K-edge Near-edge X-ray Absorption Fine Structure Spectra Research Articles

4-Mercaptobenzoic Acid Adsorption on TiO2 Anatase (101) and TiO2 Rutile (110) Surfaces

The adsorption of 4-mercaptobenzoic acid (4-MBA) on anatase (101) and rutile (110) TiO2 surfaces has been studied using synchrotron radiation photoelectron spectroscopy and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy techniques. Photoelectron spectroscopy results suggest that the 4-MBA molecule bonds to both TiO2 surfaces through the carboxyl group, following deprotonation in a bidentate geometry. Carbon K-edge NEXAFS spectra show that the phenyl ring of the 4-MBA molecule is oriented at 70° ± 5° from the surface on both the rutile (110) and anatase (101) surfaces, although there are subtle differences in the electronic structure of the molecule following adsorption between the two surfaces.

Ordered Hydrogen Bonding Structure of Water Molecules Adsorbed on Silver Iodide Particles under Subsaturated Conditions

Silver iodide (AgI) is an efficient ice-nucleating material. This has been related to the close lattice match to hexagonal ice, which helps to nucleate ice crystals on its surface under supersaturated conditions. In turn, the structure of water molecules adsorbed on its surface, embodied in the coordination of hydrogen bonds, has not been addressed so far. We suspected that AgI may induce ice-like coordination among adsorbed water molecules already under subsaturated conditions. X-ray photoelectron spectroscopy was used to probe the structure and composition at the AgI surface. We determined the chemical properties of the surface, the thickness of adsorbed water, and the amount of contaminating carbon species. Auger electron yield near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to assess the hydrogen bonding (HB) structure. The O K-edge NEXAFS spectra indicated that the HB structure of the adsorbed water on AgI under subsaturated conditions showed similarity to that of ice, which helps facilitate the stabilization of ice embryos at saturation. The approach used here opens up important perspectives for characterizing adsorbed water molecules on a wide variety of solids, which provides an important basis for understanding ice nucleation and other interfacial processes at the molecular level.

Near-Edge X-ray Absorption Fine Structure Spectroscopy of Heteroatomic Core-Hole States as a Probe for Nearly Indistinguishable Chemical Environments.

We demonstrate how the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy of single and double core-hole states created by the ionization of a heteroatom can be used to probe subtle changes in intramolecular chemical environments that are nearly indistinguishable by conventional NEXAFS spectroscopy. Using prototypical organic molecules (2/3-pentanone and pentanal), we show how new spectral features emerge in the C K-edge NEXAFS spectra, when creating single and double core-holes at the oxygen heteroatom site. The effect on the lowest unoccupied molecular orbitals is analyzed by studying the double-core-hole-induced ultrafast valence electron dynamics of the three molecules. The predicted changes from our simulations should be observable with state-of-the-art experiments at X-ray free-electron lasers.

Identifying On-Surface Site-Selective Chemical Conversions by Theory-Aided NEXAFS Spectroscopy: The Case of Free-Base Corroles on Ag(111).

We demonstrate here that theory-assisted near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy enables the site-sensitive monitoring of on-surface chemical reactions, thus, providing information not accessible by other techniques. As a prototype example, we have used free-base 5,10,15-tris(pentafluorophenyl)corroles (3H-TpFPC) adsorbed on Ag(111) and present a detailed investigation of the angle-dependent NEXAFS of this molecular species as well as of their thermally induced derivatives. For this, we have recorded experimental C and N K-edge NEXAFS spectra and interpret them based on XAS cross-section calculations by using a continuous fraction approach and core-hole including multiprojector PAW pseudopotentials within DFT. We have characterized the as-deposited low temperature (200 K) phase and unraveled the subsequent changes induced by dehydrogenation (at 330 K) and ring-closure reactions (at 430 K). By exemplarily obtaining profound insight into the on-surface chemistry of free-base corrolic species adsorbed on a noble metal this work highlights how angle-dependent XAS combined with accurate theoretical modeling can serve for the investigation of on-surface reactions, whereby even highly similar molecular structures, such as tautomers and isomers, can be distinguished.

A newly constructed photoactive system, Fe(III)-C/N-Bi2O3, for efficient visible light photocatalysis

In this study, two strategies are simultaneously applied for photocatalytic activity enhancement of Bi2O3. The first strategy is to increase the light absorption via simultaneous carbon and nitrogen doping. The second strategy is to increase the charge carrier separation via Fe grafting. Transmission electron microscopy observation reveals the presence of Fe nanoclusters resided on the Bi2O3 surface. Near edge X-ray absorption fine structure (NEXAFS) analysis confirms the valence state of the Fe species to be +3, and their coordination to be octahedral. The C K-edge and N K-edge NEXAFS spectra furthermore confirm the incorporation of C and N species in the Bi2O3 structure. This newly constructed photocatalyst, denoted as Fe(III)-C/N-Bi2O3, shows appreciably enhanced photocatalytic performance for the decomposition of 2,4-dichlorophenol under visible light as compared to bare Bi2O3.

Structural, electronic structure and antibacterial properties of graphene-oxide nano-sheets

Correlation between the structural/electronic structure properties and bio-activity of graphene-based materials need to be thoroughly evaluated before their commercial implementation in the health and environment precincts. To better investigate the local hybridization of sp2/sp3 orbitals of the functional groups of graphene-oxide (GO) and their execution in the antimicrobial mechanism, we exemplify the antibacterial activity of GO sheets towards the Escherichia coli bacteria (E. coli) by applying the field-emission scanning electron microscopy (FESEM), near edge X-ray absorption fine structure (NEXAFS) and scanning transmission X-ray microscope (STXM) techniques. C K-edge and O K-edge NEXAFS spectra have revealed lesser sp2 carbon atoms in the aromatic ring and attachment of functional oxygen groups at GO sheets. Entrapment of E. coli bacteria by GO sheets is evidenced by FESEM investigations and has also been corroborated by nano-scale imaging of bacteria using the STXM. Spectroscopy evidence of functional oxygen moieties with GO sheets and physiochemical entrapment of E. coli bacteria have assisted us to elaborate the mechanism of cellular oxidative stress-induced disruption of bacterial membrane.

A study of adventitious contamination layers on technically important substrates by photoemission and NEXAFS spectroscopies

This paper addresses the study of adventitious contamination layers on the technically important substrates by X-ray and UV photoemission (XPS and UPS) as well as near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. This investigation draws attention with regard to its application in the electronic devices of lower dimension. The Au, Pt, Ag, Cu, H passivated and SiO2 coated Si (100) are used as substrates. The XPS studies reveal that the composition of the adventitious contamination layer on studied noble substrates is distinct from those of non-metallic (H passivated and SiO2 coated Si) substrates. The ratio of the composition is different for various noble metal substrates. Furthermore, chemisorbed oxygen layer is found to present on Pt, Ag and Cu substrates in contrast to Au one. Our UPS results expose that the valence electronic structure, as well as workfunction of noble metal surfaces, are modified due to the contamination layer onto the substrate surfaces. The C K-edge NEXAFS spectra from the contamination layers on several substrates are presented to exhibit that the composition of contamination layer is different on different substrates whereas angle dependent NEXAFS data suggests that adventitious contamination layers on all substrates are amorphous in nature.

Effects of domain size on x-ray absorption spectra of boron nitride doped graphenes

Doping is an efficient way to open the zero band gap of graphene. The control of the dopant domain size allows us to tailor the electronic structure and the properties of the graphene. We have studied the electronic structure of boron nitride doped graphenes with different domain sizes by simulating their near-edge X-ray absorption fine structure (NEXAFS) spectra at the N K-edge. Six different doping configurations (five quantum dot type and one phase-separated zigzag-edged type) were chosen, and N K-edge NEXAFS spectra were calculated with large truncated cluster models by using the density functional theory with hybrid functional and the equivalent core hole approximation. The opening of the band gap as a function of the domain size is revealed. We found that nitrogens in the dopant boundary contribute a weaker, red-shifted π* peak in the spectra as compared to those in the dopant domain center. The shift is related to the fact that these interfacial nitrogens dominate the lowest conduction band of the system. Upon increasing the domain size, the ratio of interfacial atom decreases, which leads to a blue shift of the π* peak in the total NEXAFS spectra. The spectral evolution agrees well with experiments measured at different BN-dopant concentrations and approaches to that of a pristine h-BN sheet.

Electronic structure origin of conductivity and oxygen reduction activity changes in low-level Cr-substituted (La,Sr)MnO3.

The electronic structure of the (La(0.8)Sr(0.2))(0.98)Mn(1-x)Cr(x)O3 model series (x = 0, 0.05, or 0.1) was measured using soft X-ray synchrotron radiation at room and elevated temperature. O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra showed that low-level chromium substitution of (La,Sr)MnO3 resulted in lowered hybridisation between O 2p orbitals and M 3d and M 4sp valance orbitals. Mn L3-edge resonant photoemission spectroscopy measurements indicated lowered Mn 3d-O 2p hybridisation with chromium substitution. Deconvolution of O K-edge NEXAFS spectra took into account the effects of exchange and crystal field splitting and included a novel approach whereby the pre-peak region was described using the nominally filled t(2g) ↑ state. 10% chromium substitution resulted in a 0.17 eV lowering in the energy of the t(2g) ↑ state, which appears to provide an explanation for the 0.15 eV rise in activation energy for the oxygen reduction reaction, while decreased overlap between hybrid O 2p-Mn 3d states was in qualitative agreement with lowered electronic conductivity. An orbital-level understanding of the thermodynamically predicted solid oxide fuel cell cathode poisoning mechanism involving low-level chromium substitution on the B-site of (La,Sr)MnO3 is presented.

Evaluation of the fluorinated antisticking layer by using photoemission and NEXAFS spectroscopies

The electronic structures of four kinds of fluorinated self-assembled monolayers (F-SAMs) with different chain length, which were used for an antisticking layer, were investigated by the photoemission and the near-edge X-ray absorption fine structure (NEXAFS) spectroscopies. From the photoemission spectra in the wide and in the C 1s core-level regions, chemical compositions and components of the F-SAMs with different chain length were evaluated. By using the curve fitting analysis of the photoemission spectra in C 1s core-level region, it was found that the CF3 site is located at the top of the surface in the C sites of the F-SAM. From the C K-edge NEXAFS spectra of the F-SAMs as a function of the incidence angle of the excitation photon, it was shown that the σ*(C–F) and σ*(C–C) orbitals in the F-SAMs are parallel and perpendicular to the surface, respectively. This indicates that the C–C chain in (CF2) n part of the F-SAMs is perpendicular to the surface. Based on these results, the electronic structures of the F-SAMs are discussed.

Electronic Phase Transitions of δ-AgxV2O5 Nanowires: Interplay between Geometric and Electronic Structures

Vanadium oxide bronzes, with the general formula MxV2O5, provide a wealth of compositions and frameworks where strong electron correlation can be systematically (albeit thus far only empirically) tuned. In this work, we report the synthesis of single-crystalline δ-Ag0.88V2O5 nanowires and unravel pronounced electronic phase transitions induced in response to temperature and applied electric field. Specifically, a pronounced semiconductor–semiconductor transition is evidenced for these materials at ca. 150 K upon heating, and a distinctive insulator–conductor transition is observed upon application of an in-plane voltage. An orbital-specific picture of the mechanistic basis of the phase transitions is proposed using a combination of density functional theory (DFT) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Structural refinements above and below the transition temperature, angle-resolved O K-edge NEXAFS spectra, and DFT calculations suggest that the electronic phase transitions in ...

Preferential oxidation-induced etching of zigzag edges in nanographene.

We investigated the thermal oxidation process of nanographene using activated carbon fibers (ACFs) by thermogravimetry (TG), X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and electrical conductance measurements. The oxidation process started from the edge of nanographene with the formation of phenol (-OH) or ether (C-O-C) groups attached to edge carbon atoms, as verified by the XPS and NEXAFS results. While the TG results indicated a decrease in the size of the nanographene sheet during the oxidation process, the intensity of the edge-state peak, i.e., the signature of the zigzag edge, decreased in the C K-edge NEXAFS spectra. This suggests that the zigzag edge preferentially reacted with oxygen and that the nanographene terminated with the thermodynamically unstable zigzag edges converted to one terminated with stable armchair edges. As the oxidation temperature increased, the activation energy for the electron hopping transport governed by the Coulomb gap variable range hopping between the nanographene sheets increased, and the tunneling barrier decreased. This change can be understood on the basis of the decrease in the size of the nanographene sheets together with the preferential etching of nanographene edges and the decrease in the inter-nanographene-sheet distance.

Electronic structure, magnetic and structural properties of Ni doped ZnO nanoparticles

We report structural, magnetic and electronic structural properties of Ni doped ZnO nanoparticles prepared by auto-combustion method. The prepared nanoparticles were characterized by using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and dc magnetization measurements. The XRD and HR-TEM results indicate that Ni doped ZnO nanoparticles have single phase nature with wurtzite lattice and exclude the presence of secondary phase. NEXAFS measurements performed at Ni L3,2-edges indicates that Ni ions are in +2 valence state and exclude the presence of Ni metal clusters. O K-edge NEXAFS spectra indicate an increase in oxygen vacancies with Ni-doping, while Zn L3,2-edge show the absence of Zn-vacancies. The magnetization measurements performed at room temperature shows that pure and Ni doped ZnO exhibits ferromagnetic behavior.

Investigation on the Pyrazine Molecular Junction Studied by Conductance Measurement and Near Edge X-ray Absorption Fine Structure

A pyrazine molecular junction was investigated using mechanically controllable break junction (MCBJ) technique at 10 K. The conductance measurements revealed the single pyrazine molecular junctions showed two distinct conductance values of 0.27 ± 0.04 and 1.0 ± 0.2 G 0 (2e 2/h). The conductance value of the single pyrazine molecular junction was comparable with that of the metal atomic junction. The interface between pyrazine molecule and Pt surface was investigated by near edge X-ray absorption fine structure (NEXAFS). The broadening of the π* peak in N K-edge NEXAFS spectra suggested that the pyrazine molecule connected to Pt surface via a nitrogen atom. Based on the measurements of the conductance and NEXAFS, we could propose the structural models of two distinct conductance states for the single pyrazine molecular junction.

Effect of nitrogen passivation on the interfacial property and band offset of HfLaO/SiGe

The effect of NH3 plasma treatment on the interfacial and dielectric property between ultrathin HfLaO and strained Si0.65Ge0.35 substrate is investigated by high-resolution cross-sectional transmission electron micrographs (HR-TEM), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), valence-band spectra and current density–voltage (J–V). TEM, AES and XPS results confirm that the interfacial layer with N–Hf, La–N and Si–N/Si–O–N bonds acts as a barrier layer against interdiffusions during annealing to some degree. In addition, the signals of N 1s core-level and N K-edge NEXAFS spectra are seen to disappear completely after post-deposition annealing (PDA) at 900°C due to the dissociation of unstable N chemical states during high-temperature annealing. The valence-band offsets ΔEV of HfLaO/SiGe interfaces with and without nitridation are determined to be 3.38 ± 0.01 eV and 3.20 ± 0.01 eV, respectively. While the corresponding conduction-band offsets ΔEC are found to be 1.10 ± 0.01 eV and 1.28 ± 0.05 eV, respectively. Moreover, the nitrided capacitor after PDA shows a low leakage current density (J) of ∼9.15 × 10−8 A cm−2 at a gate bias of Vg = −2.0 V.

Effect of co-adsorption dye on the electrode interface (Ru complex/TiO2) of dye-sensitized solar cells

The co-adsorption interface structure of isothiocyanate (R-N=C=S) in N719 dye was investigated using a system of N719 alone and an N719 + D131 co-adsorption system. The sulfur core level (S 1s) and sulfur K absorption edge (S K-edge) were examined in detail using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS), respectively. The S 1s XPS spectra revealed that the binding energies were shifted approximately 9 eV higher in N719 alone because of interactions between the R-N=C=S of N719 and nanocrystalline TiO2. However, this strong interaction disappeared in the N719 + D131 co-adsorption system. Comparing the S K-edge NEXAFS spectra against the case of N719 alone revealed that the resonance adsorption peak at 2483 eV, which was attributed to an interaction between sulfur and the substrate, did not appear in the N719 + D131 co-adsorption system. This peak was observed under oblique incidence, but was almost indiscernible under normal incidence. These results indicate that the interface structure of sulfur atoms that strongly interacts with nanocrystalline TiO2 substrate changes to become non-interacting in the N719 + D131 co-adsorption system. We conclude that the co-adsorption dye has the unique property of inhibiting strong interactions between the S atom in the R-N=C=S group of the N719 dye and the nanocrystalline TiO2 surface.

Formation of boron‐nitride ultra‐thin films on Si(111)

AbstractChemical vapor deposition is employed to form hexagonal boron nitride (h ‐BN) monolayer films on Si(111) with a borazine precursor. The electronic structure and orientation of the films are analyzed by near edge X‐ray absorption fine structure (NEXAFS) spectroscopy and X‐ray photoelectron spectroscopy (XPS). XPS analysis shows that monolayer‐level stoichiometric BN films were formed. The B K‐edge NEXAFS spectra show two discrete peaks that have graphite‐like polarization dependence in the low‐energy region. Polarization dependence analysis of the NEXAFS indicates that the BN films had two‐dimensional orientation on the Si(111) substrate. Based on the density functional theory calculations, these discrete peaks are assigned to π* peaks at an intact h ‐BN site and edge B‐H site. Comparison with the electronic structures of bulk h ‐BN and h ‐BN/Ni(111) clarifies that the interaction between the BN layer and Si substrate was very small. This indicates that the h ‐BN monolayer was physisorbed on Si(111), making a steep interface and conserving the intrinsic electronic structure (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A theoretical interpretation of near edge X-ray absorption fine structure of hexagonal boron nitride monolayer on Ni(1 1 1)

Near edge X-ray absorption fine structure (NEXAFS) spectroscopy is devoted to study the electronic structure of hexagonal boron nitride ( h-BN) epitaxial thin film on Ni(1 1 1). The B and N K-edge NEXAFS spectra are analyzed by density functional theory calculations with some model clusters. π* band modification of h-BN is explained by orbital hybridization between π* orbital of h-BN and Ni atomic orbitals. This suggests chemisorption-like strong interaction between h-BN and Ni(1 1 1) substrate.

Observation of Au-S Interface of L-Cysteine on Gold Surface

We studied the metal-molecular interface of Au--S bonds at the L-cysteine modified gold surface using X-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS). Two kinds of L-cysteine layers, i.e., multi-layer and two-layers, were prepared on gold substrate. The formation of unique chemical bond was found at the sulfur-gold interface. The sulfur 1s core-level energy of two-layered L-cysteine on gold is higher by 8.2 eV than that of multi-layered sample. The large chemical shift was also observed in S K-edge NEXAFS spectra. Such remarkably large chemical shift in the case of two-layers has never been reported for any sulfur compounds. The direction of the S 1s energy shift for Au-S bond is opposite to that for the normal metal sulfides. This means that the electrons are donated from the sulfur atom to the gold substrate, and most likely the sulfur atoms are positively charged at likely [δ+] states. We conclude that the strong Au-S bonds that are recently used for the immobilization of molecules are due to the observed specific Au--S bond. [DOI: 10.1380/ejssnt.2009.110]

Interface electronic structure of the organic light-emitting devices: Photoemission and x-ray absorption studies of Al∕KF∕Alq3 interface

The chemistry of organic light-emitting diode interface composed of KF sandwiched between the Al and Alq3 was investigated using near-edge x-ray absorption fine structure (NEXAFS) as well as x-ray and ultraviolet photoelectron spectroscopy techniques. At the earliest stages of KF deposition on Alq3, changes in F K-edge NEXAFS spectra indicated a strong chemical reaction, which is responsible for the dipole layer formation seen in valence-band spectra. For Al deposition on KF∕Alq3, the reactions inferred from observed spectral changes are not consistent with the commonly believed KF dissociation and AlF3 formation scenarios.