Soft X-ray Absorption Spectroscopy Measurements Research Articles

Digging deeper: Buried layers and interfaces studied by modified total electron yield and soft x-ray absorption spectroscopy

We report on the soft x-ray absorption spectroscopy investigation of thin film capacitors using a modified total electron yield detection mode. This mode utilizes two ammeters instead of one as commonly employed in the classical total electron yield scheme to measure photocurrents of devices under soft x-ray irradiation. The advantage of this configuration over the surface sensitive classical total electron yield mode is that it can provide information from buried layers and interfaces up to a thickness equal to the penetration depth of soft x-rays. The method can be easily adapted to existing synchrotron end stations. We investigate dielectric capacitors with dissimilar electrodes to assess the feasibility of the modified total electron yield method. Furthermore, in operando soft x-ray absorption spectroscopy measurements are performed on ferroelectric capacitors under bias and using two ammeters. The experimental results are discussed in terms of the external and internal photoemission processes and their distribution in thin film capacitors under an external bias condition. The proposed detection method opens the way to perform electronic and chemical state analyses of the buried interfaces and layers in various devices like multiferroic tunnel junctions, memristive devices, etc., during operation under an applied bias.

Oxygen-Vacancy-Driven Orbital Reconstruction at the Surface of TiO2 Core-Shell Nanostructures.

Oxygen vacancies and their correlation with the electronic structure are crucial to understanding the functionality of TiO2 nanocrystals in material design applications. Here, we report spectroscopic investigations of the electronic structure of anatase TiO2 nanocrystals by employing hard and soft X-ray absorption spectroscopy measurements along with the corresponding model calculations. We show that the oxygen vacancies significantly transform the Ti local symmetry by modulating the covalency of titanium-oxygen bonds. Our results suggest that the altered Ti local symmetry is similar to the C3v, which implies that the Ti exists in two local symmetries (D2d and C3v) at the surface. The findings also indicate that the Ti distortion is a short-range order effect and presumably confined up to the second nearest neighbors. Such distortions modulate the electronic structure and provide a promising approach to structural design of the TiO2 nanocrystals.

Investigation of structural, magnetic and electronic properties of FeTa films for varying Ta concentration at different annealing temperatures

In the present work, temperature dependent magnetic, structural and electronic properties of FeTa films with increasing Ta concentration are investigated. X-ray reflectivity (XRR) studies reveal densification of the film with increasing Ta concentration. Thermal annealing also results in an increased electron density relative to as-deposited film, concurrent with increase in grain size as observed from X-ray diffraction (XRD) and results of Soft-X-ray absorption spectroscopy (SXAS). However, grain growth is inhibited with increasing Ta concentration by presence of Ta in grain boundary (GB) region. This also results in increase in magnetic coercivity (greater than150 %) as observed from magneto-optical Kerr effect (MOKE). The structural properties were further correlated with magnetic properties as observed from Polarized neutron reflectivity (PNR) and Ferromagnetic resonance (FMR). Effective magnetic moment of the samples, as observed from PNR decreases with increasing Ta concentration and increases with increasing annealing temperature, which is in agreement with FMR results. Scattering length density profiles, observed from simultaneous fitting of XRR and PNR show presence of an oxide layer at surface, as also confirmed from SXAS measurements.

Mixed Mott–Hubbard and charge transfer nature of 4H–SrMnO3 thin film on Si (100)

Room temperature electronic structure of polycrystalline 4H–SrMnO3 thin film grown on Si (100) substrate has been studied using resonance photo emission spectroscopy and soft x-ray absorption spectroscopy measurements. Presence of charge transfer screen Mn 3d n L final state along with the 3d n-1 final state at the valence band edge of 4H–SrMnO3 thin film confirms that the ground state is strongly mixed between Mn 3d and O 2p states. The estimated equivalent values of on-site Coulomb interaction energy (U) and O 2p to Mn 3d- charge transfer energy (Δ) (U ≈ Δ ≈ 4.8 eV) from the combination of occupied and unoccupied spectra further confirm the intermediate Mott–Hubbard and charge transfer insulator nature of 4H–SrMnO3 film. Despite having similar Mn 4+ valence state in 4H–SrMnO3 and cubic SrMnO3, 4H phase is observed to reveal much higher band gap ∼1.5 eV than the cubic phase (0.3 eV), which arises due to different MnO6 octahedra environment.

Charge Compensation Mechanism in Li-Excess Oxides Revealed By Operando Soft/Hard X-Ray Absorption Spectroscopy

Introduction Li-excess metal oxide having high charge-discharge capacity has been expected to realize higher energy density in lithium-ion batteries. Several lithium-excess metal oxides have been studied extensively in order to use effectively the charge compensation by redox reaction of oxide ions. [1-5] In 4d and 5d metal oxides such as Li2RuO3 [1] and Li2IrO3-based systems [2] , high covalent bond between 4d orbital of transition-metal and 2p orbital of oxygen enables charge compensation by the oxidation of oxide ions to peroxo-like dimer. On the other hand, in a 3d transition-metal oxide, Li1.2Ni0.13Co0.13Mn0.54O2 exhibits formation of isolated holes on oxide ions because highly ionized 3d transition-metal ions stabilize the isolated holes on the oxide ions surrounded by lithium ions. [3, 4] Other 3d transition-metal oxides such as Li1.2Ti0.4Mn0.4O2, also exhibit stable oxide ions redox. [5] According to the previous studies, covalent character or ionic character between transition-metal ions and oxide ions in Li-excess metal oxides is a key factor to stabilize oxide ions redox. However, behavior of oxide ions redox in Li-excess metal oxides with different covalent character or ionic character has not been systematically examined. In this study, we focus on four different Li-excess metal oxides, Li2MeO3 (Me = Mn and Ru) and Li1.2Ti0.4Me0.4O2 (Me = Mn and Fe), as models with different covalent character or ionic character. Developing a new operando soft X-ray absorption spectroscopy, we observed directly the electronic state of the oxide ions in these materials during charge process via the technique. [6] Experimental operando soft X-ray absorption spectroscopy measurements were performed at BL27SU in SPring-8, Japan. The absorption spectra were collected with partial fluorescence mode using home-made cell. Composite electrodes were pasted on aluminum sputtered-Si3N4 window. For Li1.2Ti0.4Mn0.4O2, half charged-LiFePO4 was used as a counter electrode, and 4.2 mol dm-3 lithium bis(trifluoromethanesulfonyl)amide/ acetonitrile was used as an electrolyte solution. Two electrode cells were assembled in the Ar-filled grovebox. XAS spectra were collected during galvanostatic measurement at rate of 1/20 C rate at 55°C. Results and Discussion The charge and discharge profiles of the Li1.2Ti0.4Mn0.4O2 showed that the voltage gradually increased at initial stage under charging process, then a plateau was observed at approximately 4.2 V. The first discharge profile was similar to the first charge one. The first discharge capacity was 270 mAh/g, which was comparable to the first charge capacity of 300 mAh/g. operando Mn L-edge XAS spectra of the Li1.2Ti0.4Mn0.4O2 showed that a board peak was observed at 642 eV before charge process. The peak separated to two peaks and shifted higher energy in the slope region of charge process, and the two peaks unchanged with subsequent charge process. The peak separation and shift toward higher energy in the slope region were caused by spin multiplicity and crystal field effect with oxidation Mn ions. No change of the spectra in the subsequently charge process means that the Mn ions did not contribute to the charge compensation. operando O K-edge XAS spectra of Li1.2Ti0.4Mn0.4O2 showed a board peak was observed around 531 eV before charge process. The peak was divided into two peaks during charge process. These peaks reflect transition from O 1s orbitals to hybridized states between 3d orbital of transition-metal and 2p orbital of oxygen. The integrated intensity of the peak at the lower energy increased in the slope region of charge process, whereas the integrated intensity of the peak at the higher energy increased with subsequent charge process. The change of the spectrum in the slope region was caused by oxidation Mn ions (Mn3+ to Mn4+) as observed in the Mn-L XANES spectra. The increase of the integrated intensity at the higher energy in the subsequently charge process was caused by oxidization of oxygen ions.

Study of local distortion and spin reorientation in polycrystalline Mn doped [formula omitted

The present work reports preparation of polycrystalline GdFe1−xMnxO3 (x = 0.0 to 0.5) samples with solid-state reaction method and characterization using x-ray diffraction, 57Fe Mössbauer spectroscopy, magnetization measurements, leakage current density and soft x-ray absorption spectroscopy measurements. Although Fe3+and Mn3+ have similar ionic radii, noticeable variations in lattice parameters are seen which are attributed to local distortions introduced by Jahn-Teller active Mn3+ doping. Antiferromagnetic transition temperature corresponding to Fe3+ ordering (TN,Fe) and Gd3+ ordering (TN,Gd) is found to decrease with Mn doping, which could be due to weakening of Fe-Fe interaction and Gd-Gd interactions respectively. The Mn substitution at Fe site in GdFeO3 is found to induce Fe3+ spin reorientation transition (TSRT) at a critical concentration and TSRT is found to increase with the Mn doping. Temperature dependent 57Fe Mössbauer measurements have been carried out to study the local distortions by estimating quadrupole shift, the evolution of magnetism and also the spin reorientation transition in GdFeO3 with Mn doping. Leakage current density is found to increase with Mn doping in GdFeO3, initially very fast and almost saturating for higher concentrations of Mn doping due to the formation of multiple oxidation states of dopant Mn at lower concentrations as evidenced from soft x-ray absorption spectroscopy measurements.

Reaction Mechanism Analysis of a Li-O2 Battery: Structure of Electrode/Electrolyte Interface Probed Via Soft-x-Ray Absorption Spectroscopy, Hard x-Ray Photoelectron Spectroscopy, and in-Situ Hard x-Ray Diffraction

Rechargeable Li-air batteries have been attracting much attention because of their extraordinary high energy capacity, viz, theoretically it reaches more than ten-times than those of existing Li-ion secondary battery. Recent materials development, such as use of glyme-series electrolytes, effective electrodes (metal based catalysts and/or high surface area carbon materials with nitrogen doping), or use of Redox-mediators, is about to find realistic solutions to overcome long-term crucial issues of Li-air battery, such as large over-potential at charge process, electrolyte degradation due oxygen radicals, and low cycleabilty presumably caused by unstable electrode properties at both anodes and cathodes. Thus, additional deeper information on the reaction mechanism, involving unwanted side reactions, could be expected to accelerate such improvement. In this context, we performed reaction mechanism analysis by utilizing various experimental techniques, such as in-situ time-resolved x-ray diffraction, soft x-ray absorption spectroscopy at Li, C, O, and S K-edges, and hard x-ray photoelectron spectroscopy (HAXPES), combined with other conventional laboratory analysis methods.Charge/discharge tests of Li-O2 battery were performed by using standard coin-type or electrochemical cells under various conditions including impact of moisture to the reaction. Soft x-ray absorption spectroscopy measurements were carried out at SR center of Ritsumeican University (BL-11) and Aichi synchrotron radiation center (BL6N1). Hard x-ray photoelectron spectroscopy and in-situ hard x-ray diffraction measurements were performed at SPring-8 on beamlines BL46XU and BL19B2.We observed formation and dissolution processes of Li2O2 via in-situ time-resolved hard x-ray diffraction, which exhibited non-linear behaviors reflecting difference of nature of deposited Li2O2 particles. Also we analyzed the structure and chemical compositions of interface between Li2O2 and/or carbon electrodes and electrolyte solutions via soft x-ray and hard x-ray spectroscopy. The result indicates the structure of interface is significantly affected by the operation condition, and consists of various species decomposed from electrolyte and/or Li-salts. More detailed discussion will be presented at the Meeting.

K=0 magnetic structure and absence of ferroelectricity in SmFeO3.

SmFeO3 has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO3 single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

Charge-order melting in charge-disproportionated perovskite CeCu3Fe4O12.

A novel quadruple perovskite oxide CeCu3Fe4O12 has been synthesized under high-pressure and high-temperature conditions of 15 GPa and 1473 K. (57)Fe Mössbauer spectroscopy displays a charge disproportionation transition of 4Fe(3.5+) → 3Fe(3+) + Fe(5+) below ∼270 K, whereas hard X-ray photoemission and soft X-ray absorption spectroscopy measurements confirm that the Ce and Cu valences are retained at approximately +4 and +2, respectively, over the entire temperature range measured. Electron and X-ray diffraction studies reveal that the body-centered cubic symmetry (space group Im3̅, No. 204) is retained at temperatures as low as 100 K, indicating the absence of any types of charge-ordering in the charge-disproportionated CeCu3Fe4O12 phase. The magnetic susceptibility and neutron powder diffraction data illustrate that the antiferromagnetic ordering of Fe ions is predominant in the charge-disproportionated CeCu3Fe4O12 phase. These findings suggest that CeCu3Fe4O12 undergoes a new type of electronic phase in the ACu3Fe4O12 series and that the melting of the charge-ordering in CeCu3Fe4O12 is caused by the substantial decrease in the Fe valence and the resulting large deviation from the ideal abundance ratio of Fe(3+):Fe(5+) = 1:1 for rock-salt-type charge-ordering.

軟X線吸収分光によるLi1^|^minus;xNi0.5Mn1.5O4の化学状態の解析

Soft X-ray absorption spectroscopy measurements were carried out in order to clarify the chemical states of Li1−xNi0.5Mn1.5O4 for the cathode in lithium ion batteries. XANES spectra exhibited that Ni valence varies from divalent to tetravalent via an intermediate state of trivalent although Mn valence remains tetravalent. These phenomena originate from the formation of the occupied d-orbital of Ni in the vicinity of the Fermi level in the valence band of Li1−xNi0.5Mn1.5O4, which was supported by the first-principles calculations. Thus, it is concluded that Ni plays an important role in maintaining charge neutrality of Li1−xNi0.5Mn1.5O4 releasing Li+ ions by the charging of the battery.

Local orbital occupation and energy levels of Co inNaxCoO2: A soft x-ray absorption study

We present a combined experimental and theoretical investigation on the orbital occupation and local symmetry of Co in ${\text{Na}}_{x}{\text{CoO}}_{2}$. We have carried out polarization dependent $\text{Co}\ensuremath{-}{L}_{2,3}$ soft x-ray absorption spectroscopy measurements. By comparing the spectra with the results of configuration-interaction cluster calculations which include the full-atomic multiplet theory, we have found that the local symmetry for the Co ions are well-described by the ${D}_{3d}$ point group with intermixing between the cubic ${t}_{2g}^{\ifmmode\pm\else\textpm\fi{}}$ and ${e}_{g}^{\ifmmode\pm\else\textpm\fi{}}$ orbitals. We have determined that the ${t}_{2g}$ holes in both the ${\text{Na}}_{0.75}{\text{CoO}}_{2}$ and ${\text{Na}}_{0.5}{\text{CoO}}_{2}$ compositions reside mainly in the ${a}_{1g}$ orbital. The spectroscopically deduced ${\text{Co}}^{4+}/{\text{Co}}^{3+}$ ratio agrees well with the nominal Na concentration. We discuss the surprising observation that the single-site cluster approach is still adequate to reproduce the spectra despite the mixed valent character of the Co ions in this compound.

Experimental estimate of electron escape depth in Fe

Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy is a powerful experimental tool to investigate the electronic and atomic structure of materials. The great power of NEXAFS derives from its elemental specificity and symmetry selection rules. Because the various elements have different core level energies, NEXAFS permits extraction of the signal from a surface monolayer or even a single buried layer in the presence of a huge background signal. We have calculated electron escape depth ( λ e ) in Fe at the L 2,3 edge using pure Fe thin films of different thickness by soft x-ray absorption spectroscopy measurements. We have recorded high-resolution x-ray absorption spectra in transmission of Fe thin films resolving the near edge x-ray absorption fine structure. As a conclusion we found the mean escape depth of emitted electrons to be λ e = 22 ± 2 Å . We believe our results provide important information for the improved understanding of theory of the photo absorption mechanism.

High-angular-resolution electron energy loss spectroscopy of hexagonal boron nitride

High-angular-resolution electron energy loss spectroscopy (EELS) is used to study the anisotropic behavior of the boron and nitrogen K ionization edges in h-BN. This work makes significant progress toward improving the anisotropy measurements. The authors show experimentally by EELS the vanishment of the π* peak existing in these K edges in agreement with electronic structure calculations and previous soft x-ray absorption spectroscopy measurements.

Ultra-Soft X-Ray Absorption Spectroscopy: A Bulk and Surface Probe of Materials

ABSTRACTDirect comparisons between the surface and bulk of diverse materials can be made by simultaneous electron yield (5 nm depth sensitivity) and fluorescence yield (200 nm) ultra soft x-ray absorption spectroscopy measurements utilizing a rapid sample interchange apparatus. For example the orientations of functional groups have been characterized at and near the surface of a series of model polymeric materials highlighting the chemical and molecular sensitivity of ultra soft x-ray absorption spectroscopy. In addition we discuss a bulk sensitive use of fluorescence yield to non destructively study a buried metal polymer interface. A second bulk sensitive example is the use of fluorescence yield oxygen K near edge x-ray spectroscopy as a method to determine the hole state density of high Tc materials.