Hard X-ray Photoemission Spectroscopy Measurements Research Articles

Bi0.5Pb0.5FeO3 with Unusual Pb Charge Disproportionation: Indication of a Systematic Charge Distribution Change in Bi0.5Pb0.5MO3 (M: 3d Transition Metal).

Bi0.5Pb0.5FeO3 with 1:1 mixture of Bi and Pb having charge degrees of freedom at the A-site of perovskite oxide ABO3 is obtained for the first time by high-pressure synthesis. Comprehensive synchrotron X-ray powder diffraction, optical second harmonic generation, Mössbauer spectroscopy, and hard X-ray photoemission spectroscopy measurements revealed that Bi0.5Pb0.5FeO3 is a canted antiferromagnetic insulator crystalizing in a nonpolar tetragonal I4/mcm structure with √2a × √2a × 2a unit cell and has unusually Pb charge disproportionated Bi3+0.5Pb2+0.25Pb4+0.25Fe3+O3 charge distribution. The valence of transition metal M in Bi0.5Pb0.5MO3 changes from 3.5+ to 3+ and finally to 2+ from Mn to Fe and to Ni, from left to right in the periodic table as the 3d-level becomes deeper. The valences of Bi and Pb increase to compensate for the decrease in the M's valence, and Pb changes from 6s2 (2+) to 6s0 (4+) before Bi changes.

The Damage Analysis for Irradiation Tolerant Spin-Driven Thermoelectric Device Based on Single-Crystalline Y₃Fe₅O₁₂/Pt Heterostructures

Spin-driven thermoelectric (STE) generation based on the combination of the spin Seebeck effect (SSE) and inverse spin Hall effect is an alternative to conventional semiconductor-based thermocouples as it is large scale, low cost, and environment-friendly. The STE device is thought to be radiation hard, making it attractive for space and nuclear technology applications. By using magnetometry, transmission electron microscopy, and the hard X-ray photoemission spectroscopy (HAXPES) measurements, we show that an STE device made of single-crystalline Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (YIG)/Pt heterostructures has tolerance to irradiation of high-energy heavy ion beams. We used 200 MeV gold ion beams modeling cumulative damages due to fission products emitted from the surface of spent nuclear fuels. By varying the dose level, we confirmed that the thermoelectric and magnetic properties of the single-crystalline YIG/Pt STE device are finite at the ion-irradiation dose up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> ions/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> fluence. In addition, the HAXPES measurements were performed to understand the effects at the interface of YIG/Pt. The HAXPES data suggest that the chemical reaction regarding Fe and O that diminishes the SSE signals is promoted with the increase of the irradiation dose. The understandings of the damage analysis in YIG/Pt are beneficial for developing better STE devices applicable to harsh environmental usages.

(Invited) Depth Analysis of Near Valence Band Maximum Defect States in Amorphous Oxide Semiconductors: Amorphous in-Ga-Zn-O

Since the thin film transistors (TFTs) using amorphous oxide semiconductor (AOS) was firstly reported in 2004[1], amorphous In–Ga–Zn–O (a-IGZO) have been studied intensively because they have large advantageous features such as large electron mobilities (larger than 10 cm2/Vs), small subthreshold voltage swing, and low temperature fabrication process [2,3]. A-IGZO with the nominal chemical composition of In:Ga:Zn=1:1:1 is now commercialized for liquid crystal displays (LCDs) for smart phone or high-resolution TV and large-size organic light-emitting diode displays, etc.. Although the a-IGZO TFTs are already commercialized, stability is still a critical issue if we use it for next generation display such as transparent or flexible. In particular, it is recognized that the negative shift of the threshold voltage (V th) is associated with the excitation of subgap photons from deep-energy defects just above the valence band maximum (called near-VBM defects), which was firstly found by hard X-ray photoemission spectroscopy (HAXPES) [4]. We recently reported that a-IGZO deposited at room temperature has large film-density-distribution in the depth direction [5]. In this study, we measured depth profile of near-VBM states including IGZO/substrate interface. It is important to understand the depth distribution also of defect density, because mobile electrons in field-effect device are induced at vicinity of substrate in case of bottom gate TFT structure. Although depth analysis was already reported for a-IGZO deposited by pulsed laser deposition (PLD), we report for a-IGZO deposited by sputtering method which is standardly used in mass-production process. As a result, we found that the depth profile is different between PLD and sputtering method. We used radio-frequency (RF) magnetron sputtering to deposit a-IGZO films on Si wafer with 150 nm SiO2 at room temperature. RF power, oxygen flow rate ratio (Ar:O2), deposition pressure were fixed at 70 W, 97:3 and 0.55 Pa, respectively. HAXPES measurements were carried out at BL15XU undulator beamline (hv = 6 keV) in SPring-8. Film thickness dependences were observed for 5 – 100 nm a-IGZO films. For extracting buried spectra, we used total reflection X-ray to modify the penetration depth [6]. From thickness dependence of VB spectra, we found that the near-VBM defect density increase with decreasing film thickness. It may suggest that a-IGZO at the vicinity of substrate has large defect density compared to bulk region. To separate the effect of surface and buried region, we extracted the buried spectra using total reflection of X-ray in HAXPES measurement and difference method. We successfully observed the depth profile in wide range from 0 to 100 nm and found that the defect density at the vicinity of substrate is several times higher than that at thick enough region. We will discuss more details including the relationship between the microstructure and near-VBM defects at the conference.

High Capacity Li Ion Battery Using Oxide-Coated Fe- and Ni-Substituted Lithium-Rich Layered Manganese Oxides Cathode

Lithium ion batteries (LIBs) have attracted much attention as large-scale power sources for hybrid electric vehicles (HEVs), plug-in HEVs, and electric vehicles (EVs) due to their high energy density, long cycle life, and environmental friendliness. Lithium-rich layered manganese oxides containing nickel, cobalt, and iron represented by the chemical formula xLi2MnO3·(1-x)LiMO2 (M= Co, Ni, and Fe) are promising candidate as an cathode material having the high operating voltage and large capacity. In particular, Fe- and Ni-substituted lithium-rich layered oxide Li1.23Mn0.46Fe0.15Ni0.15O2 (0.6Li2MnO3·0.2LiFeO2·0.2LiNiO2) with about 250 mAh/g would be a good material to make a low-cost high-capacity cathode because Mn and Fe are abundant mineral resources [1]. The large capacity has been achieved by not only the well-known redox process of transition metal but also the reversible anionic redox process between O2 2- and O2- [2]. At the present, the major problem to be solved is the capacity fades and gas generations during cycles. In this study, we succeeded in the improvement of them by using oxide-coated Li1.23Mn0.46Fe0.15Ni0.15O2. We also investigated the deterioration mechanism of cells during cycles. Li1.23Mn0.46Fe0.15Ni0.15O2 was synthesized using the coprecipitation-calcination method. A mixed solutions containing Fe, Ni, and Mn ions were co-precipitated continuously into the reactor and they was oxidized by air-bubbling for 24 h. After that ,the co-precipitated precursor was mixed with Li2CO3 and calcined at 700°C for 5 h in air and then at 850 °C for 5 h in N2 atmosphere. The obtained sample was mixed with Sm(NO3)3·6H2O and calcined at 400°C for 2 h in air (Sm/ Li1.23Mn0.46Fe0.15Ni0.15O2). Al-laminated film-packed cells were assembled using a cathode, a graphite anode, and a polyolefin porous separator containing the electrolyte solution (1M LiPF6ethylene carbonate EC/DEC =3/7 (v/v)). The stepwise activation process was carried out in the range of 1.5 to 4.5 V in the thermostatic chamber kept at 45ºC [3]. The cycle test was carried out by repeating cycles of charge and discharge processes at 40 mA/g to the lower limit voltage of 1.5 V. The positive electrodes after charge-discharge were characterized by synchrotron X-ray absorption fine structure (XAFS) and hard X-ray photoemission spectroscopy (HAXPES) measurements in the beam line BL08B2 and BL46XU at SPring-8. The initial discharge capacity of the cells with Li1.23Mn0.46Fe0.15Ni0.15O2 and Sm/Li1.23Mn0.46Fe0.15Ni0.15O2electrodes showed 265 and 258 mAh/g, respectively, which was almost same. The capacity retention and gas generation quantities during 50 cycles became 3% higher and 20% smaller, respectively, by the Sm oxide coating. XAFS results indicated that the irreversible reactions of Fe ions during cycles were suppressed by Sm oxide coating although the Mn and Ni were not affected practically. The HAXPES measurements, which can obtain the information of valence states and/or surface deposits below about 50 nm, demonstrated that the formation of the SEI (Solid Electrolyte Interphase) on cathode and irreversibility of Fe, Ni, and oxide ions were suppressed by Sm oxide coating during cycles. From the above results, we found that the oxide film on cathode materials prevented the reaction between cathode materials and electrolytes. We believe that the improvement of the cell properties with Sm/Li1.23Mn0.46Fe0.15Ni0.15O2electrodes accelerates in progress of the development of next generation storage devices. Acknowledgments This work was in part performed under the management of Research and Development of High Energy Density Lithium-Ion Battery utilizing High Capacity and Low-Cost Oxide Cathodes project supported by the New Energy and Industrial Technology Department Organization (NEDO) in Japan.

Microstructure and Compositional Defects Affects Proton Conductivity and Reactions in Y-doped BaZrO3 Thin Films

Doped perovskite oxides have been widely studied in recent years as proton conducting solid electrolytes for different electrochemical devices, e.g. Solid Oxide Fuel Cells (SOFCs). By using the proton conductor as the electrolyte in a SOFC, the fuel dilution issue can be avoided due to the water formation at the cathode side instead of the anode. Among all the alkaline-earth cerates and zirconates, Y doped BaZrO3 (BZY) has been considered one of the most promising electrolyte for the SOFCs applications for its significant proton conductivity as well as chemical stability.[1] However, most reported fuel cell performance values for this material is much lower than oxide ion conducting ceramics such as YSZ owing to an unavoidable microstructure change, i.e. high grain boundary density and low degree of the crystallinity. In order to correlate the microstructure with the conductivity, we prepared BZY thin films by Pulsed Laser Deposition and discussed the effect of the microstructure on the proton conductivity. In this presentation I will show the conductivity behavior as a function of temperature and thickness by Electrochemical Impedance Spectroscopy (EIS).[2] I will focus on the interface between BZY and substrate NdGaO3 and show the cross-sectional electrochemical activity change with nanoscale resolution by Electrochemical Strain Microscopy (ESM). [3] I will highlight the interplay between the interface defects and the electrochemical performance by comparing the EIS, ESM results with Scanning Transmission Electron microscopy (STEM). I will further show the complementary study by bulk sensitive hard x-ray photoemission spectroscopy measurements which, together with Density Functional Theory calculations, demonstrate that Y and, to some extent, also Zr ions, substitute for Ba near the interface with the substrate. [1] A. D’Epifanio, E. Fabbri, E. Di Bartolomeo, S. Licoccia, E. Traversa, Fuel Cells, 1,69-762008 (2008). [2] V. Foglietti, Nan Yang, A. Tebano, C. Aruta, E. Di Bartolomeo, S. Licoccia, C. Cantoni, G. Balestrino, Applied Physics Letters 104 , 081612 (2014). [3] Nan Yang, C. Cantoni, V. Foglietti, A. Tebano, A. Belianinov, E. Strelcov, S. Jesse, D. Di Castro, E. Di Bartolomeo, S. Licoccia, S. V. Kalinin, G. Balestrino and C. Aruta, Nano Letters, 10.1021/acs.nanolett.5b00698, (2015).

Enhanced electron correlations at theSrxCa1−xVO3surface

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide SrxCa1-xVO3. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O 2p valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O 2p electronic structure as a function of the Sr content is dominated by A-site hybridization. Near the Fermi level, the correlated V 3d Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.

“Conductive” yttria-stabilized zirconia as an epitaxial template for oxide heterostructures

We report an in situ thermochemical treatment that significantly increases the macroscopic electrical conductivity of insulating yttria-stabilized zirconia (YSZ) (001) single-crystalline substrates. We demonstrate the high-quality surface crystalline structure of the resulting “conductive” cYSZ (001) by low- and high-energy electron diffraction. Soft- and hard X-ray photoemission spectroscopy measurements reveal a sizable reduction of Zr cations to a metallic state and their homogeneous distribution within the cYSZ. We discuss the correlation between the microscopic chemical processes leading to the increased macroscopic metallicity. Finally, the heteroepitaxial growth of a functional magnetic oxide model system, ultrathin EuO on cYSZ (001), was demonstrated. cYSZ (001) thereby enables both high quality oxide heteroepitaxy and the advanced sample characterization by high electron-fluence characterization techniques.

Reactive pulsed laser deposition of Cu2ZnSnS4 thin films in H2S

Cu2ZnSnS4 (CZTS) thin films have been grown by reactive pulsed laser deposition in H2S atmosphere, combining the alternate ablation from the metallic (Cu) and alloyed (ZnxSn) targets at room temperature. The morphological, structural and optical properties of as grown CZTS thin films with varying compositions as well as upon annealing in N2 atmosphere are investigated by Rutherford backscattering spectrometry, X-ray diffraction, Raman spectroscopy and optical spectrophotometry. The chemical bonding in the “bulk” of the CZTS films is elucidated via hard X-ray photoemission spectroscopy measurements. The formation of the good quality stoichiometric polycrystalline CZTS films is demonstrated upon optimization of the growth parameters.

Fe/BaTiO3 interface: Band alignment and chemical properties

Ultrathin BaTiO3/Fe bi-layers were synthesized on MgO(001) and SrTiO3(001) substrates by pulsed laser deposition in a single vacuum cycle. The full electronic band structure at the Fe/BaTiO3 interface has been determined by hard x-ray photoemission spectroscopy measurements. Depending on the growth conditions, the valence and conduction band offsets are found 3.05–3.26 eV and 0.9–1.2 eV, respectively. Interface sensitive conversion electron Mößbauer spectroscopy demonstrates the absence of any magnetically dead layer thus proving that the determined band alignment is related to a sharp Fe/BaTiO3 interface structure down to the atomic scale.

Local moment behaviors of the valence fluctuating systems β-YbAlB4 and α-YbAlB4

β-YbAlB4 is the first example of an Yb-based heavy Fermion superconductor with Tc = 80 mK and exhibits pronounced non-Fermi-liquid behavior above Tc. On the other hand, recent hard x-ray photoemission spectroscopy measurements have revealed strong intermediate valence of both β-YbAlB4 and its locally isostructural polymorph α-YbAlB4 . Here we present the results of the specific heat and magnetization measurements of β-YbAlB4 and α-YbAlB4 . The results indicate a Fermi liquid ground state for α-YbAlB4 in contrast to the quantum criticality found in β-YbAlB4 . Interestingly, both systems exhibit Kondo lattice behavior with large Wilson ratio below T* ~ 8K, through the renormalization of a high valence fluctuation scale ~ 200 K.