Near-edge X-ray Absorption Fine Structure Results Research Articles

Synchrotron-based spectroscopic analysis of diamond-like carbon films from different source gases

Diamond-like carbon (DLC) films were deposited using the pulsed plasma-enhanced chemical vapor deposition (PECVD) method with different source gases such as methane (CH4), acetylene (C2H2), acetylene with methane (C2H2:CH4), ethylene (C2H4), and ethylene with methane (C2H4:CH4). The chemical characteristics of these DLC films were investigated using suitable spectroscopic techniques. Raman spectroscopy analysis illustrated decrease of the ID/IG ratio with the addition of CH4. Importantly, X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy not only enabled evaluation of the relative content of sp2 and sp3, but also clarified the chemical bonding of the films with different source gases. The XPS results showed lower sp2 content than the NEXAFS results. Moreover, glow discharge optical emission spectrometry (GD-OES) illustrated increase in the hydrogen content, which is in good agreement with increase of the sp3 content based on Raman, XPS, and NEXAFS analyses. Consequently, the synthesis and characterization indicate a significant role for differences of chemical composition of the films. The study described herein provides useful information for preparing high-quality DLC films, as well as for evaluating appropriately the distribution of the carbon sp2/(sp2 + sp3) ratio and the hydrogen content in the films with different source gases.

Adsorption and Reactivity of Pyridine Dicarboxylic Acid on Cu(111)

A detailed understanding of the reaction of a range of molecules on surfaces will be key to developing targeted strategies for on-surface synthesis. Here, we studied the deprotonation and decarboxylation reactions of 3,5-pyridinedicarboxylic acid (PDC) on Cu(111) using synchrotron radiation photoelectron spectroscopy (SRPES), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). PDC partially deprotonates upon deposition on Cu(111) at room temperature and adsorbs with the plane of its aromatic ring inclined at an average of ∼45° with respect to the surface. By heating to 100 °C, the deprotonation of the molecule increases. When the PDC is partially deprotonated the plane of its aromatic ring adopts a more upright orientation with respect to the surface. Additional heating to 160 °C causes complete deprotonation, upon which the molecule returns to a more planar molecular adsorption geometry. By examining both the N 1s core level and the NEXAFS results, we ascribe these changes in adsorption to a reaction-induced change in the predominant molecule–surface interaction, which is driven by the nitrogen lone pair prior to deprotonation, and by the carboxylate groups after deprotonation of the −COOH groups. These interaction channels and adsorption geometries are supported by our DFT calculations. Heating above 200 °C induces decarboxylation of the molecule; by observing the rate of reaction over a range of fixed temperatures, we extract an activation energy of 1.93 ± 0.17 eV for the decarboxylation reaction. Around this temperature we also begin to observe the ring opening of the molecule, suggesting that if PDC is to be used as a building block for on-surface synthesis of polymers, careful control of temperature is necessary for obtaining decarboxylation and covalent coupling of the molecule without destroying the aromatic core.

Morphological and chemical changes of aerosolized E. coli treated with a dielectric barrier discharge.

This study presents the morphological and chemical modification of the cell structure of aerosolized Escherichia coli treated with a dielectric barrier discharge (DBD). Exposure to DBD results in severe oxidation of the bacteria, leading to the formation of hydroxyl groups and carbonyl groups and a significant reduction in amine functionalities and phosphate groups. Near edge x-ray absorption fine structure (NEXAFS) measurements confirm the presence of additional oxide bonds upon DBD treatment, suggesting oxidation of the outer layer of the cell wall. Electron microscopy images show that the bacteria undergo physical distortion to varying degrees, resulting in deformation of the bacterial structure. The electromagnetic field around the DBD coil causes severe damage to the cell structure, possibly resulting in leakage of vital cellular materials. The oxidation and chemical modification of the bacterial components are evident from the Fourier transform infrared spectroscopy and NEXAFS results. The bacterial reculture experiments confirm inactivation of airborne E. coli upon treating with DBD.

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.

Structural and Optical Properties of Luminescent Silicon Carbonitride Thin Films

The composition, structure, and optical characteristics of amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were investigated as a function of nitrogen content. The electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) technique was utilized to fabricate two different types of a-SiCN:H thin films including films with varying nitrogen contents and films co-doped with cerium and terbium. The intensity and position of the photoluminescence (PL) peak could be tuned in the visible range by controlling the film composition, deposition conditions, and post-deposition thermal treatment. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Si K- and N K-edge were used to perform a comparative structural study on a series of CVD grown a-SiCN:H thin films. A correlation was obtained between the amount of nitrogen in the matrix and the PL behaviour, which also supported the trend in the NEXAFS results.

Dynamical Orientation of Large Molecules on Oxide Surfaces and its Implications for Dye-Sensitized Solar Cells

A dual experimental-computational approach utilizing near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory-molecular dynamics (DFT-MD) is presented for determining the orientation of a large adsorbate on an oxide substrate. A system of interest in the field of dye-sensitized solar cells is studied: an organic cyanoacrylic acid-based donor-π-acceptor dye (WN1) bound to anatase TiO2. Assessment of nitrogen K-edge NEXAFS spectra is supported by calculations of the electronic structure that indicate energetically discrete transitions associated with the two π systems of the C–N triple bond in the cyanoacrylic acid portion of the dye. Angle-resolved NEXAFS spectra are fitted to determine the orientation of these two orbital systems, and the results indicate an upright orientation of the adsorbed dye, 63° from the TiO2 surface plane. These experimental results are then compared to computational studies of the WN1 dye on an anatase (101) TiO2 slab. The ground state structure obtained from standard DFT optimization is less upright (45° from the surface) than the NEXAFS results. However, DFT-MD simulations, which provide a more realistic depiction of the dye at room temperature, exhibit excellent agreement—within 2° on average—with the angles determined via NEXAFS, demonstrating the importance of accounting for the dynamic nature of adsorbate–substrate interactions and DFT-MD’s powerful predictive abilities.

Point defects in gallium nitride: X-ray absorption measurements and multiple scattering simulations

We have studied the formation of nitrogen-related point defects in gallium nitride (GaN) using near-edge x-ray absorption fine-structure (NEXAFS) spectroscopy and ab initio FEFF calculations. The presence of several point defects, created within the GaN matrix by low-energy ion-bombardment, has been detected by NEXAFS measurements around N K-edge. FEFF simulations that take into account the formation of nitrogen antisites, interstitials, and vacancies around absorbing nitrogen atoms are consistent with NEXAFS results. The position of energy levels created by these defects within the energy gap of GaN, obtained by both NEXAFS measurements and FEFF simulations, are in good agreement with theoretical predictions.

N – K edge NEXAFS study of the defects induced by indium implantation in GaN

N – K edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy is applied in order to determine implantation-induced changes in the electronic structure of GaN. The samples were implanted with 700 keV In ions and fluencies in the range 5×1013 – 1 × 1016 ions/cm2. The NEXAFS results are discussed in combination with Rutherford backscattering (RBS) characterization which assesses the implantation induced damage. The main implantation effects on the NEXAFS spectra are: (a) a fluence-dependent broadening of the NEXAFS peaks, (b) emergence of a pre-edge shoulder (RL1) that is attributed to N split-interstitials and (c) appearance of a post-edge sharp peak (RL2) that is attributed to molecular N2 trapped in the GaN matrix. The RL2 is characterized by fine structure due to vibronic transitions that result from a change of the vibrational quantum number along with the electronic transition. The concentration of the interstitials and the N2 molecules as well as the width of the NEXAFS peaks, have a sigmoidal dependence on the logarithm of the ion fluence, following the behaviour of the defect concentration deduced from the RBS measurements.

Conformations of Polyaniline Molecules Adsorbed on Au(111) Probed by in Situ STM and ex Situ XPS and NEXAFS

In situ scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) have been used to examine the conformation of a monolayer of polyaniline (PAN) molecules produced on a Au(111) single-crystal electrode by anodization at 1.0 V [vs reversible hydrogen electrode (RHE)] in 0.10 M H(2)SO(4) containing 0.030 M aniline. The as-produced PAN molecules took on a well-defined linear conformation stretching for 500 A or more, as shown by in situ and ex situ STM. The XPS and NEXAFS results indicated that the linear PAN seen at 1.0 V assumed the form of an emeraldine salt made of PAN chains and (bi)sulfate anions. Shifting the potential from 1.0 to 0.7 V altered the shape of the PAN molecules from straight to crooked, which was ascribed to restructuring of the Au(111) electrified interface on the basis of voltammetric and XPS results. In situ STM showed that further decreasing the potential to 0.5 V transformed the crooked PAN threads into a mostly linear form again, with preferential alignment and formation of some locally ordered structures. PAN molecules could be reduced from emeraldine to leucoemeraldine as the potential was decreased to 0.2 V or less. In situ STM showed that the fully reduced PAN molecules were straight but mysteriously shortened to approximately 50 A in length. The conformation of PAN did not recuperate when the potential was shifted positively to 1.0 V.

A multi-technique investigation of TiO 2 films prepared by magnetron sputtering

We have investigated seven samples of radio frequency magnetron-sputtered TiO 2 thin films by using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). A detailed analysis of the Ti 2p and O 1s NEXAFS edges allows determining the degree of order and the polymorphs of the investigated samples. We have found that thin films grown by using a Ti target and lower deposition rates have a rutile character, while, by using a TiO 2 target and higher deposition rates, amorphous/mixed thin films are obtained. We have also found that this correlation between phase and preparation conditions is perfectly reproducible leading to films with equal characteristics for the same preparation conditions. The NEXAFS results are supported by complementary XRD results.

Coadsorption reaction between (CH3)2S and H2O on Rh(100) studied by NEXAFS and XPS

AbstractThe coadsorption reaction between (CH3)2S and H2O on a Rh(100) surface has been investigated by S K‐edge near‐edge X‐ray absorption fine structure (NEXAFS) and XPS techniques. The dissociation of (CH3)2S to methanethiolate (CH3S)—occurs at 90 K. NEXAFS and XPS results indicate that methanethiolate bonds to H2O via its sulfur atom at 170 K. At elevated temperature, the dissociation rate of molecules is much higher than in other adsorption systems, such as on a clean Rh(100) surface. Therefore, it seems that H2O damages the SC bonds of both (CH3)2S and methanethiolate upon heating. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis and characterization of Rh(PVP) nanoparticles studied by XPS and NEXAFS

Rhodium nanoparticles were synthesized by the reduction of Rh 3+ ion in ethanol solvent with use of the polyvinylpyrrolidone (PVP) of various molecular weights and the solvent of different volume ratios of water to ethanol. The formed Rh(PVP) nanoparticles have been characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) techniques. The TEM and AFM results show that the Rh(PVP) nanoparticles are monodispersed and do not agglomerate with each other. The particle size can be controlled by the molecular weight of PVP and/or the water/ethanol ratio of the solvent. The XPS and NEXAFS results indicate that the chlorine derived from RhCl 3(3H 2O) remains in the obtained nanoparticles but can be removed by heating.

Synthesis of a Series of Fluorinated Boronate Compounds and Their Use as Additives in Lithium Battery Electrolytes

A new series of anion receptors based on boronate compounds have been synthesized. These compounds can be used as anion receptors in lithium battery electrolytes. The so-called boronate means that the compounds contain a boron bonded with two oxygen atoms and one carbon atom. This series includes various boronate compounds with different fluorinated aryl and fluorinated alkyl groups. When these anion receptors are used as additives in 1,2-dimethoxyethane (DME) solutions containing various lithium salts, the ionic conductivities of these solutions are greatly increased. The electrolytes tested in this study were DME solutions containing the following lithium salts: LiF, and Without the additive, the solubility of LiF in DME (and all other nonaqueous solvents) is very low. With some of these boronate compounds as additives, LiF solutions in DME with concentration as high as 1 M were obtained. The solubilities of the other salts were also increased by these additives. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy studies show that anions are complexed with these compounds in DME solutions containing LiI salts. The degree of complexation is also closely related to the structures of the fluorinated aryl and alkyl groups which act as electron-withdrawing groups. The NEXAFS results are in good agreement with ionic conductivity studies. © 2004 The Electrochemical Society. All rights reserved.

Fundamental aspects of enantioselective heterogeneous catalysis: a NEXAFS study of methyl pyruvate and (S)-(−)-1-(1-naphthyl) ethylamine on Pt{1 1 1}

Near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy have been used to characterise the adsorption and reactivity of methyl pyruvate and (S)-(−)-1-(1-napthyl) ethylamine ((S)-NEA) on Pt{111}. Methyl pyruvate polymerises at room temperature yielding chains that contain CO bonds and no CC bonds; the CO bonds are inclined at 64±5° with respect to the metal surface. Polymerisation probably occurs by an aldol condensation that involves elimination of methanol. This mechanism is in excellent accord with the intramolecular bonding deduced from the NEXAFS results. These findings suggest that irreversible deactivation during start-up or steady state operation of Pt catalysts during enantioselective hydrogenation of alkyl pyruvates can be due to hydrogen starvation, which results in polymerisation of the prochiral reactant. For (S)-NEA, the naphthalene ring is inclined at 46±5° with respect to the metal surface. It is proposed that (S)-NEA binds to the surface via contributions from both the aromatic π-system and the lone pair of electrons on the amine nitrogen atom.

High-Energy Spectroscopic Studies of the Electronic Structures of Organic Systems Formed from Carbon and Fluorine by UPS, Vacuum-UV Optical Spectroscopy, and NEXAFS: Poly(hexafluoro-1,3-butadiene) [C(CF3) = C(CF3)] n , Fluorinated Graphites (CF, C2F, and C6F), Perfluoroalkanes n-C n F2n+2, Poly(tetrafluoroethylene) (CF2) n , and Fluorinated Fullerenes (C60F x and C70F x )

Various systems formed by carbon and fluorine were studied by UV photoelectron spectroscopy (UPS), vacuum-ultraviolet (VUV) optical spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. New data were meawred for poly(hexat1uoro-1.3-butadiene) (PHFBD), which is actually poly(hexafluoro-2-butyne), and tluorinated graphitcs CF, C2F, and C6F. These data were analyzed together with reported data for n-C24F50, poly(tetratluoroethy1ene) (PTFE), and fluoninated fiillerenes C60F x and C70)F x . The electronic structures deduced from UPS, VUV optical spectra, and MO calculations could be understood in terms of degree of IS delocalization, inductive effects of F and CF3 groups, and the molecular conformation with possible steric hindrance. The ionization threshold energy of PHFBD (10.3eV) is the largest one for unsaturated systems. The well polarized NEXAFS spectra of CF and C2F were analyzed by the comparison with related compounds. The NEXAFS results of C6F showed that the F atoms are neither in molecular form nor covalently bonded to carbon atoms.

A NEXAFS investigation of the formation and decomposition of CuO and Cu2O thin films on Cu(100)

We report the use of near-edge X-ray absorption fine structure (NEXAFS) to unambiguously characterize the oxidation state of copper in copper oxides. The method is based on the analysis of the oxygen K-edge and copper L-edge features of Cu2O and CuO powder materials. To further demonstrate its utility, NEXAFS was also used to follow spectroscopically the oxidation of a Cu(100) single crystal surface by NO2, and the thermal decomposition of the resulting oxides. In agreement with previous studies [J.A. Rodriguez, J. Hrbek, J. Vac. Sci. Technol. 12 (1994) 2140], our NEXAFS results indicate that NO2 dissociates on the metal surface, generating NO and atomic oxygen which subsequently reacts with the Cu(100) surface to produce Cu2O (300K) or a mixture of Cu2O and CuO (250K). Upon heating, the thermodynamically unstable CuO transforms into Cu2O.

The Synthesis of a New Family of Boron‐Based Anion Receptors and the Study of Their Effect on Ion Pair Dissociation and Conductivity of Lithium Salts in Nonaqueous Solutions

A new family of anion receptors based on boron compounds has been synthesized. These compounds can be used as anion receptors in lithium battery electrolytes. This family includes various borane and borate compounds with different fluorinated aryl and fluorinated alkyl groups. When these anion receptors are used as additives in 1,2‐dimethoxyethane (DME) solutions containing various lithium salts, the ionic conductivities of these solutions are greatly increased. The electrolytes tested in this study were DME solutions containing the following lithium salts: LiF, LiCl, LiBr, LiI, , and . Without the additive, the solubility of LiF in DME (and all other nonaqueous solvents) is very low. With some of these boron compounds as additives, LiF solutions in DME with concentration as high as 1 M were obtained. The solubilities of the other salts were also increased by these additives. Near edge X‐ray absorption fine structure (NEXAFS) spectroscopy studies show that and anions are complexed with these compounds in DME solutions containing LiCl or LiI salts. The degree of complexation is also closely related to the structures of the fluorinated aryl and alkyl groups, which act as electron‐withdrawing groups. The NEXAFS results are in good agreement with ionic conductivity studies.

The adsorption structure of glycine adsorbed on Cu(110); comparison with formate and [formula omitted

The molecular orientation of an ordered monolayer of glycine adsorbed on Cu(110) has been studied using X-ray Photoelectron Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS), X-ray Photoelectron Diffraction (XPD), Low-Energy Electron Diffraction (LEED) and theoretical calculations. In particular, the NEXAFS results are discussed in terms of the spectra of the related molecules ammonia (NH 3), formate (HCOO), and acetate (CH 3COO) on Cu(110). Whereas the latter two molecules chemisorb in similar geometries, glycine is found to assume a very different chemisorption geometry. Formate and acetate bond through two equivalent oxygen atoms with the molecular plane oriented nearly perpendicular to the surface, aligned along the [11̄0]-azimuth. In the case of adsorbed glycine (NH 2CH 2COO), the azimuthal orientation is still present, i.e. the bonding oxygen atoms are aligned along the [11̄0]-azimuth, but the molecule is found to bend towards the surface. A second chemisorption bond is formed at the nitrogen end of the molecule, involving copper atoms in the neighboring [11̄0]-row. We therefore have the interesting case of a chemisorption bond involving different functional groups in the same molecule.

An ARUPS/NEXAFS study of the [formula omitted] adsorbate system

The room temperature adsorption of H 2S on InP(110) may be understood as a cooperative dissociation in which H + and SH − bond to the surface anion and cation, respectively. Synchrotron radiation angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and near edge X-ray absorption fine structure (NEXAFS) are used to probe the filled and vacant orbital levels of the adsorbate complex. The NEXAFS results are consistent with the location of the sulphur in the continued layer anion site in agreement with recent X-ray standing wave measurements.

Benzene adsorption on Cu(111): Formation of a stable bilayer

The structure of benzene deposited on a Cu(111) surface has been investigated by a combination of temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and near-edge x-ray absorption fine structure (NEXAFS) measurements. The results indicate that benzene forms a stable bilayer on Cu(111) at 110 K prior to multilayer formation. The TPD studies show that the second layer benzene desorbs with a peak temperature 5 K higher than that for benzene multilayers. HREELS and NEXAFS results indicate that benzene in the first layer bonds with its π ring parallel to the surface. With increasing coverage, benzene forms a second layer with its π ring significantly tilted away from the surface. The results are consistent with an approximately perpendicular configuration between the first and second layer benzene molecules, which is analogous to the structure of crystalline benzene. Isotope labeling experiments indicate there is almost complete mixing between molecules in different layers during sequential adsorption at 110 K.