Electronic and magnetic properties strongly depend on the structure of the material, especially on the crystal symmetry and chemical environment. In nanoparticles, the break of symmetry at the surface may yield different physical properties with respect to the corresponding bulk material. A useful tool to investigate the electronic structure, magnetic behaviour and local crystallographic structure is X-ray absorption spectroscopy. In this review, recent developments in the field of extended X-ray absorption fine structure measurements and in the analysis methods for structural investigations of bimetallic nanoparticles are highlighted. The standard analysis based on Fourier transforms is compared to the relatively new field of wavelet transforms that have the potential to outperform traditional analysis, especially in bimetallic alloys. As an example, the lattice expansion and inhomogeneous alloying found in FePt nanoparticles is presented, and this is discussed below in terms of the influence of employed density functional theory calculations on the magnetic properties.
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... It not only offers the possibility to visualize the contributions depending on a particular backscatter but for instance, it allows also to distinguish contributions from light and heavy backscattering elements. The theoretical background of WT analysis and some cases of its application could be found elsewhere . ...
Modern design of superior multi-functional alloys composed of several principal components requires in-depth studies of their local structure for developing desired macroscopic properties. Here, peculiarities of atomic arrangements on the local scale and electronic states of constituent elements in the single-phase fcc- and bcc- structured high-entropy Alx-CrFeCoNi alloys (x = 0.3 and 3, respectively) are explored by element-specific X-ray absorption spectroscopy in hard- and soft- X-ray energy ranges. simulations based on the Reverse Monte Carlo approach allow to perform a simultaneous fit of extended X-ray absorption fine structure spectra recorded at K absorption edges of each 3d constituent and to reconstruct the local environment within the first coordination shells of absorbers with high precision. The revealed unimodal and bimodal distributions of all five elements are in agreement with structure-dependent magnetic properties of studied alloys probed by magnetometry. A degree of surface atoms oxidation uncovered by soft X-rays suggests different kinetics of oxide formation for each type of constituents and has to be taken into account. X-ray magnetic circular dichroism technique employed at L2,3 absorption edges of transition metals demonstrates reduced magnetic moments of 3d metal constituents in the sub-surface region of in situ cleaned fcc-structured Al0.3-CrFeCoNi compared to their bulk values. Extended to nanostructured versions of multicomponent alloys such studies would bring new insights related to effects of high entropy mixing on low dimensions.
... A possibility to probe both electronic structure and crystallographic structure is offered by XAS. While the X-ray absorption near edge structure (XANES) contains information e.g., about oxidation state of the absorbing element and hybridization effects, the extended X-ray absorption fine structure (EXAFS) can be used to analyze distance, number, and type of neighboring atoms, as well as the coordination symmetry . In contrast to diffraction methods, EXAFS analysis can be performed for both crystalline and amorphous materials. ...
Magnetic-plasmonic heterodimer nanostructures synergistically present excellent magnetic and plasmonic characteristics in a unique platform as a multipurpose medium for recently invented biomedical applications, such as magnetic hyperthermia, photothermal therapy, drug delivery, bioimaging, and biosensing. In this review, we briefly outline the less-known aspects of heterodimers, including electronic composition, interfacial morphology, critical properties, and present concrete examples of recent progress in synthesis and applications. With a focus on emerging features and performance of heterodimers in biomedical applications, this review provides a comprehensive perspective of novel achievements and suggests a fruitful framework for future research.
... where var ( ) is wavelet variance; ( , ) is the wavelet transform coefficient, which can represent the characteristics of the signal change at different times scales; and are called a scaling parameter and a translation parameter, which measures the degree of compression or scale and determines the time location of the wavelet, respectively; and * is the complex conjugate of . In the derivation process of wavelet filtering, it is assumed that the signals are infinite; however, the signals that will be processed are finite in practical progress; therefore the border of composition signals will have great error, which is called "boundary effect" . Thus when the CWT was used to eliminate the "boundary effect" on the two boundaries of the variable (i.e., the time series), symmetryextending was applied to the time series. ...
Based on monthly streamflow and precipitation data from 1960 to 2010 in the Jinjiang River Basin of China, Standardized Precipitation Index (SPI) and Standardized Streamflow Index (SSI) were used to represent meteorological and hydrological drought, respectively. The response of hydrological drought to meteorological drought under the influence of Shanmei reservoir was investigated. The results indicate that SPI and SSI have a decreasing trend during recent several decades. Monthly scales of SSI series have a significant decreasing trend from November to the following February and a significant increasing trend from May to July at Shilong hydrological station. There are three significant periodic variations with a cycle of 6-7 years, 11-12 years, and 20-21 years for annual scales of SSI series. SPI series have the same periodic variations before the 1980s, but they have not been synchronous with SSI since the 1980s at Shilong due to influences of Shanmei reservoir, especially at the periodic variations of 20-21 years. The variation of the lag time of hydrological drought in response to meteorological drought is significant at the seasonal scale. The lag time of hydrological drought to meteorological drought extends one month on average in spring, summer, and autumn but about three months in winter.
Bimetallic nanoparticles of noble metals are of high interest in imaging, biomedical devices, including nanomedicine, and heterogeneous catalysis. Synthesis, properties, characterization, biological properties, and practical applicability of nanoparticles on the basis of platinum group metals and the coin metals Ag and Au are discussed, also in comparison with the corresponding monometallic nanoparticles. In addition to the parameters that are required to characterize monometallic nanoparticles (mainly size, size distribution, shape, crystallographic nature, surface functionalization, charge), further information is required for a full characterization of bimetallic nanoparticles. This concerns the overall elemental composition of a bimetallic nanoparticle population (ratio of the two metals) and the internal distribution of the elements in individual nanoparticles (e.g., the presence of homogeneous alloys, core–shell systems, and possible intermediate stages). It is also important to ensure that all particles are identical in terms of elemental composition, that is, that the homogeneity of the particle population is given. Macroscopic properties like light absorption, antibacterial effects, and catalytic activity depend on these properties. The currently available methods for a full characterization of bimetallic nanoparticles are discussed, and future developments in this field are outlined. Bimetallic nanoparticles offer many possibilities beyond their constituents, that is, the pure metals. For instance, their imaging properties can be fine‐tuned, the antibacterial effects of silver can be changed by blending it with a second metal, and the performance in heterogeneous catalysis including electrocatalysis can be enhanced. The current concepts on synthesis and characterization are outlined.
Bimetallic silver-gold nanoparticles were prepared by co-reduction using citrate and tannic acid in aqueous solution and colloidally stabilized with poly(N-vinylpyrrolidone) (PVP). The full composition range of silver : gold from 0 : 100 to 100 : 0 (n : n) was prepared with steps of 10 mol%. The nanoparticles were spherical, monodispersed, and had a diameter of B6 nm, except for Ag : Au 90 : 10 nanoparticles and pure Ag nanoparticles which were slightly larger. The size of the nanoalloys was determined by differential centrifugal sedimentation (DCS) and transmission electron microscopy (TEM). By means of X-ray powder diffraction (XRD) together with Rietveld refinement, precise lattice parameters, crystallite size and microstrain were determined. Scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) showed that the particles consisted of a gold-rich core and a silver-rich shell. XRD and DCS indicated that the nanoparticles were not twinned, except for pure Ag and Ag : Au 90 : 10, although different domains were visible in the TEM. A remarkable negative deviation from Vegard's linear rule of alloy mixtures was observed (isotropic contraction of the cubic unit cell with a minimum at a 50 : 50 composition). This effect was also found for Ag:Au bulk alloys, but it was much more pronounced for the nanoalloys. Notably, it was much less pronounced for pure silver and gold nanoparticles. The microstrain was increased along with the contraction of the unit cell with a broad maximum at a 50 : 50 composition. The synthesis is based on aqueous solvents and can be easily scaled up to a yield of several mg of a well dispersed nanoalloy with application potential due to its tuneable antibacterial action (silver) and its optical properties for bioimaging.
Nanoparticle (NP) superlattices represent a unique material architecture for energy conversion and storage. Recent reports on carbon-coated NP superlattices have shown exciting electrochemical properties attributed to their rationally designed compositions and structures, fast electron transport, short diffusion length, and abundant reactive sites via enhanced coupling between close-packed NPs, which are distinctive from their isolated or disordered NP or bulk counterparts. In this minireview, we summarize the recent developments of highly-ordered and interconnected carbon-coated NP superlattices featuring high surface area, tailorable and uniform doping, high conductivity, and structure stability. We then introduce the precisely-engineered NP superlattices by tuning/studying specific aspects, including intermetallic structures, long-range ordering control, and carbon coating methods. In addition, these carbon-coated NP superlattices exhibit promising characteristics in energy-oriented applications, in particular, in the fields of lithium-ion batteries, fuel cells, and electrocatalysis. Finally, the challenges and perspectives are discussed to further explore the carbon-coated NP superlattices for optimized electrochemical performances.
Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick Fe60Al40 thin films along the order-disorder (B2→A2) phase transition initiated by 20-keV Ne+ ion irradiation with fluences of (0.75–6)×1014ionscm−2. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and ∼1% of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at ∼60 eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed Fe60Al40 model structures for the ordered (B2) and the disordered (A2) phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied.
Metal catalysts in nanometer size range are under worldwide investigations due to their fascinating electronic and atomic strucutures which play essential roles in tuning catalytic properties of metal catalysts. Owing to intrinsically high disorder, asymmetric bond distributions, heterogeneity in particle sizes and compositions, as well as strong coupling between the structural properties and environment, nanosized metal catalysts present a number of challenging problems in EXAFS analysis for determining the size, structure, shape, support orientation of nanocatalysts in real time and in reaction conditions. In this chapter we review methods of EXAFS analysis developed in the last two decades for structural characterization of mono- and bi-metallic nanocatalysts.
Noble metals alloyed with certain transition metals in the form of a nanoalloy exhibit enhanced catalytic or electrocatalytic activities for a number of oxidation reactions. This chapter discusses some important insights into how the interatomic distances and structures of the nanoalloy catalysts operate synergistically in activating oxygen and maneuvering surface oxygenated species by highlighting examples of nanoalloy catalysts in which Pt or Pd is alloyed with a second and/or third transition metal (M/M'. =. Co, V, Ni, Cu) for catalytic oxidation of carbon monoxide in gas phase and electrocatalytic oxidation of alcohols in an electrolyte. One important emphasis is placed on understanding atomic-scale chemical/structural ordering and coordination in correlation with the catalytic or electrocatalytic properties based on findings from ex situ and in situ synchrotron X-ray techniques such as high energy X-ray diffraction coupled to atomic pair distribution function and X-ray absorption fine structure spectroscopic analysis. The understanding of the detailed active sites of the nanoalloys has significant implications for the design of low-cost, active, and durable catalysts for sustainable energy production and conversion reactions.
Iron oxide nanoparticles are synthesised by the coprecipitation of Fe(NO3)(3)center dot 9H(2)O, FeCl2 center dot 4H(2)O and NaOH under ultrasonic irradiation with comparisons on the temperature and surfactant used. Synchrotron X-ray absorption near edge structure (XANES) indicates that the predominant phase is Fe2O3, whereas extended X-ray absorption fine structure (EXAFS) confirms the presence of FeO and Fe-Fe bonding. Effects of synthesis temperature and surfactant are also revealed by the fitting of EXAFS spectra. The largest structural distortions compared to the Fe2O3 standard is obtained in superparamagnetic particles synthesised at 60 degrees C. The magnetisation is enhanced with the temperature increasing from 60 to 80 degrees C. The addition of oleic acid as a surfactant leads to the ferromagnetic particles with increased magnetisation and a local structure closer to that of the Fe2O3 standard.
Ag nanoparticles of 1.5 to 7 nm size were produced by ion exchange of soda-lime glass for various duration. Information on local order and thermal vibrations were available by means of X-ray absorption spectroscopy using experiments at the Ag K-edge (25.514 keV). Ratio method and EXAFS fitting procedure were successfully applied to reveal the temperature dependence using the cumulant-expansion method up to third order ones. The temperature dependence of the nearest neighbor Ag-Ag distance appears different from that of polycrystalline Ag foil below 400 K. This effect can be explained by a thermoelastic model describing the mismatch of thermal expansion coefficients of Ag particles and the glass matrix. In addition, the parameter of Ag-Ag bond length of 1.5 nm particles is governed by precursor formation for crystalline Ag nanoparticles. The data of the second cumulant, the Debye-Waller factor (DWF), represent a higher static disorder, especially for nanoparticles of 1.5 - 3.5 nm size, caused by the increasing portion of surface or interface atoms. From the temperature dependent part of DWF we estimated a slightly increased Einstein temperature.
A rough calculation shows that the quadrupole term in the radiation of a forbidden line is usually larger than the dipole produced by an external electric field. This is not true, however, when there is an intermediate state, with which both initial and final states combine, and which lies close to one of them.
If the J selection rule is violated, and the Laporte rule is obeyed, the radiation cannot be due to the quadrupole term and must be ascribed to the octopole. Hg 2270 is such a line. An octopole transition will have a Zeeman effect distinctively different from that of a dipole or quadrupole.
A general theory of the extended x-ray absorption edge fine structure is
given within the framework of a one-electron approximation. An
approximate evaluation of this theory is proposed which allows a simple
calculation of the spectrum starting from theoretically calculated
electron-atom scattering phase shifts. This is shown to agree quite well
with the observed spectrum for copper in the energy range 200-800 eV
above the K edge without the use of any adjustable parameters. A
qualitative evaluation of multiple scattering effects in the general
theory is made which should be reasonably good below 100 eV. Multiple
scattering corrections are found to be very important in the low-energy
region of the spectrum for close-packed structures. It is shown that
significant cancellation of single scattering amplitudes can occur which
could account for detailed anomalies in the spectrum of Cu found by
Stern, Sayers, and Lytle.
This self-contained, comprehensive book describes the fundamental
properties of soft x-rays and extreme ultraviolet (EUV) radiation and
discusses their applications in a wide variety of fields, including EUV
lithography for semiconductor chip manufacture and soft x-ray
biomicroscopy. The author begins by presenting the relevant basic
principles such as radiation and scattering, wave propagation,
diffraction, and coherence. He then goes on to examine a broad range of
phenomena and applications. The topics covered include EUV lithography,
biomicroscopy, spectromicroscopy, EUV astronomy, synchrotron radiation,
and soft x-ray lasers. He also provides a great deal of useful reference
material such as electron binding energies, characteristic emission
lines and photo-absorption cross-sections. The book will be of great
interest to graduate students and researchers in engineering, physics,
chemistry, and the life sciences. It will also appeal to practicing
engineers involved in semiconductor fabrication and materials science.
An Auger variant of the x-ray-absorption fine-structure (EXAFS) technique has been successfully applied to study the adsorption site and adsorbate-substrate bond length in a single-crystal system. The surface-EXAFS technique should have widespread applications in surface crystallographic studies.
The X-ray absorption spectra of the 3d and 4d transition metals have been calculated within the single-particle approximation by a new linearized augmented plane wave method. The spectra, calculated with sharp atomic and band-structure single-particle levels, have been convoluted with a Lorentzian broadening function whose width is the sum of that of the core hole and the excited electrons. Plots are shown for (i) the K-edge fine structures up to at least 100 eV above the edge for Ca, Ti, Cr, Co, Cu, and Zn, (ii) the L2, 3 white lines for Ca, Ti, Cr, Co, and Cu, (iii) the L3 white lines for Sr, Zr, Nb, Ru, Rh, and Pd, and (iv) the M2, 3 and M4,5 spectrum of Pd. Systematic features which depend on the crystal structure and the placement of the Fermi level with conduction band are briefly discussed.