Angular Resolved X-ray Research Articles

Titanium infiltration into ultrathin PMMA brushes

Vapor phase infiltration (VPI) is a bottom-up process that involves the infiltration of polymers, often using atomic layer deposition compatible precursors. By exposing a polymer to an organo-metallic precursor, area selective material formation is achieved where the precursor reacts with regions covered by an infiltration-receptive polymer brush. Combining receptive and rejecting polymers that have the capability of forming complex nanopatterns could potentially allow for the creation of nanofeatures, offering a route to area selective deposition. This work is concerned with the creation and characterization of titanium-infiltrated films with a VPI process. Thin films of poly(methyl methacrylate) (PMMA) were infused with titanium isopropoxide and subsequently analyzed with angular resolved x-ray photoelectron spectroscopy. All XPS analysis and VPI treatments were completed without breaking vacuum in an integrated ultrahigh vacuum setup, with O 1s, C 1s, Ti 2p, and Si 2p core levels revealing the successful incorporation of titanium into the polymer. Grazing angle Fourier-transform infrared spectroscopy demonstrates the breaking of carbon–oxygen double bonds within the PMMA structure due to titanium incorporation.

Photogenerated charge collection on diamond electrodes with covalently linked chromophore monolayers

Encouraged by its rich surface chemistry and excellent electrochemical properties, boron-doped nanocrystalline diamond (B:NCD) is a promising p-type photoelectrode in dye-sensitized solar cells. One method of diamond surface functionalization using stable carbon-carbon bonds involves the electrochemical grafting of diazonium salts. However, this method typically leads to multilayers that may complicate the transport of photogenerated charges. Here, we establish functionalization of B:NCD electrodes by a monolayer of ethynylphenyl molecules using sterically hindered 4-(trimethylsilyl)ethynylbenzenediazonium tetrafluoroborate. Both the density and structural orientation of the grafted layer are investigated by angular resolved X-ray photoelectron spectroscopy, confirming the presence of covalently grafted monolayers. After removal of the trimethylsilyl protective groups, the resulting ethynyl functionalities are employed to immobilize organic donor-acceptor chromophores via Sonogashira cross-coupling reactions. Homogenous surface coverage is achieved even on the B:NCD electrode. Atomic scale DFT computing reveals that for the chromophore with the strongest acceptor unit, efficient charge separation of 20 Å is obtained where photogenerated holes move directly into diamond. Yet, photocurrent and photovoltage measurements suggest competitive electron recombination to the diamond electrode via the redox electrolyte. Correlation between the density of the molecular layer and photocurrents/photovoltage provides better understanding of the charge generation and recombination pathways in diamond-organic photoelectrochemical cells.

Development of broadband x-ray radiography for diagnosing magnetically driven cylindrically compressed matter

Experiments and modeling of x-ray radiography of millimeter diameter solid Al wires with laser-produced broadband x rays are reported. Experiments were performed using the 50-TW Leopard short-pulse laser in a laser and pulsed power chamber at the Nevada Terawatt Facility. To characterize broadband x rays and demonstrate a radiographic capability, bremsstrahlung, escaping electrons, and radiograph images of Al wires were simultaneously measured. The angularly resolved x-ray spectra are modeled by comparing measured bremsstrahlung signals in the range between 10 and ∼500 keV with hybrid particle-in-cell simulations. Transmission of Al wires from the radiograph images is further simulated with a Monte Carlo code. The measured transmission profiles of Al wires with three different diameters agree with calculations when a simulated x-ray spectrum composed of line emissions and bremsstrahlung is used with a source size of 600 ± 200 μm. Transmission calculations with only 22 keV Ag Kα or an exponential x-ray spectrum do not reproduce the measurement, suggesting that the accurate determination of an x-ray source spectrum, as well as the inclusion of the photon sensitivity of the detector, is critical in transmission calculations to infer the density of an object. The laser-based broadband x-ray radiography that was developed has been successfully implemented in a pulsed power chamber for future laser-pulsed-power coupled experiments.

Band bending and dipole effect at interface of metal-nanoparticles and TiO2 directly observed by angular-resolved hard X-ray photoemission spectroscopy.

This paper describes the observation of band bending and band edge shifts at the interfaces between nanoscale metals and TiO2 film over a wide depth range by angular-resolved hard X-ray photoemission spectroscopy (HAXPES). The HAXPES results indicate strong electrostatic interactions between the TiO2 semiconductor and metal nanoparticles, while density functional theory (DFT) calculations suggest that these interactions are primarily associated with charge transfer leading to electric dipole moments at the interface in the ground state. The effects of these dipole moments are not limited to the surface but also occur deep in the bulk of the semiconductor, and are highly dependent on the coverage of the metal nanoparticles on the semiconductor species.

Sub-nanometer resolution XPS depth profiling: Sensing of atoms

The development of a method capable of distinguishing a single atom in a single molecule is important in many fields. The results reported herein demonstrate sub-nanometer resolution for angularly resolved X-ray photoelectron spectroscopy (ARXPS). This is made possible by the incorporation of a Maximum Entropy Method (MEM) model, which utilize density corrected electronic emission factors to the X-ray photoelectron spectroscopy (XPS) experimental results.In this paper we report on the comparison between experimental ARXPS results and reconstructed for both inorganic and organic thin film samples. Unexpected deviations between experimental data and calculated points are explained by the inaccuracy of the constants and standards used for the calculation, e.g. emission factors, scattering intensity and atomic density through the studied thickness.The positions of iron, nitrogen and fluorine atoms were determined in the molecules of the studied self-assembled monolayers. It has been shown that reconstruction of real spectroscopic data with 0.2nm resolution is possible.

Co-Ir interface alloying induced by thermal annealing

Using angular resolved X-ray Photoelectron Spectroscopy (XPS), Magneto Optic Kerr Effect (MOKE) and X-ray Absorption Spectroscopy (XAS), we characterize the structural and magnetic evolution upon annealing of two thin Co films (8 and 9 Monolayers) deposited on Ir(111). The XAS data collected in the near Co K edge region (XANES), interpreted with ab-initio simulations, show that intermixing takes place at the Co-Ir interface. Using a linear combination analysis, we follow the intermixing during the thermally driven diffusion process. At 500 °C, the interface between Co and Ir(111) roughens slightly, but no alloy formation is detected. At 600 °C, the Co film loses integrity and MOKE data show a rearrangement of the magnetic domains. Annealing to higher temperatures results in CoxIr1 − x alloy formation and Ir segregation on the surface.

Small angle x-ray scattering with edge-illumination.

Sensitivity to sub-pixel sample features has been demonstrated as a valuable capability of phase contrast x-ray imaging. Here, we report on a method to obtain angular-resolved small angle x-ray scattering distributions with edge-illumination- based imaging utilizing incoherent illumination from an x-ray tube. Our approach provides both the three established image modalities (absorption, differential phase and scatter strength), plus a number of additional contrasts related to unresolved sample features. The complementarity of these contrasts is experimentally validated by using different materials in powder form. As a significant application example we show that the extended complementary contrasts could allow the diagnosis of pulmonary emphysema in a murine model. In support of this, we demonstrate that the properties of the retrieved scattering distributions are consistent with the expectation of increased feature sizes related to pulmonary emphysema. Combined with the simplicity of implementation of edge-illumination, these findings suggest a high potential for exploiting extended sub-pixel contrasts in the diagnosis of lung diseases and beyond.

Interpretation of X-ray photoelectron spectra of carbon-nitride thin films: New insights from in situ XPS

We report on angular-resolved x-ray photoelectron spectroscopy (XPS) studies of magnetron sputtered CNx thin films, first in situ (without air exposure), then after air exposure (for time periods ranging from minutes to several years), and finally after Ar ion etching using ion energies ranging from 500 eV to 4 keV. The as-deposited films typically exhibit two strong N1s peaks corresponding to pyridine-like, and graphite-like, at ∼398.2 eV and ∼400.7 eV, respectively. Comparison between in situ and air-exposed samples suggests that the peak component at ∼402–403 eV is due only to quaternary nitrogen and not oxidized nitrogen. Furthermore, peak components in the ∼399–400 eV range cannot only be ascribed to nitriles or pyrrolic nitrogen as is commonly done. We propose that it can also be due to a polarization shift in pyridinic N, induced by surface water or hydroxides. Argon ion etching readily removes surface oxygen, but results also in a strong preferential sputtering of nitrogen and can cause amorphization of the film surface. The best methods for evaluating and interpreting the CNx film structure and composition with ex-situ XPS are discussed.

Reflected electron-energy-loss spectra, differential inverse inelastic mean free paths, and angular resolved X-ray photoelectron spectra of a niobium sample

A technique for recovering the differential inverse inelastic mean free paths (DIIMFP) of electrons in Nb from the reflected electron energy loss spectra (REELS) at initial energies of 5 to 40 keV using a threelayer model of the sample surface is presented. The recovered DIIMFP are used for analyzing X-ray photoelectron spectra measured at different viewing angles. Comparison with experimental data is carried out.

Using neutral impact collision ion scattering spectroscopy and angular resolved X-ray photoelectron spectroscopy to analyze surface structure of surfactant solutions

In the present work, the surface structure of solutions of tetrabutylammonium iodide dissolved in formamide is in parallel analyzed by neutral impact collision ion scattering spectroscopy (NICISS) and angular resolved X-ray photoelectron spectroscopy (ARXPS). By NICISS, the concentration-depth profiles of the solute ions and solvent are achieved, whereas by ARXPS, those fraction-depth profiles are obtained. For the sake of convenient comparisons, the fraction-depth profiles are converted to concentration-depth profiles after the determination of the partial molar volume of the components. With those profiles, the surface structures of the solutions are mapped. The cations and anions distribute distinctly within the surface layer, indicating the complication of the surface structure of ionic surfactant solution. According to the definition of Gibbs dividing plane, the surface excesses of surfactant ions are determined. Those profiles and surface excesses obtained by two distinct methods are compared directly. The comparison shows good agreements in both items. The differences are also found during the comparison, and the causes are analyzed within the frame of the working mechanisms of those analytical techniques.

Determination of energy levels at the interface between O2 plasma treated ITO/P3HT : PCBM and PEDOT : PSS/P3HT : PCBM using angular-resolved x-ray and ultraviolet photoelectron spectroscopy

Interfacial energy alignments at the anode of solution processable organic photovoltaics are rarely studied. Here we use blended regio-regular poly(3-hexylthiophene) (P3HT) : phenyl-C61-butyric acid methyl ester (PCBM) deposited on top of O2 plasma cleaned indium tin oxide (ITO) and poly(3,4-ethylene-dioxythiophene) : polystyrene sulfonic acid (PEDOT : PSS) as a platform to obtain the interfacial energy alignment using angular-resolved x-ray and ultraviolet photoelectron spectroscopy. A strong downward vacuum level bending of 1.3 eV at the interface for plasma-ITO/P3HT : PCBM is observed. This results in an interfacial energetic barrier as high as 2.5 eV for holes and a reduction of barrier for electrons. This could be one of the contributing factors that result in lower device efficiency in O2 plasma-ITO/P3HT : PCBM compared to O2 plasma-ITO/PEDOT : PSS/P3HT : PCBM. The full interfacial energy diagram is determined for O2 plasma-ITO/P3HT : PCBM and PEDOT : PSS/P3HT : PCBM. Such methods can be extended to study various interfacial properties of solution processable organic semiconducting materials.

Direct observation of band bending in the topological insulator Bi2Se3

The surface band bending tunes considerably the surface band structures and transport properties in topological insulators. We present a direct measurement of the band bending on the Bi2Se3 by using the bulk sensitive angular-resolved hard x-ray photospectroscopy (HAXPES). We tracked the depth dependence of the energy shift of Bi and Se core states. We estimate that the band bending extends up to about 20 nm into the bulk with an amplitude of 0.23--0.26 eV, consistent with profiles previously deduced from the binding energies of surface states in this material.

Diffusion of In and Ga in TiN/HfO2/InGaAs nanofilms

The structure of TiN/HfO2 nanofilms grown on InxGa1−xAs substrates was assessed through angular-resolved x-ray photoelectron spectroscopy. The quantitative analysis made possible determining the thickness and composition of the various layers constituting the nanofilms treated at different temperatures (un-annealed, 500 °C/120 s and 700 °C/10 s). The TiN layer is crucial to prevent oxygen desorption from the dielectric during annealing. Small amounts of oxidized gallium and metallic arsenic are located at the HfO2/InGaAs interface. The thickness of the HfO2 layer remains stable under the thermal treatments. Annealing affects the In 3d5/2 and Ga 3p signals. From the angular dependence of the peak intensities and the detailed knowledge of the structure of the films, it was possible to determine that annealing causes In and Ga out-diffusion into the metallic layer, and also to quantify the amount of transported matter. This, along with density functional theory calculations, allowed for an estimation of the activation energy of the diffusion of indium through HfO2.

Adsorption behavior of the ternary system of phospholipid 1-palmitoyl-2-oleoyl-sn-gylycero-3-phosphocholine in 3-hydroxypropionitrile with added tetrabutyl ammonium bromide

The adsorption behavior in a ternary system composed of the non-ionic surfactant POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and the ionic surfactant TBABr (tetrabutyl ammonium bromide) in the polar solvent HPN (3-hydroxypropionitrile) is investigated utilizing surface tension measurements and ARXPS (angular resolved X-ray photoelectron spectroscopy). The latter technique allows evaluating the surface excess of both surfactants. The combined experimental information (i.e. surface excess and surface tension) allows the determination of the chemical potential and thus the activities of both surfactants by virtue of the Gibbs equation. While the activity coefficient of TBABr is hardly affected by the presence of POPC, the activity coefficient of POPC strongly depends on both, the concentration of POPC and the concentration of TBABr. We observe the activity coefficient fPOPC to vary by more than 3 orders of magnitude in the 2-dimensional parameter space (cPOPC, cTBABr). This finding reflects and explains the surprising behavior of the surface tension: adding TBABr to the binary system POPC/HPN may increase or decrease the surface tension, depending on the POPC concentration. Inspection of these results suggests that mixed POPC/TBABr aggregates are formed at low POPC concentrations while the typical features of micelle formation are absent in this ternary system even at very low TBABr concentration.

Magnetic dichroism in angular resolved hard X-ray photoelectron spectroscopy from buried magnetic layers

This work reports on the measurement of magnetic dichroism in angular-resolved photoelectron spectroscopy from in-plane magnetized buried thin films. The high bulk sensitivity of hard X-ray photoelectron spectroscopy (HAXPES) in combination with circularly polarized radiation enables the investigation of the magnetic properties of buried layers. Angular distributions of high kinetic energy (7–8keV) photoelectrons in a range of about 60° were recorded in parallel to the energy distribution. Depending on purpose, energy and angular resolutions of 150–250meV and 0.17–2° can be accomplished simultaneously in such experiments. Experiments were performed on exchange-biased magnetic layers covered by thin oxide films. More specifically, the angular distribution of photoelectrons from the ferromagnetic layer Co2FeAl layer grown on MnIr exchange-biasing layer was investigated where the magnetic structure is buried beneath a MgO layer. Pronounced magnetic dichroism is found in the Co and Fe 2p states for all angles of emission. A slightly increased magnetic dichroism was observed for normal emission in agreement with theoretical considerations.

Post-detection analysis for grating-based ultra-small angle X-ray scattering

Until recently, the hard X-ray, phase-sensitive imaging technique called grating interferometry was thought to provide information only in real space. However, by utilizing an alternative approach to data analysis we demonstrated that the angular resolved ultra-small angle X-ray scattering distribution can be retrieved from experimental data. Thus, reciprocal space information is accessible by grating interferometry in addition to real space. Naturally, the quality of the retrieved data strongly depends on the performance of the employed analysis procedure, which involves deconvolution of periodic and noisy data in this context. The aim of this article is to compare several deconvolution algorithms to retrieve the ultra-small angle X-ray scattering distribution in grating interferometry. We quantitatively compare the performance of three deconvolution procedures (i.e., Wiener, iterative Wiener and Lucy–Richardson) in case of realistically modeled, noisy and periodic input data. The simulations showed that the algorithm of Lucy–Richardson is the more reliable and more efficient as a function of the characteristics of the signals in the given context. The availability of a reliable data analysis procedure is essential for future developments in grating interferometry.

Thin layer composition profiling with angular resolved x-ray photoemission spectroscopy: Factors affecting quantitative results

Composition profiling of thin films in the nanometer range is critical to the development of future electronic devices. However, the number of techniques with such depth resolution is limited. Among them, angle-resolved x-ray photoelectron spectroscopy (ARXPS) can be used for thin layers up to a few nanometers, but it is not yet a fully established method. In order to evaluate its capabilities for use as a routine and general method, the authors evaluate both its intrinsic capabilities in comparison with other methods and the factors affecting quantification by analyzing its variability when applied at various laboratory locations with different tools and data treatments. For this purpose, dedicated samples based on multilayers of HfO2 and SiON were produced with a well-determined layer structure. The results show that ARXPS, including depth profiling reconstruction, is very efficient and compares favorably with nuclear analysis techniques. It allows the separation of the surface contamination signal from the interfacial layer signal and allows determination of the coverage quantitatively. An accuracy of ±10% is achieved for most elements except for nitrogen, where strong peak interference with hafnium and a low intensity increase the inaccuracy up to 20%. This study also highlights several technique limitations. First, the quality of the retrieved profile is strongly dependent upon the exact determination of each photoemission peak intensity. Also it demonstrates that, while favorable for chemical identification, very high resolution spectra may lead to larger errors in profile reconstruction due to larger statistical errors in the intensities, though this is true mainly for deeper layers. Finally, it points out the importance of the physical parameters used in the final obtained results.

Full depth profile of passive films on 316L stainless steel based on high resolution HAXPES in combination with ARXPS

Depth profiles of the passive films on stainless steel were based on analysis with the non-destructive hard X-ray photoelectron spectroscopy (HAXPES) technique in combination with the angular resolved X-ray photoelectron spectroscopy (ARXPS). The analysis depth with ARXPS is within the passive film thickness, while the HAXPES technique uses higher excitation energies (between 2 and 12keV) also non-destructively probing the chemical content underneath the film. Depth profiles were done within and underneath the passive film of 316L polarized in acidic solution.The passive film thickness was estimated to 2.6nm for a sample that was polarized at 0.6V and the main component in the passive film is, as expected, chromium. From the high resolution HAXPES spectra we suggest chromium in three different oxidation states present. Also for iron three oxides were detected. Gradients of chromium and iron concentrations and oxidation states within the film and an enrichment of nickel within a 0.5nm layer directly underneath the passive film are some of the results discussed.

Magnetometry of buried layers—Linear magnetic dichroism and spin detection in angular resolved hard X-ray photoelectron spectroscopy

The electronic properties of buried magnetic nano-layers were studied using the linear magnetic dichroism in the angular distribution of photoemitted Fe, Co, and Mn 2p electrons from a CoFe–Ir78Mn22 multi-layered sample. The buried layers were probed using hard X-ray photoelectron spectroscopy, HAXPES, at the undulator beamline P09 of the 3rd generation storage ring PETRA III. The results demonstrate that this magnetometry technique can be used as a sensitive element specific probe for magnetic properties suitable for application to buried ferromagnetic and antiferromagnetic magnetic materials and multilayered spintronics devices. Using the same instrument, spin-resolved Fe 2p HAXPES spectra were obtained from the buried layer with good signal quality.

Angle dependent laser nanopatterning of poly(ethylene terephthalate) surfaces

Interference effects can lead to the formation of ripple structures at laser-irradiated poly(ethylene terephthalate) surfaces. Poly(ethylene terephthalate) surface was irradiated with linearly polarized light of a pulsed 157 nm laser. In a certain range of irradiation parameters, the irradiation resulted in the formation of coherent ripples patterns. The dimension of the pattern depends on the angle of the laser beam incidence. The surface morphology of the nano-patterned poly(ethylene terephthalate) was analyzed by atomic force microscopy and focused ion beam-scanning electron microscopy. Oxygen concentration in the modified polymer surface was studied by angular resolved X-ray induced photo-electron spectroscopy. Gold nano-layers were consecutively sputtered onto the laser irradiated poly(ethylene terephthalate) surfaces. The morphology of the sputtered gold nano-layers was investigated with atomic force microscopy too. We found that the morphology of the gold nano-layers changes and depends on the surface pattern of the laser irradiated poly(ethylene terephthalate). Formation of gold “nano-hills” is observed at the ridges of the ripple structures. The amount of oxygen together with the morphology of prepared polymer pattern may be the dominant factors controlling the gold layer growth. The present results are compared with those obtained earlier on PET irradiated with krypton fluoride laser.