X-ray Standing Wave Measurements Research Articles

Probing Surface-Adlayer Conjugation on Organic-Modified Si(111) Surfaces with Microscopy, Scattering, Spectroscopy, and Density Functional Theory

Highly conjugated molecules bound to silicon are promising candidates for organosilicon electronic devices and sensors. In this study, 1-bromo-4-ethynylbenzene was synthesized and reacted with a hydrogen-passivated Si(111) surface via ultraviolet irradiation. Through an array of characterization and modeling tools, the binding configuration and morphology of the reacted molecule were thoroughly analyzed. Atomic force microscopy confirmed an atomically flat surface morphology following reaction, while X-ray photoelectron spectroscopy verified reaction to the surface via the terminal alkyne moiety. In addition, synchrotron X-ray characterization, including X-ray reflectivity, X-ray fluorescence, and X-ray standing wave measurements, enabled sub-angstrom determination of the position of the bromine atom with respect to the silicon lattice. This structural characterization was quantitatively compared with density functional theory (DFT) calculations, thus enabling the π-conjugation of the terminal carbon atom...

Molecular organization in protein-lipid film on the water surface studied by x-ray standing wave measurements under total external reflection

The x-ray standing wave method has been applied to study self-assembling processes in a protein-lipid film formed by injecting the protein-lipid mixture of alkaline phosphatase and phosphatidylinositol under the phospholipid monolayer preliminarily deposited on the water subphase by Langmuir method. X-ray standing wave measurements allowed to determine the composition of the protein-lipid film and to locate ions position in the direction normal to the film surface. The presence of trace Ni contamination incorporated in the protein-lipid film from the water subphase has been established. Numerical analysis of the X-ray standing wave fluorescence data revealed that after injection under the phospholipid monolayer, the protein-lipid mixture separated in a self-assembled manner to layered structure, molecules of alkaline phosphatase arranged themselves into a pure protein layer containing no phospholipid molecules.

Microstructural evolution, atomic migration, and FePt nanoparticle formation in ion-irradiated Pt(Fe)/C(Fe) multilayers

Ion-irradiation-induced microstructural modifications, atomic migration, and nanoparticle formation in nanoscale periodic Pt/C multilayer systems containing about 15 at. % Fe have been analyzed by x-ray reflectivity, x-ray standing wave (XSW), transmission electron microscopy, and grazing incidence x-ray diffraction (GIXRD) studies. The multilayer stack with 15 Pt(Fe)/C(Fe) layer pairs (period 4.2 nm) was irradiated with a 2 MeV Au2+ ion beam, which was rastered on the sample to obtain uniformly irradiated strips with fluences (ϕ) from 1×1014 to 1×1015 ions/cm2. Up to a certain threshold ion fluence (ϕt) we have observed mixing between Pt and C atoms. At higher fluences (ϕ>ϕt) demixing effect is pronounced. This phenomenon has been explained in terms of two competitive processes including ballistic and chemically guided atomic movements. As revealed by XSW measurements, ion irradiation causes preferential migration of Fe toward Pt layers from C layers. Consequently Fe concentration in the Pt layers increases with a corresponding decrease in the C layers. The decrease in Fe concentration in the C layers has an exponential dependence on the ion fluence. From the gradual change in shape of the Fe fluorescence yield curve, over the first order Bragg peak region, due to ion irradiation we can infer that Fe atoms migrate from C to Pt layers in the outward direction. This outward migration of Fe has been explained in terms of surface segregation of Fe in Pt under the irradiation condition. Cross-sectional transmission electron microscopy has revealed the gradual change in the microstructure, considerable atomic redistribution, and cluster formation for irradiation at the highest ion fluence (1×1015 ions/cm2), where FePt nanoparticle formation is observed in GIXRD experiments. The results and analysis presented in this work provide an understanding of the evolution of ion-beam-induced ferromagnetism and the increasing coercive field with ion fluence in the Pt(Fe)/C(Fe) multilayer system [B. N. Dev et al., Microelectronic Eng. 83, 1721 (2006)].

Bridging arsenate surface complexes on the hematite (0 1 2) surface

The fate of the oxoanion arsenate in diverse systems is strongly affected by its adsorption on the surfaces of iron (oxyhydr)oxide minerals. Predicting this behavior in the environment requires an understanding of the mechanisms of arsenate adsorption. In this study, the binding site and adsorption geometry of arsenate on the hematite (0 1 2) surface is investigated. The structure and termination of the hematite (0 1 2)-water interface were determined by high resolution X-ray reflectivity, revealing that two distinct terminations exist in a roughly 3:1 proportion. The occurrence of multiple terminations appears to be a result of sample preparation, and is not intrinsic to the hematite (0 1 2) surface. X-ray standing wave (XSW) measurements were used to determine the registry of adsorbed arsenate to the hematite structure, and thus the binding site and geometry of the resulting surface complex. Arsenate forms a bridging bidentate complex on two adjacent singly coordinated oxygen groups on each of the two distinct terminations present at the hematite surface. Although this geometry is consistent with that seen in past studies, the derived As–Fe distances are longer, the result of the topology of the FeO 6 octahedra on the (0 1 2) surface. As EXAFS-derived As–Fe distances are often used to determine the adsorption mechanism in environmental samples (e.g., mine tailings, contaminated sediments), this demonstrates the importance of considering the possible sorbent surface structures and arrangements of adsorbates when interpreting such data. As multiple functional groups are present and multiple binding geometries are possible on the hematite (0 1 2) surface, the XSW data suggest that formation of bridging bidentate surface complexes on singly coordinated oxygen sites is the preferred adsorption mechanism on this and most other hematite surfaces (provided those surfaces contain adjacent singly coordinated oxygen groups). These measurements also constrain the likely reaction stoichiometry, with only the protonation state of the surface complex undetermined. Although bridging bidentate inner-sphere surface complexes comprised the majority of the adsorbed arsenate present on the hematite (0 1 2) surface, there is an additional population of sorbed arsenate species that could not be characterized by the XSW measurements. These species are likely more disordered, and thus more weakly bound, than the bridging bidentate complexes, and may play a role in determining the fate, transport, and bioavailability of arsenate in the environment. Finally, the possibility of obtaining species-specific XSW measurements by tuning the incident beam energy to specific features in a XANES spectrum is described.

Arrangement of trace metal contaminations in thin films of liquid crystals studied by X-ray standing wave technique

X-ray standing wave method has been applied to investigate the molecular organization in Langmuir–Blodgett films of liquid-crystalline lanthanide complex deposited on silicon substrates. The X-ray standing waves measurements were carried out at BESSY II on the beamline KMC-2. Energy spectra of characteristic fluorescence emitted from the samples have been recorded at each point in the X-ray reflectivity curve as the incident angle was scanned through the total external reflection region. The integrated intensity under selected peaks was analyzed as a function of the incident angle. Incorporation of metal ions (Fe, Zn, Cu, Ca) to the Langmuir monolayer from the triply distilled water subphase has been established. Owing to the element selectivity of X-ray standing wave technique the arrangement of several types of metal ions inside the Langmuir–Blodgett films was studied individually. The experimental data revealed that the shape of the fluorescence curves for the contamination metal ions is distinctly different indicating considerable differences in the lateral distribution of these ions in the film. These results may be attributed to the phase separation in Langmuir monolayer of liquid-crystalline lanthanide complex due to the incorporation of metal ions from the water subphase.

Structure of hydrated Zn 2+ at the rutile TiO 2 (110)-aqueous solution interface: Comparison of X-ray standing wave, X-ray absorption spectroscopy, and density functional theory results

Adsorption of Zn 2+ at the rutile TiO 2 (110)-aqueous interface was studied with Bragg-reflection X-ray standing waves (XSW), polarization-dependent surface extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) calculations to understand the interrelated issues of adsorption site, its occupancy, ion–oxygen coordination and hydrolysis. At pH 8, Zn 2+ was found to adsorb as an inner-sphere complex at two different sites, i.e., monodentate above the bridging O site and bidentate between two neighboring terminal O sites. EXAFS results directly revealed a four or fivefold first shell coordination environment for adsorbed Zn 2+ instead of the sixfold coordination found for aqueous species at this pH. DFT calculations confirmed the energetic stability of a lower coordination environment for the adsorbed species and revealed that the change to this coordination environment is correlated with the hydrolysis of adsorbed Zn 2+. In addition, the derived adsorption locations and the occupancy factors of both sites from three methods agree well, with some quantitative discrepancies in the minor site location among the XSW, EXAFS, and DFT methods. Additional XSW measurements showed that the adsorption sites of Zn 2+ were unchanged at pH 6. However, the Zn 2+ partitioning between the two sites changed substantially, with an almost equal distribution between the two types of sites at pH 6 compared to predominantly monodentate occupation at pH 8.

Imaging of Atomic Layer Deposited (ALD) Tungsten Monolayers on α-TiO2(110) by X-ray Standing Wave Fourier Inversion

A single atomic layer of tungsten grown by atomic layer deposition (ALD) on a single-crystal rutile TiO2(110) support is studied by the X-ray standing wave (XSW) technique. The surface structural and chemical properties were also examined using atomic force microscopy, X-ray photoelectron spectroscopy, and low-energy electron diffraction. The XSW measured set of hkl Fourier components for the W atomic distribution function are summed together to produce a model-independent 3D map of the W atoms relative to the rutile lattice. The 3D atomic image shows surface tungsten atoms equally occupying the two nonequivalent Ti sites with a slight outward displacement. This corresponds to the atop and bridge sites with respect to the underlying lattice oxygen atoms. These XSW measurements clearly show that ALD conformal layers can be highly coherent with respect to the substrate lattice.

Structural investigations of GaN films with X‐ray standing waves

AbstractThe technique of X‐ray standing waves (XSW) was applied for the first time to thick undoped GaN films grown on (0001) sapphire substrates. The film polarity was determined unambiguously in (006) backscattering geometry, which proves the feasibility of XSW measurements on this system. Reflectivity measurements yielded a significant decrease of the crystal quality already at a Mg dopant concentration as low as 4 × 1018cm–3. For a Mg dopant concentration of (3‐4)×1019cm–3, crystallites with a difference in the lattice constant of Δc/c = 0.51‰ are found, which impedes XSW investigations on thick doped crystals. In order to solve this problem, we propose a modified sample structure, consisting of a thin doped layer on top of a thick undoped GaN film. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct Observation of Cations and Polynucleotides Explains Polyion Adsorption to Like-Charged Surfaces

We show an experimental approach for directly observing the condensation of polynucleotides and their electrolyte counterions at a liquid/solid interface. X-ray standing waves (XSW) generated by Bragg diffraction from a d = 20 nm Si/Mo multilayer substrate are used to measure the distinct distribution profiles of the polyanions and simple cations along the surface normal direction with subnanometer resolution. The 1D spatial sensitivity of this approach is enhanced by observing the XSW induced fluorescence modulations over multiple orders of Bragg peaks. We study the interesting divalent cation driven adsorption of anionic polynucleotides to anionic surfaces by exposing a hydroxyl-terminated silica surface to an aqueous solution with ZnCl2 and mercurated poly-uridylic acid (a synthetic RNA molecule). The in situ long-period XSW measurements are used to follow the evolution of both the Zn and Hg distribution profiles during the adsorption process. The conditions and physical mechanisms that govern the observed divalent cation adsorption and subsequent polynucleotide adsorption to an anionic surface are explained by a thermodynamic model that incorporates nonlinear electrostatic effects.

X-ray standing wave investigation of submonolayer barium and strontium surface phases on Si(001)

Submonolayer (2 x 1) Ba/Si(001) and (2 x 1) Sr/Si(001) surface phases have been studied with normal Si(004) and off-normal Si(022) x-ray standing wave (XSW) measurements. These results are compared to previous XSW measurements of a (2 x 3) phase of Sr on Si(001). The (004) and (022) coherent positions indicate that alkaline-earth metal (AEM) atoms occupy the same type of high symmetry site in the (2 x 1) Ba/Si(001), (2 x 1) Sr/Si(001), and (2 x 3) Sr/Si(001) surface phases. The difference between the AEM adatom heights measured for the (2 x 1) Ba/Si(001) and (2 x 1) Sr/Si(001) phases is consistent with the difference in the ionic radii of Ba and Sr. Consideration of the nearest neighbor AEM-Si bond lengths that would result from AEM atoms sitting on symmetrically dimerized Si(001) at the height specified by the Ba and Sr coherent positions strongly suggests that the AEM atoms occupy valley-bridge sites.

X-ray standing wave investigations of Si dopant incorporation in GaN

AbstractThe synchrotron radiation technique of x-ray standing waves (XSW), which allows to directly obtain structural and element-specific data, was successfully used for the investigation of the dopant site distribution in Si doped GaN films grown on (0001) sapphire substrates for the first time. The Si dopant concentration was chosen to 5×1018 cm-3 and 5×1019 cm-3. The measurements were performed on 300 nm thin doped films deposited on high-quality thick undoped GaN films. With this sample structure, influences of the Si dopant induced decrease of the crystalline quality on the XSW signal are suppressed. The XSW data are compared to those obtained from thick homogeneously doped GaN films. All XSW measurements were performed in (002) backscattering geometry. Independent of the dopant concentration, the results indicate that Si atoms are solely incorporated on substitutional Ga sites.

X-ray nanoscale profiling of layer-by-layer assembled metal/organophosphonate films.

The nanoscale structure of multilayer metal/phosphonate thin films prepared via a layer-by-layer assembly process was studied using specular X-ray reflectivity (XRR), X-ray fluorescence (XRF), and long-period X-ray standing wave (XSW) analysis. After the SiO(2) X-ray mirror surfaces were functionalized with a monolayer film terminated with phosphonate groups, the organic multilayer films were assembled by alternating immersions in (a) aqueous solutions containing Zr(4+), Hf(4+), or Y(3+) cations and then (b) organic solvent solutions of PO(3)-R-PO(3), where R was a porphyrin or porphyrin-square spacer molecule. The different heavy metal cations provided X-ray fluorescence marker layers at different heights within the different multilayer assemblies. The XSW measurements used a 22 nm period Si/Mo multilayer mirror. The long-period XSW generated by the zeroth-order (total external reflection) through fourth-order Bragg diffraction conditions made it possible to examine the Fourier transforms of the fluorescent atom distributions over a much larger q(z)() range in reciprocal space than previously achieved.

Ion Adsorption at the Rutile−Water Interface: Linking Molecular and Macroscopic Properties

A comprehensive picture of the interface between aqueous solutions and the (110) surface of rutile (alpha-TiO2) is being developed by combining molecular-scale and macroscopic approaches, including experimental measurements, quantum calculations, molecular simulations, and Gouy-Chapman-Stern models. In situ X-ray reflectivity and X-ray standing-wave measurements are used to define the atomic arrangement of adsorbed ions, the coordination of interfacial water molecules, and substrate surface termination and structure. Ab initio calculations and molecular dynamics simulations, validated through direct comparison with the X-ray results, are used to predict ion distributions not measured experimentally. Potentiometric titration and ion adsorption results for rutile powders having predominant (110) surface expression provide macroscopic constraints of electrical double layer (EDL) properties (e.g., proton release) which are evaluated by comparison with a three-layer EDL model including surface oxygen proton affinities calculated using ab initio bond lengths and partial charges. These results allow a direct correlation of the three-dimensional, crystallographically controlled arrangements of various species (H2O, Na+, Rb+, Ca2+, Sr2+, Zn2+, Y3+, Nd3+) with macroscopic observables (H+ release, metal uptake, zeta potential) and thermodynamic/electrostatic constraints. All cations are found to be adsorbed as "inner sphere" species bonded directly to surface oxygen atoms, while the specific binding geometries and reaction stoichiometries are dependent on ionic radius. Ternary surface complexes of sorbed cations with electrolyte anions are not observed. Finally, surface oxygen proton affinities computed using the MUSIC model are improved by incorporation of ab initio bond lengths and hydrogen bonding information derived from MD simulations. This multitechnique and multiscale approach demonstrates the compatibility of bond-valence models of surface oxygen proton affinities and Stern-based models of the EDL structure, with the actual molecular interfacial distributions observed experimentally, revealing new insight into EDL properties including specific binding sites and hydration states of sorbed ions, interfacial solvent properties (structure, diffusivity, dielectric constant), surface protonation and hydrolysis, and the effect of solution ionic strength.

X-ray standing wave study of the Sr/Si( [formula omitted])-( [formula omitted]) surface

Sub-monolayer surface phases of Sr on Si(0 0 1) have been studied with low-energy electron diffraction (LEED) and X-ray standing waves (XSW). A (3 × 1) phase was observed after depositing 0.6–0.8 ML Sr on room-temperature Si(0 0 1). Annealing at 750–800 °C caused a portion of the Sr to desorb and resulted in a sharp (2 × 3) LEED pattern. Normal Si(0 0 4) and off-normal Si(0 2 2) and Si(1 1 1) XSW measurements made on the (2 × 3) phase indicate that Sr atoms must sit at either cave or bridge sites. The XSW results also suggest that if a sufficiently low anneal temperature is used, the (2 × 3) phase co-exists with short-range ordered regions of Sr atoms located at valley-bridge sites.

Habit modification of nearly perfect single crystals of potassium dihydrogen phosphate (KDP) by trivalent manganese ions studied using synchrotron radiation X-ray multiple diffraction in Renninger scanning mode

The X-ray multiple diffraction technique using synchrotron radiation is applied in the preliminary study of the habit modification of KDP samples as induced by incorporation of the trivalent transition metal cation Mn3+. High-resolution Renninger scans of pure and doped KDP were carried out using 400 as the primary reflection, echoing the fact that these impurity species were segregated in the {100} growth sector. The analysis of the Renninger scans of the doped KDP crystals is consistent with the presence of the impurity species chemically bound within the KDP crystal structure, as confirmed through the suppression of the huge observed peak asymmetry, characteristic of perfect crystals. In addition, an extra Renninger-scan peak measured for the doped material is indicative of the impurity atoms occupying interstitial crystallographic sites in the lattice, a result consistent with X-ray standing-wave measurements. Renninger-scan reflection curve widths and lattice parameter measurements reveal the decrease in crystalline perfection (increased mosaic spread, η) and lattice contraction ofca0.4% in theaandclattice directions at the surface plane for the Mn3+-doped KDP samples in comparison with the undoped crystals.

Fourier-expansion solution of atom distributions in a crystal using X-ray standing waves.

Term-by-term Fourier-expansion series, each made up of components having element-specific phases and amplitudes acquired with x-ray standing wave measurements on successive orders of Bragg reflections, are used to reconstruct impurity atom distributions in muscovite mica with respect to the (001) lattice without a priori assumptions on their structures.

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.

Isotopic mass and lattice constants of Si and Ge: X-ray standing wave measurements

Isotopic mass and lattice constants of Si and Ge: X-ray standing wave measurements

Total-Electron-Yield X-Ray Standing-Wave Measurements of Multilayer X-Ray Mirrors for Interface Structure Evaluation

Total-electron-yield (TEY) X-ray standing-wave measurements of multilayer X-ray mirrors by monitoring sample photocurrent have been performed to obtain information on their layer/interface structure. This simple TEY X-ray standing-wave method enables simultaneous spectral measurement of the X-ray standing-wave and Bragg reflection. From simultaneous measurement of TEY X-ray standing-wave and Bragg reflection spectra of Mo/SiC/Si multilayers, shrinkage of the layer structure was observed along with disordering at the interface by annealing. Mapping measurements of the X-ray standing-wave signals in a Mo/Si multilayer can also be achieved on normal incidence, visibly illustrating the spatial distribution of interface structure in the sample plane.

Low-temperature interface structure ofCaF2/Si(111)studied by combining x-ray standing waves with component-resolved photoemission

X-ray photoelectrons excited by x-ray standing waves (XSW) are used to study the atomic structure of the low-formation-temperature (370°C) interface between CaF 2 and Si(111). The core-level shift of the photoemission spectra of the Ca atoms at the CaF 2 /Si interface is used to distinguish interface Ca atoms from atoms in other (bulk) layers in the XSW measurements. Therefore, we obtained quantitative structure information specific to the buried CaF 2 /Si interface avoiding some of the ambiguities of XSW. Even at the low growth temperatures used here, the interface is well ordered, with interface Ca atoms exclusively adsorbed on T 4 sites. The majority of the interface layer has CaF stoichiometry. The CaF 2 films consist of domains with type-A and type-B orientation.