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

Inducing molecular orientation in solution-processed thin films of fluorene-bithiophene-based copolymer: thermal annealing vs. solvent additive.

A deep understanding of the factors influencing the morphology of thin films based on conjugated polymers is essential to boost their performance in optoelectronic devices. Herein, we investigated the electronic structure and morphology of thin films of the copolymer poly(9,9-dioctyl-fluorenyl-co-bithiophene) (F8T2) in its pristine form as well as samples processed with the solvent additive 1,8-diiodooctane (DIO) or post-processed through thermal annealing treatment. Measurements were carried out using angle-resolved S K-edge NEXAFS (near-edge X-ray absorption fine structure) in total electron yield (TEY) and fluorescence yield (FY) detection modes. Two main transitions were observed at the S 1s NEXAFS spectra: S 1s → π* and S 1s → σ* (S-C). The observed dichroism pointed to a face-on orientation of the conjugated backbone, which was significantly increased for F8T2 films processed with DIO. Resonant Auger decay spectra were obtained and analyzed using the core-hole clock (CHC) method. An enhancement in the charge transfer process was observed for thermally annealed films, especially for samples processed with DIO, corresponding to an increase in film ordering. Furthermore, the investigated films were characterized using X-ray photoelectron spectroscopy, attesting to the presence of the thiophene unit in the samples and demonstrating that some of its sulfur atoms were positively polarized in the F8T2 films. All these experimental findings were compared with molecular dynamics (MD) simulations of film evaporation with and without DIO. The use of MD, together with mathematical modeling, was able to explain the major effects found in the experiments, including the polarization of sulfur atoms. The simultaneous use of powerful spectroscopic techniques and theoretical methods shed light on key aspects linking film morphology with fabrication procedures.

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.

Bottom-Up Fabrication of a Metal-Supported Oxo–Metal Porphyrin

In situ preparation of oxotitanium tetraphenylporphyrin (TiO-TPP) on Ag(111) under ultrahigh vacuum conditions was achieved in a multistep procedure starting from adsorbed free-base tetraphenylporphyrin (2H-TPP). The final product as well as the intermediate titanium tetraphenylporphyrin (Ti-TPP) was characterized by a suite of surface-sensitive spectroscopic tools combined with scanning tunneling microscopy and density functional theory (DFT), and compared against the parent 2H-TPP species. Facile oxidation of Ti-TPP with molecular oxygen was observed at 300 K, with X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) from the Ti 2p core levels supporting a change in the oxidation state from Ti2+ to Ti4+. N K-edge and Ti L-edge NEXAFS suggest that the tetrapyrrole macrocycle conformation is modified upon binding to oxygen, in agreement with DFT calculations that predict a marked change of the local environment of the Ti centers upon oxygen attachment. O K-edge NEXAFS and O 1s energy-scanned photoelectron diffraction from the resulting TiO-TPP monolayer provide strong evidence for the presence of a titanium–oxygen double bond, with the latter technique yielding a bond length of 1.56 ± 0.02 Å. The majority of adsorbed TiO-TPP species have the oxo group pointing away from the surface rather than toward it, and thus the oxygen atom can potentially interact with external species. Both the highly reactive, intermediate Ti-TPP species and the final product TiO-TPP are of great interest for catalytic applications.

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.

Self-association of organic solutes in solution: a NEXAFS study of aqueous imidazole.

N K-edge near-edge X-ray absorption fine-structure (NEXAFS) spectra of imidazole in concentrated aqueous solutions have been acquired. The NEXAFS spectra of the solution species differ significantly from those of imidazole monomers in the gas phase and in the solid state of imidazole, demonstrating the strong sensitivity of NEXAFS to the local chemical and structural environment. In a concentration range from 0.5 to 8.2 mol L(-1) the NEXAFS spectrum of aqueous imidazole does not change strongly, confirming previous suggestions that imidazole self-associates are already present at concentrations more dilute than the range investigated here. We show that various types of electronic structure calculations (Gaussian, StoBe, CASTEP) provide a consistent and complete interpretation of all features in the gas phase and solid state spectra based on ground state electronic structure. This suggests that such computational modelling of experimental NEXAFS will permit an incisive analysis of the molecular interactions of organic solutes in solutions. It is confirmed that microhydrated clusters with a single imidazole molecule are poor models of imidazole in aqueous solution. Our analysis indicates that models including both a hydrogen-bonded network of hydrate molecules, and imidazole-imidazole interactions, are necessary to explain the electronic structure evident in the NEXAFS spectra.

Interrogating the Carbon and Oxygen K-Edge NEXAFS of a CO2-Dosed Hyperbranched Aminosilica.

Using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, we shed light on the nature of the interaction between CO2 and the amine moieties in a hyperbranched aminosilica (HAS) material, a porous aminosilica composite with great potential for postcombustion carbon capture applications. We show that after dosing a pristine (annealed) HAS sample with CO2, the C K-edge NEXAFS spectrum presents a new π* resonance at 289.9 eV, which can be attributed to the formation of a C═O (carbonyl) bond. Additional analyses of the O K-edge using model samples containing carbamate, carbonate, and bicarbonate functional groups as reference demonstrate a carbamate bonding mechanism for the chemical adsorption of CO2 by the HAS material under the conditions employed. These findings show the capability of the C and O K-edge NEXAFS technique to identify CO2-adsorbate species despite the high concentration of C and O atoms inherently present in the sample (prior to CO2 dosing) and the significant similarities between the possible adsorbates.

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.

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)

Characterization of boroaluminosilicate glass surface structures by B K-edge NEXAFS

Techniques traditionally used to characterize bulk glass structure (NMR, IR, etc.) have improved significantly, but none provide direct measurement of local atomic coordination of glass surface species. Here, we used Near-Edge X-ray Absorption Fine Structure (NEXAFS) as a direct measure of atomic structure at multicomponent glass surfaces. Focusing on the local chemical structure of boron, we address technique-related issues of calibration, quantification, and interactions of the beam with the material. We demonstrate that beam-induced adsorption and structural damage can occur within the timeframe of typical measurements. The technique is then applied to the study of various fracture surfaces where it is shown that adsorption and reaction of water with boroaluminosilicate glass surfaces induces structural changes in the local coordination of boron, favoring B IV species after reaction.

Direct observation of defects in hexagonal boron nitride by near-edge X-ray absorption fine structure and X-ray photoemission spectroscopy

Formation of defects in hexagonal boron nitride (h-BN) under low-energy argon or nitrogen ion bombardment has been studied by near-edge X-ray absorption fine structure (NEXAFS) around boron and nitrogen K-edges and X-ray photoemission spectroscopy (XPS) from B1s and N1s core levels. Breaking of B–N bonds and formation of nitrogen vacancies have been identified in the B K-edge NEXAFS and B1s XPS measurements, followed by the formation of molecular nitrogen, N 2, at interstitial positions for both argon and nitrogen bombardments. The formation of N 2 produces a sharp resonance in the low-resolution NEXAFS spectra around the N K-edge, showing characteristic vibrational fine structure in high-resolution measurements.

Characterization of benzenethiolate self-assembled monolayer on Cu(1 0 0) by XPS and NEXAFS

The composition, electronic structure and molecular orientation of a self-assembled monolayer (SAM) of benzenethiolate on the Cu(1 0 0) surface have been investigated by means of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). An ordered benzenethiolate SAM with c(2 × 6) superstructure has been obtained by the reaction of diphenyldisulfide with the Cu(1 0 0) surface at room temperature. S 2 p and C 1 s XP spectra show S–Cu bond formation without significant thiolate decomposition. The strong dichroic signal of C K-edge NEXAFS upon varying polarization angle indicates upstanding of the adsorbed molecules with the benzene ring tilted by 20° from the surface normal.

Structural Study of NO Adsorbed on the Reconstructed Pt(110)-(1 × 2) Surface with X-ray Photoelectron Diffraction and Near-Edge X-ray Absorption Fine Structure Spectroscopy

The adsorption structure of NO on the reconstructed Pt(110)-(1 x 2) surface was studied with X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low-energy scanned-angle photoelectron diffraction (LESA-PD), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The experiments were performed at 180 K, where no surface lifting from (1 x 2) to (1 x 1) takes place after NO adsorption. XPS indicates that the (1 x 2) unit cell of the Pt(110) surface contains 1.5 NO molecules at the saturated coverage. XPD and LESA-PD analyses allow us to propose a structural model for the NO adlayer, where two-thirds of the NO molecules in the (1 x 2) unit cell are adsorbed on the atop site of the close-packed Pt rows (ridges) along the [10] direction with an inclined geometry and one-third of the NO molecules adsorb on the bridge site between the Pt ridges with an upright configuration. This model is supported by the N K-edge NEXAFS experiments and is consistent with the recently reported model based on the density functional theory (Orita, H.; Nakamura, I.; Fujitani, T. J. Phys. Chem. B 2005, 109, 10312).

Adsorption structures of NO on Pt(1 1 1) studied by the near edge X-ray absorption fine structure spectroscopy

Abstract Near edge X-ray absorption fine structure (NEXAFS) spectra were measured and analyzed to reveal the adsorption structures of NO on Pt(1 1 1) at 290 and 230 K. N K-edge NEXAFS spectra show a single π* resonance at 290 K, while they exhibit double peaks at 230 K. Simple polarization analysis cannot be applied to such a complicated system. Simulations based on the multiple scattering theory confirmed that NO is uniquely adsorbed at the hollow site at 290 K and there are two kinds of adsorption species at 230 K: one at atop site with a tilt angle of about 50° and another at hollow site with upright direction. They are in close agreement with the results by recent LEED I/V analysis.

Adsorption behavior of dimethylsulfide (CH 3SCH 3) on Ni(1 1 0) surface studied by NEXAFS and XPS

Orientation and charge transfer upon adsorption of dimethylsulfide (CH 3SCH 3) on Ni(1 1 0) surface has been studied by near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) techniques. It is found that dimethylsulfide adsorbs molecularly on Ni(1 1 0) surface. Polarization dependence S K-edge NEXAFS indicates that the S–C bond is tilted 19 ± 8° from the surface. Above room temperature, the S–C bond breaks and creates atomic sulfur. An amount of charge transfer (0.62 electron) from nickel to dimethylsulfide molecule has measured using S 1s XPS.

Thioethanol on Cu(111) at room temperature: A near edge x-ray absorption fine structure and x-ray photoelectron spectroscopy study

Orientation and charge transfer upon adsorption of thioethanol on copper surface at room temperature has been studied by near edge x-ray absorption fine structure (NEXAFS) and x-ray photoelectron spectroscopy (XPS) techniques. It is found that thioethanol adsorbs dissociatively on the Cu(111) surface. Polarization dependent S K-edge NEXAFS indicates that the S–C bond is tilted 47±7° from the surface. Above 400 K, the S–C bond breaks and creates atomic sulfur. An amount of charge transfer (one electron) from copper to thioethanol molecules was measured using S 1s XPS.

Characterization of coke deposited on catalysts by carbon K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy

C K-edge NEXAFS spectra could distinguish between the structures of coke on the catalysts used in fluid catalytic cracking, hydrotreatment and dry reforming of methane from the characteristic ;π* features. A particular advantage of NEXAFS spectroscopy was found in the characterization of highly aromatic coke that could not be analyzed by cross-polarization 13C-NMR due to the lack of magnetization transfer from 1H to 13C. The NEXAFS results elucidated two different structures of coke on Ni catalysts used in dry reforming of methane; one with a graphitic structure formed at lower temperatures and the other with a non-graphitic structure formed at higher temperatures.

Electronic properties of SO 2 adsorbed on Ni(100) studied by UPS and O K-edge NEXAFS

Surface electronic properties of submonolayer sulfur dioxide SO 2 molecularly adsorbed on Ni(100) have been investigated by means of ultraviolet photoelectron spectroscopy (UPS) and O K-edge near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. NEXAFS has confirmed that the SO 2 molecule is lying flat on the surface and significant charge transfer occurs from the substrate to the π ∗ orbital. UPS has directly clarified a partial occupancy of the π ∗ level, and correspondingly the π level was found to shift to a lower binding energy more significantly than other orbitals. It has been clearly elucidated that the SO 2 molecule acts as a π acceptor upon adsorption on the Ni(100) surface.