Simple derivation of L-absorption spectra of 3d transition metal elements by the self-absorption effect observed in soft X-ray emission spectra.

This study proposes a simple evaluation method for deriving L-absorption information from two L-emission spectra of 3d transition metal (TM) elements obtained at two different accelerating voltages. This method realizes a spatial identity for X-ray emission and absorption spectroscopies. This method was evaluated for the Fe L-emission spectra of Fe and its oxides, and was applied to the TM L-emission spectra of MnO, Co, CoO, and NiO. The derived absorption peak positions were consistent with those obtained previously at synchrotron orbital radiation facilities, which considered the core-hole effect. This simple derivation method could be useful for obtaining X-ray absorption spectroscopy distribution images from X-ray emission spectroscopy mapping data obtained by scanning electron microscopy.

Chemistry of zipping reactions in mesoporous carbon consisting of minimally stacked graphene layers.

The structural evolution of highly mesoporous templated carbons is examined from temperatures of 1173 to 2873 K to elucidate the optimal conditions for facilitating graphene-zipping reactions whilst minimizing graphene stacking processes. Mesoporous carbons comprising a few-layer graphene wall display excellent thermal stability up to 2073 K coupled with a nanoporous structure and three-dimensional framework. Nevertheless, advanced temperature-programmed desorption (TPD), X-ray diffraction, and Raman spectroscopy show graphene-zipping reactions occur at temperatures between 1173 and 1873 K. TPD analysis estimates zipping reactions lead to a 1100 fold increase in the average graphene-domain, affording the structure a superior chemical stability, electrochemical stability, and electrical conductivity, while increasing the bulk modulus of the framework. At above 2073 K, the carbon framework shows a loss of porosity due to the development of graphene-stacking structures. Thus, a temperature range between 1873 and 2073 K is preferable to balance the developed graphene domain size and high porosity. Utilizing a neutron pair distribution function and soft X-ray emission spectra, we prove that these highly mesoporous carbons already consist of a well-developed sp2-carbon network, and the property evolution is governed by the changes in the edge sites and stacked structures.

Characteristics and exchange interactions observed in L-emission spectra of Fe, Mn and their oxides by using a high-energy-resolution soft X-ray emission spectroscopy instrument.

For examining the characteristics of L-emission spectra of Fe, Mn and their oxides, a larger energy-dispersion spectrometer for an electron probe microanalyser was constructed. The energy resolution was evaluated to be 0.3 eV at the Fermi edge observed for the B K-emission of LaB6. The Lα,ß-emission profiles and peak positions of those oxides were different from those of pure metals, reflecting the different density of states of valence bands and different charge states of metal elements. The Lℓ-emission profiles of the oxides showed shoulder structures, even though the emission is caused by transitions between two inner shell levels. The presence of the shoulder structures was assigned to the result of the 3s3d exchange interaction in the final state of the Lℓ emission, in which the 3s state has a spin. The Lℓ profiles were decomposed into two peaks by Lorentz fit, and the energy separation was evaluated to be ∼3 eV.

Analytical technique for self-absorption structure of iron L-emission spectra obtained by soft X-ray emission spectrometer.

The method deriving the L self-absorption spectrum from Lα,ß emission spectra obtained at different accelerating voltages has been optimized for analyzing the chemical state of Fe in solid materials. Fe Lα,ß emission spectra obtained are fitted using Pseudo-Voigt functions and normalized by the integrated intensity of each Fe Ll line, which is not affected by L2,3 absorption edge. The self-absorption spectrum is calculated by dividing the normalized intensity profile collected at low accelerating voltage by that collected at a higher accelerating voltage. The obtained profile is referred to as soft X-ray self-absorption structure (SX-SAS). This method is applied to six Fe-based materials (Fe metal, FeO, Fe3O4, Fe2O3, FeS and FeS2) to observe different chemical states of Fe in those materials. By comparing the self-absorption spectra of iron oxides, one can observe the L3 absorption peak structure shows a shift to the higher energy side as ferric (3+) Fe increases with respect to ferrous (+2) Fe. The intensity profiles of self-absorption spectra of metallic Fe and FeS2 shows shoulder structures between the L3 and L2 absorption peaks, which were not observed in spectra of Fe oxides. These results indicate that the SX-SAS technique is useful to examine X-ray absorption structure as a means to understand the chemical states of transition metal elements.

In situ observation of the chemical bonding state of Si in the molten state of eutectic Au-Si alloy of Au81Si19 by using a soft X-ray emission spectroscopy electron microscope.

Phase diagram of Au-Si binary alloy system shows a large drop in melting temperature of about 1000K compared with that of Si at a composition of Au:Si = 81:19, where the melting temperature is about 636K. Mixing of Au and Si below the melting temperature was observed by transmission electron microscopy experiment, and it was found that the mixed region shows a diffraction pattern of a diffuse ring intensity indicating an amorphous structure of the mixed area. Si L-emission spectra, which reflect the energy state of bonding electrons of Si atom, of molten Au81Si19 alloy were measured for the first time to investigate the energy state of valence electrons of Si. The Si L-emission spectrum showed a characteristic loss of L1 peak, which is related to sp3 directional bonding in crystalline Si. The intensity profile is also different from that of molten Si reported. This suggests a characteristic atomic arrangement that exists in the molten state. The intensity profile also indicated a small density of state in the molten state at Fermi energy. The obtained spectrum was compared with the calculated density of state of possible crystal structures reported. The comparison suggested that Si atoms are surrounded by eight Au atoms in the molten state of Au81Si19 alloy. The formation of this local atomic arrangement can be an origin of a large drop of melting temperature at about Au:Si = 81:19.

Design and experimental evaluation of enhanced diffraction efficiency of lanthanum-based material coated laminar-type gratings in the boron K-emission region.

Numerical and experimental studies have been performed to evaluate the enhancement of diffraction efficiency of diffraction gratings around B $K$-emission by overcoating lanthanum series layers on conventional metal-coated laminar-type gratings. We propose an optical design method based on the concept of spectral flux given by collection efficiency and diffraction efficiency. A diffraction grating with a small angle of incidence provides an advantage to soft x-ray spectrographs because it collects the emission at a larger solid angle compared to that of conventional grazing incidence diffraction gratings. Numerical calculations indicated that La and ${\rm{La}}{{\rm{F}}_3}$ were promising as overcoating materials on a laminar-type Ni-coated diffraction grating, and we performed an experimental study using ${\rm{La}}{{\rm{F}}_3}$ and La/C overcoatings, considering their producibility and durability. The diffraction efficiencies were measured using a reflectometer at a synchrotron facility. The diffraction efficiencies observed at 183.4 eV were 29.4% and 34.3% at angles of incidence of 85.1° and 84.9° for ${\rm{Ni}}/{\rm{La}}{{\rm{F}}_3}$ and Ni/La/C gratings, respectively.

Electron transfer in LiMn1.5Ni0.5O4 during charging studied with soft X-ray spectrometry.

This is the first report on analyzing the chemical state of Li-ion battery electrodes at different states of charge by using a wavelength-dispersive spectrometer, which has a two-order improved energy resolution in the soft X-ray energy region compared with that of a conventional energy-dispersive X-ray analyzer. Electrodes containing LiMn1.5Ni0.5O4 were charged to prepare Li0.5Mn1.5Ni0.5O4 and λ-Mn0.75Ni0.25O2. The soft X-ray emission spectra obtained from those materials show that the O-K emission signal was drastically decreased throughout the charging process. This suggests that O-2p electron contributed to the electrochemical oxidation. The density of states and Bader charge evaluated from ab initio calculation support this result.

Evaluation of TEM specimen quality prepared by focused ion beam using symmetry breaking index of convergent-beam electron diffraction.

Degradation of the crystalline quality of transmission electron microscopy specimens in silicon prepared with different conditions has been examined using convergent-beam electron diffraction (CBED). The specimens are prepared using focused ion beam (FIB) with different accelerating voltages, Ar-ion milling and crushing method. Symmetry breaking of CBED patterns was quantitatively evaluated by symmetry breaking index S, which has been previously reported. The degradation and inhomogeneity of the FIB specimen were suppressed by decreasing the accelerating voltages of the FIB fabrication in the final process.

Microdiamond in a low-grade metapelite from a Cretaceous subduction complex, western Kyushu, Japan.

Microdiamonds in metamorphic rocks are a signature of ultrahigh-pressure (UHP) metamorphism that occurs mostly at continental collision zones. Most UHP minerals, except coesite and microdiamond, have been partially or completely retrogressed during exhumation; therefore, the discovery of coesite and microdiamond is crucial to identify UHP metamorphism and to understand the tectonic history of metamorphic rocks. Microdiamonds typically occur as inclusions in minerals such as garnet. Here we report the discovery of microdiamond aggregates in the matrix of a metapelite from the Nishisonogi unit, Nagasaki Metamorphic Complex, western Kyushu, Japan. The Nishisonogi unit represents a Cretaceous subduction complex which has been considered as an epidote-blueschist subfacies metamorphic unit, and the metapelite is a member of a serpentinite mélange in the Nishisonogi unit. The temperature condition for the Nishisonogi unit is 450 °C, based on the Raman micro-spectroscopy of graphite. The coexistence of microdiamond and Mg-carbonates suggests the precipitation of microdiamond from C-O-H fluid under pressures higher than 2.8 GPa. This is the first report of metamorphic microdiamond from Japan, which reveals the hidden UHP history of the Nishisonogi unit. The tectonic evolution of Kyushu in the Japanese Archipelago should be reconsidered based on this finding.

Information of valence charge of 3d transition metal elements observed in L-emission spectra.

L-emission spectra of 3d transition metal elements from Sc to Zn and some oxides were measured to examine the relation between L-emission intensities of Lα, Lß, Lℓ, and Lη and valences of those elements by using a soft X-ray emission spectrometer attached to a scanning electron microscope. Lα,ß emission intensity due to transitions from valence bands to core 2p levels compared with Lℓ,η emission intensity due to transitions from core 3 s to deeper 2p levels, Lα,ß/Lℓ,η was found to be a key parameter. A linear relation was found between the number of 3d electrons and the intensity ratio of Lα,ß/(Lα,ß+ Lℓ,η) from Sc to Ni, except for Cr. It takes into account not only a change in N3d but also a change of transition probability due to a change in N3d In the case of 3d metal oxides, the evaluation based on the equation showed an overestimation of the calculated number of 3d electrons, which could be due to a charge transfer from ligand oxygen atoms to the transition metal element, resulting from a core-hole effect in the intermediate state.

Non-uniform Excitation States in Photoinduced Deformation of Amorphous Carbon Nitride Films.

Amorphous carbon nitride (a-CNx) films prepared via reactive radio frequency magnetron sputtering deform under on-off visible light illumination. We investigate the relationship between photoinduced deformation and surface electrical states via scanning electron microscopy with Ar+ laser irradiation (SEM-L). Two samples with different levels of photoinduced deformation are prepared. For the film with small photoinduced deformation, uniform secondary electron emission is observed on the film surface, regardless of whether the laser is on or off. On the a-CNx film, which has fifty times larger photoinduced deformation than the previous film, light and dark patches, similar to a speckle pattern, appear on the film surface in SEM-L images. This anomalous phenomenon indicates non-uniformity of the electrical states excited by laser light irradiation. A size of the patches is well correlated with an inhomogeneous distribution of sp3C and sp2C, Isp3C/Isp2C, obtained using soft X-ray emission spectroscopy (SXES). Simultaneously, temporal decrease in the sp3C component under illumination is obtained via SXES.

Direct observation of Mn and Ni ordering in LiMn1.5Ni0.5O4 using atomic resolution scanning transmission electron microscopy.

LiMn1.5Ni0.5O4 is an excellent candidate as a cathode-active material in high-voltage lithium-ion batteries and studied using atomic resolution scanning transmission electron microscope. High-angle annular dark-field (HAADF) images obtained at [100] orientation demonstrate that Mn and Ni atoms are regularly ordered at octahedral sites in a spinel structure, in a 3:1 ratio between columns with high and low intensities. Simulations of HAADF images revealed that atomic columns including Mn exhibit a larger intensity than that by Ni columns, primarily because of the effect of the Debye-Waller factor.

Soft X-ray emission spectroscopy study of characteristic bonding states and its distribution of amorphous carbon-nitride (a-CNx) films.

Soft X-ray emission spectroscopy based on electron microscopy was applied to investigate bonding electron states of amorphous carbon nitride (a-CNx) films with different nitrogen contents of x. Carbon K-emission spectrum showed characteristic intensity distribution of not only sp2 bonding but also sp3 bonding. The a-CNx film with lager x, which has a larger macroscopic electric resistivity, shows a larger content of the carbon sp3: C-C bonding signal. Furthermore, the dependence of spectral intensity distribution on x suggests the presence of sp2: C-N and sp3: C-N bonding. Those results show that the relation between macroscopic electrical resistivity of a-CNx film and its nitrogen content is because of the decrease of sp2: C-C bonding and the formation of sp2: C-N and sp3: C-C and C-N bonding conformation induced by an introduction of nitrogen atoms. Spatial variation of a signal ratio of sp3/sp2 was visualized and was confirmed as a relation between sp3 boding amount and nitrogen content x.

Soft X-ray emission spectroscopy study of electronic structure of sodium borosilicide Na8B74.5Si17.5.

Chemical bonding state of sodium borosilicide Na8B74.5Si17.5, which is a new member of B12-cluster materials, is investigated by soft X-ray emission spectroscopy. The material is composed of B12 cluster network and characteristic silicon chains of [-Si-(Si-Si)3-Si-] connected by sp3 bonding, in which bonding distances and bonding angles are close to those in cubic Si crystal. B K-emission spectrum of the material showed a similar but a broader intensity distribution with those of B12 cluster materials of α-r-B, B4C and ß-r-B. The broader intensity distribution can be due to a variation of B-B bond length in B12 cluster. The density of states (DOS) of silicon chains of [-Si-(Si-Si)3-Si-] was experimentally derived. It shows a similar energy width, and peak or shoulder structures in intensity distribution with those of L-emission spectrum of cubic Si. From comparisons between experimental spectra and corresponding calculated DOS, covalent bonding between Si chain and B12 cluster network is suggested. Those are discussed by using a theoretically calculated density of state of Na8B74.5Si17.5 by using WIEN2k code.

Na-Ga-Si type-I clathrate single crystals grown via Na evaporation using Na-Ga and Na-Ga-Sn fluxes.

Single crystals of a Na-Ga-Si clathrate, Na8Ga5.70Si40.30, of size 2.9 mm were grown via the evaporation of Na from a Na-Ga-Si melt with the molar ratio of Na : Ga : Si = 4 : 1 : 2 at 773 K for 21 h under an Ar atmosphere. The crystal structure was analyzed using X-ray diffraction with the model of the type-I clathrate (cubic, a = 10.3266(2) Å, space group Pm3̄n, no. 223). By adding Sn to a Na-Ga-Si melt (Na : Ga : Si : Sn = 6 : 1 : 2 : 1), single crystals of Na8Ga x Si46-x (x = 4.94-5.52, a = 10.3020(2)-10.3210(3) Å), with the maximum size of 3.7 mm, were obtained via Na evaporation at 723-873 K. The electrical resistivities of Na8Ga5.70Si40.30 and Na8Ga4.94Si41.06 were 1.40 and 0.72 mΩ cm, respectively, at 300 K, and metallic temperature dependences of the resistivities were observed. In the Si L2,3 soft X-ray emission spectrum of Na8Ga5.70Si40.30, a weak peak originating from the lowest conduction band in the undoped Si46 was observed at an emission energy of 98 eV.

Postsynthesis of h-BN/Graphene Heterostructures Inside a STEM.

Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in-plane graphene growth from the step-edge of hexagonal BN (h-BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h-BN and graphene is atomically identified as largely N-C bonds. This postgrowth method can form graphene nanoribbons connecting two h-BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h-BN layer. The electronic properties of the vertically stacked h-BN/graphene heterostructures are investigated by electron energy-loss spectroscopy (EELS). Low-loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h-BN layers.

ɛ-TiO, a Novel Stable Polymorph of Titanium Monoxide.

For the Ti/O system, three titanium monoxide (TiO) phases (α, ß, and γ) with defective NaCl-type structures and a high-temperature hexagonal phase (H) have been known for decades. In this work, single crystals of a novel polymorph, ɛ-TiO, were synthesized by using a bismuth flux. X-ray diffraction (XRD) revealed a hexagonal crystal structure (a=4.9936(3) Å, c=2.8773(2) Å, P6‾ 2m) that is isotypic with ɛ-TaN. While the Ti atoms are surrounded by trigonal prismatic (sixfold coordination) and trigonal planar (threefold coordination) arrangements of O atoms, the O atoms are found in a pseudo-square-pyramidal arrangement of Ti atoms. First-principles calculations of the formation enthalpy and the electron and phonon density of states and crystal orbital Hamilton population (COHP) analysis revealed that ɛ-TiO is more stable than α-TiO, which had previously been regarded as the most stable phase at low temperatures.

High energy-resolution electron energy-loss spectroscopy study of the dielectric properties of multi-shell nanoparticles.

Nanoparticles, which have multi-shell structure, are expected to be stable and high efficient for the light-emitting devices. The efficiency of luminescence is considered to be affected by the multi-shell structure. In order to understand the mechanism of high efficiency luminescence, it is necessary to evaluate the multi-shell structure and dielectric properties from each particle. High energy-resolution electron energy-loss spectroscopy (HR-EELS) based on TEM is a powerful tool for this purpose. By comparing between the experimental and the simulated results, it is possible to evaluate the effect of the size and physical property of each shell material on the dielectric properties of multi-shell nanoparticles. In this study, simulations of EELS spectra of multi-shell nanoparticle (core: CdSe, inner shell: CdS, outer shell: ZnS) and mono-shell nanoparticle (core: CdSe, shell: CdS) were conducted by the dielectric continuum theory[1].Figure 1 shows calculated EELS spectra of multi and mono shell nanoparticles. The spectra are calculated from dielectric functions of single CdSe, CdS and ZnS crystals, which were experimentally derived from HR-EELS spectra by using Kramers-Kronig analysis. The radius of 6.9 nm for the nanoparticle in the simulation corresponds to the average size of actual synthesized nanoparticles. Energy positions of arrows in the inset correspond to band gap energies of CdSe, CdS and ZnS[2]. In the spectrum of multi-shell nanoparticle, the intensity corresponding to interband transition near band gap of CdSe is suppressed comparing with that of the mono shell nanoparticle. This result indicates that ZnS outer shell affects the intensity profile of EELS spectrum near band gap. This effect should be sensitive for the thickness of the shells. Thus, there is a possibility that the effect of size and thickness of each core and shell on dielectric properties of multi-shell nanoparticles could be evaluated by using HR-EELS technique.jmicro;63/suppl_1/i18/DFU039F1F1DFU039F1Fig. 1.Calculated EELS spectra.

Optical properties of Group X-XII intermetallic compounds studied by HR-EELS.

Electronic structure of d orbital states in transition metals is a key factor for their physical properties and chemical functions. Copper and intermetallic compound PdZn have good catalysis function for the methanol steam reforming reaction. Tsai et al. showed that from results of XPS measurements the d electronic structure of PdZn was similar with that of copper, and the catalysis function should be related to the d electron states [1]. This similarity of d electronic states leads to another view point of the mechanism for coloring the intermetallic compounds. It is well-known that the characteristic red color of copper is caused by interband transition from the d electrons. Therefore, PdZn and Group X-XII intermetallic compounds are expected to be colored and the optical properties should depend on the d electronic states. In this study, the relations between optical properties and d electron states of Group X-XII intermetallic compounds were investigated by using high energy-resolution electron energy-loss spectroscopy (HR-EELS) based on transmission electron microscopy (TEM). From the relation between optical properties and d electronic states, the mechanism of colored intermetallic compounds will be discussed.Figure shows the optical reflectivity of NiZn, PdZn and PtZn, which were derived from EELS spectra by Kramers-Kronig analysis. Intensity drops (arrows) of the reflectivity were observed in visible energy region. These are caused by the interband transitions from d electronic states. The energy positions of the reflectivity drops have tendency of shifting to higher energy side with increasing atomic number of Group X elements (Ni → Pd → Pt). This indicates that the transition energies of d electrons become larger with the atomic number of the elements. First principle calculations (WIEN2k) confirmed that the interband transitions of d electronic states were excitations from bonding d states to hybrid states of anti-bonding s, p, and d states of Group X elements. The bonding anti-bonding energy split increase with the atomic numbers because of increasing crossover of wave function. This implies the intermetallic compounds should be colored and the color should be changed gradually depending on the atomic number of Group X elements.

High-energy resolution electron energy-loss spectroscopy study of interband transitions characteristic to single-walled carbon nanotubes.

An electron energy-loss spectroscopic (EELS) study using a monochromator transmission electron microscope was conducted for investigating the dielectric response of isolated single-walled carbon nanotubes (SWCNTs) owing to interband transitions characteristic to chiral structures. Individual chiral structures of the SWCNTs were determined by electron diffraction patterns. EELS spectra obtained from isolated SWCNTs showed sharp peaks below π plasmon energy of 5 eV, which were attributed to the characteristic interband transitions of SWCNTs. In addition, unexpected shoulder structures were observed at the higher energy side of each sharp peak. Simulations of EELS spectra by using the continuum dielectric theory showed that an origin of the shoulder structures was because of the surface dipole mode along the circumference direction of the SWCNT. It was noticed that the electron excitation energies obtained by EELS were slightly higher than those of optical studies, which might be because of the inelastic scattering process with the momentum transfers. To interpret the discrepancy between the EELS and optical experiments, it is necessary to conduct more accurate simulation including the first principle calculation for the band structure of SWCNTs.