Neutron Reflectivity Data Research Articles

Observation of a cubic phase in mesoporous silicate films grown at the air/water interface

X-Ray diffraction is reported from mesoporous silicate films grown at the air/water interface. The films were studied both as powdered films, and oriented on silicon or mica sheets. At early stages of growth we observe Bragg diffraction from a highly ordered cubic phase, with both long and short d-spacing peaks. We have assigned this as a discontinuous micellar Pm3n phase in which the silica is partly ordered. Later films retain only the known hexagonal p6m peaks and have lost any order both at short d-spacings and the longer d-spacing Bragg peaks characteristic of the cubic structure. The silica framework is considerably expanded from that in bulk amorphous silica, average Si···Si distances are some 30% greater. Incorporation of glycerol or polyethylene glycol preserves the earlier cubic structure. To be consistent with earlier, insitu, X-ray and neutron reflectivity data we infer that both structures are produced after a phase transition from a less-ordered film structure late in the induction phase. The structural relations between the film Pm3n and p6m phase(s) and the known bulk SBA-1 and MCM-41 phases are briefly discussed.

Layer-by-layer assembled protein/polymer hybrid films: nanoconstruction via specific recognition

In the present study it is shown that streptavidin-containing multilayer films with varying numbers of polyelectrolyte spacer layers can be fabricated reproducibly using optimized deposition conditions. Direct alternation of streptavidin and PLB leads to multilayer systems with an average streptavidin thickness of 5.3 nm which is in good agreement with the dimensions of the protein. When the streptavidin layers are spacered by more polyelectrolyte layers the distance between the protein sheets is increased up to e.g. 6.5 nm in the case of (PLB/PSS/PAH/PSS/PLB) as spacer layer. X-ray reflectivity reveals that streptavidin increases the surface roughness of the films probably due to the rigid three-dimensional structure of the protein. The control of surface roughness seems to be essential for a successful multilayer build-up. The property of PLB to provide for multilayer construction by two different interactions (electrostatic and specific) allowed to probe the interpenetration depth of adjacent layers. For the [PLB/(PSS/PL) 2/streptavidin] system an interpenetration depth of about 4 polymer layers corresponding to approximately 3.4 nm has been derived. These data are in agreement with a model for pure polyelectrolyte films obtained from neutron and X-ray reflectivity data.

Deuterium phase behavior in thin-film Pd

The absorption of deuterium from the gas phase into two Pd thin films 668 \AA{} and 1207 \AA{} thick was measured at room temperature with in situ neutron reflectometry. Room-temperature solubility isothermal curves, out-of-plane film expansion, and deuterium depth profiles were determined from fits to the neutron reflectivity data. The measurements demonstrate that the deuterium solubility behavior, both in solid solution and within the two-phase region, is strongly perturbed by the thin-film geometry, consistent with previous solubility measurements in the published literature. The phase behavior investigated here was observed to depend on film thickness and on deuterium cycling through the two-phase region. The 668-\AA{} film exhibited the greatest initial phase perturbation and most significant changes upon cycling. Upon repeated cycling, both films approach nearly identical deuterium isothermal solubility and out-of-plane expansion behaviors. The observed equilibrium out-of-plane expansion behavior was consistent with the films expanding under an in-plane clamping constraint imposed by the substrate. The effect of this substrate constraining force is to amplify the out-of-plane expansion beyond that expected in bulk Pd. Taken together, these measurements implicate the film/substrate interfacial clamping interaction as the origin of the perturbed hydrogen phase behavior in thin-film geometry.

On the reflectivity of reactively sputtered Ni/Ti multilayers

X-ray and neutron reflectivities of multilayers are strongly influenced by interfacial properties. Typical examples are Ni/Ti multilayers sputtered in Ar showing low reflectivity because of the large interface roughness and interdiffusion. Reactive sputtering of Ni layers in an Ar/air mixture has been chosen for accomplishing the task of improving the reflectivity. Bilayer samples with 20 and 300 periods, sputtered at different air flows clearly demonstrate a substantial change in the interface structure as confirmed by the X-ray reflectivity measurements. The contour maps of the diffuse X-ray scattering data also exhibit similar features. These findings are also supported by neutron reflectivity data on supermirrors.

Neutron reflection studies of titanium segregation to metal–ceramic interfaces

Titanium is an extremely reactive element which is known to promote the wetting of ceramics by liquid metals. The wetting process is believed to be controlled by the segregation of a very thin layer of Ti to the metal/ceramic interface where it undergoes a localised redox reaction with the ceramic. This hypothesis is consistent with cross-sectional microanalysis studies of metal–ceramic interfaces where thin layers of substoichiometric titanium oxides have been identified. The study of titanium segregation is ideally suited to neutron reflection because of the negative scattering length of the dominant isotope. In this study we have characterised the chemical composition of three interfaces between metals and a sapphire (Al 2O 3) single crystal using specular neutron reflection. The interfaces are: Sn/sapphire, Sn/sapphire containing a thin (≈20 nm) Ti interlayer introduced by physical vapour deposition (PVD), and an interface between sapphire and a commercial Ag-Cu-Ti alloy used to braze ceramics. We have evaluated the neutron reflection data collected from the CRISP reflectometer at the ISIS facility using a multilayer model of the interface. The results indicate that the 20 nm evaporated layer cannot be distinguished from interface roughness while the interface between sapphire and the commercial braze alloy contains a thin (≈70 nm) titanium suboxide layer.

Measurement of the Dynamic Surface Excess in an Overflowing Cylinder by Neutron Reflection

Neutron reflection has been used to measure the dynamic surface concentration, Γdyn, of the cationic surfactant CTAB in an overflowing cylinder (OFC). This is the first direct measurement of Γdyn for a surfactant at an expanding liquid surface. Γdyn was measured for bulk concentrations between 1.8 and 0.125 mM, corresponding to areas per molecule of 42 to >300 A2. Careful account had to be taken of the curvature of the surface in establishing calibration procedures. Γdyn was less than the equilibrium surface concentration Γe at all concentrations below the cmc but approached Γe at bulk concentrations well above the cmc, as expected for an expanding surface. Ellipsometry has been used previously to measure the coefficient of ellipticity of surfactant solutions in an OFC. The neutron reflection data were used to calibrate the ellipticity of the surface in terms of Γdyn, and ellipsometry was then used to map changes in Γdyn across the surface of the OFC with a precision of 2 × 10-8 mol m-2. The ellipticity c...

Influence of Anchor Block Size on the Thickness of Adsorbed Block Copolymer Layers

We present surface force data on three different polystyrene/poly(2-vinylpyridine) block copolymers (PS/P2VP) with a fixed size of the nonadsorbing PS block but widely varying sizes of the adsorbing P2VP block. With respect to the sizes of the two blocks, they range from moderately to highly asymmetric. The equilibrium force profiles are almost overlapping, which means that the variation in layer thickness is very small over a large range of anchor block size. This finding is in disagreement with the predictions of simple scaling models for polymer brushes. However it agrees with findings from neutron reflectivity data on a comparable series of PS/P2VP block copolymers. We also find agreement with recent neutron reflectivity experiments on poly(dimethylsiloxane)/polystyrene and with self-consistent field calculations on selectively adsorbed block copolymers.

Polarised neutron reflectivity from a [UAs/Co] multilayer

We present X-ray and polarised neutron reflectivity data from a Co(200 Å)/12[UAs(80 Å)/Co(20 Å)] multilayer. There is reasonable agreement between values of the thicknesses determined from the X-ray and those from the neutron data; both indicate slightly larger UAs thicknesses than of the nominal structure. Analysis of the polarised neutron data indicates that there is magnetism in all layers. However, due to the large ferromagnetic signal from the underlying cobalt, magnitudes of the moments in the constituent layers have not been determined unambiguously.

The Influence of Sorbitol on the Adsorption of Surfactants at the Air–Liquid Interface

Neutron reflection and surface tension have been used to study the adsorption of the nonionic surfactant monododecyl hexaethylene glycol (C12E6) and the mixed nonionic–anionic surfactantsn-dodecyl triethylene glycol (C12E3) and Sodium dodecyl sulfate (SDS) at the air–liquid interface. Water and a sorbitol (d-glucitol, CH2OH(HCOH)4CH2OH)/water mixture were selected as the solvents. The addition of 300 g/liter of sorbitol to an aqueous solution of C12E6reduces the critical micellar concentration, cmc, from ∼7 × 10−5to ∼3 × 10−5M,and increases the surface pressure at the cmc; the effect on the C12E3/SDS mixture is less. Despite these changes, the pattern of adsorption at the air–liquid interface is essentially unaltered by the addition of sorbitol, and the surface tension and neutron reflectivity data are in good agreement. The consequences of the sorbitol addition are however seen directly in the structure of the C12E6monolayer. The ethylene oxide (EO) chain is more extended than in water, and more displaced from the solvent, consistent with dehydration of the ethylene oxide group.

Antiferromagnetic interlayer correlations in annealed Ni80Fe20/Ag multilayers.

Sputtered ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$/Ag multilayers, annealed post-growth, exhibit giant magnetoresistance (GMR) with pronounced field sensitivity [T. L. Hylton et al., Science 261, 1021 (1993)]. We have characterized a series of ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$(20 \AA{})/Ag(40 \AA{}) multilayers annealed at temperatures ranging from 305 to 335\ifmmode^\circ\else\textdegree\fi{}C using x-ray and polarized neutron reflectivity techniques. For all of the samples, specular x-ray measurements reveal that the laterally averaged interfaces between the ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ and Ag layers are not well defined. The growth-plane morphology of the multilayers, determined from off-specular x-ray diffraction, shows a dependence on annealing temperature. Specular and off-specular polarized neutron reflectivity data indicate that the GMR in the annealed samples does not arise from long-range antiferromagnetic alignment of coherent ferromagnetic sheets, as generally observed in related materials. Instead, annealing promotes the formation of planar ferromagnetic domains of micrometer size within each ${\mathrm{Ni}}_{80}$${\mathrm{Fe}}_{20}$ layer that are antiferromagnetically correlated along the growth axis. The length scales of these domains are consistent with a model in which weak dipolar forces dominate the interactions between them. \textcopyright{} 1996 The American Physical Society.

Antiphase domains in Langmuir-Blodgett films

For bilayer stacks of non-centrosymmetrical molecules, formed by the Langmuir-Blodgett (LB) technique (Y deposition), 001 peaks for the bilayer period are usually observed in the X-ray specular reflectivity curve. In this study, we show that the introduction of stacking disorder at lower transfer ratios and the subsequent partial repair of defects by a parallel shift of oppositely oriented molecules into the vacancies may result in the formation of antiphase domains and in an increase in the average monolayer symmetry followed by quenching of the bilayer peaks. The X-ray reflectivity data, obtained for multilayers of penta-alkynes, are used to demonstrate this real structural effect, which is expected to be of general importance for X-ray and neutron reflectivity data reduction and for the structural modelling of multilayers.

Theoretical and experimental investigations of adsorbed protein structure at a fluid interface

AbstractThe structure of adsorbed layers of the disordered milk protein β‐casein at the air‐water interface determined by specular neutron reflectivity measurements is presented and compared with results obtained from self‐consistent‐field modelling of a β‐casein look‐alike molecule at a solid‐water interface. Changes in the layer structure with substrate pH in the range pH 5.5‐7.0 have been determined experimentally and have been estimated theoretically. At pH 7 the neutron reflectivity data are fitted using a two‐layer model of the adsorbed protein film consisting of a thin (≈Å) protein‐rich region (φ≈0.95) at the interface and a thicker (≈45 Å), more diffuse (φ≈0.15), layer extending into the bulk aqueous phase. On reducing the substrate pH towards the protein isoelectric point, a thickening of the adsorbed layer is observed which is attributed to secondary adsorption of protein parallel to the monolayer observed at pH 7. The thicknesses of both layers in the fitted model, and the volume fraction of protein in the diffuse layer, increase to accommodate the extra protein. A model, linear chain polyelectrolyte consisting of apolar, polar (uncharged) and polar (pH‐dependent charge) residues is constructed in a sequence which mirrors the known primary structure of β‐casein. The adsorption behaviour of this casein look‐alike at a solid‐water interface is modelled using the self‐consistent‐field theories of Scheutjens and Fleer, under similar conditions of pH as for the specular neutron reflection experiments. Total segment density profiles are obtained which show very good qualitative agreement with the experimental results. At neutral pH, the monolayer profile φ(z) falls off in a smooth fashion from very high values at the interface (φ≈0.95) to values close to the bulk concentration over about 50 Å. On reducing the pH, the adsorbed amount is seen to increase, with the additional material appearing as a shoulder on the φ(z) profile, as observed in the neutron experiments.

The simultaneous analysis of multi-contrast neutron and X-ray grazing incidence reflection data from amphiphilic layers

When adsorbed from solution amphiphilic molecules form thin layers, the density of which varies smoothly across the interface. Because the reflectivity of such interfaces varies subtly from that of the underlying solution, much effort is required to optimise both the experiments performed and the analysis of experimental data in order to determine the structure of these interfaces to high resolution. In particular, it is vital to make use of contrast variation, both by manipulating the neutron scattering length densities of all or some of the species present at the interface, but also by measuring X-ray reflection which provides another contrast possibility, and has the advantage of a larger momentum transfer range than is currently possible for neutron reflection. Here we describe a new method of data analysis, based on the kinematic approximation, which is particularly suited to the analysis of multi-contrast neutron and X-ray reflection data. The method performs a non-linear least-squares minimisation on all reflection measurements simultaneously. The minimisation may be performed subject to various physical constraints. This is important if the structure of the interface is to be obtained to high resolution. We apply the method to the study of amphiphiles adsorbed from solution. The examples we take demonstrate its use when contrast variation is possible to different extents.

Toward the solution of the inverse problem in neutron reflectometry

We show that the chemical depth profile of a film of unknown structure can be retrieved unambiguously from neutron reflection data by adding to the system a known magnetic layer. Three independent reflectivities are obtained by taking measurements with the sample magnetized in a magnetic field perpendicular to the surface and subsequently parallel to it, and using in the latter geometry neutrons polarized either in the direction of the field or opposite to it. The procedure consists of two steps. First, from the three reflectivities both the real and imaginary parts of the reflection coefficient of the unknown film are extracted within the framework of the rigorous dynamical theory. Second, the neutron scatteringlength density (and consequently the chemical depth profile) is obtained by a suitable numerical technique for the conventional Schrödinger inverse scattering problem. Computer experiments were conducted for selected cases: starting from the profiles the reflectivities were calculated in a limited range of q and then the original profiles were successfully recovered. The influence on the accuracy of the recovered depth profile of the counting statistics and the cutoffs at low and high q are discussed.

Phase effects in neutron reflection by microemulsions and polymers

The effect of the reflection phase on the determination of scattering-length density profiles by inversion of neutron reflection data is investigated. Use is made of a logarithmic dispersion relation between the phase and the reflectivity. Examples of neutron reflection by microemulsions and polymers are considered.

Direct Fitting of Grazing-Incidence Neutron and X-ray Reflectivity Profiles using the Kinematic Approximation

At present, neutron and X-ray reflection data have been analysed either by modelling, using the optimal-matrix method, or by the fitting of partial structure factors arising from application of the kinematic approximation. Here the direct and simultaneous fitting of neutron and X-ray reflection profiles measured for a series of isotopic variations of the same interface is described. The interface is described as consisting of several components, which are either groups of atoms in a molecule or a whole molecule itself. In this work, the density distributions of the components perpendicular to the interface are described as Gaussian or tanh functions. These forms are chosen because they have analytical Fourier transforms. Fitting reflectivity profiles rather than partial structure factors reduces the sensitivity of the analysis to imperfections in one or more data sets. With existing kinematic methods, it is difficult to combine X-ray and neutron measurements. The new method readily allows such a combination, which is of importance if the capacity for isotopic substitution is limited or information about the counter-ion distribution is desired. It is also demonstrated that constraints posed by the stoichiometry of the interfacial species, by volume filling and molecular geometry, or from the results of the application of other experimental techniques, may readily be incorporated into the new fitting procedure.

A Neutron Reflectivity Study of Spread Monolayers of Eicosanoic Acid, Valinomycin, and Their Equimolar Mixture

Neutron reflectivity profiles and surface pressure (Π)-area (A) isotherms were obtained for spread monolayers of eicosanoic acid, valinomycin, and their equimolar mixture at pH 5.8 and 296 K. The neutron data for the individual components showed that the molecules of deuterated eicosanoic acid were oriented almost vertically at a surface pressure as low as 12 mN m-1, in agreement with the low compressibility of this portion of the Π-A isotherm, whereas the molecules of valinomycin lie flat in the surface. The Π-A isotherm for the mixture was the average of that for the individual components, which implies little interaction between unlike molecules. It was not possible to model the neutron reflectivity data for this system using an intimately mixed monolayer of the two molecular species. However, the assumption that the monolayer consisted of separate islands of eicosanoic acid and valinomycin gave good-quality fits to the data on both a D2O and a contrast-matched water subphase. This lack of cross interaction among the molecules of an archipelago of two species explains the Π-A data.

Problems in the analysis and interpretation of neutron reflection data

We consider several sources of error in the interpretation of specular reflectivity data to obtain structural detail of an interface. We classify the errors into four types; ambiguity in the derivation of the scattering length density profile; ambiguity in the subsequent derivation of the profile of the different components at the interface; errors resulting from inaccuracies or approximations; sample dependent errors. Examples of each are given and analysed.

Magnetic profile as a function of structural disorder in Fe/Cr superlattices

Structural disorder has been systematically introduced in an [Fe(30 Å)/Cr(17 Å)]10 superlattice to investigate its effects on the magnetization profile. Low angle x-ray and neutron scattering results indicate progressive structural degradation in samples produced with increased argon pressure (3, 6, and 12 mTorr). The x-ray spectra from the more disordered samples are characterized by a decreased intensity and increased linewidth of the superlattice Bragg peaks, along with a systematic disappearance of the finite size intensity oscillations. Elastic, polarized, low angle, neutron reflectivity data exhibit decreasing superlattice Bragg intensities and increasing linewidths, indicating increased disorder in agreement with x-ray data. Antiferromagnetic alignment of adjacent Fe layers is inferred by the appearance of magnetic peaks between the structural peaks and by the lack of magnetic contribution to the structural superlattice peaks. Interestingly, the magnetic superlattice peak does not exhibit appreciable degradation with increasing disorder in the structure, indicating a magnetic coherence length significantly greater than that of the chemical superlattice structure.

Analysis of neutron and X-ray reflectivity data by constrained least-squares methods

Two methods for the determination of scattering length density profiles from specular reflectivity data are discussed. For either method kinematical or dynamical theory can be used to calculate the reflectivity. In the first method the scattering density is parametrized using cubic splines. The coefficients in the series are determined by constrained nonlinear least-squares methods, in which the smoothest solution that agrees with the data is chosen. In the second approach the profile is expressed as a series of sine and cosine terms. A smoothness constraint is used which reduces the coefficients of the higher harmonics in the Fourier series. The two methods work equally well in most situations, and they are able to recover the original profiles. In general, the method using splines as the basis functions is better suited for aperiodic than for periodic structures, whereas the sine/cosine basis is well suited for periodic and nearly periodic structures.