X-ray Standing-wave Research Articles

X‐ray standing wave assisted XANES for depth dependent chemical state analysis of Cr in Cr2O3/Cr bilayer structure

We report the detailed depth‐dependent structural and chemical analysis of the Cr2O3/Cr bilayer structure deposited on Si ˂100˃ substrate. The non‐destructive simultaneous X‐ray reflectivity and grazing incidence X‐ray fluorescence measurements were used for this purpose. Corresponding variation in the chemical state of Cr atoms as a function of depth has been studied using X‐ray standing wave (XSW) assisted X‐ray absorption near edge structure (XANES) measurements. Various oxidation states of Cr atoms present in the Cr2O3/Cr bilayer structure were determined using X‐ray photoelectron spectroscopy (XPS). Depth‐resolved XANES measurements confirmed the presence of chromium oxide (Cr2O3) and hydroxide (Cr (OH)3) at the top surface of the Cr2O3/Cr bilayer structure. The results also reveal the presence of metallic Cr along with its compounds Cr2O3 and Cr (OH)3 at the interface medium, showing significant mixing between the Cr2O3 and Cr layers. Our results clearly demonstrate that the XSW assisted XANES technique is extremely efficient for determining the variation of chemical states at the surface, interface, and different depths of a thin film structure. Such types of analysis are particularly useful for differentiating the presence of a metal from its own oxides, even at higher depths inside a thin film medium.

Structure of Graphene Grown on Cu(111): X-Ray Standing Wave Measurement and Density Functional Theory Prediction.

We report the quantitative adsorption structure of pristine graphene on Cu(111) determined using the normal incidence x-ray standing wave technique. The experiments constitute an important benchmark reference for the development of density functional theory approximations able to capture long-range dispersion interactions. Electronic structure calculations based on many-body dispersion-inclusive density functional theory are able to accurately predict the absolute measure and variation of adsorption height when the coexistence of multiple moiré superstructures is considered. This provides a structural model consistent with scanning probe microscopy results.

Atomic-Scale Interface for Pt Nanoparticles on SrTiO3 (001).

The interfacial structure formed by Pt nanoparticles grown epitaxially on a SrTiO3 (001) surface by pulsed laser deposition was studied by X-ray standing-wave (XSW) excited core-level photoelectron emission. The XSW-generated 3D atomic map of the Pt and interfacial oxygens for the oxidized Pt/SrTiO3 interface differs significantly from that of the as-deposited interface. After oxidation, the Pt atoms shifted upward and their atomic occupation at fcc-like sites evolved as the oxidation temperature increased. Interfacial oxygen atoms were differentiated from bulk O atoms by the chemical shift in the binding energy of their 1s electrons. After oxidation, the interfacial oxygen atoms rearranged to form a TiO2 bilayer at the interface. These results provide a more complete description of the strong metal-support interaction process at the interface.

X-ray standing wave characterization of the strong metal-support interaction in Co/TiOx model catalysts.

The strong metal-support interaction (SMSI) is a phenomenon observed in supported metal catalyst systems in which reducible metal oxide supports can form overlayers over the surface of active metal nanoparticles (NPs) under a hydrogen (H2) environment at elevated temperatures. SMSI has been shown to affect catalyst performance in many reactions by changing the type and number of active sites on the catalyst surface. Laboratory methods for the analysis of SMSI at the nanoparticle-ensemble level are lacking and mostly based on indirect evidence, such as gas chemisorption. Here, we demonstrate the possibility to detect and characterize SMSIs in Co/TiOx model catalysts using the laboratory X-ray standing wave (XSW) technique for a large ensemble of NPs at the bulk scale. We designed a thermally stable MoNx/SiNx periodic multilayer to retain XSW generation after reduction with H2 gas at 600°C. The model catalyst system was synthesized here by deposition of a thin TiOx layer on top of the periodic multilayer, followed by Co NP deposition via spare ablation. A partial encapsulation of Co NPs by TiOx was identified by analyzing the change in Ti atomic distribution. This novel methodological approach can be extended to observe surface restructuring of model catalysts in situ at high temperature (up to 1000°C) and pressure (≤3 mbar), and can also be relevant for fundamental studies in the thermal stability of membranes, as well as metallurgy.

Structure affinity of the Langmuir monolayer and the corresponding Langmuir-Blodgett film revealed by X-ray techniques.

The possibility of reproducing the structural organization and functional abilities of a Langmuir monolayer in a film formed from it is one of the fundamental problems of ultrathin film science. This work is devoted to the comparison of monolayer and Langmuir-Blodgett (LB) film characteristics using the example of 2D systems based on the dithia-aza-crown substituted hemicyanine dye HCS. As was shown earlier, the investigated systems are promising for the preparation of selective sensors and extractors for mercury ions in aqueous solutions with a subnanomolar sensitivity threshold. Therefore, the study of the analyte binding mechanism by such a film is of great importance. The study carried out using an ultra-highly brilliant X-ray source (ESRF) allows the application of highly sensitive techniques such as X-ray reflectometry (XRR) and X-ray standing wave (XSW). Comparison of the electron density depth profile of the HCS Langmuir monolayer at the air/water interface and the HCS film transferred to a silicon substrate shows the preservation of the film structure and its functional features. The XSW measurements in turn reveal the similarities in the fine structure of preorganized Langmuir monolayers and Langmuir-Blodgett films of HCS. The integration of X-ray techniques with molecular modeling methods allowed us to show that the crown-ether groups of HCS molecules in the pre-organized monolayer and in the corresponding LB film lie on the surface of water or silicon, and the bound mercury ion is located above the crown-ether, partially binding to the nitrogen atom. The latter loses conjugation to the chromophore group, thereby altering the UV-vis spectrum and providing a response signal. The revealed mechanism of imprinting preorganization allows the proposed approach to be extended to other crown-substituted amphiphilic dyes to significantly enhance the sensory response.

Atypical magnetism in Pt/MgO/FeCoB/Pt waveguide structure by exchanging the order of MgO and FeCoB layers

Interfaces in FeCoB/MgO/FeCoB magnetic tunnel junction play a vital role in controlling their magnetic and transport properties for various applications in spintronics and magnetic recording media. In this work, interface structures of a few nm thick FeCoB layers in FeCoB/MgO and MgO/FeCoB bilayers are comprehensively studied using x-ray standing waves (XSW) generated by depositing bilayers between Pt waveguide structures. High interface selectivity of nuclear resonance scattering (NRS) under the XSW technique allowed to measure structure and magnetism at the two interfaces, namely FeCoB-on-MgO and MgO-on-FeCoB, yielding an interesting result that electron density and hyperfine fields are not symmetric at both interfaces. The formation of a high-density FeCoB layer at the MgO/FeCoB (FeCoB-on-MgO) interface with an increased hyperfine field (∼34.65 T) is attributed to the increasing volume of FeCo at the interface due to boron diffusion from 57FeCoB to the MgO layer. Furthermore, it caused unusual angular-dependent magnetic properties in MgO/FeCoB bilayer, whereas FeCoB/MgO is magnetically isotropic. In contrast to the literature, where the unusual angular dependent in FeCoB based system is explained in terms of in-plane magnetic anisotropy, present findings attributed the same to the interlayer exchange coupling between bulk and interface layer within the FeCoB layer.

Interface-resolved study of magnetism in MgO/FeCoB/MgO trilayers using x-ray standing wave techniques

Interfaces in the MgO-FeCoB-MgO trilayer have been studied with grazing incident nuclear resonance scattering (GINRS) using the x-ray standing waves (XSW) technique. High depth selectivity of the present method allows one to measure magnetism and structure at the two interfaces of FeCoB, namely, FeCoB-on-MgO and MgO-on-FeCoB, independently, yielding an intriguing result that both interfaces are not symmetric. A high-density layer with an increased magnetic hyperfine field at the FeCoB-on-MgO interface suggests different growth mechanisms at the two interfaces. The azimuthal angle-dependent magneto-optic Kerr effect measurements reveal the presence of unusual uniaxial magnetic anisotropy (UMA) in the trilayer. An in-situ temperature-dependent study discovered that this UMA systematically reduces with temperature. The trilayer becomes isotropic at 450C with an order-of-magnitude increase in coercivity. The asymmetry at the interfaces is, in turn, explained by boron diffusion from the FeCoB interface layer into the nearby MgO layer. Stress-induced UMA is observed in the boron-deficient FeCoB layer, superimposed with the bulk FeCoB layer, and found to be responsible for unusual UMA. The temperature-dependent variation in the UMA and coercivity can be understood in terms of variations in the internal stresses and coupling between FeCoB bulk and the interface layer.

Structural and magnetic asymmetry at the interfaces of MgO/FeCoB/MgO trilayer: Precise study under x-ray standing wave conditions

Magnetic tunnel junctions based on FeCoB as a magnetic electrode and MgO as a tunneling barrier gained much attention because of their applications in random access memories and magnetic sensors in disk drives. In this work, the structural and magnetic properties of the MgO/FeCoB/MgO trilayer have been studied precisely under x-ray standing wave (XSW) conditions, where XSW is generated through a high-density (Pt) waveguide structure. The combined x-ray scattering and fluorescence data obtained under XSW conditions revealed the formation of a high-density FeCoB layer at the MgO/FeCoB interface (FeCoB-on-MgO) in the as-deposited trilayer. Diffusion of B from the FeCoB layer into MgO is attributed to the formation of Fe- and Co-rich high-density layer (B-deficient FeCoB layer) at the interface. Angular-dependent magnetism of the trilayer structure revealed the presence of in-plane magnetic anisotropy (IMA), which disappeared with thermal annealing at a temperature of 450 °C. Stress in B-deficient FeCoB layer at the interface is attributed to the origin of IMA through magneto-elastic anisotropy energy minimization. The disappearance of anisotropy after annealing is mainly due to the removal of long-range stress and the formation of crystalline bcc-FeCo phase.

Atomic-Scale View of Redox Induced Changes for Monolayer MoOx on α-TiO2(110) with Chemical-State Sensitivity.

Supported molybdenum oxide (MoOx) plays an important role in catalytic transformations from alcohol dehydrogenation to transesterification. During these reactions, molybdenum and oxygen surface species undergo structural and chemical changes. A detailed, chemical-state specific, atomic-scale structural analysis of the catalyst under redox conditions is important for improving catalytic properties. In this study, a monolayer of Mo grown on α-TiO2(110) by atomic-layer deposition is analyzed by X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy (XPS). The chemical shifts for Mo 2p3/2 and O 1s peaks are used to distinguish Mo6+ from Mo4+ and surface O from bulk O. Excitation of XPS by XSW allows pinpointing the location of these surface species relative to the underlying substrate lattice. Measured 3D composite atomic density maps for the oxidized and reduced interfaces compare well with our density functional theory models and collectively create a unique view of the redox-driven dynamics for this complex catalytic structure.

Atomic-Site-Specific Surface Valence-Band Structure from X-Ray Standing-Wave Excited Photoemission.

X-ray standing-wave (XSW) excited photoelectron emission was used to measure the site-specific valence band (VB) for ½ monolayer (ML) Pt grown on a SrTiO_{3} (001) surface. The XSW induced modulations in the core level (CL), and VB photoemission from the surface and substrate atoms were monitored for three hkl substrate Bragg reflections. The XSW CL analysis shows the Pt to have a face-centered-cubic-like cube-on-cube epitaxy with the substrate. The XSW VB information compares well to a density functional theory calculated projected density of states from the surface and substrate atoms. Overall, this Letter represents a novel method for determining the contribution to the density of states by valence electrons from specific atomic surface sites.

Spatial variation of geometry, binding, and electronic properties in the moiré superstructure of MoS2 on Au(111)

The lattice mismatch between a monolayer of MoS2 and its Au(111) substrate induces a moiré superstructure. The local variation of the registry between sulfur and gold atoms at the interface leads to a periodic pattern of strongly and weakly interacting regions. In consequence, also the electronic bands show a spatial variation. We use scanning tunneling microscopy and spectroscopy (STM/STS), x-ray photoelectron spectroscopy (XPS) and x-ray standing wave (XSW) for a determination of the geometric and electronic structure. The experimental results are corroborated by density functional theory. We obtain the geometric structure of the supercell with high precision, identify the fraction of interfacial atoms that are strongly interacting with the substrate, and analyze the variation of the electronic structure in dependence of the location within the moiré unit cell and the nature of the band.

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.

Structure of monolayer 2H−TaS2 on Au(111)

We determined the structure of epitaxial $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ on Au(111) using the method of x-ray standing waves (XSW), supported by density functional theory (DFT) calculations and scanning tunneling microscopy (STM). The lattice mismatch between substrate and overlayer gives rise to a moir\'e superstructure, which modulates the structural and electronic properties. For a specific registry (S atoms directly above Au substrate atoms), local covalentlike bonds form, whereas globally weak van der Waals bonding prevails. Still, the $\mathrm{Ta}{\mathrm{S}}_{2}$ layer remains rather flat. Significant charge transfer from Au(111) into the conduction band of the two-dimensional material is found.

Atomic-Scale Structure of Chemically Distinct Surface Oxygens in Redox Reactions.

During redox reactions, oxide-supported catalytic systems undergo structural and chemical changes. Improving subsequent catalytic properties requires an understanding of the atomic-scale structure with chemical state specificity under reaction conditions. For the case of 1/2 monolayer vanadia on α-TiO2(110), we use X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy to follow the redox induced atomic positional and chemical state changes of this interface. While the resulting XSW 3D composite atomic maps include the Ti and O substrate atoms and V surface atoms, our focus in this report is on the previously unseen surface oxygen species with comparison to density functional theory predictions.

Layer-resolved many-electron interactions in delafossite PdCoO2 from standing-wave photoemission spectroscopy

When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO2, which consists of alternating layers of metallic Pd and Mott-insulating CoO2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO2, we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO2 layers. Computationally, we find that many-body interactions in Pd and CoO2 layers are highly different. Holes in the CoO2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials.

Structural characterisation of molecular conformation and the incorporation of adatoms in an on-surface Ullmann-type reaction

The on-surface synthesis of covalently bonded materials differs from solution-phase synthesis in several respects. The transition from a three-dimensional reaction volume to quasi-two-dimensional confinement, as is the case for on-surface synthesis, has the potential to facilitate alternative reaction pathways to those available in solution. Ullmann-type reactions, where the surface plays a role in the coupling of aryl-halide functionalised species, has been shown to facilitate extended one- and two-dimensional structures. Here we employ a combination of scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and X-ray standing wave (XSW) analysis to perform a chemical and structural characterisation of the Ullmann-type coupling of two iodine functionalised species on a Ag(111) surface held under ultra-high vacuum (UHV) conditions. Our results allow characterisation of molecular conformations and adsorption geometries within an on-surface reaction and provide insight into the incorporation of metal adatoms within the intermediate structures of the reaction.

Suspension assisted analysis of sulfur in petroleum coke by total-reflection X-ray fluorescence

The influence of the particle size distribution of a petroleum coke sample compared to its sulfur content was investigated. For this matrix, an optimization procedure of the Suspension Assisted Analysis (SAA) by Total-reflection X-Ray Fluorescence (TXRF) quantitative method was developed. SAA-TXRF sulfur recoveries were evaluated for three particle size distributions of the same coke sample. The sulfur recovery increased when the particle size distribution was smaller. The observed behaviour was correlated and validated with CHNS elemental analysis and microwave-assisted digestion TXRF measurements. The results indicate that the sulfur signal is strongly influenced by the particle size distribution and the deposition morphology of the petroleum coke material. This effect could be explained by the presence of a strong absorption effect for the sulfur signal in combination with the distortion of the X-ray Standing Waves (XSW) field observed between the analyte, S, and the elements Ti and Co, which were used as an internal standard. The variation in sulfur observed was up to 45.5% lower than the higher recovery obtained by CHNS. This investigation suggests that for an adequate application of the SAA-TXRF method in petroleum coke or similar matrices, careful optimization of the final particle dispersion of the ground coke is crucial and necessary. In this case, the use of a high- power ultrasound probe was the key.

Impact of fluorination on interface energetics and growth of pentacene on Ag(111).

We studied the structural and electronic properties of 2,3,9,10-tetrafluoropentacene (F4PEN) on Ag(111) via X-ray standing waves (XSW), low-energy electron diffraction (LEED) as well as ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS). XSW revealed that the adsorption distances of F4PEN in (sub)monolayers on Ag(111) were 3.00 Å for carbon atoms and 3.05 Å for fluorine atoms. The F4PEN monolayer was essentially lying on Ag(111), and multilayers adopted π-stacking. Our study shed light not only on the F4PEN–Ag(111) interface but also on the fundamental adsorption behavior of fluorinated pentacene derivatives on metals in the context of interface energetics and growth mode.

How microbial biofilms impact the interactions of Quantum Dots with mineral surfaces?

The increasing use of Quantum Dots (QDs) - nanoparticles exhibiting unique optical properties – and their incorporation in multiple engineering products is likely to result in the release of this new class of contaminants into natural systems. In soils, bacterial biofilms and mineral surfaces form highly reactive interfaces, which may control QDs' environmental fate. However, little is known regarding QDs' stability in, and modes of interactions with, biofilm/mineral interfaces. This study examines the interactions, distributions and stability of thioglycolic acid-capped CdSe/ZnS QDs at the corundum (α-Al2O3)/Shewanella oneidensis MR-1 interface, for exposure times ranging between 1 h to 24 h. Long Period – X-ray Standing Wave – Fluorescence Yield spectroscopy and Grazing Incidence – X-ray Absorption Spectroscopy were used. Results indicate increases in Zn and Se concentrations within the biofilm/crystal system with time, demonstrating its high accumulation capacity over 24 h. In addition, dissolution of a part of the ZnS shell occurs within 1 h, highlighting the potential degradation of QDs when exposed to the biofilm/crystal compartment. Once released, Zn(II) migrates toward the biofilm-crystal interface and interacts preferentially with the crystal surface. In contrast, the remaining CdSe core is mostly preserved, and stays within the biofilm thickness. However, at 24 h, Se and Zn present similar distribution profiles indicating a general reduction in ZnS shell dissolution at this longer exposure time.

X-ray atomic mapping of quantum dots

We report a model-independent atomic-mapping technique for quantum dots (QDs) by combining Bragg reflection x-ray standing wave (XSW) and grazing incidence diffraction (GID) measurements. In this study, we choose GaAs capped InGaAs QDs/GaAs(001) as a model system to show the locations and arrangements of indium atoms within the QDs along various $[hkl]$ directions. This technique directly reveals the actual amount of positional anisotropy and ordering fraction of indium atoms within the QDs by probing the ($\overline{1}11$), (111), (311), ($\overline{1}31$), (113), and ($\overline{1}13$) crystallographic planes. We find that indium atoms are outwardly shifted along the [001] direction by small fractions of the lattice constant, $0.04{a}_{\mathrm{GaAs}}$ and $0.06{a}_{\mathrm{GaAs}}$ from Ga sites for 50- and 150-\AA{} GaAs capped InGaAs QDs, respectively. We observe that an improved coherency factor of the indium atoms within the QDs by 45--60% along the [001] and [011] directions reduces the photoluminescence linewidth by 22%, thus making the QDs efficient for QD-laser and optoelectronic device applications. We also find that the position and ordering of In atoms along the (113) and ($\overline{1}13$) planes are most sensitive to the thickness of the GaAs cap layer. Our XSW-based results are supported by numerical calculations using a QD-macroscopic structural model based on our GID study. We thus show that this atomic-mapping technique will be useful for studying various quantum structures and tuning their properties.