High Angle Annular Dark Field Research Articles

Phase transformation and radiation resistance of B-site high entropy pyrochlores

Two high entropy pyrochlores (HEPs), Gd2(Ti0.2Zr0.2Sn0.2Hf0.2Ta0.2)2O7 and Gd2(Ti0.2Zr0.2Sn0.2Hf0.2Nb0.2)2O7 have been synthesized and irradiated with 800 keV Kr2+ ions and examined by in situ transmission electron microscopy (TEM). The irradiation-induced order-to-disorder phase transformation from pyrochlore to the fluorite structure in these two HEPs and Gd2Sn2O7 was observed by using selected area electron diffraction (SAED), high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive spectroscopy (EDS) mapping. The cation antisite defect (CAD) formation energy and the electron localization function (ELF) were calculated using density functional theory (DFT). Both the experimental results and calculated results suggest that the HEPs exhibit a higher resistance to phase transformation and amorphization than Gd2Sn2O7.

Compositionally tailored order-disorder of cation-anion sublattices in Cu2Te1-xSx solid solution thermoelectric materials

High performance thermoelectric Cu2Te1-xSx solid solutions can surprisingly be formed over a wide compositional range even with big atomic size difference between S and Te. The Cu sublattice has been found to be partially ordered and even shows amorphous substructures in some compositions, while S/Te is considered to form a chemically disordered yet crystalline sublattice. However, precise order-disorder configurations of cation-anion sublattices and their evolution upon compositional change still remain unclear. Meanwhile, the appearance of diffuse reflections on the electron diffraction patterns suggests occurrence of short-range order whose structures are not yet solved. Here, atomic structures of Cu2Te1-xSx (x = 0.1, 0.2, 0.4, 0.5, 0.6 and 0.8) have been investigated by transmission electron microscopy in both real and reciprocal spaces. The high-angle annular dark field atomic imaging indicates an approximate hexagonal stacking for S/Te atoms in all solution compositions. However, location of Cu atoms in numerous interstices of the chalcogen sublattice varies by S/Te atomic ratio, leading to change in order-disorder configurations of Cu or even nanoscale phase separation. Diffuse scattering with different geometries are identified to arise from chemical short-range order in forms of sulphur-rich atomic layers on (0001) and lath-like tellurium aggregation on {10-10}. The as-observed compositionally tailored solid solution structures and short-range order structures are found to be strongly correlated to the electron and phonon transportation in Cu2Te1-xSx. The present work indicates a promising strategy for designing solid solutions having large atomic-size mismatch which may generate abundant order/disorder atomic configurations hence greatly improve the thermoelectric properties.

STEM analysis of deformation and B distribution In nanosecond laser ultra-doped Si1−x B x

We report on the structural properties of highly B-doped silicon (up to 10 at.% of active doping) realised by nanosecond laser doping. The crystalline quality, lattice deformation and B distribution profile of the doped layer are investigated by scanning transmission electron microscopy followed by high-angle annular dark field contrast studies and geometrical phase analysis, and compared to the results of secondary ions mass spectrometry and Hall measurements. When increasing the active B concentration above 4 at.%, the fully strained, perfectly crystalline, Si:B layer starts showing dislocations and stacking faults. These only disappear around 8 at.% when the Si:B layer is well accommodated to the substrate. With increasing B incorporation, an increasing number of small precipitates is observed, together with filaments with a higher active B concentration and stacking faults. At the highest concentrations studied, large precipitates form, related to the decrease of active B concentration. The structural information, defect type and concentration, and active B distribution are connected to the initial increase and subsequent gradual loss of superconductivity.

Hardening effect and precipitation evolution of an isothermal aged Mg-Sm based alloy

The age-hardening behavior and precipitation evolution of an isothermal aged Mg−5Sm−0.6Zn−0.5Zr (wt.%) alloy have been systematically investigated by means of transmission electron microscopy (TEM) and atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The Vickers hardness of the present alloy increases first and then decreases with ageing time. The sample aged at 200 °C for 10 h exhibits a peak-hardness of 90.5 HV. In addition to the dominant β0′ precipitate (orthorhombic, a = 0.642 nm, b = 3.336 nm and c = 0.521 nm) formed on {11-20}α planes, a certain number of γ” precipitate (hexagonal, a = 0.556 nm and c = 0.431 nm) formed on basal planes are also observed in the peak-aged alloy. Significantly, the basal γ” precipitate is more thermostable than prismatic β0′ precipitate in the present alloy. β0′ precipitates gradually coarsened and were even likely to transform into β1 phase (face centered cubic, a = 0.73 nm) with the increase of ageing time, which accordingly led to a gradual decrease in number density of precipitates and finally resulted in the decreased hardness and mechanical property in the over-aged alloys.

Microstructure evolution and mechanical properties of the Al-Cu-Mg-Ag alloy during non-isothermal aging

In this work, the effect of non-isothermal aging treatment with the heating rate of 20, 40 and 60 °C/h on the precipitation behavior and mechanical properties of the Al-Cu-Mg-Ag alloy were investigated by differential scanning calorimetry (DSC) analysis, transmission electron microscopy (TEM) and high-angle annular dark field – scanning transmission electron microscopy (HAADF-STEM) observation, as well as hardness and tensile tests. The activation energy and Avrami exponent associated with the precipitation of Ω phase were calculated based on the non-isothermal DSC experiments with various heating rates. The size distribution and number density of Ω plates in the α-Al matrix under peak-aged condition were quantitatively counted. Besides, A non-isothermal aging kinetics model proposed was applied to evaluate the heating aging treatment (HAT) process of the Al-Cu-Mg-Ag alloy. Results indicate that HAT modifies the size and density of Ω phase, thereby enhancing the mechanical properties of the Al-Cu-Mg-Ag alloy. HAT promotes the precipitation of Ω phase and shortens the aging time for the alloy to reach the peak-aged condition. It is interesting that the size distribution of Ω plates shifts to larger values with the increase of the heating rate during the HAT process.

Nanoscale chemical and structural investigation of solid solution polyelemental transition metal oxide nanoparticles

Although it has been shown that configurational entropy can improve the structural stability in transition metal oxides (TMOs), little is known about the oxidation state of transition metals under random mixing of alloys. Such information is essential in understanding the chemical reactivity and properties of TMOs stabilized by configurational entropy. Herein, utilizing electron energy loss spectroscopy (EELS) technique in an aberration-corrected scanning transmission electron microscope (STEM), we systematically studied the oxidation state of binary (Mn,Fe)3O4, ternary (Mn, Fe, Ni)3O4, and quinary (Mn, Fe, Ni, Cu, Zn)3O4 solid solution polyelemental transition metal oxides (SSP-TMOs) nanoparticles. Our findings show that the random mixing of multiple elements in the form of solid solution phase not only promotes the entropy stabilization but also results in stable oxidation state in transition metals spanning from binary to quinary transition metal oxide nanoparticles.

Structure of V-defects in long wavelength GaN-based light emitting diodes

The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990s. Strategic use of V-defects in pre-quantum well superlattices or equivalent preparation layers has enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semi-polar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The “large” V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector b=±a±c) character threading dislocations. In addition, “small” V-defects often form in the multi-quantum well region and are believed to be deleterious to high-efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast transmission electron microscopy that during V-defect filling, the threading dislocation, which runs up the center of the V-defect, will “bend” onto one of the six {101¯1} semi-polar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs.

Novel strategy for space group determination in real space

Traditional space group determination methods are all in reciprocal space, which involves ambiguous identification on some space groups which have glide plane and screw axes. The novel strategy herein for space group determination in real space is based on the atom resolution high angle annular dark field (HAADF) technology. Three HAADF images in three specific crystal zone axes are needed at most. The proposed strategy for space group determination is easy and effective.

Exploiting the interaction between halloysite and charged PNAs for their controlled release.

The design and development of nanomaterials that could be used in nanomedicine are of fundamental importance to obtain smart nanosystems for the treatment of several diseases. Halloysite, because of its interesting features, represents a suitable nanomaterial for the delivery of different biologically active species. Among them, peptide nucleic acids (PNAs) have attracted considerable attention in recent decades for their potential applications in both molecular antisense diagnosis and as therapeutic agents, although up to now, the actual clinical applications have been very limited. Herein we report a systematic study on the supramolecular interaction of three differently charged PNAs with halloysite. Understanding the interaction mode of charged molecules with the clay surfaces represents a key factor for the future design and development of halloysite based materials which could be used for the delivery and subsequent intracellular release of PNA molecules. Thus, three different PNA tetramers, chosen as models, were synthesized and loaded onto the clay. The obtained nanomaterials were characterized using spectroscopic studies and thermogravimetric analysis, and their morphologies were studied using high angle annular dark field transmission electron microscopy (HAADF/STEM) coupled with Energy Dispersive X-ray spectroscopy (EDX). The aqueous mobility of the three different nanomaterials was investigated by dynamic light scattering (DLS) and ζ-potential measurements. The release of PNA tetramers from the nanomaterials was investigated at two different pH values, mimicking physiological conditions. Finally, to better understand the stability of the synthesized PNAs and their interactions with HNTs, molecular modelling calculations were also performed. The obtained results showed that PNA tetramers interact in different ways with HNT surfaces according to their charge which influences their kinetic release in media mimicking physiological conditions.

Atomic scale analysis of Zn2+ storage in robust tunnel frameworks.

Realizing rapid and reversible Zn2+ storage at the cathode is imperative for the advancement of aqueous Zn-ion batteries (ZIBs), which offer an excellent option for large-scale electrochemical energy storage. However, owing to limitations of the structural stability of previously investigated frameworks, the Zn2+ storage processes remain unclear, thus hindering progress towards the above goal. Herein, we present the novel application of MoVTe oxide with an M1 phase (MVT-M1) as a potential cathode material for ZIBs. MVT-M1 features broad and robust tunnels that facilitate reversible Zn2+ insertion/extraction during cycling, as well as rich redox centers (Mo, V, and Te) to aid in charge redistribution, resulting in good performances in ZIBs. The exceptional resilience of MVT-M1 to high-energy electron beams allows for direct observation of Zn2+ insertion/extraction at the atomic scale within the tunnels for the first time using high-angle annular dark field scanning transmission electron microscopy; the storage location of zinc ions within the cathode is accurately determined layer by layer from the surface to the bulk phase by employing time-of-flight secondary ion mass spectrometry. Additionally, solvent molecules (H2O and methanol) are also found inside the tunnels along with Zn2+. Due to the broader heptagonal tunnels and Te ions in the hexagonal tunnels, MVT-M1 exhibits good cycling stability, outperforming MoVTe oxide with the M2 phase (no heptagonal tunnels) and MoV oxide with the M1 phase (no Te). These findings hold significant importance in advancing our understanding of the Zn2+ storage mechanism and enable the design of novel materials specifically optimized for efficient Zn2+ storage.

Lead-chlorine synergistic immobilization mechanism in municipal solid waste incineration fly ash (MSWIFA)-based magnesium potassium phosphate cement.

The high chlorine (Cl) and lead (Pb) content characteristics of municipal solid waste incineration fly ash (MSWIFA) pose environmental risks and hinder resource utilization. Herein, an MSWIFA-based magnesium potassium phosphate cement (MKPC) preparation strategy was developed, which allowed the MSWIFA recycling and the Pb-Cl synergistic immobilization without the washing pretreatment. The compressive strength of the resulting 10wt% MSWIFA-based MKPC was 28.44MPa, with over 99.2% reduction in leaching toxicity of Pb and Cl. The high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) and X-ray absorption spectroscopy (XAS) analyzes showed that Pb, phosphate and Cl- formed Pb5(PO4)3Cl in MKPC. In-situ X-ray diffraction (XRD) tests showed that Pb3(PO4)2 was gradually transformed to Pb5(PO4)3Cl through a dissolution-precipitation process. The formation energy, Bader charge, charge density difference and density of states (DOS) of Pb5(PO4)3Cl were analyzed by first-principles calculations, confirming that Pb5(PO4)3Cl was more thermodynamically stable than Pb3(PO4)2 and PbCl2 and that electronic interactions between Pb-p, O-p, P-p and Cl-p orbits were the origin of Pb-Cl synergistic immobilization. This work provides a new strategy for the resource utilization of MSWIFA without washing pretreatment, and provides an in-depth understanding of the Pb-Cl synergistic immobilization mechanism.

Single-atom thermoelectric materials: a new opportunity

Single-atom materials show great potential in the field of thermoelectrics due to their distinguishing features such as maximum atom utilization efficiency, unique electronic structure, guest−host interactions, and a tunable coordination environment. Herein, the concept of single-atom thermoelectric materials is presented. Thereafter, we introduce characterization techniques including high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption fine structure spectroscopy (XAFS) for identifying the specific coordination environment of single atoms. Furthermore, a typical work demonstrating the effect of single atoms on the thermoelectric transport properties of Bi2S3 is provided. Finally, we propose possible future development paths for single-atom thermoelectric materials. This paper provides a reference for further studies of single-atom thermoelectric materials.

The element occupation preference of μ phase in ternary Ni-Cr-Mo model superalloy

Topologically close-packed (TCP) phases seriously endanger the properties of Superalloy. Investigating the microstructure of the TCP phases will provide a crystallographic reference for the design of superalloys with better properties. The μ phase is a common TCP phase in superalloys. It belongs to the space group R-3m, and has a hexagonal structure with five Wyckoff sites of 3a, 18h, 6c1, 6c2 and 6c3. In this paper, occupation of the three kinds of atoms at five Wyckoff sites of the μ phase in Ni-Cr-Mo model superalloy was determined using aberration-corrected High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), QSTEM software simulation and first-principles calculations. It was found that Mo atoms with larger size and smaller electronegativity tend to occupy 6c3, 6c1 and 6c2 sites in order of priority. Ni atoms mainly occupy the sites with coordination number (CN) of 12 at 3a and 18 h, and 3a sites contain more Cr atoms than 18 h sites.

TEM study of ∼PbCr2S4 columnar misfit phases

The disordered structure of the columnar misfit ​∼ ​PbCr2S4 phase (∼ character indicates that it is close to the stoichiometric composition but slightly away) has been revealed by means of transmission electron microscopy (TEM) varied techniques such as selected area electron diffraction (SAED), high-resolution TEM (HRTEM), dark-field TEM and high-angle annular dark field (HAADF) in scanning-transmision (STEM) mode. The structure consists of a hexagonal framework (Cr21S36), with unit cell parameters a0 ​= ​b0 ​= ​21.407 (1) Å and c0 ​= ​3.476 (1), and with hexagonal (Pb6Cr2S6) and trigonal columns (Pb3S) with the same a and b parameters and different c: c3 ​= ​3.98 Å; c6 ​= ​5.62 ​Å, measured by electron diffraction. While the framework exhibit no crystalline disorder, the arrangement of the two types of columns is disordered. This disorder is shown by electron diffraction as diffuse scattering planes which are perpendicular to the framework c axis and parallel to the a∗b∗ plane. While the disorder between hexagonal columns is due to the lack of correlation between different columns, we observe in the trigonal columns also the presence of intra-columnar atomic disorder. X-ray wavelength dispersive spectroscopy (XWDS) analyses produce Pb0.81(1)Cr2S3.97(6) composition. TEM imaging techniques suggest lack of correlation between trigonal columns as the cause for the observed disorder as well as inside-the-column structural disorder in Pb3S trigonal columns.

Blue-green luminescence properties and charge compensation of Ca9MgLi(PO4)7 doped with Tb3+ and alkali metal ions

Abstract Ca9MgLi(PO4)7: 4%Tb3+, 4%R+ (R = Li, Na, K) phosphors were synthesized by a solid-state reaction method. The crystal structure, morphology, composition and luminescent properties of as-prepared samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrometer (EDS), high-angle annular dark field (HAADF) and spectroscopy technique. The effect of a small concentration of charge compensators like Li+, Na+, K+ on Ca9MgLi(PO4)7: Tb3+ phosphor has also investigated. Ca9MgLi(PO4)7: Tb3+, R+ (R = Li, Na, K) exhibit superior blue-green luminescence around 5D4 → 7F5 (543 nm) to Ca9MgLi(PO4)7: Tb3+ and can be effectively excited by 370 nm light, which implies that efficient charge compensation can promote the emitting of Tb3+ in Ca9MgLi(PO4)7. The CIE coordinates and decay lifetimes of typical samples were also studied. The Ca9MgLi(PO4)7: Tb3+, R+ (R = Li, Na, K) have promising application as a blue-green phosphor for near-ultraviolet chip excited white LEDs.

3D Nanoprinting of All-Metal Nanoprobes for Electric AFM Modes.

3D nanoprinting via focused electron beam induced deposition (FEBID) is applied for fabrication of all-metal nanoprobes for atomic force microscopy (AFM)-based electrical operation modes. The 3D tip concept is based on a hollow-cone (HC) design, with all-metal material properties and apex radii in the sub-10 nm regime to allow for high-resolution imaging during morphological imaging, conductive AFM (CAFM) and electrostatic force microscopy (EFM). The study starts with design aspects to motivate the proposed HC architecture, followed by detailed fabrication characterization to identify and optimize FEBID process parameters. To arrive at desired material properties, e-beam assisted purification in low-pressure water atmospheres was applied at room temperature, which enabled the removal of carbon impurities from as-deposited structures. The microstructure of final HCs was analyzed via scanning transmission electron microscopy-high-angle annular dark field (STEM-HAADF), whereas electrical and mechanical properties were investigated in situ using micromanipulators. Finally, AFM/EFM/CAFM measurements were performed in comparison to non-functional, high-resolution tips and commercially available electric probes. In essence, we demonstrate that the proposed all-metal HCs provide the resolution capabilities of the former, with the electric conductivity of the latter onboard, combining both assets in one design.

Different atomic contrasts in HAADF images and EELS maps of rutile TiO2.

High-angle annular dark-field (HAADF) imaging and elemental mapping at the atomic scale by scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) are widely used for material characterization, in which quantitative understanding of the contrast of the image is required. Here, we report an unexpected image contrast in the elemental mapping of rutile TiO2, where the Ti L2,3 map shows an anisotropic elliptical shape that extends along the long axis in the octahedral structure, while the atomic contrast of Ti columns in the HAADF image is almost circular. Multi-slice simulation reveals that unique electron channeling related to the rutile structure and the difference of the potentials between HAADF and EELS cause the different atomic contrasts in the two images.

Occupancy of lattice positions probed by X-ray photoelectron diffraction: A case study of tetradymite topological insulators

Occupancy of different structural positions in a crystal lattice often seems to play a key role in material properties. Several experimental techniques have been developed to uncover this issue, all of them being mostly bulk sensitive. However, many materials including topological insulators (TIs), which are among the most intriguing modern materials, are intended to be used in devices as thin films, for which the sublattice occupancy may differ from the bulk. One of the possible approaches to occupancy analysis is X-ray Photoelectron Diffraction (XPD), a structural method in surface science with chemical sensitivity. We applied this method in a case study of Sb2(Te1-xSex)3 mixed crystals, which belong to prototypical TIs. We used high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) as a reference method to verify our analysis. We revealed that the XPD data for vacuum cleaved bulk crystals are in excellent agreement with the reference ones. Also, we demonstrate that the anion occupancy near a naturally formed surface can be rather different from that of the bulk. The present results are relevant for a wide range of compositions where the system remains a topological phase, as we ultimately show by probing the transiently occupied topological surface state above the Fermi level by ultrafast photoemission.

Synthesis of the Elusive Doublewall Nanotubes and Nanocones(Horns) of MoS2 via Focused Solar Ablation

AbstractThe synthesis of fundamentally small MoS2 nanotubes and nanocones(horns) that have proven elusive in prior studies has been achieved via ablation of a precursor mixture of crystallites of MoS2 + MoO3 by highly concentrated solar radiation. The special far‐from‐equilibrium conditions achieved in the solar furnace prove conducive to the generation of these singular nanostructures. Extensive electron microscopy and characterization results (transmission electron microscopy (TEM), electron diffraction (ED), X‐ray diffraction (XRD), scanning TEM (STEM), and high angle annular dark field (HAADF)) reveal a range of nanoparticle shapes and sizes based on which reaction mechanisms are proposed. Molecular dynamics simulations indicate that the sizable thermal fluctuations intrinsically produced in the high‐temperature solar reactor soften the MoS2 nanostructures, yielding corrugated layers that favor nanostructures with only a few layers, in agreement with the experimental observations.

Carbon encapsulated iron oxide for simultaneous Fenton degradation and adsorption of cationic and anionic dyes from water

Adsorption and Fenton oxidation processes are considered the most effective ways to treat industrial dye effluent. However, the separation of adsorbent materials and the leaching of secondary pollutants are big hurdles. To resolve these issues, here, we have developed a unique hybrid structure (Fe3O4 @C) by encapsulating iron oxide into carbon shells using olive mill waste in a one-step process. The resulting unique core-shell Fe3O4 @C structure possessed both tunned surface functional groups (carboxylic, carbonyl, and amines) and magnetic properties, which can be easily separated from treated water using a magnet. Upon using Fe3O4 @C as a catalyst, it simultaneously removed/ degraded both Congo red (CR) and methylene blue (MB) up to 94% through Fenton oxidation. ICP-MS analysis detected negligible (6.6 ppb) iron leaching even in Fenton degradation, which is far lower than the allowable leaching limit of 300 ppb showing the efficacy of the developed structure. This is due to the core-shell structure where Fe3O4 remains active inside the core due to dual protection by encapsulating inside a semicrystalline carbon shell, which is then protected by a porous carbon matrix. This double protection assists high adsorption and maintains efficient charge transfer that favors high catalytic activity with limited iron leaching. Further to prove this Fe3O4 @C was used in 5 consecutive adsorption cycles, where it removed 94.4% MB after 5th cycle and even preserved the pristine core-shell structure and chemical bonds as indicated by ex-situ Energy-dispersive X-ray spectroscopy, High-angle annular dark field, High-angle annular bright field, and X-ray photoelectron spectroscopy analysis. Hence, this study successfully showed that olive waste could be converted into a functional material that can further be used at a large scale for pollutant removal from wastewater.