Search

Applied surface science 590, 153063 (2022). doi:10.1016/j.apsusc.2022.153063, Interfaces of the magnetic layer in the heterostructure Ta/Co$_2$FeAl/MgO have been studied using hard X-ray photoelectron spectroscopy (HAXPES). Depth-selective information with sub-nanometer resolution is obtained using X-ray standing waves. It is found that the two magnetic species namely, Co and Fe behave very differently at the interfaces; 54% of Fe in the magnetic layer gets either oxidised or alloyed with Ta at the interfaces, as against 48% of Co. Oxidation state of both Co and Fe decreases, as one moves away from the MgO interface. On the average about 0.36 nm of Co$_2$FeAl gets oxidised at MgO interface and 0.66 nm gets alloyed with Ta at the bottom interface. Thermal annealing at 200℃ results in further oxidation of Fe, accompanied by a partial reduction of Co. It is suggested that this opposite behaviour of Fe and Co at the MgO interface may be the cause of the thermal annealing induced shift of the centroid of Fe distribution towards MgO layer, as observed in some STEM studies in such heterostructures., Published by Elsevier, Amsterdam

During the past 10 years, photon science activities at DESY in Hamburg, Germany, have expanded significantly and this development is expected to continue in the coming years. The soft X-ray free-electron laser (FEL) FLASH has been in user operation for over 10 years and the high-brilliance hard X-ray synchrotron radiation source PETRA III started serving the user community five years ago. Access to both light sources has since been highly demanded by scientists not only from Germany and Europe, but from all over the world. The request for beamtime far exceeded the capacity of available experimental infrastructure at both facilities and, for this reason, it was necessary to add further beamlines and also to broaden the portfolio of techniques. Therefore, new facilities have recently been built to almost double the existing capacity for user beamtime at both sources (Figure 1).

Ni and Cu KLL Auger spectra excited by X-rays from polycrystalline metal foils were measured with good energy resolution and intensity earlier. Auger spectra of 3d transition metals contain satellite peaks due to the atomic excitation processes. Because of the complexity of the measured spectral shape a complete explanation of the spectra was not given in the previous works. A new analysis of the measured spectra is presented here, with improved description of effects of inelastic electron scattering of the electrons in the solid sample and using complex peak shapes to model the satellite structure that follows each diagram line. The energy loss part of measured spectra due to the bulk plasmon excitations, surface plasmon excitations and intrinsic loss processes was removed using the Partial Intensity Analysis method based on energy loss distributions obtained from experimental reflection electron energy loss spectra of the same Cu and Ni metal foils. Relative Auger-transition energies derived from measured spectra of copper are in good agreement with previous experimental works and the results of cluster molecular orbital multielectron (DV-ME) calculations. The intensity ratio I( 3 P 2 / 3 P 0 ) shows better agreement with the result of relativistic calculations than in previous works. In the case of nickel the relative Auger-transition energies are in good agreement with the previous results. According to the new evaluation four satellite peaks were identified on the low energy side of each diagram line in the Auger spectra of Ni.

The paper presents photoemission study of core level binding energy shifts caused by local crystalline structure collapse in cubic Pb1−xCdxTe crystal. Photoemission spectra of two kinds of semiconductor samples are compared. The first one is ternary crystal of Pb0.94Cd0.06Te with the frozen rock salt structure where the crystalline local structure collapse is expected due to the difference of ion radii of Cd and Pb cations. The second sample was the CdTe(22 nm)/PbTe(6 nm)/CdTe(4 μm)/GaAs(1 1 1)B nanostructure grown by molecular beam epitaxy (MBE) method, where crystalline local structure is not expected to be created. The photoemission spectra show that for the crystal with local structure the electron binding energies of cations are higher (e.g. +0.2 eV) whereas for anions they are lower (e.g. −0.08 eV) than in the multilayer structure. A model is proposed to explain obtained results by the local crystalline structure collapse in Pb0.94Cd0.06Te crystal.

Three different methods to determine the oxide-phase concentration in mixed cerium oxide by hard X-ray photoelectron spectroscopy are applied and quantitatively compared. Synchrotron-based characterization of the O 1s region was used as a benchmark to introduce a method based on the weighted superposition of the Ce 3d spectra of the pure Ce(3+) and Ce(4+) phases, which was shown to lead to reliable and highly accurate determination of the mean oxidation state in mixed cerium oxides. The results obtained reveal a linear relation between the third distinct final state (u''') satellite peak intensity of the Ce(4+) phase and the Ce(4+) concentration by proper inclusion of Ce(3+)-related plasmon satellite peaks, which contradicts previous claims of nonlinear behavior. In contrast, quantitative conventional peak-fitting procedures were shown to be well suited for the Ce 2p region due to its relatively simple structure. Additional satellite features observed in the Ce 3d spectrum of CeO2 were proposed to originate from plasmon contributions.

epl 123(4), 47003 (2018). doi:10.1209/0295-5075/123/47003, A proper investigation of the valence band electronic structure is the essential first step towards understanding the intriguing co-existence of several exotic electronic phases like ferromagnetism and superconductivity, in $LaAlO_3-SrTiO_3$ heterostructures. In order to comprehend the electronic structure across the $LaAlO_3-SrTiO_3$ oxide interface, a detailed valence band investigation has been carried out using variable energy hard X-ray photoelectron spectroscopy and signatures of both low-energy coherent and high-energy features have been observed in the valence band spectra. Our combined experimental and theoretical study suggests that the charge carriers at the interface are weakly correlated and the high-energy feature does not arise from the lower Hubbard band. Instead, this high-energy feature is attributed to in-gap states induced by oxygen vacancies and possible polaron formation., Published by EDP Sciences, Les-Ulis

Review of scientific instruments 90(10), 103102 - (2019). doi:10.1063/1.5109321, We present a new high-pressure x-ray photoelectron spectroscopy system dedicated to probing catalytic reactions under realistic conditions at pressures of multiple bars. The instrument builds around the novel concept of a “virtual cell” in which a gas flow onto the sample surface creates a localized high-pressure pillow. This allows the instrument to be operated with a low pressure of a few millibar in the main chamber, while simultaneously a local pressure exceeding 1 bar can be supplied at the sample surface. Synchrotron based hard x-ray excitation is used to increase the electron mean free path in the gas region between sample and analyzer while grazing incidence, Published by American Institute of Physics, [S.l.]

Using hard x-ray photoelectron spectroscopywith variable photon energy (2-8 keV), we address the distribution of charge carriers in the prototypical LaAlO3 (LAO) and SrTiO3 (STO) oxide heterostructures with high carrier densities (10(17) cm(-2)). Our results demonstrate the presence of two distinct charge distributions in this system: one tied to the interface with a similar to 1-nm width and similar to 2-5 x 10(14)-cm(-2) carrier concentration, while the other appears distributed nearly homogeneously through the bulk of STO corresponding to a much larger carrier contribution. Our results also establish bimodal oxygen vacancies, namely on top of LAO and throughout STO, quantitatively establishing these as the origin of the observed bimodal depth distribution of charge carriers in these highly doped sample.

The electronic properties of buried magnetic nano-layers were studied using the linear magnetic dichroism in the angular distribution of photoemitted Fe, Co, and Mn 2p electrons from a CoFe–Ir78Mn22 multi-layered sample. The buried layers were probed using hard X-ray photoelectron spectroscopy, HAXPES, at the undulator beamline P09 of the 3rd generation storage ring PETRA III. The results demonstrate that this magnetometry technique can be used as a sensitive element specific probe for magnetic properties suitable for application to buried ferromagnetic and antiferromagnetic magnetic materials and multilayered spintronics devices. Using the same instrument, spin-resolved Fe 2p HAXPES spectra were obtained from the buried layer with good signal quality.

In this study of the changes induced by exposing MWCNTs to a nitrogen plasma, it was found by HRTEM that the atomic nitrogen exposure does not significantly etch the surface of the carbon nanotube (CNT). Nevertheless, the atomic nitrogen generated by a microwave plasma effectively grafts amine, nitrile, amide, and oxime groups onto the CNT surface, as observed by XPS, altering the density of valence electronic states, as seen in UPS. (C) 2008 Elsevier Ltd. All fights reserved.

The effect of the oxygen plasma treatment on the electronic states of multi-wall carbon nanotubes (MWCNTs) is analyzed by X-ray photoemission measurements (XPS) and UPS, both using synchrotron radiation. It is found that the plasma treatment effectively grafts oxygen at the CNT-surface. Thereafter, the interaction between evaporated Pd and pristine or oxygen plasma-treated MWCNTs is investigated. Pd is found to nucleate at defective sites, whether initially present or introduced by oxygen plasma treatment. The plasma treatment induced a uniform dispersion of Pd clusters at the CNT-surface. The absence of additional features in the Pd 3d and C 1s core levels spectra testifies that no Pd–C bond is formed. The shift of the Pd 3d core level towards high-binding energy for the smallest clusters is attributed to the Coulomb energy of the charged final state.

A method is presented for analysing the energy loss part (dominated by contributions from extrinsic, intrinsic and surface plasmon excitations) of electron spectra excited from free-electron-like solids by X-rays. It is an improved version of the model originally proposed by Steiner, Hochst and Hufner and it accounts for contributions of electrons suffering multiple energy losses of different origin. Applied for analysing a Ge 2s photoelectron spectrum excited from a thin Ge layer, the applicability of the method to separate contributions to the inelastic background of the spectra due to intrinsic, extrinsic and surface plasmons is demonstrated. The aim of this work to give a detailed description of the simple statistical model applied in ref. [M. Novak, S. Egri, L. Kover, I. Cserny, W. Drube, W.S.M. Werner, Surf. Sci. 601 (2007) 2344] for practical analysis of photoelectron spectra. The effects of the application of energy loss distributions (for single bulk and single surface excitations) derived by using different methods are also investigated.

We have revisited the valence band electronic structure of NiO by means of hard x-ray photoemission spectroscopy (HAXPES) together with theoretical calculations using both the GW method and the local density approximation + dynamical mean-field theory (LDA+DMFT) approaches. The effective impurity problem in DMFT is solved through the exact diagonalization (ED) method. We show that the LDA+DMFT method alone cannot explain all the observed structures in the HAXPES spectra. GW corrections are required for the O bands and Ni-s and p derived states to properly position their binding energies. Our results establish that a combination of the GW and DMFT methods is necessary for correctly describing the electronic structure of NiO in a proper ab-initio framework. We also demonstrate that the inclusion of photoionization cross section is crucial to interpret the HAXPES spectra of NiO.We argue that our conclusions are general and that the here suggested approach is appropriate for any complex transition metal oxide., 16 pages, 5 figures

We analyze the excitation of secondary electrons by hard-X-rays in subsurface layers. By studying core-excited photoelectron lines and their plasmon satellites in photoemission spectra, we show how electrons excited by hard-X-rays can carry information from deep regions deep within the sample to the surface. It is believed that the decay of high-energy photoelectrons via plasmon-loss is strongly related to the production of secondary electrons. For high-energy electrons, however, the momentum transfer to plasmons is small compared to the electron’s initial momentum, so the lateral position on the surface from which the secondary electrons are emitted is close to that of the atom initially excited by the hard-X-rays. This explains why the spatial resolution of hard-X-ray photoelectron emission microscope (HX-PEEM) images is good even if the buried interface is covered by a film with a thickness many times the inelastic mean free path of the primary electrons. This argument explains well recent HX-PEEM results.

Multi-wall carbon nanotubes (MWCNTs) were decorated with metal clusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of metal coverage can be varied by plasma treating the MWCNT surface. The metal clusters on oxygen plasma treated arc-discharge MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized are-discharge MWCNTs. In contrast, the plasma treatment did not affect the cluster distribution on CVD MWCNTs. Analyses of the valence band and the core levels by X-ray photoelectron spectroscopy suggest poor charge transfer between gold clusters and MWCNTs; on the contrary suggest good charge transfer between Ni clusters and MWCNTs. (c) 2007 Elsevier B. V. All rights reserved.

Carbon nanotubes (CNTs) were decorated with Ni clusters by thermal evaporation. It is shown that the CNT decoration with Ni varies from well-organized clusters to the complete coverage of the surface resulting in electronic charge transfer and formation of Ni–C bonds at the interface between these two materials, depending on the evaporated amount of Ni. The Ni–CNTs interaction induces an increase in the metallicity of the system suggesting that CNTs decorated with metal atoms can be a good candidate as template for production of metal nanowires.

High energy resolution KL 23 L 23 Auger spectra of polycrystalline Cu and Ni were measured using photon energies up to about 50 eV above the K-absorption edge and down to 5 eV (Cu KLL) and 4 eV (Ni KLL) below threshold. The spectra show strong satellite structures varying considerably as a function of the photon energy. In the sub-threshold region the linear dispersion of the diagram line energy positions and a distortion of the line shape as a function of photon energy, attributable to the Auger resonant Raman process, is clearly observed, indicating the one-step nature of the Auger emission. These changes in the resonant spectra are interpreted using a simple model based on resonant scattering theory in combination with partial density of states obtained from cluster molecular orbital (DV- X α) calculations.

The effect of the oxygen plasma treatment on the electronic states of multiwall carbon nanotubes (MWCNTs) is analysed by XPS and UPS, both using synchrotron radiation. It is found that the plasma treatment effectively grafts oxygen at the CNT surface. Thereafter, the interaction between evaporated gold and pristine or oxygen-plasma-treated MWCNTs is investigated. Gold is found to nucleate at defect sites, whether initially present or introduced by oxygen plasma treatment. Gold cluster uniform dispersion is observed on plasma-treated CNTs. The absence of additional features in the Au 4f and C 1s core levels spectra testifies that no Au–C bond is formed. The Au 4f core level energy shift towards high binding energy for the smallest clusters indicates cluster charging during the photoionization process, suggesting poor electrical contact between gold clusters and CNT surface. Cluster size effects are observed on the valence band spectra.

The Si 1s and 2p photoemission and the Si KL2,3L2,3 Auger emission from SiO2/Si(1 0 0) surfaces were measured for oxide thicknesses from 0.4 to 3.8 nm using high-energy photoemission (hν=3000 eV). The energy of the SiO2 emission depends on the layer thickness. The Si4+–Si0 energy differences ΔE1s, ΔE2p and ΔEKLL were combined to separate the initial and final state contribution to the energy shift of the Si4+ 2p line. Within the experimental error, the 2p initial state is constant up to about 2 nm oxide thickness, where differential charging gradually sets in. The observed variation of ΔE2p is entirely due to final state relaxation. It is found that ΔE1s−ΔE2p=0.62 eV, independent of the oxide thickness.

Angle resolved photoemission studies of SiO2/SiC samples grown ex situ in N2O on polar and non-polar 4H-SiC surfaces are reported. Data from the Si 1s and Si 2p core levels and the Si KL2,3L2,3 Auger transitions are analyzed and compared to data from a sample grown in O-2 on the (0001) surface. The results show oxide growth without nitride or oxy-nitride formation. Presence of two oxidation states, SiO2 and a sub-oxide explains recorded Si 1s. Si 2p and Si KLL spectra. Estimates of the oxide layer thickness show that the oxidation rate is highest for the (10 (1) under bar0) surface, somewhat smaller and similar for the (11 (2) under bar0) and (000 (1) under bar) surfaces, and smaller by a factor of about two for the (0001) surface.

Photoemission studies of oxidized SiC samples grown ex situ in N2O, at a temperature of 900 °C, on the (0 0 0 1), (0 0 0 1), (1 1 2 0) and (1 0 1 0) surfaces are reported. Angle resolved data from the Si 1s and Si 2p core levels and the Si KL2,3L2,3 Auger transitions are analyzed and compared to data from a sample grown in O2 on the (0 0 0 1) surface. The results show oxide growth and no oxy-nitride formation. The growth rate is found to be smallest for the Si-terminated (0 0 0 1) surface and highest for the nonpolar (1 0 1 0) surface. The presence of two oxidation states, Si+4 and a suboxide, are required to explain and model recorded Si 1s, Si 2p and Si KLL spectra. The SiO2 shift is found to be smaller on the (0 0 0 1) surface than on the other three surfaces, which is attributed to an oxide thickness dependence of the shift. A layer attenuation model describes satisfactorily the intensity variations observed in the core level components versus electron emission angle when assuming the suboxide at the interface. Estimates made of the thickness of the oxide layers show that the oxidation rate for the (0 0 0 1) surfaces is about half of that for the (1 0 1 0) surface and that the oxidation rate for the (1 1 2 0) and (0 0 0 1) surfaces are similar but somewhat smaller than for the (1 0 1 0) surface. The amount of suboxide is found to be smaller on the nonpolar than on the polar surfaces.

Angle resolved studies of SiO2/SiC samples utilizing the Si 2p and Si 1s core levels and the Si KLL Auger transitions are reported. Samples with total oxide thicknesses from ca. 5 to 118 A are investigated. The data collected show that two oxidation states only, Si+1 and Si+4, are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra. For all samples investigated the intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model when assuming that the sub-oxide, Si+1, is located at the interface. The SiO2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and moreover to depend on the thickness of the oxide layer.

Angle resolved photoemission studies of the Si 2p and Si 1s core levels and the Si KL 2,3 L 2,3 Auger transitions from SiO 2 /SiC samples are reported. Most samples investigated were grown in situ on initially clean and well ordered √3×√3 reconstructed 4H–SiC(0 0 0 1) surfaces but some samples were grown ex situ using a standard dry oxidation procedure. The results presented cover samples with total oxide thicknesses from about 5 to 118 A. The angle resolved data show that two oxidation states only, Si +1 and Si +4 , are required to explain and model recorded Si 2p, Si 1s and Si KLL spectra. The intensity variations observed in the core level components versus electron emission angle are found to be well described by a layer attenuation model for all samples when assuming a sub-oxide (Si 2 O) at the interface with a thickness ranging from 2.5 to 4 A. We conclude that the sub-oxide is located at the interface and that the thickness of this layer does not increase much when the total oxide thickness is increased from about 5 to 118 A. The SiO 2 chemical shift is found to be larger in the Si 1s level than in the Si 2p level and to depend on the thickness of the oxide layer. The SiO 2 shift is found to be fairly constant for oxides less than about 10 A thick, to increase by 0.5 eV when increasing the oxide thickness to around 25 A and then to be fairly constant for thicker oxides. An even more pronounced dependence is observed in the Si KLL transitions where a relative energy shift of 0.9 eV is determined. The relative final state relaxation energy Δ R (2p) is determined from the modified Auger parameter. This yields a value of Δ R (2p)=−1.7 eV and implies, for SiO 2 /SiC, a “true” chemical shift in the Si 2p level of only ≈0.4 eV for oxide layers of up to 10 A thick.

Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.

Si 2p and C 1s core level spectra recorded at different electron emission angles from SiO2/SiC samples using a photon energy of 3.0 keV show two components. These are identified as originating from SiO2 and SiC for Si 2p while for C 1s they are identified to originate from graphite like carbon and SiC. The relative intensity of these components are extracted and compared to calculated intensity variations assuming different models for the elemental distribution in the surface region. For both samples investigated best agreement between experimental and calculated intensity variations with emission angle is obtained when assuming a graphite like layer on top of the oxide layer. Contribution from carbon at the SiC/SiO2 interface could not be identified.

Energy research is to a large extent materials research, encompassing the physics and chemistry of materials, including their synthesis, processing toward components and design toward architectures, allowing for their functionality as energy devices, extending toward their operation parameters and environment, including also their degradation, limited life, ultimate failure and potential recycling. In all these stages, X-ray and electron spectroscopy are helpful methods for analysis, characterization and diagnostics for the engineer and for the researcher working in basic science.This paper gives a short overview of experiments with X-ray and electron spectroscopy for solar energy and water splitting materials and addresses also the issue of solar fuel, a relatively new topic in energy research. The featured systems are iron oxide and tungsten oxide as photoanodes, and hydrogenases as molecular systems. We present surface and subsurface studies with ambient pressure XPS and hard X-ray XPS, resonant photoemission, light induced effects in resonant photoemission experiments and a photo-electrochemical in situ/operando NEXAFS experiment in a liquid cell, and nuclear resonant vibrational spectroscopy (NRVS). (C) 2012 Elsevier B.V. All rights reserved.

We introduce a new method for determining accurate values of the valence-band maximum in x-ray photoemission spectra. Specifically, we align the sharpest peak in the valence-band region of the experimental spectrum with the corresponding feature of a theoretical valence-band density of states curve from ab initio GW theory calculations. This method is particularly useful for soft and hard x-ray photoemission studies of materials with a mixture of valence-band characters, where strong matrix element effects can render standard methods for extracting the valence-band maximum unreliable. We apply our method to hydrogen-terminated boron-doped diamond, which is a promising substrate material for novel solar cell devices. By carrying out photoemission experiments with variable light polarizations, we verify the accuracy of our analysis and the general validity of the method., Comment: 15 pages, 7 figures, accepted for publication in Journal of Applied Physics

Copper deposited by thermal evaporation onto pristine and oxygen plasma treated carbon nanotubes (CNTs) diffuse over the CNT surface, coalescing and forming crystalline islands. The nucleation sites of the islands are preferentially defects, and more homogeneous island dispersion was observed at the CNT oxygen functionalized surface. The presence of weakly bound oxygen atoms at the CNT surface induces the formation of CuO bonds at the Cu/CNT interface, as described through density functional calculations. Exposure to air allows further oxidation to facetted crystalline Cu2O. Oxygen plasma pre-treatment represents a promising route for homogenous disperse Cu2O nanoparticle decoration of CNTs.

The bulk single crystals of Pb 1− x Cd x Te were obtained by self-selecting vapor growth (SSVG) method with proper quenching. Tunable high energy X-ray photoemission spectroscopy (THE-XPS) and ultraviolet photoemission spectroscopy (UPS) were applied to study the crystals electronic structure of valence band and core level regions. The valence band density of states distribution spectra remarkably changes with the crystal composition. In most cases obtained valence band spectra of ternary crystal with dominating of ionic bonding can be in first approximation treated as a sum of the components spectra. In the case of measured Pb 1− x Cd x Te ternary crystal this first order approximation does not follow the rule and inclination of ternary crystal spectra possesses some differences from the sum of spectra measured for crystal PbTe and CdTe. The increase of Cd contents in the crystals leads to appearance of an additional density of states in the large part of the valence band, for binding energy range up to 10 eV.

International audience; The interaction between evaporated gold and pristine or oxygen plasma treated multi-walled carbon nanotubes (MWCNTs) is investigated. Experimental and theoretical results indicate that gold nucleation occurs at defect sites, whether initially present or introduced by oxygen plasma treatment. Uniform gold cluster dispersion is observed on plasma treated carbon nanotubes (CNTs) and associated with the presence of uniformly dispersed oxidized vacancy centres on the CNT surface.

International audience; We study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs significantly weakens the Ti-CNT interaction. Ti atoms at the surface will preferentially bond to oxygenated sites. Potential sources of oxygen impurities are examined, namely oxygen from any residual atmosphere and pre-existing oxygen impurities on the nanotube surface, which we enhance through oxygen plasma surface pretreatment. Variation in literature data concerning Ohmic contacts between Ti and carbon nanotubes is explained via sample pre-treatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier-free Ti-nanotube contact formation.

Hard x-ray photoelectron spectroscopy (HAXPES) is a new variant of the well-established photoemission technique, which extends its range to much higher photoelectron energies up to 10 keV and thus to enhanced probing depths of the order of 10 nm and beyond. This not only facilitates direct access to the intrinsic bulk electronic structure of solids, but also allows extended depth profiling and the study of buried interfaces not possible by conventional photoemission. Here, we present two HAXPES case studies on transition metal hybrid and heterostructures demonstrating the potential of the method. We also discuss a new HAXPES setup at the high-brilliance hard x-ray synchrotron radiation source PETRA III at DESY (Hamburg) currently under construction.

The interaction between evaporated Pt and pristine or oxygen-plasma-treated multiwall carbon nanotubes (CNTs) is investigated. Pt is found to nucleate at defect sites, whether initially present or introduced by oxygen plasma treatment. The plasma treatment induces a uniform dispersion of Pt nanoparticles at the CNT surface. The absence of additional features in the C 1s core level spectrum indicates that no mixed Pt-C phase is formed. The formation of C-O-Pt bonds at the cluster-CNT interface is suggested to reduce the electronic interaction between Pt nanoparticles and the CNT surface. (c) 2008 Elsevier B. V. All rights reserved.

KLM Auger spectra of Ni metal were measured with high energy resolution and high statistical accuracy using monochromatic synchrotron radiation. The spectra were corrected for the background from inelastically scattered electrons using partial intensity analysis where the electron trajectories were calculated by Monte Carlo simulation. Following background correction, the corrected spectra were fitted to model peaks taking into account intrinsic excitations. The measured transition energy of the most intense KL2M3 (1D2) Auger line is 7388.1 eV (0.4). The obtained relative intensities and energies of the Auger diagram lines are compared to published calculations as well as to experimental data for other 3d transition metals. In addition, the presence of Ni 3s, 3p photoelectron peaks in the spectra excited internally by Ni Kα X-rays is shown.

The changes induced by exposing multi-walled carbon nanotubes (CNTs) to atomic nitrogen were analysed by high-resolution transmission electron microscopy (HRTEM), x-ray and ultraviolet photoelectron spectroscopy. It was found that the atomic nitrogen generated by a microwave plasma effectively grafts chemical groups onto the CNT surface altering the density of valence electronic states. HRTEM showed that the exposure to atomic nitrogen does not significantly damage the CNT surface.

Deep core Ge 2s photoelectron spectra from polycrystalline Ge films induced by monochromatic synchrotron radiation, of 4, 6 and 8 keV were measured and analysed using two different methods, the partial intensity analysis and the extended Hufner method to determine the spectral contributions from different electron energy loss processes due to bulk extrinsic, intrinsic and surface excitations. The obtained photon energy dependence of the ratio of these contributions was compared as a function of the photoelectron kinetic energy. It was found that the relative contribution of intrinsic excitations increase with the photon energy.

A XAFS‐experiment allowing for in‐situ experiments has been set up at DORIS bending magnet beamline C. For that purpose, a new double‐crystal, UHV‐compatible monochromator with fast scanning capability was designed. This fixed‐exit monochromator uses two crystal sets on a common central rotation axis driven by an ex‐vacuo goniometer. Bragg angles range from 5° to 55.5° resulting in a total energy range 2.3 – 43.4 keV using Si(111)/(311) crystal sets. Crystal pairs can be remotely selected by translating the vacuum chamber. Energy encoding is performed using an optical encoder system. The standard XAFS sample environment is set‐up in vacuo and can be adapted for special sample environments. For in‐situ experiments, the beamline is equipped with twelve gas lines. An exhaust line allows toxic/reactive gases to be handled. As an initial performance test of the instrument, Ti, Cr, Fe and Cu XAFS and Ce K‐shell QEXAFS measurements were performed.

High-energy resolution $K{L}_{23}{L}_{23}$ Auger spectra of polycrystalline Cu and Ni metals were measured while tuning the energy of the exciting x-rays in the near threshold region up to $50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ above the $K$ absorption edge. The intensity evolution of the pronounced satellite lines as a function of photon energy reveals different behavior for particular satellites as a consequence of initial state and final state shake excitations. The satellite intensity development near threshold is compared to predictions of different simple models based on the free atom approximation. The observed energy separations between the satellites and the $^{1}D_{2}$ main diagram line is in accord with results from a cluster molecular orbital model and confirms the assignment based on the satellite intensity evolution curves concerning the origin of the satellites.

In the quantitative analysis of buried interfaces or the exploration of local electronic structures around component atoms in bulk solids, spectroscopy of transitions in the deep atomic core has an increasingly significant role to play. A key issue in understanding the deep core photoelectron spectra is the identification and separation of the contributions that are attributable to different excitations leading to loss of energy of the emitted photoelectrons. In this work, Ge 1s and 2s photoelectron spectra are analyzed using various models to interpret the measured spectral shapes. A detailed comparison of the results derived from the different approaches is presented. Copyright © 2006 John Wiley & Sons, Ltd.

An instrument for high‐energy X‐ray photoelectron spectroscopy is operated at the BW2 wiggler beamline of HASYLAB. The photon energy ranges from 2.3 to 10 keV, the electron energy reaches up to 7.5 keV (including an optional 2.5 keV sample bias, the current electron analyzer limit is 5 keV). Using a Si(311) double crystal monochromator, the total energy resolution is 0.37 eV at 4.3 keV as determined from the Si 2p photoemission. For Si(111) typical values are 0.45 eV at 3 keV. The photon flux typically amounts to 5×1012 1/s for Si(111). Focusing is achieved by the combination of a sagittally bent monochromator crystal and a tangentially bent mirror in the monochromatic beam. The focus size (FWHM) at the sample is 0.25 mm (vertical) × 2 mm (horizontal) which is well matching the acceptance of the SCIENTA SES‐200 electron analyzer. Selected results are shown to demonstrate the instrument performance.

We report an in situ thermochemical treatment that significantly increases the macroscopic electrical conductivity of insulating yttria-stabilized zirconia (YSZ) (001) single-crystalline substrates. We demonstrate the high-quality surface crystalline structure of the resulting “conductive” cYSZ (001) by low- and high-energy electron diffraction. Soft- and hard X-ray photoemission spectroscopy measurements reveal a sizable reduction of Zr cations to a metallic state and their homogeneous distribution within the cYSZ. We discuss the correlation between the microscopic chemical processes leading to the increased macroscopic metallicity. Finally, the heteroepitaxial growth of a functional magnetic oxide model system, ultrathin EuO on cYSZ (001), was demonstrated. cYSZ (001) thereby enables both high quality oxide heteroepitaxy and the advanced sample characterization by high electron-fluence characterization techniques.

The results of photoemission studies of SiO2/SiC samples for the purpose of revealing presence of any carbon containing by-products at the interface are reported. Two components could be identified in recorded Si 2p and C ls core level spectra. For Si 2p these were identified to originate from SiO2 and SiC while for C ls they were interpreted to originate from graphite like carbon and SiC. The variation in relative intensity of these components with emission angle was first investigated. Thereafter the intensity of the different components were studied after successive Ar+-sputtering cycles. Both experiments showed contribution from graphite like carbon on top of the oxide but not at the interface.

Conducting polypyrrole (PPY) nanotubes is a classical model system for strongly correlated disordered materials showing intriguing switching transitions from low to high conductivity states at low temperature. This switching behaviour can be tuned by incorporating gold nanoclusters to form composite nanotubes (AuPPY). Here, we present core level electronic structure studies on PPY and AuPPY nanotubes with different diameters using hard X-ray photoemission spectroscopy at room temperature. The spectroscopic data provide information on the role of diameter as well as metal cluster incorporation for a modification of the electronic structure of this important class of nanotubes. Furthermore, electrical transport measurements were performed at low temperature to extract the change in localization length and doping level in these nanotubes. Based on these results, we explain the observed electrical transport behavior of these nanotubes as the interplay of disorder and carrier concentration.

Ceria-based mixed oxides have proved to be good alternatives to pure ceria as redox materials for advanced three-way catalysts. In this work, the behaviour of a Ce 0.8 Tb 0.2 O 2-x mixed-oxide sample and the corresponding oxide-supported Rh and Pt samples has been investigated. Special attention has been paid to the low-temperature redox response. Temperature-programmed desorption (TPD), temperature-programmed reduction and oxygen buffering capacity were used as experimental techniques. The evolution of reducibility was also followed by tunable high-energy x-ray photoemission spectroscopy, focussing on the surface information provided by Tb 3d and Ce 3d core levels. The oxidation state of both Ce and Tb was followed during ultrahigh vacuum annealing of the three samples up to 750 K. Comparison of the XPS spectra for the different samples before and after annealing showed that the Ce 4+ reduction was negligible but Tb 4+ was gradually reduced to Tb 3+ . Furthermore, the TPD and XPS experiments are in good agreement, with the highest reduction rate corresponding to the Rh/CeTbO x sample.