SEXAFS Research Articles

An improved model for N2 adsorption on graphitic adsorbents and graphitized thermal carbon black—The importance of the anisotropy of graphene

Computer simulations of N2 adsorption on graphite frequently use the 10-4-3 equation with Steele’s molecular parameters to describe the dispersive-repulsive interaction between a molecule and graphite. This model assumes that graphite is a uniformly homogeneous continuum solid, and its derivation implies the following assumptions: (1) the solid is built from stacked, equally spaced graphene layers, (2) there is an infinite number of layers, and (3) the carbon atom molecular parameters are invariant for all layers (collision diameter of 0.34 nm and reduced well depth of interaction energy of 28 K). Despite the fact that this model can give an acceptable description of experimental data for this system, there are experimental observations that simulation results fail to account for. First, the isotherm does not exhibit a step in the sub-monolayer coverage region at 77 K, which is attributed to a transition from the supercritical state of the adsorbate to the commensurate state, and therefore fails to reproduce the cusp and heat spike in the experimental isosteric heat curve versus loading at close to monolayer coverage. Second, the simulation results overpredict the experimental data in the multilayer region. These discrepancies suggest that (1) the absence of lateral corrugation in the 10-4-3 potential misses the commensurate to incommensurate transition and (2) the long-range solid-fluid potential, experienced by the second and higher layers onwards, is too strong. Here we examine a revised graphite potential model that incorporates three features absent from the 10-4-3 model: (1) an energetic corrugation of the potential arising from the discrete atom structure of the adsorbent, (2) the unequal spacing of the graphene layers due to the anisotropic force field acting on graphene layers at the surface, and (3) the different polarizabilities of carbon atoms in graphite, parallel and normal to the graphene surface. These features are corroborated by a number of experimental measurements and quantum-mechanical calculations: (1) the Low-Energy Electron Diffraction (LEED) and Surface-Extended X-ray Absorption Fine Structure (SEXAFS) experiments show that the first adsorbate layer is smaller than predicted by the 10-4-3 model with the traditional molecular parameters suggested by Steele, and (2) the potential well depth for atoms in graphene is stronger than for C-atoms in graphite. The simulation results using this revised graphite model give an improved description of the fine features of adsorption of N2 on graphite: the sub-step in the first layer of the isotherm, the spike in the isosteric heat curve versus loading, and the coverage at higher loadings.

Surface EXAFS via differential electron yield.

Surface-sensitive analysis via extended X-ray absorption fine-structure (EXAFS) spectroscopy is demonstrated using a thickness-defined SiO2 (12.4 nm)/Si sample. The proposed method exploits the differential electron yield (DEY) method wherein Auger electrons escaping from a sample surface are detected by an electron analyzer. The DEY method removes local intensity changes in the EXAFS spectra caused by photoelectrons crossing the Auger peak during X-ray energy sweeps, enabling EXAFS analysis through Fourier transformation of wide-energy-range spectral oscillations. The Si K-edge DEY X-ray absorption near-edge structure (XANES) spectrum appears to comprise high amounts of SiO2 and low Si content, suggesting an analysis depth, as expressed using the inelastic mean free path of electrons in general electron spectroscopy, of approximately 4.2 nm. The first nearest neighbor (Si-O) distance derived from the Fourier transform of the Si K-edge DEY-EXAFS oscillation is 1.63 Å. This value is within the reported values of bulk SiO2, showing that DEY can be used to detect a surface layer of 12.4 nm thickness with an analysis depth of approximately 4.2 nm and enable `surface EXAFS' analysis using Fourier transformation.

Strain and composition of ultrasmall Ge quantum dots studied by x-ray scattering and in situ surface x-ray absorption spectroscopy

Investigations of the structural properties of ultrasmall Ge dots, grown through a thin silicon oxide layer on Si(001), have been performed by combining (i) grazing incidence x-ray diffraction, (ii) grazing incidence small angle x-ray scattering (GISAXS), and (iii) surface extended x-ray absorption fine structure (SEXAFS). Grazing incidence x-ray diffraction observations show that the dots are highly strained to the Si(001) substrate. The dot shape and composition, as determined by GISAXS experiments and in situ SEXAFS, strongly depend on the thickness and the composition of the silicon oxide layer covering the surface of the silicon substrate. Ge dots grown through 1.2nm thick SiO2 are round shaped with a Ge content of about 60% whereas dots grown through 0.3nm thick SiOx are facetted and composed of almost pure Ge.

Magnetic and structural properties of Mn/InSb(001)

We have investigated the temperature effect on Mn dilution into InSb substrates by means of high-resolution core level spectroscopy, surface EXAFS, and by X-ray Magnetic Circular Dichroism (XMCD) using synchrotron radiation. Samples were prepared by manganese deposition of 1, 2, 4 and 8 ML at room temperature (RT) and at 500K on InSb(001) (4 x 2)c(8 x 2) surfaces. By fitting In 4d and Sb 4d core level spectra with Voigt-functions, we identify metallic In and Sb atoms segregated on the surface. EXAFS results show MnSb-like compound formation with hexagonal structure in which Mn atoms replace some antimony atoms in the first nearest neighbours shell.

Low-coverage condensation of K on TiO 2(1 1 0) 1 × 1

The adsorption site of K on TiO 2(1 1 0) 1 × 1 has been investigated at low coverage (<0.15 ML, where 1 monolayer corresponds to the number of surface unit cells). A combination of surface extended X-ray absorption fine structure (SEXAFS), scanning tunneling microscopy (STM), and non-contact atomic force microscopy (NC-AFM) was used for this purpose. SEXAFS identifies a threefold O hollow site in which K bonds to two bridging O atoms and one in-plane O atom at K–O bond distances of 2.49 ± 0.12 Å and 2.73 ± 0.07 Å. STM and NC-AFM images evidence a condensation of K atoms on either side of the bridging O row.

Substrate Dependence of Self-Assembly of Alkanethiol: X-ray Absorption Fine Structure Study

We have investigated effects of substrates on self-assembly of alkanethiol molecules by means of X-ray absorption fine structure spectroscopy. Molecular orientation of hexanethiolate adsorbed on various substrates such as Cu(111), Ag(111), Ag(100), Si(111), and their oxygen-covered surfaces has been explored by C K-edge and S K-edge near-edge X-ray absorption fine structure (NEXAFS). It was revealed that the alkyl-chain orientation is not correlated to the S−C bond orientation but primarily determined by the molecular density that definitely depends on the substrate surface. Precovered oxygen overlayers do not prevent thiol molecules from being adsorbed but promote their adsorption rate. The surface oxygen atoms are removed via formation and desorption of H2O. The interface structure of a hexanethiolate monolayer on O/Cu(100) has been studied in detail by use of S K-edge surface extended X-ray absorption fine structure (SEXAFS). The SEXAFS results indicated that hexanethiol adsorption induces a lateral displacement of surface Cu atoms toward the sulfur atom of the thiolate with a concomitant breaking of the −O−Cu−O− one-dimensional chain.

Crystallographic structure and morphology of cobalt films on Cu(1111)

Abstract We present a detailed study of the crystallographic structure and morphology of Co/Cu(1 1 11) films by surface extended X-ray absorption fine structure (SEXAFS) and scanning tunneling microscopy (STM). At the very beginning of the growth, the step array is profoundly altered, and large terraces about two times larger than on the clean substrate are formed. For higher thicknesses, islands are observed on the surface. They are first isotropic, and then begin to elongate along the initial step edge direction around 1 ML, and relative straight steps are retrieved around 5 ML. The SEXAFS measurements show that the lattice parameter of cobalt is expanded parallel to the surface but Co is not in a pseudomorphic epitaxy at the beginning of growth because of the relaxation processes at islands free edges. An in-plane structural anisotropy is found only for a Co coverage around 0.7 ML. Upon increasing the thickness, Co adopts a face centred tetragonal structure like on a flat Cu(0 0 1). These results show that the step-induced magnetic anisotropy does not originate from an in-plane crystallographic anisotropy but it is more likely due to the shape anisotropy (i.e. morphology of the films).

On the possibility of eliminating the energy threshold for electron detection in semiconductor detectors

A method of eliminating the energy threshold for electron detection in semiconductor devices is described. The class of devices used for the detection and measurement of electron emission is sufficiently large, including electrostatic analyzers, gas-filled devices, microchannel plates, etc. An alternative to these types of devices for electron detection is offered by semiconductor radiation detectors based on the p-n junctions, which are widely used in the spectrometry of nuclear particles including medium-and high-energy electrons (102 keV and above). These detectors are obviously advantageous in comparison to the devices of other types, but there are several factors hindering the use of semiconductor detectors for the detection and analysis of low-energy electrons (in the kiloelectronvolt range). We propose an approach that allows the energy threshold for electron detection in semiconductor detectors to be eliminated by means of preliminary acceleration of the detected particles in an electrostatic field created between the emitter and the detector. This approach removes the basic factor limiting the use of semiconductor detectors in a number of diagnostic methods based on the analysis of electron emission, such as the extended X-ray absorption fine structure (EXAFS), surface EXAFS, and X-ray absorption near-edge structure (XANES) techniques.

Atomic structure and magnetic properties of Mn on InAs(1 0 0)

We have studied Mn/InAs(001)(4×2)c(8×2) interfaces by high-resolution core-level spectroscopy, surface EXAFS and by X-ray magnetic circular dichroism (XMCD). The growth of manganese is found to depend strongly on the temperature of InAs. At low temperatures (room and liquid nitrogen temperature (LNT)), Mn atoms form the compound MnAs in the surface region leaving metallic In islands on the surface. Manganese deposition at 530K results in strong diffusion of Mn into the substrate followed by formation of the compound In1−xMnxAs in which Mn atoms are substituted in indium-sites. We show by XMCD that In1−xMnxAs is ferromagnetic with a relatively high value of an average magnetic moment per Mn atom, exceeding 0.7μB at low temperatures and with an individual magnetic moment of about 2.4μB. The temperature dependence of the Mn magnetic moment follows the Brillouin function and it drops to zero at temperature of about 150K for measurements in an external magnetic field of 5T.

Magnetism, structure and morphology of thin cobalt films deposited on Cu(1 1 5)

The growth of a cobalt film on a vicinal Cu(1 1 n) surface induces an uniaxial magnetic anisotropy. If the steps are clearly at the origin of this behaviour, the exact mechanism (lower coordination at steps, anisotropic strain) remains obscure. For Co films grown on Cu(1 1 5), a twofold magnetic anisotropy is observed by in situ Kerr effect for thicknesses above 5 monolayers (ML), the easy magnetization axis being always parallel to the step edge. Detailed studies of the morphology by scanning tunneling microscopy and of the structure by Surface EXAFS allow to conclude that the step-induced magnetic anisotropy does not originate from an in-plane anisotropic structural relaxation but is more likely related to the stepped film morphology.

Sulphur adsorption on the fivefold surface of the i-Al–Pd–Mn quasicrystal

The adsorption of sulphur on the fivefold flat-terraced surface of the icosahedral Al–Pd–Mn quasicrystal ( i-Al–Pd–Mn) has been studied using extended X-ray absorption fine structure (EXAFS), Auger electron spectroscopy and low energy electron diffraction (LEED). Surface EXAFS data collected by Auger electron yield from the S saturated surface give average bond lengths for S–Al and S–Pd of 2.10±0.06 and 2.40±0.06 Å respectively. The data indicate that single-site adsorption on the unreconstructed surface may be ruled out; this is consistent with near-edge EXAFS data and the disappearance of the LEED pattern upon adsorption of sub-monolayer amounts of S. The implication of this finding is that adsorbate-induced reconstruction and/or multiple-site adsorption occurs in this system.

Surface EXAFS – A mathematical model

Extended X-ray absorption fine structure (EXAFS) studies are a powerful technique for studying the chemical environment of specific atoms in a molecular or solid matrix. The study of the surface layers of “thick” materials introduces special problems due to the different escape depths of the various primary and secondary emission products which follow X-ray absorption. The processes are governed by the properties of the emitted fluorescent photons or electrons and of the material. Their interactions can easily destroy the linear relation between the detected signal and the absorption cross-section. Also affected are the probe depth within the surface and the background superimposed on the detected emission signal. A general mathematical model of the escape processes is developed which permits the optimisation of the detection modality (X-rays or electrons) and the experimental variables to suit the composition of any given surface under study.

Surface EXAFS study of metastable magnetic thin films.

Epitaxial thin films provide new opportunities to explore the relationship between structure and magnetism. The bidimensionnal character of magnetic films deposited on single-crystal substrates and the occurrence of singular crystallographic structures often confer on these systems electronic and magnetic properties that cannot be found in the bulk solids. Although shape anisotropy would favour an in-plane easy axis of magnetization for thin films, Ni layers deposited on Cu(001) present a perpendicular magnetic anisotropy in a very wide thickness range. It is shown that this can be explained by a distorted structure of Ni, originating from the strain induced by the epitaxy on the Cu substrate. In the field of low-dimensional magnetism, nanostructures with a reduced lateral dimension are now being widely investigated in view of their technological applications. Thin Fe layers on MgO(001) can be cut into strips by the 'atomic saw' method: a compression of the substrate induces a dislocation slipping which 'saws' both the substrate and the Fe film into regular and separated ribbons. The observed magnetic anisotropy, with the easy axis perpendicular to the strips, is explained by a structural relaxation occurring during the structuration process. In these two studies, a precise structural characterization and simple magnetoelastic models allow the magnetic behaviour of the systems to be described. The structure of the films can be described as an elastic deformation of the bulk structure.

Surface EXAFS study of the p4 g(2×2) reconstruction of C on Ni(100) and C on Ni films

We report temperature- and angular-dependent surface extended X-ray absorption fine structure spectroscopy (SEXAFS) measurements of the p4 g(2×2)C/Ni(100) system. Both a Ni(100) single crystal and a 4 monolayer Ni film evaporated on a Cu(100) single crystal were used as substrates. The k-space used for the data analysis needs to be truncated at low k values. This is due to a strong increase of the atomic background close to the edge, which prevents a clear separation of near-edge features from the SEXAFS oscillations. For both substrates the local structure can be described by the ’clockwise’ reconstruction of the surface. This leads to a strong anisotropy of the vibrational amplitudes: the vibrational amplitude for the out-of-plane CNi bond at room temperature is determined to be twice as large as that for the in-plane motion.

Extended X-ray absorption fine structure investigation of the structure of iron overlayers on Cu(111)

Surface extended X-ray absorption fine structure (SEXAFS) has been used to probe the structure of ultrathin films of iron on Cu(111). With increasing film thickness, the interatomic separation falls from 2.57 Å rapidly towards that of bulk iron (2.48 Å). The degree of order and coordination in the films also fall over this region. These changes are consistent with a loss of face-centred cubic structure. However, the structure does not exhibit a clear body-centred cubic SEXAFS fingerprint until around eight layers.

Electrical Double-Layer Structure at the Rutile–Water Interface as Observed in Situ with Small-Period X-Ray Standing Waves

X-Ray standing wave (XSW) measurements were made of Rb and Sr adsorbed from aqueous solutions at the rutile (110)–water interface. These experiments were performed to address the extent to which direct measurements of electrical double-layer structure are possible. The experimental results show that the Bragg XSW technique, using small-period standing waves generated by Bragg diffraction from the substrate, can precisely measure ion locations within the condensed layer and the in situ partitioning of ions between the condensed and diffuse layers. Differences in condensed layer ion positions were observed for Sr ions (measured in situ) as compared with Rb ions (in situ) and also for Sr ions (ex situ). An additional constraint on the ex situ Sr site geometry was provided by polarization-dependent surface EXAFS measurements. Such measurements can provide important constraints for the development and verification of electrical double-layer theory especially as applied to ion adsorption at the solid–water interface.

New opportunities in soft X-ray absorption to characterise the adsorbate bonding

A useful and necessary step to understand the complexity of catalysis is to first study the simple prototype systems which involve elements active in such reactions. Studies throwing light on gas–metal interaction are of vital importance. The X-ray absorption spectroscopy can provide vital information for catalysis as one probes the system on a very local scale. We present state of the art surface EXAFS measurements of carbon, nitrogen and oxygen adsorbed on well characterised metal surfaces (Ni and Cu). The present contribution highlights from experimental measurements supported by improved theoretical calculations that temperature and angular dependent studies are essential for a reliable structural analysis and to gain an insight into absorbate–substrate bonding and kinetics. The low Z metal systems were found suitable to detect unambiguously the photoelectron backscattering by interstitial charge densities.

Surface extended X-ray absorption fine structure investigation of Fe islands grown on c(2×2) N/Cu(100) surface

We present the results of a surface extended X-ray absorption fine structure (SEXAFS) investigation of the growth of Fe islands on the c(2×2) N/Cu(100) surface for different N doses. It is known from scanning tunnelling microscopy (STM) experiments that at subsaturation coverage adsorbed N atoms on Cu(100) form square islands of c(2×2) periodicity and about 5nm×5nm in size. These islands can self-organize in regular patterns. From the persistence of the c(2×2) low energy electron diffraction patterns obtained for any Fe and N coverage we deduce (in agreement with previous STM results) that Fe islands grow on surface areas which are not passivated by N atoms. In this study we estimate that we were able to produce Fe islands thicker than 10 atomic layers, a value higher than that observed using STM. The first shell analysis of SEXAFS data shows that Fe atoms are arranged in an fcc lattice with tetragonal distortion, as observed in Fe/Cu(100) films, with the formation of bilayers and/or coalescence in small islands at the lowest coverage studied (one monolayer).

THE CORRELATION OF LOCAL STRUCTURE AND MAGNETISM IN THIN FERROMAGNETIC FILMS

The most important quantity in magnetism is the magnetic anisotropy energy (MAE). Small MAE corresponds to soft magnets, good for transformers; large MAE corresponds to hard magnets, good for storage media. On a surface and nanometer scale LEED, MEED, X-ray and photoelectron diffraction, surface EXAFS and other techniques have recently made enormous progress in the determination of the real nearest neighbor distances and interlayer spacings within a tenth of an angstrom. These parameters serve as input for ab initio calculations of the MAE. Recent experiments on ultrathin Ni and Co films on Cu and W substrates will be discussed. We show that small displacements from ideal the fcc structure by only 2% may change the MAE by orders of magnitude! Thus for the first time the origin of the MAE in thin films can be understood on the basis of first-principles calculations taking into account the real local, i.e. noncubic perturbed, structure. This will result in an intimate contact between surface structure determination and magnetic properties.

SEXAFS and NIXSW investigation of the [formula omitted] surface

P K-edge surface extended X-ray absorption fine structure (SEXAFS) of the Rh(111)-( 7 × 7 ) R19.1°- P surface is reported. The results indicate that a recently proposed model for this surface is valid in which a mixed overlayer of composition Rh 5P 3 is formed on the Rh(111) surface. However, some modifications to atomic positions are necessary to acheive consistency with the RhP bond lengths determined by SEXAFS; this leads to a reappraisal of our normal incidence X-ray standing wavefield (NIXSW) data from the same surface.