Low-voltage Scanning Transmission Electron Research Articles

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.

Thin, Conformal Carbon-Coating of Silicon-Phosphorus Nanoparticle Agglomerates for Lithium-Ion Battery Anodes

Efforts to further enhance the capabilities of silicon as a lithium-ion battery anode are continuing into the realm of chemical composition. Adding varying amounts of other elements can significantly alter the material properties or side reactions, which influence electrochemical performance. Additionally, tuning the chemical composition combined with surface modifications can better enable application of silicon anodes in other devices such as hybrid supercapacitors.Prior work, among others, has shown different performance enhancement regimes depending on how much phosphorus is added to silicon-containing anodes [1, 2]. In particular, the amount of phosphorus from ppm levels to majority atomic fraction can result in enhancement in the conductivity, diffusivity, or forming altogether a wholly new protective matrix [3]. However, while the rate capability has been enhanced only primarily for delithiation, silicon-phosphorus (SiPx) anodes still suffer from similar capacity degradation issues as pure silicon anodes.In the present work, we attempt to address the capacity degradation issue by using a thin, conformal carbon coating to modify the solid-electrolyte interface (SEI) reaction. Firstly, amorphous nanoparticles of SiPx with a few atomic percent of phosphorus are prepared using silane and phosphine co-pyrolysis. Using an oscillating furnace at 800 degrees Celsius under a low flow of acetylene in Ar carrier gas, we are able to produce consistent, uniform carbon films on nanoparticle agglomerates with select thicknesses between 2 and 10 nm, validated using low voltage scanning transmission electron microscopy (LV-STEM), as shown in Figure 1. By comparing Si- and SiPx-containing anodes in electrochemical half cells, we can reproduce the rate capability results while demonstrating the effect of carbon coating on cycle life and capacity retention. Furthermore, the carbon coating’s effect on volume expansion for Si- and SiPx-containing anodes is studied in situ using an electrochemical dilatometer in half cell configuration.[1] Y. Domi, H. Usui, M. Shimizu, Y. Kakimoto, H. Sakaguchi, ACS Applied Materials & Interfaces 2016, 8, 7125-7132.[2] S. Huang, L.-Z. Cheong, D. Wang, C. Shen, ACS Applied Materials & Interfaces 2017, 9, 23672-23678.[3] F. T. Huld, S. Y. Lai, W. M. Tucho, R. Batmaz, I. T. Jensen, S. Lu, O. E. Eleri, A. Y. Koposov, Z. Yu, F. Lou, ChemistrySelect 2022, 7, e202202857. Figure 1

Pulsed electrophoretic deposition of hybrid coatings from aqueous suspensions as surface functionalization and sealing technique of anodized AA2024. Part I: Morphological characterization, analysis of the interfacial interactions, and evaluation of pore impregnation of the anodic layer

In this two-part series, pulsed direct current for cathodic electrophoretic deposition (pulsed-CEPD) was studied as sealing technique of anodized AA2024 by depositing hybrid sol-gel films from aqueous suspensions. The coating was obtained using tetraethyl orthosilicate and hexadecyltrimethoxysilane as precursors and two pulse widths were used: 10 microseconds and 100 milliseconds. For comparison, continuous direct current for cathodic electrophoretic deposition was also performed. In this first article, it was developed a low-cost automated apparatus capable of creating pulsed fields using a commercial power supply. This was necessary since the automation system provided by the power supply manufacturer did not have a short enough response time able to create pulse widths in the order of microseconds. Field emission scanning electron microscopy (FESEM) and focused ion beam milling techniques were utilized to evaluate the surface morphology and interaction mechanisms in the interface anodic layer/film. Low voltage scanning transmission electron microscopy (STEM)-in-FESEM, and transmission electron microscopy (TEM) were used to evaluate the pore impregnation provided by pulsed-CEPD. Results demonstrated that the developed apparatus successfully created pulsed fields. FESEM micrographs of the anodic layer sealed by pulsed-CEPD showed that the shortest pulse provided the best films, likely due to the greater control of hydrogen bubbles generation and the promotion of a more regular deposition. Also, they revealed two interfacial interactions between the anodic layer and the film. STEM-in-FESEM images showed that the shortest pulse width provided greater pore impregnation by particles. This observation was confirmed by TEM, which demonstrated an incomplete pore impregnation for the longest pulse. Discussion regarding result interpretations is presented based on the generation and dissipation of H2(g) (considering the current interruption); on an alternative to the colloidal stability theory due to the presence of a pulsed field; and on the electrokinetic effects at the electrodes during pulsed-CEPD, that discusses how electroosmotic flow influences the deposition.

Radiation damage and defect dynamics in 2D WS2: a low-voltage scanning transmission electron microscopy study

Modern low-voltage scanning transmission electron microscopes (STEMs) have been invaluable for the atomic scale characterization of two-dimensional (2D) materials. Nevertheless, the observation of intrinsic structures of semiconducting and insulating 2D materials with 60 kV-microscopes has remained problematic due to electron radiation damage. In recent years, ultralow-voltage microscopes have been developed with the prospects of minimizing radiation damage of such 2D materials, however, to date only ultralow-voltage TEM investigations of semiconducting and insulating 2D materials have been reported, but similar results using STEM, despite being more widely adopted, are still missing. Here we report a quantitative analysis of radiation damage and beam-induced defect dynamics in semiconducting 2D WS2 during 30 kV and 60 kV-STEM imaging, particularly by recording atomic resolution electrostatic potential movies using integrated differential phase contrast to visualize both the light sulfur and heavy tungsten atoms. Our results demonstrate that electron radiation damage of 2D WS2 aggravates by a factor of two when halving the electron beam energy from 60 keV to 30 keV, from which we conclude electronic excitation and ionization to be the dominant mechanism inducing defects and damage during low-voltage STEM imaging of semiconducting 2D materials.

Enhanced Atomic-Scale Imaging of PGM-Free Catalysts By Low-Voltage Scanning Transmission Electron Microscopy

Platinum group metal (PGM)-free catalysts are promising candidates to supplant costly precious metal catalysts in hydrogen fuel cells and enable low-cost, commercially viable devices [1]. Recent developments in PGM-free catalysts have enabled performance comparable to Pt catalysts by some metrics [2], but durability remains a significant challenge and limits practical application of these devices [3]. To improve the durability of PGM-free catalysts, understanding and controlling atomic-scale active site structure and the corresponding degradation mechanisms is imperative [1]. Active sites in these catalysts are commonly thought to be FeN4 complexes within a graphene lattice. Many potential active site structures have been predicted by computational methods, however, and a technique capable of directly validating these models is needed to reveal the true nature of these sites and enable increased control over degradation [1].Scanning transmission electron microscopy (STEM) is capable of resolving a wide range of materials properties down to the atomic scale, such as structure, composition, and bonding information. The characteristics of PGM-free materials makes obtaining atomic-scale information about active site structure by STEM challenging, however. These materials are often defect-rich, containing a large number of dopants, holes, etc., and a large portion of proposed active sites exist at edges of the material, all of which increase susceptibility to beam damage. Conventional high-angle annular dark-field (HAADF) STEM imaging exacerbates this problem since only a small proportion of the incident beam electrons generate signal, and this signal is most sensitive to heavy elements. A large electron dose is therefore required to obtain a sufficient signal from light elements such as carbon and nitrogen, which can result in beam-induced atomic displacements at susceptible sites when using standard accelerating voltages. As a consequence, direct structural analysis of active sites is often not possible, impeding our understanding of degradation pathways.In this talk, we will demonstrate advancements in STEM techniques that significantly enhance our ability to directly analyze atomic-scale active site structure in PGM-free materials. We utilize a recently developed dose-efficient imaging technique based on four-dimensional (4D)-STEM [4,5] to reduce the required electron dose and simultaneously increase sensitivity to light elements. To minimize beam damage at sensitive active sites, we perform this dose-efficient 4D-STEM technique in a low-voltage aberration-corrected instrument operated at 30 kV, producing an atomic-scale probe below the threshold energy for beam-induced atomic displacements at many sites. We perform these experiments on a model PGM-free catalyst that contains many monolayer regions, which is ideal for low-voltage 4D-STEM and enables experimental validation through simulations of proposed active site structures. This work demonstrates how low-voltage 4D-STEM will provide new atomic-scale insights into the structure and degradation pathways of PGM-free catalyst active sites, accelerating the development of low-cost hydrogen fuel cells.

Visualization of unstained homo/heterogeneous DNA nanostructures by low-voltage scanning transmission electron microscopy

Three-dimensional (3D) homo/heterogeneous DNA nanostructures were studied with low-voltage scanning transmission electron microscopy (LV-STEM). Four types of 3D DNA nanostructures were designed and fabricated by the origami method including newly proposed protocols. The low-energy electron probe and optimized dark-field STEM detector enabled individual unstained DNA nanostructures to be clearly imaged by the single acquisition without the averaging process. For the vertically stacked double structures, assembled through modified single-stranded domains, and the structures containing a square opening (i.e., a hole) in the center, the LV-STEM successfully reveals the vertical information of these 3D structures as the contrast differences compared to the reference. For the heterogeneous structures, the LV-STEM visualized both regions of the functionalized gold nanoparticles and the DNA base structure with distinct contrasts. This study introduces a straightforward method to fabricate stackable DNA nanostructures or nanoparticles by replacing a relatively small number of incumbent DNA strands, which could realize the simple and sophisticated fabrication of higher-order 3D DNA homo/hetero nanostructures. Together with these design techniques of DNA nanostructures, this study has demonstrated that the LV-STEM is the swift and simple method for visualizing the 3D DNA nanostructures and certifying the fabricated products as the specified design, which is applicable to various research fields on soft materials including DNA nanotechnology.

Formation of Metal-Nitrogen Sites in Atomically-Dispersed Catalysts Observed By in Situ Microscopy

Platinum group metal-free (PGM-free) catalysts are under intense development for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Recent research efforts have focused on catalysts derived from metal organic frameworks doped with transition metals such as Fe, Ni, Co, and Mn. High temperature pyrolysis is a key synthesis step in the formation of the active catalyst, and aims to produce porous, mesographtic carbons with a high density of single transition metal atoms bound to nitrogen. In this work, correlated in situ characterization techniques have been employed to explore the mechanisms that influence catalytic activity and provide guidance for synthesis protocols. The coordination of advanced synctrotron and electron microscopy methods provide new insight into the atomic-level transformations which occur during pyrolysis. Micro-electro-mechanical system (MEMS)-based heating devices were utilized to directly observe transformations occurring at the atomic scale as the temperature was ramped to 1100oC within the electron microscope. It was observed that significant amounts of the transition metal and nitrogen were lost as the pyrolysis temperature increased from 900-1100oC, which could limit the active site density. However, rotating disk electrode measurements indicate that pyrolysis temperatures beyond 900oC appear to be required to maximize mass activity. As shown in Figure 1, low voltage scanning transmission electron microscopy observations indicated that the degree of carbon graphitization increased as the temperature was elevated 1100oC and nitrogen-bound transition metal sites were also more readily observed within the graphene lattice by electron energy loss spectroscopy. Consequently, approaches are being explored to maintain higher Fe and N content while also generating highly graphitized domains. Beyond active site density, the microporosity of the support can also play a key role in the performance of the catalyst. Ongoing in situ studies will also aim to explore the mechanisms of microporosity formation within the MOF support and its subsequent influence on fuel cell performance. Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, as part of the ElectroCat Consortium, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility. Figure 1

Graphene-supporting films and low-voltage STEM in SEM toward imaging nanobio materials without staining: Observation of insulin amyloid fibrils

Utilization of graphene-supporting films and low-voltage scanning transmission electron microscopy (LV-STEM) in scanning electron microscopy (SEM) is shown to be an effective means of observing unstained nanobio materials. Insulin amyloid fibrils, which are implicated as a cause of type II diabetes, are formed in vitro and observed without staining at room temperature. An in-lens cold field-emission SEM, equipped with an additional homemade STEM detector, provides dark field (DF)-STEM images in the low energy range of 5–30keV, together with secondary electron (SE) images. Analysis based on Lenz’s theory is used to interpret the experimental results. Graphene films, where the fibrils are deposited, reduce the background level of the STEM images compared with instances when conventional amorphous carbon films are used. Using 30keV, which is lower than that for conventional TEM (100–300keV), together with low detection angles (15–55mrad) enhances the signals from the fibrils. These factors improve image quality, which enables observation of thin fibrils with widths of 7–8nm. STEM imaging clearly reveals a twisted-ribbon structure of a fibril, and SE imaging shows an emphasized striped pattern of the fibril. The LV-STEM in SEM enables acquisition of two types of images of an identical fibril in a single instrument, which is useful for understanding the structure. This study expands the application of SEM to other systems of interest, which is beneficial to a large number of users. The method in this study can be applied to the observation of various nanobio materials and analysis of their native structures, thus contributing to research in materials and life sciences.

Structure and local chemical properties of boron-terminated tetravacancies in hexagonal boron nitride.

Imaging and spectroscopy performed in a low-voltage scanning transmission electron microscope are used to characterize the structure and chemical properties of boron-terminated tetravacancies in hexagonal boron nitride. We confirm earlier theoretical predictions about the structure of these defects and identify new features in the electron energy-loss spectra of B atoms using high resolution chemical maps, highlighting differences between these areas and pristine sample regions. We correlate our experimental data with calculations which help explain our observations.

Characterization of conductive nanobiomaterials derived from viral assemblies by low-voltage STEM imaging and Raman scattering

New technologies require the development of novel nanomaterials that need to be fully characterized to achieve their potential. High-resolution low-voltage scanning transmission electron microscopy (STEM) has proven to be a very powerful technique in nanotechnology, but its use for the characterization of nanobiomaterials has been limited. Rotavirus VP6 self-assembles into nanotubular assemblies that possess an intrinsic affinity for Au ions. This property was exploited to produce hybrid nanobiomaterials by the in situ functionalization of recombinant VP6 nanotubes with gold nanoparticles. In this work, Raman spectroscopy and advanced analytical electron microscopy imaging with spherical aberration-corrected (Cs) STEM and nanodiffraction at low-voltage doses were employed to characterize nanobiomaterials. STEM imaging revealed the precise structure and arrangement of the protein templates, as well as the nanostructure and atomic arrangement of gold nanoparticles with high spatial sub-Angstrom resolution and avoided radiation damage. The imaging was coupled with backscattered electron imaging, ultra-high resolution scanning electron microscopy and x-ray spectroscopy. The hybrid nanobiomaterials that were obtained showed unique properties as bioelectronic conductive devices and showed enhanced Raman scattering by their precise arrangement into superlattices, displaying the utility of viral assemblies as functional integrative self-assembled nanomaterials for novel applications.

New Low Voltage Scanning Transmission Electron Microscope Detector for Fastest Image Acquisition in BF, DF and HAADF

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Influence of strain gradients on the grain refinement during industrial scale ECAP

AbstractThe microstructure of the aluminium alloy 6060 after industrial scale (cross‐section 50×50 mm2) equal‐channel angular pressing (ECAP) is studied. We focus on processing without additional backpressure in order to allow the formation of a corner gap and the corresponding development of a distinct strain gradient after one pass. Low voltage scanning transmission electron microscopy (STEM) and electron backscatter diffraction analysis (EBSD) are used to determine the average grain size, grain boundary misorientation angles and grain shape aspect ratios. These microstructural parameters are analysed for three characteristic locations from top to bottom within the billet. Our EBSD observations clearly document the occurrence of global strain gradients and microstructural heterogeneity. The STEM analyses expand the information on this heterogeneous deformation for the investigated locations in the sub‐micrometer range. In good agreement with previous studies, our results provide detailed information on how a heterogeneous induced shear strain affects microstructure and mechanical properties when ECAP is performed without backpressure.

Imaging with low-voltage scanning transmission electron microscopy: A quantitative analysis

A dedicated specimen holder has been designed to perform low-voltage scanning transmission electron microscopy in dark field mode. Different test samples, namely InGaAs/GaAs quantum wells, InGaAs nanowires and thick InGaAs layers, have been analysed to test the reliability of the model based on the proportionality to the specimen mass-thickness, generally used for image intensity interpretation of scattering contrast processes. We found that size of the probe, absorption and channelling must be taken into account to give a quantitative interpretation of image intensity. We develop a simple procedure to evaluate the probe-size effect and to obtain a quantitative indication of the absorption coefficient. Possible artefacts induced by channelling are pointed out. With the developed procedure, the low voltage approach can be successfully applied for quantitative compositional analysis. The method is then applied to the estimation of the In content in the core of InGaAs/GaAs core-shell nanowires.

Microstructural Features and Mechanical Properties after Industrial Scale ECAP of an Al 6060 Alloy

Future applications of ultrafine-grained, high performance materials produced by equal-channel angular pressing (ECAP) will most likely require processing on an industrial scale. There is a need for detailed microstructural and mechanical characterisation of large-scale, ECAP-processed billets. In the present study, we examine the microstructure and mechanical properties as a function of location and orientation within large (50 x 50 x 300 mm³) billets of an Al 6060 alloy produced by ECAP (90° channel angle) with different magnitudes of backpressure. The internal deformation is analysed using a grid-line method on split billets. Hardness is recorded in longitudinal and cross-sectional planes. In order to further characterise the local, post-ECAP mechanical properties, tensile tests in different layers are performed. Moreover, low voltage scanning transmission electron microscopy observations highlight relevant microstructural features. We find that the homogeneity and anisotropy of mechanical properties within the billets depend significantly on the geometry of the shear zone. We demonstrate that deformation gradients can be reduced considerably by increasing the backpressure: The opening-angle of the fan-shaped shear zone is reduced from ψ ≈ 20 ° to ψ ≈ 7 ° when the backpressure is increased from 0 to 150 MPa. Backpressures of 150 MPa result in excellent homogeneity, with a relative variation of tensile mechanical properties of less than 7 %. Our investigation demonstrates that ECAP is suitable for processing homogenous, high performance materials on a large scale, paving the way for advanced industrial applications.

Improvement of strength and ductility for a 6056 aluminum alloy achieved by a combination of equal-channel angular pressing and aging treatment

Incited by the aim of improving the strength and preserving a moderate ductility, the promising approach of a combined equal-channel angular pressing (ECAP) and aging treatment was applied to the 6056 Al–Mg–Si–(Cu) alloy. This method has been proven to be most effective for a small number of ECAP passes in the solid-solution condition and a following aging treatment, which is timed to enable precipitation hardening up to the peak strength on the one hand and microstructural recovery just before the onset of softening on the other hand. In this work, the evolution of hardness during post-ECAP aging and the effect of aging temperature and time on strength and ductility will be discussed. By means of low-voltage scanning transmission electron microscopy, the underlying microstructural features in terms of dislocation structure and precipitation characteristics will be presented. It has been found that peak aging especially at low-aging temperatures is suitable for achieving the desired high-strength combination, whereas—on the expense of some percent in the strengthening—the ductility is best in slightly underaged conditions. Compared to the initial peak-aged condition, an increase in strength of 27%, combined with a moderate ductility of 6.5% uniform elongation was achieved for peak aging at 150 °C after two passes of ECAP.

Cyclic behavior and microstructural stability of ultrafine-grained AA6060 under strain-controlled fatigue

The strain-controlled fatigue behavior of AA6060, a precipitation hardening aluminum alloy, was investigated in ultrafinegrained (UFG) conditions after severe plastic deformation (SPD) by equal-channel angular pressing (ECAP). Two as-processed conditions, representing different stages of strain hardening and grain refinement as well as a ductility-optimized condition, achieved by a combined ECAP and aging treatment were considered. Low-voltage scanning transmission electron microscopy on samples stopped at characteristic stages of the fatigue process was applied to investigate the microstructural development. The as-processed as well as the optimized condition showed cyclic softening, which was found to be dependent on the amount of prestrain induced by ECAP processing. This is linked to dynamic recovery processes in the microstructure, indicated by a clearer distinction of grain boundaries and a reduction of dislocations in the grain interior. For all applied plastic strain amplitudes, ranging from Δεpl/2=1×10−3 to 5×10−3, the fatigue life of the ductility-optimized condition did not reach that of the severely work-hardened counterpart. For explaining this unexpected result, the differing (size-dependent) effectiveness of precipitates for the pinning of dislocations during cyclic loading was considered.

Plastic deformation mechanism of ultra-fine-grained AA6063 processed by equal-channel angular pressing

Equal-channel angular pressing (ECAP) is an established method to produce ultra-fine-grained (UFG) materials with extraordinary mechanical properties. However, different methods to characterize the complex microstructure give contradictory results. Therefore an ECAP-processed UFG aluminum alloy AA6063 was characterized by various electron-microscopy-based methods such as backscattered electron contrast, electron backscatter diffraction, transmission electron microscopy and low-voltage scanning transmission electron microscopy. Only a skilful combination of all methods reveals the complete information about the microstructure which is needed to understand the results of the mechanical testing by nanoindentation, tensile tests and strain-rate jump test. The main difference is the amount of low-angle grain boundaries and high-angle grain boundaries which determine hardness, tensile and yield strength and strain-rate sensitivity of ECAP materials produced by different numbers of passes and routes. This is explained by the interaction of dislocations with the different grain boundaries.

Lipid Nanotube Encapsulating Method in Low-Energy Scanning Transmission Electron Microscopy Analyses

The lipid nanotube (LNT) encapsulating method is a rational sample fixation method that can be used to mount samples for transmission electron microscopy analyses. By employing the LNT encapsulating method in 30 kV low-voltage scanning transmission electron microscopy (LV-STEM), it is possible to record multiangle images of ferritin without using the negative staining method. We have also recorded a tilted series of high-contrast LV-STEM images and reconstructed three-dimensional images. These results show that LNTs have sufficient durability for LV electron beam, and indicate the potential of the LNT encapsulating method as a sample fixation method of LV electron microscopy.

Fast Throughput Low Voltage Scanning Transmission Electron Microscope Imaging of Nano-Resolution Three Dimensional Tissue

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Anatomy of a low temperature zircon outgrowth

Outgrowths of zircon and xenotime are abundant on detrital zircon in a greenschist facies regional metamorphic slate from the Scottish Highlands. Back-scattered electron images reveal that ca. 3-μm wide zircon outgrowths are porous, inclusion-rich and contain fine-grained intergrowths with xenotime. Focused ion beam milling, transmission electron microscopy and low-voltage scanning transmission electron microscopy show that the outgrowth microstructure is complex, composed of mis-orientated nano-crystalline zircon and a poorly crystalline zircon–xenotime complex. Well-defined micron-sized xenotime is also present within the outgrowth. Micro-textures show that the outgrowth developed in a polyphase history during cooling and exhumation. Accommodation space for the outgrowth is probably generated by a combination of thermal decompaction and the migration of defects in adjacent quartz driven by the force of crystallisation. Zircon may be unique in its ability to record textural evidence of events during uplift and exhumation that are not recorded in major silicate phases.