Soft X-ray Photoemission Spectroscopy Research Articles

Surface Mediated Synthesis of 2D Covalent Organic Networks: 1,3,5‐Tris(4‐bromophenyl)benzene on Au(111)

Covalent organic networks derived from surface‐mediated self‐assembly of 1,3,5‐tris(4‐bromophenyl)benzene into two‐dimensional networks on Au(111) have been studied by scanning tunneling microscopy (STM) and by X‐ray spectroscopic methods. High resolution soft X‐ray photoemission spectroscopy (SXPS) using synchrotron radiation have been used to study the formation of the 1,3,5‐tris(4‐bromophenyl)benzene‐derived 2D networks by observing temperature‐dependent C 1s and Br 3d core level XPS spectra. X‐ray absorption (XA) measurements of the formation of these 2D networks have been obtained at the C K‐edge where their temperature and angular dependence are examined. The results of these XPS and XA spectroscopic measurements are compared to detailed ab initio electronic structure calculations of 1,3,5‐tris(4‐bromophenyl)benzene molecules to aid the interpretation of the features of these spectra.

Effects of Air Exposure on Hard and Soft X-ray Photoemission Spectra of Ultrananocrystalline Diamond/Amorphous Carbon Composite Films

Hard X-ray photoemission spectroscopy (HAXPES) was employed for the structural evaluation of ultrananocrystalline diamond/amorphous carbon (UNCD/a-C) composite films deposited on cemented carbide substrates, at substrate temperatures up to 550 °C by coaxial arc plasma deposition. The results were compared with those of soft X-ray photoemission spectroscopy (SXPES). Since nanocrystalline diamond grains are easily destroyed by argon ion bombardment, the structural evaluation of UNCD/a-C films, without the argon ion bombardment, is preferable for precise evaluation. For samples that were preserved in a vacuum box after film preparation, the sp3 fraction estimated from HAXPES is in good agreement with that of SXPES. The substrate temperature dependencies also exhibited good correspondence with that of hardness and Young’s modulus of the films. On the other hand, the sp3 fraction estimated from SXPES for samples that were not preserved in the vacuum box had an apparent deviation from those of HAXPES. Since it is possible for HAXPES to precisely estimate the sp3 fraction without the ion bombardment treatment, HAXPES is a feasible method for UNCD/a-C films, comprising nanocrystalline diamond grains.

Unexpected metal-insulator transition in thick Ca1-xSrxVO3 film on SrTiO3 (100) single crystal

Epitaxial Ca1-xSrxVO3 (0 ≦ x ≦ 1) thin films were grown on (100)-oriented SrTiO3 substrates by using the pulsed laser deposition technique. In contrast to the previous report that metal-insulator transition (MIT) in Ca1-xSrxVO3 (CSVO) was achieved only for extremely thin films (several nm thick), MIT was observed at 39, 72, and 113 K for films with a thickness of 50 nm. The electronic structure was investigated by hard and soft X-ray photoemission spectroscopy (HX-PES and SX-PES). The difference between these PES results was significant due to the variation in an escape depth of photoelectrons of PES. While HX-PES showed that the V 2p3/2 spectra consisted of four peaks (V5+, V4+, V3+, and V2+/1+), SX-PES showed only three peaks (V5+, V4+, and V3+). This difference can be caused by a strain from the substrate, which leads to the chemical disorder (V5+, V4+, V3+, and V2+/1+). The thin film near the substrate is affected by the strain. The positive magnetoresistance is attributed to the effect of electron-electron interactions in the disorder system. Therefore, the emergence of MIT can be explained by the electron-electron interactions from the chemical disorder due to the strain.

Microscopic origin of the mobility enhancement at a spinel/perovskite oxide heterointerface revealed by photoemission spectroscopy

The spinel/perovskite heterointerface $\gamma$-Al$_2$O$_3$/SrTiO$_3$ hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO$_3$/SrTiO$_3$ by more than an order of magnitude despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and \textit{ab initio} calculations we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO$_3$ layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

Dielectric collapse at the LaAlO3/SrTiO3 (001) heterointerface under applied electric field

The fascinating interfacial transport properties at the LaAlO3/SrTiO3 heterointerface have led to intense investigations of this oxide system. Exploiting the large dielectric constant of SrTiO3 at low temperatures, tunability in the interfacial conductivity over a wide range has been demonstrated using a back-gate device geometry. In order to understand the effect of back-gating, it is crucial to assess the interface band structure and its evolution with external bias. In this study, we report measurements of the gate-bias dependent interface band alignment, especially the confining potential profile, at the conducting LaAlO3/SrTiO3 (001) heterointerface using soft and hard x-ray photoemission spectroscopy in conjunction with detailed model simulations. Depth-profiling analysis incorporating the electric field dependent dielectric constant in SrTiO3 reveals that a significant potential drop on the SrTiO3 side of the interface occurs within ~2 nm of the interface under negative gate-bias. These results demonstrate gate control of the collapse of the dielectric permittivity at the interface, and explain the dramatic loss of electron mobility with back-gate depletion.

Ce 4f electronic states of CeO1−xFxBiS2 studied by soft x-ray photoemission spectroscopy

We use soft x-ray photoemission spectroscopy (SXPES) to investigate Ce 4f electronic states of a new BiS2 layered superconductor CeO1-xFxBiS2, for polycrystalline and single-crystal samples. The Ce 3d spectrum of the single crystal of nominal composition x = 0.7 has no f(0) component and the spectral shape closely resembles the ones observed for Ce trivalent insulating compounds, strongly implying that the CeO layer is still in an insulating state even after the F doping. The Ce 3d-4f resonant SXPES for both polycrystalline and single-crystal samples shows that the prominent peak is located around 1 eV below the Fermi level (E-F) with negligible spectral intensity at EF. The F-concentration dependence of the valence band spectra for single crystals shows the increases of the degeneracy in energy levels and of the interaction between Ce 4f and S 3p states. These results give insight into the nature of the CeO1-xFx layer and the microscopic coexistence of magnetism and superconductivity in CeO1-xFxBiS2.

Correlation effect in Sr1−xLaxRuO3 studied by soft x-ray photoemission spectroscopy

To clarify how the electronic state of Sr1-xLaxRuO3 evolves with La doping, we conducted photoemission (PES) experiments using soft x-rays. The spectral shape of the Ru 4d derived peak near the Fermi level changes significantly with increasing x. This variation indicates that a spectral weight transfer from the coherent to incoherent component occurs due to an enhancement of the electron correlation effect. Resonant PES experiments at the La 3d_{5/2} edge have confirmed that there is no significant contribution of the La 5d state in the energy range where the spectral weight transfer is observed. Using the dependence of the photoelectron mean free path on the photon energy, we subtracted the surface components from the PES spectra and confirmed that the enhancement of the electron correlation effect with La doping is an intrinsic bulk phenomenon. On the other hand, a large portion of the coherent component remains at the Fermi level up to x = 0.5, reflecting that the Ru 4d state still has itinerant characteristics. Moreover, we found that the PES spectra hardly depend on the temperature and do not exhibit a discernible change with magnetic ordering, suggesting that the temperature variation of the exchange splitting does not follow the prediction of the Stoner theory. The presently obtained experimental results indicate that the electron correlation effect plays an important role in Sr1-xLaxRuO3 and that the Ru 4d electrons possess both local and itinerant characteristics.

Adsorption of Dimethyl Sulfoxide on LiCoO2 Thin Films: Interface Formation Studied by Photoemission Spectroscopy

The formation of electrode–electrolyte interfaces is of key importance for the stability and performance of lithium-ion batteries. To increase efficiency and lifetime, a detailed understanding of the processes at these interfaces is necessary. Both chemical reactions resulting in the formation of interface layers as well as double layer formation, i.e., band bending, influence the batteries’ properties. In this contribution, we investigate the interface formation between a thin film LiCoO2 cathode material and a solvent adsorbate. Using a surface-science approach, dimethyl sulfoxide (DMSO, (CH3)2SO) was used as molecular probe and adsorbed stepwise onto LiCoO2 at low temperature. After each step, the interface was analyzed by synchrotron-based soft X-ray photoemission spectroscopy (SXPS). We established the energy level alignment of the LiCoO2−DMSO interface, showing a band bending downward in the cathode material attributed to Li-ion transport into the adsorbate phase. Lithium transport results in the fo...

(Invited) LLZO Ceramic Electrolytes: A Path Forward to Solid-State Batteries?

The appeal of solid-state batteries is clear; there are no flammable liquid components to leak or catch fire, and the possibility of using lithium anodes suggests that higher energy density than currently obtainable with Li-ion configurations should be possible. The emergence of garnet-structured electrolytes based on Li7La3Zr2O12 (LLZO) are particularly attractive for such devices because of their high conductivities and apparent stability against lithium. Still, there are many challenges associated with LLZO, including difficulties in densifying the materials and high interfacial resistances at the lithium electrodes. Work in our group has been directed towards overcoming these obstacles. Classical ceramics processing techniques have been used to produce samples up to 94% dense, as thin as 200 microns, with excellent control over grain size and grain boundary chemistry. Synchrotron techniques such as soft x-ray absorption spectroscopy (XAS) and x-ray photoemission spectroscopy (XPS) reveal that the main culprit leading to high interfacial resistance at lithium electrodes is lithium carbonate, either present as a by-product of synthesis or produced upon exposure to air. Polishing in inert atmosphere removes this impurity and lowers the interfacial impedance, but is not practical commercially. Through grain boundary engineering, it is possible to substantially reduce the sensitivity of samples to air exposure, greatly reducing the tendency towards formation of lithium carbonate and resulting in much lower interfacial impedances at lithium electrodes.

Soft X-ray absorption and photoemission spectroscopy study of semiconductor oxide nanoparticles for dye-sensitized solar cell: ZnSnO3 and Zn2SnO4

Abstract The electronic structures of the Zn-stannate nanoparticles of ZnSnO3 and Zn2SnO4, which are the potential nano-structured semiconductor oxides for a dye sensitized solar cell (DSSC), have been investigated by employing photoemission spectroscopy (PES) and soft X-ray absorption spectroscopy (XAS), and compared to those of reference materials. The divalent and tetravalent valence states of Zn2+ and Sn4+ ions are confirmed experimentally. The energy levels of both the valence-band and conduction-band edges are determined experimentally. The top of the valence band in PES is slightly higher in Zn2SnO4 than in ZnSnO3. The onset energies of the O 1s XAS spectra of the Zn-stannates are found to be similar to each other, but higher than that of TiO2. The O 1 s XAS spectrum of ZnSnO3 exhibits the higher unoccupied density of states near the bottom of the conduction band than those of Zn2SnO4, SnO2 and ZnO, reflecting the larger number of holes in the Zn 3 d bands of ZnSnO3. Hence, the easier electron transfer is expected from the LUMO (lowest unoccupied molecular orbital) of a dye molecule to the conduction band of ZnSnO3 nanoparticles on the transparent conductive electrode of a DSSC.

Temperature Dependence of Magnetically Active Charge Excitations in Magnetite across the Verwey Transition.

We study the electronic structure of bulk single crystals and epitaxial films of Fe_{3}O_{4}. Fe 2p core level spectra show clear differences between hard x-ray (HAX) and soft x-ray photoemission spectroscopy (PES). The bulk-sensitive spectra exhibit temperature (T) dependence across the Verwey transition, which is missing in the surface-sensitive spectra. By using an extended impurity Anderson full-multiplet model-and in contrast to an earlier peak assignment-we show that the two distinct Fe species (A and B site) and the charge modulation at the B site are responsible for the newly found double peaks in the main peak above T_{V} and its T-dependent evolution. The Fe 2p HAXPES spectra show a clear magnetic circular dichroism (MCD) in the metallic phase of magnetized 100-nm-thick films. The model calculations also reproduce the MCD and identify the contributions from magnetically distinct A and B sites. Valence band HAXPES shows a finite density of states at E_{F} for the polaronic half metal with a remnant order above T_{V} and a clear gap formation below T_{V}. The results indicate that the Verwey transition is driven by changes in the strongly correlated and magnetically active B-site electronic states, consistent with resistivity and optical spectra.

Soft X-Ray Absorption and Photoemission Study on the Electronic Structure of (La,Sr)CoO3

A perovskite-type oxide (La,Sr)CoO3 (LSC) has been extensively studied as a cathode material of SOFC due to its high performance. In comparison to the other cathode materials, the bulk property of LSC can be characterized by a large oxygen nonstoichiometry, high oxide ion diffusivity, and high electronic (hole) conductivity, while it is not clear which is the factor governing the high catalytic and cathode performance. On the other hand, discussions have recently been made on the correlation between electronic structure and cathode properties. After evaluation of surface exchange coefficient and defect chemical analyses on a series of solid solution SrTi1-x Fe x O3, it was suggested that the overall cathode reaction kinetics is mainly determined by the density of minority electronic carrier in bulk, in other words, the position of the Fermi energy relative to the electronic band structure [1]. It is also proposed that the oxygen reduction reaction (ORR) activity of a series of perovskite oxides, although the reaction in aqueous electrolyte is concerned, is primarily correlated with the electronic state of the B-site transition element, that is, the occupation of e g orbital and extent of covalency against oxygen [2]. The fundamental understanding on the electronic structure of cathode materials is thus required. While there have been many studies on the electronic structure of LSC by means of x-ray spectroscopies and DFT calculations, most of them are concerned with the electronic transport and magnetic property, and sometimes lack determination of chemical situation of the specimens. In this study, the authors report the comprehensive analyses of the electronic structure for LSC by using soft x-ray absorption and photoemission spectroscopy in consideration of chemistry in bulk and the cathode reaction process. Dense specimens of LSC have been prepared via a citrate process, and the surface has been polished finally with 0.1um diamond abrasive. The specimens have been heat-treated under some conditions of temperature and oxygen partial pressure (Po2), and subjected to x-ray spectroscopic measurements. O K-edge and Co L-edge XAS spectra have been collected at BL-11A and -19B of Photon Factory, KEK, Japan. XPS spectra of the constituent core level and valence band have been collected with an x-ray photoemission spectrometer (PHI5000 / ULVAC-PHI). Fig. 1 shows the O K-edge XAS spectra of La0.6Sr0.4CoO3- d (LSC40) collected in electron yield mode. The main absorption feature (C) observed around 536eV is rather insensitive to the heat-treatment condition, and can be assigned mainly to La5d and Sr4d hybridization orbital. On the other hand, the low energy feature (A-B) ranging 526-531eV show a significant change depending on the heat-treatment condition. It is noted that among the T-Po2 conditions examined, the oxygen nonstoichiometry (3-d) changes in the following order: 2.95 under (T, Po2)=(873K, 1atm), 2.89 (1073K, 10-1atm), 2.83 (1073K, 10-3atm(Ar atmosphere)) [3]. The absorption intensity at the lowest energy part (A) drastically decreases upon reduction. From the XPS measurements, it is observed that the binding energies of the constituent core level, except for Co2p, increases upon reduction. The above observations suggest the following picture. With decreasing oxygen content, the electron hole introduced by Sr-doping reduces, leading to the decrease in the unoccupied density of state corresponding to the absorption feature (A). At the same time, the occupied density of states in the valence band increases and the Fermi energy upward shifts. In contrast to the O K-edge XAS spectra, the Co L-edge absorption spectra are overall less sensitive to the heat-treatment condition, apart from slight energy shift and appearance of shoulder structure. It is widely known that L-edge absorption structure of Co and also the other transition elements significantly changes according to the valence and spin states. It is therefore suggested that the charge compensation for extrinsic defects is practically taken by oxide ion rather than by Co; that is, the electron hole may be expressed as O-. This behavior can be ascribed to the large extent of covalency, and has been observed also for Fe-doped BaZrO3[4]. [1] W.C. Jung and H.L. Tuller, Adv. Energy Mater.1 (2011) 1184. [2] J. Suntivich et al., Nature Chem.3 (2011) 546. [3] M. Kuhn et al., J. Solid State Chem.197 (2013) 38. [4] D. Kim et al., Chem. Mater.26 (2014) 927. Figure 1

Soft X-ray photoemission study of Co2(Cr1−xFex)Ga Heusler compounds

We have performed soft X-ray photoemission spectroscopy (SXPES) and X-ray absorption spectroscopy (XAS) of the Co-based Heusler compounds Co2(Cr1−xFex)Ga (x = 0.0, 0.4, and 1.0) in order to study their electronic structures. Band-structure calculation was carried out and compared with the experimental results. SXPES spectra show hν-dependence, revealing the contributions of the Co, Cr, and Fe 3d electronic states in the valence band. The band width observed by the SXPES seems to be narrower than that predicted by the band-structure calculation. XAS spectra depend strongly on the the value of x in Co2(Cr1−xFex)Ga. The electron correlation effects are found to be stronger as x changes from 0.0 to 1.0.

Soft X-Ray Absorption and Photoemission Study on the Electronic Structure of (La,Sr)CoO3

In order to reveal the electronic structure of (La,Sr)CoO3-d relevant to the chemistry in SOFC cathode, soft x-ray absorption and photoemission spectroscopy were employed. The lowest energy component in O K-edge XAS spectra of the La0.6Sr0.4CoO3-d is ascribed to the electron holes introduced by Sr-doping, and sensitively changes in intensity with oxygen nonstoichiometry. On the other hand, the Co2p XAS spectrum is insensitive to the oxidation state of the specimen. The increase in oxygen nonstoichiometry is accompanied with upward shift of the Fermi energy, while the binding energy of the Co2p level is almost unchanged. It is suggested that the relevant electron holes are rather associated with oxide ion rather than simple unoccupied state of cobalt ion.

Soft X-ray Photoemission Study of New BiS2-Layered Superconductor LaO1−xFxBiS2

We use core level and valence band soft x-ray photoemission spectroscopy (SXPES) to investigate electronic structure of new BiS$_{2}$ layered superconductor LaO$_{1-x}$F$_{x}$BiS$_{2}$. Core level spectra of doped samples show a new spectral feature at the lower binding energy side of the Bi 4${f}$ main peak, which may be explained by core-hole screening with metallic states near the Fermi level ($E_{\rm F}$). Experimental electronic structure and its ${x}$ dependence (higher binding energy shift of the valence band as well as appearance of new states near $E_{\rm F}$ having dominant Bi 6${p}$ character) were found to be consistent with the predictions of band structure calculations in general. Noticeable deviation of the spectral shape of the states near $E_{\rm F}$ from that of calculations might give insight into the interesting physical properties. These results provide first experimental electronic structure of the new BiS$_{2}$ layered superconductors.

Valence states and electronic structures of Co and Mn substituted spin gapless semiconductor PbPdO2

Electronic structures of Pb(Pd0.9T0.1)O2 (T = Mn, Co) spin gapless semiconductors have been investigated by employing soft X-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). The valence states of Co and Mn ions are found to be mixed-valent (∼2.7) and tetravalent, respectively. The measured valence-band PES and O 1s XAS spectra show that both PbPdO2 and PbPd0.9Co0.1O2 are small-gap semiconductors. This finding is supported by the calculated band structures, obtained in the density functional theory with the modified Becke-Johnson potential (mBJ) scheme. This work also shows evidence for the existence of the phase separation in Mn-substituted PbPd0.9Mn0.1O2.

High-temperature thermal stability study of 1 nm Al2O3 deposited on InAs surfaces investigated by synchrotron radiation based photoemission spectroscopy

High-resolution soft x-ray photoemission spectroscopy (SXPS) has been used to study the high-temperature thermal stability of ultra-thin atomic layer deposited (ALD) Al2O3 layers (∼1 nm) on sulfur passivated and native oxide covered InAs surfaces. While the arsenic oxides were removed from both interfaces following a 600 °C anneal, a residual indium oxide signal remained. No significant differences were observed between the sulfur passivated and native oxide surfaces other than the thickness of the interfacial oxide layer while the Al2O3 stoichiometry remained unaffected by the anneals. The energy band offsets were determined for the Al2O3 on the sulfur passivated InAs surface using both valence band edge and shallow core-level photoemission measurements.

Impact of Electrode Oxidation on the Current Transport Properties at Platinum/(Niobium-Doped Strontium-Titanate) Schottky Junctions

The current transport properties and resistance-switching (RS) behavior at platinum/niobium-doped strontium-titanate (Pt/NSTO) Schottky junctions were investigated for junctions with different degrees of oxidation (PtOx) in the Schottky electrode. It was found that the rectifying current-voltage (I–V) profile strongly depends on the heat-treatment of the NSTO crystal and oxidation of the Pt electrode. The hysteresis in the I–V relationships was larger for the PtOx/NSTO junction than for the pure Pt/NSTO junction, and heat-treatment of the NSTO crystal suppresses the hysteresis in I-V and increases the leakage current. The change in the rectifying I–V profiles can be reasonably explained by the increase in the built-in potential caused by partial oxidation of the electrode interface, as determined by soft X-ray photoemission spectroscopy. The experimental results indicate that control of the built-in potential and the partial oxidation of the electrode, which affects the homogeneity of the potential profile, is the key to improving and controlling the rectification and RS properties in Pt/NSTO junctions.

Observation of strontium segregation in LaAlO3/SrTiO3 and NdGaO3/SrTiO3 oxide heterostructures by X-ray photoemission spectroscopy

LaAlO3 and NdGaO3 thin films of different thicknesses have been grown by pulsed laser deposition on TiO2-terminated SrTiO3 single crystals and investigated by soft X-ray photoemission spectroscopy. The surface sensitivity of the measurements has been tuned by varying photon energy hν and emission angle Θ. In contrast to the core levels of the other elements, the Sr 3d line shows an unexpected splitting for higher surface sensitivity, signaling the presence of a second strontium component. From our quantitative analysis we conclude that during the growth process Sr atoms diffuse away from the substrate and segregate at the surface of the heterostructure, possibly forming strontium oxide.

High temperature thermal stability of the HfO2/Ge (100) interface as a function of surface preparation studied by synchrotron radiation core level photoemission

High resolution soft x-ray photoemission spectroscopy (SXPS) have been used to study the high temperature thermal stability of ultra-thin atomic layer deposited (ALD) HfO2 layers (∼1nm) on sulphur passivated and hydrofluoric acid (HF) treated germanium surfaces. The interfacial oxides which are detected for both surface preparations following HfO2 deposition can be effectively removed by annealing upto 700°C without any evidence of chemical interaction at the HfO2/Ge interface. The estimated valence and conduction band offsets for the HfO2/Ge abrupt interface indicated that effective barriers exist to inhibit carrier injection.