Local Atomic Order Research Articles

Control of intrinsic ferroelectricity and structural phase in pure HfO2 films via crystalline orientation

The ferroelectricity of hafnia-based thin films has garnered considerable attention in both academic researches and industrial applications. However, the fundamental properties, such as high coercivity, the wake-up effect, and the mechanism of ferroelectricity have not been fully elucidated. Here we report the crystallization orientation control of structural phase and ferroelectricity in pure HfO2 thin films. Both (001)- and (111)-oriented HfO2 thin films exhibit a mixture of ferroelectric orthorhombic and non-ferroelectric monoclinic phases. With decreasing film thickness, the orthorhombic phase ratio increases for both orientations, with a consistently higher proportion for (111)-oriented film. Consequently, the ferroelectricity is significantly enhanced in thinner (111)-oriented film. Remarkably, both (001)- and (111)-oriented pure HfO2 thin films demonstrate an intrinsic ferroelectricity. Moreover, the coercive field of the (001)-oriented film appears to be lower than that of the (111)-oriented film. Additionally, oxygen ions migrate more easily in the (001)-oriented film, which exhibits distinct electronic structure and local atomic ordering compared to the (111)-oriented film. These results provide valuable insights into the ferroelectricity of HfO2 and suggest that crystalline orientation is an effective approach to explore the ferroelectric properties in hafnia-based films.

Determinism of Gold-Monolayers’ Local Atomic Ordering in the Formation of Their Electronic Structure

Determinism of Gold-Monolayers’ Local Atomic Ordering in the Formation of Their Electronic Structure

Multilevel atomic structural model for interstratified opal materials

The structure of opal has long fascinated scientists. It occurs in a number of structural states, ranging from amorphous to exhibiting features of stacking disorder. Opal-CT, where C and T signify cristobalite- and tridymite-like interstratification, represents an important link in the length scales between amorphous and crystalline states. However, details about local atomic (dis)order and arrangements extending to long-range stacking faults in opal polymorphs remain incompletely understood. Here, a multilevel modeling approach is reported that considers stacking states in correlation with the abundance of C and T segments as a high-level structural parameter (i.e. not each atom). Optimization accounting for inter-tetrahedral bond lengths and angles and the regularity of the silicate tetrahedra is included as lower levels of structural parameters. Together, a set of parameters with both coarse-grained and atomistic features for different levels of structural details is refined. Structural disorder at the ∼10–100 Å distance scale is evaluated using experimental pair distribution function and diffraction datasets, comparing peak intensities, widths and asymmetry. This work presents a complete multilevel structural description of natural opal-CT and explains many of the unusual features observed in X-ray powder diffraction patterns. This modeling approach can be adopted generally for analyzing layered materials and their assembly into 3D structures.

Role of local atomic short-range order distribution in alloys: Why it matters in Si-Ge-Sn alloys

Role of local atomic short-range order distribution in alloys: Why it matters in Si-Ge-Sn alloys

Acoustic shock wave-induced short-range ordered graphitic domains in amorphous carbon nanoparticles and correlation between magnetic response and local atomic structures

Over the years, a wide range of spectral research has been carried out on various carbon forms to explore deeply their structure-property relationship. The structural stability of carbon allotropies remains to be a challenging endeavor yet to be achieved under extreme conditions, especially under acoustical shocked conditions. From the experimental findings, we report the acoustic shock wave response of the amorphous carbon nanoparticles and the resultant degree of crystalline nature, morphological characteristics and magnetic response. Moreover, analytical techniques such as X-ray diffractometry (XRD), Raman spectrometer, High resolution transmission electron microscopy (HR-TEM) and vibrational sample magnetometer (VSM) have been employed to arrive at the relationships of structure – morphology – magnetic properties under shocked conditions. Interestingly, under shocked conditions (0, 250, 500 and 750 shocks), compelling evidence has been found for the short-range amorphous pockets which turn into spatially short-range ordered graphitic domains within the sphere-shaped amorphous nanoparticles whereby the obtained values of the ratio ID/IG of the samples are 0.94, 0.93, 0.95 and 1.01 for 0, 250, 500 and 750 shocked conditions, respectively. Moreover, the saturation magnetization is found to have reduced in the base of the local atomic ordering and the obtained values are found to be 1, 0.64, 0.41, 0.725 emu/g for 0, 250, 500 and 750 shocked conditions, respectively. The structure-property relationship is to be elaborated in the upcoming sections.

Atomistic study of liquid fragility and spatial heterogeneity of glassy solids in model binary alloys

Fragility is a fundamental property of glass-forming liquids. Here, we evaluated the liquid fragility and structural and dynamic heterogeneity of glassy solids for four model binary alloys. The most fragile alloy exhibited the maximum dynamic heterogeneity in the mechanical unfreezing process. The local atomic order contributed to structural and dynamic heterogeneities in the glassy solid. We observed that atomic displacement significantly correlated with degrees of clustering of local atomic orders. The clustering produced during the glass-forming quenching process enhanced structural and dynamic heterogeneities, especially in fragile glass alloys. Therefore, this alloy system exhibited correlations among liquid fragility, dynamic heterogeneity in liquid alloys, and dynamic and structural heterogeneities in glassy solids. We discussed the underlying physics of the correlation based on a theoretical model for fragility. These structural and dynamic analyses also provided deeper insights into the features of structural heterogeneity in glassy solids. The alloy with the most fragility exhibited the largest difference in atomic mobility between the densely and loosely packed local atomic orders, implying the greatest heterogeneity in the degree of packing density.

Study of structure evolution of ultra-rapidly annealed Fe75.3Ni10B14Cu0.7 alloys

The article concerns modern soft magnetic materials made by rapid solidification and subsequent ultra-rapid annealing. Due to the lower amount of non-magnetic elements, higher magnetisation is possible. Unfortunately, the lack of those elements causes difficulty during isothermal nanocrystallisation operation, being crucial from the final product properties point of view. Instead of isothermal heat treatment, ultra-rapid annealing (URA) is implemented. The seconds-long annealing at a higher temperature than the temperature of conventional nanocrystallisation enables material with a higher amount of ferromagnetic elements and optimal structure. It reflects in materials with good magnetic softness and higher possible magnetisation values. The amorphous alloy with Fe75.3Ni10B14Cu0.7 chemical composition in the shape of a ribbon was heat treated between cooper blocks at 773 K (500 °C) for different times in the 1–60 s range. To study structure evolution, we brought XRD, DSC, Flash DSC, Mössbauer spectroscopy, TEM, and HRTEM techniques to play. Based on the DSC results, we estimated the minimum heating speed to obtain optimal structure at 6000 K/min. The Mössbauer results allowed us to study annealing time-dependent local atomic ordering in the amorphous state. A comparison of X-Ray and synchrotron diffraction results drove us to the conclusion that X-Ray measurements are sufficient to determine the material structure. TEM shows the formation of α-Fe crystallites even after an annealing time of 0.5 s. The increase in annealing time leads to an increase in the crystalline phase volume fraction and growth of α-Fe crystallite size.

Correlation of phase composition, magnetic properties and hyperthermia efficiency of silica-coated FeCo nanoparticles for therapeutic applications

The local atomic order, magnetic properties, hyperthermia efficiency, drug loading capacity and in-vitro cytotoxicity are investigated for the core–shell (Fe25Co75)x(SiO2)100-x (70 ≤ x ≤ 100, wt.%) nanoparticles (NPs) sintered by the metals co-precipitation followed with SiO2 TEOS deposition. Electron microscopy reveal the formation of nearly spherical structures with medium diameter from 80 nm to 220 nm containing 20–50 nm cores identified by XRD as α-FeCo bcc alloy. Mössbauer spectra and magnetometry reflects the formation of magnetically-interacting net of single-domain FeCo cores with broad size distribution. Observed decrease in magnetization (MS) and effective magnetic anisotropy (Keff) for NPs with × is the consequence of the increasing thickness of the SiO2 shells. Calculations indicate that there is an optimal thickness δ of SiO2 shells ranging between 90 ÷ 120 nm for the highest value of specific absorption rate (SAR) originating from their effect on Keff values. Experimentally proved that SAR has the highest value for (FeCo)80(SiO2)20 NPs in correlation with the Linear Response Theory approach. High drug loading capacity (up to 98%) and low in-vitro cytotoxicity (IC50 index below 230 µg/ml) experimentally proved for (FeCo)x(SiO2)100-x NPs make them perfect candidates for designing smart drug delivery systems.

Singlet Molecular Oxygen Generation via Unexpected Emission Color-Tunable CdSe/ZnS Nanocrystals for Applications in Photodynamic Therapy

It is highly desirable in biomedical sciences to utilize the multifunctional nanoparticles of similar size with tunable emission. Since the optoelectronic properties of quantum dots (QDs) originate from size-dependent quantum confinement effects, we developed an alternate approach to synthesize color-tunable CdSe/ZnS QDs based on interfacial ion exchange (predominantly exchange of Se2– by S2– anions), using 1-dodecanethiol and oleylamine solvent systems as a sensitive parameter. The wide-range color-tunability (490–570 nm) was achieved unexpectedly as a result of interfacial alloying without inducing a significant change in the size (from 4.45 to 4.81 nm) of QDs. The local atomic structure order, chemical composition, and nature of alloying in QDs were unraveled by XAFS data analysis. Owing to the molecular-like sensitization behavior, the QDs were evaluated for singlet molecular oxygen (1O2) efficiency. They were further studied in RAW 264.7 macrophages for biocompatibility, bioimaging, and delivering pathways for use in future photodynamic therapy (PDT). The QDs demonstrated efficient singlet molecular oxygen (1O2) quantum yields (ΦQDs) of 14, 12, and 18% for QDs (I), QDs (II), and QDs (III), respectively. The QD-treated cells presented high cell viability above 85% and induced no cell activation. Fluorescence and transmission electron microscopy (TEM) images of cells manifested a considerable amount of QDs in the vicinity of the cell membrane and intracellular regions. The pathway-specific inhibition measurements revealed that the QDs were internalized by cells via energy-dependent endocytosis, predominantly macropinocytosis and other receptor-mediated endocytic pathways, and accumulated them presumably in endosome/lysosomes. This study will open new possibilities for engineering interfacial alloying-based tunable emission QDs and pathway-specific delivery of QD-based theranostics into a site of interest for simultaneous bioimaging and PDT.

Local ordering in Ge/Ge–Sn semiconductor alloy core/shell nanowires revealed by extended x-ray absorption fine structure (EXAFS)

Short-range atomic order in semiconductor alloys is a relatively unexplored topic that may promote design of new materials with unexpected properties. Here, local atomic ordering is investigated in Ge–Sn alloys, a group-IV system that is attractive for its enhanced optoelectronic properties achievable via a direct gap for Sn concentrations exceeding ≈10 at. %. The substantial misfit strain imposed on Ge–Sn thin films during growth on bulk Si or Ge substrates can induce defect formation; however, misfit strain can be accommodated by growing Ge–Sn alloy films on Ge nanowires, which effectively act as elastically compliant substrates. In this work, Ge core/Ge1−xSnx (x ≈ 0.1) shell nanowires were characterized with extended x-ray absorption fine structure (EXAFS) to elucidate their local atomic environment. Simultaneous fitting of high-quality EXAFS data collected at both the Ge K-edge and the Sn K-edge reveals a large (≈ 40%) deficiency of Sn in the first coordination shell around a Sn atom relative to a random alloy, thereby providing the first direct experimental evidence of significant short-range order in this semiconductor alloy system. Comparison of path length data from the EXAFS measurements with density functional theory simulations provides alloy atomic structures consistent with this conclusion.

Local atomic structure order and electrochemical properties of NiO based nano-catalysts for ethanol sensing at room temperature

The increased emission of volatile organic compounds (VOCs) and their adverse effects on environment led to the development of more economical and sustainable sensing technologies. Ethanol is a member of VOC family and has harmful effect on human health and environment. Therefore, an efficient electrocatalyst for ethanol detection at ambient temperature is highly desirable. Here we present the successful synthesis of NiO based sensing material where modification of NiO was carried out by transition metals doping (8% Cu and 8% Zn) and codoping (4% Cu + 4% Zn). The synthesized catalysts were characterized by several techniques like powder X ray diffraction, Raman spectroscopy, scanning electron microscopy, X-ray absorption fine structure, Brunauer–Emmett–Teller and diffuse reflectance spectroscopy. The electrochemical responses showed that the codoped NiO has a better catalytic performance towards ethanol detection with a wide linear range from 3 mM to 12 mM, and a low detection limit (LOD) of 2.2 mM. The striking analytical performance and facile preparation technique make this catalyst effective for efficient enzyme-free electrochemical ethanol sensing.

Temperature-dependent effect of cooling rate on the melt-quenching process of metallic glasses

The cooling rate during the melt-quenching process is a crucial factor in the formation of metallic glasses. This study reveals the temperature- and thermal-history-dependent effects of cooling rate on the relaxation state of metallic glasses prepared by the rapid quenching of a liquid alloy. Using molecular dynamics techniques, we conducted melt-quenching simulations in which the cooling rates were varied during the quenching process. Maps were constructed to demonstrate the effect of the cooling rate on the quenching process as a function of temperature. The critical temperature range varied depending on the thermal history of the cooling process. Analyses of the local atomic order and atomic dynamics revealed the atomistic origin of the temperature- and thermal-history-dependent effects of the cooling rate. This study also discusses the physical background of the upper and lower ends of the critical temperature range based on the potential energy landscape and atomic dynamics.

Unidirectional Rashba spin splitting in single layer WS2(1−x)Se2x alloy

Atomically thin two-dimensional (2D) layered semiconductors such as transition metal dichalcogenides have attracted considerable attention due to their tunable band gap, intriguing spin-valley physics, piezoelectric effects and potential device applications. Here we study the electronic properties of a single layer WS1.4Se0.6 alloys. The electronic structure of this alloy, explored using angle resolved photoemission spectroscopy, shows a clear valence band structure anisotropy characterized by two paraboloids shifted in one direction of the k-space by a constant in-plane vector. This band splitting is a signature of a unidirectional Rashba spin splitting with a related giant Rashba parameter of 2.8 ± 0.7 eV Å. The combination of angle resolved photoemission spectroscopy with piezo force microscopy highlights the link between this giant unidirectional Rashba spin splitting and an in-plane polarization present in the alloy. These peculiar anisotropic properties of the WS1.4Se0.6 alloy can be related to local atomic orders induced during the growth process due the different size and electronegativity between S and Se atoms. This distorted crystal structure combined to the observed macroscopic tensile strain, as evidenced by photoluminescence, displays electric dipoles with a strong in-plane component, as shown by piezoelectric microscopy. The interplay between semiconducting properties, in-plane spontaneous polarization and giant out-of-plane Rashba spin-splitting in this 2D material has potential for a wide range of applications in next-generation electronics, piezotronics and spintronics devices.

Local atomic ordering strategy for high strength Mg alloy design by first-principle calculations

In this study, solute X (X = Li, Al, Mn, Zn, Y, Zr, Nd, and Gd) solution strengthening in Mg alloys have been screened by ab initio density functional theory calculations to quantify not only the substitutional stacking-fault configurations but also the solute ordering sequence as a function of local segregation. Interestingly, it has been found that the strengthening effects of single atom addition to a supercell made of 64 atoms can be mostly attributed to lattice distortions (Mn>Nd>Gd>Y>Zn>Al>Zr>Li), while the local ordering arrangements of Mg-X complex actually contribute most to the strengthening when the solute concentration rises. For example, Nd can induce a large local atomic ordering and significantly increase the basal critical-resolved shear stress (CRSS). A linear relationship between solute concentrations and ideal strength, and the quasi-quadratic relationship between solute atomic radii and ideal strength have been observed. Simultaneously, the higher the solute concentration, the higher degree of the solid solution strengthening, resulting in a smaller quasi-quadratic function curve opening. Based on the screening of the chemical (including stacking fault energy and atomic bonds) and strain (lattice distortion energy) energy calculations, we have discovered that the solute strengthening follows the ordering sequence of Nd> Mn> Gd> Y> Zn> Al> Zr> Li. • Local ordering of Mg-X alloys was found by thermodynamic evaluations using DFT. • Both chemical and strain energy contribute to the solute strengthening ordering. • Local ordering configuration in solid solution strengthening guide the alloy by design.

Magnetic Properties and Local Atomic Ordering in Ce(Fe1 – xSix)2 Compounds with a Silicon Content x ≤ 0.05

Magnetic Properties and Local Atomic Ordering in Ce(Fe1 – xSix)2 Compounds with a Silicon Content x ≤ 0.05

Effect of composition and nanostructure on the mechanical properties and thermal stability of Zr100-xCux thin film metallic glasses

Thin film metallic glasses (TFMGs) are a novel class of materials showing a mutual combination of large plastic deformation in tension (>10% strain) and superior yield strength up to ∼ 3.5 GPa, which make them ideal candidates for applications such as flexible electronics. Nevertheless, a clear relationship between the atomic structure and mechanical properties of TFMGs has not yet been achieved. In particular, the role of composition in determining a different local atomic order and the effect of nanostructure on TFMGs properties must be further investigated. In this work, the mechanical properties and thermal stability of several amorphous Zr100-xCux TFMGs with either compact or fine columnar nanostructure were studied. The mediating role of composition in controlling crystallization temperature and hardness is here reported, which was found to increase from 4.6 to 7.7 GPa with increasing Cu content from 26 to 76 at.%. Moreover, plastic behavior and fracture resistance are shown to be highly dependent on both composition and nanostructure, with the Cu-rich and homogeneous film able to withstand elongation up to 2% strain before crack initiation. These results underline how atomic structure changes induced by composition can effectively influence TFMG properties, while demonstrating an approach to tune their behavior for various technological applications.

Local Atomic Order, Morphology, and Magnetic Properties in Silica‐Coated FeCo Nanoparticles Synthesized by Coprecipitation Technique

Nanoparticles (NPs) of pure Fe25Co75 are sintered by Fe and Co coprecipitation followed by SiO2 deposition by tetraethoxysilane hydrolysis to form silica‐coated Fe25Co75 NPs ((Fe25Co75)x(SiO2)100–x (90 ≤ x ≤ 10, wt%)). Study of the morphology and phase composition of (Fe25Co75)x(SiO2)100–x nanopowders evidences the formation of agglomerations of NPs with individual medium diameter ≈20–50 nm containing partially oxidized cores of α‐FeCo bcc alloy covered with amorphous SiO2. Mössbauer spectroscopy clearly reveals that coverage with SiO2 shells promotes up to four‐time enhancement of resistance of FeCo cores to undesirable formation of surface Fe(Co) hydroxides. Temperature dependencies of magnetization recorded under field‐cooled (FC) and zero‐field‐cooled (ZFC) protocols for uncoated Fe25Co75 and (Fe25Co75)70(SiO2)30 nanopowders reveal that some part of NPs being in single‐domain state form superferromagnetic long‐range ordering.

Revealing the nano-structures of low-dimensional germanium on Ag(1 1 0) using XPS and XPD

In this work, we present a structural investigation of sub-monolayer films of germanium on Ag(1 1 0) by means of photoelectron spectroscopy (XPS) and diffraction (XPD), as well as low-energy electron diffraction (LEED). Since the rising progress in the synthesis of various kinds of nanoribbons, also germanium nanoribbons (Ge-NR) have been synthesized on Ag(1 1 0), recently. Here, we focus on their structural evolution and found the formation of two different phases of germanium at coverages of {0.5}, hbox {ML} and {0.7},hbox {ML}, differing fundamentally from predicted nanoribbon structures. By means of LEED measurements, we obtained evidence for germanium superstructures which are not aligned along the [{overline{1}};1;0]-direction, as expected for nanoribbon growth. Using synchrotron-based high-resolution XPS and XPD experiments of the Ge 3d and Ag 3d core-levels, we resolved the local chemical and atomic order of the germanium films. Thus, the strong internal bonding of the buckled germanium film and a weak Van-der-Waals interaction between silver and germanium were discovered. Moreover, XPD-simulations delivered a detailed model of the structural arrangement of the preliminary nanoribbon phase, which also provided an approach to identify the origin of the two chemically shifted components in the Ge 3d signal by applying a component-wise decomposition of the XPD data.

Pure and Ni2O3-decorated CeO2 nanoparticles applied as CO gas sensor: Experimental and theoretical insights

In the present work, the structural, morphological and electrical properties of CeO2 nanoparticles with spherical and rod-like morphologies and rods decorated with Ni2O3 were investigated. Morphological, structural and electronic analyses showed a relationship between the shape/size of particles and the respective surface types, number of carriers, density of vacancies and local structural atomic order, in addition to an influence on the electrical properties of the materials. Electrical analyses revealed that the rod-like morphology has a better CO sensor response than the spherical one. In order to complement and rationalize the experimental findings, simulations at the density functional theory (DFT) were carried out to investigate the two possible mechanisms (Langmuir˗Hinshelwood and Mars-Van Krevelen) acting on the CO adsorption process and elucidate the behavior of the sensor in heterojunction-type samples.

Reduction-Driven 3D to 2D Transformation of Cu Nanoparticles.

The interaction between metal and metal oxides at the nanoscale is of uttermost importance in several fields, thus its enhancement is highly desirable. In catalysis, the performance of the nanoparticles is dependent on a wide range of properties, including its shape that is commonly considered stable during the catalytic reaction. In this study, highly reducible CeO2-x nanoparticles are synthesized aiming to provide Cu/CeO2-x nanoparticles, which are classically active catalysts for the CO oxidation reaction. It is observed that the Cu nanoparticles shape changes during reduction treatment (prior to the CO oxidation reaction) from a nearly spherical 3D to a planar 2D shape, then enhances the Cu-CeO2-x interaction. The spread of the Cu nanoparticles over the CeO2-x surface during the reduction treatment occurs due to the minimization of the total system energy. The shape change is accompanied by migration of O atoms from CeO2 surface to the border of the Cu nanoparticles and the change from the Cu0 to Cu+1 state. The spreading of the Cu nanoparticles influences on the reactivity results toward the CO oxidation reaction since it changes the local atomic order around Cu atoms. The results show a timely contribution for enhancing the interaction between metal and metal oxide.