Ce 3d Core Level Research Articles

Oxygen vacancies mediated changes in ferromagnetic, optical, Photoluminescent and electronic structure properties of Ho-doped CeO2 nanoparticles

The undoped CeO2 and Ce1-xHoxO2 (x = 0.03, 0.05 and 0.07) nanoparticles prepared using microwave assisted co-precipitation route are investigated using X-Ray Diffraction (XRD), UV–Vis–NIR, Raman, photoluminescence, X-ray Photoelectron Spectroscopy (XPS) and SQUID-VSM magnetometer. The XRD validates the inclusion of Ho3+ at the Ce3+/Ce4+ site while maintaining the crystal structure with development of defects and oxygen vacancies with expansion of the CeO2 nanolattice. The composition of the elements with desired stoichiometry is confirmed by Energy Dispersive X-ray spectra analysis. The absorption spectroscopy analysis reports the Urbach energy along with refractive index increases with Ho3+ doping and the optical band gap is decreased indicating red shift. All the nanoparticles are subjected to PL investigations to ascertain the oxygen vacancy and defect structure of nanoparticles. Raman active modes at ∼500 - 630 cm−1 verifies the existence of oxygen vacancies (Vo)., which are seen to rise with concentration of Ho-doping in the CeO2 lattice. When Ce4+ transforms into Ce3+, oxygen vacancies are generated in the lattice to maintain the charge neutrality, as can be seen by the Ce-3d, O-1s, and Ho-4d core level XPS spectra. To demonstrate that Ho3+ doping favours ferromagnetic ordering in the CeO2 lattice, the magnetic properties using the Bound Magnetic Polaron model and F centre exchange mechanism are discussed. The development of various complexes that involve host and doped cations are mediated by Vo suggests the existence of F0, F+, and F2+ centres. The weak ferromagnetic behaviour in Ho3+-doped CeO2 nanoparticles reinforce the relationship between Vo to explain the observed magnetic properties of nanoparticles.

Impact of the states in the electronic structure of the intermediate-valence superconductor CeIr3

The electronic structure of the f-based superconductor was studied by photoelectron spectroscopy. The energy distribution of the states were revealed by the resonant photoelectron spectroscopy. The states were mostly distributed in the vicinity of the Fermi energy, suggesting the itinerant character of the states. The contribution of the states to the density of states (DOS) at the Fermi energy was estimated to be nearly half of that of the states, implying that the states have a considerable contribution to the DOS at the Fermi energy. The core-level and x-ray absorption spectra were analyzed based on a single-impurity Anderson model. The number of the states in the ground state was estimated to be , which is much larger than the values obtained in the previous studies (i.e. ).

Enhanced Oxygen Storage Capacity of Porous CeO2 by Rare Earth Doping.

CeO2 is an important rare earth (RE) oxide and has served as a typical oxygen storage material in practical applications. In the present study, the oxygen storage capacity (OSC) of CeO2 was enhanced by doping with other rare earth ions (RE, RE = Yb, Y, Sm and La). A series of Undoped and RE-doped CeO2 with different doping levels were synthesized using a solvothermal method following a subsequent calcination process, in which just Ce(NO3)3∙6H2O, RE(NO3)3∙nH2O, ethylene glycol and water were used as raw materials. Surprisingly, the Undoped CeO2 was proved to be a porous material with a multilayered special morphology without any additional templates in this work. The lattice parameters of CeO2 were refined by the least-squares method with highly pure NaCl as the internal standard for peak position calibrations, and the solubility limits of RE ions into CeO2 were determined; the amounts of reducible-reoxidizable Cen+ ions were estimated by fitting the Ce 3d core-levels XPS spectra; the non-stoichiometric oxygen vacancy (VO) defects of CeO2 were analyzed qualitatively and quantitatively by O 1s XPS fitting and Raman scattering; and the OSC was quantified by the amount of H2 consumption per gram of CeO2 based on hydrogen temperature programmed reduction (H2-TPR) measurements. The maximum [OSC] of CeO2 appeared at 5 mol.% Yb-, 4 mol.% Y-, 4 mol.% Sm- and 7 mol.% La-doping with the values of 0.444, 0.387, 0.352 and 0.380 mmol H2/g by an increase of 93.04, 68.26, 53.04 and 65.22%. Moreover, the dominant factor for promoting the OSC of RE-doped CeO2 was analyzed.

Layer-resolved electronic behavior in a Kondo lattice system, CeAgAs2

We investigate the electronic structure of an antiferromagnetic Kondo lattice system CeAgAs2 employing hard x-ray photoemission spectroscopy. CeAgAs2, an orthorhombic variant of HfCuSi2 structure, exhibits antiferromagnetic ground state, Kondo like resistivity upturn and compensation of magnetic moments at low temperatures. The photoemission spectra obtained at different photon energies suggest termination of the cleaved surface at cis-trans-As layers. The depth-resolved data show significant surface-bulk differences in the As and Ce core level spectra. The As 2p bulk spectrum shows distinct two peaks corresponding to two different As layers. The peak at higher binding energy correspond to cis-trans-As layers and is weakly hybridized with the adjacent Ce layers. The As layers between Ce and Ag-layers possess close to trivalent configuration due to strong hybridization with the neighboring atoms and the corresponding feature appear at lower binding energy. Ce 3d core level spectra show multiple features reflecting strong Ce-As hybridization and strong correlation. Intense f 0 peak is observed in the surface spectrum while it is insignificant in the bulk. In addition, we observe a features at binding energy lower than the well-screened feature indicating the presence of additional interactions. This feature becomes more intense in the bulk spectra suggesting it to be a bulk property. Increase in temperature leads to a spectral weight transfer to higher binding energies in the core level spectra and a depletion of spectral intensity at the Fermi level as expected in a Kondo material. These results reveal interesting surface-bulk differences, complex interplay of intra- and inter-layer covalency, and electron correlation in the electronic structure of this novel Kondo lattice system.

Effect of Cerium Oxide Nanostructures on CO Oxidation.

Cerium oxide particles with different morphologies, namely nanoparticles, nanofibers, nanocubes, and rice grains have been prepared by simple chemical routes. The shape and size of the synthesized morphologies have been studied using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) and selected area electron diffraction (SAED) techniques have been used to determine their crystal phases. Both nanoparticles and nanocubes of cerium oxide exclusively crystallize in fluorite structure of CeO₂ as observed in XRD patterns, whereas nanofibers and rice grains are characterized by the presence of CeO₂, Ce₂O₃, and Ce(OH)₃ phases. X-ray photoelectron spectroscopy (XPS) has been employed to evaluate Ce species present in the different cerium oxide morphologies and to estimate their relative surface concentrations. As evident from Ce 3d core level spectra cerium oxide nanoparticles and nanocubes are basically CeO₂ having Ce in the +4 oxidation state along with some amount of Ce3+ species. In contrast, Ce is in +3 oxidation state on its surface in cerium oxide nanofibers and rice grains that contain intermediate phases like Ce₂O₃ and Ce(OH)₃ as endorsed by XRD patterns. CO oxidation has been carried out over these cerium oxide morphologies and among all morphologies lowest temperature CO oxidation has been demonstrated by the nanocube morphology.

Quantitative study of the f occupation in CeMIn5 and other cerium compounds with hard X-rays

We present bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) data of the Ce3d core levels and lifetime-reduced L-edge X-ray absorption spectroscopy (XAS) in the partial fluorescence yield (PFY) mode of the CeMIn5 family with M=Co, Rh, and Ir. The HAXPES data are analyzed quantitatively with a combination of full multiplet and configuration interaction model which allows correcting for the strong plasmons in the CeMIn5 HAXPES data, and reliable weights wn of the different fn contributions in the ground state are determined. The CeMIn5 results are compared to HAXPES data of other heavy fermion compounds and a systematic decrease of the hybridization strength Veff from CePd3 to CeRh3B2 to CeRu2Si2 is observed, while it is smallest for the three CeMIn5 compounds. The f-occupation, however, increases in the same sequence and is close to one for the CeMIn5 family. The PFY-XAS data confirm an identical f-occupation in the three CeMIn5 compounds and a phenomenological fit to these PFY-XAS data combined with a configuration interaction model yields consistent results.

Quantitative study of valence and configuration interaction parameters of the Kondo semiconductors CeM2Al10 (M = Ru, Os and Fe) by means of bulk-sensitive hard X-ray photoelectron spectroscopy

The occupancy of the 4fn contributions in the Kondo semiconductors CeM2Al10 (M=Ru, Os and Fe) has been quantitatively determined by means of bulk-sensitive hard X-ray photoelectron spectroscopy (HAXPES) on the Ce 3d core levels. Combining a configuration interaction scheme with full multiplet calculations allowed to accurately describe the HAXPES data despite the presence of strong plasmon excitations in the spectra. The configuration interaction parameters obtained from this analysis – in particular the hybridization strength Veff and the effective f binding energy Δf – indicate a slightly stronger exchange interaction in CeOs2Al10 compared to CeRu2Al10, and a significant increase in CeFe2Al10. This shows the existence of a substantial amount of Kondo screening in these magnetically ordered systems and places the entire CeM2Al10 family in the region of strong exchange interactions.

Valence States in CeVO4 and Ce0.5Bi0.5VO4 Probed by Density Functional Theory Calculations and X-ray Photoemission Spectroscopy

Ce is one of the few lanthanide elements to exhibit well-defined (III) and (IV) oxidation states in solid-state environments, and there is therefore ambiguity as to whether CeVO4 should be formulated as Ce(III)V(V)O4 or Ce(IV)V(IV)O4. To address this question, CeVO4 and Ce0.5Bi0.5VO4 have been studied by density functional theory calculations and X-ray photoemission spectroscopy. A peak above the main O 2p valence band in photoemission is attributed to localized Ce 4f states, in agreement with the calculations which show occupation of Ce 4f states. The Ce 3d core level spectrum is diagnostic of Ce(III) with no sign of a peak associated with 4f0 final states that are characteristic of Ce(IV) compounds. The experimental and theoretical results thus confirm that both compounds contain Ce(III) and V(V), rather than Ce(IV) and V(IV). In agreement with experiment, the calculations also show that the tetragonal zircon phase adopted by CeVO4 is more stable for Ce0.5Bi0.5VO4 than the monoclinic clinobisvanite phase adopted by BiVO4, so that the formation of the stereochemically active Bi(III) lone pairs is suppressed by Ce doping.

GROWTH, STRUCTURAL CHARACTERIZATION AND INTERFACIAL REACTION OF MAGNETRON SPUTTERED CeO2 THIN FILMS ON DIFFERENT SUBSTRATES

CeO 2 thin films were deposited on Si , Al , Ti –6 Al –4 V alloy, Si 3 N 4 and glass substrates by magnetron sputtering at room temperature. Growth of CeO 2 films on Si and Si 3 N 4 and effect of annealing were investigated by XRD, FESEM and AFM. Interaction between deposited CeO 2 films and Si , Al , Ti –6 Al –4 V alloy, Si 3 N 4 and glass substrates was investigated by XPS. XRD studies show that films are oriented preferentially to (200)-direction of CeO 2 and no significant change is observed in the XRD patterns of films after heat treatment. CeO 2 film on Si 3 N 4 exhibits rough morphology, whereas very fine morphology is observed in CeO 2 film on Si . CeO 2 film on Si shows lower roughness in relation to that on Si 3 N 4 as demonstrated by AFM studies. XPS results show that Ce is present as both +4 and +3 oxidation states in CeO 2 film deposited on Si and Al substrates, whereas Ce 4+ is the main species in CeO 2 films deposited on Ti –6 Al –4 V alloy, Si 3 N 4 and glass substrates. Ce3d , Si2p and O1s core level spectra demonstrate that Ce 2 O 3 or cerium silicate and SiO x type of species are formed at the interface of CeO 2 and Si . Similarly, formation of interfacial species like Ce 2 O 3 or cerium aluminate is evident in CeO 2 film on Al as demonstrated by XPS studies. On the other hand, interfacial reactions between CeO 2 and Ti –6 Al –4 V alloy, Si 3 N 4 and glass substrates are limited in the respective films.

X-ray photoemission study of CeTIn5 (T = Co, Rh, Ir)

We investigated CeTIn5 (T = Co, Rh, Ir) using temperature- and angle-dependent x-ray photoemission spectroscopy. The Ce 3d core level has a very similar shape for all three materials and is indicative of weak f-hybridization. The spectra were analyzed using a simplified version of the Anderson impurity model, which yields a Ce 4f occupancy that is larger than 0.9. The temperature dependence shows a continuous, irreversible and exclusive broadening of the Ce 3d peaks, due to oxidation of Ce at the surface.

X-ray photoelectron spectroscopy analysis and band offset determination of CeO2deposited on epitaxial (100), (110), and (111)Ge

The oxidation states, interface, and band alignment properties of physical vapor deposited CeO2 films on epitaxial (100), (110), and (111)Ge were investigated by x-ray photoelectron spectroscopy (XPS). The cross-sectional transmission electron microscopy demonstrated the polycrystalline nature of the CeO2 film. XPS analysis showed multiple Ce3d and Ce4d oxidation states with a mixture of Ce3+ and Ce4+ components existing in CeO2. Angular resolved XPS investigations indicate that the CeO2 films mostly consist of Ce4+ oxidation states while the Ce3+ oxidation states are preferentially present near the surface. The CeO2/(100)Ge, CeO2/(110)Ge, and CeO2/(111)Ge structures showed almost identical valence band offset (VBO) values of 1.6, 1.5, and 1.6 eV, respectively, using XPS measurements from Ce3d core level (CL) peaks. These (VBO) values were also supported by XPS measurements from shallow Ce4d CL binding energy peaks. The conduction band offset values between CeO2/Ge were ∼1.3 eV using the measured optical bandgap of CeO2. The XPS spectral analysis of cerium oxidation states and the measured band offset parameters for carrier confinement would offer an important path for the future design of Ge-based metal-oxide semiconductor devices.

XPS studies on the interaction of CeO2 with silicon in magnetron sputtered CeO2 thin films on Si and Si3N4 substrates

CeO2 thin films were deposited on silicon and silicon nitride substrates by magnetron sputtering at room temperature and annealed at 400 and 600°C in air and vacuum. Interaction between deposited CeO2 and Si in CeO2/Si and CeO2/Si3N4 systems was investigated by XPS. The results show that Ce is present as both Ce4+ and Ce3+ oxidation states in CeO2 film deposited on Si substrate, whereas Ce4+ is the main species in as-deposited CeO2/Si3N4 film. Detailed analyses of Ce3d, Si2p and O1s core level spectra demonstrate that Ce2O3 and SiOx or cerium silicate type of species are formed at the interface of CeO2 and Si. Concentrations of Ce3+ species increase drastically in CeO2/Si thin films after annealing at 400°C in vacuum due to enhanced interfacial reaction. On the other hand, interfacial reaction between CeO2 and Si3N4 substrate is limited in as-deposited as well as heat treated films.

RHEED and XPS study of cerium interaction with SnO2 (110) surface

Abstract Interaction of cerium with thin epitaxial film of tin dioxide with (110) surface orientation was investigated by methods of X-ray Photoelectron Spectroscopy (XPS) and Reflection High Energy Electron Diffraction (RHEED). Strong interaction of cerium and tin leads to cerium diffusion into the tin dioxide bulk and formation of tin–cerium mixed oxide indicated by highly non-stoichiometric cerium oxide Ce 3d core level spectrum. This process is more pronounced with higher temperature during the deposition of cerium. With higher amount of deposited cerium, the tin dioxide substrate is gradually saturated with cerium atoms and the cerium atoms start to create 3D epitaxial clusters of cerium oxide with the (100) and (111) crystallographic planes parallel to the surface. The clusters show a higher oxidation state than the cerium atoms located in the tin oxide bulk. Oxygen required for the creation of these clusters comes from the tin dioxide substrate, which is partially reduced to metallic tin in this process. At the end of the experiment, we obtained a multilayer system consisting of layers of partially reduced tin dioxide, tin–cerium mixed oxide and top layer of partially reduced epitaxial grains of cerium oxide.

Room temperature redox reaction by oxide ion migration at carbon/Gd-doped CeO2 heterointerface probed by an in situ hard x-ray photoemission and soft x-ray absorption spectroscopies

In situ hard x-ray photoemission spectroscopy (HX-PES) and soft x-ray absorption spectroscopy (SX-XAS) have been employed to investigate a local redox reaction at the carbon/Gd-doped CeO2 (GDC) thin film heterointerface under applied dc bias. In HX-PES, Ce3d and O1s core levels show a parallel chemical shift as large as 3.2 eV, corresponding to the redox window where ionic conductivity is predominant. The window width is equal to the energy gap between donor and acceptor levels of the GDC electrolyte. The Ce M-edge SX-XAS spectra also show a considerable increase of Ce3+ satellite peak intensity, corresponding to electrochemical reduction by oxide ion migration. In addition to the reversible redox reaction, two distinct phenomena by the electrochemical transport of oxide ions are observed as an irreversible reduction of the entire oxide film by O2 evolution from the GDC film to the gas phase, as well as a vigorous precipitation of oxygen gas at the bottom electrode to lift off the GDC film. These in situ spectroscopic observations describe well the electrochemical polarization behavior of a metal/GDC/metal capacitor-like two-electrode cell at room temperature.

Kondo scaling of the pseudogap inCeOs4Sb12 andCeFe4P12

CeOs4Sb12 and CeFe4P12 are classified as Kondo semiconductors, which show coupled changesin electrical transport, thermodynamic and magnetic properties with alow-temperature semiconductor-like electrical resistivity. We have carried outcore level and valence band photoemission spectroscopy on single crystalCeOs4Sb12 andCeFe4P12 tostudy their electronic structure and the evolution of states at the Fermi level as a function of temperature(∼10–300 K). The Ce 3d core level spectra show the presence off0,f1 andf2 final states with very different relative intensities in the two compounds.Single-impurity Anderson model calculations provide f electron counts ofnf = 0.97 and 0.86 per Ce atom, suggestive of a low- and high-TK (= single ion Kondotemperature) for CeOs4Sb12 and CeFe4P12, respectively. The high-resolution temperature-dependent near-Fermi level spectra show pseudogaps ofenergy ∼ 50 meV and ∼ 110 meV in the valence band density of states (DOS) ofCeOs4Sb12 andCeFe4P12, respectively. The temperature dependence of the DOS at the Fermi levelfollows the change in effective magnetic moment estimated from magneticsusceptibility for both materials, confirming the Kondo nature of the pseudogap inCeOs4Sb12 andCeFe4P12. Acompilation of measured pseudogaps using photoemission and optical spectroscopy identifies the chargegaps ΔC for Ce-based Kondo semiconductors and provides a direct relation withTK givenby ΔC ∼ 2kBTK. In conjunction with the known behaviour of the spin gapsΔS ∼ kBTK, the results establish the coupled energy scaling of the spin and charge gaps in Kondosemiconductors.

Oxidation of thin Ce layers on Rh(110)

Oxidation of Rh(110) covered by thin (5–9 Å) overlayers of Ce was investigated by X-ray photoelectron spectroscopy and low energy electron diffraction (LEED). The oxidation was studied for O 2 exposure between 1 and 5 · 10 10 L, and at a sample temperature of 500 °C. The Ce 3d core level spectrum indicated that Ce was almost completely oxidised to CeO 2 after exposure of 50 L O 2 with only a thin overlayer of Ce 2O 3. The LEED pattern observed at this exposure was diffuse, but a ring shaped structure indicated long-range order at the surface. Rhodium oxide features were only observed in the core level spectra when the sample was exposed to an O 2 atmosphere of ∼ 1 · 10 5 Pa for 1 min (∼ 5 · 10 10 L). The oxidation rate was estimated to be up to 2 times higher when the substrate was covered with a thin layer of Ce before oxidation as compared to oxidation of the clean Rh(110) substrate. The thermal stability of the rhodium and the cerium oxides was investigated by subsequently annealing the oxidized sample to temperatures between 600 and 1000 °C. Rhodium oxide disappeared completely after heating to 800 °C, while CeO 2 started to deoxidize after heating to 900 °C.

Electronic structure of CenMmIn2m+3n, where n = 1, 2; m = 0, 1;M = Co, Rh or Ir: experiment and calculations

We present a detailed study of the electronic structure of theCenMmIn2m+3n (M = Co, Rh,Ir; n = 1, 2and m = 0, 1) series of Ce intermetallic compounds and of the referenceLaIn3 compound. The ground state of these heavy-fermion (HF) materials can be tunedbetween antiferromagnetic (AF) and superconducting (SC), with pressure ordoping as the tuning parameter. Performing the x-ray photoelectron spectroscopy(XPS) measurements on the Ce 3d core levels as well as on the valence bandstates we analyse the dependence of both the Ce 4f band character and physicalproperties on the kind of transition metal atom M and on the number ofCeIn3 layersintervened by the MIn2 layers in the investigated family of compounds. We draw a parallel between theXPS valence band spectra and ab initio band structure calculations based onthe full-potential linearized augmented plane-wave (FP-LAPW) method. Weanalyse changes in valence band states within the whole family of materials. Wecompare the experimental magnetic moments on Ce atoms with the theoretical onescalculated within different approximations for exchange–correlation potential.Finally, we have shown that the Ce 4f electrons participate in bonding formation forall investigated compounds. Our study indicates that the observed changes inthe 4f band on-site hybridization energy result from the reconstruction of thecharge density distribution driven by transition metal atoms inserted into theCeIn3 structure.

Epitaxial growth of continuous CeO 2(111) ultra-thin films on Cu(111)

Cerium oxide was grown epitaxially on a Cu(111) substrate at 520 K in an oxygen atmosphere and resulted in a high quality CeO 2(111) overlayers. The continuous single crystalline thin films were characterized by low energy electron diffraction, X-ray photoelectron spectroscopy of the Ce 3d core levels and resonant photoelectron spectroscopy of the valence band. The Ce oxide on Cu(111) grew initially in the form of islands giving a sharp (1.5 × 1.5) hexagonal diffraction pattern of the CeO 2(111) structure. It covered the substrate surface completely after deposition of a 2.5-monolayer thick overlayer. Ce 3d spectra and resonant enhancement of the valence band emission show that Ce is present in the Ce 4+ oxidation state.

Surface alloy formation after deposition of Ce on Rh(1 1 0)

Vapour deposition of Ce onto a Rh(1 1 0) single crystal at room temperature is studied by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low energy electron diffraction (LEED). The thicknesses of the deposited Ce layers are estimated to be between 2 and 9 Å. To study the changes in the Ce–Rh surface layer, the samples are annealed at temperatures between 500 and 1000 °C after Ce deposition. After heating, a c(2 × 2) LEED pattern appears for the sample with the thinnest deposited Ce layer (2.4 Å). For samples with thicker Ce-films, the LEED pattern co-exists of a c(2 × 2) structure and a more diffuse 6% contracted (2 × 1) structure. This appears at the same temperature as the Ce 3d and Rh 3d core levels exhibit sharp intensity changes and binding energy shifts. The intensity of the f 0, f 1 and f 2 multiplets in the Ce 3d core level spectra change when the annealing temperature is increased. The relative intensity of the Ce 3d f 0 and f 2 features compared to the Ce 3d f 1 features is largest after annealing to 500 °C. This is below the temperature at which the ordered surface alloy is formed. When the sample is heated above the formation temperature of the surface alloy, the relative intensity of the Ce 3d f 0 and f 2 features decrease.

Magnetic Properties and Electronic Structure of CeTIn (T = Ni, Cu, Pd, Au) Compounds

The electronic structure of the ternary CeTIn (T = Ni, Cu, Pd, Au) compounds was investigated by means of X-ray photoelectron spectroscopy. Our interest was aimed mainly to the valence bands and the Ce3d core levels of investigated compounds. Analysis of the valence bands indicates that they are mainly determined by the Tnd band, whereas a share of the R4f states can be estimated to be about a few percent only. The analysis of the difierential valence bands spectra between CeTIn and LaTIn analogues suggests that the Ce4f 1 ground states shift deeper below the Fermi level with an increase in the 4f level occupation factor. The analysis of the ⁄ £