Atomic-scale Characterization Research Articles

Real-time imaging of atomic electrostatic potentials in 2D materials with 30 keV electrons

Scanning transmission electron microscopy (STEM) is the most widespread adopted tool for atomic scale characterization of two-dimensional (2D) materials. Many 2D materials remain susceptible to electron beam damage, despite the standardized practice to reduce the beam energy from 200 keV to 80 or 60 keV. Although, all elements present can be detected by atomic electrostatic potential imaging using integrated differential phase contrast (iDPC) STEM or electron ptychography, capturing dynamics with atomic resolution and enhanced sensitivity has remained a challenge. Here, by using iDPC-STEM, we capture defect dynamics in 2D WS$_2$ by atomic electrostatic potential imaging with a beam energy of only 30 keV. The direct imaging of atomic electrostatic potentials with high framerate reveals the presence and motion of single atoms near defects and edges in WS$_2$ that are otherwise invisible with conventional annular dark-field STEM or cannot be captured sufficiently fast by electron ptychography.

Atomic-Scale Structural Characterization of Silver-Doped Phosphate-Based Glasses Prepared by Coacervation

Phosphate-based glasses (PBGs) are traditionally prepared using the high-temperature melt-quenching (MQ) route or via the more recent sol–gel (SG) method that requires the use of organic solvents. The coacervation method represents an excellent inexpensive and green alternative to MQ and SG, being performed in aqueous solution and at room temperature. Coacervation is particularly applicable for the production of PBGs designed for biomedical applications because it allows for the inclusion of temperature-sensitive molecules and does not require the use of toxic solvents. Whereas the atomic structure of the MQ and SGPBGs is known, the atomic structure of those prepared via coacervation has yet to be investigated. In this study, a comprehensive advanced structural characterization has been performed on phosphate-based glasses in the system P2O5–CaO–Na2O–Ag2O (Ag2O mol % = 0, 1, 3, 5, 9, and 14) prepared via the coacervation method. Glasses within this system should find application as bioresorbable biomaterials thanks to their ability to release bioactive ions in a controlled manner. In particular, they possess antibacterial properties, inferred by the release of Ag+ over time. High-energy X-ray diffraction (HEXRD), 31P and 23Na solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and X-ray absorption Spectroscopy (XAS) at the Ag K-edge were used to probe the atomic structure of the glasses after drying in vacuum and after calcination at 300 °C. The length of the polyphosphate chains in the solid state appears to be independent of silver concentration; however, significant degradation of these chains is seen after calcination at 300 °C. Atomic-scale characterisation results indicate that the structure of these glasses is akin to that of other silver-doped phosphate glasses prepared using the MQ and SG methods. This suggests that phosphate-based glasses prepared using milder and greener conditions may have similar chemical and physical properties such as solubility, biocompatibility, and antibacterial properties.

Theory of Atomic-Scale Vibrational Mapping and Isotope Identification with Electron Beams.

Transmission electron microscopy and spectroscopy currently enable the acquisition of spatially resolved spectral information from a specimen by focusing electron beams down to a sub-angstrom spot and then analyzing the energy of the inelastically scattered electrons with few-meV energy resolution. This technique has recently been used to experimentally resolve vibrational modes in 2D materials emerging at mid-infrared frequencies. Here, on the basis of first-principles theory, we demonstrate the possibility of identifying single isotope atom impurities in a nanostructure through the trace that they leave in the spectral and spatial characteristics of the vibrational modes. Specifically, we examine a hexagonal boron nitride molecule as an example of application, in which the presence of a single isotope impurity is revealed through changes in the electron spectra, as well as in the space-, energy-, and momentum-resolved inelastic electron signal. We compare these results with conventional far-field spectroscopy, showing that electron beams offer superior spatial resolution combined with the ability to probe the complete set of vibrational modes, including those that are optically dark. Our study is relevant for the atomic-scale characterization of vibrational modes in materials of interest, including a detailed mapping of isotope distributions.

The hidden structure dependence of the chemical life of dislocations.

Dislocations are one-dimensional defects in crystals, enabling their deformation, mechanical response, and transport properties. Less well known is their influence on material chemistry. The severe lattice distortion at these defects drives solute segregation to them, resulting in strong, localized spatial variations in chemistry that determine microstructure and material behavior. Recent advances in atomic-scale characterization methods have made it possible to quantitatively resolve defect types and segregation chemistry. As shown here for a Pt-Au model alloy, we observe a wide range of defect-specific solute (Au) decoration patterns of much greater variety and complexity than expected from the Cottrell cloud picture. The solute decoration of the dislocations can be up to half an order of magnitude higher than expected from classical theory, and the differences are determined by their structure, mutual alignment, and distortion field. This opens up pathways to use dislocations for the compositional and structural nanoscale design of advanced materials.

Atomic-Scale Local Work Function Characterizations of Br Islands on Cu(111)

The electronic properties of halogens on the surface of metals have attracted substantial attention due to their promotion effects on surface molecular dehydrogenation reactions. Herein, the electr...

Atomic-scale preparation and characterization ofhigh-temperature superconducting thin films

High-temperature ( T c) superconductors have been at the forefront of physics research for more than 30 years due to their rich physical properties and potential applications. Exploring and discovering new high- T c superconductors, as well as hereby clarifying the microscopic mechanism of unconventional superconductivity are among the long-desired goals of the world’s physicists. In this article, starting from the history and current challenges of high- T c superconductors, we review the atomic-scale understanding of conventional high- T c superconductivity based on heteroepitaxial films, especially focusing on the recent experimental advances on molecular beam epitaxy growth and quantum manipulation of high- T c superconducting thin films. We emphasize the atomic-scale exploration of epitaxial superconducting films by means of scanning tunneling microscopy, while a strategy of designing interfacial superconductors between two dissimilar materials is also introduced. We mainly devote to the experimental wisdom, methods, the microscopic understanding of high- T c superconductivity, and hope to inspire the related scientific researchers.

Three-dimensional atomic scale characterization of [formula omitted] twin boundaries in titanium

The {112¯2}<1123¯> compression twin can accommodate a considerable amount of strain under c-axis compression in Ti. However, unlike the tensile {101¯2}〈1¯011〉 twin, the structure of {112¯2}<1123¯> compression twins has not been completely characterized. Here, we apply a combined technique of HR-TEM characterization, topological analysis, and atomistic simulations to explore the facets that bound the {112¯2}<1123¯> twin in Ti. In addition to the currently known facets (CTB and B-Py), six new facets are observed and categorized for the first time from atomic-scale TEM observations along five crystallographic directions. The six new facets are (112¯0)//(112¯6), PrPr1, PyPy1, (211¯1)//(1¯21¯1), (11¯04)//(011¯1¯), and (011¯0)//(211¯4). Results from the topological and computational analysis are in reasonable agreement with and support the HRTEM observations. Specifically, (1) the observed facets align with low-index interfaces in both twin and matrix domains, (2) the facets with lower surface energies are found to form extended interfaces, and (3) high-surface-energy facets are found at the twin tip region and explained by the fact that the energy of the combined facet and facet junction configuration is energetically preferred in the twin tip region. These results not only provide a comprehensive understanding of the 3D structure of {112¯2}<1123¯> compression twins in Ti, but also validate the MD procedure and the Ti interatomic potential employed. This is extremely important for future study of {112¯2} twin mobility and interactions with other defects, both features that remain extremely challenging to capture in experiments.

Atomic Scale Characterization of Fluxional Cation Behavior on Nanoparticle Surfaces: Probing Oxygen Vacancy Creation/Annihilation at Surface Sites.

Oxygen vacancy creation and annihilation are key processes in nonstoichiometric oxides such as CeO2. The oxygen vacancy creation and annihilation rates on an oxide's surface partly govern its ability to exchange oxygen with the ambient environment, which is critical for a number of applications including energy technologies, environmental pollutant remediation, and chemical synthesis. Experimental methods to probe and correlate local oxygen vacancy reaction rates with atomic-level structural heterogeneities would provide significant information for the rational design and control of surface functionality; however, such methods have been unavailable to date. Here, we characterize picoscale fluxional behavior in cations using time-resolved in situ aberration-corrected transmission electron microscopy to locate atomic-level variations in oxygen vacancy creation and annihilation rates on oxide nanoparticle surfaces. Low coordination number sites such as steps and edges, as well as locally strained sites, exhibited the greatest number of cation displacements, implying enhanced surface oxygen vacancy activity at these sites. The approach has potential applications to a much wider class of materials and catalysis problems involving surface and interfacial transport functionalities.

Atomic-Scale Characterization of Droplet Epitaxy Quantum Dots.

The fundamental understanding of quantum dot (QD) growth mechanism is essential to improve QD based optoelectronic devices. The size, shape, composition, and density of the QDs strongly influence the optoelectronic properties of the QDs. In this article, we present a detailed review on atomic-scale characterization of droplet epitaxy quantum dots by cross-sectional scanning tunneling microscopy (X-STM) and atom probe tomography (APT). We will discuss both strain-free GaAs/AlGaAs QDs and strained InAs/InP QDs grown by droplet epitaxy. The effects of various growth conditions on morphology and composition are presented. The efficiency of methods such as flushing technique is shown by comparing with conventional droplet epitaxy QDs to further gain control over QD height. A detailed characterization of etch pits in both QD systems is provided by X-STM and APT. This review presents an overview of detailed structural and compositional analysis that have assisted in improving the fabrication of QD based optoelectronic devices grown by droplet epitaxy.

Atomic scale characterization of three-dimensional structure, magnetic properties and dynamic evolutions of materials by transmission electron microscopy

Atomic scale characterization and manipulation is one of the physical bottlenecks, which needs to be broken when realizing atom manufacturing. The aberration-corrected transmission electron microscopy (TEM) is a powerful tool for structural characterization due to its exceptional spatial resolution. Therefore, it is very crucial to co-characterize atomic-scale three-dimensional structure and properties of atomic manufacturing materials by using TEM, which allows us to further understand the physics mechanism of atomic manipulation of materials. Nano-clusters and nanoparticles are two of the main objects in the studies of atomic manufacturing materials and devices, and possess rich physical and chemical properties and high manoeuverability. In this paper, we summarize the recent progress of quantitatively determining three-dimensional structures and magnetic properties of nanocluster, nanoparticles and nanograins, as well as their dynamic evolutions under the working conditions. The methodological breakthrough and development of electron microscopy techniques provide a solid foundation for precisely controlling atomic manufacturing materials.

Growth and Atomic-Scale Characterization of Ultrathin Silica and Germania Films: The Crucial Role of the Metal Support.

The present review reports on the preparation and atomic‐scale characterization of the thinnest possible films of the glass‐forming materials silica and germania. To this end state‐of‐the‐art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side‐by‐side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.

Highly charged 180 degree head-to-head domain walls in lead titanate

Charged domain walls (DWs) in ferroelectric materials are an area of intense research. Microscale strain has been identified as a method of inducing arrays of twin walls to meet at right angles, forming needlepoint domains which exhibit novel material properties. Atomic scale characterisation of the features exhibiting these exciting behaviours was inaccessible with the piezoresponse force microscopy resolution of previous work. Here we use aberration corrected scanning transmission electron microscopy to observe short, stepped, highly charged DWs at the tip of the needle points in ferroelectric PbTiO3. Reverse Ti4+ shift polarisation mapping confirms the head-to-head polarisation in adjacent domains. Strain mapping reveals large deviations from the bulk and a wider DW with a high Pb2+ vacancy concentration. The extra screening charge is found to stabilise the DW perpendicular to the opposing polarisation vectors and thus constitutes the most highly charged DW possible in PbTiO3. This feature at the needle point junction is a 5 nm × 2 nm channel running through the sample and is likely to have useful conducting properties. We envisage that similar junctions can be formed in other ferroelastic materials and yield exciting phenomena for future research.

Precipitation of γ’ during cooling of nickel-base superalloy Haynes 282

ABSTRACT Cooling-induced precipitation of the strengthening γ’ phase is commonly investigated in Ni-base superalloys with a high γ’ volume fraction, where it is used to control the final microstructure and properties. Less is known about the phase separation in low-volume-fraction alloys during cooling, although the microstructural state after cooling from solution treatment is known to affect subsequent heat-treatment steps. We use atomic-scale characterisation of Ni-base superalloy Haynes 282 (equilibrium γ’ volume fraction around 20%) to show that air cooling after solution or carbide stabilisation results in precipitation of nm-sized γ’ particles, whereas precipitation was suppressed during water quenching. The solution treatment has a significant effect on the hardness and γ’ precipitation during air cooling from the subsequent carbide stabilisation temperature. Also, the carbide-stabilisation treatment itself affects the γ’ precipitation during subsequent air cooling.

Enhancement of Cu nano-precipitation by Al addition in a Cu-Ni bearing low alloy steel

The effect of Al addition on the evolution of Cu precipitates in an Fe-2Cu-1.5Mn-1.5Ni alloy during tempering at 500 °C was investigated. The atomic-scale characterization revealed that the Al addition enhanced Cu nucleation in the matrix and on dislocations (or PAGB). Furthermore, the dislocation density was greater in steel with Al, probably because a larger number of Cu particles delayed the recovery and annihilation of dislocations in the martensite matrix, which resulted in a significant increase of hardness after tempering. The concentrations of Cu and Mn in Cu clusters were lower and those of Al and Ni were higher than the values of both metastable equilibrium bcc-Cu and equilibrium fcc-Cu phases.

High resolution atomic scale characterization of dislocations in high entropy alloys: Critical assessment of template matching and geometric phase analysis

The paper assesses the applicability of advanced atomic resolution displacement measurement techniques to characterize dislocation character in metallic materials using simulated images derived from anisotropic elasticity and actual measurements in high entropy alloys. We draw attention to two techniques: the real space method of template matching (TeMA) and the reciprocal space method of geometric phase analysis (GPA) and provide a critical assessment. These techniques have limitations for direct evaluation of full dislocations Burgers vector or when local displacements are exceeding 50% lattice spacing. This is clearly illustrated with simulated arctangent displacement profiles reminiscent of dislocation cores. An approach for circumventing this limitation is suggested in the form of a nearest neighbor correction. Additionally, a methodology for determination of the Burgers vector is introduced on the basis of a vectorial rendering of the displacement field upon consideration of two zone axis measurements and applied to TeMA and GPA. The experimental results conform to the Burgers vector of a full lattice dislocation in the FCC crystal structure of the High-Entropy Alloy (HEA). The comparison of simulated and experimental images proves the efficacy of the HR-TEM (High Resolution Transmission Electron Microscopy) displacement mapping techniques while pointing to the need for caution in case of large displacements.

Atomic-scale characterization of mature HIV-1 capsid stabilization by inositol hexakisphosphate (IP6).

Inositol hexakisphosphates (IP6) are cellular cofactors that promote the assembly of mature capsids of HIV. These negatively charged molecules coordinate an electropositive ring of arginines at the center of pores distributed throughout the capsid surface. Kinetic studies indicate that the binding of IP6 increases the stable lifetimes of the capsid by several orders of magnitude from minutes to hours. Using all-atom molecular dynamics simulations, we uncover the mechanisms that underlie the unusually high stability of mature capsids in complex with IP6 We find that capsid hexamers and pentamers have differential binding modes for IP6 Ligand density calculations show three sites of interaction with IP6 including at a known capsid inhibitor binding pocket. Free energy calculations demonstrate that IP6 preferentially stabilizes pentamers over hexamers to enhance fullerene modes of assembly. These results elucidate the molecular role of IP6 in stabilizing and assembling the retroviral capsid.

Influence of codoping with Hf and La on grain‐boundary transport in alumina

AbstractThe role of reactive elements (RE) is an important topic in understanding the oxidation behavior of high‐temperature alloys. In this work, the influence of codoping alumina with two different RE elements (500 ppm Hf + 500 ppm La) was studied. The kinetics of oxygen grain‐boundary (GB) transport were studied at 1400°C using metallic nickel particles as markers. The results were compared with data obtained on the corresponding singly doped compositions; alumina‐500 ppm La, and alumina‐500 ppm Hf. The results showed that singly doping with La did not have any benefit compared to undoped alumina, whereas singly doping with Hf resulted in a slowing of transport by a factor of ~7. The behavior of the codoped sample was very similar to that of the singly doped Hf composition. For all the studied compositions, atomic scale characterization using high‐angle annular dark‐field scanning transmission electron microscopy and atom probe tomography (APT) revealed strong segregation of the dopant ions to the alumina grain boundaries. In the codoped sample, APT revealed evidence of oxygen excess and aluminum depletion at the GB core.

Atomic scale characterization of complex stacking faults and their configurations in cold deformed Fe42Mn38Co10Cr10 high-entropy alloy

Atomic-resolution scanning transmission electron microscopy is used to study stacking faults and their configurations formed in cold deformed samples of a face-centered cubic (FCC) Fe42Mn38Co10Cr10 (at.%) high-entropy alloy (HEA). It is found that the deformed microstructures contain at least four types of intrinsic stacking faults and two types of extrinsic stacking faults that are bounded by different partial dislocations, including Shockley, Frank and unusual 1/6<411> partials. Additionally, seventeen types of stacking fault configurations are also found, including the well-known Lomer-Cottrell lock, Hirth lock and faulted dipole, the rarely reported intrinsic-extrinsic fault bend, and thirteen hitherto unreported configurations containing different stacking faults and partial dislocations. Among these seventeen configurations, fifteen of them are constructed by conjugate stacking faults connecting at their edges, with a stair-rod partial belonging to 1/6<110>, 1/3<110> or 1/3<100> lying at every joint point of the stacking faults; and the rest two configurations have two intersecting stacking faults but contain different stair-rod dipoles at the intersection. Fourteen of these configurations are all comprised exclusively of intrinsic stacking faults, while three of them consist of a mixture of intrinsic and extrinsic stacking faults. Moreover, three out of the seventeen configurations contain surprisingly a Frank partial at one edge, in contrast to previously reported configurations that are all bounded exclusively by Shockley partials. Based on Burgers vector analysis, the formation mechanisms of the six types of stacking faults and the seventeen types of configurations are proposed and discussed.

Phase decomposition in a nanocrystalline CrCoNi alloy

Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it.

Defect-Selective Charge-Density-Wave Condensation in 2H-NbSe_{2}.

Defects have been known to substantially affect quantum states of materials including charge density wave (CDW). However, the microscopic mechanism of the influence of defects is often elusive due partly to the lack of atomic scale characterization of defects themselves. We investigate native defects of a prototypical CDW material 2H-NbSe_{2} and their microscopic interaction with CDW. Three prevailing types of atomic scale defects are classified by scanning tunneling microscope, and their atomic structures are identified by density functional theory calculations as Se vacancies and Nb intercalants. Above the transition temperature, two distinct CDW structures are found to be induced selectively by different types of defects. This intriguing phenomenon is explained by competing CDW ground states and local lattice strain fields induced by defects, providing a clear microscopic mechanism of the defect-CDW interaction.