Specular X-ray Reflectivity Data Research Articles

Membrane Cholesterol Modulates Superwarfarin Toxicity

Superwarfarins are modified analogs of warfarin with additional lipophilic aromatic rings, up to 100-fold greater potency, and longer biological half-lives. We hypothesized that increased hydrophobicity allowed interactions with amphiphilic membranes and modulation of biological responses. We find that superwarfarins brodifacoum and difenacoum increase lactate production and cell death in neuroblastoma cells. In contrast, neither causes changes in glioma cells that have higher cholesterol content. After choleterol depletion, lactate production was increased and cell viability was reduced. Drug-membrane interactions were examined by surface X-ray scattering using Langmuir monolayers of dipalmitoylphosphatidylcholine and/or cholesterol. Specular X-ray reflectivity data revealed that superwarfarins, but not warfarin, intercalate between dipalmitoylphosphatidylcholine molecules, whereas grazing incidence X-ray diffraction demonstrated changes in lateral crystalline order of the film. Neither agent showed significant interactions with monolayers containing >20% cholesterol. These findings demonstrate an affinity of superwarfarins to biomembranes and suggest that cellular responses to these agents are regulated by cholesterol content.

Interfacial and electrical properties of radio frequency sputtered ultra-thin TiO2 film for gate oxide applications

The electrical responses of ultra-thin TiO2 films in the form of Al/TiO2/Si capacitors prepared by rf sputtering, have been studied in a wide frequency range as a function of post deposition annealing temperature. Thickness, interfacial and surface roughnesses of the heterostructures were extracted by fitting the specular X-ray reflectivity data. The TiO2 film annealed at 800 °C exhibited negligible hysteresis in the capacitance–voltage curve with the lowest interface trap density of 3.18 × 1011 cm−2 eV−1 extracted using Hill-Coleman method. Minimum equivalent oxide thickness (EOT) of 1.67 nm was obtained for the film annealed at annealed at 800 °C with leakage current density of 7.4 × 10−4 A/cm2 at −2 V. Space charge limited current (SCLC) conduction mechanism has been found to be dominant for the film annealed at 400 °C; however for the film annealed at 800 °C, it was found that at lower voltage (<2.5 V) the conduction mechanism is governed by SCLC with a transition to Fowler–Nordheim tunnelling at voltages higher than 2.5 V.

Complex impedance spectroscopy of high-k HfO2 thin films in Al/HfO2/Si capacitor for gate oxide applications

The dielectric responses of ultrathin (~6.65 nm) HfO2 films, in the form of Al/HfO2/Si capacitors were prepared by rf sputtering technique, has been studied in the wide frequency range as a function of deposition temperatures. Deposition temperatures were varied from room temperature (30 °C) to 500 °C. Thickness and the interfacial and surface roughness of heterostructures were extracted by fitting the specular X-ray reflectivity data. The impedance analysis combined with modulus spectroscopy was performed to get insight of the microscopic features like grain, grain boundary and film–electrode interfaces and their effects in the film properties. The films exhibited maximum frequency dispersion in both real and imaginary part of impedance at low frequency range. The frequency analysis of the modulus and impedance studies showed the distribution of the relaxation times due to the presence of grains and grain boundaries in the films. Impedance analysis revealed that the interfacial polarization caused by space charges in the film/electrode interfaces plays an important role in the dielectric behavior of the capacitor. In order to explain effectively that the impedance plots contain one or two arcs due to more than one relaxation contributions, the results are interpreted using the approach proposed by Abrantes (Z / vs. |Z // |/f representation). The dielectric loss (tan δ) curves exhibited the fact that there is possibility of existence of a Schottky barrier at the insulator semiconductor interface, which is due to traps distributed throughout the semiconductor-insulator interface and it is believed to be due to auto doping during deposition process. The ac conductivity, σ ac (ω), varies as σ ac (ω) = Bω n with n in the range 0.06–0.71.

Is the Calcite–Water Interface Understood? Direct Comparisons of Molecular Dynamics Simulations with Specular X-ray Reflectivity Data

New insights into the understanding of calcite–water interface structure are obtained through direct comparisons of multiple classical molecular dynamics (MD) simulations with high-resolution specular X-ray reflectivity (XR) data. This set of comparisons, with four different state-of-the-art force fields (including two nonpolarizable, one polarizable, and one reactive force field), reveal new insights into the absolute accuracy of the simulated structures and the uniqueness of the XR-derived structural results. These four simulations, though qualitatively similar, have visibly distinct interfacial structures and are distinguished through a quantitative comparison of the XR signals calculated from these simulations with experimental XR data. The results demonstrate that the simulated calcite–water interface structures, taken as a whole, are not consistent with the XR data (i.e., within the precision and accuracy of the XR data). This disagreement is largely due to the simulated calcite interfacial structure. The simulated interfacial water profiles show a higher level of consistency with the XR data, but with substantially different levels of agreement, with the rigid-ion model (RIM) simulations showing semiquantitative agreement. Further comparisons of the structural parameters that describe the interfacial structure (derived from both the MD simulations and the XR data) provide further insight into the sources of differences between these two approaches. Using the new insights from the RIM simulations, new structures of the calcite–water interface consistent with both the experimental data and the simulation are identified and compared to recent results.

Characteristics of Strontium Bismuth Tantalate Film with ZrO2 Buffer Layer for Non-Volatile Memory Applications

We have investigated the interfacial and electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi2Ta2O9 ferroelectric films grown on ZrO2 buffer layer coated Si. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. For as-deposited film, no interfacial layer is found between SBT and ZrO2 from simulation data, however, an accurate fit can only be possible if a thin SiO2 layer is assumed at ZrO2/Si interface. On annealing, the formation of an inhomogeneous interfacial layer between SBT/ZrO2 interfaces having thickness 1.3 nm is observed.

Stochastic fitting of specular X-ray reflectivity data usingStochFit

Specular X-ray reflectivity data provide detailed information on the electron density distribution at an interface. Typical modeling methods involve choosing a generic electron density distribution based on an initial speculation of the electron density profile from the physical parameters of the experimental system. This can lead to a biased set of solutions.StochFitprovides stochastic model-independent and model-dependent methods for analyzing X-ray reflectivities of thin films at an interface.StochFitdivides an electron density profile into many small boxes and stochastically varies the electron density of these boxes to locate the best fit to a measured reflectivity. Additionally, it provides the ability to perform model-dependent fitting with a stochastic search of the parameter space in order to locate the best possible fit. While model-independent profile search algorithms have been described previously, they are difficult to implement because of the heavy computational requirements, and there is a dearth of software available to the general scientific public utilizing these techniques. Several cases that illustrate the usefulness of these techniques are presented, with a demonstration of how they can be used in tandem.

Interfacial and electrical properties of SrBi2Ta2O9/ZrO2/Si heterostructures for ferroelectric memory devices

We have investigated the interfacial and frequency dependent electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi2Ta2O9 (SBT) ferroelectric films grown on ZrO2 buffer layer coated Si. Heterostructure SBT and ZrO2 thin films were deposited using rf magnetron sputtering. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. The structure exhibited clockwise capacitance-voltage hysteresis with a maximum memory window of 2.0 V at a bias voltage of ±7 V. Frequency dependent (5 kHz–1 MHz) measurements at room temperature indicated that the clockwise hysteresis originates from the ferroelectric domain reversal. A minimum leakage current density of 4×10−8 A/cm2 of fabricated capacitors at an applied voltage of ±5 V revealed that the ZrO2 buffer layer prevents the interfacial diffusion between SBT thin film and the substrate, resulting in an improvement of interface quality. The charge retention time of the ferroelectric capacitor was studied as a function of buffer layer thickness.

Adsorption of Rb + and Sr 2+ at the orthoclase (0 0 1)–solution interface

Adsorption of Rb + and Sr 2+ at the orthoclase (0 0 1)–solution interface is probed with high-resolution X-ray reflectivity and resonant anomalous X-ray reflectivity. Specular X-ray reflectivity data for orthoclase in contact with 0.01 m RbCl solution at pH 5.5 reveal a systematic increase in electron density adjacent to the mineral surface with respect to that observed in contact with de-ionized water (DIW). Quantitative analysis indicates that Rb + adsorbs at a height of 0.83 ± 0.03 Å with respect to the bulk K + site with a nominal coverage of 0.72 ± 0.10 ions per surface unit mesh (55.7 Å 2). These results are consistent with an ion-exchange reaction in which Rb + occupies an inner-sphere adsorption (IS) site. In contrast, X-ray reflectivity data for orthoclase in contact with 0.01 m Sr(NO 3) 2 solution at pH 5.3 reveal few significant changes with respect to DIW. Resonant anomalous X-ray reflectivity was used to probe Sr 2+ adsorption and to image its vertical distribution. This element-specific measurement reveals that Sr 2+ adsorbs with a total coverage of 0.37 ± 0.02 ions per surface unit mesh, at a substantially larger height (3.28 ± 0.05 Å) than found for Rb +, and with a relatively broad density distribution (having a root-mean-square width of 1.88 ± 0.08 Å for a single-peak model), implying that Sr 2+ adsorbs primarily as a fully-hydrated outer-sphere (OS), species. Comparison to a two-height model suggests that 13 ± 5% of the adsorbed Sr 2+ may be present as an IS species. This partitioning implies a ∼5 kJ/mol difference in free energy between the IS and OS Sr 2+ on orthoclase. Differences in the partitioning of Sr 2+ between IS and OS species for orthoclase (0 0 1) and muscovite (0 0 1) suggest control by the geometry of the IS adsorption site. Results for the OS distribution are compared to predictions of the Poisson–Boltzmann equation in the strong coupling regime, which predicts an intrinsically narrow vertical diffuse ion distribution; the OS distribution might thus be thought of as the diffuse ion profile in the limit of high surface charge.

Capillary wave fluctuations and intrinsic widths of coupled fluid-fluid interfaces: An x-ray scattering study of a wetting film on bulk liquid

An x-ray specular reflectivity (XR) and off-specular diffuse scattering (XDS) study of the coupled thermal capillary fluctuations and the intrinsic profiles of two interacting fluid-fluid interfaces is presented. The measurements are carried out on complete wetting films of perfluoromethylcyclohexane (PFMC) on the surface of bulk liquid eicosane (C20), as a function of film thickness 30<D<160 A. In order to facilitate the analysis and interpretation of the data with minimal complexity, approximate methods for calculating scattering intensities are developed to take into account the subtleties of thermal diffuse scattering from layered liquid surfaces. With these methods, the calculations of XR/XDS intensities are reduced to a single numerical integration of simple functions in real space. In addition, an analytic expression is derived for small-angle XR that contains Debye-Waller-like factors with effective capillary roughness and takes into account the partial correlations of the two interfaces. The expression for the XR is quantitatively accurate so long as the reflection angle is small enough that the scattering from interfaces is distinguishable from bulk scattering. The results of the XR and XDS data analysis indicate that the capillary fluctuations at the two interfaces of the wetting films are partially correlated and their coupling is consistent with the van der Waals interactions. The relatively large intrinsic width (4 approximately 6A) of the liquid-liquid interface observed for thicker films (D greater than or similar to 50 A) is comparable to the value expected for the bulk liquid-liquid interface (D-->infinity), determined by either the radius of gyration (5.3 A) or the bulk correlation length (4.8 A) of the alkane C20. The intrinsic liquid-vapor interfacial width is sharper (approximately 2 A) and remains essentially constant over the entire probed range of D .

On the use of CCD area detectors for high-resolution specular X-ray reflectivity

The use and application of charge coupled device (CCD) area detectors for high-resolution specular X-ray reflectivity is discussed. Direct comparison of high-resolution specular X-ray reflectivity data measured with CCD area detectors and traditional X-ray scintillator ('point') detectors demonstrates that the use of CCD detectors leads to a substantial (approximately 30-fold) reduction in data acquisition rates because of the elimination of the need to scan the sample to distinguish signal from background. The angular resolution with a CCD detector is also improved by a factor of approximately 3. The ability to probe the large dynamic range inherent to high-resolution X-ray reflectivity data in the specular reflection geometry was demonstrated with measurements of the orthoclase (001)- and alpha-Al2O3 (012)-water interfaces, with measured reflectivity signals varying by a factor of approximately 10(6) without the use of any beam attenuators. Statistical errors in the reflectivity signal are also derived and directly compared with the repeatability of the measurements.

Lipid Headgroup Discrimination by Antimicrobial Peptide LL-37: Insight into Mechanism of Action

Interaction of the human antimicrobial peptide LL-37 with lipid monolayers has been investigated by a range of complementary techniques including pressure-area isotherms, insertion assay, epifluorescence microscopy, and synchrotron x-ray scattering, to analyze its mechanism of action. Lipid monolayers were formed at the air-liquid interface to mimic the surface of the bacterial cell wall and the outer leaflet of erythrocyte cell membrane by using phosphatidylglycerol (DPPG), phosphatidylcholine (DPPC), and phosphatidylethanolamine (DPPE) lipids. LL-37 is found to readily insert into DPPG monolayers, disrupting their structure and thus indicating bactericidal action. In contrast, DPPC and DPPE monolayers remained virtually unaffected by LL-37, demonstrating its nonhemolytic activity and lipid discrimination. Specular x-ray reflectivity data yielded considerable differences in layer thickness and electron-density profile after addition of the peptide to DPPG monolayers, but little change was seen after peptide injection when probing monolayers composed of DPPC and DPPE. Grazing incidence x-ray diffraction demonstrated significant peptide insertion and lateral packing order disruption of the DPPG monolayer by LL-37 insertion. Epifluorescence microscopy data support these findings.

Model-independent one-dimensional imaging of interfacial structures at&lt;1Åresolution

The feasibility of model-independent error-correction algorithms is established for imaging buried interfacial structures in one dimension. Using the experimental structure factor modulus from x-ray specular reflectivity data, we directly compare structures derived by a modified Fienup algorithm using a known reference structure and that derived by traditional (model-dependent) least-squares fitting. This demonstrates the utility of this approach for determining one-dimensional interfacial electron density profiles with spatial resolution of $&lt;1\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The potential applications and limitations of this approach are discussed.

Determination of interface roughness of Gd films deposited on Si surface using improved wavelet transform of X-ray reflectivity data

An improved wavelet transform method for the analysis of specular X-ray reflectivity data has been developed. It permits the evaluation of the thickness and roughness of particular layers in the thin film without assuming a certain film structure. The advantage of this method is that it can be applied to the analysis of a multilayer with unknown chemical and physical properties. It is useful in the characterization of structures with a complex composition, particularly oxide or diffuse layers. The present approach was successfully applied to the study of experimental data obtained on Gd thin nanoparticle films.

LB films of rodlike phthalocyanine aggregates: specular X-ray reflectivity studies of the effect of interface modification on coherence and microstructure.

We present here X-ray specular reflectivity (XRR) characterization of the ordering of Langmuir-Blodgett films of the liquid crystalline phthalocyanine (Pc) 2,3,9,10,16,17,23,34-octakis(2-benzyloxyethoxy)copper(II) phthalocyanine, 1, on Si(100) wafers with a native oxide layer and these same substrates modified with monolayers of varying percentages of methyl- and phenyl-terminated silanes. The central copper atom in these Pc's provides for high contrast in X-ray reflectivity for single-bilayer films of 1. The XRR data are modeled as arising from multiple layers of organic material, above and below the rows of copper atoms in the aligned Pc cores; variations in the total thickness of these films, and the spacing between the rows of copper atoms, are attributed to changes in the interaction with the substrate, and changes in the Pc orientation and side chain packing. The most pronounced effect on these parameters comes from variations in the ratio of the phenyl-silane versus methyl-silane content of the substrate modifiers and annealing of these films past their crystalline (K) --> liquid crystalline (LC) mesophase transition temperature. Transfer of multiple bilayers of this Pc leads to additional changes in the thickness of each layer, eventually forming a hexagonal close-packed array, reminiscent of bulk fibers of this material, as revealed by X-ray diffraction (XRD).

Characterization of pore structure in a nanoporous low-dielectric-constant thin film by neutron porosimetry and X-ray porosimetry.

A small-angle neutron scattering (SANS) porosimetry technique is presented for characterization of pore structure in nanoporous thin films. The technique is applied to characterize a spin-on organosilicate low dielectric constant (low-k) material with a random pore structure. Porosimetry experiments are conducted using a "contrast match" solvent (a mixture of toluene-d8 and toluene-h8) having the same neutron scattering length density as that of the nanoporous film matrix. The film is exposed to contrast match toluene vapor in a carrier gas (air), and pores fill with liquid by capillary condensation. The partial pressure of the solvent vapor is increased stepwise from 0 (pure air) to P0 (saturated solvent vapor) and then decreased stepwise to 0 (pure air). As the solvent partial pressure increases, pores fill with liquid solvent progressively from smallest to largest. SANS measurements quantify the average size of the empty pores (those not filled with contrast match solvent). Analogous porosimetry experiments using specular X-ray reflectivity (SXR) quantify the volume fraction of solvent adsorbed at each step. Combining SXR and SANS data yields information about the pore size distribution and illustrates the size dependence of the filling process. For comparison, the pore size distribution is also calculated by application of the classical Kelvin equation to the SXR data.

Large and small angle x-ray scattering studies of Si/SiGe superlattices grown by gas-source molecular beam epitaxy

We have performed large and small angle x-ray scattering measurements on Si/Si0.7Ge0.3 superlattices grown by gas-source molecular beam epitaxy. The thickness of the Si and SiGe layers as well as their Ge content were extracted through a correlation between large-angle x-ray diffraction and photoluminescence measurements. A SiGe growth rate enhancement was observed during the first stages of the SiGe layer deposition, which might be due to a slow build-up of the Ge surface population. The Fresnel optical method was used to analyse the small-angle specular x-ray reflectivity data. The interface root mean square roughness, 7 ± 1 Å for the samples grown at 520 and 580 °C, and 5 ± 1 Å for the sample grown at 550 °C, are quite comparable to those found in Si/SiGe superlattices grown by solid source MBE. The crystallographic quality of those stackings is very good; no measurable period dispersion has been obtained in x-ray diffraction.

X-Ray Reflectivity and FTIR Measurements of N 2 Plasma Effects on the Density Profile of Hydrogen Silsesquioxane Thin Films

The density depth profile and chemical bond structure of hydrogen silsesquioxane (HSQ) thin films treated with an plasma with varying power and exposure time were measured using specular X-ray reflectivity (SXR) and Fourier transform infrared (FTIR) spectroscopy. The SXR data indicated that the density profile of an untreated HSQ film is not uniform. At least four layers with different electron densities were required to fit the SXR data. For HSQ films treated with either increasing plasma power or plasma exposure time, the film roughness increased and a densified layer was observed at the film/air interface. The thickness of the densified layer increased with both plasma power and plasma exposure time. In the FTIR spectra of the plasma-treated films, the intensities of the Si-O peaks due to the network structure and the Si-OH peak due to water absorption increased and the intensities of the Si-H peaks decreased. The FTIR data show that the plasma converts the HSQ structure into a -like structure and are consistent with the densification observed in the SXR measurements. In general, the HSQ film is more sensitive to increasing plasma power than to increasing plasma exposure time. © 2001 The Electrochemical Society. All rights reserved.

Study of thin film multilayers using X-ray reflectivity and scanning probe microscopy

We have presented new schemes to analyse grazing incidence specular X-ray reflectivity data to obtain structural and chemical information of thin films. Analysis of specular reflectivity data gives information along the depth of the film, whereas, analysis of non-specular data reveals the structural information across the film surface and interfaces. The schemes proposed are based on the Born approximation and the distorted wave born approximation (DWBA). Surface structural parameters such as, height–height correlation and roughness exponent of the film obtained from the analysis of X-ray reflectivity was compared with results obtained from atomic force microscopy

Investigation of N2 Plasma Effects on the Density Profile of Hydrogen Silsesquioxane Thin Films

ABSTRACTThe density depth profile and chemical bond structure of hydrogen silsesquioxane (HSQ) thin films treated with an N2 plasma with varying power and exposure time were measured using specular x-ray reflectivity (SXR) and Fourier transform infrared (FTIR) spectroscopy. The SXR data indicate that the density profile of an untreated HSQ film is not uniform and four layers with different electron densities were required to fit the SXR data. For HSQ films treated with either increasing plasma power or plasma exposure time, the film roughness increased and a densified layer was observed at the film/air interface. The thickness of the densified layer increased with both plasma power and plasma exposure time. As a result, up to seven distinct layers were used to model the experimental SXR data from plasma treated films. The FTIR data show that the plasma transforms the Si-H bonds in the HSQ film into Si-O bonds leaving more oxygen atoms around a Si atom. These data are also consistent with the densification observed in the SXR measurements. In general, the HSQ film is more sensitive to increasing plasma power than to increasing plasma exposure time.

Structure refinement of infinite-layer superlattices

The structure of (BaCuO 2 ) 2 /(CaCuO 2 ) 2 superconducting superlattices (SLs), obtained by the pulsed-laser deposition technique, was investigated by X-ray diffraction (XRD) and X-ray specular reflectivity (XSR). Simulations of XRD and XSR data obtained by varying the structure factor and, consequently, the chemical composition of the hypothesized structure lead to an accurate structural determination of the investigated SLs. For the (BaCuO 2 ) 2 layers of the investigated superconducting SLs we found that the copper-to-barium ratio obtained by the simulations is less than one. Moreover, the copper vacancies are localized only in the CuO 2 planes between barium atoms. Non-stoichiometric compounds with an excess of oxygen are compatible with the experimental data, indicating the Ba 2 Cu 2-x O 4-y layers to be the reservoir charge blocks' in the superconducting SLs.