Core-level Electron Research Articles

実験室系硬Ｘ線光電子分光

Hard X-ray photoelectron spectroscopy (HAXPES) has recently attracted attention for its unique capabilities. High energy X-ray enables to excite deep core level electrons and in parallel the information depth from the sample surface is significantly larger than conventional XPS, which enables to investigate electronic structures beneath the sample surface and buried interface without destructive ion etching. Although most of the previous HAXPES studies have been conducted with synchrotron radiation source, laboratory HAXPES is getting more and more attracted in practical applications. In this review, we report unique capabilities of the laboratory HAXPES with our recent applications, which has both of soft and hard X-ray as excitation sources.

Electronic structure of (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals

We report on studies of the X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion bombarded surfaces of (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals grown by solution-melt technique. The (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystals were found to be very rigid with respect to Ar+-ion bombardment. In particular, such a treatment did not cause any significant changes of values of the binding energies of core-level electrons as well as of the shapes of the XPS core-level and valence-band spectra of (Ga55In45)2S300 and (Ga54.59In44.66Er0.75)2S300 single crystal surfaces. We measured for the (Ga55In45)2S300 compound the X-ray emission bands representing the energy distribution of mainly the valence S p and Ga p states and compared them on a common energy scale with the XPS valence-band spectrum. The comparison allows us for concluding that the S 3p and Ga 4p states contribute mainly at the top and in the central portion of the valence band of (Ga55In45)2S300, respectively. In addition, we have also detected at ambient conditions a strong photoluminescence of the (Ga54.59In44.66Er0.75)2S300 single crystal with a maximum at 810 nm for the excitation wavelength of 532 nm. Our results indicate that the (Ga54.59In44.66Er0.75)2S300 single crystal is a very promising material for its application in optoelectronic technologies.

Exploring the Single Atom Spin State by Electron Spectroscopy

To control the spin state of an individual atom is an ultimate goal for spintronics. A single atom magnet, which may lead to a supercapacity memory device if realized, requires the high-spin state of an isolated individual atom. Here, we demonstrate the realization of well isolated transition metal (TM) atoms fixed at atomic defects sparsely dispersed in graphene. Core-level electron spectroscopy clearly reveals the high-spin state of the individual TM atoms at the divacancy or edge of the graphene layer. We also show for the first time that the spin state of single TM atoms systematically varies with the coordination of neighboring nitrogen or oxygen atoms. These structures can be thus regarded as the smallest components of spintronic devices with controlled magnetic behavior.

RETRACTED ARTICLE: HFCVD Application for Growth of Monoclinic Tungsten Trioxide Crystal Nano-walls

In this research the hot filament chemical vapor deposition technique application for growth of monoclinic tungsten trioxide crystal nano-walls on Si substrates without using catalysts was presented. The products were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and four-point probe instrument. The FESEM images and XRD patterns show that crystallinity of the samples was improved when the substrate temperature increases to 800 °C. The peaks intensity of WO3 are enhanced by increasing the substrate temperature, which can be understood based on the fact that the thermal energy enable atoms to diffuse, as a result, a more perfect crystal structure was obtained. From FESEM images, it can be seen that by increasing the temperature the nano-walls structure appears and continues to grow sharper. The width of the nano-walls is in the range of 80–300 nm and lengths of them are about 5 μm. The electrical measurement results show that the resistivity decreased with increasing the substrate temperature, in such a way high crystallinity samples with sheet resistance of 36 Ω/sq and sharper nano-walls morphology were obtained with a temperature of 800 °C. Also measurements of binding energies of both the XPS spectra W4f and O1s core-level electrons indicated that the sample is stoichiometric with maximum substrate temperature.

Ultrafast molecular dynamics illuminated with synchrotron radiation

Synchrotron radiation is a powerful tool for studying molecular dynamics in small molecules in spite of the absence of natural matching between the X-ray pulse duration and the time scale of nuclear motion. Promoting core level electrons to unoccupied molecular orbitals simultaneously initiates two ultrafast processes, nuclear dynamics on the potential energy surfaces of the highly excited neutral intermediate state of the molecule on the one hand and an ultrafast electronic decay of the intermediate excited state to a cationic final state, characterized by a core hole lifetime. The similar time scales of these processes enable core excited pump-probe-type experiments to be performed with long duration X-ray pulses from a synchrotron source. Recent results obtained at the PLIEADES beamline concerning ultrafast dissociation of core excited states and molecular potential energy curve mapping facilitated by changes in the geometry of the short-lived intermediate core excited state are reviewed. High brightness X-ray beams combined with state-of-the art electron and ion-electron coincidence spectrometers and highly sophisticated theoretical methods are required to conduct these experiments and to achieve a full understanding of the experimental results.

Stability and Core-Level Signature of Nitrogen Dopants in Carbonaceous Materials

Nitrogen doping is an important strategy in tuning the properties and functions of carbonaceous materials. But the chemical speciation of the nitrogen groups in the sp2-carbon framework has not been firmly established. Here we address two important questions in nitrogen doping of carbonaceous materials from a computational approach: the relative stability of different nitrogen groups and their X-ray photoelectron spectrum (XPS) signatures of the core-level (N 1s) electron binding energies. Four types of nitrogen groups (graphitic, pyrrolic, aza-pyrrolic, and pyridinic) in 69 model compounds have been examined. Computed formation energies indicate that pyrrolic and pyridinic nitrogens are significantly more stable (by about 110 kJ/mol) than graphitic and aza-pyrrolic nitrogens. This stability trend can be understood from the Clar’s sextet rule. Predicted N 1s binding energies show relatively high consistency among each dopant type, thereby offering a guide to identify nitrogen groups. The relative stability coupled with predicted N 1s binding energies can explain the temperature-dependent change in the experimental XPS spectra. The present work therefore provides fundamental insights into nitrogen dopants in carbonaceous materials, which will be useful in understanding the applications of nitrogen-doped carbons in electric energy storage, electrocatalysis, and carbon capture.

Electronic structure of oxygen-containing bismuth compounds

Systematic X-ray photoelectron spectroscopic (XPES) measurements were performed for bismuth compounds: α-Bi2O3, BaBiO3, and NaBiO3. Data on the valence band structure and positions of core electron levels in atoms were gained. Electronic band structure calculations were performed by the fullpotential linear-muffin-tin-orbital (FP-LMTO) method. A comprehensive analysis of the electronic structures was carried out on the basis of comparison of experimental and theoretical data. Our results imply the occurrence of noticeable electronic correlations in the compounds under study, which should be taken into account in interpreting the unusual properties discovered earlier in α-Bi2O3 and in some other oxygen-containing bismuth compounds.

Core-level spectroscopy to probe the oxidation state of single europium atoms.

The valence of individual europium atoms confined in carbon nanotubes is successfully measured by using core-level electron energy loss spectroscopy. Changes in the oxidation state at the atomic scale have been observed in Eu atomic chains exposed to oxygen. A transitory behavior has been identified where multiple atoms show a signal consistent with a sum of Eu^{2+} and Eu^{3+}. This indicates that single atoms change their valence state multiple times during the reaction, suggesting that oxidation in confined spaces and with extra energy input (from the electron beam) might not be a simple one step electron transfer event.

Electronic structure of Cu2CdGeSe4 single crystal as determined from X-ray spectroscopy data

We report on studies of the electronic structure of Cu2CdGeSe4 single crystal by using X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). In particular, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+ ion-irradiated surfaces of the Cu2CdGeSe4 single crystal have been recorded. Our XPS results indicate that Cu2CdGeSe4 is rather rigid with respect to Ar+ ion-irradiation: such a treatment does not induce any significant changes of values of the binding energies of core-level electrons as well as of the shapes of the XPS core-level and valence-band spectra of the Cu2CdGeSe4 surface. For the Cu2CdGeSe4 single crystal, the XES bands representing the energy distribution of mainly the valence Cu d, Cd d, Ge p and Se p states have been measured and compared on a common energy scale with its XPS valence-band spectrum. Such a comparison allows for concluding that the Cu 3d and Cd 4d states contribute mainly at the top and at the bottom of the valence band, respectively, while the Ge 4p and Se 4p states in the central and upper portions of the band. These experimental findings are found to be in agreement with literature data of band-structure calculations for this compound.

Selecting core-hole localization or delocalization in CS2 by photofragmentation dynamics.

Electronic core levels in molecules are highly localized around one atomic site. However, in single-photon ionization of symmetric molecules, the question of core-hole localization versus delocalization over two equivalent atoms has long been debated as the answer lies at the heart of quantum mechanics. Here, using a joint experimental and theoretical study of core-ionized carbon disulfide (CS2), we demonstrate that it is possible to experimentally select distinct molecular-fragmentation pathways in which the core hole can be considered as either localized on one sulfur atom or delocalized between two indistinguishable sulfur atoms. This feat is accomplished by measuring photoelectron angular distributions within the frame of the molecule, directly probing entanglement or disentanglement of quantum pathways as a function of how the molecule dissociates.

Gold atomic clusters extracting the valence electrons to shield the carbon monoxide passivation on near-monolayer core-shell nanocatalysts in methanol oxidation reactions.

Atomic-scale gold clusters were intercalated at the inter-facet corner sites of Pt-shell Ru-core nanocatalysts with near-monolayer shell thickness. We demonstrated that these unique clusters could serve as a drain of valence electrons in the kink region of the core-shell heterojunction. As jointly revealed by density functional theory calculations and valence band spectra, these Au clusters extract core-level electrons to the valence band. They prevent corrosion due to protonation and enhance the tolerance of CO by increasing the electronegativity at the outermost surface of the NCs during the methanol oxidation reaction (MOR). In these circumstances, the retained current density of Pt-shell Ru-core NCs is doubled in a long-term (2 hours) MOR at a fixed voltage (0.5 V vs. SCE) by intercalating these sub-nanometer gold clusters. Such novel structural confinement provides a possible strategy for developing direct-methanol fuel cell (DMFC) modules with high power and stability.

H-bonds and Protons in Molecular Crystals: Insight from Combined XPS/ssNMR/XRD

Physicochemical properties of molecular crystals are significantly influenced by non-covalent interactions and proton transfer. A well known application is the tuning of solubility, bioavailability and stability of pharmaceutical actives through co-crystal (hydrogen-bonding) or salt (ionic, Brønsted acceptors) formation. X-ray Photoelectron Spectroscopy (XPS) is an intrinsically local structural probe, providing information on the chemical state and chemical environment of atoms in molecules and crystals through the photoemission of core level electrons. We have recently studied a wide range of acid-base complexes in molecular crystals and found that analyzing the chemical shifts of N1s core level binding energies provides a facile route for characterizing the chemical and structural changes at functional groups involved in hydrogen bonding and proton transfer [1]. Very importantly, XPS unequivocally distinguishes protonated (salt) from hydrogen-bonded (co-crystal) nitrogen moieties. We have complemented our results for nitrogen species with 15N Solid-State Nuclear Magnetic Resonance (ssNMR) chemical shifts, which reveal low frequency shifts with protonation, but the magnitude of these shifts is additionally influenced by the wider chemical environment [2]. When crystallographic structure information is available, ssNMR shifts can be computationally predicted and thereby related to H-bond lengths, giving a measure of H-bond strength (NMR crystallography). The wide variety of donor/acceptor systems we have investigated has covered a large range of pKa values and demonstrates the generic nature of taking an XPS/ssNMR/XRD approach to organic molecule crystallography (Fig 1). The excellent agreement between the conclusions drawn by XPS and combined ssNMR/CASTEP investigations opens up a reliable avenue for local structure characterization in molecular systems even in the absence of crystal structure information, for example with non-crystalline or amorphous matter.

Quantitative ADF-STEM and STEM-EELS at the atomic level

The ability to count, locate and distinguish the atoms in a material is one of the ultimate pursuits of nanomaterials characterization. In recent decades, significant advances toward this goal have occurred in the field of scanning transmission electron microscopy (STEM), with the establishment of a class of imaging modes capable of ~0.1 nm spatial resolution and high chemical sensitivity. High-angle annular dark-field (ADF) imaging and chemical mapping via core-level spectroscopy are prominent techniques in this class, with applicability to a wide variety of nanostructured materials, such as nanoparticles, interfaces, and embedded phases. With such advances comes the ability to perform fully quantitative imaging for unprecedented accuracy in nanostructure characterization. However, full realization of this goal requires that we must be able to isolate and quantify all of the experimental parameters pertinent to imaging at 0.1 nm resolution, so that the only remaining unknown is the nanostructure itself. This is demonstrated in the present work. We present a systematic study of the influence of experimental factors pertinent to 0.1 nm ADF-STEM. We demonstrate that ADF-STEM images can be interpreted on a quantitative basis, in terms of the number, positions and species of atoms in the material, *without* recourse to adjustable parameters [1]. The figure presents a demonstration for [001]-oriented LaB6. A similar demonstration will be shown for atomic-resolution chemical mapping based on core-level electron energy-loss spectroscopy (EELS) [2]. The approach demonstrated here improves on previous works by removing instrumental unknowns from the analysis. Applications of this approach will be presented.

Computational study of graphene growth on copper by first-principles and kinetic Monte Carlo calculations.

In this work the growth of a graphene monolayer on copper substrate, as typically achieved via chemical vapor deposition of propene (C3H6), was investigated by first-principles and kinetic Monte Carlo calculations. A comparison between calculated C1s core-level binding energies and electron spectroscopy measurements showed that graphene nucleates from isolated carbon atoms adsorbed on surface defects or sub-superficial layers upon hydrocarbon fragmentation. In this respect, ab initio nudged elastic band simulations yield the energetic barriers characterizing the diffusion of elemental carbon on the Cu(111) surface and atomic carbon uptake by the growing graphene film. Our calculations highlight a strong interaction between the growing film edges and the copper substrate, indicative of the importance of the grain boundaries in the epitaxy process. Furthermore, we used activation energies to compute the reaction rates for the different mechanisms occurring at the carbon-copper interface via harmonic transition state theory. Finally, we simulated the long-time system growth evolution through a kinetic Monte Carlo approach for different temperatures and coverage. Our ab initio and Monte Carlo simulations of the out-of-equilibrium system point towards a growth model strikingly different from that of standard film growth. Graphene growth on copper turns out to be a catalytic, thermally-activated process that nucleates from carbon monomers, proceeds by adsorption of carbon atoms, and is not self-limiting. Furthermore, graphene growth seems to be more effective at carbon supersaturation of the surface-a clear fingerprint of a large activation barrier for C attachment. Our growth model and computational results are in good agreement with recent X-ray photoelectron spectroscopy experimental measurements.

In situ soft X-ray absorption spectroscopy of flames

The feasibility of in situ soft X-ray absorption spectroscopy for imaging carbonaceous species in hydrocarbon flames is demonstrated using synchrotron radiation. Soft X-rays are absorbed by core level electrons in all carbon atoms regardless of their molecular structure. Core electron spectroscopy affords distinct advantages over valence spectroscopy, which forms the basis of traditional laser diagnostic techniques for combustion. In core level spectroscopy, the transition linewidths are predominantly determined by the instrument response function and the decay time of the core–hole, which is on the order of a femtosecond. As a result, soft X-ray absorption measurements can be performed in flames with negligible Doppler and collisional broadening. Core level spectroscopy has the further advantage of measuring all carbonaceous species regardless of molecular structure in the far-edge region, whereas near-edge features are molecule specific. Interferences from non-carbon flame species are unstructured and can be subtracted. In the present study, absorption measurements in the carbon K-edge region are demonstrated in low-pressure (P total = 20–30 Torr) methane jet flames. Two-dimensional imaging of the major carbonaceous species, CH4, CO2, and CO, is accomplished by tuning the synchrotron radiation to the respective carbon K-edge, near-edge X-ray absorption fine structure (NEXAFS) transitions and scanning the burner.

X-ray spectroscopy study of the electronic structure of non-centrosymmetric Ag2CdSnS4 single crystal

We report on studies of the electronic structure of Ag2CdSnS4 single crystal grown by the horizontal gradient freeze technique. For the crystal under consideration, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that Ag2CdSnS4 single crystal surface is very rigid with respect to Ar+ ion-irradiation. In particular, Ar+ bombardment of the single crystal surface does not induce any significant changes of values of the binding energies of core-level electrons as well as the energy distribution of electronic states within the valence-band region. The Ar+ bombardment does not change the elemental stoichiometry of the Ag2CdSnS4 surface. For the Ag2CdSnS4 compound, the X-ray emission bands representing the energy distribution of the valence Ag d, Cd d and S p states were recorded and compared on a common energy scale with the XPS valence-band spectrum. Results of such a comparison indicate that the S 3p states contribute predominantly in the upper and central portions of the valence band of the Ag2CdSnS4 single crystal. In addition, our data reveal that the Ag 4d and Cd 4d states contribute mainly in the central portion and at the bottom of the valence band, respectively.

Nature of Pd and Ti Metals in the Structure of CMAS Glass and Ceramics

The structure of electrically conductive CMAS-TiO2-Pd glass and ceramics was investigated by transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), and extended X-ray absorption fine structure spectroscopy (EXAFS). The XANES spectra of Ti do not show any significant difference between the glasses ceramized in air or in a reducing “forming” gas, as well as between Pd-containing versus Pd-free samples, nor between surface versus bulk of the glass-ceramic samples. However, EXAFS and XANES data recorded at the Pd K-edge show significant dependences on whether the glass-ceramic was ceramized in air or in “forming” gas. The XPS spectra of Ti 2p core-level electrons for glasses ceramized in air or “forming” gas also show a strong difference depending on whether the samples did or did not contain Pd. STEM mapping confirms the existence of grains in the form of main crystalline phases identified with XRD, and also reveals the existence of Pd nanoparticles in glasses ceramized in both air and in “forming” gas.

Near-Edge X-ray Absorption Fine Structure Spectra and Site-Selective Dissociation of Phenol

A time-of-flight mass spectrometer with orthogonal acceleration and soft X-rays from synchrotron radiation were utilized to measure near-edge X-ray absorption fine structure (NEXAFS) spectra of carbon and oxygen in phenol and the corresponding ionic fragments following core excitation. The photon energies were in the range of 284-298 eV for the carbon K-edge and 529.5-554.5 eV for the oxygen K-edge. The total ion yield, ion intensity for each ionic fragment, and ion intensity ratio, defined as ion intensity divided by total ion yield, were measured as a function of photon energy. Possible mechanisms of dissociation are proposed and enhancements of specific products of dissociation are reported. In general, the enhancement of these specific products is small in the carbon K-edge region but is clear for some products at the oxygen K-edge. In particular, elimination of the H atom from the hydroxyl group was observed only at the oxygen K-edge. One remarkable result is that an excitation of a core-level electron of oxygen greatly enhanced the cleavage of specific C-C bonds.

Fundamentals of XAFS

The basic physical principles of X-ray Absorption Fine-Structure (XAFS) are presented. XAFS is an element-specific spectroscopy in which measurements are made by tuning the X-ray energy at and above a selected core-level binding energy of a specified element. Although XAFS is a well-established technique providing reliable and useful information about the chemical and physical environment of the probe atom, its requirement of an energy-tunable X-ray source means it is primarily done with synchrotron radiation sources and so is somewhat less common than other spectroscopic analytical methods. XAFS spectra are especially sensitive to the oxidation state and coordination chemistry of the selected element. In addition, the extended oscillations of the XAFS spectra are sensitive to the distances, coordination number and species of the atoms immediately surrounding the selected element. This Extended X-ray Absorption Fine-Structure (EXAFS) is the main focus of this chapter. As it is element-specific, XAFS places few restrictions on the form of the sample, and can be used in a variety of systems and bulk physical environments, including crystals, glasses, liquids, and heterogeneous mixtures. Additionally, XAFS can often be done on low-concentration elements (typically down to a few ppm), and so has applications in a wide range of scientific fields, including chemistry, biology, catalysis research, material science, environmental science, and geology. Special attention in this chapter is given to the basic concepts used in analysis and modeling of EXAFS spectra. X-ray absorption fine structure (XAFS) is the modulation of an atom’s X-ray absorption probability at energies near and above the binding energy of a core-level electron of the atom. The XAFS is due to the chemical and physical state of the absorbing atom. XAFS spectra are especially sensitive to the formal oxidation state, coordination chemistry, and the distances, coordination number and species of the atoms immediately surrounding the selected …

The effect of noble gas bombarding on nitrogen diffusion in steel

The low energy (∼50–350 eV) noble gases ion bombardment of the steel surface shows that the pre-treatments increase nitrogen diffusion by modifying the outermost structure of the material. The surface microstructure and morphology of the studied samples were characterized by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The crystalline and chemical structures in the outermost layers of the surface were analyzed by grazing angle X-ray diffraction (GAXRD) and photoemission electron spectroscopy (XPS). Temperature effusion studies of the implanted ions are used to elucidate the noble gases site localization in the network. The local compressive stress induced by the nearby iron atoms on the core level electron wave functions of the trapped noble gases are studied by photoemission electron spectroscopy (XPS) and interpreted considering a simple mechanical model. Nano-hardness measurements show the dependence of the material elastic constant on the energy of the implanted noble gases. Although the ion implantation range is about few nanometers, the atomic attrition effect is larger enough to modify the material structure in the range of micrometers. Two material stress zones were detected where the outermost layers shows compressive stress and the underneath layers shows tensile stress. The implanted noble gases can be easily removed by heating. A diffusion model for polycrystalline-phase systems is used in order to discuss the influence of the atomic attrition on the N diffusion coefficient. The concomitant effect of grain refining, stress, and surface texture on the enhancing nitrogen diffusion effect is discussed.