X-ray Photoemission Experiments Research Articles

Structure Matters: Asymmetric CO Oxidation at Rh Steps with Different Atomic Packing.

Curved crystals are a simple but powerful approach to bridge the gap between single crystal surfaces and nanoparticle catalysts, by allowing a rational assessment of the role of active step sites in gas-surface reactions. Using a curved Rh(111) crystal, here, we investigate the effect of A-type (square geometry) and B-type (triangular geometry) atomic packing of steps on the catalytic CO oxidation on Rh at millibar pressures. Imaging the crystal during reaction ignition with laser-induced CO2 fluorescence demonstrates a two-step process, where B-steps ignite at lower temperature than A-steps. Such fundamental dissimilarity is explained in ambient pressure X-ray photoemission (AP-XPS) experiments, which reveal partial CO desorption and oxygen buildup only at B-steps. AP-XPS also proves that A-B step asymmetries extend to the active stage: at A-steps, low-active O–Rh–O trilayers buildup immediately after ignition, while highly active chemisorbed O is the dominant species on B-type steps. We conclude that B-steps are more efficient than A-steps for the CO oxidation.

Evolution of the electronic structure and correlations accompanied by suppression of itinerant ferromagnetism in Sr1−x(La0.5K0.5)xRuO3

We have carried out soft x-ray photoemission experiments on itinerant ferromagnet ${\mathrm{Sr}}_{1\ensuremath{-}x}{({\mathrm{La}}_{0.5}{\mathrm{K}}_{0.5})}_{x}{\mathrm{RuO}}_{3}$ to investigate how the electronic state varies with doping concentration. The Ru $4d$-derived coherent part of the valence spectra develops significantly with increasing $x$ for $x\ensuremath{\leqslant}0.3$, which can be explained by the suppression of the ferromagnetic exchange splitting. With further increasing $x$, this development is overwhelmed by the spectral weight transfer from the coherent to the incoherent parts due to the electron correlation. The enhancement of the electron correlation effect with doping is also confirmed by the Ru $3d$ core-level spectra as the suppression of the well-screened peak. In contrast to the remarkable variation of the Ru $4d$ spectral intensity as a function of $x$, the valence spectra hardly depend on temperature and do not show any noticeable change across the magnetic transition temperatures, indicating that the temperature dependence of the exchange splitting cannot be explained by a simple Stoner picture. We compare the present photoemission results with those for isostructural ${\mathrm{Sr}}_{1\ensuremath{-}x}{A}_{x}{\mathrm{RuO}}_{3}$ ($A=\mathrm{La}$ and Ca) and discuss the origin of the difference in the magnetic property between these doped compounds.

On the role of nano-confined water at the 2D/SiO2 interface in layer number engineering of exfoliated MoS2 via thermal annealing

We studied the effects of thermal annealing (in air and in ultra-high vacuum from room temperature up to 300 °C) of mechanically exfoliated mono-layer and few layer MoS2 onto 270 nm SiO2/Si(1 0 0). The experiments were performed with optical microscopy, atomic force microscopy, non resonant Raman spectroscopy, and photoluminescence (PL) spectroscopy on the mono-layer flakes. We demonstrate the presence of a nano-confined water layer at the interface with the silicon substrate. The thickness of this water layer can be increased by immersing the exfoliated samples in water for one hour, or decreased by post exfoliation annealing. Then, we directly demonstrate the sublimation with annealing of the bottom layer at the interface with SiO2. PL experiments performed on the mono-layers in the 250 °C–300 °C annealing range, together with previous x-ray photoemission experiments, demonstrate the direct correlation of the PL integrated spectral intensity with the concentration of sulfur vacancies that are passivated by oxygen after exposure to air. Namely, the vacancy formation is triggered in the 250 °C–300 °C range independently of the annealing environment, and correspondingly in the same temperature range the PL spectral intensity proportionally increases. With respect to pristine exfoliated MoS2 we estimate via PL an increase by a factor three of the vacancy concentration upon annealing to 300 °C. This, considering the typical vacancy concentration values of mechanical exfoliated MoS2, leads to sulfur vacancy concentration values close to 1014 cm−2. Considering such high value, the sublimation of the bottom layer is then explained via oxidation of the sulfur vacancies upon dissociation of water preferentially supplied by the film present at the interface with the SiO2 substrate.

Impact of covalent functionalization by diazonium chemistry on the electronic properties of graphene on SiC.

Plenty of strategies focused on covalent interaction have been developed to functionalize graphene's surface in order to employ it in a wide range of applications. Among them, the use of radical species including nitrene, carbene and aryl diazonium salts is regarded as a promising strategy to establish the covalent functionalization of graphene. In this work, we highlight the effect of diazonium chemistry on the electronic properties of graphene on SiC. On the basis of X-ray and synchrotron-based photoemission experiments, we were able to prove that 3,4,5-trimethoxybenzenediazonium (TMeOD) units, reduced and chemisorbed onto graphene using electrochemistry, preserve the electronic structure of the Dirac cone, through inducing a slightly additional n-type doping of graphene, as revealed by a downshift of the Dirac cone probed by angle-resolved photoemission experiments.

Probing energy landscapes in multilayer heterostructures: Challenges and opportunities

Quantitative characterization of energy landscapes at buried interfaces is essential for assessing their functionality. This perspective highlights recent developments in reconstructing internal potential profiles based on hard x-ray photoemission experiments that have proved to be a rich source of information. We show that band-edge profiles can be reconstructed from core-level photoelectron spectra by performing a comprehensive search for the best-fit set of associated layer-resolved spectra. The use of hard x-rays allows heterostructures to be probed over length scales comparable to relevant electronic screening lengths. Significantly, this information takes our understanding of such systems to a new level that is not currently achievable using any other experimental technique.

Synthesis, Crystal Structure, and Selected Properties of [Au(S2 CNH2 )2 ]SCN: A Precursor for Gold Macro-Needles Consisting of Gold Nanoparticles Glued by Graphitic Carbon Nitride.

A new preparation route is developed for the synthesis of needle-like crystals of [Au(S2 CNH2 )2 ]SCN, which avoids disproportionation of the AuI salt used as a starting material. In the crystal structure, the two crystallographically independent AuIII centers are in a square-planar environment of two S2 CNH2 ligands. The Hirshfeld surface analysis reveals the presence of noncovalent intermolecular S⋅⋅⋅S interactions, which are essential for the spatial arrangement of the molecules. Density functional theory (DFT) calculations including dispersion and damping corrections result in a unit cell volume very close to the value determined experimentally. Thermal decomposition in an inert atmosphere generates black needles with lengths of up to 500 μm. X-ray powder diffraction and pair distribution function analyses demonstrate that the needles are composed of nanosized crystals with a volume-weighted average domain size of 20(1) nm. According to results of X-ray photoemission experiments, the black needles are covered by a nitrogen-rich carbon nitride with composition near (CN)2 N. 13 C solid-state NMR investigations indicate that two different carbon species are present, with signals corresponding well to heptazine units as in melon and triazine units as in poly(triazin imide) type compounds. Scanning transmission electron microscopy tomography evidences that the needles are composed of slightly elongated nanoparticles.

FT-IR, XPS, and DFT Study of Adsorption Mechanism of Sodium Acetohydroxamate onto Goethite or Hematite

The adsorption of sodium acetohydroxamate on the goethite or hematite surface was investigated by Fourier transform infrared spectroscopy (FT-IR), X-ray photoemission spectroscopy and periodic plane-wave density functional theory (DFT) calculations. The core-level shifts and charge transfers of the adsorbed surface iron sites calculated by DFT with periodic interfacial structures were confronted to the X-ray photoemission experiments. FT-IR results reveal that the interfacial structure of sodium acetohydroxamate adsorbed on the goethite or hematite surface may be assigned to a five-membered ring complex. In agreement with the adsorption energies determined by the DFT calculations, a five-membered ring complex is formed via bonding of one surface iron atom of goethite (101) or (100) to both oxygen atoms of hydroxamate group, and these two oxygen atoms of the hydroxamate group correspondingly attach to two neighboring iron atoms of the goethite surface. But a five-membered ring complex between two oxygen atoms of the hydroxamate group and one surface iron atom of hematite (001) is formed without any extra attachments. The calculated core-level shifts of Fe2p for the interfacial structures are correspondingly in good agreement with the experimental observed one, which confirmed the reliability of the calculated results.

Layer-Resolved Cation Diffusion and Stoichiometry at the LaAlO3/SrTiO3 Heterointerface Probed by X-ray Photoemission Experiments and Site Occupancy Modeling.

The layer-resolved cation occupancy for different conducting and insulating interfaces of LaAlO3 (LAO) thin films on SrTiO3 (STO) has been determined by angle-resoled X-ray photoelectron spectroscopy (AR-XPS). Three STO interfaces with LAO have been considered, namely, a conducting interface with a 5 unit cell (u.c.) LAO layer, an insulating interface with a 5 u.c. LAO layer, and an insulating interface with a 3 u.c. LAO layer. Considering inelastic and elastic scattering processes in the transport approximation, the core-level signal attenuation has been modeled on the basis of Monte Carlo calculations of the electron trajectories across the heterostructures. Different effects involving cation stoichiometry and diffusion through the interface have been considered to interpret data. Beyond a mere abrupt interface modeling, the LaAlO3/SrTiO3 heterojunction is shown to host cation diffusion processes within 3-4 unit cells in the bulk layer, along with a clear Sr substoichiometry, an issue so far virtually neglected in the analysis of these systems. The present results show the capability of the AR-XPS modeling to explore element-sensitive properties at the oxide interfaces, matching and completing the information that can be provided by probes based on electron microscopy or X-ray scattering.

Synchrotron Stories from Stanford—From a Parking Lot to First Beam

Our journey in synchrotron radiation started in July 1972 when we joined a group at Stanford led by Seb Doniach and Bill Spicer to build a “Pilot Project” to test the feasibility of performing X-ray photoemission experiments on the newly commissioned SPEAR storage ring at SLAC. The SPEAR ring was expressly built for high-energy physics using colliding electron and positron beams (ultimately discovering the existence of quarks and garnering two Nobel prizes). As a result, anything we did could not interfere with the high-energy physics experiments.

Polarization dependent hard X-ray photoemission experiments for solids: Efficiency and limits for unraveling the orbital character of the valence band

We have investigated the efficiency and limits of polarization dependent hard X-ray photoelectron spectroscopy (HAXPES) in order to establish how well this method can be used to unravel quantitatively the contributions of the orbitals forming the valence band of solids. By rotating the energy analyzer rather than the polarization vector of the light using a phase retarder, we obtained the advantage that the full polarization of the light is available for the investigation. Using NiO, ZnO, and Cu2O as examples for solid state materials, we established that the polarization dependence is much larger than in photoemission experiments utilizing ultra-violet or soft X-ray light. Yet we also have discovered that the polarization dependence is less than complete on the basis of atomic calculations, strongly suggesting that the trajectories of the outgoing electrons are affected by appreciable side-scattering processes even at these high kinetic energies. We have found in our experiment that these can be effectively described as a directional spread of ±18° of the photoelectrons. This knowledge allows us to identify, for example, reliably the Ni 3d spectral weight of the NiO valence band and at the same time to demonstrate the importance of the Ni 4s for the chemical stability of the compound.

Charge transfer satellites in x-ray spectra of transition metal oxides

Strongly correlated materials such as transition metal oxides (TMOs) often exhibit large satellites in their x-ray photoemission (XPS) and x-ray absorption spectra (XAS). These satellites arise from localized charge-transfer (CT) excitations that accompany the sudden creation of a core hole. Here we use a two-step approach to treat such excitations in a localized system embedded in a condensed system and coupled to a photoelectron. The total XAS is then given by a convolution of a spectral function representing the localized excitations and the XAS of the extended system. The local system is modeled roughly in terms of a simple three-level model, leading to a double-pole approximation for the spectral function that represents dynamically weighted contributions from the dominant neutral and charge-transfer excitations. This method is implemented using a resolvent approach, with potentials, radial wave-functions and matrix elements from the real-space Green's function code FEFF, and parameters fitted to XPS experiments. Representative calculations for several TMOs are found to be in reasonable agreement with experiment.

Hard x-ray photoemission spectroscopy on the trilayer system MgO/Au/Fe using standing-wave excitation

The trilayer system MgO/Au monolayer/Fe was investigated by hard x-ray photoemission experiments in combination with the standing-wave technique. The insertion of the Au layer into the Fe/MgO tunnel junction provides an additional handle to influence the properties of the interface. The recently explored method of standing-wave excited hard x-ray photoemission was used to investigate both the structural properties and chemical states of the interfacial layers in one experiment. The results show that the Au monolayer does not grow as a closed layer, but intermixes strongly with the Fe below. This behaviour results in a very sharp interface between the Au/Fe and the MgO layer on top. However, the XPS spectra show no hint for a formation of FeO at the interface.

Band alignment of rutile and anatase TiO2

The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO₂. The discovery of the photolysis of water on the surface of TiO₂ in 1972 launched four decades of intensive research into the underlying chemical and physical processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO₂ (ref. ). We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments, that a type-II, staggered, band alignment of ~ 0.4 eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust separation of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts.

Core–hole effects in fullerene molecules and small-diameter conducting nanotubes: a density functional theory study

Core–hole induced electron excitations in fullerene molecules, and small-diameter conducting carbon nanotubes, are studied using density functional theory with minimal, split-valence, and triply-split-valence basis sets plus the generalized gradient approximation by Perdew–Burke–Ernzerhof for exchange and correlation. Finite-size computations are performed on the carbon atoms of a C60 Bucky ball and a piece of (3, 3) armchair cylindrical network, terminated by hydrogen atoms, while periodically boundary conditions are imposed on a (3, 3) nanotube unit cell. Sudden creation of the core state is simulated by replacing a 1s electron pair, localized at a central site of the structures, with the effective pseudo-potentials of both neutral and ionized atomic carbon. Excited states are obtained from the ground-state (occupied and empty) electronic structure of the ionized systems, and their overlaps with the ground state of the neutral systems are computed. These overlaps enter Fermi’s golden rule, which is corrected with lifetime and finite-temperature effects to simulate the many-electron response of the nanoobjects. A model based on the linked cluster expansion of the vacuum persistence amplitude of the neutral systems, in a parametric core–hole perturbation, is developed and found to be reasonably consistent with the density functional theory method. The simulated spectrum of the fullerene molecule is found to be in good agreement with x-ray photoemission experiments on thick C60 films, reproducing the low energy satellites at excitation energies below 4 eV within a peak position error of ca. 0.3 eV. The nanotube spectra show some common features within the same experiments and describe well the measured x-ray photoelectron lineshape from nanotube bundles with an average diameter of 1.2 nm.

Application of hard X-ray photoelectron spectroscopy to electronic structure measurements for various functional materials

The revolver undulator beamline BL15XU at SPring-8 has been constructed by National Institute for Materials Science (NIMS). We have started hard X-ray photoemission experiments for various functional materials to measure the bulk-sensitive and buried interface electronic structures at BL15XU of SPring-8 since 2006. In this paper we report the performance of the NIMS contract beamline for hard X-ray photoelectron spectroscopy (HAXPES) and recent HAXPES results for several functional materials. By utilizing the large probing depth of HAXPES, it is possible to measure bulk and buried interface electronic structures of solids. We also describe the perspectives on HAXPES at the NIMS contract beamline.

Sensitivity of Structural and Electronic Properties of Gold–Thiolate Nanoclusters to the Atomic Composition: A Comparative X-ray Study of Au19(SR)13 and Au25(SR)18

With the recent successful synthesis of an increasing number of atomically precise thiolate–gold nanoclusters (NCs), there are growing demands for a better understanding of their atomic structures and structure–property relationships in response to composition change. Herein, we present X-ray experimental results on structural and electronic properties of a recently synthesized gold–thiolate nanocluster, Au19(SR)13, in comparison with the structurally well-known Au25(SR)18 reference. The atomic structure of Au19(SR)13 was first probed with extended X-ray absorption fine structure (EXAFS). A multishell EXAFS fitting method (consisting of one Au–S and three Au–Au shells) allowed for a detailed structural comparison of the two nanoclusters, which revealed a sizable increase of the bond distances between the surface–surface and surface–staple Au atoms upon the decrease of Au atoms in the nanocluster. X-ray photoemission experiments and ab initio calculations further demonstrated the sensitive change of electr...

Impact of valence states on the superconductivity of iron telluride and iron selenide films with incorporated oxygen

We report on the local electronic structure of oxygen incorporated FeTe and FeSe films and how this relates to superconductivity observed in these films. In the case of FeTe, initially grown films are measured to be nonsuperconducting, but become superconducting following oxygen incorporation. In FeSe the opposite happens; initially grown films are measured to be superconducting, but experience a quenching of superconductivity following oxygen incorporation. X-ray photoemission and total fluorescence yield (TFY) x-ray absorption experiments show that oxygen incorporation changes the initial Fe valence state in both the as grown FeTe and FeSe films to mainly Fe${}^{3+}$ in the oxygen incorporated films. In contrast we observe that while Te moves to a mixed Te${}^{0}$/Te${}^{4+}$ valence state, the Se always remains Se${}^{0}$. This work highlights how different responses of the electronic structure by the respective chalcogenides to oxidation could be related to the mechanisms which are inducing superconductivity in FeTe and quenching superconductivity in FeSe.

Detection of filament formation in forming-free resistive switching SrTiO3 devices with Ti top electrodes

We investigated the influence of Ti top electrodes on the resistive switching properties of SrTiO3 thin film devices. Above a Ti layer thickness of 5 nm, the initial resistance is strongly reduced, giving rise to forming-free devices. Hard x-ray photoemission experiments reveal the Ti layer to be composed of several oxide phases, induced by the redox-reaction at the Ti/SrTiO3 interface. Grazing incidence small angle x-ray scattering measurements indicate that the reduction of the SrTiO3 thin film occurs in a filamentary way. We attribute this behavior to the preferential reduction of SrTiO3 thin films along highly defective areas.

Hole polaron formation and migration in olivine phosphate materials

By combining first principles calculations and experimental XPS measurements, we investigate the electronic structure of potential Li-ion battery cathode materials LiMPO4 (M=Mn,Fe,Co,Ni) to uncover the underlying mechanisms that determine small hole polaron formation and migration. We show that small hole polaron formation depends on features in the electronic structure near the valence-band maximum and that, calculationally, these features depend on the methodology chosen for dealing with the correlated nature of the transition-metal d-derived states in these systems. Comparison with experiment reveals that a hybrid functional approach is superior to GGA+U in correctly reproducing the XPS spectra. Using this approach we find that LiNiPO4 cannot support small hole polarons, but that the other three compounds can. The migration barrier is determined mainly by the strong or weak bonding nature of the states at the top of the valence band, resulting in a substantially higher barrier for LiMnPO4 than for LiCoPO4 or LiFePO4.

Surface valence states and stoichiometry of non-superconducting and superconducting FeTe films

We report the surface electronic structure and stoichiometry of FeTe films following the incorporation of oxygen by three different methods: air exposure, dry oxygen exposure and low temperature oxygen annealing. X-ray photoemission experiments show that oxygen incorporation changes the initial valence state of Fe from 0 to mainly 3+. We also observe that the Te changes valence from initially 0 to mixed 0 and 4+. The rate of valence changes is seen to depend on the method of incorporation. In addition, it is observed that the surface of the FeTe films is left in a Te deficient state following any type of exposure to oxygen.