounts of dissipation. In this study we investigate the deposition of monodisperse, amine functionalized silica nanoparticles on gold surfaces using QCM with dissipation (QCM-D) to obtain frequency and dissipation changes during adsorption from the liquid phase. These investigations are combined with ex situ scanning electron microscopy (SEM) measurements to study both coverage as well as lateral arrangement of the particles. An ordered layer of particles is found at saturation coverage due to the charged particle surface resulting in a repulsive interaction between the particles. The repulsion ensures a minimal distance between the particles, which leads to a saturation coverage of 15% for particles of 137 nm diameter. The frequency shift is shown to be a linear function of coverage which is a behavior expected for an elastic medium according to the Sauerbrey equation. However, the system shows a strong dependence of the normalized frequency shift on the overtones as well as a large dissipation, which is a clear indication for a system with viscoelastic properties. The analysis of the data show that a reliable determination of the adsorbed mass solely on the basis of QCM-D results is not possible, but additional information as determined by SEM in the present case is required to determine the coverage. From a correlation of the QCM-D results with the structural characterization it is possible to infer that the dissipation is a long ranged phenomenon. A lower boundary of the interaction length could be derived being twice the particle diameter for the particles studied here. In contrast to that the frequency response behaves like local phenomenon.