Depth-resolved X-ray Absorption Research Articles

Chemical States of Stainless Steel in nm to Several Tens of nm Region from Surface Observed by the Depth-Resolved X-ray Absorption Spectroscopy.

Chemical states of Fe, Cr, Ni, Mn, and O in stainless steel, SS304, are nondestructively observed over a wide range of depth from nanometer (nm) to several tens of nm by means of the depth-resolved X-ray absorption spectroscopy in the soft X-ray region. It is revealed that Cr is more oxidized in the surface region than the inner region, while Fe is also oxidized at the surface though the oxidation is less prominent compared to Cr. The chemical states of Cr are described by a mixture of Cr-O and Cr-OH coordinations, with more OH components at the surface. Mn and Ni spectra show MnO and metallic Ni features, respectively, throughout the depth of at least up to several tens of nm.

Nanometer-scale depth-resolved hard x-ray absorption spectroscopy based on the detection of energy-loss Auger electrons with low energies

X-ray absorption spectroscopy (XAS) provides information on the chemical state and atomic structure of a target element even without long-range periodicity. A depth-resolved surface-sensitive XAS method for K-edge using hard x rays is proposed herein. The principle of this method is based on the selective detection of low-energy electrons using an electron analyzer. The detected electrons originate from the LMM Auger electrons that cascade from the KLL Auger process caused by K-shell absorption and lose the energy corresponding to their travel distance in a solid. The analysis depth was confirmed to be in the nanometer range using a GaN substrate with a thin oxide film of the defined thickness. In addition, depth-resolved information regarding the local atomic structure, including the interatomic distance and crystallinity, was obtained via the Fourier transformation of the spectral oscillations. Because the proposed technique does not require a change in the detection angle, it is also expected to be used for particles and samples with uneven surfaces.

Effect of cesium for Cu(In,Ga)Se2 and Cu(In,Ga)(S,Se)2 films studied by depth-resolved XAFS

Alkali metal treatment is an indispensable technology for obtaining highly efficient CuInSe2 (CIS) solar cells. This study evaluated the structural change of Cu(In,Ga)Se2 (CIGSe) and Cu(In,Ga)(S,Se)2 (CIGSSe) films with and without the CsF post-deposition treatment (CsF-PDT) (for CIGSe) and Cs treatment (for CIGSSe) by the Se K-edge depth-resolved X-ray absorption fine structure. The CIGSe films were deposited by a three-stage process, and the CIGSSe films were deposited by the sulfurization after the selenization method. Although CsF-PDT can re-form the surface of CIGSe film, the Cs treatment cannot modify the surface of the CIGSSe film. Our result suggests that the improvement is due to not only the surface reforming but also an effect on the inside of the CIGSe and CIGSSe films. This study provides useful information for the preparation of highly efficient CIS solar cells.

Understanding the Overlithiation Properties of LiNi0.6Mn0.2Co0.2O2 Using Electrochemistry and Depth-Resolved X-ray Absorption Spectroscopy

One known drawback of Ni-containing layered cathodes is their poor first cycle efficiency of 85%–90%, upon cycling in a practical potential window. The poor first cycle efficiency is likely a result of surface overlithiation due to significant lithium ion diffusion limitation at this bulk state of charge, but the overlithiation properties of Ni-containing cathodes are currently insufficiently understood. This work focuses on one Ni-containing cathode, LixNi0.6Mn0.2Co0.2O2, and performs detailed characterization of its intercalation properties both in the poor cycling efficiency region as well as in the overlithiation region, where the bulk lithium ion content rises above the value of 1. The results of the study first demonstrate that it is possible to recover the capacity this cathode “loses” in the first cycle by lowering the applied potential. Then, they establish the possibility to overlithiate LixNi0.6Mn0.2Co0.2O2 cathodes by as much as 300 mAhg−1 relative to the pristine electrode. Through complementary characterization using ex situ X-ray diffraction and X-ray absorption spectroscopy both the structural changes and the oxidation state variations in the material throughout the overlithiation process are elucidated. The generated knowledge can be used in developing more accurate physics-based models of industrially-relevant batteries.

Structural analysis of Cu(In,Ga)Se2 thin-films by depth-resolved XAFS

Cu(In,Ga)Se2 (CIGS) is a promising material for thin-film photovoltaic devices. Elucidation of the crystal structure of CIGS thin-films is significant for understanding the properties of CIGS solar cells. This study demonstrates the structural evaluation of CIGS thin-films prepared by a three-stage process using Se K-edge depth-resolved X-ray absorption fine structure (XAFS). The CIGS films changed from a Cu-poor to Cu-rich composition during the second stage of the three-stage process. Notably, Cu2−xSe is generated at the surface of the film at the end of the second stage. After the third stage process, CIGS films have a Cu-poor composition from the surface to a depth of 200 nm. Depth-resolved XAFS technique is sufficiently applicable for evaluation of electronic and crystal structure of CIGS thin-film. This study will provide useful information for the elucidation of the structure of CIGS thin films.

Effect of introducing interlayers into electrode/electrolyte interface in all-solid-state battery using sulfide electrolyte

Introduction of an interlayer between a cathode and a sulfide solid electrolyte is a well-known method for reducing the interfacial resistance and improving the performance of all-solid-state batteries. However, the mechanism responsible for the interlayer remains unclear because it is difficult to observe the reactions at the nanometer-scale range. In this study, thin-film model interface of LiCoO2/80Li2S·20P2S5 and LiCoO2/Li3PO4/80Li2S·20P2S5 are fabricated by pulsed-laser deposition. The model interfaces are investigated by performing electrochemical measurements and depth-resolved X-ray absorption spectroscopy to clarify the effect of the Li3PO4 interlayer. The results indicate that a reaction product layer forms between the LiCoO2 cathode and the 80Li2S·20P2S5 electrolyte during charge/discharge processes, resulting in high interfacial resistance. Meanwhile, the formation of the reaction product layer can be suppressed by the introduction of a Li3PO4 interlayer.

Development of high signal-to-background ratio depth-resolved soft X-ray absorption spectroscopy by fluorescence energy selection

High signal-to-background (S/B) ratio depth-resolved soft X-ray absorption spectroscopy (XAS) is achieved by an energy-selective detection of the fluorescence with a diffraction grating at different emission angles. We apply this technique to a Au/FeCo/BaTiO3 thin film, and selectively observe the Fe and Co Lα fluorescences. The S/B ratio for the Co L-edge depth-resolved XAS is then enhanced by nearly two orders of magnitude. We observe the spectra at different probing depths, and find from the layer-resolved analysis that ∼60% of bottom Co is oxidized. A significant improvement in the observation of the buried interface state is thus successfully demonstrated.

Observation of an electric field-induced interface redox reaction and magnetic modification in GdOx/Co thin film by means of depth-resolved X-ray absorption spectroscopy.

We study an electric field-induced redox reaction at the interface of GdOx/Co thin film, by means of soft X-ray absorption spectroscopy (XAS). The fluorescence-yield depth-resolved XAS analysis reveals that the interfacial Co layer at GdOx/Co is composed of ∼50% Co oxide when the negative field is applied, while metallic Co is dominant in the case of the positive field. We suppose that the interfacial layer is oxidized by oxygen migration from the GdOx layer with the negative field while the interfacial layer shows the metallic state with the positive field, which means that the redox reaction is induced by the electric field. In addition, it is found from the X-ray magnetic circular dichroism (XMCD) measurement that the orbital magnetic moment of Co is larger when the negative electric field is applied to the film, as compared to the positive field. Moreover, the depth-resolved XMCD analysis reveals that the interfacial Co layer shows no or little magnetization regardless of the electric field, while for the inner layer, an increase of the orbital magnetic moment is suggested with the negative field. The field-induced magnetic modification could be attributed to the change of the orbital moment in the inner Co layer due to interfacial modifications. We thus succeed in the direct observation of the redox reaction-induced change in the interface magnetic state.

Depth-resolved chemical speciation of a W-B4C multilayer structure

We report results of simultaneous x-ray reflectivity and grazing incidence x-ray fluorescence measurements in combination with x-ray standing-wave-assisted depth-resolved near-edge x-ray absorption measurements to reveal new insights on chemical speciation of W in a ${\text{W-B}}_{4}\mathrm{C}$ multilayer structure. Interestingly, our results show the existence of various unusual electronic states for the W atoms especially those near to the surface and interface boundary of a thin film medium as compared to those of the bulk. These observations are found to be consistent with the results obtained using first-principles calculations. Unlike the conventional x-ray absorption measurements the present approach has an advantage that it permits the determination of the depth-resolved chemical nature of an element in the thin-layered materials at atomic length scale resolution.

Nanometer-resolution depth-resolved measurement of florescence-yield soft x-ray absorption spectroscopy for FeCo thin film.

We develop a fluorescence-yield depth-resolved soft x-ray absorption spectroscopy (XAS) technique, which is based on the principle that the probing depth is changed by the emission angle of the fluorescence soft x rays. Compared with the electron-yield depth-resolved XAS technique, which has been established in this decade, we can observe wider range in-depth XAS distribution up to several tens of nm. Applying this technique to a 30 ML (∼4.3 nm) FeCo thin film, we observe Fe L-edge XAS spectra at the probing depth of 0.3-6 nm and find that the film has 22 ML (∼3.1 nm) surface oxide layer while its inner layer shows metallic state. We thus successfully obtain nanometer-resolution depth-resolved XAS spectra and further expect that operando measurement under the electric and/or magnetic fields is possible.

Elucidation of Reaction Mechanism at the Cathode/Electrolyte Interface in All-Solid-State Battery By Using Depth-Resolved X-Ray Absorption Spectroscopy

As one of the next-generation batteries, all-solid-state batteries have attracted a lot of attention due to their outstanding safety performance and energy density. However, their power performance are still far from traditional lithium ion batteries using liquid electrolyte, which is considered as a result of interfacial resistance existing between cathode and solid electrolyte. To overcome this inherent interfacial problem of all-solid-state batteries, there is an urgent need to understand the reaction mechanism at the cathode/electrolyte interface. As the origin of the interfacial resistance, so far there are three hypotheses have been proposed: the space-charge layer [1], the reaction product layer [2] and the mechanical structure change [3]. In this study, two types of LiCoO2 thin-film model electrodes with and without interface modification by using Li3PO4 interlayer were fabricated by pulsed laser deposition (PLD). The electrodes were applied for electrochemical measurement and depth resolved X-ray absorption spectroscopy (DR-XAS) measurement to investigate the reaction mechanism of interface between LiCoO2 cathode and Li2S-P2S5 solid electrolyte. In order to analyze the interfacial reaction, thin-film model electrode with LiCoO2 cathode and Li2S-P2S5 solid electrolyte was deposited on mirror-polished platinum substrate by PLD. A Li3PO4 interlayer was also deposited between cathode and solid electrolyte by PLD. Electrochemical measurements were performed by using liquid electrolyte (1M LiClO4 in propylene carbonate) and lithium metal anode. Cyclic voltammetry (CV) measurement was carried out from 3.2 V to 4.4 V at 0.1 mV/s. Electrochemical impedance spectroscopy (EIS) measurement was performed at 3.9 V, 4.0 V, 4.1 V, 4.2 V, 4.3 V and 4.4 V with a 30 mV amplitude during first charge and discharge cycle. The frequency range of EIS was limited from 10-2 to 106 Hz. DR-XAS measurements were conducted using the BL37XU beamline, SPring-8, Japan, with a two-dimensional pixel array detector, PILATUS (Dectris, Switzerland). Comparing to the thin-film electrode with Li3PO4 interlayer, the first cycle of CV curves of the electrode without Li3PO4 interlayer shows a current peak around 3.2 V, which is considered to come from the reaction between cathode and electrolyte. EIS measurements show that Li3PO4 interlayer can suppress the increase of interface resistant during charge and discharge, which means a more stable interface can be kept by interface modification. In order to make a more detailed observation of the interfacial structure, DR-XAS near edge structure of the electrodes are analyzed at Co K-edge. The energy shift around the interface in the electrodes with Li3PO4 interlayer is smaller than that of samples without Li3PO4 interlayer. These results indicate that valence changes of cobalt ions near cathode/solid electrolyte interface are suppressed by interface modification. The introducing of Li3PO4 interlayer reduces the inter-diffusion between cathode and solid electrolyte. The results of both electrochemical and DR-XAS measurements present that the reaction mechanism is the reaction product layer at cathode/electrolyte interface in all-solid-state battery using sulfide electrolyte. Reference [1] K. Takada, N. Ohta, L. Zhang, K. Fukuda, I. Sakaguchi, R. Ma, M. Osada, T. Sasaki , Solid State Ionics, 179 (2008) 1333. [2] A. Sakuda, A. Hayashi, M. Tatsumisago, Chem Mater, 22 (2010) 949. [3] K. Kishida, N. Wada, H. Adachi, K. Tanaka, H. Inui, C. Yada, Y. Iriyama, Z. Ogumi., Acta Materialia, 55 (2007) 4713. Acknowledgement This research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovative Next Generation Batteries (SPRING) Project.

Depth-Resolved X-Ray Absorption Spectroscopic Studies on Reaction Mechanism at the Cathode/Electrolyte Interface in All-Solid-State Battery

As one of the next-generation batteries, all-solid-state batteries have attracted attention due to their outstanding safety performance and energy density. However, their power performance are still far from the increasing demand of electric vehicles, which seems to be caused by the large interfacial resistance between cathode and solid electrolyte. To overcome the inherent interfacial problem of all-solid-state batteries, there is an urgent need to understand the reaction mechanism at the cathode/electrolyte interface. As the origin of the interfacial resistance, three hypotheses have been proposed: the space-charge layer [1], the reaction product [2] and the mechanical structure change [3]. In this study, we applied thin-film model electrode for electrochemical measurement and depth resolved X-ray absorption spectroscopy (DR-XAS) measurement to investigate the reaction mechanism of interface between LiCoO2 cathode and Li2S-P2S5 solid electrolyte with interface modification by using Li3PO4 interlayer. In order to analyze the interfacial reaction between cathode and electrolyte, thin-film model electrode with LiCoO2 cathode and Li2S-P2S5 solid electrolyte was deposited on mirror-polished platinum substrate by pulsed laser deposition (PLD). A Li3PO4 interlayer was also deposited between cathode and solid electrolyte by PLD. Electrochemical measurements were performed by using liquid electrolyte (1M LiClO4 in propylene carbonate) and lithium metal anode. Cyclic voltammetry measurement (CV) was carried out from 3.2 V to 4.4 V at 0.1 mV/s. EIS measurement was performed at 3.9 V, 4.0 V, 4.1 V, 4.2 V, 4.3 V and 4.4 V with a 30 mV amplitude. The frequency range of EIS was limited from 10-2 to 106 Hz. Depth-resolved XAS measurements were conducted using the BL37XU beamline, SPring-8, Japan, with a two-dimensional pixel array detector, PILATUS (Dectris, Switzerland). The first cycle of CV curves of the film without Li3PO4 interlayer shows a current peak around 3.2 V, which is considered to come from the reaction between cathode and electrolyte. EIS measurements show that the interfacial resistance of the films with Li3PO4 interlayer does not increase compared with the film without Li3PO4 interlayer. In order to discuss the interfacial structure, depth resolved X-ray near edge structure at Co K-edge is analyzed. The energy shift around the interface in the film with Li3PO4 interlayer is smaller than that of sample without Li3PO4 interlayer. These results indicate that valence changes of cobalt ions near cathode/solid electrolyte interface are suppressed by using Li3PO4 interlayer. The introducing of Li3PO4 interlayer reduces the inter-diffusion between cathode and solid electrolyte. Reference [1] K. Takada, N. Ohta, L. Zhang, K. Fukuda, I. Sakaguchi, R. Ma, M. Osada, T. Sasaki., Solid State Ionics, 179 (2008) 1333. [2] A. Sakuda, A. Hayashi, M. Tatsumisago., Chem Mater, 22 (2010) 949. [3] K. Kishida, N. Wada, H. Adachi, K. Tanaka, H. Inui, C. Yada, Y. Iriyama, Z. Ogumi., Acta Materialia, 55 (2007) 4713. Acknowledgement This research was financially supported by the Japan Science and Technology Agency (JST), Advanced Low Carbon Technology Research and Development Program (ALCA), Specially Promoted Research for Innovative Next Generation Batteries (SPRING) Project.

Chemical state of Ag in Conducting Bridge Random Access Memory cells: a depth resolved X-ray Absorption Spectroscopy investigation.

Conducting Bridge Random Access Memories (CBRAM) are a promising substitute for FLASH technology but problems with limited retention of the low resistance ON state still hamper their massive deployment. Depth resolved X-ray Absorption Spectroscopy has been used to describe the chemical state of the atoms of the active electrode (in this case Ag) and to reveal the role of Sb as stabilizer of the metallic state.

Confocal depth-resolved micro-X-ray absorption spectroscopy study of chemically strengthened boroaluminosilicate glasses

A set of chemically strengthened boroaluminosilicate glasses containing 1 mol% Fe2O3are here studied using depth-resolved confocal X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy.

Depth-Resolved X-ray Absorption Spectroscopy by Means of Grazing Emission X-ray Fluorescence.

Grazing emission X-ray fluorescence (GEXRF) is well suited for nondestructive elemental-sensitive depth-profiling measurements on samples with nanometer-sized features. By varying the grazing emission angle under which the X-ray fluorescence signal is detected, the probed depth range can be tuned from a few to several hundred nanometers. The dependence of the XRF intensity on the grazing emission angle can be assessed in a sequence of measurements or in a scanning-free approach using a position-sensitive area detector. Hereafter, we will show that the combination of scanning-free GEXRF and fluorescence detected X-ray absorption spectroscopy (XAS) allows for depth-resolved chemical speciation measurements with nanometer-scale accuracy. While the conventional grazing emission geometry is advantageous to minimize self-absorption effects, the use of a scanning-free setup makes the sequential scanning of the grazing emission angles obsolete and paves the way toward time-resolved depth-sensitive XAS measurements. The presented experimental approach was applied to study the surface oxidation of an Fe layer on the top of bulk Si and of a Ge bulk sample. Thanks to the penetrating properties and the insensitivity toward the electric conduction properties of the incident and emitted X-rays, the presented experimental approach is well suited for in situ sample surface studies in the nanometer regime.

Surface Strontium Segregation of Solid Oxide Fuel Cell Cathodes Proved by In Situ Depth-Resolved X-ray Absorption Spectroscopy

Surface Strontium Segregation of Solid Oxide Fuel Cell Cathodes Proved by In Situ Depth-Resolved X-ray Absorption Spectroscopy

Ar+ ion milling-induced suppression of surface oxidation in Fe70Co30 thin films

We show from the depth-resolved X-ray absorption analysis that the degree of surface oxidation of Fe70Co30 thin films is reduced by Ar+ ion milling even if the film is exposed to air after ion milling. The milled film has thinner surface oxide layer and smaller ratio of Fe oxide component in the surface layer than those of the as-deposited film. It is also indicated that selective oxidation of Fe proceeds accompanied by the movement of Fe atoms toward the surface. The suppression of oxidation is explained by the obstruction of Fe movement toward the surface by the milling-induced disorder.

Spin orientation transition across the single-layer graphene/nickel thin film interface

The spin-polarized electronic structures across the interface between single-layer graphene and a Ni(111) thin film are explored by employing depth-resolved X-ray absorption and magnetic circular dichroism spectroscopy with atomic layer resolution. The depth-resolved Ni L2,3-edge analysis clarifies that the Ni atomic layers adjacent to the interface show a transition of the spin orientation to the perpendicular one in contrast to the in-plane one in the bulk region. The C K-edge analysis reveals the intensification of the spin–orbit interactions induced by the π–d hybridization at the interface as well as out-of-plane spin polarization in the π band region of graphene. The present study indicates the importance of the interface design at the atomic layer level for graphene-based spintronics.

Effects of ZrO2 Coating on LiCoO2 Thin-Film Electrode Studied by In Situ X-ray Absorption Spectroscopy

Surface modification effects on interfacial reaction for zirconia (ZrO2) coated lithium cobalt oxide (LiCoO2) are studied. ZrO2-coated LiCoO2 thin-film electrodes are prepared by a pulsed laser deposition with different preparation periods. Cyclic voltammetry measurements clearly show that cycle performance and high-potential durability are improved by the ZrO2 coating. AC impedance spectroscopy reveals that there are two types of resistance associated with ZrO2 coating; one is the charge transfer resistance which depends on the Li+-containing electrolyte concentration, and the other is the ZrO2-layer resistance which depends on the deposition periods of ZrO2 coatings. In situ total-reflection fluorescence X-ray absorption spectroscopy for ZrO2-coated LiCoO2 thin-film electrodes reveals that the ZrO2 coating layer suppresses the reduction reaction by electrolyte contact, leading to improving its cycle performance. Depth-resolved X-ray absorption spectroscopy reveals that the ZrO2 coating layer can prevent the increase of local distortion of the LiCoO2 electrode when overcharged, leading to high-potential durability.

XAFS and XMCD Spectra at the Surface and Interface of Ultrathin Films Observed by the Depth-Resolved XAFS/XMCD Technique

A depth-resolved X-ray absorption fine structure (XAFS) technique has been developed by using the detection-angle dependence of the effective escape depth of the Auger electrons in the electron-yield XAFS measurement. The technique is applied to a Ni thin film covered with oxygen adsorbates in order to extract the XAFS and X-ray magnetic circular dichroism (XMCD) spectra at the surface layer. The chemical state and the magnetic structure of the surface layer are clarified from the extracted XAFS/XMCD spectra. [DOI: 10.1380/ejssnt.2012.521]