X-ray Standing Wave Experiments Research Articles

An Efficient Algorithm for Automatic Structure Optimization in X-ray Standing-Wave Experiments

X-ray standing-wave photoemission experiments involving multilayered samples are emerging as unique probes of the buried interfaces that are ubiquitous in current device and materials research. Such data require for their analysis a structure optimization process comparing experiment to theory that is not straightforward. In this work, we present a new computer program for optimizing the analysis of standing-wave data, called SWOPT, that automates this trial-and-error optimization process. The program includes an algorithm that has been developed for computationally expensive problems: so-called black-box simulation optimizations. It also includes a more efficient version of the Yang X-ray Optics Program (YXRO) [Yang, S.-H., Gray, A.X., Kaiser, A.M., Mun, B.S., Sell, B.C., Kortright, J.B., Fadley, C.S., J. Appl. Phys. 113, 1 (2013)] which is about an order of magnitude faster than the original version. Human interaction is not required during optimization. We tested our optimization algorithm on real and hypothetical problems and show that it finds better solutions significantly faster than a random search approach. The total optimization time ranges, depending on the sample structure, from minutes to a few hours on a modern computer, and can be up to 100x faster than a corresponding manual optimization. These speeds make the SWOPT program a valuable tool for real-time analyses of data during synchrotron experiments.

Long Vertical Distance Bonding of the Hexagonal Boron Nitride Monolayer on the Cu(111) Surface

The hexagonal boron nitride (hBN) monolayer on the Cu(111) surface has recently been considered an example of an extremely weak hBN/metal interaction, as indicated, e.g., from the presence of an only electronic Moire-like superstructure that was observed in scanning tunneling microscopy images. From these results, a large bonding distance of the hBN sheet to the topmost Cu layer can be envisaged but has not been proven so far. We report a structural analysis of the hBN/Cu(111) interface based on high resolution low energy electron diffraction and normal incidence X-ray standing wave experiments. We find that both the boron and nitrogen atoms are located at very large vertical distances of dB = 3.25 ± 0.02 A and dN = 3.22 ± 0.03 A with respect to the nominal position of the topmost Cu(111) layer. Significant vertical buckling and lateral distortions of the hBN layer can be excluded. These results demonstrate that the hBN monolayer on the Cu(111) surface is indeed well described by a rigid and geometrically...

Non-dipolar effects in photoelectron-based normal incidence X-ray standing wave experiments

The normal incidence X-ray standing waves technique is one of the most well-established methods for investigating the geometric structure at interfaces and surfaces. It is able to measure vertical positions and distances of individual atomic species with very high precision (typically <0.02Å). These data not only yield valuable structural information, but also represent an excellent benchmark for density functional theory and ab initio calculations. Non-dipolar effects are well known to strongly affect the result, in particular when light elements are involved. A correction mechanism for these effects is established, but in its commonly-used form it is based on one essential restriction, namely the assumption of perfect normal incidence of the X-rays with respect to the relevant lattice planes of the crystal. Here, we show that small deviations from normal incidence, as they are unavoidable in typical experimental setups, lead to significant systematic errors in the NIXSW results. The magnitude of this effect depends on the specific conditions in a non-linear way and may reach up to 5%, corresponding to several tenths of an Ångström in the adsorption height. We present a straightforward way of accounting for this effect, and demonstrate that recording the photoelectron yield in an angular-resolved mode is indispensable, since the correction parameters strongly depend on the electron take-off angle.

Chemisorption of manganese phthalocyanine on Cu(001) surface promoted by van der Waals interactions

van der Waals (vdW) interactions within density functional theory are shown to strongly reduce the distance between manganese phthalocyanine (MnPc) and a Cu(001) surface to that found by x-ray standing wave experiments. Thus, the physisorbed ground state that is predicted within the generalized-gradient approximation formalism is replaced by a chemisorbed ground state once vdW interactions are taken into account. These findings indicate how to systematically obtain the correct theoretical adsorption distance for complex molecules and thus accurately predict the properties of the ensuing molecule/metal interface. The reduction of the experimental work function upon molecular adsorption is satisfactorily accounted for and explained in terms of Friedel-like oscillations of the charge density at the vicinity of the MnPc molecule that change the sign of the charge transfer electric dipole. This shows how vdW interactions can strongly impact charge injection in organic electronic devices.

Atomic Structure and Crystalline Order of Graphene-Supported Ir Nanoparticle Lattices

We present the atomic structure of Ir nanoparticles with 1.5nm diameter at half height and three layers average height grown on graphene/Ir(111). Using surface x-ray diffraction, we demonstrate that Ir nanoparticles on graphene/Ir(111) form a crystallographic superlattice with high perfection. The superlattice arrangement allows us to obtain detailed information on the atomic structure of the nanoparticles themselves, such as size, shape, internal layer stacking and strain. Our experiments disclose that the nanoparticles reside epitaxially on top of the graphene moiré structure on Ir(111), resulting in significant lateral compressive intraparticle strain. Normal incidence x-ray standing wave experiments deliver additional information on the particle formation induced restructuring of the graphene layer.

Theoretical study of PTCDA adsorbed on the coinage metal surfaces, Ag(111), Au(111) and Cu(111)

A thorough understanding of the adsorption of molecules on metallic surfaces is a crucial prerequisite for the development and improvement of functionalized materials. A prominent representative within the class of π-conjugated molecules is 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) which, adsorbed on the Ag(111), Au(111) or Cu(111) surfaces, shows characteristic trends for work-function modification, alignment of molecular levels with the substrate Fermi energy and binding distances. We carried out density functional theory (DFT) calculations to investigate to what extent these trends can be rationalized on a theoretical basis. We used different density functionals (DF) including a fully non-local van der Waals (vdW) DF capable of describing dispersion interactions. We show that, rather independent of the DF, the calculations yield level alignments and work-function modifications consistent with ultra-violet photoelectron spectroscopy when the monolayer is placed onto the surfaces at the experimental distances (as determined from x-ray standing wave experiments). The lowest unoccupied molecular orbital is occupied on the Ag and Cu surfaces, whereas it remains unoccupied on the Au surface. Simultaneously, the work function increases for Ag but decreases for Cu and Au. Adsorption distances and energies, on the other hand, depend very sensitively on the choice of the DF. While calculations in the local density approximation bind the monolayer consistently with the experimental trends, the generalized gradient approximation in several flavors fails to reproduce realistic distances and energies. Calculations employing the vdW-DF reveal that substantial bonding contributions arise from dispersive interactions. They yield reasonable binding energies but larger binding distances than the experiments.

Electron Core-Hole Interaction and Its Induced Ionic Structural Relaxation in Molecular Systems under X-Ray Irradiation

The experiments on perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride and 16FCuPc on Ag(111) surfaces by the normal incidence x-ray standing wave are poorly reproduced by first-principles ground state calculations but well reproduced by considering initial- and final-state effects, which include the response of the valence electrons to screen a core hole created by x-ray photoemission and further induced ionic structural relaxation. This study shows that the initial- and final-state effects due to screening are of great importance in characterizing molecules on metal surfaces in the normal incidence x-ray standing wave experiments.

True Nature of an Archetypal Self-Assembly System: Mobile Au-Thiolate Species on Au(111)

Alkanethiol self-assembled monolayer (SAM) phases on Au(111) have been assumed to involve direct S head group bonding to the substrate. Using x-ray standing wave experiments, we show the thiolate actually bonds to gold adatoms; self-organization in these archetypal SAM systems must therefore be governed by the movement of these Au-S-R moieties on the surface between two distinct local hollow sites on the surface. The results of recent ab initio total energy calculations provide strong support for this description, and a rationale for the implied significant molecular mobility in these systems.

Molecular Distortions and Chemical Bonding of a Largeπ-Conjugated Molecule on a Metal Surface

Normal incidence x-ray standing wave experiments and density functional theory reveal that 3,4,9,10-perylene-tetracarboxylic-dianhydride chemisorbs on Ag(111) in a nonplanar but vertically distorted configuration. The carboxylic O atoms are 0.18 +/- 0.03 angstroms closer to the surface than the perylene core. The distortion is related to weak, local bonds between carboxylic O atoms and the Ag surface which are coupled--through charge transfer into the former lowest unoccupied molecular orbital--to the primary, extended chemisorption bond via the perylene skeleton.

Application of X-ray standing wave (XSW) technique for studies of Zn incorporation in InP epilayers

We present a new approach to determine the site incorporation of p-type impurities in the doped layers of microelectronic device structures based on X-ray standing wave (XSW) technique. Activation behavior of Zn in InP epitaxial layers grown by metal organic vapor phase epitaxy (MOVPE) on InP(1 0 0) substrates has been studied. The XSW experiments were performed at the A2 beamline of the Cornell high-energy synchrotron source (CHESS). Angular dependences of the Zn–K fluorescence intensity excited by the XSW field inside the InP layer for symmetrical (4 0 0) reflection have been measured along with the Indium L-edge and phosphorus K-edge fluorescence and X-ray reflectivity. Analysis of the XSW data based on the dynamical diffraction theory in layered crystal structures allowed us to determine the fractions of both Zn atoms incorporated into crystal lattice and interstitial Zn. In our example, a 1 μm thick MOCVD-grown InP layer with the nominal concentration of Zn atoms of (2.4 ± 0.2) × 10 18 cm −3 has the fraction of the substitutional Zn of (65 ± 5)%. This result is in a good agreement with electrical activation of Zn measured with a combination of SIMS and CV profilometry. The accurate knowledge of the interstitial-to-substitutional ratio as a function of growth conditions is required to optimize electrical activation of Zn and to control its diffusion in the device structures.

Surface complexation of neodymium at the rutile-water interface: A potentiometric and modeling study in NaCl media to 250°C

The adsorption of Nd 3+ onto rutile surfaces was examined by potentiometric titration from 25 to 250°C, in 0.03 and 0.30 m NaCl background electrolyte. Experimental results show that Nd 3+ sorbs strongly, even at low temperature, with adsorption commencing below the pH znpc of rutile. In addition, there is a systematic increase in Nd 3+ adsorption with increasing temperature. The experimental results were rationalized and described using surface oxygen proton affinities computed from the MUlti SIte Complexation or MUSIC model, coupled with a Stern-based three-layer description of the oxide/water interface. Moreover, molecular-scale information was incorporated successfully into the surface complexation model, providing a unique geometry for the adsorption of Nd 3+ on rutile. The primary mode of Nd 3+ adsorption was assumed to be the tetradentate configuration found for Y 3+ adsorption on the rutile (110) surface from previously described in situ X-ray standing wave experiments, wherein the sorbing cations bond directly with two adjacent “terminal” and two adjacent “bridging” surface oxygen atoms. Similarly, the adsorption of Na + counterions was also assumed to be tetradentate, as supported by MD simulations of Na + interactions with the rutile (110) surface, and by analogous X-ray standing wave results for Rb + adsorption on rutile. Fitting parameters for Nd 3+ adsorption included binding constants for the tetradentate adsorption complex and capacitance values for the inner-sphere binding plane. In addition, hydrolysis of the tetradentate adsorption complex was permitted and resulted in significantly improved model fits at higher temperature and pH values. The modeling results indicate that the Stern-based MUSIC surface-complexation model adequately accommodates molecular-scale information to uniquely rationalize and describe multivalent ion adsorption systematically into the hydrothermal regime.

A comparison between Auger and photoelectron emission detection in an X-ray standing-wave analysis of adsorbates

We studied the contribution of secondary electrons emission to the Auger intensity when the adsorption edge is low compared to the photon energ y. We performed a normal incidence X-ray standing-wave (XSW) experiment comparing photoemission K 2s and Auger K-LVV yields of Ni(1 1 1)- p(2×2)K system. The analysis of both profiles permits us to estimate that 5% of the total secondary electrons contributes to the K-L 23VV excitation strongly modifying the line-shape of the potassium Auger profile. In this case, the use of the Auger peak intensity profile as a monitor during an XSW experiment for surface structural studies gives completely unreliable results.

Modeling the surface complexation of calcium at the rutile-water interface to 250°C

The adsorption behavior of metal-(hydr)oxide surfaces can be described and rationalized using a variety of surface complexation models. However, these models do not uniquely describe experimental data unless some additional insight into actual binding mechanisms for a given system is available. This paper presents the results of applying the MUlti SIte Complexation or MUSIC model, coupled with a Stern-based three layer description of the electric double layer, to Ca 2+ adsorption data on rutile surfaces from 25 to 250°C in 0.03 and 0.30 m NaCl background electrolyte. Model results reveal that the tetradentate adsorption configuration found for Sr 2+ adsorbed on the rutile (110) surface in the in situ X-ray standing wave experiments of Fenter et al. (2000) provides a good fit to all Ca 2+ adsorption data. Furthermore, it is also shown that equally good fits result from other plausible adsorption complexes, including various monodentate and bidentate adsorption configurations. These results amply demonstrate the utility of in situ spectroscopic data to constrain surface complexation modeling, and the ability of the MUSIC model approach to accommodate this spectroscopic information. Moreover, this is the first use of any surface complexation model to describe multivalent ion adsorption systematically into the hydrothermal regime.

A structure study of the electroless deposition of Au on Si( [formula omitted]):H

Electroless deposition of gold in monolayer amounts on hydrogen terminated Si(1 1 1) was studied by a variety of structure sensitive techniques. Rutherford back scattering data revealed a linear relationship between the Au coverage and the concentration of the Au solution used for deposition. Atomic force microscopy and surface X-ray diffraction studies indicated cluster formation, with the Au(1 1 1) face epitaxially aligned to the Si(1 1 1):H substrate. However, X-ray standing wave experiments at coverage below one monolayer suggested the formation of a gold layer. Both clusters and isolated Au atoms appear to be present at all coverages, which is explained by the formation of type of Au `wetting layer', in addition to cluster formation which increases as the coverage increases.

Non-dipole effects in photoelectron-monitored X-ray standing wave experiments: characterisation and calibration

The results are reported of a series of experiments designed to characterise and calibrate the role of non-dipolar (notably electric quadrupole–dipole interference) effects in the angular distribution of photoelectron emission on structural studies of surfaces using normal incidence X-ray standing wavefield absorption. The values of the resulting backward/forward asymmetry parameter (relative to the photon propagation direction) have been measured for 1s emission at photon energies around 3 keV for first and second row elements from C to Cl, and are compared with recent theoretical determinations. The influence of additional non-dipole interference terms in the coherent X-ray standing wave has also been determined for the case of Al 1s emission and found to be weak, typically introducing systematic errors in atomic positions of only around 0.02–0.04 Å.

Laterally graded multilayers and their applications

Laterally graded multilayers consisting of uniform W layers and wedge-shaped C layers have been made at the Advanced Photon Source deposition lab for tunable x-ray double-monochromator applications. The double monochromator has two identical graded multilayers in series, as in the conventional double-crystal monochromator arrangement. By letting the x-ray beam hit slightly different (bilayer) d spacing on each multilayer, one can adjust the bandpass and peak energy of the transmitted beam. Also, since the Bragg angles of the two multilayers are not constrained to be the same, the angle of the transmitted beam can be varied in the vertical plane. This option may be an attractive alternative to the conventional way of studying liquid surfaces in reflectivity and grazing-incidence diffraction measurements. The graded multilayer comprised 60 bilayers of W and C on 100×25×3 mm float glass with a d spacing varying from 35 to 60 Å and an average gradient of 0.27 Å/mm in the long direction. The films were made by dc magnetron sputtering with the sputtered atoms passing a contoured mask while the substrate was moving. Two different masks were designed to produce either a uniform (for W) or graded (for C) thickness profile. The multilayer and graded multilayers have many other novel applications. Potential applications in x-ray fluorescence detection and x-ray standing wave experiments will be discussed.

X-Ray Interface Characterization of Buried InAs Layers on GaAs (001)

Ultra thin buried InAs layers on GaAs (001) crystals prepared by molecular beam epitaxy are structurally characterized using synchrotron radiation. Grazing incidence X-ray reflectivity and crystal truncation rods were utilized to determine the average layer thickness, interface roughness, and the stoichiometry of the layers. From X-ray standing wave experiments the In lattice site and vertical distribution are determined. We discuss our results in view of the structural transition of the layer system with In deposition from 1.0 to 2.1 ML.

Ultrahigh-Vacuum Facility for High-Resolution Grazing-Angle X-ray Diffraction at a Vertical Wiggler Source of Synchrotron Radiation

A versatile ultrahigh-vacuum chamber has been designed for grazing-angle X-ray standing-wave and diffraction experiments at the vertical wiggler source of the Photon Factory. Unlike at other sources, the vertically polarized X-rays from the wiggler favour the use of a horizontal scattering geometry. The X-ray chamber is equipped with a hemispherical beryllium window, which allows any scattering angle to be attained and secondary emissions to be measured. The chamber is of a compact design, sitting on a precision rotary table which is rotated for scans. Samples are introduced from a portable vessel. The whole procedure can be performed in a vacuum better than 10(-7) Pa. The system has successfully been applied to a grazing-angle X-ray standing-wave experiment, which determined the dimer bond length and the domain structure of Si(001) surfaces deposited with monolayer arsenic.

Membrane Structure Characterization Using Variable-Period X-Ray Standing Waves

The variable-period x-ray standing wave (XSW) technique is emerging as a powerful tool for studying membrane structure. However, two significant problems arise when the method is used to characterize membranes of thickness d L < 100 Å. First, the surface roughness, σ r, of the supporting reflecting mirror convolutes with the intrinsic half-width of the marker atom distribution in the membrane, σ in, and contributes to an apparent half-width, σ, which is measured in the XSW experiment. Here we show how the latter terms are related quantitatively [ σ in = ( σ 2 − σ r 2) 1/2], such that rough mirrors give rise to larger marker atom distribution widths, σ, and how the required quantity σ in can be determined in the XSW measurement. Second, when the mean position of the marker atom layer, 〈 z〉, is close to one or both boundaries of the membrane, its distribution function is truncated at the boundary. In such cases, we show why marker atom distribution should be expressed in terms of its first and second moments. We also demonstrate by numerical simulations of realistic samples how the physical parameters, σ r, σ, 〈 z〉, and d L, affect x-ray reflectivity and fluorescence yield profiles as an aid in their interpretation.

A New Apparatus for Polarized X-ray Absorption Fine Structure Using a Grazing-Incidence Fluorescence Excitation

A new apparatus is described for surface-sensitive X-ray absorption fine structure (XAFS) at the 27-pole wiggler beamline BL13B at the Photon Factory. The apparatus is designed for polarized XAFS experiments at low temperature. A closed-cycle helium cryostat is rotated around the omega axis of a vertically mounted high-precision goniometer. The apparatus can be used for XAFS and X-ray standing-wave experiments, sharing a 19-element pure-Ge detector array as a fluorescence detector. The available temperature range is 15-300 K. The performance is demonstrated by in-plane polarized Cu K-EXAFS for a YBa2Cu307 thin film (approximately 1000 A) epitaxically grown on an SrTiO3 single crystal. An in-situ study of amorphous selenium under optical excitation revealed that neutral defect pairs (C3(0)-C3(0)) are formed by photoexcitation of lone pairs. This indicates that grazing-incidence fluorescence excitation is advantageous for 'pump-and-probe' experiments which can minimize the mismatch in the probing depth between X-ray and optical excitation.