Resonant Photoemission Spectroscopy Research Articles

Soft x-ray irradiation induced metallization of layered TiNCl

We have performed soft x-ray spectroscopy in order to study the photoirradiation time dependence of the valence band structure and chemical states of layered transition metal nitride chloride TiNCl. Under the soft x-ray irradiation, the intensities of the states near the Fermi level (E F) and the Ti3+ component increased, while the Cl 2p intensity decreased. Ti 2p–3d resonance photoemission spectroscopy confirmed a distinctive Fermi edge with Ti 3d character. These results indicate the photo-induced metallization originates from deintercalation due to Cl desorption, and thus provide a new carrier doping method that controls the conducting properties of TiNCl.

Charge screening effects in the resonant photoemission of Rh2O3, RuO2 , and MoO2

We have performed valence-band resonant photoemission spectroscopy across the transition-metal (TM) ${L}_{3}$ edge of ${\mathrm{Rh}}_{2}{\mathrm{O}}_{3}, {\mathrm{RuO}}_{2}$, and ${\mathrm{MoO}}_{2}$. This technique allows us to access important information on the partial contribution of each respective TM. We show that an extended cluster model, coupled with transitions between discrete and continua states, describes well the overall resonance behavior as well as the experimental constant initial-state (CIS) spectra. Moreover, we analyze the resonant enhancement of the main lines and satellite structures to discuss the influence of the different charge regimes, the TM--O $2p$ hybridization, and additional charge screenings to the CIS signal. Finally, we argue that this approach can be extended to the study of other transition-metal oxides.

Electronic Structure and Interface Energetics of CuBi2O4 Photoelectrodes.

CuBi2O4 exhibits significant potential for the photoelectrochemical (PEC) conversion of solar energy into chemical fuels, owing to its extended visible-light absorption and positive flat band potential vs the reversible hydrogen electrode. A detailed understanding of the fundamental electronic structure and its correlation with PEC activity is of significant importance to address limiting factors, such as poor charge carrier mobility and stability under PEC conditions. In this study, the electronic structure of CuBi2O4 has been studied by a combination of hard X-ray photoemission spectroscopy, resonant photoemission spectroscopy, and X-ray absorption spectroscopy (XAS) and compared with density functional theory (DFT) calculations. The photoemission study indicates that there is a strong Bi 6s–O 2p hybrid electronic state at 2.3 eV below the Fermi level, whereas the valence band maximum (VBM) has a predominant Cu 3d–O 2p hybrid character. XAS at the O K-edge supported by DFT calculations provides a good description of the conduction band, indicating that the conduction band minimum is composed of unoccupied Cu 3d–O 2p states. The combined experimental and theoretical results suggest that the low charge carrier mobility for CuBi2O4 derives from an intrinsic charge localization at the VBM. Also, the low-energy visible-light absorption in CuBi2O4 may result from a direct but forbidden Cu d–d electronic transition, leading to a low absorption coefficient. Additionally, the ionization potential of CuBi2O4 is higher than that of the related binary oxide CuO or that of NiO, which is commonly used as a hole transport/extraction layer in photoelectrodes. This work provides a solid electronic basis for topical materials science approaches to increase the charge transport and improve the photoelectrochemical properties of CuBi2O4-based photoelectrodes.

Origin of the high Seebeck coefficient of the misfit [Ca2CoO3]0.62[CoO2] cobaltate from site-specific valency and spin-state determinations

Layered misfit cobaltate [Ca$_2$CoO$_3$]$_{0.62}$[CoO$_2$], which emerged as an important thermoelectric material~[A. C. Masset et al. Phys. Rev. B, 62, 166 (2000)], has been explored extensively in the last decade for the exact mechanism behind its high Seebeck coefficient. Its complex crystal and electronic structures have inhibited consensus among such investigations. This situation has arisen mainly due to difficulties in accurate identification of the chemical state, spin state, and site symmetries in its two subsystems (rocksalt [Ca$_2$CoO$_3$] and triangular [CoO$_2$]). By employing resonant photoemission spectroscopy and x-ray absorption spectroscopy along with charge transfer multiplet simulations (at the Co ions), we have successfully identified the site symmetries, valencies and spin states of the Co in both layers. Our site-symmetry observations explain the experimental value of the high Seebeck coefficient and also confirm that the carriers hop within the rocksalt layer, which is in contrast to earlier reports where hopping within triangular CoO$_2$ layer has been held responsible for the large Seebeck coefficient.

Localization of electronic states resulting from electronic topological transitions in the Mo1−xRex alloys: A photoemission study

We present the results of resonant photoemission spectroscopy experiments on Mo1−xRex alloy compositions spanning over two electronic topological transitions (ETTs) at critical concentrations xC1=0.05 and xC2=0.11. Photoelectrons show an additional resonance (R3) in constant initial state spectra of alloys along with two resonances (R1 and R2), which are similar to those observed in molybdenum. All the resonances show Fano-like line shapes. Asymmetry parameter q of resonances R1 and R3 of alloys is observed to be large and negative. Our analysis suggests that the origin of large negative q is associated with phonon assisted interband scattering between Mo-like states and the narrow band that appeared due to ETT.

Role of point and line defects on the electronic structure of LaAlO3/SrTiO3 interfaces

Realization of heterostructures containing multiple two-dimensional electron liquids requires a fine control of the fabrication process. Here, we report a structural and spectroscopy study of LaAlO3/SrTiO3/LaAlO3 trilayers grown on the SrTiO3 substrate by pulsed-laser deposition. Scanning transmission electron microscopy with the help of ab initio calculations reveals that antisite defects associated with oxygen vacancies are primarily present in the SrTiO3 film (STO-f) close to the p-type interface (STO-f/LaAlO3), while oxygen vacancies prevail close to the top n-type interface (LaAlO3/STO-f). At the same interface, misfit dislocations relax the tensile strain of the top LaAlO3 layer. Combining x-ray absorption spectroscopy, x-ray linear dichroism, resonant photoemission spectroscopy, and electron energy loss spectroscopy, we observe that the 3d orbital reconstruction at the interface between LaAlO3 and the SrTiO3 substrate is confined over a few interfacial Ti planes while, at the top n-type interface (LaAlO3/STO-f), the absence of a dichroic signal can be related to the blurring of the interfacial orbital reconstruction due to the heterogeneity of defects.

Origin of itinerant carriers in antiferromagnetic LaFe1−xMoxO3 studied by x-ray spectroscopies

We report on the electronic structure of doped $\mathrm{LaFe}{\mathrm{O}}_{3}$ at the crossover from an insulating-to-metallic phase transition. Comprehensive x-ray spectroscopic methodologies are used to understand core and valence electronic structure as well as crystal structure distortions associated with the electronic transition. Despite the antiferromagnetic (AFM) ordering at room temperature, we show direct evidence of itinerant carriers at the Fermi level revealed by resonant photoemission spectroscopy (RPES) at the Mo ${L}_{3}$ edge. RPES data taken at the Fe ${L}_{3}$ edge show spectral weight near the valence band edge and significant hybridization with O $2p$ states required for AFM ordering. Resonant inelastic x-ray scattering spectra taken across Fe ${L}_{2,3}$ edges show electron correlation effects ($U$) driven by Coulomb interactions of $d$ electrons as well as broad charge-transfer excitations for $x\ensuremath{\ge}0.2$ where the compound crosses over to a metallic state. Site substitution of Fe by Mo ions in the Fe-${\mathrm{O}}_{6}$ octahedra enhances the separation of the two Fe-O bonds and Fe-O-Fe bonding angles relative to the orthorhombic $\mathrm{LaFe}{\mathrm{O}}_{3}$, but no considerable distortions are present to the overall structure. Mo ions appear to be homogeneously doped, with average valency of both metal sites monotonically decreasing with increasing Mo concentration. This insulator-to-metal phase transition with AFM stability is primarily understood through intermediate interaction strengths between correlation ($U$) and bandwidth ($W$) at the Fe site, where an estimation of this ratio is given. These results highlight the important role of extrinsic carriers in stabilizing a unique phase transition that can guide future efforts in antiferromagnetic-metal spintronics.

Ultra-fast charge transfer between fullerenes and a gold surface, as prepared by electrospray deposition

Monolayers of C60 and the functionalised fullerene phenyl-C61-butyric acid methyl ester (PCBM) have been prepared on Au(111) by in situ electrospray deposition. The observation of super-spectator/super-Auger decay in resonant photoemission spectroscopy (RPES) of C60 confirms monolayers prepared using electrospray undergo ultra-fast charge transfer as per previous studies of C60 monolayers on Au(111) prepared by sublimation. For PCBM, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) show the lowest unoccupied molecular orbital (LUMO) of the molecule overlaps the Fermi-level of the metal surface. Despite this, no evidence of ultra-fast charge transfer is observed in the RPES of PCBM.

Nanoscale architecture of ceria-based model catalysts: Pt–Co nanostructures on well-ordered CeO2(111) thin films

We have prepared and characterized atomically well-defined model systems for ceria-supported Pt–Co core–shell catalysts. Pt@Co and Co@Pt core–shell nanostructures were grown on well-ordered CeO2(111) films on Cu(111) by physical vapour deposition of Pt and Co metals in ultrahigh vacuum and investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The deposition of Co onto CeO2(111) yields Co–CeO2(111) solid solution at low Co coverage (0.5 ML), followed by the growth of metallic Co nanoparticles at higher Co coverages. Both Pt@Co and Co@Pt model structures are stable against sintering in the temperature range between 300 and 500 K. After annealing at 500 K, the Pt@Co nanostructure contains nearly pure Co-shell while the Pt-shell in the Co@Pt is partially covered by metallic Co. Above 550 K, the re-ordering in the near surface regions yields a subsurface Pt–Co alloy and Pt-rich shells in both Pt@Co and Co@Pt nanostructures. In the case of Co@Pt nanoparticles, the chemical ordering in the near surface region depends on the initial thickness of the deposited Pt-shell. Annealing of the Co@Pt nanostructures in the presence of O2 triggers the decomposition of Pt–Co alloy along with the oxidation of Co, regardless of the thickness of the initial Pt-shell. Progressive oxidation of Co coupled with adsorbate-induced Co segregation leads to the formation of thick CoO layers on the surfaces of the supported Co@Pt nanostructures. This process is accompanied by the disintegration of the CeO2(111) film and encapsulation of oxidized Co@Pt nanostructures by CeO2 upon annealing in O2 above 550 K. Notably, during oxidation and reduction cycles with O2 and H2 at different temperatures, the changes in the structure and chemical composition of supported Co@Pt nanostructures were driven mainly by oxidation while reduction treatments had little effect regardless of the initial thickness of the Pt-shell.

Synergetic effect on catalytic activity and charge transfer in Pt-Pd bimetallic model catalysts prepared by atomic layer deposition.

Pt-Pd bimetallic nanoparticles were synthesized on TiO2 support on the planar substrate as well as on high surface area SiO2 gel by atomic layer deposition to identify the catalytic performance improvement after the formation of Pt-Pd bimetallic nanoparticles by surface analysis techniques. From X-ray absorption near edge spectra of Pt-Pd bimetallic nanoparticles, d-orbital hybridization between Pt 5d and Pd 4d was observed, which is responsible for charge transfer from Pt to Pd. Moreover, it was found from the in situ grazing incidence X-ray absorption spectroscopy study that Pt-Pd nanoparticles have a Pd shell/Pt core structure with CO adsorption. Resonant photoemission spectroscopy on Pt-Pd bimetallic nanoparticles showed that Pd resonant intensity is enhanced compared to that of Pd monometallic nanoparticles because of d-orbital hybridization and electronic states broadening of Pt and Pd compared monometallic catalysts, which results in catalytic performance improvement.

Charge transfer and spillover phenomena in ceria-supported iridium catalysts: A model study.

Iridium-based materials are among the most active bifunctional catalysts in heterogeneous catalysis and electrocatalysis. We have investigated the properties of atomically defined Ir/CeO2(111) model systems supported on Cu(111) and Ru(0001) by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, near ambient pressure X-ray photoelectron spectroscopy (NAP XPS), scanning tunneling microscopy, and temperature programmed desorption. Electronic metal-support interactions in the Ir/CeO2(111) system are accompanied by charge transfer and partial reduction of CeO2(111). The magnitude of the charge transfer depends strongly on the Ir coverage. The Ir/CeO2(111) system is stable against sintering upon annealing to 600 K in ultrahigh vacuum (UHV). Annealing of Ir/CeO2(111) in UHV triggers the reverse oxygen spillover above 450 K. The interaction of hydrogen with Ir/CeO2(111) involves hydrogen spillover and reversible spillover between 100 and 400 K accompanied by the formation of water above 190 K. Formation of water coupled with the strong reduction of CeO2(111) represents the dominant reaction channel upon annealing in H2 above 450 K. The interaction of Ir/CeO2(111) with oxygen has been investigated at moderate and NAP conditions. Additionally, the formation and stability of iridium oxide prepared by deposition of Ir in oxygen atmosphere was investigated upon annealing in UHV and under exposure to H2. The oxidation of Ir nanoparticles under NAP conditions yields stable IrOx nanoparticles. The stability of Ir and IrOx nanoparticles under oxidizing conditions is hampered, however, by encapsulation by cerium oxide above 450 K and additionally by copper and ruthenium oxides under NAP conditions.

Quantitative Analysis of the Oxidation State of Cobalt Oxides by Resonant Photoemission Spectroscopy.

Quantitative assessment of the charge transfer phenomena in cobalt oxides and cobalt complexes is essential for the design of advanced catalytic materials. We propose a method for the evaluation of the oxidation state of cobalt oxides with mixed valence states using resonant photoemission spectroscopy. The method is based on the calculation of the resonant enhancement ratio (RER) from the heights of the resonant features associated with the Co3+ and Co2+ states. The nature of the corresponding states was corroborated by means of density functional calculations. We employed a well-ordered Co3O4(111) film to calibrate the RER with respect to the atomic Co3+/Co2+ ratio. The method was applied to monitor the reduction of a well-ordered Co3O4(111) film to CoO(111) upon annealing under exposure to isopropanol. We demonstrate that this method yields the stoichiometry of cobalt oxides at a level of accuracy that cannot be achieved when fitting the Co 2p core level spectra.

Temperature-dependent evolution of Ti 3d spectral features at surface of BaxTi8O16+δ

We have studied temperature-driven effects on photoemission spectra of hollandite-type ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$ across its metal-insulator transition (MIT) at 220 K by using surface-sensitive photon energies from 27 to 70 eV. The surface Ti $3d$ spectral weight was suppressed and shifted towards higher binding energy from the bulk if the surface was obtained by cleavage above the MIT temperature. The Ti $3d$ spectral weight was recovered once the sample was cooled across the MIT, indicating a transfer of Ti $3d$ electrons from the bulk to the surface across the MIT. The recovered Ti $3d$ spectral shape is consistent with the bulk-sensitive result and indicates strong localization character of the Ti $3d$ electron even in the metallic phase. Resonant photoemission spectroscopy with absorption from the Ti $3p$ core level exhibits resonances of the O $2p$-Ti $3d$ and O $2p$-Ti $4s,p$ hybridized states. The result suggests that the O $2p$-Ti $4s,p$ hybridization plays a role in the interplay between the bulk and the surface in ${\mathrm{Ba}}_{x}{\mathrm{Ti}}_{8}{\mathrm{O}}_{16+\ensuremath{\delta}}$.

First-principles study of the electronic structure and the Fermi surface in rare-earth filled skutterudites RPt4Ge12

Experiments on rare-earth filled skutterudites demonstrate an intriguing array of thermodynamic, transport and superconducting properties, and bring to fore theoretical challenges posed by f-electron systems. First principle calculations based density functional theory and its extensions for strongly correlated systems such as the Hubbard U correction, provide valuable information about electronic structure that can be used to understand experiments. We present a comprehensive study of the electronic structure and Fermi surface of a series of rare earth filled skutterudites, RPt4Ge12 (where R = La, Ce, Pr), aimed at shedding light on: consequences of progressive increase of f-orbital occupancy in the series; the effects of the Hubbard parameter U; the Fermi surfaces, band structures and densities of states. The calculated Fermi surfaces may be relevant to the question of multi-band versus single-band superconductivity. Computed densities of states qualitatively explain the available resonant photoemission spectroscopy experiments, and (together with available specific heat measurements) provide estimates of the effective masses. We also show the existence of pseudogaps in the total density of states which may be relevant for the thermoelectric properties of these systems.

Temperature driven Mott-Hubbard to charge-transfer insulator transition in hexagonal Sr0.6Ba0.4MnO3

We have probed the electronic structure across the ferroelectric transition temperature (TC = 355 K) of hexagonal Sr0.6Ba0.4MnO3 (SBMO) which shows a magneto-electric effect at room temperature. The valence and conduction bands are characterized by resonant photoemission and X-ray absorption spectroscopy measurements, respectively. The valence band spectrum is found to be hugely modified across TC. Above TC, SBMO behaves like a Mott-Hubbard insulator, whereas at 300 K, it shows a charge-transfer-type insulator behavior. Modulation in the hybridization of Mn-3d and O-2p orbitals due to local structural variations with temperature unravels the rational for the observed electronic structure transition vis a vis magnetic and electrical behaviors of SBMO.

Magnetic and Electronic Properties of Gd-Doped Topological Insulator Bi1.09Gd0.06Sb0.85Te3

The recent realization of quantum anomalous Hall effect and Majorana fermions observation enhance interest in magnetism investigation in topological insulators. In this work, the electronic and magnetic structure of the Gd-doped topological insulator Bi1.09Gd0.06Sb0.85Te3 were systematically studied by means of angle-resolved photoemission spectroscopy, resonance photoemission spectroscopy (ResPES) and SQUID magnetometry. Resonant features related to the Gd density of states near the Fermi level are experimentally observed. Study of magnetic structure showed antiferromagnetic ordered bulk at low temperatures as well as presence of hysteresis loop at elevated temperatures. Finally, possible mechanism of magnetism and its relation to observed electronic features are discussed.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0401000, 2015CB921300, 2016YFA0300303, 2016YFA0401002 and 2017YFA0303103, the National Natural Science Foundation of China under Grant Nos 11674371, 11774401 and 11874330, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB07000000, and

We utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature , which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization. However, the Fermi vector and the Fermi surface volume have little change around Tcoh, which challenges the widely believed evolution from a high-temperature small Fermi surface to a low-temperature large Fermi surface. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Resonant inelastic X-ray scattering of a Ru photosensitizer: Insights from individual ligands to the electronic structure of the complete molecule.

N 1s Resonant Inelastic X-ray Scattering (RIXS) was used to probe the molecular electronic structure of the ruthenium photosensitizer complex cis-bis(isothiocyanato) bis(2,2'-bipyridyl-4,4'-dicarboxylato) ruthenium(II), known as "N3." In order to interpret these data, crystalline powder samples of the bipyridine-dicarboxylic acid ligand ("bi-isonicotinic acid") and the single ring analog "isonicotinic acid" were studied separately using the same method. Clear evidence for intermolecular hydrogen bonding is observed for each of these crystalline powders, along with clear vibronic coupling features. For bi-isonicotinic acid, these results are compared to those of a physisorbed multilayer, where no hydrogen bonding is observed. The RIXS of the "N3" dye, again prepared as a bulk powder sample, is interpreted in terms of the orbital contributions of the bi-isonicotinic acid and thiocyanate ligands by considering the two different nitrogen species. This allows direct comparison with the isolated ligand molecules where we highlight the impact of the central Ru atom on the electronic structure of the ligand. Further interpretation is provided through complementary resonant photoemission spectroscopy and density functional theory calculations. This combination of techniques allows us to confirm the localization and relative coupling of the frontier orbitals and associated vibrational losses.

Anion-mediated electronic effects in reducible oxides: Tuning the valence band of ceria via fluorine doping

Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce3+ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce4+ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F--Ce4+ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria.

Electronic structure of the high- TC ferromagnetic semiconductor (Ga,Fe)Sb: X-ray magnetic circular dichroism and resonance photoemission spectroscopy studies

The electronic structure and the magnetism of the novel ferromagnetic semiconductor (Ga,Fe)Sb, whose Curie temperature $T_{\rm C}$ can exceed room temperature, were investigated by means of x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and resonance photoemission spectroscopy (RPES). The line-shape analyses of the XAS and XMCD spectra suggest that the ferromagnetism is of intrinsic origin. The orbital magnetic moments deduced using XMCD sum rules were found to be large, indicating that there is a considerable amount of 3$d^{6}$ contribution to the ground state of Fe. From RPES, we observed a strong dispersive Auger peak and non-dispersive resonantly enhanced peaks in the valence-band spectra. The latter is a fingerprint of the correlated nature of Fe 3$d$ electrons, whereas the former indicates their itinerant nature. It was also found that the Fe 3$d$ states have finite contribution to the DOS at the Fermi energy. These states presumably consisting of majority-spin $p$-$d$ hybridized states or minority-spin $e$ states would be responsible for the ferromagnetic order in this material.