Resonant Photoemission Spectroscopy Research Articles

Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

The evolution of the Mn charge state, chemical composition, and electronic structure of (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O -edge and Mn -edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment ( ambient pressure of ) alone results in La deficiency (severest near the surface with Sr doping ) and a stronger contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O -edge X-ray absorption measurements support enrichment nearer the surface, along with the formation of mixed and/or passive and SrO species.

Electronic structure ofLiCoO2thin films: A combined photoemission spectroscopy and density functional theory study

The electronic properties of ${\text{LiCoO}}_{2}$ have been studied by theoretical band-structure calculations (using density functional theory) and experimental methods (photoemission). Synchrotron-induced photoelectron spectroscopy, resonant photoemission spectroscopy (ResPES), and soft x-ray absorption (XAS) have been applied to investigate the electronic structure of both occupied and unoccupied states. High-quality PES spectra were obtained from stoichiometric and highly crystalline ${\text{LiCoO}}_{2}$ thin films deposited ``in situ'' by rf magnetron sputtering. An experimental approach of separating oxygen- and cobalt-derived (final) states by ResPES in the valence-band region is presented. The procedure takes advantage of an antiresonant behavior of cobalt-derived states at the $3p\text{\ensuremath{-}}3d$ excitation threshold. Information about the unoccupied density of states has been obtained by $\text{O}\text{ }K$ XAS. The structure of the $\text{Co}\text{ }L$ absorption edge is compared to semiempirical charge-transfer multiplet calculations. The experimental results are furthermore compared with band-structure calculations considering three different exchange potentials [generalized gradient approximation (GGA), using a nonlocal Hubbard $U$ $(\text{GGA}+U)$ and using a hybrid functional (Becke, three-parameter, Lee-Yang-Parr [B3LYP])]. For these different approaches total density of states and partial valence-band density of states have been investigated. The best qualitative agreement with experimental results has been obtained by using a $\text{GGA}+U$ functional with $U=2.9\text{ }\text{eV}$.

Interaction between Cobalt Phthalocyanine and Gold Studied by X-ray Absorption and Resonant Photoemission Spectroscopy

Cobalt phthalocyanine (CoPc) monolayer films evaporated on Au- (100) are studied by photoexcited electron spectroscopies: X-ray absorption (XAS) andresonantphotoemissionspectroscopy(ResPES).AccordingtoXASdata,theCo 2p absorption spectra look different for monolayer and thicker organic films, pointing to a possible interaction of the molecules with the substrate via the Co d electrons. The contribution of Co-related states in the valence band of CoPc was observed with ResPES. The conclusions are discussed in combination with core- level X-ray photoemission spectroscopy (XPS), X-ray excited Auger electron spectroscopy (XAES), and valence band ultraviolet photoemission spectroscopy (UPS) results previously obtained. SECTION Surfaces, Interfaces, Catalysis

Resonance photoemission studies of (111) oriented CeO2 thin film grown on Si (100) substrate by pulsed laser deposition

The electronic structure of CeO2 thin film grown by pulsed laser deposition on Si (100) substrate has been investigated using resonance photoemission spectroscopy (RPES). X-ray photoemission study on the film suggests that Ce has 3+ and 4+ valence states. Valence band spectra of the film show a feature at 2.1 eV of binding energy and a broad band at higher binding energy due to O 2p derived state. RPES measurements performed in the Ce 4d→4f photoabsorption region show maximum intensity for 2.1 eV feature at photon energy of 122 eV confirming it to be due to Ce3+ (4f1) state. RPES measurements also show maximum intensity for binding energy position of 4.4 eV in the broad band at photon energy of 125 eV, suggesting it to be due to Ce4+ (4f0) state. Constant initial state (CIS) versus photon energy plots also confirm these findings and suggest that the broad band is admixture of O 2p and Ce 4f and 5d derived states.

Electronic structure ofCeAg2Ge2studied by resonant photoemission spectroscopy

The electronic structure of ${\text{CeAg}}_{2}{\text{Ge}}_{2}$ single crystal has been investigated by using valence-band photoemission at different photon energies ranging from 110 to 150 eV. Resonant photoemission has been observed near the $4d$ threshold of Ce at 121 eV. The constant initial-state spectra shows two photoemission features having $4f$ character near the Fermi level at $\ensuremath{-}0.4$ and $\ensuremath{-}1.7\text{ }\text{eV}$ which exhibits Fano-type sharp resonance character. The experimental spectra have been interpreted with the help of calculations based on full-potential linearized augmented plane-wave method using density-functional theory. Excellent agreement has been obtained between the theory and the experiment. The origin of the feature near to Fermi level is related to the $\text{Ce}\text{ }4f$ states and the feature at $\ensuremath{-}1.7\text{ }\text{eV}$ is related to the strong hybridization between the $\text{Ce}\text{ }4f$ and $5d$, $\text{Ag}\text{ }4d$ and $\text{Ge}\text{ }4p$ states.

Relating Energy Level Alignment and Amine-Linked Single Molecule Junction Conductance

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant X-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope-based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) that presents Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

Resonant photoemission spectroscopy study of electronic structures of LiMn2O4

The valence-band resonant photoemission spectra (RPES) of LiMn2O4 have been measured throughout the Mn3p absorption edge. Based on the RPES data, the contribution of Mn3d states to the valence band of LiMn2O4 has been described and, consequently, the detailed hybridization between O2p and Mn3d states in the valence-band was determined.

Evidence for a Correlated Insulator to Antiferromagnetic Metal Transition in CrN

We investigate the electronic structure of chromium nitride (CrN) across the first-order magnetostructural transition at T(N)∼286 K. Resonant photoemission spectroscopy (PES) shows a gap in the 3d partial density of states at the Fermi level and an on-site Coulomb energy U∼4.5 eV, indicating strong electron-electron correlations. Bulk-sensitive high-resolution (6 meV) laser PES reveals a clear Fermi edge indicating an antiferromagnetic metal below T(N). Hard x-ray Cr 2p core-level PES shows T-dependent changes across T(N) which originate from screening due to coherent states as substantiated by cluster model calculations using the experimentally observed U. Electrical resistivity confirms an insulator above T(N) (E(g)∼70 meV) becoming a disordered metal below T(N). Thus, CrN transforms from a correlated insulator to an antiferromagnetic metal, coupled to the magnetostructural transition.

Electronic structure of cobalt phthalocyanine studied by resonant photoemission: Localization of Co-related valence band states

We study the electronic structure of cobalt phthalocyanine (CoPc) thin films on Au(1 0 0) using resonant photoemission spectroscopy and X-ray absorption spectroscopy (XAS). From XAS we conclude that the molecules are highly ordered and oriented parallel to the substrate surface. Cobalt related valence band states are identified using resonant photoemission. A strong enhancement of particular features is observed at the resonance energies, depending on the angle of incidence. This behavior can be understood by participator decay processes, the (de-)localization of Co-related valence band states is discussed.

Carrier Compensation by Excess Oxygen Atoms in Anatase Ti0.94Nb0.06O2+δ Epitaxial Thin Films

We report the effect of post-deposition annealing on the electrical transport properties of anatase Ti0.94Nb0.06O2 (TNO) epitaxial thin films. Annealing TNO films in pure oxygen drastically suppressed the carrier density (ne). A high ne of the order of 1021 cm-3 was recovered by successive annealing in pure hydrogen. Core-level X-ray photoemission spectroscopy revealed that Ti and Nb respectively exist as tetravalent and pentavalent ions in fully oxidized samples. The concentration of Nb5+ relative to that of Nb4+ tends to increase with O2 annealing, suggesting that carriers released by Nb donors are compensated by electron-killing impurity states created by O2 annealing. Based on these findings, we propose that excess oxygen atoms incorporated by O2 annealing occupy interstitial sites and behave as deep acceptor states, which compensate electron carriers generated by Nb doping. Resonant valence-band photoemission spectroscopy directly confirmed the formation of deep acceptor states associated with oxygen annealing.

Electronic structure and magnetism of the diluted magnetic semiconductor Fe-doped ZnO nanoparticles

We have studied the electronic structure of Fe-doped ZnO nanoparticles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy, resonant photoemission spectroscopy, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe3+ ionic state with mixture of a small amount of Fe2+ and that Fe3+ ions are dominant in the surface region of the nanoparticles. It is shown that the room temperature ferromagnetism in the Fe-doped ZnO nanoparticles primarily originated from the antiferromagnetic coupling between unequal amounts of Fe3+ ions occupying two sets of nonequivalent positions in the region of the XMCD probing depth of ∼2–3 nm.

Resonant photoemission of LaNiO3-δthin films

Thin LaNiO3−δ films with pseudocubic (100) preferred orientation were prepared by reactive DC magnetron sputtering and annealed at ultra high vacuum above dehydration temperature. The resonant photoemission spectroscopy was used to study the surface composition and electronic structure under La 4d→4f and Ni 3p→3d photoexcitation. The resonance features observed in core level and valence band spectra under La 4d→4f transition were explained in terms of autoionization process and lanthanum–oxygen valence band states’ hybridization. No resonant features were observed in the valence band spectra under Ni 3p→3d excitation indicating that nickel species are not present at the LaNiO3−δ film surface after heat treatment. Keywords: rare earth alloys and compounds, oxide materials, resonant photoelectron spectroscopy

Angle-Resolved and Resonant Photoelectron Spectroscopy Study of Ni2P (10-10) Single-Crystal Surface

The electronic structure of Ni2P (10−10) has been investigated by photoemission spectroscopy (PES) utilizing synchrotron radiation. The surface which was cleaned by Ar+ ion sputtering (3 kV, 15 min) and annealing (400oC) gave a c (2×4) low-energy electron diffraction pattern. In the PES spectra measured for this surface, a Ni 3d-P 3p hybrid band (main band) and a satellite were observed at 0—4 eV and at 8 eV, respectively. Resonant PES (RPES) study showed that the satellite is associated with the photoemission process leading to a two-hole bound final state. The angle-resolved PES measurements showed that there were at least two surface states ; one was observed at about 0.4 eV around the Γ− point and the other was observed at about 0.6 eV along the boundary of the surface Brillouin zone along the [0001] direction. The results of RPES measurements suggested that the latter state should have little contribution of the Ni 3d component.

Electronic properties of single-walled carbon nanotubes encapsulating a cerium organometallic compound

Single-walled carbon nanotubes are filled with a cerium organometallic compound. Using core-level excitation spectroscopy we study the electronic structure of specific atoms in this complex compound. From X-ray absorption spectroscopy we show that the electronic configuration of cerium is nearly trivalent. Resonant photoemission spectroscopy provides further details of the valence electronic properties of cerium as well as of carbon. Thermal annealing modifies the chemical identity of the filling that varies the electronic interactions between the nanotubes and filling material. Schematic for a single-walled carbon nanotube encapsulating cerocene molecules.

Observation of Itinerant Ce 4f Electronic States in CeIrSi3 Studied by Angle-Resolved Ce 3d→4f Resonance Photoemission Spectroscopy

We have applied angle-resolved Ce 3 d →4 f resonance photoemission spectroscopy ( h ν∼882 eV) in the soft X-ray region to the analysis of the non-centrosymmetric pressure-induced superconductor CeIrSi 3 and obtained the 4 f band structure and Fermi surfaces. We have found that the Ce 4 f states are located mainly near the Fermi level and that the photoemission feature corresponding to the dispersive conduction bands across the Fermi level shows considerable resonant enhancement. In addition, the band structure and Fermi surfaces of CeIrSi 3 differ from those of the non- f reference compound LaIrSi 3 , and the differences are well explained by the band structure calculated within local density approximation, in which the Ce 4 f states are assumed to be itinerant. These results strongly suggest that the Ce 4 f electrons in CeIrSi 3 hybridized well with conduction bands and form itinerant electronic states.

Electronic structure of bulk ferromagnetic Ge 0.86Mn 0.14Te

The electronic structure of polycrystalline Ge 0.86Mn 0.14Te—ferromagnetic semiconductor with T C=110 K, has been studied by means of resonant photoemission spectroscopy for photon energies close to the Mn 3p→3d excitation. The contribution of Mn 3d states to the electronic structure of the system was revealed and position of the Mn ions in the crystal, characteristic of GeTe-based diluted magnetic semiconductors was proved.

Microscopic (AFM) and resonant photoemission study of Gd/Si(1 1 1) interface

Atomic force microscopy (AFM) and Fano-type resonant photoemission spectroscopy (RPS) of Gd atoms deposited on Si(1 1 1) surface are presented. Layers of thicknesses 0.2, 2 and 15 nm were deposited under UHV conditions. Anomalously deep craters were observed on the Si(1 1 1) surface side of the interface below the Gd layer. We expect that anomalously high chemical activity of Gd and a dominating diffusion of Si atoms are responsible for the effect. Application of synchrotron radiation in the energy region hν corresponding to the Gd 4d–4f transition (140–170 eV) enabled Fano-type resonant photoemission spectra. Si(1 1 1) surface valence band spectra with contributions from Gd 4f electrons were detected for a 0.2-nm-thick Gd layer deposited onto a clean Si(1 1 1) surface. The photoemission study revealed a Fano resonance shape of the curve with resonance and antiresonance energies hv=151.8 and 146.8 eV, respectively. The Gd 4f localized electrons contribute to the valence band density of states located at 9.8 eV below the valence band edge.

Crystalline electric field effects inCe 3dcore-level spectra of heavy-fermion systems: Hard x-ray photoemission spectroscopy onCeNi1−xCoxGe2

High-resolution hard X-ray photoemission measurements have been performed to clarify the electronic structure originating from the strong correlation between electrons in bulk Ce 3$d$ core-level spectra of CeNi$_{1-x}$Co$_x$Ge$_2$ (0 $\leq$ $x$ $\leq$ 1). In the Ce 3$d_{5/2}$ core-level spectra, the variation of satellite structures ($f^2$ peaks) shows that the hybridization strength between Ce 4$f$- and conduction electrons gradually increases with Co concentration in good agreement with the results of Ce 3$d-4f$ and 4$d-4f$ resonant photoemission spectroscopies. Particularly, in Ce 3$d_{3/2}$ core-level spectra, the multiplet structures of $f^1$ peaks systematically change with the degeneracy of $f$-states which originates from crystalline electric field effects.

Unusual energy shifts in resonant photoemission spectra of organic model molecules

We study the electronic structure of zinc phthalocyanine (ZnPc) and 1,4-octa-decyl substituted zinc phthalocyanine [(Dec)(8)PcZn] thin films (approximately 6-15 nm) using resonant photoemission spectroscopy and X-ray absorption spectroscopy (XAS) at room temperature and at liquid He temperature. From XAS we conclude that the probability amplitude of the lowest unoccupied molecular orbital is located predominantly at the inner C and N atoms of the molecules. Nonlinear energy shifts in resonant photoemission were observed; large shifts are explained by reduced electrical conductivity of inhomogeneously oriented molecules.

Investigation of valence states and electronic structure of ferromagnetic double-perovskite La2MnNiO6 by using synchrotron radiation

The electronic structure of ferromagnetic double-perovskite La2MnNiO6 has been investigated by employing Mn and Ni 2p→3d resonant photoemission spectroscopy (RPES) and soft x-ray absorption spectroscopy (XAS). The Mn and Ni 2p XAS measurements show that Mn ions are formally tetravalent (3d3) and that Ni ions are formally divalent (3d8). Mn and Ni 2p→3d RPES shows that the occupied Mn 3d3 (t2g3↑) states are sharp and located around 2 eV below EF, while the occupied Ni 3d8 (t2g3↑t2g3↓eg2↑) states are broader than the Mn 3d states and they extend from EF to ∼5 eV below EF.