X-ray Standing Wave Analysis Research Articles

Laboratory-based 3D X-ray standing-wave analysis of nanometre-scale gratings.

The increasing structural complexity and downscaling of modern nanodevices require continuous development of structural characterization techniques that support R&D and manufacturing processes. This work explores the capability of laboratory characterization of periodic planar nanostructures using 3D X-ray standing waves as a promising method for reconstructing atomic profiles of planar nanostructures. The non-destructive nature of this metrology technique makes it highly versatile and particularly suitable for studying various types of samples. Moreover, it eliminates the need for additional sample preparation before use and can achieve sub-nanometre reconstruction resolution using widely available laboratory setups, as demonstrated on a diffractometer equipped with a microfocus X-ray tube with a copper anode.

Self-metalation of tetraphenyl porphyrin on Au(111): Structural characterisation via X-ray standing wave analysis

The study of surface-confined reactions provides a route towards characterising the mechanistic processes which underpin heterogeneous catalysis. On-surface reactions provide alternative pathways to solution phase synthesis, and can be studied by a range of techniques which can provide chemical and structural information. Importantly, these kinds of studies give us the insight required to potentially control the selectivity and efficiency of these reactions, as well as allowing for the design of molecular systems with specific functionalities. Porphyrin molecules are frequently selected as participants in these systems as they offer significant customisability, with respect to the reactivity of pendant groups and metal atoms which can be incorporated within the porphyrin macrocycle. Utilising the normal incidence X-ray standing wave (NIXSW) technique in conjunction with scanning probe methods and X-ray photoelectron spectroscopy (XPS) enables the generation of detailed models of on-surface systems, including the adsorption geometries of individual molecules. Here the NIXSW technique has been used to perform structural characterisation of three distinct phases of an on-surface reaction of tetraphenyl porphyrin (TPP) on Au(111); close-packed, diffuse and metalated. Using the chemical specificity of XPS and NIXSW we gain insight into chemical and structural changes exhibited by nitrogen atoms within the porphyrin macrocycle at several stages within the reaction process.

Evolution of adsorption heights in the on-surface synthesis and decoupling of covalent organic networks on Ag(111) by normal-incidence X-ray standing wave.

Structural characterization in on-surface synthesis is primarily carried out by Scanning Probe Microscopy (SPM) which provides high lateral resolution. Yet, important fresh perspectives on surface interactions and molecular conformations are gained from adsorption heights that remain largely inaccessible to SPM, but can be precisely measured with both elemental and chemical sensitivity by Normal-Incidence X-ray Standing Wave (NIXSW) analysis. Here, we study the evolution of adsorption heights in the on-surface synthesis and post-synthetic decoupling of porous covalent triazine-phenylene networks obtained from 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (TBPT) precursors on Ag(111). Room temperature deposition of TBPT and mild annealing to ∼150 °C result in full debromination and formation of organometallic intermediates, where the monomers are linked into reticulated networks by C-Ag-C bonds. Topologically identical covalent networks comprised of triazine vertices that are interconnected by biphenyl units are obtained by a thermally activated chemical transformation of the organometallic intermediates. Exposure to iodine vapor facilitates decoupling by intercalation of an iodine monolayer between the covalent networks and the Ag(111) surface. Accordingly, Scanning Tunneling Microscopy (STM), X-ray Photoelectron Spectroscopy (XPS) and NIXSW experiments are carried out for three successive sample stages: organometallic intermediates, covalent networks directly on Ag(111) and after decoupling. NIXSW analysis facilitates the determination of adsorption heights of chemically distinct carbon species, i.e. in the phenyl and triazine rings, and also for the organometallic carbon atoms. Thereby, molecular conformations are assessed for each sample stage. The interpretation of experimental results is informed by Density Functional Theory (DFT) calculations, providing a consistent picture of adsorption heights and molecular deformations in the networks that result from the interplay between steric hindrance and surface interactions. Quantitative adsorption heights, i.e. vertical distances between adsorbates and surface, provide detailed insight into surface interactions, but are underexplored in on-surface synthesis. In particular, the direct comparison with an in situ prepared decoupled state unveils the surface influence on the network structure, and shows that iodine intercalation is a powerful decoupling strategy.

X-ray standing wave analysis of nanostructures using partially coherent radiation

The effect of longitudinal (or temporal) coherence on total reflection assisted x-ray standing wave (TR-XSW) analysis of nanoscale materials is quantitatively demonstrated by showing how the XSW fringe visibility can be strongly damped by decreasing the spectral resolution of the incident x-ray beam. The correction for nonzero wavelength dispersion (δλ ≠ 0) of the incident x-ray wave field is accounted for in the model computations of TR-XSW assisted angle dependent fluorescence yields of the nanostructure coatings on x-ray mirror surfaces. Given examples include 90 nm diameter Au nanospheres deposited on a Si(100) surface and a 3 nm thick Zn layer trapped on top a 100 nm Langmuir-Blodgett film coating on a Au mirror surface. Present method opens up important applications, such as enabling XSW studies of large dimensioned nanostructures using conventional laboratory based partially coherent x-ray sources.

Model independent x-ray standing wave analysis of periodic multilayer structures

We present a model independent approach for the analysis of X-ray fluorescence yield modulated by an X-ray standing wave (XSW), that allow a fast reconstruction of the atomic distribution function inside a sample without fitting procedure. The approach is based on the direct regularized solution of the system of linear equations that characterizes the fluorescence yield. The suggested technique was optimized for, but not limited to, the analysis of periodic layered structures where the XSW is formed under Bragg conditions. The developed approach was applied to the reconstruction of the atomic distribution function for LaN/BN multilayers with 50 periods of 43 Å thick layers. The object is especially difficult to analyze with traditional methods, as the estimated thickness of the interface region between the constituent materials is comparable to the individual layer thicknesses. However, using the suggested technique, it was possible to reconstruct width of the La atomic distribution showing that the La atoms stay localized within the LaN layers and interfaces and do not diffuse into the BN layer. The analysis of the reconstructed profiles showed that the positions of the center of the atomic distribution function can be estimated with an accuracy of 1 Å.

Atomic-Scale Study of Ambient-Pressure Redox-Induced Changes for an Oxide-Supported Submonolayer Catalyst: VOx/α-TiO2(110)

The activity of supported catalysts is strongly linked to the structure of the surface species. However, direct in situ structural measurements of submonolayer catalysts are lacking. Using in situ X-ray standing wave analysis combined with ex situ X-ray photoelectron spectroscopy and atomic force microscopy, we observed reversible redox-induced surface cation dynamics for three-fourths of a monolayer of vanadia grown by atomic-layer deposition on an α-TiO2(110) surface. On the basis of first-principles density functional theory calculations, we propose vanadia submonolayer structures that agree with and suggest explanations for the observed redox-induced structural changes. Under oxidation conditions, the correlated submonolayer structure is a 2D trimer-decorated polymeric chain along the rutile [−1 1 0] direction; under reducing conditions, the more stable polymeric chain remains unreduced, whereas the less stable decorating trimers become reduced and uncorrelated with the support lattice. These results ...

Terthiophene on Au(111): A scanning tunneling microscopy and spectroscopy study

Terthiophene (3T) molecules adsorbed on herringbone (HB) reconstructed Au(111) surfaces in the low coverage regime were investigated by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra-high vacuum conditions. The 3T molecules adsorb preferentially in fcc regions of the HB reconstruction with their longer axis oriented perpendicular to the soliton walls of the HB and at maximum mutual separation. The latter observation points to a repulsive interaction between molecules probably due to parallel electrical dipoles formed during adsorption. Constant-separation (I-V) and constant-current (z-V) STS clearly reveal the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, which are found at −1.2 eV and +2.3 eV, respectively. The HOMO–LUMO gap corresponds to that of a free molecule, indicating a rather weak interaction between 3T and Au(111). According to conductivity maps, the HOMO and LUMO are inhomogeneously distributed over the adsorbed 3T, with the HOMO being located at the ends of the linear molecule, and the LUMO symmetrically with respect to the longer axis of the molecule at the center of its flanks. Analysis of spectroscopic data reveals details of the contrast mechanism of 3T/Au(111) in STM. For that, the Shockley-like surface state of Au(111) plays an essential role and appears shifted outwards from the surface in the presence of the molecule. As a consequence, the molecule can be imaged even at a tunneling bias within its HOMO–LUMO gap. A more quantitative analysis of this detail resolves a previous discrepancy between the fairly small apparent STM height of 3T molecules (1.4–2.0 nm, depending on tunneling bias) and a corresponding larger value of 3.5 nm based on X-ray standing wave analysis. An additionally observed linear decrease of the differential tunneling barrier at positive bias when determined on top of a 3T molecule is compared to the bias independent barrier obtained on bare Au(111) surfaces. This striking difference of the barrier behavior with and without adsorbed molecules is interpreted as indicating an adsorption-induced dimensionality transition of the involved tunneling processes.

Ion-Irradiated Laterally Graded Ni/C Multilayers: A Combined X-ray Standing Wave and X-ray Reflectivity Analysis

An X-ray standing wave (XSW) experimental facility was set up at the Rossendorf Beam Line (ROBL) at the European Synchrotron Radiation Facility (ESRF). Using this facility, the microstructures and ion-beam induced microstructural modifications of a nanoscale laterally graded Ni/C multilayer systems have been studied by the combined X-ray reflectivity (XRR) and XSW technique. The multilayer stack with 15 Ni/C layer pairs was fabricated on a glass substrate by ion beam sputtering. A 2 MeV Cu2+ ion beam was rastered on the samples to obtain uniformly irradiated strips with fluences from 1×1014 to 7×1014 ions/cm2. We have observed that X-ray reflectivity at the first order Bragg peak gradually increases due to increase in multilayer period in the virgin samples. The multilayer period has expanded and interfaces broadened due to ion irradiation. X-ray standing wave analysis indicates that, during deposition, a significant amount of Ni diffuses into C layers. Up to a certain fluence, we have observed that more and more Ni atoms are incorporated into C layers. At higher fluences, Ni is progressively segregated from the C layers. These mixing and demixing phenomena of Ni in C layers as a function of ion fluence have been explained in terms of two competitive processes including ballistic mixing and chemically guided atomic movements.

Ion-Irradiated Laterally Graded Ni/C Multilayers: A Combined X-ray Standing Wave and X-ray Reflectivity Analysis

Ion-Irradiated Laterally Graded Ni/C Multilayers: A Combined X-ray Standing Wave and X-ray Reflectivity Analysis

Characterization of trace embedded impurities in thin multilayer structures using synchrotron X‐ray standing waves

AbstractX‐ray standing wave (XSW) field generated under Bragg reflection condition in a periodic Mo/Si multilayer structure has been used to determine the concentration and location of various trace element contaminants embedded in different layers of that multilayer structure. We have used intense synchrotron X rays for XSW analysis. It is observed that various trace element impurities such as Cr, Fe, Ni and W get embedded unintentionally in the multilayer structure during the deposition process. Consequences of such impurity incorporation on the optical properties of the multilayer structure are discussed in hard and soft X‐ray regions. Present measurements are important in order to optimize the deposition methods on one hand and to better correlate the measured optical properties of a multilayer structure with theoretical models on the other. Copyright © 2010 John Wiley & Sons, Ltd.

Study of swift heavy-ion-induced modification in Ti/Si using x-ray standing waves

Intermixing in a Si/Ti/Si tri-layer induced by 120 MeV Au ions has been studied. X-raystanding wave analysis combined with x-ray reflectivity has been used to get a depth profileof the Ti marker layer with an accuracy of a fraction of a nanometer. Two differentthicknesses of the Ti marker layer have been used to study the possible effect of layerthickness on intermixing. In the case of a 2 nm thick Ti layer intermixing is stronger ascompared to a 6 nm Ti film, which can be understood in terms of a stronger confinementof the dissipated energy in the Ti layer due to increased interface scattering ofδ-electrons in the case of the 2 nm thick Ti layer. In the 6 nm thick Ti layer, intermixing isasymmetric at the two interfaces, which may be due to a possible asymmetry in theinterface structure in the as-deposited film itself.

X-ray standing wave analysis of overlayer-induced substrate relaxation: The clean and Bi-covered (110) GaP surface

The relaxation of the surface P atoms, for both the clean and Bi-covered GaP(110) surface, was studied with x-ray standing wave (XSW) spectroscopy using surface-sensitive x-ray photoelectron as the XSW modulated signal. The photoemission signal of the outermost surface layer is mixed with the signal from the remaining near surface of the underlying substrate, so further analysis is required to calculate the geometry of the relaxation of the surface atoms. We present a general analysis method for extracting the geometry of the surface reconstruction that minimizes the propagation of the uncertainties associated with fitting XSW data. It takes advantage of the fact that the coherent distance may be more accurately determined than the coherent fraction in XSW data analysis. This method makes use of the electron attenuation length, and shows that the relaxation is only weakly dependent on the uncertainties of this parameter. Results indicate that, for the clean GaP surface, P relaxes with a small outward rotational displacement, with the axis of the rotation located at the second-layer Ga site, whereas, for the Bi-covered case, relaxation consists of a rotation in the opposite direction. The magnitude of the contraction is not negligible, and might be important in the interpretation of low-energy electron diffraction data and in ab initio calculations.

Normal incidence X-ray standing wave analysis of thin gold films

Normal incidence X-ray standing wave (NIXSW) analysis has been successfully performed on epitaxial gold films on mica substrates using reflection from the (1 1 1) planes parallel to the surface. We show that NIXSW can be used to monitor the decrease in order within the gold film caused by annealing, and the position of sulfur within a monolayer of methyl thiolate (CH 3S–) on the surface. The Au–S layer spacing was found to be 2.54 ± 0.05 Å, in close agreement with previous work on a single crystal system.

Chemically Resolved Structure of theSn/Ge(111)Surface

The structure and chemical states of the Sn/Ge(111) surface are characterized by x-ray standing waves combined with photoemission. For the room temperature square root 3xsquare root 3 phase two chemical components, approximately 0.4 eV apart, are observed for both Sn 3d and 4d core levels. Our model-independent, x-ray standing wave analysis shows unambiguously that the two components originate from Sn adatoms located at two different heights separated vertically by 0.23 A, in favor of a model composed of a fluctuating Sn layer. Contrary to the most accepted scenario, the stronger Sn 3d and 4d components, which appear at the lower binding-energy sides and account for 2/3 of the Sn adatoms, are identified to be associated with the higher Sn position, manifesting their filled valence state character.

Kinematical x-ray standing waves for crystal structure investigations

A theoretical and experimental investigation of x-ray standing wave formation in crystals under weak diffracted beam conditions is presented. The standing wave phase dependence is described using the kinematical approximation and is valid at large deviations from the Bragg angle for nonperfect crystals as well as for ideal crystals. This formalism was successfully applied to experimental data from perfect GaAs and CaF{sub 2}/Si and from mosaic Cu{sub 3}Au crystals. Based on these results, a novel method for kinematical x-ray standing wave analysis of nonperfect crystals is proposed.

Comparative X-ray Standing Wave Analysis of Metal−Phosphonate Multilayer Films of Dodecane and Porphyrin Molecular Square

The nanoscale structures of multilayer metal-phosphonate thin films prepared via a layer-by-layer assembly process using Zr(4+) and 1,12-dodecanediylbis(phosphonic acid) (DDBPA) or porphyrin square bis(phosphonic acid) (PSBPA) were studied using specular X-ray reflectivity (XRR), X-ray fluorescence, and long-period X-ray standing wave (XSW) analysis. The films were prepared in 1, 2, 3, 4, 6, and 8 layer series on both Si(001) substrates for XRR and on 18.6 nm period Si/Mo layered-synthetic microstructure X-ray mirrors for XSW. After functionalizing the SiO(2) substrate surfaces with a monolayer film terminated with phosphonate groups, the organic multilayer films were assembled by alternating immersions in (a) aqueous solutions containing Zr(4+)or Hf(4+) (final metal layer only) cations and then (b) organic solvent solutions of PO(3)-R-PO(3)(4-), where R was DDBPA or PSBPA spacer molecule. The Hf(4+) cation served as the marker for the top surface of the films, whereas the Zr(4+) cation was present in all other layers. The PSBPA also contained Zn and Re atoms at its midline which served as heavy-atom markers for each layer. The long-period XSW generated by the 0th- (total external reflection) through 4th-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 which permitted simultaneous analysis of Hf, Zn/Re, and Zr atomic distributions.

X-ray standing wave analysis of theSn∕Si(111)−√3×√3surface

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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.

Depth selective XANES study of swift heavy ion irradiation effects in metal/Si systems

Swift heavy ion induced intermixing of thin metallic layers of Ti, Fe and W with Si has been studied. A single specimen containing 3.0 nm thick layers of the three metals at different depths in Si, ensures a more reliable comparison of the mixing efficiencies of the three systems. X-ray reflectivity and X-ray standing wave analysis has been used to obtain the concentration profile of the three metals before and after irradiation with 100 MeV Au ions. While Ti and Fe exhibit considerable mixing with Si after an irradiation fluence of 1 × 10 13 ions/cm 2, W profile is only slightly broadened. XANES measurements under X-ray standing wave conditions have been able to differentiate between the structure of the center of a layer from that of the interfacial region. It is found that irradiation results in only a further disordering of the central as well as interfacial regions of the W layer. In contrast, the Fe layer gets completely mixed with Si and forms an amorphous silicide.

X-ray standing wave and reflectometric characterization of multilayer structures

Microstructural characterization of synthetic periodic multilayers by x-ray standing waves have been presented. It has been shown that the analysis of multilayers by combined x-ray reflectometry (XRR) and x-ray standing wave (XSW) techniques can overcome the deficiencies of the individual techniques in microstructural analysis. While interface roughnesses are more accurately determined by the XRR technique, layer composition is more accurately determined by the XSW technique where an element is directly identified by its characteristic emission. These aspects have been explained with an example of a 20 period Pt/C multilayer. The composition of the C-layers due to Pt dissolution in the C-layers, Pt$_{x}$C$_{1-x}$, has been determined by the XSW technique. In the XSW analysis when the whole amount of Pt present in the C-layers is assumed to be within the broadened interface, it l eads to larger interface roughness values, inconsistent with those determined by the XRR technique. Constraining the interface roughness values to those determined by the XRR technique, requires an additional amount of dissolved Pt in the C-layers to expl ain the Pt fluorescence yield excited by the standing wave field. This analysis provides the average composition Pt$_{x}$C$_{1-x}$ of the C-layers .