Direct Atomic-scale Imaging Research Articles

Probing Defectivity Beneath the Hydrocarbon Blanket in 2D hBN Using TEM-EELS.

The controlled creation and manipulation of defects in 2D materials has become increasingly popular as a means to design and tune new material functionalities. However, defect characterization by direct atomic-scale imaging is often severely limited by surface contamination due to a blanket of hydrocarbons. Thus, analysis techniques that can characterize atomic-scale defects despite the contamination layer are advantageous. In this work, we take inspiration from X-ray absorption spectroscopy and use broad-beam electron energy loss spectroscopy (EELS) to characterize defect structures in 2D hexagonal boron nitride (hBN) based on averaged fine structure in the boron K-edge. Since EELS is performed in a transmission electron microscope (TEM), imaging can be performed in-situ to assess contamination levels and other factors such as tears in the fragile 2D sheets, which can affect the spectroscopic analysis. We demonstrate the TEM-EELS technique for 2D hBN samples irradiated with different ion types and doses, finding spectral signatures indicative of boron-oxygen bonding that can be used as a measure of sample defectiveness depending on the ion beam treatment. We propose that even in cases where surface contamination has been mitigated, the averaging-based TEM-EELS technique can be useful for efficient sample surveys to support atomically resolved EELS experiments.

Ingenious Architecture and Coloration Generation in Enamel of Rodent Teeth.

Teeth exemplify architectures comprising an interplay of inorganic and organic constituents, resulting in sophisticated natural composites. Rodents (Rodentia) showcase extraordinary adaptations, with their continuously growing incisors surpassing human teeth in functional and structural optimizations. In this study, employing state-of-the-art direct atomic-scale imaging and nanoscale spectroscopies, we present compelling evidence that the release of material from ameloblasts and the subsequent formation of iron-rich enamel and surface layers in the constantly growing incisors of rodents are complex orchestrated processes, intricately regulated and independent of environmental factors. The synergistic fusion of three-dimensional tomography and imaging techniques of etched rodent́s enamel unveils a direct correlation between the presence of pockets infused with ferrihydrite-like material and the acid resistant properties exhibited by the iron-rich enamel, fortifying it as an efficient protective shield. Moreover, observations using optical microscopy shed light on the role of iron-rich enamel as a microstructural element that acts as a path for color transmission, although the native color remains indistinguishable from that of regular enamel, challenging the prevailing paradigms. The redefinition of "pigmented enamel" to encompass ferrihydrite-like infusion in rodent incisors reshapes our perception of incisor microstructure and color generation. The functional significance of acid-resistant iron-rich enamel and the understanding of the underlying coloration mechanism in rodent incisors have far-reaching implications for human health, development of potentially groundbreaking dental materials, and restorative dentistry. These findings enable the creation of an entirely different class of dental biomaterials with enhanced properties, inspired by the ingenious designs found in nature.

Atomic-Scale Time-Resolved Imaging of Krypton Dimers, Chains and Transition to a One-Dimensional Gas.

Single-atom dynamics of noble-gas elements have been investigated using time-resolved transmission electron microscopy (TEM), with direct observation providing for a deeper understanding of chemical bonding, reactivity, and states of matter at the nanoscale. We report on a nanoscale system consisting of endohedral fullerenes encapsulated within single-walled carbon nanotubes ((Kr@C60)@SWCNT), capable of the delivery and release of krypton atoms on-demand, via coalescence of host fullerene cages under the action of the electron beam (in situ) or heat (ex situ). The state and dynamics of Kr atoms were investigated by energy dispersive X-ray spectroscopy (EDS), electron energy loss spectroscopy (EELS), and X-ray photoelectron spectroscopy (XPS). Kr atom positions were measured precisely using aberration-corrected high-resolution TEM (AC-HRTEM), aberration-corrected scanning TEM (AC-STEM), and single-atom spectroscopic imaging (STEM-EELS). The electron beam drove the formation of 2Kr@C120 capsules, in which van der Waals Kr2 and transient covalent [Kr2]+ bonding states were identified. Thermal coalescence led to the formation of longer coalesced nested nanotubes containing more loosely bound Krn chains (n = 3-6). In some instances, delocalization of Kr atomic positions was confirmed by STEM analysis as the transition to a one-dimensional (1D) gas, as Kr atoms were constrained to only one degree of translational freedom within long, well-annealed, nested nanotubes. Such nested nanotube structures were investigated by Raman spectroscopy. This material represents a highly compressed and dimensionally constrained 1D gas stable under ambient conditions. Direct atomic-scale imaging has revealed elusive bonding states and a previously unseen 1D gaseous state of matter of this noble gas element, demonstrating TEM to be a powerful tool in the discovery of chemistry at the single-atom level.

Direct atomic-scale observation of thermally driven grain-boundary migration associated with triple-junction motion

The internal structure of metals is defined by a complex network of grain boundaries (GBs) and their intersections---triple junctions (TJs). Heat or stress forces GBs to migrate, thus altering many physical properties of the material, and TJs play a key role in this migration as they control GB edges. The GB-TJ interaction strongly depends on the local crystallography, so it can only be properly described at the atomic scale. Although recent direct atomic-scale imaging of GB migration provides some insight into their dynamic behavior, the nanoscale details of the coupled motion of TJs and GBs in a natural nanocrystalline system remain elusive. Here, using in situ transmission electron microscopy (TEM) imaging, we describe the dynamics of the GB-TJ system in a nanocrystalline Co film with mixed fcc and hcp crystal phases during thermal annealing at both the mesoscale and the atomic scale. Our observations reveal that GB evolution proceeds through destabilization and displacement of TJs followed by GB relaxation within the new TJ geometry, which modifies the dihedral angles. This process is mediated by atomic-scale disconnection dynamics, where TJs serve as their sources and sinks. These findings advance our understanding of grain evolution in metal nanostructures, which is crucial for engineering their properties.

Direct Atomic-Scale Insight into the Precipitation Formation at the Lanthanum Hydroxide Nanoparticle/Solution Interface

Understanding precipitation formation at lanthanum hydroxide (La(OH)3) nanoparticle-solution interfaces plays a crucial role in catalysis, adsorption, and electrochemical energy storage applications. Liquid-phase transmission electron microscopy enables powerful visualization with high resolution. However, direct atomic-scale imaging of the interfacial metal (hydro)oxide nanostructure in solutions has been a major challenge due to their beam-driven dissolution. Combining focused ion beam and aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, we present an atomic-scale study of precipitation formation at La(OH)3 nanoparticle interfaces after reaction with phosphate. The structure transformation is observed to occur at high- and low-crystalline La(OH)3 nanoparticle surfaces. Low-crystalline La(OH)3 mostly transformed and high-crystalline ones partly converted to LaPO4 precipitations on the outer surface. The long-term structure evolution shows the low transformation of high-crystalline La(OH)3 nanoparticles to LaPO4 precipitation. Because precipitation at solid-solution interfaces is common in nature and industry, these results could provide valuable references for their atomic-scale observation.

Structural investigation of the temperature-stable relaxor dielectric Ba0.8Ca0.2TiO3-Bi(Mg0.5Ti0.5)O3

Aberration corrected scanning transmission electron microscopy (STEM) and electron diffraction have been used to disclose local structure and nano-chemistry in a Ca modified BaTiO3-Bi(Mg0.5Ti0.5)O3 relaxor dielectric ceramic which exhibits high and near-invariant relative permittivity over a wide temperature range. High-resolution, synchrotron X-ray diffraction indicated a globally cubic structure (Pm3̅m), but direct atomic-scale imaging by STEM revealed local tetragonal distortions. Nanopolar clusters were identified from B-site atomic displacement vectors measured relative to oxygen ion positions along < 100 > and < 110 > zone axes of integrated differential phase contrast (iDPC) STEM images, highlighting cluster sizes of 2–5 nm. Chemical analysis by STEM-energy dispersive X-ray spectroscopy and full pattern refinements of X-ray powder diffraction data each implied high levels of Bi vacancies within the matrix. The possibility that these A-site vacancies modulate the nanopolar structure and promote flattening of the permittivity-temperature response in this class of dielectric is discussed.

Revealing the Origin of Low-Temperature Activity of Ni-Rh Nanostructures during CO Oxidation Reaction with Operando TEM.

In bimetallic heterostructured nanoparticles (NPs), the synergistic effect between their different metallic components leads to higher catalytic activity compared to the activity of the individual components. However, how the dynamic changes through which these NPs adopt catalytically active structures during a reaction and how the restructuring affects their activity are largely unknown. Here, using operando transmission electron microscopy, structural changes are studied in bimetallic Ni–Rh NPs, comprising of a Ni core whose surface is decorated with smaller Rh NPs, during a CO oxidation reaction. The direct atomic‐scale imaging reveals that, under O2‐rich conditions, Ni core partially transforms into NiO, forming a (Ni+NiO)–Rh hollow nanocatalyst with high catalytic activity. Under O2‐poor conditions, Rh NPs alloy with the surface of the core to form a NiRh‐alloy surface, and the NPs display significantly lower activity. The theoretical calculations indicate that NiO component that forms only under O2‐rich conditions enhances the activity by preventing the CO poisoning of the nanocatalysts. The results demonstrate that visualizing the structural changes during reactions is indispensable in identifying the origin of catalytic activity. These insights into the dynamic restructuring of NP catalysts under a reactive environment are critical for the rational design of high‐performance nanocatalysts.

Evolution of the conductive filament system in HfO2-based memristors observed by direct atomic-scale imaging

The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO2 has been accepted as one of the most promising resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO2-based memristors. Here, the conductive filament system in the amorphous HfO2-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf6O and its crystalline surroundings (monoclinic or tetragonal HfOx). The phase of the HfOx shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO2 is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO2-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.

Shearing mechanisms of co-precipitates in IN718

The Ni-base superalloy 718 is the most widely used material for turbomachinery in the aerospace industry and land-based turbines. Although the relationship between processing and the resulting properties is well known, an understanding of the specific deformation mechanisms activated across its application temperature range is required to create more mechanistically accurate property models. Direct atomic-scale imaging observations with high angle annular dark-field scanning transmission electron microscopy, complemented by phase-field modeling informed by generalized stacking fault surface calculations using density functional theory, were employed to understand the shear process of γ″ and γ′/γ″ co-precipitates after 1% macroscopic strain at lower temperature (ambient and 427 °C). Experimentally, intrinsic stacking faults were observed in the γ″, whereas the γ′ was found to exhibit anti-phase boundaries or superlattice intrinsic stacking faults. Additionally, the atomically flat γ′/γ″ interfaces in the co-precipitates were found to exhibit offsets after shearing, which can be used as tracers for the deformation events. Phase-field modeling shows that the developing fault-structure is dependent on the direction of the Burgers vector of the a2〈110〉 matrix dislocation (or dislocation group) due to the lower crystal symmetry of the γ″ phase. The interplay between γ′ and γ″ phases results in unique deformation pathways of the co-precipitate and increases the shear resistance. Consistent with the experimental observations, the simulation results indicate that complex shearing mechanisms are active in the low-temperature deformation regime and that multiple a2〈110〉 dislocations of non-parallel Burgers vectors may be active on the same slip plane.

Direct confirmation of the dopant site in indium-doped SrTiO3 photocatalyst via atomic-scale analytical transmission electron microscopy imaging

Confirming the dopant site of In3+-doped SrTiO3 (In–STO) is essential to reveal the mechanism of its photocatalytic activity. In a previous study, x-ray absorption spectroscopic analysis and theoretical investigations were performed to discuss the dopant site, and In3+–Ti4+ substitution was proposed. However, direct confirmation of the In3+ dopant site has not yet been reported. Here, we performed direct atomic-scale imaging of In–STO crystals via analytical transmission electron microscopy and revealed the dopant site based on real-space elemental mapping. The Ti and Sr sites in the SrTiO3 crystal lattice were well identified by atomic column elemental mapping using energy dispersive x-ray spectroscopy (EDS). The EDS signal of indium has a stronger intensity at the Ti site than at the Sr site, based on the total analysis of each Ti and Sr atomic column. By applying principal component analysis on the raw EDS spectral imaging data cube, the indium site was clearly imaged; it completely fit into the Ti atomic column positions. These results provide direct evidence of In–Ti substitution in In-STO photocatalysts.

Direct Imaging of Kinetic Pathways of Atomic Diffusion in Monolayer Molybdenum Disulfide.

Direct observation of atomic migration both on and below surfaces is a long-standing but important challenge in materials science as diffusion is one of the most elementary processes essential to many vital material behaviors. Probing the kinetic pathways, including metastable or even transition states involved down to atomic scale, holds the key to the underlying physical mechanisms. Here, we applied aberration-corrected transmission electron microscopy (TEM) to demonstrate direct atomic-scale imaging and quasi-real-time tracking of diffusion of Mo adatoms and vacancies in monolayer MoS2, an important two-dimensional transition metal dichalcogenide (TMD) system. Preferred kinetic pathways and the migration potential-energy landscape are determined experimentally and confirmed theoretically. The resulting three-dimensional knowledge of the atomic configuration evolution reveals the different microscopic mechanisms responsible for the contrasting intrinsic diffusion rates for Mo adatoms and vacancies. The new insight will benefit our understanding of material processes such as phase transformation and heterogeneous catalysis.

Twin boundary-assisted lithium ion transport.

With the increased need for high-rate Li-ion batteries, it has become apparent that new electrode materials with enhanced Li-ion transport should be designed. Interfaces, such as twin boundaries (TBs), offer new opportunities to navigate the ionic transport within nanoscale materials. Here, we demonstrate the effects of TBs on the Li-ion transport properties in single crystalline SnO2 nanowires. It is shown that the TB-assisted lithiation pathways are remarkably different from the previously reported lithiation behavior in SnO2 nanowires without TBs. Our in situ transmission electron microscopy study combined with direct atomic-scale imaging of the initial lithiation stage of the TB-SnO2 nanowires prove that the lithium ions prefer to intercalate in the vicinity of the (101̅) TB, which acts as conduit for lithium-ion diffusion inside the nanowires. The density functional theory modeling shows that it is energetically preferred for lithium ions to accumulate near the TB compared to perfect neighboring lattice area. These findings may lead to the design of new electrode materials that incorporate TBs as efficient lithium pathways, and eventually, the development of next generation rechargeable batteries that surpass the rate performance of the current commercial Li-ion batteries.

Dislocation motion and grain boundary migration in two-dimensional tungsten disulphide.

Dislocations have a significant effect on mechanical, electronic, magnetic and optical properties of crystals. For a dislocation to migrate in bulk crystals, collective and simultaneous movement of several atoms is needed. In two-dimensional crystals, in contrast, dislocations occur on the surface and can exhibit unique migration dynamics. Dislocation migration has recently been studied in graphene, but no studies have been reported on dislocation dynamics for two-dimensional transition metal dichalcogenides with unique metal-ligand bonding and a three-atom thickness. This study presents dislocation motion, glide and climb, leading to grain boundary migration in a tungsten disulphide monolayer. Direct atomic-scale imaging coupled with atomistic simulations reveals a strikingly low-energy barrier for glide, leading to significant grain boundary reconstruction in tungsten disulphide. The observed dynamics are unique and different from those reported for graphene. Through strain field mapping, we also demonstrate how dislocations introduce considerable strain along the grain boundaries and at the dislocation cores.

Direct Atomic-Scale Imaging of Multistep Phase Transition during the Lithiation of Nanowires by In-Situ (S)TEM

Direct Atomic-Scale Imaging of Multistep Phase Transition during the Lithiation of Nanowires by In-Situ (S)TEM

A Correlative Study of Direct Atomic-Scale Imaging of Hydrogen and Oxygen Interstitials in Niobium Utilized Atom-Probe Tomography and Aberration-Corrected Scanning Transmission Electron Microscopy

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

Nonstoichiometric doping and Bi antisite defect in single crystal Bi2Se3

We studied the defects of Bi${}_{2}$Se${}_{3}$ generated from Bridgman growth of stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size, and transport properties are strongly affected by the types of defects generated. Major defect types of the Bi${}_{\text{Se}}$ antisite and partial Bi${}_{2}$-layer intercalation are identified through combined studies of direct atomic-scale imaging with scanning transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy, x-ray diffraction, and Hall effect measurements. We propose a consistent explanation to the origin of defect type, growth morphology, and transport property.

Direct Atomic-Scale Imaging about the Mechanisms of Ultralarge Bent Straining in Si Nanowires

To safely and reliably use nanowires (NWs) for exploring new functions for different nanodevices, the mechanical properties and structural evolution of the nanowires under external stress become highly important. Large strain (up to 14%) bending experiments of Si NWs were conducted in a high-resolution transmission electron microscope at atomic resolution. The direct dynamic atomic-scale observations revealed that partial and full dislocation nucleation, motion, escape, and interaction were responsible for absorbing the ultralarge strain of up to 14% in bent Si nanowires. The prevalent full dislocation movement and interactions induced the formation of Lomer lock dislocations in the Si NWs. Finally, in contrast to the unlock process of Lomer dislocations that can happen in metallic materials, we revealed that the continuous straining on the Lomer dislocations induced a crystal-amorphous (c-a) transition in Si NWs. Our results provide direct explanation about the ultralarge straining ability of Si at the nanometer scale.

Scanning tunnelling microscopy study on hydrogen removal from Si(0 0 1)-(2 × 1):H surface excited with low-energy electron beams

Direct atomic-scale imaging using scanning tunnelling microscopy (STM) shows the formation of un-terminated single dangling bond (DB) on Si(0 0 1)-(2 × 1):H irradiated with low-energy electron beams below 13 eV. The concentration of DB sites increases initially with excitation dose, followed by saturation at the levels that depend on the beam-current density. It is shown that re-termination of DBs with hydrogen, accelerated under electron irradiation, is responsible for the saturation. We conclude that re-terminating hydrogen is issued from bulk of Si. Quantitative analyses of the results determine the precise Si–H bond rupture cross section to be 1.3 × 10 −20 cm 2, which is constant for electron energy from 2.6 to 12.6 eV. Mechanism of Si–H bond rupture is discussed based on the results.

Direct atomic-scale imaging of ceramic interfaces

Understanding the atomic structure and chemistry of internal interfaces is often critical to developing interface structure–property relationships. Results are presented from several studies in which Z-contrast scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) have been employed to solve the atomic structures of oxide interfaces. The Z-contrast imaging technique directly reveals the projected cation sublattices constituting the interface, while EELS provides chemical and local electronic structure information. Because Z-contrast imaging and EELS can be performed simultaneously, direct correlations between structure and chemistry can be made at the atomic scale. The utility of Z-contrast imaging and EELS is demonstrated in three examples: a ZrO 2 24° [100] symmetric tilt grain boundary, a NiO–cubic ZrO 2 eutectic interface and a Ni–cubic ZrO 2 metal–ceramic interface. The power and versatility of Z-contrast and EELS for solving interface structures in oxide systems is clearly demonstrated in these three material systems.

Direct surface imaging in small metal particles

Atomic-level information about the surfaces of small metal particles has been recorded directly in recent observations with a 600-kV high-resolution electron microscope. Here, we have studied small polycrystalline particles of silver and gold tilted to bring their surfaces parallel to the electron beam. However, unlike previous workers using this normal reflection electron microscopy (REM) configuration, we have used conventional bright field axial imaging thereby considerably facilitating image interpretation. As well as clean, sharp surface images, morphological details of catalytic significance, such as the distribution of surface steps, particle facetting and the nature of surface reconstructions, have been obtained. Moreover, detailed computer simulations confirmed that the electron micrographs can be interpreted in terms of atomic columns and, in particular, established that some micrographs showed, for the first time in a transmission electron microscope (TEM), direct atomic-scale imaging of a reconstructed metal surface.