Local Atomic Scale Research Articles

Impact of interatomic structural characteristics of aluminosilicate hydrate on the mechanical properties of metakaolin-based geopolymer

This study explores the influence of the interatomic structure of sodium aluminosilicate hydrate (N-A-S-H) with varying silica contents on the mechanical properties of metakaolin-based geopolymer. Geopolymer pastes comprising Si/Al ratios between 2.0 and 3.0 were synthesized. A larger number of Si-O-Si linkages compared to Si-O-Al linkages and a higher atomic number density were found in the geopolymers with higher silica contents, which enhanced the compressive strength of the geopolymer pastes up to the optimal Si/Al ratio of 2.5. The paste with a Si/Al = 2.5 exhibited a greater portion of Q4(1Al and 2Al) and denser morphology compared to the other geopolymer pastes. Furthermore, in-situ high-energy synchrotron X-ray scattering experiments were conducted to assess the elastic modulus of the aluminosilicate structure at a local atomic scale. The modulus value in real space decreases with increasing silica contents up to Si/Al = 2.5 and increases with the presence of excessive unreacted silica fume. The modulus value in reciprocal space for the axial and lateral directions both presented a positive value at the geopolymer comprising a Si/Al ratio higher than 2.5, indicating that the load-bearing property of N-A-S-H changed at higher Si/Al ratios. Moreover, the smallest difference between the strains along the axial and lateral directions was detected for the geopolymer with Si/Al = 2.5 in both the real and reciprocal space, owing to the most interconnected and flexible nanostructure, which led to the highest mechanical strength.

Universal basis underlying temperature, pressure and size induced dynamical evolution in metallic glass-forming liquids

The dramatic temperature-dependence of liquids dynamics has attracted considerable scientific interests and efforts in the past decades, but the physics of which remains elusive. In addition to temperature, some other parameters, such as pressure, loading and size, can also tune the liquid dynamics and induce glass transition, which makes the situation more complicated. Here, we performed molecular dynamics simulations for Ni50Zr50 bulk liquid and nanodroplet to study the dynamics evolution in the complex multivariate phase space, especially along the isotherm with the change of pressure or droplet size. It is found that the short-time Debye–Waller factor universally determines the long-time relaxation dynamics no matter how the temperature, pressure or size changes. The basic correlation even holds at the local atomic scale. This finding provides general understanding of the microscopic mechanism of dynamic arrest and dynamic heterogeneity.

Surface Boron Modulation on Cobalt Oxide Nanocrystals for Electrochemical Oxygen Evolution Reaction

AbstractHerein, we show that coupling boron with cobalt oxide tunes its structure and significantly boost its electrocatalytic performance for the oxygen evolution reaction (OER). Through a simple precipitation and thermal treatment process, a series of Co−B oxides with tunable morphologies and textural parameters were prepared. Detailed structural analysis supported first the formation of an disordered and partially amorphous material with nanosized Co3BO5 and/or Co2B2O6 being present on the local atomic scale. The boron modulation resulted in a superior OER reactivity by delivering a large current and an overpotential of 338 mV to reach a current density of 10 mA cm−2 in 1 M KOH electrolyte. Identical location transmission electron microscopy and in situ electrochemical Raman spectroscopy studies revealed alteration and surface re‐construction of materials, and formation of CoO2 and (oxy)hydroxide intermediate, which were found to be highly dependent on crystallinity of the samples.

Surface Boron Modulation on Cobalt Oxide Nanocrystals for Electrochemical Oxygen Evolution Reaction.

Herein, we show that coupling boron with cobalt oxide tunes its structure and significantly boost its electrocatalytic performance for the oxygen evolution reaction (OER). Through a simple precipitation and thermal treatment process, a series of Co-B oxides with tunable morphologies and textural parameters were prepared. Detailed structural analysis supported first the formation of an disordered and partially amorphous material with nanosized Co3 BO5 and/or Co2 B2 O6 being present on the local atomic scale. The boron modulation resulted in a superior OER reactivity by delivering a large current and an overpotential of 338 mV to reach a current density of 10 mA cm-2 in 1 M KOH electrolyte. Identical location transmission electron microscopy and in situ electrochemical Raman spectroscopy studies revealed alteration and surface re-construction of materials, and formation of CoO2 and (oxy)hydroxide intermediate, which were found to be highly dependent on crystallinity of the samples.

Atomic Scale Structure of (Ag,Cu)2ZnSnSe4 and Cu2Zn(Sn,Ge)Se4 Kesterite Thin Films

Kesterite based materials are being researched and developed as affordable, efficient, and mechanically flexible absorber materials for thin film photovoltaics. Both (Ag,Cu)2ZnSnSe4 and Cu2Zn(Sn,Ge)Se4 based devices have shown great potential in overcoming some of the remaining challenges for further increasing the conversion efficiency of kesterite based solar cells. This study therefore investigates the long range crystallographic structure and the local atomic scale structure of technologically relevant thin films by means of grazing incidence X-ray diffraction and low temperature X-ray absorption spectroscopy. As expected, the unit cell dimensions change about an order of magnitude more than the element specific average bond lengths. In case of Cu2Zn(Sn,Ge)Se4, the thin film absorbers show a very similar behavior as Cu2Zn(Sn,Ge)Se4 powder samples previously studied. Small amounts of residual S in the thin films were taken into account in the analysis and the results imply a preferential formation of Sn-S bonds instead of Ge-S bonds. In (Ag,Cu)2ZnSnSe4, the dependence of the Ag-Se and Cu-Se bond lengths on Ag/(Ag+Cu) might indicate an energetic advantage in the formation of certain local configurations.

Anomalous Behavior in the Atomic Structure of Nb3Sn under High Pressure

In the present study, the local atomic structure of a Nb3Sn superconductor sample has been probed by X-ray absorption fine structure (XAFS) as a function of hydrostatic pressure (from ambient up to 26 GPa) using a diamond anvil cell set-up. The analysis of the Nb-K edge extended X-ray absorption fine structure (EXAFS) data was carried out combining standard multi shell structural refinement and reverse Monte Carlo method to provide detailed in situ characterization of the pressure-induced evolution of the Nb local structure in Nb3Sn. The results highlight a complex evolution of Nb chains at the local atomic scale, with a peculiar correlated displacement of Nb–Nb and Nb–Nb–Nb configurations. Such a local effect appears related to anomalies evidenced by X-ray diffraction in other superconductors belonging to the same A15 crystallographic structure.

In-situ XAFS and SERS study of self-healing of passive film on Ti in Hank's physiological solution

The self-healing of passive film on Ti after being damaged in Hank's physiological solution greatly fluctuates with time and its mechanism has been investigated by in-situ X-ray absorption fine structure spectroscopy (XAFS) and surface enhanced Raman spectroscopy (SERS) in a novel electrochemical repassivation set-up. The self-healing response is instant and the current density decays within 50 s due to the increase of the ratio of O and Ti atoms. The increase of the coordination number of TiO contributes to the decrease of current density until 6000 s. Moreover, the in-situ structure evolution is vastly different from that of ex-situ. Two time-dependent adsorbed intermediates exist during self-healing, the adsorption layers of TiOHads and TiOH, and finally the interface forms a stable passive film of OTiOH after 1000 s with good corrosion resistance. Besides, in-situ XAFS and SERS exhibit a high disorder and amorphous nature of passive film on Ti, and the crystallinity increases from 300 s to 6000 s. Additionally, the time-dependent structure evolution and mechanism of self-healing of titanium passive film in local atomic scale will help to understand the corrosion science and protection in healthcare.

57Fe Mössbauer study of epitaxial TiN thin film grown on MgO (1 0 0) by magnetron sputtering

The properties and performance of TiN thin films are closely related to the concentration and mobility of lattice defects in the thin film structures of TiN. This makes a local atomic scale study of TiN thin films an ever-growing demand. Emission 57Fe Mössbauer spectroscopy (eMS) is a powerful tool in this regard, which we apply here to study an ultrathin TiN film epitaxially grown on MgO (1 0 0). With the help of theoretical calculations, our results show that most implanted Fe ions adopt a 2+ valence state and locate at the Ti sublattice in the bulk-like single crystalline grains, with the rest Fe residing at the grain boundaries as interstitials. A small percentage of nitrogen point defects (vacancy VN and interstitial NI) are observed in the bulk-like crystalline grains. A temperature-dependent, interstitial NI mediated site-exchange between NI and VN inside the crystal grain are deduced via a N2 dimmer like diffusion of NI through the crystal grains in the temperature range of 540–620 K. This is interesting in the perspective of exploring the catalytic property of TiN nanostructures. The titanium vacancy (VTi) is only detected at the grain boundaries. Annealing up to 813 K, both the VN and NI are annihilated in the crystalline grains and the VTi is fully recovered with healing of the grain boundaries. However, no evidence of ferromagnetism due to dilute implantation of 57Mn/57Fe and or structural defects in the film is obtained. This suggests that the so far reported dilute magnetism and defect-induced ferromagnetism in TiN nanostructures requires a further systematic investigation.

Crystal Structures, Local Atomic Environments, and Ion Diffusion Mechanisms of Scandium-Substituted Sodium Superionic Conductor (NASICON) Solid Electrolytes

The importance of exploring new solid electrolytes for all-solid-state batteries has led to significant interest in NASICON-type materials. Here, the Sc3+-substituted NASICON compositions Na3ScxZr2–x(SiO4)2–x(PO4)1+x (termed N3) and Na2ScyZr2–y(SiO4)1–y(PO4)2+y (termed N2) (x, y = 0–1) are studied as model Na+-ion conducting electrolytes for solid-state batteries. The influence of Sc3+ substitution on the crystal structures and local atomic environments has been characterized by powder X-ray diffraction (XRD) and neutron powder diffraction (NPD), as well as solid-state 23Na, 31P, and 29Si nuclear magnetic resonance (NMR) spectroscopy. A phase transition between 295 and 473 K from monoclinic C2/c to rhombohedral R3̅c is observed for the N3 compositions, while N2 compositions crystallize in a rhombohedral R3̅c unit cell in this temperature range. Alternating current (AC) impedance spectroscopy, molecular dynamics (MD), and high temperature 23Na NMR studies are in good agreement, showing that, with a higher Sc3+ concentration, the ionic conductivity (of about 10–4 S/cm at 473 K) decreases and the activation energy for ion diffusion increases. 23Na NMR experiments indicate that the nature of the Na+-ion motion is two-dimensional on the local atomic scale of NMR although the long-range diffusion pathways are three-dimensional. In addition, a combination of MD, bond valence, maximum entropy/Rietveld, and van Hove correlation methods has been used to reveal that the Na+-ion diffusion in these NASICON materials is three-dimensional and that there is a continuous exchange of sodium ions between Na(1) and Na(2) sites.

TDPAC study of Fe-implanted titanium dioxide thin films

Fe-doping in TiO2 has been proven to improve several of its properties, including the photocatalytic activity. Time-differential perturbed angular correlation (TDPAC) as the applied spectroscopy method is particularly interesting because it can probe the electric and magnetic interactions on a local atomic scale. In this work the hyperfine interactions on 111Cd atoms substituting Ti atoms in TiO2 due to nearby Fe atoms also diluted within the TiO2 lattice were measured as a function of temperature. The results review two fractions with distinct quadrupole interaction parameters. One site, occupied by the 111Cd probes, presents the smaller quadrupole interaction frequency, namely υq1 = 45 MHz, and can be ascribed to sites that are more distant from the Fe substitutional site whereas the second site characterized with υq2 = 62 MHz is related to Cd probe atoms that are closer to the Fe defect. Additionally, the system has been characterized using electron dispersive spectroscopy.

Automatic Tuning Matching Cycler (ATMC) in situ NMR spectroscopy as a novel approach for real-time investigations of Li- and Na-ion batteries

We have developed and explored the use of a new Automatic Tuning Matching Cycler (ATMC) in situ NMR probe system to track the formation of intermediate phases and investigate electrolyte decomposition during electrochemical cycling of Li- and Na-ion batteries (LIBs and NIBs). The new approach addresses many of the issues arising during in situ NMR, e.g., significantly different shifts of the multi-component samples, changing sample conditions (such as the magnetic susceptibility and conductivity) during cycling, signal broadening due to paramagnetism as well as interferences between the NMR and external cycler circuit that might impair the experiments. We provide practical insight into how to conduct ATMC in situ NMR experiments and discuss applications of the methodology to LiFePO4 (LFP) and Na3V2(PO4)2F3 cathodes as well as Na metal anodes. Automatic frequency sweep 7Li in situ NMR reveals significant changes of the strongly paramagnetic broadened LFP line shape in agreement with the structural changes due to delithiation. Additionally, 31P in situ NMR shows a full separation of the electrolyte and cathode NMR signals and is a key feature for a deeper understanding of the processes occurring during charge/discharge on the local atomic scale of NMR. 31P in situ NMR with “on-the-fly” re-calibrated, varying carrier frequencies on Na3V2(PO4)2F3 as a cathode in a NIB enabled the detection of different P signals within a huge frequency range of 4000ppm. The experiments show a significant shift and changes in the number as well as intensities of 31P signals during desodiation/sodiation of the cathode. The in situ experiments reveal changes of local P environments that in part have not been seen in ex situ NMR investigations. Furthermore, we applied ATMC 23Na in situ NMR on symmetrical Na–Na cells during galvanostatic plating. An automatic adjustment of the NMR carrier frequency during the in situ experiment ensured on-resonance conditions for the Na metal and electrolyte peak, respectively. Thus, interleaved measurements with different optimal NMR set-ups for the metal and electrolyte, respectively, became possible. This allowed the formation of different Na metal species as well as a quantification of electrolyte consumption during the electrochemical experiment to be monitored. The new approach is likely to benefit a further understanding of Na-ion battery chemistries.

Putting a New Spin on Scanning Transmission Electron Microscopy

Local atomic scale ordering and structural distortions can significantly modify material properties. Aberration correction dramatically improves spatial resolution into the sub-Angstrom regime, unlocking information about material defects previously beyond reach. While STEM has proven essential to the atomic scale characterization of materials, for example at defects, interfaces, or even in perfect crystals, measurement of atomic displacements and distances has remained challenging due to the presence of sample drift. Distortion proportional to the rate of sample drift during image acquisition hinders the accurate measurement or even representation of atomic structure. Though modern STEM installations are optimized to reduce vibration, air flow/fields, and temperature fluctuations, sample drift generally remains.

The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism.

The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the (57)Fe Mössbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale into the mechanically induced changes in the hexagonal structure of the material is obtained. It is revealed that the milling of BaFe12O19 results in the deformation of its constituent polyhedra (FeO6 octahedra, FeO4 tetrahedra and FeO5 triangular bi-pyramids) as well as in the mechanically triggered transition of the Fe(3+) cations from the regular 12k octahedral sites into the interstitial positions provided by the magnetoplumbite structure. The response of the hexaferrite to the mechanical treatment is found to be accompanied by the formation of a non-uniform nanostructure consisting of an ordered crystallite surrounded/separated by a structurally disordered surface shell/interface region. The distorted polyhedra and the non-equilibrium cation distribution are found to be confined to the amorphous near-surface layers of the ferrite nanoparticles with the thickness extending up to about 2 nm. The information on the mechanically induced short-range structural disorder in BaFe12O19 is complemented by an investigation of its magnetic behaviour on a macroscopic scale. It is demonstrated that the milled ferrite nanoparticles exhibit a pure superparamagnetism at room temperature. As a consequence of the far-from-equilibrium structural disorder in the surface shell of the nanoparticles, the mechanically treated BaFe12O19 exhibits a reduced magnetization and an enhanced coercivity.

Imaging Catalysts at Work: A Hierarchical Approach from the Macro‐ to the Meso‐ and Nano‐scale

AbstractThis review highlights the importance of developing multi‐scale characterisation techniques for analysing operating catalysts in their working environment. We emphasise that a hierarchy of in situ techniques that provides macro‐, meso‐ and nano‐scale information is required to elucidate and optimise catalyst performance fully. This combined methodology should ideally embrace spatially resolved and spatio‐temporal monitoring of a) the structure, catalytic activity, temperature and heat/mass transfer pattern in axial and radial directions in real reactors, b) the structure and temperature/heat/mass transport gradients in shaped catalysts and catalyst grains and c) meso‐ and nano‐scale information about particles and clusters, whose physical and electronic properties are linked directly to the micro‐kinetic behaviour of the catalysts. Techniques such as X‐ray diffraction (XRD), infrared (IR), Raman, X‐ray photoelectron spectroscopy (XPS), UV/Vis, and X‐ray absorption spectroscopy (XAS), which have mainly provided global atomic scale information, are being developed to provide the same information on a more local scale, often with sub‐second time resolution. X‐ray microscopy, both in the soft and more recently in the hard X‐ray regime, allows two‐ and three‐dimensional information to be collected down to 10 nm spatial resolution in a gas atmosphere or liquid, although this improvement is at the expense of temporal resolution. Electron microscopy, which provides excellent local atomic scale structural and spectroscopic information, is being developed towards tomographic imaging and realistic conditions, allowing gaps to be bridged in pressure, reaction conditions and length scales, up to the meso‐ and macro‐scale. In addition, new techniques such as single molecule fluorescence spectroscopy and non‐linear spectroscopic techniques are emerging. In this review, we discuss prospects for the development and combined application of both existing and new techniques for in situ catalyst characterisation.

Visualization of highly graded oxygen vacancy profiles in lead-zirconate-titanate by spectrally resolved cathodoluminescence spectroscopy

The ultraviolet and visible cathodoluminescence (CL) emitted at room temperature from bulk hard lead-zirconate-titanate polycrystalline perovskite has been systematically collected before and after an annealing cycle conducted in a reducing atmosphere. Spectroscopic assessments have been made of the in-depth stoichiometric profile developed upon annealing from the sample surface toward the subsurface. Trapping of electronic charge and local atomic scale distortions in the perovskite oxygen octahedron influences the variation observed in visible CL emission, while lattice distortions upon annealing directly arise from the formation of oxygen vacancies.

Cationic Ordering and Microstructural Effects in the Ferromagnetic Perovskite La0.5Ba0.5CoO3: Impact upon Magnetotransport Properties

The synthesis and structural study of the stoichiometric perovskite La0.5Ba0.5CoO3 have allowed three forms to be isolated. Besides the disordered La0.5Ba0.5CoO3 and the perfectly ordered layered LaBaCo2O6, a third form called nanoscale-ordered LaBaCo2O6 is obtained. As evidenced by transmission electron microscopy investigations, the latter consists of 112-type 90° oriented domains fitted into each other at a nanometer scale which induce large strains and consequently local atomic scale lattice distortions. These three ferromagnetic perovskites exhibit practically the same TC ≅ 174–179 K, but differently from the other phases, the nanoscale-ordered LaBaCo2O6 is a hard ferromagnet, with HC ≈ 4.2 kOe, due to the strains which may pin domain walls, preventing the reversal of the spins in a magnetic field. The magnetotransport properties of these phases show that all of them exhibit a maximum intrinsic magnetoresistance, close to 6–7% around TC under ±70 kOe, but that the ordered phase exhibits a much higher...

Local atomic structure in (Zr 1 − xU x)N

(Zr 1 − x U x )N solid solutions were prepared for EXAFS measurements by a sol–gel route combined with infiltration and carbothermic reduction. The lattice parameter and the more distant coordination shells (Me 2 and Me 3) around the Zr and U atoms follow the Vegard law. In the first coordination shell, the U–N distance also follows the Vegard law. Though the Zr–N bond distance increases with the lattice expansion caused by increasing U content, it remains constant at 232–235 pm in U-rich (Zr 1 − x U x )N ( x > 0.6). The measurements indicate that U accommodates the lattice contraction with increasing Zr content, whereas Zr is able to expand its Zr–N bond only at lower U content. In the composition range of transmutation fuels, (Zr 1 − x U x )N is homogeneous at the local atomic scale.

Structural properties of semiconductor nanostructures determined via X-ray anomalous diffraction (DAFS) and absorption (EXAFS)

We will report novel results on the structural properties of encapsulated semiconductor nanostructures obtained by an ultimate application of anomalous diffraction combined with ray absorption spectroscopy. We have been studying InAs/InP Quantum Sticks (QSs) and GaN/AlN Quantum Dots (QDs). Reciprocal space mapping and fixed-Q anomalous diffraction, mesured as a function of energy, in grazing incidence, gives access to the the partial structure factor of the embedded nanostructures (QSs or QDs), allowing to determine their size, strain and atomic composition. Quantitative analysis of the Grazing Incidence Diffraction Anomalous Fine Structure (GIDAFS) oscillations above the resonant edges, gives strain accommodation and coordination at a local atomic scale for the diffraction-selected isostrain region inside the nanostructures. On the other hand EXAFS measurements provide a comparison with the average atomic environment in the whole island. These methods have been applied succesfully both to InAs sticks and to the GaN dots.

Co 59 nuclear magnetic resonance study of molecular-beam epitaxy grown Co/V multilayers

Co 59 nuclear magnetic resonance (NMR) measurements have been undertaken to determine the local atomic environment of Co in Co/V multilayers, previously studied by magnetometry. These NMR measurements provide confirmation, on a local atomic scale, of the results deduced from the earlier magnetization study. Co/V multilayers show considerable interfacial mixing with a magnetically dead, vanadium rich layer penetrating approximately 2 Å into an adjoining Co layer. The presence of this dead layer has the effect of modifying the hyperfine field experienced by the Co nuclei at a distance of several atomic planes from the interface.

A study of n-type conduction in amorphous chalcogenide sputtered films

A detailed study of the X-ray photoemission spectroscopy (XPS) and the transport properties of amorphous sputtered films with compositions approximated by a formula Ge25Se75-xBix where x=0-17.4 is reported. The XPS result indicates that, on a local atomic scale, the chemical states of the films are a mixture of different bonding environments of GeSe2 and Bi2Se4 for all compositions and that the Bi is introduced as a positive charged centre. The addition of 15.6 at.% Bi leads to an increase of ten orders of magnitude in the electrical conductivity as well as a decrease in the optical and electrical gaps. A negative sign of the thermoelectric power as observed for x>10, indicating that the conduction type changed from p type (low x) to n type, which is in agreement with the glass. The results are interpreted as the effect of the shift of the Fermi level closer to the conduction band due to the incorporation of the positive-U defects that unpin the Fermi energy.