High-resolution X-ray Reflectivity Research Articles

Optical constants and thickness determination of La[formula omitted]Sr[formula omitted]MnO3 thin films on Nb:SrTiO3 substrates by spectro-ellipsometry: Combination of optical and X-ray techniques

40 nm thick La2/3Sr1/3MnO3 (LSMO) thin films were epitaxially grown by Pulsed Laser Deposition (PLD) onto niobium doped SrTiO3 (Nb:STO) substrates, with different Nb concentration from 0.01%wt to 0.5%wt. The optical characterization of the heterostructures by spectroscopic ellipsometry enables us to extract the optical constants of the manganite heteroepitaxial layer at room temperature. Performing spectrophotometry in the same wavelength range brings a useful cross-validation of the extracted results. In addition, the thickness evaluation of the LSMO layer by spectro-ellipsometry is further validated by both High Resolution X-ray diffraction and X-ray reflectivity, as well as a Transmission Electron Microscopy cross section, taken as a physical reference. This study validates quantitatively the spectro-ellipsometry as a suitable routine tool to measure accurately both thickness and complex refractive index of the LSMO thin film, picturing their peculiar electrical behaviour between both metallic and insulating phases. The relative error on the thickness measurement between X-ray and ellipsometry is less than 5%. The LSMO complex refractive index enabled also a simultaneous estimation of further material properties, such as the optical gap ωg or the mass density ρm, determined with less than 1.5% relative error compared to X-ray reflectivity results.

Unveiling the Origin of Fast Hydride Ion Diffusion at Grain Boundaries in Nanocrystalline TiN Membranes.

Nanocrystalline titanium nitride (TiN) has been determined to be a promising alternative to noble metal palladium (Pd) for fabricating base membranes for the energy-efficient production of pure hydrogen. However, the mechanism of transport of hydrogen through a TiN membrane remains unclear. In this study, we established an atomistic model of the transport of grain boundary hydride ions through such a membrane. High-resolution transmission electron microscopy and X-ray reflectivity confirmed that a nanocrystalline TiN1.0 membrane with a (100) preferred growth orientation retained about 4 Å-wide interfacial spaces along its grain boundaries. First-principles calculations based on the density functional theory showed that these grain boundaries allowed the diffusion of interfacial hydride ion defects with very small activation barriers (<12 kJ mol-1). This was substantiated by the experiment. In addition, the narrow boundary produced a sieving effect, resulting in a selective H permeation. Both the experimental and theoretical results confirmed that the granular microstructures with the 4 Å-wide interlayer enabled the transition metal nitride to exhibit pronounced hydrogen permeability.

Epitaxial growth and characterization of (001) [NiFe/M]20 (M = Cu, CuPt and Pt) superlattices

We present optimization of [(15Å) Ni80Fe20/(5Å) M]20 single crystal multilayers on (001) MgO substrates, with M being Cu, Cu50Pt50 and Pt. These superlattices were characterized by high resolution X-ray reflectivity (XRR) and diffraction (XRD) as well as polar mapping of important crystal planes. It is shown that cube on cube epitaxial relationship can be obtained when depositing at substrate temperature of 100 °C regardless of the lattice mismatch (5% and 14% for Cu and Pt, respectively). At lower substrate temperatures poly-crystalline multilayers were obtained while at higher substrate temperatures {111} planes appear at ∼10° off normal to the film plane. It is also shown that as the epitaxial strain increases, the easy magnetization axis rotates towards the direction that previously was assumed to be harder, i.e. from [110] to [100], and eventually further increase in the strain makes the magnetic hysteresis loops isotropic in the film plane. Higher epitaxial strain is also accompanied with increased coercivity values. Thus, the effect of epitaxial strain on the magnetocrystalline anisotropy is much larger than what was observed previously in similar, but polycrystalline samples with uniaxial anisotropy (Kateb et al. 2021).

Uptake of Pb and the Formation of Mixed (Ba,Pb)SO4 Monolayers on Barite During Cyclic Exposure to Lead-Containing Sulfuric Acid

Barite (BaSO4) is a common additive in lead-acid batteries, where it acts as a nucleating agent to promote the reversible formation and dissolution of PbSO4 during battery cycling. However, little is known about the molecular-scale mechanisms that control the nucleation and cyclic evolution of PbSO4 over a battery's lifetime. In this study, we explore the responses of a barite (001) surface to cycles of high and low lead concentrations in 100 mM sulfuric acid solution using in situ atomic force microscopy and high-resolution X-ray reflectivity. We find that PbSO4 epitaxial films readily nucleate on the barite surface, even from solutions that are undersaturated relative to bulk PbSO4. Despite this, barite (001) proves to be an ineffective nucleator of bulk PbSO4, as multilayer growth is suppressed even in highly supersaturated solutions. Instead, we find evidence that Pb2+ ions can directly exchange with Ba2+ to create mixed (Ba,Pb)SO4 surfaces. These chemically mixed surfaces do not host PbSO4 monolayers as readily as pristine barite, and the original reactivity is not regained until a fresh surface is re-established by aggressive etching. Our results can be partly explained by traditional models of thin-film growth, which predict a Stranski-Krastanov (S-K) growth mode, where monolayer films are stabilized by a reduction in surface energy, but multilayer growth is inhibited by epitaxial strain. Complementary density functional theory calculations confirm the basic energetic terms of the model but also show evidence for thickness-dependent energetics that are more complex than would be predicted from traditional models. The experimental results are better understood by extending the model to consider the formation of mixed surfaces and films, which have reduced strain and interfacial energies relative to pure films while also being stabilized by entropy of mixing. These insights into nonstoichiometric heteroepitaxy will enable better predictions of how barite affects PbSO4 nucleation in battery environments.

Y(III) Sorption at the Orthoclase (001) Surface Measured by X-ray Reflectivity.

Interactions of heavy metals with charged mineral surfaces control their mobility in the environment. Here, we investigate the adsorption of Y(III) onto the orthoclase (001) basal plane, the former as a representative of rare earth elements and an analogue of trivalent actinides and the latter as a representative of naturally abundant K-feldspar minerals. We apply in situ high-resolution X-ray reflectivity to determine the sorption capacity and molecular distribution of adsorbed Y species as a function of the Y3+ concentration, [Y3+], at pH 7 and 5. With [Y3+] ≥ 1 mM at pH 7, we observe an inner-sphere (IS) sorption complex at a distance of ∼1.5 Å from the surface and an outer-sphere (OS) complex at 3-4 Å. Based on the adsorption height of the IS complex, a bidentate, binuclear binding mode, in which Y3+ binds to two terminal oxygens, is proposed. In contrast, mostly OS sorption is observed at pH 5. The observed maximum Y coverage is ∼1.3 Y3+/AUC (AUC: area of the unit cell = 111.4 Å2) for all the investigated pH values and Y concentrations, which is in the expected range based on the estimated surface charge of orthoclase (001).

Water Adsorption on Mica Surfaces with Hydrophilicity Tuned by Counterion Types (Na, K, and Cs) and Structural Fluorination.

The stability of adsorbed water films on mineral surfaces has far-reaching implications in the Earth, environmental, and materials sciences. Here, we use the basal plane of phlogopite mica, an atomically smooth surface of a natural mineral, to investigate water film structure and stability as a function of two features that modulate surface hydrophilicity: the type of adsorbed counterions (Na, K, and Cs) and the substitution of structural OH groups by F atoms. We use molecular dynamics simulations combined with in situ high-resolution X-ray reflectivity to examine surface hydration over a range of water loadings, from the adsorption of isolated water molecules to the formation of clusters and films. We identify four regimes characterized by distinct adsorption energetics and different sensitivities to cation type and mineral fluorination: from 0 to 0.5 monolayer film thickness, the hydration of adsorbed ions; from 0.5 to 1 monolayer, the hydration of uncharged regions of the siloxane surface; from 1 to 1.5 monolayer, the attachment of isolated water molecules on the surface of the first monolayer; and for >1.5 monolayer, the formation of an incipient electrical double layer at the mineral-water interface.

Relativistic X-Ray Reflection Models for Accreting Neutron Stars

Abstract We present new reflection models specifically tailored to model the X-ray radiation reprocessed in accretion disks around neutron stars, in which the primary continuum is characterized by a single-temperature blackbody spectrum, emitted either at the surface of the star or at the boundary layer. These models differ significantly from those with a standard power-law continuum, typically observed in most accreting black holes. We show comparisons with earlier reflection models and test their performance in the NuSTAR observation of the neutron star 4U 1705−44. Simulations of upcoming missions such as XRISM-Resolve and Athena X-IFU are shown to highlight the diagnostic potential of these models for high-resolution X-ray reflection spectroscopy. These new reflection models xillverNS, and their relativistic counterpart relxillNS, are made publicly available to the community as an additional flavor in the relxill suite of reflection models.

Trivalent ion overcharging on electrified graphene

The structure of the electrical double layer (EDL) formed near graphene in aqueous environments strongly impacts its performance for a plethora of applications, including capacitive deionization. In particular, adsorption and organization of multivalent counterions near the graphene interface can promote nonclassical behaviors of EDL including overcharging followed by co-ion adsorption. In this paper, we characterize the EDL formed near an electrified graphene interface in dilute aqueous YCl3 solution using in situ high resolution x-ray reflectivity (also known as crystal truncation rod) and resonant anomalous x-ray reflectivity (RAXR). These interface-specific techniques reveal the electron density profiles with molecular-scale resolution. We find that yttrium ions (Y3+) readily adsorb to the negatively charged graphene surface to form an extended ion profile. This ion distribution resembles a classical diffuse layer but with a significantly high ion coverage, i.e., 1 Y3+ per 11.4 ± 1.6 Å2, compared to the value calculated from the capacitance measured by cyclic voltammetry (1 Y3+ per ∼240 Å2). Such overcharging can be explained by co-adsorption of chloride that effectively screens the excess positive charge. The adsorbed Y3+ profile also shows a molecular-scale gap (⩾5 Å) from the top graphene surfaces, which is attributed to the presence of intervening water molecules between the adsorbents and adsorbates as well as the lack of inner-sphere surface complexation on chemically inert graphene. We also demonstrate controlled adsorption by varying the applied potential and reveal consistent Y3+ ion position with respect to the surface and increasing cation coverage with increasing the magnitude of the negative potential. This is the first experimental description of a model graphene-aqueous system with controlled potential and provides important insights into the application of graphene-based systems for enhanced and selective ion separations.

Formation of ultrathin cobalt ferrite films by interdiffusion ofFe3O4/CoObilayers

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (${\mathrm{Co}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}$) films by interdiffusion of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO bilayers. Bilayer samples with different ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped ${\mathrm{SrTiO}}_{3}$(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ annealing temperature and is completed for temperatures above $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ film with partial inversion has been formed exhibiting homogeneously distributed ${\mathrm{Co}}^{2+}$ and mainly ${\mathrm{Fe}}^{3+}$ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.

Comprehensive structural and optical characterization of AlAs/GaAs distributed Bragg reflector

30-pair AlAs/GaAs distributed Bragg reflector (DBR), which has 1030 nm center reflectivity, is studied extensively by means of High Resolution X-ray Diffraction (HR-XRD) and reflectivity measurements. Theta/2-Theta measurements and dynamical simulations have been done for (002), (004) and (006) planes to determine strain and thickness of AlAs and GaAs layers in the DBR stack. Reciprocal space mappings (RSMs) are measured for same planes and also for (224) plane to find out tilt and relaxation of the DBR stack. Relaxation is not observed and it is confirmed with symmetric in-plane (400) Theta/2-Theta and RSM measurements. This is a first study in the literature according to the best of our knowledge. Finally, we have shown sensitivity of high angle diffraction planes to disorders in crystal. Angle-dependent reflectivity simulations have been also done and compared with measurements. 99.99% reflectivity is obtained with 99.5 nm stop bandwidth and 482.7 nm penetration depth.

Insights on the Alumina–Water Interface Structure by Direct Comparison of Density Functional Simulations with X-ray Reflectivity

Density functional theory molecular dynamics (DFT-MD) simulations are frequently used to predict the interfacial structures and dynamical processes at solid–water interfaces in efforts to gain a deeper understanding of these systems. However, the accuracy of these predictions has not been rigorously quantified. Here, direct comparisons between large-scale DFT-MD simulations and high-resolution X-ray reflectivity (XR) measurements of the well-defined Al2O3(001)/water interface reveal the relative accuracy of these two methods to describe interfacial structure, a comparison that is enabled by XR’s high sensitivity to atomic-scale displacements. The DFT-MD simulated and XR model-fit structures are qualitatively similar, but XR signals calculated directly from the DFT-MD predictions deviate significantly from the experimental data, revealing discrepancies in these two approaches. Differences in the derived interfacial Al2O3 relaxation profiles of ∼0.02 A within the top five layers are significant to XR, but a...

Epitaxial graphene-encapsulated surface reconstruction of Ge(110)

Understanding and engineering the properties of crystalline surfaces has been critical in achieving functional electronics at the nanoscale. Employing scanning tunneling microscopy, surface x-ray diffraction, and high-resolution x-ray reflectivity experiments, we present a thorough study of epitaxial graphene (EG)/Ge(110) and report a Ge(110) ``6 \ifmmode\times\else\texttimes\fi{} 2'' reconstruction stabilized by the presence of epitaxial graphene unseen in group-IV semiconductor surfaces. X-ray studies reveal that graphene resides atop the surface reconstruction with a 0.34 nm van der Waals (vdW) gap and provides protection from ambient degradation.

Structural differences between superconducting and non-superconducting CaCuO2/SrTiO3 interfaces

A study of the interface structure of superconducting and non-superconducting CaCuO2/SrTiO3 heterostructures grown on NdGaO3(110) substrates is reported. Using the combination of high resolution x-ray reflectivity and surface diffraction, the crystallographic structure of superconducting and non-superconducting samples has been investigated. The analysis has demonstrated the excellent sharpness of the CaCuO2/SrTiO3 interface (roughness smaller than one perovskite unit cell). Furthermore, we were able to discriminate between the superconducting and the non-superconducting phase. In the former case, we found an increase of the spacing between the topmost Ca plane of CaCuO2 block and the first TiO2 plane of the overlaying STO block, relative to the non-superconducting case. These results are in agreement with the model that foresees a strong oxygen incorporation in the interface Ca plane in the superconducting heterostructures.

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.

Zinc Adsorption and Hydration Structures at Yttria-Stabilized Zirconia Surfaces

Zinc adsorption and interfacial hydration on yttria-stabilized zirconia (YSZ) surfaces in contact with aqueous zinc solutions at room temperature and neutral pH have been probed with combined specular high-resolution X-ray reflectivity and element-specific (Zn) resonant anomalous X-ray reflectivity techniques. The total and partial zinc-specific electron density profiles in the surface normal direction show the detailed interfacial hydration structures with zinc adsorption. Strongly depending on its crystallographic orientations, the YSZ (110) surface adsorbs zinc species only within adsorbed water layers above the terminal plane, while on (111) surface, zinc further penetrates the substrate (below the terminal plane). Considering that both surfaces are enriched with oxygen vacancies and metal-depleted sites, on which chemisorbed water species are expected, the observed contrast indicates that specific zinc adsorption is controlled strongly by the intrinsic surface chemistry that results from orientation-...

Long-Term Cyclability of Electrochromic Poly(3-hexyl thiophene) Films Modified by Surfactant-Assisted Graphene Oxide Layers.

Organic electrochromic (EC) materials are generally known to be electrochemically unstable during the ion intercalation/deintercalation process. One effective method to stabilize them is incorporating graphene derivatives in the polymer matrix, thereby creating strong interaction between graphene derivatives and polymer. However, previous studies are limited to specific polymers and bulk-blended systems, such as mixing the polymer with graphene derivatives. In this study, we developed a polymer-graphene derivative complex with the chemical assistance of a surfactant (octadecylamine, ODA). Graphene oxide (GO) was introduced as a protective layer on the electrochromic poly(3-hexyl thiophene) (P3HT) films by the Langmuir-Schaefer method. The deposition of the GO-ODA protective layer with high coverage was confirmed by atomic force microscopy and high-resolution X-ray reflectivity. The strong interactions between GO-ODA and P3HT were examined with UV-vis spectrophotometry and X-ray photoelectron spectroscopy. Electrochemical and electrochromic investigations revealed that the GO-ODA layer greatly improved the long-term cyclability of the P3HT film. These findings imply that the GO-ODA complex can significantly stabilize the EC cycling, due to its strong interaction with the P3HT film.

Morphological anisotropy of nano-dimensional arsenic trisulfide glass films and liquid crystal photoalignment

ABSTRACTThe results of high-resolution x-ray reflectivity study of nano-dimensionally thin arsenic trisulfide (As2S3) glass films reveal surface roughness anisotropy upon prolonged irradiation with polarized 436 nm blue light. The anisotropy correlates with the photoinduced anisotropy and LC photoalignment observed on these thin films. However, the As2S3 film's thickness reduces drastically during the reflectivity experiments, and ambient oxygen and moisture penetrate the film and create a second altered thin layer on top of a thicker film of As2S3. The results confirm the viability of As2S3 films for non-contact LC photoalignment material but suggests the need for further process optimization to reduce the films' degeneration to increase their effectiveness.

Ion induced intermixing and consequent effects on the leakage currents in HfO2/SiO2/Si systems

Atomic layer deposited (ALD) samples with layer stacks of HfO2 (3 nm)/SiO2 (0.7 nm)/ Si were subjected to 120 MeV Au ion irradiation at different fluences to study intermixing effects across the HfO2/SiO2 interface. High-resolution Rutherford backscattering spectrometry (HRBS) and X-ray reflectivity (XRR) measurements confirm an increase in the interlayer thickness as a result of SHI induced intermixing effects. Current–voltage (I–V) measurements reveal an order of magnitude difference in the leakage current density between the pristine and irradiated samples. This can be explained by considering the increased physical thickness of interlayer (HfSiO). Furthermore, the samples were subjected to rapid thermal annealing (RTA) process to analyze annealing kinetics.

Effect of defects on reaction of NiO surface with Pb-contained solution

In order to understand the role of defects in chemical reactions, we used two types of samples, which are molecular beam epitaxy (MBE) grown NiO(001) film on Mg(001) substrate as the defect free NiO prototype and NiO grown on Ni(110) single crystal as the one with defects. In-situ observations for oxide-liquid interfacial structure and surface morphology were performed for both samples in water and Pb-contained solution using high-resolution X-ray reflectivity and atomic force microscopy. For the MBE grown NiO, no significant changes were detected in the high-resolution X-ray reflectivity data with monotonic increase in roughness. Meanwhile, in the case of native grown NiO on Ni(110), significant changes in both the morphology and atomistic structure at the interface were observed when immersed in water and Pb-contained solution. Our results provide simple and direct experimental evidence of the role of the defects in chemical reaction of oxide surfaces with both water and Pb-contained solution.

Complementary study of the internal porous silicon layers formed under high-dose implantation of helium ions

The surface layers of Si(001) substrates subjected to plasma-immersion implantation of helium ions with an energy of 2–5 keV and a dose of 5 × 1017 cm–2 have been investigated using high-resolution X-ray reflectivity, Rutherford backscattering, and transmission electron microscopy. The electron density depth profile in the surface layer formed by helium ions is obtained, and its elemental and phase compositions are determined. This layer is found to have a complex structure and consist of an upper amorphous sublayer and a layer with a porosity of 30–35% beneath. It is shown that the porous layer has the sharpest boundaries at a lower energy of implantable ions.