X-ray Reflectivity Patterns Research Articles

Method to Determine the Distribution of Substituted or Intercalated Ions in Transition-Metal Dichalcogenides: FexVSe2 and Fe1–xVxSe2

Multiple techniques are combined to determine the amount of intercalation and/or substitution in transition-metal dichalcogenides. Kiessig fringes in the X-ray reflectivity pattern are used to calculate thickness. Laue oscillations in the specular diffraction pattern of the crystallographically aligned samples determine the number of unit cells in the coherently diffracting domains. The amount of impurity phase(s) is estimated by the difference between the thickness of the films and the size of the domains. If the difference is small relative to the total film thickness, the composition of the crystalline phase can be determined from X-ray fluorescence measurements. The number of unit cells possible can be calculated from the amount of each element determined by X-ray fluorescence measurements using in-plane lattice parameters, and the amount of the anion should agree with the number of unit cells determined from the Laue oscillations. The total amount of the metal relative to that required for the number of unit cells from the Laue oscillations provides a direct determination of the relative amount of intercalation and/or substitution in the crystalline dichalcogenide. The utility of this approach is illustrated in FexV1–ySe2 samples. The relative amount of intercalation versus substitution was determined independently using electron microscopy and Rietveld refinement of diffraction patterns and is consistent with this new approach.

Ultralow thermal conductivity of turbostratically disordered MoSe2 ultra-thin films and implications for heterostructures

Films containing 8, 16, 24, 32 and 64 MoSe2 layers were synthesized using the modulated elemental reactants method. X-ray reflectivity patterns showed that the annealed films were the targeted number of MoSe2 layers thick with atomically smooth interfaces. In-plane x-ray diffraction (XRD) scans contained only hk0 reflections for crystalline MoSe2 monolayers. Specular XRD patterns contained only 00l reflections, also indicating that the hk0 plane of the MoSe2 layers are parallel to the substrate. Both XRD and electron microscopy techniques indicated that the hk0 planes are rotationally disordered with respect to one another, with all orientations equally probable for large areas. The rotational disorder between MoSe2 layers is present even when analyzed spots are within 10 nm of one another. Cross-plane thermal conductivities of 0.07–0.09 W m−1 K−1 were measured by time domain thermoreflectance, with the thinnest films exhibiting the lowest conductivity. The structural analysis suggests that the ultralow thermal conductivity is a consequence of rotational disorder, which increases the separation between MoSe2 layers. The bonding environment of the Se atoms also becomes significantly distorted from C3v symmetry due to the rotational disorder between layers. This structural disorder efficiently reduces the group velocity of the transverse phonon modes but not that of longitudinal modes. Since rotational disorder between adjacent layers in heterostructures is expected if the constituents have incommensurate lattices, this study indicates that these heterostructures will have very low cross-plane thermal conductivity.

Dynamics of altered surface layer formation on dissolving silicates

The extrapolation of mineral dissolution kinetics experiments to geological timescales has frequently been challenged by the observation that mineral dissolution rates decrease with time. In the present study, we report a detailed investigation of the early stages of wollastonite dissolution kinetics, linking time-resolved measurements of wollastonite dissolution rate as a function of crystallographic orientation to the evolution of physicochemical properties (i.e., diffusivity, density, and thickness) of amorphous silica-rich layers (ASSLs) that developed on each surface. Batch dissolution experiments conducted at room temperature and at far-from-equilibrium conditions revealed that the initial (i.e., ASSL-free) dissolution rate of wollastonite (R(hkl)) based on Ca release observe the following trend: R(010)≈R(100)>R(101)>R(001). A gradual decrease of the dissolution rate of some faces by up to one order of magnitude resulted in a modification of this trend after two days: R(010)≫R(100)⩾R(101)≈R(001). In parallel, the diffusivity of ASSLs developed on each face was estimated based on the measurement of the concentration profile of a conservative tracer (methylene blue) across the ASSL using nanoSIMS. The apparent diffusion coefficients of methylene blue as a function of the crystallographic orientation (Dapp(hkl)) observe the following trend: Dapp(010)⩾Dapp(100)>Dapp(101)≫Dapp(001), and decreases as a function of time for the (100) and (101) faces. Finally, the density of ASSL was estimated based on the modeling of X-ray reflectivity patterns acquired as a function of time. The density of ASSLs developed on the (010) faces remains low and constant, whereas it increases for the ASSLs developed on the (001) faces. On the whole, our results suggest that the impact of the formation of ASSLs on the wollastonite dissolution rate is anisotropic: while some crystal faces are weakly affected by the formation of non-passivating ASSLs (e.g., the (010) face), the dissolution of other faces is hampered by passivating ASSLs within a few hours. The observed passivation is suggested to originate from the progressive densification of the ASSL, which limits the transport of reactive species from and to the dissolving wollastonite surface, as evidenced by the estimated diffusivity of the ASSLs. Because the apparent face-specific diffusivity of the ASSLs is correlated with the face-specific initial (i.e., ASSL-free) dissolution rate of wollastonite, we propose that the extent of ASSL densification (and the resulting impact on ion transport) is (at least partly) controlled by the absolute mineral dissolution rate. Overall, this study argues that the formation and microstructural evolution of ASSLs are likely candidates for mineral ageing, highlighting the need for determining the parameters controlling the spontaneous changes of ASSL diffusivity as a function of the reaction progress.

A Study of High Temperature Stability of Zirconia Surface in Pressurized Water with High Resolution X-Ray Reflectivity

Metals and metal-alloys under hydrothermal conditions undergo accelerated corrosion and degradation which is detrimental to the integrity and safety of many industrial operations. Owing to their low thermal neutron cross-section, reasonable mechanical properties, and adequate corrosion resistance in high temperature water, zirconium alloys have been used as nuclear power plant fuel cladding materials. The presence of a passive oxide layer on the zirconium surface acts as a barrier to mitigate further corrosion of the metal, which plays a key role in sustaining the integrity of cladding. However, the continuous growth of zirconia on the alloy surface limits the life span of the cladding material and therefore reduces the burn-up efficiency of the nuclear fuel and also introduces potential safety risks. Despite the factor that there exists numerous studies on the mechanisms of in-reactor corrosion of zirconium alloys, the very initial stage of the degradation mechanism still remains unclear. With development of a hydrothermal cell dedicated for in-situ synchrotron X-ray reflectivity measurement of oxide-water interface, we successfully measured the interfacial structure at yttria-stabilized zirconia, or YSZ, (111) surface for a series of temperatures ranging from room temperature up to 360 ºC in pressurized water condition of ~20 MPa. As the temperature increases, small changes in the X-ray reflectivity are observed which is presumably attributed to atomic level modification of the interfacial structure. With increasing duration time for hydrothermal reaction at higher temperatures above 340 ºC, the change in the X-ray reflectivity patterns becomes distinct and a layered structure in addition to the ambient hydration structure could be qualitatively deduced from the measured X-ray reflectivity data. Interestingly, however, when the temperature goes back to 50 ºC, the ambient hydration structure was recovered although it was not perfectly reversible but qualitatively similar. The quantitative analysis of the X-ray patterns at different temperatures and hydrothermal reaction time will provide valuable information on the interfacial hydration structure and processes from the molecular level insight.

Surface and grain boundary interdiffusion in nanometer-scale LSMO/BFO bilayer

Epitaxial 150nm thick LSMO/BFO bilayer is deposited on STO (100) substrate by pulsed laser deposition, to study magnetoelectric effect. Unexpected low value of room temperature magnetization in bilayer indicates towards the possibility of interdiffusion. Further, sharp fall in the value of TC (53K) also added our anxiety towards possible interdiffusion in BFO/LSMO system. Low-angle x-ray diffraction technique is used to investigate interdiffusion phenomena, and the temperature-dependent interdiffusivity is obtained by accurately monitoring the decay of the first-order modulation peak as a function of annealing time. It has been found that the diffusivity at different temperatures follows Arrhenius-type behavior. X-ray reflection (XRR) pattern obtained for the bilayer could not be fitted in the Parratt’s formalism, which confirms the interdiffusion in it. Depth profiles of 209Bi, 56Fe ions measured by secondary ion mass spectroscope (SIMS) further substantiate the diffusion of these ions from upper BFO layer into lower LSMO layer.

X-ray reflectivity analysis of titanium dioxide thin films grown by cathodic arc deposition.

TiO2 thin films deposited by a vacuum arc on a glass substrate were characterized by X-ray reflectivity (XRR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Several thin films with different amounts of deposited TiO2 mass and different deposition and annealing temperatures were studied. A qualitative analysis of the XRD patterns indicated the presence of the anatase and/or rutile crystalline phases in most of the studied samples. From the analysis of the experimental XRR curves--which exhibited a wide angular range of oscillatory behavior--the thickness, mass density and interface roughness were determined. All XRR patterns were well fitted by modeled curves that assume the presence of a single and homogeneous TiO2 layer over which a very thin H2O layer is adsorbed. The thickest H2O adsorption layers were developed in films with the highest anatase content. Our overall results of the XRR analyses are consistent with those derived from the imaging techniques (SEM and AFM).

ZnO: Hydroquinone superlattice structures fabricated by atomic/molecular layer deposition

Here we employ atomic layer deposition in combination with molecular layer deposition to deposit crystalline thin films of ZnO interspersed with single layers of hydroquinone in an effort to create hybrid inorganic–organic superlattice structures. The ratio of the ZnO and hydroquinone deposition cycles is varied between 199:1 and 1:1, and the structure of the resultant thin films is verified with X-ray diffraction and reflectivity techniques. Clear evidence of the formation of a superlattice-type structure is observed in the X-ray reflectivity patterns and the presence of organic bonds in the films corresponding to the structure of hydroquinone is confirmed with Fourier transform infrared spectroscopy measurements. We anticipate that hybrid superlattice structures such as the ones described in this work have the potential to be of great importance for future applications where the precise control of different inorganic and organic layers in hybrid superlattice materials is required.

Effect of broadband light on Ag/As2S3 multilayers

A sandwich structure of four double layers of Ag (5nm)/As2S3 (82.7nm) has been prepared by Pulsed Laser Deposition (PLD) method. The effect of broadband light on the multilayer structure has been investigated. The X-ray reflectivity (XRR) patterns after different irradiation times have been measured. Although the fully intermixing of Ag and As2S3 layers should be expected during irradiation with visible light, however even after 5h of irradiation the intermixing is not completed. The characteristic features of XRR diagrams for long irradiation times have been interpreted by scattering of X-ray radiation on clusters of Ag or Ag–As2S3.

Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers

Ni/NiO multilayers were grown by magnetron sputtering at room temperature, with the aid of the natural oxidation procedure. That is, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (∼1.2 nm) is formed. Simulated x-ray reflectivity patterns reveal that layering is excellent for individual Ni-layer thickness larger than 2.5 nm, which is attributed to the intercalation of amorphous NiO between the polycrystalline Ni layers. The magnetization of the films, measured at temperatures 5–300 K, has almost bulk-like value, whereas the films exhibit a trend to perpendicular magnetic anisotropy (PMA) with an unusual significant positive interface anisotropy contribution, which presents a weak temperature dependence. The power-law behavior of the multilayers indicates a non-negligible contribution of higher order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the temperature dependence of the magnetization in the spin-wave regime show that the magnetization in the multilayers decreases faster as a function of temperature than the one of bulk Ni. Finally, when the individual Ni-layer thickness decreases below 2 nm, the multilayer stacking vanishes, resulting in a dramatic decrease of the interface magnetic anisotropy and consequently in a decrease of the perpendicular magnetic anisotropy.

Preparation of High Permittivity GdScO3 Films by Liquid Injection MOCVD

We describe preparation and properties of GdScO3 thin films. The films were prepared by liquid injection metal-organic chemical vapor deposition, MOCVD at 600 ºC on (100) Si substrate. The as-deposited films were amorphous with a smooth surface and sharp GdScO3/Si interface. X-ray diffraction showed that the amorphous phase is well preserved upon rapid thermal annealing up to 1000 ºC. However, modification of the X-ray reflectivity pattern after annealing at 1000 ºC indicates increasing of the film thickness, presumably due to diffusion of silicon from the substrate into the whole volume of the film. Capacitance-voltage measurement resulted in dielectric constant of 22. It is shown, that exact stoichiometry of GdScO3 is not necessary to achieve dielectric constant above 20.

Segregation and interdiffusion in (Fe,Co)/Pt superlattices

We report on the chemical structure of (Fe,Co)/Pt superlattices, which recently have shown high uniaxial magnetocrystalline anisotropy combined with high saturation magnetic moments. In particular, the homogeneity of the (Fe,Co) alloy is studied with a combination of x-ray and neutron reflectometry---the latter in a configuration where magnetic scattering is negligible. It is deduced, with support from off-specular x-ray reflectivity patterns and corresponding simulations, that the lower (Fe,Co)-on-Pt interface contains more Co than the upper Pt-on-(Fe,Co) interface. This can occur as Co interdiffuses into Pt more easily than Fe, as shown by density-functional calculations. The effect of this interdiffusion and segregation on the uniaxial anisotropy is discussed, and it is found that the previously observed discrepancy between experimental and theoretical anisotropy values can be quantitatively accounted for.

Investigation of interface properties of sputter deposited TiN/CrN superlattices by low angle x-ray reflectivity

Approximately 1.8 µm thick nanolayered multilayer coatings of TiN/CrN (also known as superlattices) were deposited on silicon (1 0 0) substrates at different modulation wavelengths (4.6–12.8 nm), substrate temperatures (50–400 °C) and substrate bias voltages (−50 to −200 V) using a reactive direct current magnetron sputtering system. The x-ray reflectivity (XRR) technique was employed to determine various properties of the multilayers such as interface roughness, surface roughness, electron density, critical angle and individual layer thicknesses. The modulation wavelengths of the TiN/CrN superlattice coatings were calculated using a modified Bragg's law. Furthermore, the experimental XRR patterns were simulated using theoretically generated patterns and a good fit was obtained for a three layer model, i.e. (1) top surface roughness layer, (2) TiN/CrN multilayer coating (approximately 1.8 µm) and (3) Ti interlayer (∼0.5 µm) at the film–substrate interface. For the superlattice coatings prepared at a modulation wavelength of 9.7 nm, a substrate bias of −200 V and a substrate temperature of 400 °C the XRR patterns showed Bragg reflections up to 5th order, indicating well-defined periodicity of the constituent layers and relatively sharp interfaces. The simulation showed that the superlattice coatings prepared under the above conditions exhibited low surface and interface roughnesses. We also present the effect of substrate temperature and substrate bias, which are critical parameters for controlling the superlattice properties, onto the various interface properties of TiN/CrN superlattices.

Wheat glutenin proteins assemble into a nanostructure with unusual structural features

The proteins of wheat have a known propensity to aggregate into a variety of forms. We report here a novel nanostructure from wheat proteins, derived from a crude extract of high molecular weight glutenins. The structure was characterised by a significant thioflavin T (ThT) fluorescence and a fibrillar morphology by transmission electron microscopy (TEM). The ThT fluorescence and TEM data are suggestive of an amyloid structure, but the X-ray fibre diffraction data show a reflection pattern (4.02, 4.2–4.3, 4.6, 12.9, 19.3 and 38.7 Å) inconsistent with both the classic amyloid form and the previously described β-helix structure. The 4.6 Å reflection is consistent with that predicted for the amyloid inter-β-strand, and the absence of the inter-β-sheet distance at ≈10–11 Å is not unprecedented in amyloid-like structures. However, our observed X-ray reflection pattern has not been previously reported and suggests a novel wheat glutenin nanostructure.

Interface smoothing of soft x-ray Mo/Y multilayer mirror by thermal treatment

A systematic study of structure and thermal stability of a [(Mo/Y/) x 5] soft x-ray multilayer (ML) mirror prepared by electron beam evaporation system has been performed by means of x-ray reflectivity (XRR) and grazing incidence x-ray diffraction (GIXRD). The GIXRD patterns show that the Mo and Y layers are polycrystalline in nature. The obtained XRR patterns confirm the very good quality of the ML stack. Isochronal thermal annealing up to the temperature of 400 °C leads to a decrease in interface roughness which in turn results in sharpening of the ML interfaces. Further elevating the annealing temperature above 400 °C oxidizes the Y layers and the interface roughness increases. Thus increase in interface roughness beyond this temperature can be understood in terms of evolution of yttrium oxide, which significantly affects the ML structural properties. The modulation structure is well maintained even after annealing at 600 °C, which confirms very good thermal stability of the Mo/Y ML mirror. Our results demonstrate that low-temperature annealing (up to 400 °C) is an effective method to improve the quality of an as deposited Mo/Y soft x-ray ML mirror. Diffuse scattering measurements show very good correlation with the XRR fitting parameters.

Luminescence property of Ce 3+-doped silica decorated with S 2− and Cl − anions

Ce 3+-doped silica was synthesized by sol–gel technique and was further decorated with S 2− and Cl − anions through chemical exchange in controlled ambient at elevated temperature. The structure and optical property of samples were examined by X-ray diffraction spectrum, XPS pattern, reflection pattern, and photoluminescence patterns. There is a broad luminescence band at 445 nm under the excitation at 320 nm in the Ce 3+-doped silica heat-treated in air at 1000 °C. The heat-treatment of the sample in vacuum at 800 °C can increase the intensity of luminescence but have no effect on the wavelength of luminescence. The decoration of S 2− and Cl − anions cannot only increase the luminescent intensity but also shift the luminescent wavelength to shorter wavelength.

Uncertainty of Film Structures Evaluated by X-Ray Reflectivity

X-ray reflectivity (XRR) is a powerful technique for evaluating nanometer scale film structures. In principle, it is rested on interference of X-rays reflected on surface and interface of the film stacks. If we have a proper structural model for film stacks, calculation of XRR pattern is a physically well-defined procedure based on the Fresnel formula that only needs well known physical constants, wavelength of X-ray and refractive indices of film materials for X-ray. Therefore, XRR is capable of providing standard nano-film thickness ensuring SI unit traceability. We discuss roots of errors for the evaluated film structures, for example, miss-alignment of sample positions andso on. We also present several trials to diminish uncertainties of measured film structures.

In situ energy dispersive x-ray reflectometry measurements on organic solar cells upon working

The change in the morphology of plastic solar cells was studied by means of time-resolved energy dispersive x-ray reflectivity (XRR). This unconventional application of the XRR technique allowed the follow up of in situ morphological evolution of an organic photovoltaic device upon working. The study consisted of three steps: A preliminary set of XRR measurements on various samples representing the intermediate stages of cell construction, which provided accurate data regarding the electronic densities of the different layers; the verification of the morphological stability of the device under ambient condition; a real-time collection of XRR patterns, both in the dark and during 15h in artificial light conditions which allowed the changes in the system morphology at the electrode-active layer interface to be monitored. In this way, a progressive thickening of this interface, responsible for a reduction in the performances of the device, was observed directly.

Phase formation by ion beam mixing in the Ti/Si multilayer system

Abstract The irradiation effect of 350 MeV Au+ ions on Ti/Si multilayers has been studied using Rutherford backscattering spectroscopy, X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXRD). Intermixing effects have been studied as a function of fluences of 0.46 × 1014, 1.82 × 1014 and 4.62 × 1014 cm−2. Rutherford backscattering spectra (RBS) confirm mixing at the interface. X-ray reflectivity patterns show damage at the interfaces with the absence of a continuous fringe pattern at high fluence doses in comparison to the pristine interface. Mixing leads to titanium di-silicide (TiSi2) phase formation as a shown by grazing incidence X-ray diffraction patterns. The observed intermixing is attributed to energy deposited by the incident ions in the electronic system of the target. Swift heavy ion irradiation induced intermixing increases with fluence.

Morphological and mechanical properties study of [WO 3/W] n nanoscale multilayers

Microstructure, morphology and hardness of W, WO 3 single layers and WO 3/W multilayers with various period thicknesses have been studied. Films have been deposited via RF sputtering at a working pressure of 0.5 Pa and with a power density of 1.18 W/cm 2. W layers are crystallised. WO 3 single layers were investigated with various oxygen partial pressure ( P O 2 =10–30%). The roughness increases with P O 2 . This phenomenon is ascribed to active resputtering of the deposited layer by oxygen ions. For a P O 2 =10%, the layer possesses a stoichiometric composition WO 3, an amorphous structure and a homogeneous surface morphology. P O 2 =10% was chosen to performed WO 3/W multilayers, in order to elaborate amorphous/crystallised multilayers and to study their mechanical properties. Several period thicknesses ( Λ) have been investigated and X-ray reflectivity patterns reveal a multilayered structure even for the lowest Λ of 2 nm. Nevertheless, no effects of the period thickness on the hardness were observed which is in disagreement with Hall Petch and Lehoczky theories.

Fabrication and characterisation of ultra-thin tungsten–carbon (W/C) and platinum–carbon (Pt/C) multilayers for X-ray mirrors

Ultra-thin tungsten–carbon (W/C) and platinum–carbon (Pt/C) multilayers were fabricated using dc magnetron sputtering. The bilayer period and the total number of layers were varied for both systems to ascertain the X-ray reflectance response. As a preliminary investigation, in situ resistance measurements were made to determine the minimum continuous single layer thickness that could be produced for W and Pt. We used X-ray reflectivity to monitor the reflectance from the first order peaks and the number of layers in the multilayered structures. X-ray photoelectron spectroscopy showed that a distinct intermixing layer develops in both W/C and Pt/C. A repeating four-layer model was developed to characterise the X-ray reflectivity patterns with mixed interface layers. The model accounts for all the essential features in the observed reflectivity data with particular attention given to layer thickness and interfacial roughness. Assessment of the films by indentation indicated that they are mechanically reliable with no cracking events or delamination obvious from the load–displacement curves indicating excellent adhesion. An anomalous improvement in hardness and Young’s modulus with increasing number of layers was observed. The present results indicate that high-quality ultra-thin W/C and Pt/C tailored multilayer films produced by dc magnetron sputtering show considerable promise as mirrors for high energy X-ray applications.