Soft X-ray Photoemission Spectroscopy Research Articles

Differences in the Ti4+/Ti3+ Redox Reactions between Li4Ti5O12 and LiTi2(PO4)3 Observed By Soft X-Ray Spectroscopy

Clarification of the redox reaction in electrode materials for Li-ion batteries is essential to obtain strategies to improve the performances such as charge-discharge capacity, voltage and cyclability. We focus on the difference in voltage, i.e., redox potential between Li4Ti5O12 (LTO) and LiTi2(PO4)3 (LTP). Spinel-type LTO with a redox potential around 1.5 V vs Li/Li+ is one of the anode materials [1], while the main redox potential of LTP (NASICON-type structure) is around 2.4 V vs Li/Li+ [2]. Although the difference in redox potential is as large as ~0.9 V, it has been thought to be that Ti4+/Ti3+ redox couple is identical for both LTO and LTP. Here, we demonstrate the electronic-structure analysis of LTO and LTP by using soft X-ray absorption spectroscopy (XAS) and soft X-ray photoemission spectroscopy (XPS). Operando Ti L 2,3-edge XAS by use of an all-solid-state cell [3,4] successfully revealed reversible Ti4+/Ti3+ redox reactions by discharge/charge for both materials, but the XAS line shapes were different. By analyzing the spectra using multiplet calculation [5], differences in the crystal-field splitting for the TiO6 octahedron and O 2p -> Ti 3d charge-transfer (CT) effect were found. The different CT effect was also confirmed by Ti 2p-3d resonant XPS measurements for the initial-state LTO and LTP samples. Moreover, the difference in redox potential is mostly explained by the energy levels of the pre-edge structures in the O K-edge XAS calibrated with the O 1s XPS spectra. In the presentation, the understandings of the spectroscopic results will be discussed in detail.

Effect of Mn substitution on the electronic structure for Mn-doped indium-tin oxide films studied by soft and hard x-ray photoemission spectroscopy

Effect of Mn substitution on the electronic structure for Mn-doped indium-tin oxide films studied by soft and hard x-ray photoemission spectroscopy

Direct Evidence of Dynamic Metal Support Interactions in Co/TiO2 Catalysts by Near-Ambient Pressure X-ray Photoelectron Spectroscopy.

The interaction between metal particles and the oxide support, the so-called metal-support interaction, plays a critical role in the performance of heterogenous catalysts. Probing the dynamic evolution of these interactions under reactive gas atmospheres is crucial to comprehending the structure-performance relationship and eventually designing new catalysts with enhanced properties. Cobalt supported on TiO2 (Co/TiO2) is an industrially relevant catalyst applied in Fischer-Tropsch synthesis. Although it is widely acknowledged that Co/TiO2 is restructured during the reaction process, little is known about the impact of the specific gas phase environment at the material's surface. The combination of soft and hard X-ray photoemission spectroscopies are used to investigate in situ Co particles supported on pure and NaBH4-modified TiO2 under H2, O2, and CO2:H2 gas atmospheres. The combination of soft and hard X-ray photoemission methods, which allows for simultaneous probing of the chemical composition of surface and subsurface layers, is one of the study's unique features. It is shown that under H2, cobalt particles are encapsulated below a stoichiometric TiO2 layer. This arrangement is preserved under CO2 hydrogenation conditions (i.e., CO2:H2), but changes rapidly upon exposure to O2. The pretreatment of the TiO2 support with NaBH4 affects the surface mobility and prevents TiO2 spillover onto Co particles.

Synthesis and comprehensive synchrotron-based structural analysis of Si-doped nanodiamond composite films deposited on cemented carbide

Si-doped nanodiamond composite (NDC) films were deposited on unheated WC − Co substrates by coaxial arc plasma deposition for advanced cutting tools applications. The nanoindentation test revealed changes in films hardness and Young's modulus attributed to various Si doping concentrations (0, 1, 5, and 20 at.%) and the catalytic effects of Co diffused from the substrate surface into the films. To reveal the physical origin behind these changes, the films' structure was examined by synchrotron-based Auger electron spectroscopy (AES), soft X-ray photoemission spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). The AES and XPS analysis showed a consistent correlation between nanoindentation measurements and the estimated C sp3 fraction in the films. The NEXAFS spectra revealed intense C − C σ* resonance, consistent with the measured film hardness. The 1 at.% Si-doped film, deposited with an undoped NDC buffer layer to mitigate the Co catalytic effects, exhibited the highest hardness of 60 GPa, the largest C sp3 fraction, and the most intense CC σ* peak. Si doping resulted in the formation of C − Si sp3 bonds at the expense of C sp2 bonds, increasing the fraction of C sp3 bonds and enhancing film hardness. The synchrotron-based analysis effectively revealed the electronic states of NDC hard coatings, particularly after the removal of surface contaminants through mechanical polishing.

Quasiparticle State in 4f Localized Ferromagnet CeRu2Ge2 Studied by Soft X-ray Photoemission Spectroscopy

The electronic state of 4f localized ferromagnet CeRu2Ge2 has been investigated by angle-resolved photoemission spectroscopy (ARPES) experiments using soft x rays. The band structure and Fermi surfaces consisting of non-4f bands are almost consistent with a calculation based on the density-functional theory for LaRu2Ge2, thus confirming a localized character of the 4f electrons. On the other hand, the Ce 3d–4f resonant ARPES experiments demonstrated that the 4f spectral function is dominated by f1 components in the vicinity of the Fermi level and thus is different from that expected for a localized electron system. We also revealed that the intensities of the f1 components have almost no temperature dependence up to at least 100 K, which is two orders of magnitude larger than the Kondo temperature of CeRu2Ge2.

Investigation of the Electronic Structure of the Mg<sub>99.2</sub>Zn<sub>0.2</sub>Y<sub>0.6</sub> Alloy Using X-ray Photoelectron Spectroscopy

We investigated the electronic structure of the Mg99.2Zn0.2Y0.6 alloy using hard and soft X-ray photoemission spectroscopy and electronic band structure calculations to understand the mechanism of the phase stability of this material. Electronic structure of the Mg99.2Zn0.2Y0.6 alloy showed a semi-metallic electronic structure with a pseudo-gap at the Fermi level. The observed electronic structure of the Mg99.2Zn0.2Y0.6 alloy suggests that the presence of a pseudogap structure is responsible for phase stability.

Bulk and surface chemical compositions and microstructure properties of CaF2:Y3+ material

The structural and chemical compositions of Y3+ ion-doped CaF2 were investigated as a possible lanthanide hosting material. A series of CaF2 nanomaterials doped with various concentrations of Y3+ ions under the chelating agent ethylene diamine tetraacetic acid (EDTA) were synthesized using the hydrothermal method. The x-ray diffraction results demonstrated that a mixture of cubic CaF2 and [CaY]F2 phases gradually formed with an increasing Y3+ ion concentration. A single [CaY]F2 cubic phase was formed when EDTA was added as the chelating agent. Scanning electron microscopy results demonstrated that the particle size and the morphology of the material depended on the Y3+ concentration and that EDTA (0.5 g) produced a spherical morphology. The surface and bulk chemical compositions were determined using a combined system of soft x-ray photoemission spectroscopy (XPS) (Al-Kα 1486.7 eV) and hard x-ray photoemission spectroscopy (HAXPES) (Cr-Kα 5414.7 eV). The relative changes in the chemical composition of the surface and subsurface/bulk were investigated. The combined XPS-HAXPES analysis demonstrated that the surface impurities in the accessible peaks (Ca 2p, Y 3d, and F 1s) completely diminished in the subsurface region. However, XPS-HAXPES analysis confirmed that the composition of the [CaY]F2 nanomaterial on the surface differs from that in the subsurface region.

Valence fluctuation in Yb3Si5 probed by synchrotron X-ray photoemission spectroscopy

We report the direct evidence of valence fluctuation in the thermoelectric material Yb3Si5 by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb3Si5 single crystal shows clear separation of the Yb2+ peak and Yb3+ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb3Si5.

Electronic structure of superconducting nickelates probed by resonant photoemission spectroscopy

The discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni$^{1+}$ cations possess nominally the same 3$d^9$ configuration as Cu$^{2+}$ in cuprates, the electronic structure variances remain elusive. Here, we present a soft x-ray photoemission spectroscopy study on parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison to density functional theory + U (on-site Coulomb repulsion energy) calculations, we estimate U ~5 eV, smaller than the charge transfer energy $\Delta$ ~8 eV, confirming the Mott-Hubbard electronic structure in contrast to charge-transfer cuprates. Near the Fermi level ($E_F$), we observe a signature of occupied rare-earth states in the parent compound, which is consistent with a self-doping picture. Our results demonstrate a correlation between the superconducting transition temperature and the oxygen 2$p$ hybridization near $E_F$ when comparing hole-doped nickelates and cuprates.

Electronic structure of the intermediate-valence compound EuNi2P2 studied by soft x-ray photoemission spectroscopy

We carried out angle-resolved photoemission (ARPES) experiments using soft x rays to investigate the electronic structure of the intermediate-valence compound ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$. Both the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ components arising from the $4{f}^{6}$ and $4{f}^{5}$ final states were observed in the valence spectra, directly confirming an intermediate-valence character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and the ARPES results were compared with calculations based on the density-functional theory for the non-4$f$ reference compounds ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ and ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. We found that the ARPES spectra up to just below the Fermi level are better reproduced by the calculation of ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ rather than that of ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. The heavy-fermion bands in ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$ are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$, and the ${\mathrm{Eu}}^{2+}$ components located at the very vicinity of the Fermi level.

Hard x-ray photoemission spectroscopy of LaVO3/SrTiO3 : Band alignment and electronic reconstruction

The heterostructure consisting of the Mott insulator LaVO$_3$ and the band insulator SrTiO$_3$ is considered a promising candidate for future photovoltaic applications. Not only does the (direct) excitation gap of LaVO$_3$ match well the solar spectrum, but its correlated nature and predicted built-in potential, owing to the non-polar/polar interface when integrated with SrTiO$_3$, also offer remarkable advantages over conventional solar cells. However, experimental data beyond the observation of a thickness-dependent metal-insulator transition is scarce and a profound, microscopic understanding of the electronic properties is still lacking. By means of soft and hard X-ray photoemission spectroscopy as well as resistivity and Hall effect measurements we study the electrical properties, band bending, and band alignment of LaVO$_3$/SrTiO$_3$ heterostructures. We find a critical LaVO$_3$ thickness of five unit cells, confinement of the conducting electrons to exclusively Ti 3$d$ states at the interface, and a potential gradient in the film. From these findings we conclude on electronic reconstruction as the driving mechanism for the formation of the metallic interface in LaVO$_3$/SrTiO$_3$.

Oxide Fermi liquid universality revealed by electron spectroscopy

We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_4$ (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through analysis of this self-energy and the Fermi-liquid cut-off energy scale, we find -- in contrast to hole-doped cuprates -- a momentum isotropic and comparatively weak electron correlation in PLCCO. Yet, the self-energies extracted from multiple oxide systems combine to demonstrate a logarithmic divergent relation between the quasiparticle scattering rate and mass. This constitutes a spectroscopic version of the Kadowaki-Woods relation with an important merit -- the demonstration of Fermi liquid quasiparticle lifetime and mass being set by a single energy scale.

Attractive Coulomb interaction, temperature-dependent hybridization, and natural circular dichroism in 1T−TiSe2

We study the electronic structure of single crystal $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$ using temperature ($T$)-dependent soft x-ray photoemission spectroscopy (PES) and natural circular dichroism (NCD) across the charge-density-wave (CDW) transition at ${T}_{\mathrm{cdw}}\ensuremath{\sim}200$ K. We investigate detailed changes in energy positions and widths of the Ti $2p$ and Se $3d$ core-level spectra across ${T}_{\mathrm{cdw}}$. The Ti $2p\ensuremath{-}3d$-resonant PES shows a clear Ti $3d$ character two-hole correlation satellite in the valence band. A Cini-Sawatzky analysis indicates an effective attractive two-hole Coulomb interaction energy ${U}_{dd}$ =\ensuremath{-}1.3 eV. This attractive Coulomb energy is significantly larger than the known energy scale of excitonic correlations ($<$ 100 meV) in $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$. On-resonant $T$-dependent measurements indicate that the strongly hybridized Ti $3d$-Se $4p$ bands show increased hybridization in the CDW phase. On-resonant valence band spectra show a finite, possibly experimental geometry-related NCD of the states at and near the Fermi level which do not show $T$ dependence across ${T}_{\mathrm{cdw}}$. Given the presence of excitonic effects and the periodic lattice distortion, our results suggest that theoretical models need to take into account the attractive Coulomb interaction and hybridization changes for a complete understanding of the CDW in $1T\ensuremath{-}{\mathrm{TiSe}}_{2}$.

Hard and soft x-ray photoemission spectroscopy study of the new Kondo system SmO thin film

SmO thin film is a new Kondo system showing a resistivity upturn around 10 K and was theoretically proposed to have a topologically nontrivial band structure. We have performed hard x-ray and soft x-ray photoemission spectroscopy to elucidate the electronic structure of SmO. From the Sm 3$d$ core-level spectra, we have estimated the valence of Sm to be $\sim$2.96, proving that the Sm has a mixed valence. The valence-band photoemission spectra exhibit a clear Fermi edge originating from the Sm 5$d$-derived band. The present finding is consistent with the theory suggesting a possible topological state in SmO and show that rare-earth monoxides or their heterostructures can be a new playground for the interplay of strong electron correlation and spin-orbit coupling.

Elucidating the Mechanistic Origins of Photocatalytic Hydrogen Evolution Mediated by MoS2/CdS Quantum-Dot Heterostructures.

Solar fuel generation mediated by semiconductor heterostructures represents a promising strategy for sustainable energy conversion and storage. The design of semiconductor heterostructures for photocatalytic energy conversion requires the separation of photogenerated charge carriers in real space and their delivery to active catalytic sites at the appropriate overpotentials to initiate redox reactions. Operation of the desired sequence of light harvesting, charge separation, and charge transport events within heterostructures is governed by the thermodynamic energy offsets of the two components and their photoexcited charge-transfer reactivity, which determine the extent to which desirable processes can outcompete unproductive recombination channels. Here, we map energetic offsets and track the dynamics of electron transfer in MoS2/CdS architectures, prepared by interfacing two-dimensional MoS2 nanosheets with CdS quantum dots (QDs), and correlate the observed charge separation to photocatalytic activity in the hydrogen evolution reaction. The energetic offsets between MoS2 and CdS have been determined using hard and soft X-ray photoemission spectroscopy (XPS) in conjunction with density functional theory. A staggered type-II interface is observed, which facilitates electron and hole separation across the interface. Transient absorption spectroscopy measurements demonstrate ultrafast electron injection occurring within sub-5 ps from CdS QDs to MoS2, allowing for creation of a long-lived charge-separated state. The increase of electron concentration in MoS2 is evidenced with the aid of spectroelectrochemical measurements and by identifying the distinctive signatures of electron-phonon scattering in picosecond-resolution transient absorption spectra. Ultrafast charge separation across the type-II interface of MoS2/CdS heterostructures enables a high Faradaic efficiency of ∼99.4 ± 1.2% to be achieved in the hydrogen evolution reaction (HER) and provides a 40-fold increase in the photocatalytic activity of dispersed photocatalysts for H2 generation. The accurate mapping of thermodynamic driving forces and dynamics of charge transfer in these heterostructures suggests a means of engineering ultrafast electron transfer and effective charge separation to design viable photocatalytic architectures.

Direct insight into the band structure of SrNbO3

We present the results of a photon energy and polarization dependent angle-resolved photoemission spectroscopy (ARPES) study on high quality, epitaxial ${\mathrm{SrNbO}}_{3}$ thin films prepared in situ by pulsed laser deposition (PLD). We show that the Fermi surface is composed of three bands mainly due to ${\mathrm{t}}_{2g}$ orbitals of Nb $4d$, in analogy with the $3d$-based perovskite systems. The bulk band dispersion for the conduction and valence states obtained by density functional theory (DFT) is generally consistent with the ARPES data. The small discrepancy in the bandwidth close to the Fermi level seems to result from the interplay of correlation effects and the presence of vacancies. The ARPES results are complemented by soft x-ray photoemission spectroscopy measurements in order to provide indications on the chemical states and the stoichiometry of the material.

Electronic and Magnetic Properties of Stoichiometric and Off-stoichiometric SrMnO3 Thin Films

Two thin films of SrMnO3 (SMO) were grown on LAO (LaAlO3) (110) substrate by pulsed laser deposition (PLD) technique with different oxygen stoichiometry. From low-temperature magnetization measurements, it is observed that both films show a clear ferromagnetic kind of behavior. Electronic structure of both films has been investigated by using soft x-ray absorption spectroscopy (XAS) and photoemission spectroscopy (PES). It is revealed that the Mn-O hybridization is hugely modified due to oxygen stoichiometry, with its consequences on their magnetic properties.

Electronic states of EuCu2Ge2 and EuCu2Si2 studied by soft x-ray photoemission spectroscopy

We have carried out angle-integrated photoemission spectroscopy (AIPES) and angle-resolved photoemission spectroscopy (ARPES) experiments using soft x rays on single crystals of ${\mathrm{EuCu}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{EuCu}}_{2}{\mathrm{Si}}_{2}$ grown by the Bridgman method to investigate their electronic structures. The AIPES results showed that the Eu ions in ${\mathrm{EuCu}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{EuCu}}_{2}{\mathrm{Si}}_{2}$ are in a divalent state and a nearly trivalent state, respectively, in accordance with the previously reported magnetic properties. The three-dimensional band structures and shapes of the Fermi surfaces of ${\mathrm{EuCu}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{EuCu}}_{2}{\mathrm{Si}}_{2}$ were studied by ARPES measurements. We found that the band structures near the Fermi level and Fermi surfaces of ${\mathrm{EuCu}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{EuCu}}_{2}{\mathrm{Si}}_{2}$ are very different from each other and are well reproduced by the band structure calculations based on density-functional theory for ${\mathrm{SrCu}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{YCu}}_{2}{\mathrm{Si}}_{2}$. This suggests that a charge transfer from the localized $4f$ states into the valence bands is responsible for the difference in the electronic states between ${\mathrm{EuCu}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{EuCu}}_{2}{\mathrm{Si}}_{2}$.

Identification of Lone-Pair Surface States on Indium Oxide

Indium oxide is widely used as a transparent electrode in optoelectronic devices and as a photocatalyst with activity for reduction of CO2. However, very little is known about the structural and electronic properties of its surfaces, particularly those prepared under reducing conditions. In this report, directional “lone-pair” surface states associated with filled 5s2 orbitals have been identified on vacuum-annealed In2O3(111) through a combination of hard and soft X-ray photoemission spectroscopy and density functional theory calculations. The lone pairs reside on indium ad-atoms in a formal +1 oxidation state, each of which traps two electrons into a localized hybrid orbital protruding away from the surface and lying just above the valence band maximum in photoemission spectra. The third electron associated with the ad-atoms is delocalized into the conduction band, thus producing the surface electron accumulation layer identified previously on vacuum-annealed In2O3(111) (1 × 1) surfaces. The surface str...

Photoemission and soft X-ray absorption spectroscopy study of Gd-substituted thermoelectric and topological Bi2Te3

The electronic structures of Gd-substituted Bi2−xGdxTe3 single crystals have been investigated by employing synchrotron-radiation spectroscopies, such as soft X-ray absorption spectroscopy (XAS), soft X-ray magnetic circular dichroism (XMCD), photoemission spectroscopy (PES), and scanning photoelectron microscopy (SPEM). SPEM measurements confirm that Gd ions are substituted homogeneously in Bi2−xGdxTe3 crystals. The measured Gd 3d XAS and XMCD spectra provide evidence that the substituted Gd ions are trivalent and magnetic. The hν-dependence of the valence-band (VB) PES spectra are described well by the hν-dependent contributions of the Te 5p and Bi 6p states. A good agreement is found between the measured VB PES spectra of Bi2−xGdxTe3 and the calculated partial densities of states, revealing that the Te 5p states will play an important role in determining the thermoelectric and topological properties of Bi2−xGdxTe3.