Resonant Photoemission Spectroscopy Research Articles

Generation of Fe 3O 4 Nanoparticle Aggregates in a Ferrofluid Driven by External Electric Field

The electronic structure of Yb implanted ZnO has been studied by the resonant photoemission spectroscopy (RPES). The contribution of Yb 4f partial density of states (PDOS) is predominant in the binding energy (BE) about 7.5 and ~11.7 eV below the VB maximum. At the resonance energy, around 182 eV, the multiplet structure with the highest BE shows the strongest resonance (around 11.7 eV) that corresponds to the 1I multiplet which is almost exclusively responsible for this resonance, while 3H and 3F states are responsible for the resonance around 7.5 eV. It was also found that the Yb 4f PDOS distribution shows some similarity to Yb2O3.

NbOx/CeO2-rods catalysts for oxidative dehydrogenation of propane: Nb–CeO2 interaction and reaction mechanism

A series of NbOx/CeO2-rod catalysts were synthesized and employed as catalysts for oxidative dehydrogenation of propane (ODHP) reaction. Nb–CeO2 interaction and reaction mechanisms were studied with synchrotron-radiation-based techniques. Synchrotron radiation photoemission spectroscopy and resonant photoemission spectroscopy results demonstrate that the Nb5+ precursor preferentially interact with surface oxygen vacancies on CeO2 rods. Surface redox reactions occur between Nb5+ and Ce3+ (surface oxygen vacancy) to produce Nb4+ and Ce4+, resulting in decreased surface oxygen vacancy concentrations on CeO2 rods. CeO2 rods are active in catalyzing propane combustion at low temperatures (below 400°C) via the MvK mechanism. The loading of NbOx suppresses the surface reducibility of CeO2 rods, leading to decreased activity but greatly enhanced propene selectivity. The ODHP reaction catalyzed by CeO2 rods and NbOx/CeO2-rods catalysts at high temperatures (above 450°C) involves the radical mechanism, in which the gas-phase propyl radical was observed with synchrotron radiation VUV photoionization time-of-flight mass spectroscopy. These results greatly deepen the fundamental understanding of CeO2-based catalysts for ODHP reaction.

Interaction at the F16CuPc/TiO2 Interface: A Photoemission and X-ray Absorption Study

The interfacial interaction and charge transfer dynamics between a F16CuPc molecular thin film and rutile TiO2(110) (1×1) surface have been studied by photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and resonant photoemission spectroscopy (RPES). The evolution of PES spectra as a function of F16CuPc film thickness shows strong coupling between the molecules and the TiO2 surface. Adsorbed molecules experience substrate mediated charge transfer. Electrons being pulled away from nitrogen atoms toward to carbon ring results in an opposite direction binding energy shift for C 1s and N 1s. Moreover, the molecule gets deformed due to their strong interaction with the TiO2 surface. Ultrafast charge transfer from F16CuPc molecules to the TiO2 substrate takes place on the time scale of 10 fs due to their strong electronic coupling. The results pave the way for the design and realization of F16CuPc based electronic devices.

Molecular Alignment and Electronic Structure of N,N'-Dibutyl-3,4,9,10-perylene-tetracarboxylic-diimide Molecules on MoS2 Surfaces.

The molecular orientation of organic semiconductors on a solid surface could be an indispensable factor to determine the electrical performance of organic-based devices. Despite its fundamental prominence, a clear description of the emergent two-dimensional layered material-organic interface is not fully understood yet. In this study, we reveal the molecular alignment and electronic structure of thermally deposited N,N'-dibutyl-3,4,9,10-perylene-dicarboximide (PTCDI-C4) molecules on natural molybdenum disulfide (MoS2) using near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The average tilt angle determination reveals that the anisotropy in the π* symmetry transition of the carbon K-edge (284-288 eV range) is present at the sub-monolayer regime. Supported by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and resonant photoemission spectroscopy (RPES) measurements, we find that our spectroscopic measurements indicate a weak charge transfer established at the PTCDI-C4/MoS2 interface. Sterical hindrance due to the C4 alkyl chain caused tilting of the molecular plane at the initial thin film deposition. Our result shows a tunable interfacial alignment of organic molecules on transition metal dichalcogenide surfaces effectively enhancing the electronic properties of hybrid organic-inorganic heterostructure devices.

Resonant photoemission and X-ray absorption spectroscopies of lithiated magnetite thin film

Resonant photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) were used to investigate the effect of lithiation on the electronic structure of Fe3O4 thin film relevant to the operation mechanism of nanoionic devices to enable magnetic property tuning. Comparison of the Fe 2p XAS spectrum for lithiated Fe3O4 (Li–Fe3O4) with that for pristine Fe3O4 clearly demonstrated that the number of Fe2+ ions at octahedral B sites is increased by lithiation. The valence band RPES spectra of Li–Fe3O4 further showed that lithiation increases the density of states near the Fermi level originating Fe2+ ions at octahedral B sites. These findings agree well with the observed decrease in the saturation magnetization in the magnetization–magnetic field (M–H) loop of Li–Fe3O4 thin film, indicating that minority spins (down spins) increase (i.e., total spins decrease) due to lithiation. The variation in the number of Fe2+ ions at B sites is suggested to be an underlying operating mechanism of a nanoionics-based magnetic property tuning device.

Electronic structure of a 3, 4, 9, 10-perylene-tetracarboxylic-dianhydride thin film revealed by synchrotron-based resonant photoemission spectroscopy

The electronic structure of a 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) thin film is investigated in situ using synchrotron-based near edge X-ray absorption fine structure (NEXAFS) spectroscopy and resonant photoemission spectroscopy (RPES).The NEXAFS spectroscopy can monitor the electronic transitions from core level to unoccupied states.The C K-edge NEXAFS spectrum of the PTCDA thin film shows four distinct absorption peaks below 290 eV,which are attributed to the transitions from 1s core level of C-atoms in different chemical environments (perylene core C-atoms vs anhydride C-atoms) into lowest unoccupied molecular orbitals (LUMOs) with * symmetry. The RPES spectra are collected in the valence band region by sweeping photon energy across the C 1s * absorption edge.Three typical features of the C 1s signals excited by second-order harmonic X-ray,resonant photoemission and resonant Auger features are observed in RPES spectra,and are identified,relying on the development of kinetic energy of the emitted photoelectrons upon the change of incident photons energy.It is found that the C 1s signals excited by second-order harmonic X-ray are present at high kinetic energy side of spectrum.The kinetic energy of this feature shows photon energy dependence,that is,this feature shifts to higher kinetic energy by photon energy increasing twice.Resonant Auger peaks in RPES spectra are located on the low kinetic energy side with constant kinetic energy regardless the change of photon energy.The resonant Auger may originate from deeper molecular orbitals with binding energy large than 4.1 eV,suggesting that the resonant Auger decay process involved in deeper molecular orbitals occurs on a time scale comparable to C 1s core hole lifetime of 6 femtoseconds.Resonant enhancement of highest occupied molecular orbitals (HOMOs) derived valence band features or HOMO-1 and HOMO-2 derived resonant photoemission features in our case are lying between the C 1s signals and the resonant Auger signals.The Kinetic energy increases as the photon energy sweeps across the absorption edge,whereas their binding energy remains constant.In addition, the enhancements of two resonances show photon energy dependence that enhancement of HOMO-1 related resonance dominates over HOMO-2 related resonance at energies corresponding to perylene core C 1s to LUMOs transitions, whereas HOMO-2 related resonance becomes dominant at transitions from anhydride C 1s to LUMOs.This behavior can be related to the wavefunction character and symmetry of the frontier molecular orbitals.Clarifying each resonant feature in RPES spectra and their origin will pave the way for accurately determining the ultrafast charge transfer time at organic/electrode interfaces using synchrotron-based core hole clock technique implementation of RPES.

Amorphous Gallium Oxide as an Improved Host for Inorganic Light-Emitting Thin Film Semiconductor Fabricated at Room Temperature on Glass

We report new amorphous oxide semiconductor (AOS)-based thin film phosphor, Eu-doped a-Ga2Ox (a-GO:Eu), to solve the issues of previously reported a-In–Ga–Zn–O:Eu (a-IGZO:Eu). The internal quantum efficiencies (IQE) of a-GO:Eu (2.3% for unannealed, 8.3% for annealed films) are improved from those of a-IGZO:Eu (0.9% and 1.6%, respectively) because of the much wider bandgap (4.26 eV), subsequent low residual electron density, and higher available annealing temperature. We found that the annealing temperature to improve IQE is limited by crystallization temperature. Another issue of a-IGZO:Eu is that the initial state of Eu3+ 4f is deeper than the valence band maximum (VBM), which is not suitable for light-emitting diode. We expected that Eu3+ 4f would locate above the VBM in a-GO:Eu because the VBM of a-Ga2Ox is ∼0.8 eV deeper than that of a-IGZO. However, resonant photoemission spectroscopy revealed that the Eu 4f states are bound more to the O 2p valence band than to the vacuum level, and the Eu3+ 4f states in a-GO:Eu are still buried in the valence band.

Orbital Symmetry of the Kondo State in Adsorbed FePc Molecules on the Au(110) Metal Surface

The Kondo ground state can arise in surface-supported organometallic systems, such as the 3d transition metal phthalocyanines, as a result of the hybridization of the 3d localized orbitals with the underlying metal substrate electrons. Low-temperature scanning tunneling spectroscopy can identify the Kondo states as a zero-bias anomaly in the differential conductance curves, often localized in close proximity with the 3d metal center. However, the information on the symmetry of the Kondo state, on the exact shape of the spectral function, and on the magnetic state is generally missing from the experimental data. We apply complementary techniques, i.e., scanning tunneling spectroscopy, resonant photoemission spectroscopy, and X-ray magnetic circular dichroism, to identify the Kondo state, the orbital symmetry, and the magnetic moment of Fe-Phthalocyanine chains formed by two structural phases with increasing molecular density: first ×5 and at higher coverage ×7 FePc ordered chain structures assembled on the...

Resonant core spectroscopies of the charge transfer interactions between C60 and the surfaces of Au(111), Ag(111), Cu(111) and Pt(111)

Charge transfer interactions between C60 and the metal surfaces of Ag(111), Cu(111), Au(111) and Pt(111) have been studied using synchrotron-based photoemission, resonant photoemission and X-ray absorption spectroscopies. By placing the X-ray absorption and valence band spectra on a common binding energy scale, the energetic overlap of the unoccupied molecular orbitals with the density of states of the underlying metal surface have been assessed in the context of possible charge transfer pathways. Resonant photoemission and resonant Auger data, measuring the valence region as a function of photon energy for C60 adsorbed on Au(111) reveals three constant high kinetic energy features associated with Auger-like core-hole decay involving an electron transferred from the surface to the LUMO of the molecule and electrons from the three highest occupied molecular orbitals, respectively and in the presence of ultra-fast charge transfer of the originally photoexcited molecule to the surface. Data for the C60/Ag(111) surface reveals an additional Auger-like feature arising from a core-hole decay process involving more than one electron transferred from the surface into the LUMO. An analysis of the relative abundance of these core-hole decay channels estimates that on average 2.4 ± 0.3 electrons are transferred from the Ag(111) surface into the LUMO. A core-hole clock analysis has also been applied to assess the charge transfer coupling in the other direction, from the molecule to the Au(111) and Ag(111) surfaces. Resonant photoemission and resonant Auger data for C60 molecules adsorbed on the Pt(111) and Cu(111) surfaces are shown to exhibit no super-Auger features, which is attributed to the strong modification of the unoccupied molecular orbitals arising from stronger chemical coupling of the molecule to the surface.

Impurity states in the magnetic topological insulator V:(Bi,Sb)2Te3

The ferromagnetic topological insulator $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_{2}{\mathrm{Te}}_{3}$ has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of $\mathrm{V}:{(\mathrm{Bi},\mathrm{Sb})}_{2}{\mathrm{Te}}_{3}$ thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V $3d$ impurity band was extracted and exhibits a high density of states near the Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.

Resonant photoemission spectroscopic studies of Eu2O3 thin film

In the present paper, we have investigated the structural and electronic properties of Eu2O3 thin film deposited by pulsed laser deposition technique on Si (100) substrate. The film was characterized by X-ray diffraction and photoemission measurements. X-ray photoemission studies suggest that Eu has 2+ (∼8%) and 3+ (∼92%) valence states in the film. Valence band spectrum of the film shows four features (2 eV, 4.8 eV, 7.7 eV, and 10.4 eV) with the prominent one at 7.7 eV due to the Eu3+ (4f 6) state. Resonance photoemission spectroscopy measurements have been performed in the Eu 4d → 4f photo absorption region, and thus, obtained constant initial state intensity plots suggest that the feature close to the Fermi level (∼2 eV) results from the hybridization of the O 2p and Eu2+ (4f7) states, while other features appearing at higher binding energies are due to the hybridization of the O 2p and Eu3+ (4f6) states.

Epitaxial synthesis and physical properties of double-perovskite oxide Sr2CoRuO6 thin films

We report epitaxial structures and physical properties of double-perovskite Sr2CoRuO6 films grown using pulsed-laser deposition. Samples with a degree of Co/Ru order of 2–73% were obtained by changing growth temperature. X-ray absorption spectroscopy (XAS) on the highest ordered sample revealed that Co ions were trivalent with a high-spin configuration and Ru ions were pentavalent. We found large differences in magnetization and resistivity between the highest and lowest ordered samples as well as the absence of strong magnetism and metallicity, which are common characteristics of SrCoO3 and SrRuO3. Using resonant photoemission spectroscopy and XAS, dominant d-orbital components at the top of the occupied state (the bottom of the unoccupied state) were identified to be Ru 4d t2g (Co 3d and Ru 4d t2g). These results suggest that the ground state of double-perovskite Sr2CoRuO6 is a ferrimagnetic insulator.

X-ray spectroscopy and electronic structure of MoO2

We study the electronic structure of MoO2 using an extended cluster model. The calculations confirm that the compound is in a highly mixed covalent Mott-Hubbard regime. These results are then compared to other calculations and to x-ray absorption and (resonant) photoemission spectroscopies. The good agreement with the experimental spectra indicates a strong covalent character for the Mo 4d-O 2p bond. Finally, we show that, not only local charge fluctuations from the oxygen ions are necessary, but also nonlocal screenings, from the Mo dimer and from coherent (metallic) states, must be included in the description of the electronic structure of MoO2.

Localized and mixed valence state of Ce4fin superconducting and ferromagneticCeO1−xFxBiS2revealed by x-ray absorption and photoemission spectroscopy

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) and Ce $4d$-$4f$ resonant photoemission spectroscopy (PES) on single crystals of CeO$_{1-x}$F$_x$BiS$_2$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$ electronic states. In the Ce $L_3$-edge XAS, mixed valence of Ce was found in the $x=0.0$ sample and the F-doping suppresses it, which is consistent with the results on polycrystalline samples. As for the resonant PES, we found that the Ce $4f$ electrons in both $x=0.0$ and $0.5$ systems respectively form a flat band at 1.0 eV and 1.4 eV below the Fermi level and there is no contribution to the Fermi surfaces. Interestingly, Ce valence in CeOBiS$_2$ deviates from Ce$^{3+}$ even though Ce $4f$ electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce $4f$ in CeOBiS$_2$ is mixed with the unoccupied Bi $6p_z$, which is consistent with the previous local structural study. Based on the analysis of the Ce $L_3$-edge XAS spectra using Anderson's impurity model calculation, we found that the transfer integral becomes smaller increasing the number of Ce $4f$ electrons upon the F substitution for O.

Electronic structure of FeAl alloy studied by resonant photoemission spectroscopy and Ab initio calculations

Resonant photoemission spectroscopy has been used to investigate the character of Fe 3d states in FeAl alloy. Fe 3d states have two different character, first is of itinerant nature located very close to the Fermi level, and second, is of less itinerant (relatively localized character), located beyond ∼2 eV below the Fermi level. These distinct states are clearly distinguishable in the resonant photoemission data. Comparison between the results obtained from experiments and first principle based electronic structure calculations show that the origin of the itinerant character of the Fe 3d states is due to the ordered B2 structure, whereas the relatively less itinerant (localized) Fe 3d states are from the disorders present in the sample. The exchange splitting of the Fe 3s core level peak confirms the presence of local moment in this system. It is found that the itinerant electrons arise due to the hybridization between Fe 3d and Al 3s and 3p states. Presence of hybridization is observed as a shift in the Al 2p core-level spectra as well as in the X-ray near edge absorption spectra towards lower binding energy. Our photoemission results are thus explained by the co-existence of ordered and disordered phases in the system.

Spectroscopy and electronic structure ofSr2YRuO6andSr2YRu0.75Ir0.25O6

We study the electronic structure of the ${\mathrm{Sr}}_{2}{\mathrm{YRuO}}_{6}$ and ${\mathrm{Sr}}_{2}{\mathrm{YRu}}_{0.75}{\mathrm{Ir}}_{0.25}{\mathrm{O}}_{6}$ compounds using x-ray (resonant) photoemission and absorption spectroscopies. The experimental results are interpreted with first-principles calculations, which give a good agreement with all the spectra. These results show that, although the spin-orbit coupling does not induce orbital anisotropies in these systems, it is responsible for the reduction of magnetic moments within the the ${\mathrm{Ir}}^{5+}{\text{O}}_{6}$ octahedra, weakening the magnetic ordering in the substituted system. Finally, our findings support the idea that the canting of ${\mathrm{Ru}}^{5+}$ magnetic moments actually plays an important role in the stabilization of the intriguing magnetic ordering in the ${\mathrm{Sr}}_{2}{\mathrm{YRuO}}_{6}$ compound.

Reduction of Pt2+ species in model Pt–CeO2 fuel cell catalysts upon reaction with methanol

The stability of atomically dispersed Pt2+ species on the surface of nanostructured CeO2 films during the reaction with methanol has been investigated by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy. The isolated Pt2+ species were prepared at low Pt concentration in Pt–CeO2 film. Additionally, Pt2+ species coexisting with metallic Pt particles were prepared at high Pt concentration. We found that adsorption of methanol yields similar decomposition products regardless of Pt concentration in Pt–CeO2 films. A small number of oxygen vacancies formed during the methanol decomposition can be replenished in the Pt–CeO2 film with low Pt concentration by diffusion of oxygen from the bulk. In the presence of supported Pt particles, a higher number of oxygen vacancies leads to a partial reduction of the Pt2+ species. The isolated Pt2+ species are reduced under rather strongly reducing conditions only, i.e. during annealing under continuous exposure to methanol. Reduction of isolated Pt2+ species results in the formation of ultra-small Pt particles containing around 25 atoms per particle or less. Such ultra-small Pt particles demonstrate excellent stability against sintering during annealing of Pt–CeO2 film with low Pt concentration under reducing conditions.

Electronic structure of Mo1−xRex alloys studied through resonant photoemission spectroscopy

We studied the electronic structure of Mo-rich Mo1−xRex alloys () using valence band photoemission spectroscopy in the photon energy range 23–70 eV and density of states calculations. Comparison of the photoemission spectra with the density of states calculations suggests that, with respect to the Fermi level EF, the d states lie mostly in the binding energy range 0 to −6 eV, whereas s states lie in the binding energy range −4 to −10 eV. We observed two resonances in the photoemission spectra of each sample, one at about 35 eV photon energy and the other at about 45 eV photon energy. Our analysis suggests that the resonance at 35 eV photon energy is related to the Mo 4p–5s transition and the resonance at 45 eV photon energy is related to the contribution from both the Mo 4p–4d transition (threshold: 42 eV) and the Re 5p–5d transition (threshold: 46 eV). In the constant initial state plot, the resonance at 35 eV incident photon energy for binding energy features in the range EF (BE = 0) to −5 eV becomes progressively less prominent with increasing Re concentration x and vanishes for x > 0.2. The difference plots obtained by subtracting the valence band photoemission spectrum of Mo from that of Mo1−xRex alloys, measured at 47 eV photon energy, reveal that the Re d-like states appear near EF when Re is alloyed with Mo. These results indicate that interband s–d interaction, which is weak in Mo, increases with increasing x and influences the nature of the superconductivity in alloys with higher x.

Electronic properties of atomic layer deposition films, anatase and rutile TiO2 studied by resonant photoemission spectroscopy

The TiO2 films are prepared by atomic layer deposition (ALD) method using titanium isopropoxide precursors at 250 °C and analyzed using resonant photoemission spectroscopy (resPES). We report on the Ti2p and O1s core levels, on the valence band (VB) spectra and x-ray absorption spectroscopy (XAS) data, and on the resonant photoelectron spectroscopy (resPES) profiles at the O1s and the Ti3p absorption edges. We determine the elemental abundance, the position of the VB maxima, the partial density of states (PDOS) in the VB and in the conduction band (CB) and collect these data in a band scheme. In addition, we analyze the band-gap states as well as the intrinsic states due to polarons and charge-transfer excitations. These states are found to cause multiple Auger decay processes upon resonant excitation. We identify several of these processes and determine their relative contribution to the Auger signal quantitatively. As our resPES data allow a quantitative analysis of these defect states, we determine the relative abundance of the PDOS in the VB and in CB and also the charge neutrality level.The anatase and rutile polymorphs of TiO2 are analyzed in the same way as the TiO2 ALD layer. The electronic properties of the TiO2 ALD layer are compared with the anatase and rutile polymorphs of TiO2. In our comparative study, we find that ALD has its own characteristic electronic structure that is distinct from that of anatase and rutile. However, many details of the electronic structure are comparable and we benefit from our spectroscopic data and our careful analysis to find these differences. These can be attributed to a stronger hybridization of the O2p and Ti3d4s states for the ALD films when compared to the anatase and rutile polymorphs.

Atomically Dispersed Pd, Ni, and Pt Species in Ceria-Based Catalysts: Principal Differences in Stability and Reactivity

We have investigated the stability and the reactivity of atomically dispersed Pt, Pd, and Ni species on nanostructured CeO2 films by means of synchrotron radiation photoelectron spectroscopy and resonant photoemission spectroscopy in combination with density functional calculations. All three metals reveal specific similarities associated with the high adsorption energy of atomically dispersed Pt2+, Pd2+, and Ni2+ species that exceeds the corresponding cohesive energies of the bulk metals. The corresponding Pt–CeO2, Pd–CeO2, and Ni–CeO2 model catalysts have been prepared in the form of thin films on CeO2(111)/Cu(111) substrates and investigated experimentally under ultrahigh vacuum conditions. The atomically dispersed Pt2+, Pd2+, and Ni2+ species were formed exclusively at low concentrations of the corresponding metals. High concentrations resulted in the presence of additional metal oxide phases and emergence of metallic particles. We found that under the employed experimental conditions the Pd–CeO2 films closely resemble the Pt–CeO2 system with respect to the redox behavior upon reaction with hydrogen. Unlike Pt–CeO2, the Pd–CeO2 system shows a strong tendency to stabilize Pd2+ not only at the surface but also in the ceria bulk. In sharp contrast to both Pt–CeO2 and Pd–CeO2, the Ni–CeO2 system does not exhibit the redox functionality required for hydrogen activation due to the remarkably high stability of Ni2+ species.