X-ray Standing Wave Study Research Articles

Selective hydrogenation of graphene on Ir(111): an X-ray standing wave study.

A combined high resolution X-ray photoelectron spectroscopy and X-ray standing wave study into the adsorption structure of hydrogenated graphene on Ir(111) is presented. By exploiting the unique absorption profiles and significant modulations in signal intensity found within the X-ray standing wave results, we refine the fitting of the C 1s X-ray photoelectron spectra, allowing us to disentangle the contributions from hydrogenation of graphene in different high-symmetry regions of the moiré supercell. We clearly demonstrate that hydrogenation in the FCC regions results in the formation of a graphane-like structure, giving a standalone component that is separated from the component assigned to the similar structure in the HCP regions. The contribution from dimer structures in the ATOP regions is found to be minor or negligible. This is in contrast to the previous findings where a dimer structure was assumed to contribute significantly to the sp3 part of the C 1s spectra. The corrugation of the remaining pristine parts of the H-graphene is shown to increase with the H coverage, reflecting an increasing number and size of pinning centers of the graphene to the Ir(111) substrate with increasing H exposure.

X-ray standing wave studies of molecular adsorption: why coherent fractions matter

The normal incidence x-ray standing wave (NIXSW) technique is the primary source of quantitative experimental information on the adsorption height of many near-planar molecules on metal surfaces, of relevance to organic electronics. The technique yields two structural parameters, the coherent position and the coherent fraction. For high values of the coherent fraction the coherent position can be related directly to the adsorption height, but because the coherent fraction is effectively an order parameter, low values of this parameter are often attributed to partial disorder. It is certainly true that in depositing these large molecules on a surface it is very challenging to produce a perfectly ordered surface. However, is the type of disorder that is likely to occur able to account for no other effect on measured NIXSW data than a lowering of the coherent fraction? Here we show, by considering a wide range of possible types of ‘disorder’, that in almost all such situations it is improbable that the coherent positions associated with very low coherent fractions actually provide a reliable measure of the adsorption height of the ordered component, as is often assumed to be the case. As such, presentations of NIXSW data that only consider coherent position values are likely to be extremely misleading, and provide an unreliable benchmark for detailed understanding of these interfaces.

X-ray diffraction and X-ray standing-wave study of the lead stearate film structure

A new approach to the study of the structural quality of crystals is proposed. It is based on the use of X-ray standing-wave method without measuring secondary processes and considers the multiwave interaction of diffraction reflections corresponding to different harmonics of the same crystallographic reflection. A theory of multiwave X-ray diffraction is developed to calculate the rocking curves in the X-ray diffraction scheme under consideration for a long-period quasi-one-dimensional crystal. This phase-sensitive method is used to study the structure of a multilayer lead stearate film on a silicon substrate. Some specific structural features are revealed for the surface layer of the thin film, which are most likely due to the tilt of the upper layer molecules with respect to the external normal to the film surface.

X-ray standing wave study of Si clusters on a decagonal Al-Co-Ni quasicrystal surface

Quantitative adsorption structure determinations on quasicrystals are scarce because most techniques for measuring surface structures are not well suited to the complex and infinite unit cells of quasicrystals. The normal incidence standing x-ray wave field technique presents a solution to these problems because it can be made inherently surface sensitive and does not involve extensive computational effort. We describe a method for applying this technique to adsorbates on quasicrystals, with specific application to a submonolayer of Si atoms on a decagonal Al-Co-Ni surface. We demonstrate the sensitivity of the technique to both adsorption site and geometry, leading to the conclusion that the Si atoms, which form six-atom pentagonal clusters, have an average height of $1.77\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\AA{}$ above pentagonal hollow sites, with a significant height variation among the Si atoms in the cluster. In particular, the central Si atom sits more deeply than the five surrounding Si atoms, which are, on average, $2.7\phantom{\rule{0.16em}{0ex}}\AA{}$ away from the central Si atom. Although this study was performed on a decagonal quasicrystal that is periodic perpendicular to the surface, we describe how the technique can be applied to cases with no periodicity.

Normal-incidence x-ray standing-wave study of copper phthalocyanine submonolayers on Cu(111) and Au(111)

Understanding the adsorption and growth mechanisms of large $\ensuremath{\pi}$-conjugated molecules on noble metal surfaces is a crucial aspect for designing and optimizing electronic devices based on organic materials. The investigation of adsorption heights for these molecules on different surfaces can be a direct measure for the strength of the adsorbate-substrate interaction, and gives insight into the fundamental bonding mechanisms. However, the adsorption strength is often also influenced by intermolecular (lateral) interactions which cause, e.g., island formation in the submonolayer regime and influence the adsorption geometry of individual molecules. The lateral structure can then dominate the vertical structure formation and influence the adsorbate-substrate interaction. In this context, the adsorption of copper-phthalocyanines on noble metal surfaces [Au(111), Ag(111), and Cu(111)] represents an ideal model system since the lateral structure formation, as well as the molecular adsorption geometries, strongly depend on coverage and temperature, and hence can be tuned easily. We demonstrate that for CuPc/Au(111), a system dominated by physisorption, the adsorption height of the molecules is independent from the lateral adsorption geometry. In contrast, a strong chemisorption of CuPc on Cu(111) shows a clear gradient in the interaction strength: Individual molecules in diluted phases are significantly stronger bonded than molecules in dense phases. This finding quantifies the increase of the exchange correlation in the binding process, which goes along with the tendency to a more site-specific adsorption geometry at small coverages.

Ab Initio Calculation of the Dispersion Interaction between a Polyaromatic Molecule and a Noble Metal Substrate: PTCDA on Ag(110)

The geometry of PTCDA adsorbed on a (110)-oriented silver crystal is optimized on a finite metal cluster used as a rigid substrate. In a comparison between second-order Moller−Plesset perturbation theory (MP2) and density functional theory (DFT), we find pronounced differences; MP2 gives a nearly flat adsorbate, in agreement with the geometries deduced from X-ray standing wave studies on similar substrates, whereas the lack of dispersion interactions in DFT results in a strongly bent geometry. In MP2, the van-der-Waals attraction counterbalances the overlap repulsion, resulting in an average height of the PTCDA adsorbate above the topmost substrate layer of 2.68 A for C atoms, comparing favorably with recent measurements based on X-ray standing wave techniques. The carboxylic oxygens interact more strongly with the substrate than the anhydride groups, so that we predict a height difference of 0.13 A between both types of oxygen atoms.

Molecular distortion of NTCDA upon adsorption on Ag(111): a normal incidence x-ray standing wave study

We investigated the adsorption geometry of the model system NTCDA (1,4,5,8-naphthalin-tetracarboxylicacid-dianhydride) on Ag(111) using the normal incidence x-ray standing wave (NIXSW) technique. For the relaxed monolayer structure a significant vertical distortion of the molecule is found upon adsorption. The carboxylic oxygen atoms at the corners of the molecule are located 0.25(3) Å closer to the topmost Ag atoms than the naphthalene core at 2.997(16) Å. This distortion effect is similar to that of the larger mole- cule (3,4,9,10-perylene tetracarboxylicacid-dianhydride) (PTCDA)/Ag(111) (Hauschild A et al 2005 Phys. Rev. Lett. 95 209602), but the chemisorptive bonding is weaker. Our structural investigation is based on photoelectron and Auger emission NIXSW data, the independent measurement of which allows us to correct in a self-consistent way for non-dipolar contributions to the photoelectron yield as well as for electron induced Auger processes, two effects which may significantly influence the structural results if not considered.

Substrate-dependent bonding distances of PTCDA: A comparative x-ray standing-wave study on Cu(111) and Ag(111)

We study the adsorption geometry of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on Ag(111) and Cu(111) using X-ray standing waves. The element-specific analysis shows that the carbon core of the molecule adsorbs in a planar configuration, whereas the oxygen atoms experience a non-trivial and substrate dependent distortion. On copper (silver) the carbon rings resides 2.66 A (2.86 A) above the substrate. In contrast to the conformation on Ag(111), where the carboxylic oxygen atoms are bent towards the surface, we find that on Cu(111) all oxygen atoms are above the carbon plane at 2.73 A and 2.89 A, respectively.

Arsenite sorption and co-precipitation with calcite

Sorption of As(III) by calcite was investigated as a function of As(III) concentration, time and pH. The sorption isotherm, i.e. the log Γ As(III) vs. log [As(OH) 3° / As sat] plot is S-shaped and has been modelled on an extended version of the surface precipitation model [Farley, K.J., Dzombak, D.A., Morel, F.F.M., 1985. A surface precipitation model for the sorption of cations on metal oxides. Journal of Colloid and Interface Science 106 (227–242); Wersin, P., Charlet, L., Kathein, R., Stumm, W., 1989. From adsorption to precipitation: sorption of Mn 2+ to FeCO 3(s). Geochimica et Cosmochimica Acta 53, 2787–2796]. At low concentrations, As(OH) 3° is adsorbed by complexation to surface Ca surface sites, as previously described by the X-ray standing wave technique [Cheng, L.W., Fenter, P., Sturchio, N.C., Zhong, Z., Bedzyk, M.J., 1999. X-ray standing wave study of arsenite incorporation at the calcite surface. Geochimica et Cosmochimica Acta 63 (19–20), 3153–3157]. The inflexion point of the isotherm, where As(OH) 3° is limited by the amount of surface sites ( S T), yields 6 sites nm − 2 in good agreement with crystallographic data. Beyond this value, the amount of sorbed arsenic increases linearly with solution concentration, up to the saturation of arsenic with respect to the precipitation of CaHAsO 3(s). The solid solutions formed in this concentration range were examined by X-ray and neutron diffraction. The doped calcite lattice parameters increase with arsenic content while c/ a ratio remains constant. Our results made on bulk calcite on the atomic displacement of As atoms along [0001] direction extend those published by Cheng et al. [Cheng, L.W., Fenter, P., Sturchio, N.C., Zhong, Z., Bedzyk, M.J., 1999. X-ray standing wave study of arsenite incorporation at the calcite surface. Geochimica et Cosmochimica Acta 63 (19–20), 3153–3157] on calcite surface. This study provides a molecular-level explanation for why As(III) is trapped by calcite in industrial treatments.

Publisher's Note: Adsorption-induced distortion of F16CuPc on Cu(111) and Ag(111): An x-ray standing wave study [Phys. Rev. B 71, 205425 (2005)

Publisher's Note: Adsorption-induced distortion of F16CuPc on Cu(111) and Ag(111): An x-ray standing wave study [Phys. Rev. B 71, 205425 (2005)

X-ray standing wave studies of strained InxGa1−xAs/InP short-period superlattices

We report an x-ray standing wave (XSW) study on a set of structurally well-characterized InxGa1−xAs/InP short-period superlattices grown by metal–organic chemical vapor deposition and chemical-beam epitaxy techniques. It was possible to model the x-ray standing wave profiles only once the superlattice period has been assumed to be constituted by four layers of well-defined chemical composition [barrier (InP), first interface (InAs0.7P0.3), well (In0.53Ga0.47As), and second interface (In0.53Ga0.47As0.7P0.3)], and of variable thickness. The thickness of the four layers have been obtained by fitting the high resolution x-ray diffraction profiles of the heterostructures. The presence of partially disordered interface layers, as evidenced by a transmission electron microscopy study, causes a significant reduction of the coherent fraction, F, of both Ga and As atoms. The difference in F values among measured samples illustrates how the XSW can provide important information on the quality of semiconductor superlattices. Comparison with a “long period (160 Å)” In0.53Ga0.47As/InP superlattice, where the role played by InAs0.7P0.3 and In0.53Ga0.47As0.7P0.3 interface layers is negligible, confirms this picture. The coherent fraction of both As and Ga correlates well with the average perpendicular lattice misfit 〈Δa⊥/a〉 determined by x-ray diffraction.

X-ray standing wave study of Si/Ge/Si( [formula omitted]) heterostructures grown with Bi as a surfactant

X-ray standing wave (XSW) analysis was used for an atomic-scale structural study of ultra-thin Si/Ge heterostructures grown on Si(0 0 1) by surfactant mediated epitaxy with Bi as the surfactant. XSW measurements were performed for the Si(0 0 4) and Si(0 2 2) Bragg reflections for Ge coverages from 1 to 10 monolayers. The measured Ge coherent positions agree with the calculated Ge positions for a tetragonally distorted Ge lattice formed on Si(0 0 1) using continuum elasticity theory. However, the measured Ge coherent fractions are smaller than expected. The quality of the Si cap layer and its registry with the underlying Si(0 0 1) substrate lattice was also determined by combining the XSW technique with evanescent-wave emission.

Normal incidence X-ray standing wave study of Fe on Cu( [formula omitted

The structure of annealed ultra-thin Fe films on Cu(1 1 1) has been investigated using normal incidence X-ray standing wave (NIXSW). Films of between 0.5 and 5.3 monolayer equivalence (MLE) have been deposited in situ and briefly annealed to 513 K. NIXSW measurements of the (1 1 1) and ( 1 ̄ 1 1) reflections using 2p 3/2 photoemission show that the film is pseudomorphic up to 2 MLE, with near complete occupation of the fcc hollow sites. For this regime, the value of the (1 1 1) coherent position is equivalent to a film spacing relative to the lattice of 2.00±0.03 Å. For the 5.3 MLE film, there is a drastic reduction in the ( 1 ̄ 1 1) coherent fraction and shift in the ( 1 ̄ 1 1) coherent position that is consistent with the transformation of the entire film to a Kurjumov-Sachs oriented bcc(1 1 0) structure.

X-ray Standing Wave Studies of Minerals and Mineral Surfaces: Principles and Applications

Research Article| January 01, 2002 X-ray Standing Wave Studies of Minerals and Mineral Surfaces: Principles and Applications Michael J. Bedzyk; Michael J. Bedzyk 1Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, 60208, U.S.A.2Materials Science Division Search for other works by this author on: GSW Google Scholar Likwan Cheng Likwan Cheng 3Environmental Research Division, Argonne National Laboratory, Argonne, Illinois, 60439, U.S.A. Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2002) 49 (1): 221–266. https://doi.org/10.2138/gsrmg.49.1.221 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Michael J. Bedzyk, Likwan Cheng; X-ray Standing Wave Studies of Minerals and Mineral Surfaces: Principles and Applications. Reviews in Mineralogy and Geochemistry 2002;; 49 (1): 221–266. doi: https://doi.org/10.2138/gsrmg.49.1.221 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search With a penetration depth ranging from microns to millimeters, Ångstrom-wavelength X-rays are an ideal probe for studying atomic-scale buried structures found in the natural environment, such as impurities in minerals and adsorbed ions at mineral-water interfaces. But this penetration depth also makes an X-ray beam inherently less useful as a spatially localized probe. Using the superposition of two coherently coupled X-ray beams, however, makes it possible to localize the X-ray intensity into interference fringes of an X-ray standing wave (XSW) field, as illustrated in Figure 11, and thereby attain a spatially localized periodic probe with a length scale... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Interpretation and prediction of triple-layer model capacitances and the structure of the oxide-electrolyte-water interface

The interpretation of mineral-water interactions in processes such as surface-charge development, adsorption of aqueous ions onto surfaces, and the kinetics of dissolution at the scale of the oxide-electrolyte-water interface has been greatly facilitated by models of the electric double layer. In models that explicitly account for adsorption of the electrolyte ions, a critical parameter is the integral electric capacitance that expresses the charge at the surface relative to the drop in electric potential at some distance away from the surface where the electrolyte ions adsorb. Despite the widespread application of such surface complexation models, much uncertainty surrounds the choice of values of the integral capacitance because it appears to depend on the specific oxide and type of electrolyte, yet it cannot be directly measured.In the present study, it is shown that triple-layer model capacitances (C1), obtained in a consistent manner from regression of surface-charge data referring to a wide range of ionic strengths, electrolyte types, and mineral surfaces, fall into two groups: on rutile, anatase, and magnetite, values of C1 increase with decreasing crystallographic radius of the electrolyte cation from Cs+ to Li+; on quartz, amorphous silica, goethite, hematite, and alumina, values of C1 increase with decreasing hydrated electrolyte cation radius from Li+ to Cs+. The triple-layer model capacitances on both groups of solids can be described by a model of the mineral-water interface with physically reasonable parameters consistent with X-ray standing-wave studies of the rutile-water interface (Fenter et al., 2000). The model specifies a layer of chemisorbed water molecules at the surface and a layer of adsorbed electrolyte cations. On rutile, anatase, and magnetite, the layer of chemisorbed water molecules is interpenetrated by the layer of electrolyte cations that adsorb close to the surface as dehydrated, inner-sphere complexes. On quartz, amorphous silica, goethite, hematite, and alumina, the layer of chemisorbed water molecules varies from 0.0 to as much as 6.0 Å and the electrolyte cations form hydrated, outer-sphere complexes. The model capacitances are consistent with interfacial dielectric constants ranging from 20 to 62. Triple-layer model capacitances can now be predicted for oxides in either alkali or alkaline earth electrolyte solutions that have not been studied experimentally. In addition, predictions can be made of the structure of the oxide-electrolyte-water interface for many oxides and electrolytes.

Direct Profiling and Reversibility of Ion Distribution at a Charged Membrane/Aqueous Interface: An X-ray Standing Wave Study

Thermodynamic reversibility of ion binding at a membrane/aqueous interface has been demonstrated using variable period X-ray standing waves generated above a gold mirror. The interface consists of a negatively charged and polymerized phospholipid monolayer bathed in a dilute zinc chloride solution. Zinc ion distribution in the diffuse double layer was monitored as the pH of the aqueous medium was titrated from a value of 5.8 to 2.0 and back again to pH 5.8. The ion distribution was modeled by an exponentially decaying function. As H+ concentration increased upon changing pH from 5.8 to 2.0, the amount of zinc in the diffuse double layer (integrated as a two-dimensional projection along the surface normal) decreased from 1.4 × 1013 to 1.0 × 1012 ions/cm2 and the concentration of bound zinc at the surface dropped by a factor of 6.4 from an initial value of 122 mM to a low of ca. 19 mM. Recovery of the pH 5.8 distribution profile was essentially complete upon back-titration from pH 2.0. These results demonstrate clearly that within the experimental error of the measurement ion distribution at the charged membrane interface is reversible upon adjusting the pH of the electrolyte solution which corresponds to altering the degree of protonation and, thus, the net charge on the exposed phosphate headgroups.

X-ray standing wave study of MnTe fractional monolayers inserted in CdTe (001)

The x-ray standing wave method is used in correlation with reflection high-energy electron diffraction and scanning tunneling microscopy to investigate the crystallographic features of MnTe monolayers inserted in CdTe (001). Either conventional molecular beam epitaxy or atomic layer epitaxy were employed for the formation of the CdTe starting surface, the deposition of the MnTe fractional monolayer, and its encapsulation by CdTe. Significant differences concerning the ratio of Mn atoms involved in MnTe clusters to those incorporated as part of a CdMnTe alloy are observed between the samples. Those differences are due to differing CdTe starting surface roughness.

X-ray standing wave study of wet-etch sulphur-treated InP(100) surfaces

Normal incidence X-ray standing wave (NIXSW) measurements are presented for sulphur terminated InP(100) surfaces, prepared by wet-etch in (NH 4) 2S solution followed by UHV anneal. Standing wave profiles for sulphur 1s photoyield in three planes (220, 311 and 31 1 ) indicate that sulphur is close to the phosphorus site. Coherent positions and fractions are compared with the predictions of a number of models of the surface, including novel 2×2 structures. The experimental data can be reproduced by a general two-dimer model, but it is more likely that the data are reflecting significant disorder.

Chemical-shift X-ray standing wave studies: coadsorption site determination of PFx fragments on Ni(111)

Abstract Using P 1s photoemission as a spectral fingerprint, the X-ray-induced fragmentation of PF 3 adsorbed on Ni(111) has been characterised at both room temperature and 140 K. At room temperature only a single fragment is seen, but at low temperature three different phosphorus states are seen, corresponding to two different fragments, in addition to intact PF 3 . Full normal-incidence X-ray standing wave (NIXSW) {111} site triangulation has been performed on each of the chemically shifted P 1s photoemission signals in order to establish the local adsorption sites, and to aid the species identification. At room temperature the single fragment is proposed to be atomic P, which occupies fcc hollow sites with a P–Ni nearest neighbour distance consistent with a phosphide. At 140 K the three P states are assigned to PF 2 occupying bridge sites, and the two inequivalent P atoms of a P 2 F x species, bonded to the surface through one P atom that occupies fcc hollow sites. A scheme of surface reactions is proposed to account for these conclusions and to reconcile the results with those of earlier structural and spectroscopic studies of this system.

Angular dependence in photoemission: from atoms to surfaces to atoms

Abstract Some aspects of atomic and solid-state contributions to the angular dependence of photoemission from core levels, and their relevance and utility in the study of surfaces, are reviewed from a personal and historical perspective. In particular, the role played by dipole angular dependence in photoelectron diffraction and X-ray absorption spectroscopy, and by both dipole and non-dipole effects in X-ray standing wave studies are highlighted.