Resonant Photoemission Spectroscopy Research Articles

Cation diffusion and hybridization effects at the Mn-GaSe(0001) reacted interface:Ab initiocalculations and soft x-ray electron spectroscopy studies

The electronic properties of the Mn:GaSe interface, produced by evaporating Mn at room temperature on a $\ensuremath{\epsilon}$-GaSe(0001) single-crystal surface, have been studied by soft x-ray spectroscopies, and the experimental results are discussed at the light of ab initio DFT+U calculations of a model ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se} (x=0.055)$ surface alloy. Consistently with these calculations that also predict a high magnetic moment for the Mn ions $(4.73\text{--}4.83 {\ensuremath{\mu}}_{B})$, XAS measurements at the Mn $L$ edge indicate that Mn diffuses into the lattice as a ${\mathrm{Mn}}^{2+}$ cation with negligible crystal-field effects. Ab initio calculations also show that the most energetically favorable lattice sites for Mn diffusion are those where Mn substitutes Ga cations in the Ga layers of the topmost Se-Ga-Ga-Se sandwich. Mn $s$ and $p$ states are found to strongly hybridize with Se and Ga $p$ states, while weaker hybridization is predicted for Mn $d$ states with Se $s$ and $p$ orbitals. Furthermore, unlike other Mn-doped semiconductors, there is strong interaction between the $\mathrm{Ga}\text{\ensuremath{-}}s$ and $\mathrm{Mn}\text{\ensuremath{-}}{d}_{{z}^{2}}$ states. The effects of hybridization of Mn $3d$ electrons with neighboring atoms are still clearly detectable from the characteristic charge-transfer satellites observed in the photoemission spectra. The Mn $3d$ spectral weight in the valence band is probed by resonant photoemission spectroscopy at the Mn $L$ edge, which also allowed an estimation of the charge transfer ($\mathrm{\ensuremath{\Delta}}=2.95$ eV) and Mott-Hubbard ($U=6.4$ eV) energies on the basis of impurity-cluster configuration-interaction model of the photoemission process. The Mott-Hubbard correlation energy $U$ is consistent with the ${U}_{\mathrm{eff}}$ on-site Coulomb repulsion parameter (5.84 eV) determined for the ab initio calculations.

Resonant PhotoEmission Spectroscopy Investigation of Fe2O3 – TiO2 Heterojunctions for Solar Water Splitting

The implementation of semiconductor heterojunctions photo-anodes appears as a very promising way to improve the performances of devices for solar water splitting (sun light assisted hydrogen production from water). However, assembling different materials together results in the existence of interfaces which usually do not have the same electronic structure than the simply stacked individual layers. The electronic structure of the valence band being a key parameter for water splitting, it is necessary to investigate it for each layer and for the interfaces. A very powerful technique to tackle these issues is Resonant PhotoEmissionSpectroscopy (RPES). In this work, we present RPES results of the valence band performed on epitaxial TiO2/Ti-doped Fe2O3 heterojunctions. The provided insights concerning the interface and the electronic structure are correlated to the water splitting performances.

Ultrafast Bidirectional Charge Transport and Electron Decoherence at Molecule/Surface Interfaces: A Comparison of Gold, Graphene, and Graphene Nanoribbon Surfaces.

We investigate bidirectional femtosecond charge transfer dynamics using the core-hole clock implementation of resonant photoemission spectroscopy from 4,4'-bipyridine molecular layers on three different surfaces: Au(111), epitaxial graphene on Ni(111), and graphene nanoribbons. We show that the lowest unoccupied molecular orbital (LUMO) of the molecule drops partially below the Fermi level upon core-hole creation in all systems, opening an additional decay channel for the core-hole, involving electron donation from substrate to the molecule. Furthermore, using the core-hole clock method, we find that the bidirectional charge transfer time between the substrate and the molecule is fastest on Au(111), with a 2 fs time, then around 4 fs for epitaxial graphene and slowest with graphene nanoribbon surface, taking around 10 fs. Finally, we provide evidence for fast phase decoherence of the core-excited LUMO* electron through an interaction with the substrate providing the first observation of such a fast bidirectional charge transfer across an organic/graphene interface.

Resonant photoemission spectroscopy for intermediate band materials

Resonant photoemission spectroscopy is used to study the intermediate-band material Cr doped ZnS. Using resonant photoemission, we show that the intermediate-band can be characterized, revealing the filling and specific orbital character of the states contributing to the resonant photoemission signal. We demonstrate that resonant photoemission spectroscopy is a powerful approach for understanding the origin of intermediate bands in doped ZnS. The methodology can be widely extended to a large variety of materials, providing useful information towards engineering of high efficiency intermediate band solar cells and of other optoelectronic devices.

Effects of Cr substitution on the magnetic and transport properties and electronic states ofSrRuO3epitaxial thin films

The effect of Cr substitution in a SrRuO3 epitaxial thin film on SrTiO3 substrate was investigated by measuring the magnetic and transport properties and the electronic states. The ferromagnetic transition temperature of the SrRu0.9Cr0.1O3 film (166 K) was higher than that of the SrRuO3 film (147 K). Resonant photoemission spectroscopy experimentally revealed that the Cr 3dt2g orbital is hybridized with the Ru 4dt2g orbital in the SrRu0.9Cr0.1O3 film, supporting the assumption that the enhancement of the ferromagnetic transition temperature through Cr substitution stems from the widening of energy bands due to the hybridization of Cr 3dt2g and Ru 4dt2g orbitals. Furthermore, we found that the Hall resistivity of the SrRu0.9Cr0.1O3 film at low temperature is not a linear function of magnetic field in the high-field region where the out-of-plane magnetization was saturated; this result suggests that the SrRu0.9Cr0.1O3 film undergoes a structural transition at low temperature accompanied with the modulation of the Fermi surface.

Universal electronic structure of polar oxide hetero-interfaces.

The electronic properties of NdGaO3/SrTiO3, LaGaO3/SrTiO3, and LaAlO3/SrTiO3 interfaces, all showing an insulator-to-metal transition as a function of the overlayer-thickness, are addressed in a comparative study based on x-ray absorption, x-ray photoemission and resonant photoemission spectroscopy. The nature of the charge carriers, their concentration and spatial distribution as well as the interface band alignments and the overall interface band diagrams are studied and quantitatively evaluated. The behavior of the three analyzed heterostructures is found to be remarkably similar. The valence band edge of all the three overlayers aligns to that of bulk SrTiO3. The near-interface SrTiO3 layer is affected, at increasing overlayer thickness, by the building-up of a confining potential. This potential bends both the valence and the conduction band downwards. The latter one crossing the Fermi energy in the proximity of the interface and determines the formation of an interfacial band offset growing as a function of thickness. Quite remarkably, but in agreement with previous reports for LaAlO3/SrTiO3, no electric field is detected inside any of the polar overlayers. The essential phenomenology emerging from our findings is discussed on the base of different alternative scenarios regarding the origin of interface carriers and their interaction with an intense photon beam.

Electronic structure origin of conductivity and oxygen reduction activity changes in low-level Cr-substituted (La,Sr)MnO3.

The electronic structure of the (La(0.8)Sr(0.2))(0.98)Mn(1-x)Cr(x)O3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La,Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t(2g) ↑ state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t(2g) ↑ state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La,Sr)MnO3 is presented.

Decomposition of Acetic Acid on Model Pt/CeO2 Catalysts: The Effect of Surface Crowding

Adsorption and decomposition of acetic acid were studied by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature-programmed desorption on Pt/CeO2(111) model catalysts prepared on Cu(111). Reference experiments under identical conditions were performed on stoichiometric CeO2(111), partially reduced CeO2–x, and oxygen pre-exposed O/Pt/CeO2(111)/Cu(111). The principal species formed on all samples during adsorption of acetic acid at 150 K were acetate and molecularly adsorbed acetic acid. On the basis of the differences in the splitting between the methyl and carboxyl/carboxylate groups in the C 1s spectra, we identified the adsorption sites for acetate and molecularly adsorbed acetic acid on Pt/CeO2. During annealing, we detected an increase in the concentration of acetate on CeO2(111) support exclusively in the presence of supported Pt particles. The effect is caused by the decomposition of molecularly adsorbed acetic acid on Pt particles followed ...

Photoinduced reduction of surface states in Fe:ZnO.

We report on the electronic structure of nano-crystalline Fe:ZnO, which has recently been found to be an efficient photocatalyst. Using resonant photoemission spectroscopy, we determine the binding energy of Fe 3d states corresponding to different valencies and coordination of the Fe atoms. The photo-activity of ZnO reduces Fe from 3+ to 2+ in the surface region of the nano-crystalline material due to the formation of oxygen vacancies. Electronic states corresponding to low-spin Fe(2+) are observed and attributed to crystal field modification at the surface. These states are potentially important for the photocatalytic sensitivity to visible light due to their location deep in the ZnO bandgap. X-ray absorption and x-ray photoemission spectroscopy suggest that Fe is only homogeneously distributed for concentrations up to 3%. Increased concentrations does not result in a higher concentration of Fe ions in the surface region. This is limiting the photocatalytic functionality of ZnO, where the most efficient Fe doping concentration has been shown to be 1%-4%.

Study of electronic structure of Co2MnSn Heusler alloy by resonant photoemission spectroscopy and ab initio calculations

The electronic structure of a ferromagnetic Heusler alloy Co2MnSn has been investigated using resonant photoemission spectroscopy to probe the Mn 3d and Co 3d states in the valence band region. The motivation of the present work is to understand the low value of spin polarization reported for this system, which mainly depends on the density of states present at the Fermi level. Valence band of Co2MnSn shows a large density of delocalized Co 3d states present near the Fermi level. The Mn 3d states are more localized and exist below the Fermi level. First principles calculations using density functional theory with GGA and GGA+U formalisms have been performed to understand and analyze the experimental results. The experimental density of states and magnetic moment show a good matching with the calculation carried out with on-site Coulomb correlation U of ∼3eV on the Mn 3d states. The presence of U on the Mn 3d states also reduces the spin polarization at the Fermi level, which is one of the reasons for the observed low value of spin polarization reported for this system.

Resonant photoemission study of multiferroic LuFe2O4 across three-dimensional to two-dimensional charge ordering

Electronic structures modification of polycrystalline LuFe2O4 across 3 dimensional to 2 dimensional charge ordering (CO) has been studied by resonance photoemission spectroscopy. Constant-initial-state spectra measured across 3p → 3d resonance energy indicate its charge transfer insulating behavior. Valance band spectra study shows states lying closer to the Fermi level are due to Fe2+ ions. As the temperature is varied from 3D to 2D CO state, a weight transfer of DOS from low binding energy to higher binding energy is observed which is attributed to the modification in the Fe-O hybridized states.

Photoemission and x-ray absorption studies of the isostructural to Fe-based superconductors diluted magnetic semiconductorBa1−xKx(Zn1−yMny)2As2

The electronic and magnetic properties of a new diluted magnetic semiconductor (DMS) Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$, which is isostructural to so-called 122-type Fe-based superconductors, are investigated by x-ray absorption spectroscopy (XAS) and resonance photoemission spectroscopy (RPES). Mn $L_{2,3}$-edge XAS indicates that the doped Mn atoms have the valence 2+ and strongly hybridize with the $4p$ orbitals of the tetrahedrally coordinating As ligands. The Mn $3d$ partial density of states (PDOS) obtained by RPES shows a peak around 4 eV and relatively high between 0-2 eV below the Fermi level ($E_{F}$) with little contribution at $E_{F}$, similar to that of the archetypal DMS Ga$_{1-x}$Mn$_{x}$As. This energy level creates $d^{5}$ electron configuration with $S=5/2$ local magnetic moments at the Mn atoms. Hole carriers induced by K substitution for Ba atoms go into the top of the As $4p$ valence band and are weakly bound to the Mn local spins. The ferromagnetic correlation between the local spins mediated by the hole carriers induces ferromagnetism in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$

Direct growth of metallic TiH2 thin films by pulsed laser deposition

We report the layer-by-layer growth and electronic properties of (111)-oriented, nearly stoichiometric TiH2 films (δ-phase) pulsed-laser-deposited on α-Al2O3(0001) using a TiH2 ceramic target. The content of the δ-phase increased as the decomposition to the Ti metal was suppressed at low temperatures. Moreover, long-lasting oscillations of reflection high-energy electron diffraction intensity were observed during the initial growth of the δ-phase film. The film showed metallic conductivity down to low temperatures. The results of Ti 2p–3d resonant photoemission spectroscopy and Hall measurement were consistent with those of the conducting electrons residing in the Ti 3d states.

Resonant photoemission spectroscopy study of the d and f states in GdNi5 − x Cu x intermetallics

The electronic structure of the GdNi5 − xCux (x = 0, 1, and 2) compounds is studied by resonant photoemission spectroscopy, the main regularities of the electronic strucutre transformation upon substitution of copper for nickel are established, and the energies of the d and f states that form its valence bands are measured. X-ray absorption and resonant photoemission spectra at the gadolinium M4, 5 and N4, 5 edges and nickel and copper L2, 3 edges are recorded. It is shown that a core photohole and an excited electron in gadolinium decay along the elastic channel; in the final state, one hole forms in the valence band and Gd 4f photoemission is enhanced tenfold. Since the inelastic Auger-type relaxation channel dominates in the transition elements with the formation of two holes in the valence band, the resonant photoemission spectrum allows us to measure the energy of two-particle interactions. The intra-atomic repulsion energy in nickel and copper is found to be ∼7 eV.

The electronic properties of mixed valence hydrated europium chloride thin film.

We investigate the electronic properties of a model mixed-valence hydrated chloride europium salt by means of high resolution photoemission spectroscopy (HRPES) and resonant photoemission spectroscopy (RESPES) at the Eu 3d → 4f and 4d → 4f transitions. From the HRPES spectra, we have determined that the two europium oxidation states are homogeneously distributed in the bulk and that the hydrated salt film is exempt from surface mixed valence transition. From the RESPES spectra, the well separated resonant contributions characteristic of divalent and trivalent europium species (4f(6) and 4f(7) final states, respectively) are accurately extracted and quantitatively determined from the resonant features measured at the two edges. The partial absorption yield spectra, obtained by integrating the photoemission intensity in the valence-band region, can be well reproduced by atomic multiplet calculation at the M(4,5) (3d-4f) absorption edge and by an asymmetric Fano-like shape profile at the N(4,5) (4d-4f) absorption edge. The ratio of Eu(2+) and Eu(3+) species measured at the two absorption edges matches with the composition of the mixed valence europium salt as determined chemically. We have demonstrated that the observed spectroscopic features of the mixed valence salt are attributed to the mixed-valence ground state rather than surface valence transition. HRPES and RESPES spectra provide reference spectra for the study of europium salts and their derivatives.

Valence Band of Ce_2Co_{0.8}Si_{3.2} and Ce_2RhSi_3 Studied by Resonant Photoemission Spectroscopy and FPLO Calculations

This work presents studies of the valence band of two Kondo lattice systems: Ce2Co0.8Si3.2, which is paramagnetic with the Kondo temperature T-K approximate to 50 K and Ce2RhSi3, which is antiferromagnetic below T-N = 4.5 K and exhibits TK approximate to 9 K. The photoemission spectra, which are obtained with photon energy tuned to Ce - 4d 4f resonance, reveal a Kondo peak at the Fermi energy (E-F), its spin orbit splitting partner at 0.24 eV and a broad maximum related to Ce f(0) final state. The spectra indicate that Kondo peak has a higher intensity for Ce2Co0.8Si3.2. The off-resonance photoemission data reveal that a maximum in the 3d electron density of states is shifted towards EF for Ce2Co0.8Si3.2 as compared to Ce2RhSi3. Full-potential local-orbital calculations were realized with local spin density approach +U approach for 213 stoichiometry. They show that a higher density of states near EF is observed for Ce2CoSi3. The calculations also reveal the existing tendencies for antiferromagnetic and ferromagnetic ground states in a case of Ce2RhSi3 and Ce2CoSi3, respectively. (Less)

Electronic Structures of CeM2Al10 (M = Fe, Ru, and Os) Studied by Soft X-ray Resonant and High-Resolution Photoemission Spectroscopies

We have performed a photoemission spectroscopy (PES) study of CeM2Al10 (M = Fe, Ru, and Os) to directly observe the electronic structure involved in the unusual magnetic ordering. Soft X-ray resonant (SXR) PES provides spectroscopic evidence of the hybridization between conduction and Ce 4f electrons (c-f hybridization) and the order of the hybridization strength (Ru < Os < Fe). High-resolution (HR) PES of CeRu2Al10 and CeOs2Al10, as compared with that of CeFe2Al10, identifies two structures that can be ascribed to structures induced by the c-f hybridization and the antiferromagnetic ordering, respectively. Although the c-f hybridization-induced structure is a depletion of the spectral intensity (pseudogap) around the Fermi level (EF) with an energy scale of 20-30 meV, the structure related to the antiferromagnetic ordering is observed as a shoulder at approximately 10-11 meV within the pseudogap. The energies of the shoulder structures of CeRu2Al10 and CeOs2Al10 are approximately half of the optical gap (20 meV), indicating that EF is located at the midpoint of the gap.

Electronic structure of amorphous ZnO films

AbstractWe use resonant photoemission spectroscopy (resPES) to study the electronic properties of amorphous ZnO (a‐ZnO) layers. We report on the core levels, the valence band (VB) PES data, and the X‐ray absorption (XAS) data which we use for the conduction band (CB) density of states. From these results we are able to derive the partial density of states (pDOS) of O2p and Zn4s4p states in the VB and CB, respectively as well as a band scheme. At the O1s resonance we observe a band of localized defect states which is located between the Fermi energy and the CBM. At the Zn2p edge the XAS data indicate that localized Zn4s4p states are involved in the DOS starting already at the Fermi energy. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Charge transfer from an adsorbed ruthenium-based photosensitizer through an ultra-thin aluminium oxide layer and into a metallic substrate.

The interaction of the dye molecule N3 (cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4'-dicarbo-xylato)-ruthenium(II)) with the ultra-thin oxide layer on a AlNi(110) substrate, has been studied using synchrotron radiation based photoelectron spectroscopy, resonant photoemission spectroscopy, and near edge X-ray absorption fine structure spectroscopy. Calibrated X-ray absorption and valence band spectra of the monolayer and multilayer coverages reveal that charge transfer is possible from the molecule to the AlNi(110) substrate via tunnelling through the ultra-thin oxide layer and into the conduction band edge of the substrate. This charge transfer mechanism is possible from the LUMO+2 and 3 in the excited state but not from the LUMO, therefore enabling core-hole clock analysis, which gives an upper limit of 6.0 ± 2.5 fs for the transfer time. This indicates that ultra-thin oxide layers are a viable material for use in dye-sensitized solar cells, which may lead to reduced recombination effects and improved efficiencies of future devices.

Electronic structure ofCeCuAs2

We study the electronic structure of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$, which is known to show an anomalous negative temperature coefficient of resistivity below room temperature, while the thermopower shows metallic behavior. We carry out hard x-ray photoemission spectroscopy (PES), x-ray absorption spectroscopy (XAS), and resonant PES across the Ce $3d\text{\ensuremath{-}}4f$ and Cu $2p\text{\ensuremath{-}}3d$ thresholds to investigate the role of Kondo screening in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$. The Ce $3d$ core-level PES shows the ${f}^{0}$, ${f}^{1}$, and ${f}^{2}$ features, and the Ce $3d\text{\ensuremath{-}}4f$ XAS shows corresponding features due to transitions into the ${f}^{1}$ and ${f}^{2}$ states, as in Kondo systems. The spectral feature assignments are confirmed by single-impurity Anderson model calculations and indicate a nearly trivalent-Ce configuration with small but finite mixed valency. The resonant PES valence band spectra across the Ce $3d\text{\ensuremath{-}}4f$ threshold show an intense Ce $4{f}^{1}$ resonance just below the Fermi level, while the Ce $4{f}^{0}$ feature is observed at a binding energy of 2.5 eV. The obtained $f$ partial density of states shows an unusual pseudogaplike dip at the Fermi level in $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$. The results indicate the importance of Kondo screening and a pseudogap in the $f$ partial density of states for the anomalous transport and magnetic properties of $\mathrm{Ce}\mathrm{Cu}{\mathrm{As}}_{2}$.