X-ray Reflectivity Analysis Research Articles

Atomic interdiffusion in Pt-Ni thin films at low temperatures: An in-situ X-ray reflectivity study

We have investigated the kinetic process of the atomic interdiffusion across the interface in thin platinum‑nickel (Pt-Ni) bilayer films at temperatures ranging from 230 °C to 300 °C by in-situ X-ray reflectivity (XRR). The Pt-Ni interface width obtained by analyzing the XRR curve provides the time evolution of the diffusion length in nanometer scale and the diffusivity of atoms across the interface. We find that the interdiffusion is dominated by Ni atoms with an associated activation energy Q of 0.87 ± 0.05 eV. The analysis of the anomalous XRR measured with various X-ray energies across the K absorption edge of Ni provides more accurate electron density profile near the interface.

Low-energy ion polishing of Si in W/Si soft X-ray multilayer structures

The effect of ion polishing in sputter deposited W/Si multilayer mirrors with a d-spacing of 2.5 nm was studied. 0.1 to 0.5 nm of Si were etched with 100 eV Ar+ ions. This process resulted in a pronounced reduction in diffused scattering, measured at wavelengths about 0.1 nm. However, CuKa X-ray specular reflectivity and AFM showed only a marginal reduction of the roughness amplitude in the systems. Furthermore, the soft X-ray reflectivity at 0.84 and 2.4 nm did not show any changes after the ion polishing as compared to the nonpolished structures. Grazing incidence X-ray reflectivity (GIXR) analysis revealed that there was no pure W present in the deposited multilayers, with WSi2 being formed instead. As a result, it was concluded that the initial roughness in W/Si multilayers grown by magnetron sputtering is not the major factor in the reflectivity deviation from the calculated value for an ideal system. Nevertheless, the grazing incidence small-angle X-ray scattering (GISAXS) analysis revealed that ion polishing reduces the vertical propagation of roughness from layer to layer by a factor of two, as well as favorably affecting the lateral correlation length and Hurst parameter. These improvements explain the reduction of diffused X-ray scattering at 0.1 nm by more than an order of magnitude, which is relevant for applications like high resolution XRD analysis.

Wear-resistive and electrically conductive nitrogen-containing DLC film consisting of ultra-thin multilayers prepared by using filtered arc deposition

Nitrogen-containing diamond-like carbon (N-DLC) multilayer films approximately 500 nm thick were fabricated on tungsten carbide substrates as surface protective films with high wear-resistive and conductive properties. Each layer thickness of the N-DLC multilayer films was approximately 10 nm, and the films were a periodic bilayer structure consisting of hard and soft N-DLC layers. Owing to the high abrasion resistance of the hard layer and the low aggressiveness and high adhesion of the soft layer, the multilayer films showed good polishing and wear resistances compared with the hard N-DLC monolayer film, and the electrical resistivity was about half. In the case of DLC multilayer films consisting of hard N-free DLC and N-DLC films, the decrease of each layer thickness leads to the reduction of the polishing resistance. From X-ray reflectivity analysis of ultra-thin N-free DLC films, it was indicated that the film density of an ultra-thin N-free DLC film is lower than that of a thick N-free DLC film. In the DLC multilayer film with thin N-free DLC layers, it is possible that the polishing resistance of the whole DLC film reduced because the hardness the N-free DLC layer was decreased due to the low film density of each N-free DLC layer.

Towards x-ray waveguide formation upon ion irradiation of a Co thin film on Si(111)

We have obtained the essential features of x-ray waveguide formation in an ion-irradiated cobalt thin film on Si substrate. Ion irradiation was carried out with 1 MeV Si+ ions at a fluence of 5 × 1016 ions cm−2. X-ray reflectivity (XRR) experiment shows characteristic dips in the total external reflection region indicating formation of resonant electromagnetic modes—an essential requirement for x-ray waveguides. XRR analysis further reveals formation of a low electron-density layer sandwiched between higher electron-density layers. These are two key requirements for an x-ray waveguide. Computation of x-ray intensity within the irradiated sample shows about 15 times flux enhancement of the electric field within the low electron-density guiding layer.

REFLEX: a program for the analysis of specular X-ray and neutron reflectivity data

The use of X-ray and neutron reflectivity has been generalized worldwide for scientists who want to determine specific physical properties (such as electron-density profile, scattering-length density, roughness and thickness) of films less than 200 nm thick deposited on a substrate. This paper describes a freeware program namedREFLEX, which is a standalone program dedicated to the simulation and analysis of X-ray and neutron reflectivity from multilayers. This program was first written two decades ago and has been constantly improved since, but never published until now. The latest version ofREFLEXcovers generalized types of calculation of reflectivity curves including both neutron and X-ray reflectivity. In the case of X-rays, the program can deal with both s and p polarization, which is quite important in the soft X-ray region where the two polarizations can yield different results. Neutron reflectivity is calculated within the framework of non-spin-polarized neutrons.REFLEXhas also been designed to include any type of fluid (such as supercritical CO2) on top of the analysed film and includes corrections of the footprint effect for analysis on an absolute scale.

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoOx (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoOx. Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoOx molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoOx/BHJ. The high recombination resistance (~5 kΩ cm2) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

Ionic Liquid Mixture at the Vacuum Interface and the Peaks and Antipeaks Analysis of X-ray Reflectivity

This article presents work on the structure of the ionic liquid mixture between 1-methyl-3-octylimidazolium octylsulfate and 1-ethyl-3-methylimidazolium ethylsulfate at the vacuum interface. In par...

Preparation, characterization and electrical behaviors of PEDOT:PSS-Au/Ag nanocomposite thin films: an ecofriendly approach

Structure, morphology and electrical behaviors of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) or PEDOT:PSS thin films are investigated in the presence of protein-mediated green chemically synthesized positively charged gold and silver nanoparticles. The pure and PEDOT:PSS nanocomposite thin films are prepared by spin coating method. The presence of both nanoparticle and polymer is confirmed from X-ray diffraction, whereas composite formation is confirmed from Raman and FTIR spectroscopy. Atomic force microscopy (AFM) images show the surface morphologies of both pure and composite films, whereas average film thicknesses are obtained from AFM and X-ray reflectivity analysis. The presence of electrostatic interaction between the positively charged metallic nanoparticles and negatively charged PSS chains leads to the electrostatic shielding between cationic PEDOT and anionic PSS, which favors better charge transfer through PEDOT–PEDOT conducting paths. The increase in electrical conductivity is visualized from the current–voltage (I–V) curves, which show that the conductivity is relatively higher in the presence of silver than gold nanoparticles in the composite thin films. The conductivity of nanocomposite films is approximately five to six times enhanced in comparison with the pristine PEDOT:PSS thin films.

Si-doping effect on solution-processed In-O thin-film transistors

In this work, silicon-doped indium oxide thin-film transistors (TFTs) have been fabricated for the first time by a solution processing method. By varying the Si concentration in the In2O3–SiO2 binary oxide structure up to 15 at%, the thicknesses, densities, and crystallinity of the resulting In–Si–O (ISO) thin films were investigated by x-ray reflectivity (XRR) and x-ray diffraction techniques, while the produced TFTs were characterized by a conventional three-probe method. The results of XRR analysis revealed that the increase in the content of Si dopant increased the thickness of the produced film and reduced its density, and that all the Si-doped ISO thin films contained only a single amorphous phase even after annealing at temperatures as high as 800 °C. The manufactured ISO TFTs exhibited a reduction in the absolute value of threshold voltage VT close to 0 V and low current in the off-state, as compared to those of the non-doped indium oxide films, due to the reduced number of oxygen defects, which was consistent with the behavior of ISO TFTs fabricated by a sputtering method. The ISO TFT with a Si content of 3 at% annealed at 400 °C demonstrated the smallest subthreshold swing of 0.5 V/dec, VT of −5 V, mobility of 0.21 cm2 V−1s−1, and on/off current ratio of about 2 × 107.

Microstructure, Surface Plasmon, Magneto-Optic Surface Plasmon, and Sensitivity Properties of Magneto-Plasmonic Co/Au Multilayers

Microstructure properties of Co/Au multilayers prepared using dc-magnetron sputtering is reported using X-ray reflectivity (XRR) and X-ray diffraction (XRD) analysis. XRR profiles of these multilayers showed excellent bilayer periodicity. The XRD spectra displayed Co layer thickness-dependent properties. However, annealing increased lateral tensile strain decreased compressive strain along the normal substrate direction. While surface roughness, crystallite grain size, and strain are affected by Co layer thickness, microstructure and periodicity are dominated by fcc-Au (111). Surface plasmon resonance (SPR) and magneto-optic SPR study of an optimized Co/Au multilayer in Kretschmann configuration for air-He and water-Methanol media showed an enhanced sensitivity by over 4 times when excited at the wavelength of 785 nm as compared to the conventional SPR configuration when excited at the wavelength of 632.8 nm. This enhancement in MOSPR sensitivity means the detection limit of this class of transducers can be substantially improved by tuning Co/Au layer thickness, wavelength, and incident angle of optical radiation.

Data-reduction procedure for correction of geometric factors in the analysis of specular X-ray reflectivity of small samples

For small samples, the modification of the X-ray reflectivity (XRR) profile by the geometric factors due to the profile and size of the beam and the size of the sample is significant. These geometric factors extend the spill-over angle, which is often greater than the critical angle for small samples. To separate the geometric factors, it is necessary to know the spill-over angle. Since the geometric factors are smoothly varying functions and extend beyond the critical angle, it is impossible to determine the spill-over angle from the XRR profile of small samples. It is shown that the spill-over angle can be determined by comparing the normal XRR profile of a small sample with the XRR profile taken with a surface-contact knife edge on the same sample. Thus, a procedure has been developed for data reduction for small samples and validated with suitable experiments. Unlike the methods used hitherto, which have drawbacks, this is a self-consistent method for data reduction.

Magneto-Optic Surface Plasmon Resonance Ti/Au/Co/Au/Pc Configuration and Sensitivity

Magneto-optic surface plasmon resonance (MOSPR)-based sensors are highly attractive as next-generation biosensors. However, these sensors suffer from oxidation leading to degradation of performance, reproducibility of the sensor surface, because of the difficulty of removing adsorbed materials, and degradation of the sensor surface during surface cleaning and these limit their applications. In this paper, I propose MOSPR-based biosensors with 0 to 15 nm thick inert polycarbonate laminate plastic as a protective layer and theoretically demonstrate the practicability of my approach in water-medium for three different probing samples: ethanol, propanol, and pentanol. I also investigate microstructure and magnetic properties. The chemical composition and layered information of the sensor are investigated using X-ray reflectivity and X-ray diffraction analyses and these show distinct face-centered-cubic (fcc)-Au (111) phases, as dominated by the higher density of conduction electrons in Au as compared to Co. The magnetic characterization measured with the in-plane magnetic field to the sensor surface for both the as-deposited and annealed multilayers showed isotropic easy axis magnetization parallel to the multilayer interface at a saturating magnetic field of <100 Oersted (Oe). The sensor showed a maximum sensitivity of 5.5 × 104%/RIU (refractive index unit) for water–ethanol media and the highest detection level of 2.5 × 10−6 for water-pentanol media as the protective layer is increased from 0 to 15 nm.

Evaluation of spontaneous superlattice ordering in MOCVD grown AlxGa1-xAs epilayer on GaAs (100) using X-ray reflectivity and rocking curve analysis

Spontaneous superlattice structure in few monolayer length scale has been observed very recently in AlxGa1-xAs epilayer on (001) GaAs substrate grown by Metal Organic Chemical Vapor Deposition. Here we have reported a detailed study of this structure using high resolution x-ray reflectivity (XRR) and rocking curve (XRC) analysis and tried to extract composition modulation in the superlattice. A model-independent Distorted Wave Born Approximation (DWBA) has been applied to the XRR results to successfully obtain the electron density profile of the structure from which the compositional information could be extracted. The XRR profile of the AlxGa1-xAs sample (x = 0.17) showed sharp equidistant peaks giving clear evidence of superlattice ordering in the epilayer. Simulation of the XRR results showed a bilayer segregation with alternating Al composition of 0.1 and 0.4 of thickness 20 and 8 Å, respectively with interfacial thicknesses of 12 Å on both sides, giving a periodicity of 52 Å. The cross-sectional transmission electron microscopic (XTEM) image also showed similar length scale. A kinematical model was then used successfully to fit the sharp peaks observed in the XRC using the characteristic length and compositional parameters obtained from the XRR simulation. These results also showed that the individual bilayers were of single crystalline zinc blende structures. Both the simulation techniques were first applied to a test structure of AlAs/GaAs multilayer of known thickness before applying the same to the spontaneous superlattice to validate the model.

Growth of HfN thin films by reactive high power impulse magnetron sputtering

Thin hafnium nitride films were grown on SiO2 by reactive high power impulse magnetron sputtering (HiPIMS) and reactive direct current magnetron sputtering (dcMS). The conditions during growth were kept similar and the film properties were compared as growth temperature, nitrogen flow rate, and in the case of HiPIMS, duty cycle were independently varied. The films were characterized with grazing incidence X-ray diffraction (GIXRD), X-ray reflection (XRR) and X-ray stress analysis (XSA). HiPIMS growth had a lower growth rate for all grown films, but the films surfaces were smoother. The film density of HiPIMS deposited films grown at low duty cycle was comparable to dcMS grown films. Increasing the duty cycle increased the density of the HiPIMS grown films almost to the bulk density of HfN as well as increasing the growth rate, while the surface roughness did not change significantly. The HiPIMS grown films had large compressive stress while the dcMS grown films had some tensile stress. The dcMS grown films exhibit larger grains than HiPIMS grown films. The grain size of HiPIMS grown films decreases with increasing nitrogen flow rate, while the dcMS grain size increased with increasing nitrogen flow rate. This work shows that duty cycle during HiPIMS growth of HfN films has a significant effect on the film density and growth rate while other film properties seem mostly unaffected.

Restructuring of polyelectrolyte thin films in the presence of nonsolvent

Effects of nonsolvent (toluene) on two different polyelectrolyte thin films are studied by investigating their out-of-plane structures and in-plane surface morphologies. X-ray reflectivity analysis shows that the thicknesses of sodium poly(acrylic acid) (PAA) and poly(sodium 4-styrenesulfonate) (PSS) thin films increase if the films are kept for longer time inside toluene and nearly a linear relation is maintained between the film thickness and seasoning time. Surface topographies obtained from atomic force microscopy show that the surface morphologies and roughnesses change after dipping the films inside toluene as restructuring takes place on the surfaces of the films. Although toluene is nonsolvent for both PAA and PSS, however, restructuring of nanometer-thick polyelectrolyte is clearly visible and the effect is much more pronounced for thicker PAA and PSS films than the thinner one. Nonsolvent-induced structural relaxation of stressed structures developed under 2D confinement is the most probable reason for such structural and morphological modifications.

Thermal Atomic Layer Etching of Titanium Nitride Using Sequential, Self-Limiting Reactions: Oxidation to TiO2 and Fluorination to Volatile TiF4

The thermal atomic layer etching (ALE) of TiN was demonstrated using a new etching mechanism based on sequential, self-limiting oxidation and fluorination reactions. The oxidation reactant was either O3 or H2O2, and the fluorination reactant was hydrogen fluoride (HF) derived from HF-pyridine. In the proposed reaction mechanism, the O3 reaction oxidizes the surface of the TiN substrate to a TiO2 layer and gaseous NO. HF exposure to the TiO2 layer then produces TiF4 and H2O as volatile reaction products. The overall reaction can be written as TiN + 3O3 + 4HF → TiF4 + 3O2 + NO + 2H2O. Quartz crystal microbalance studies showed that HF can spontaneously etch TiO2 films. Spectroscopic ellipsometry and X-ray reflectivity analysis showed that TiN films were etched linearly versus the number of ALE cycles using O3 and HF as the reactants. The TiN etching also occurred selectively in the presence of Al2O3, HfO2, ZrO2, SiO2, and Si3N4. The etch rate for TiN ALE was determined at temperatures from 150 to 350 °C. Th...

Coupling X-Ray Reflectivity and In Silico Binding to Yield Dynamics of Membrane Recognition by Tim1

The dynamic nature of lipid membranes presents significant challenges with respect to understanding the molecular basis of protein/membrane interactions. Consequently, there is relatively little known about the structural mechanisms by which membrane-binding proteins might distinguish subtle variations in lipid membrane composition and/or structure. We have previously developed a multidisciplinary approach that combines molecular dynamics simulation with interfacial x-ray scattering experiments to produce an atomistic model for phosphatidylserine recognition by the immune receptor Tim4. However, this approach requires a previously determined protein crystal structure in a membrane-bound conformation. Tim1, a Tim4 homolog with distinct differences in both immunological function and sensitivity to membrane composition, was crystalized in a closed-loop conformation that is unlikely to support membrane binding. Here we have used a previously described highly mobile membrane mimetic membrane in combination with a conventional lipid bilayer model to generate a membrane-bound configuration of Tim1 in silico. This refined structure provided a significantly improved fit of experimental x-ray reflectivity data. Moreover, the coupling of the x-ray reflectivity analysis with both highly mobile membrane mimetic membranes and conventional lipid bilayer molecular dynamics simulations yielded a dynamic model of phosphatidylserine membrane recognition by Tim1 with atomic-level detail. In addition to providing, to our knowledge, new insights into the molecular mechanisms that distinguish the various Tim receptors, these results demonstrate that in silico membrane-binding simulations can remove the requirement that the existing crystal structure be in the membrane-bound conformation for effective x-ray reflectivity analysis. Consequently, this refined methodology has the potential for much broader applicability with respect to defining the atomistic details of membrane-binding proteins.

X-ray Reflectivity Studies on the Mixed Langmuir-Blodgett Monolayers of Thiol-Capped Gold Nanoparticles, Dipalmitoylphosphatidylcholine, and Sodium Dodecyl Sulfate.

Langmuir-Blodgett monolayers of thiolated gold nanoparticles mixed with dipalmitoylphosphatidylcholine/sodium dodecyl sulfate (DPPC/SDS) were investigated by combining the X-ray reflectivity, grazing-incident scattering, and TEM analyses to reveal the in-depth and in-plane organization and the 2D morphology of such mixed monolayers. It was found that the addition of a charged single-tail surfactant to the thiolated Au nanoparticle monolayer helps to stabilize the Au nanoparticle monolayer and to strengthen the mechanical property of the mixed monolayer film. For mixing with lipids, it was found that the thiolated gold nanoparticles could be pushed on top of the lipid monolayer when the mixed monolayer is compressed. At a typical comparable total surface area ratio of gold nanoparticle to lipid, the thiolated gold nanoparticles could form a uniform domain on top of the DPPC monolayer. When there are more thiolated gold nanoparticles than that could be supported by the lipid monolayer, domain overlapping could occur to form bilayer gold nanoparticle domains at some regions. At low total surface area ratio of thiolated gold nanoparticle to lipid, the thiolated gold nanoparticles tend to form a connected threadlike aggregation structure. Evidently, the morphology of the thiolated gold nanoparticle monolayer is highly depending on the total surface area ratio of the thiolated gold nanoparticle to lipid. SDS is found to have a dispersion power capable of dispersing the originally uniform Au-8C nanoparticle domain of the mixed Au-8C/DPPC monolayer into a foamlike structure for the mixed Au-8C/SDS/DPPC monolayer. It is evident that not only the concentration ratio but also the size and shape of the template formed by the amphiphilic molecules and their interaction with the thiolated gold nanoparticles can all have great effects on the organizational structure as well as morphology of the thiolated gold nanoparticle monolayer.

High mobility 2-dimensional electron gas at LaAlO3/SrTiO3 interface prepared by spin coating chemical methods

Highly mobile 2-dimensional electron gases (2DEGs) at the (001), (011) and (111)-oriented LaAlO3/SrTiO3 (LAO/STO) interfaces are obtained using spin coating chemical method, which is a gentle technique without plasma bombardment of the pulsed laser deposition. As revealed by x-ray diffraction spectrum and x-ray reflectivity analysis, the LAO over layer is epitaxially grown, and has a uniform thickness of ∼15 nm, ∼20 nm and ∼26 nm for (001), (011) and (111) orientations, respectively. The interfaces are well metallic down to 2 K. The carrier mobilities are ∼28 000 cm2 V−1 s−1, ∼22 000 cm2 V−1 s−1 and ∼8300 cm2 V−1 s−1 at 2 K for the (001), (011) and (111) LAO/STO interfaces, respectively, and ∼8 cm2 V−1 s−1, ∼4 cm2 V−1 s−1 and ∼4 cm2 V−1 s−1 at room temperature. The present work shows that the spin coating chemical method is a feasible approach to get high quality 2DEG at both the polar/non-polar and polar/polar interfaces.

Domain Spacing and Composition Profile Behavior in Salt-Doped Cyclic vs Linear Block Polymer Thin Films: A Joint Experimental and Simulation Study

Herein, we leveraged lithium salt doping of linear and cyclic block polymers (BPs) in thin film geometries to demonstrate how BP architecture influences self-assembled nanofeature sizes and interfacial widths in nanostructured thin films as a function of segregation strength. To mitigate potential issues with sample-to-sample variability and time-consuming synthesis, we used a single linear BP specimen that was ring-closed to generate an analogous cyclic BP, and we mixed the cyclic and/or parent linear BP with a lithium salt to modulate effective segregation strength (χeffN). Based on X-ray reflectivity analyses, cyclic polystyrene-block-poly[oligo(ethylene glycol) methacrylate] BP thin film specimens had ∼20% smaller domain spacings than their linear counterparts at all χeffNs and lower absolute sensitivities to changes in χeff at identical molecular weights. We also report the first direct measurements of interfacial widths in cyclic BP assemblies, which quantitatively demonstrated that interfacial mixi...