Specular Neutron Reflectivity Research Articles

Real-space neutron scattering methods

Using precession devices with tilted interfaces with respect to the polarised neutron beam enables measurement of structures by scattering. The scattering techniques can be small-angle neutron scattering, neutron diffraction and both specular and non-specular neutron reflectivity. We describe in this article all these techniques in a real-space concept. In all the techniques this concept describes in which direction structural information is obtained. In some cases the concept shows directly how to interpret the measured data in real-space.

Structural analysis for a poly(methyl methacrylate) ultrathin film in water by neutron reflectivity

Density profiles of a perdeuterated poly(methyl methacrylate) (dPMMA) film spin-coated on a substrate in water (H2O) and deuterated water (D2O) were examined along the direction normal to the interface by specular neutron reflectivity (NR). Although H2O and D2O were typical non-solvents for dPMMA, their interfaces were more diffuse than the air/dPMMA interface due to the swelling caused by the sorption of H2O and D2O molecules. An isotopic effect on aggregation structure of the dPMMA film in water was also discussed.

A comparison of modern data analysis methods for X-ray and neutron specular reflectivity data

Data analysis methods for specular X-ray or neutron reflectivity are compared. The methods that have been developed over the years can be classified into different types. The so-called classical methods are based on Parrat's or Abelès' formalism and rely on minimization using more or less evolved Levenberg–Marquardt or simplex routines. A second class uses the same formalism, but optimization is carried out using simulated annealing or genetic algorithms. A third class uses alternative expressions for the reflectivity, such as the Born approximation or distorted Born approximation. This makes it easier to invert the specular data directly, coupled or not with classical least-squares or iterative methods using over-relaxation or charge-flipping techniques. A fourth class uses mathematical methods founded in scattering theory to determine the phase of the scattered waves, but has to be coupled in certain cases with (magnetic) reference layers. The strengths and weaknesses of a number of these methods are evaluated using simulated and experimental data. It is shown that genetic algorithms are by far superior to traditional and advanced least-squares methods, but that they fail when the layers are less well defined. In the latter case, the methods from the third or fourth class are the better choice, because they permit at least a first estimate of the density profile to be obtained that can be refined using the classical methods of the first class. It is also shown that different analysis programs may calculate different reflectivities for a similar chemical system. One reason for this is that the representation of the layers is either described by chemical composition or by scattering length or electronic densities, between which the conversion of the absorptive part is not straightforward. A second important reason is that routines that describe the convolution with the instrumental resolution function are not identical.

Combined magnetic X-ray and polarized neutron reflectivity study of the origins of exchange bias in the Co / FeF2 system

Abstract We discuss studies of the magnetic specular reflectivity of neutrons and X-rays from the exchange bias system consisting of a single crystal film of antiferromagnetic FeF2 capped with a ferromagnetic Co film cooled in an applied magnetic field below the T N of the FeF2. This system exhibits a shift of the magnetic hysteresis loop along the direction of the cooling field H c (positive exchange bias) or in the opposite direction (negative exchange bias) depending on the magnitude of the cooling field. The use of neutrons with polarization analysis enables the spatial distribution of different vector components of the magnetization to be determined, and the use of resonant magnetic X-ray scattering enables magnetization in a compound system to be determined element-selectively. Our results show that the coupling across the interface between the relatively few uncompensated Fe spins and the more numerous Co spins is antiferromagnetic. In a large cooling field which overrides this coupling, the Fe spins are oriented along H c and some get pinned in this direction, in turn pinning the Co spins above the interface oppositely and thus creating positive exchange bias. For sall values of H c the Fe spins get locked in the opposite direction producing negative exchange bias. In addition a significant fraction of the Fe spins at the interface are unpinned and always align opposite to the Co magnetization.

Additive-induced phase transition of a spin-coated lipid film

We have measured specular neutron reflectivity for dimyristoylphosphatidylcholine (DMPC) films spin-coated on Si substrates with and without NaI. The obtained reflectivity profiles were analyzed using Parratt's equation with taking the distribution of the stacking number of bilayers into consideration, since the bilayer stacking still remains on the film surface and it forms terrace-like islands. The repeat distance of the bilayer stacking for the DMPC/NaI mixture film was drastically decreased at 80 °C, and the evaluated value was consistent with that in an “interdigitated gel” phase. Since this change was not observed for the pure DMPC film, we concluded that the phase transition to an “interdigitated gel” phase was induced by the addition of NaI.

Structure and stability of DPPE planar bilayers.

Biomembrane mimics in the form of supported planar bilayers allow the application of a wide range of surface and interface analytical techniques. The structure and phase-behavior of single and double bilayers of 1,2-dipalmitoylphosphoethanolamine (DPPE) were investigated by specular neutron reflectivity for their viability as biomembrane mimics. Whilst single bilayer samples were found to exhibit stable gel and fluid structures, double bilayers were found to be intrinsically unstable in the fluid phase as a planar structure. A Bragg peak was observed in the reflectivity data at just above the gel-to-fluid transition temperature, indicating the partial rearrangement of the upper bilayer into a repeat stacked structure. The lower bilayer was structurally stable. The structure and phase-behaviour of a double bilayer containing a ratio of 9 : 1 DPPE/cholesterol was also investigated to assess the stabilising effect of cholesterol on the upper bilayer. The presence of cholesterol completely destabilised the upper bilayer, causing it to detach 7 °C below the gel-to-fluid transition temperature of DPPE. It is possible that the cholesterol increases the overall conical shape of DPPE molecule by residing in the chain region of the lipid.

Distribution of reaction products in phospholipase A 2 hydrolysis

We have monitored the composition of supported phospholipid bilayers during phospholipase A 2 hydrolysis using specular neutron reflection and ellipsometry. Porcine pancreatic PLA 2 shows a long lag phase of several hours during which the enzyme binds to the bilayer surface, but only 5 ± 3% of the lipids react before the onset of rapid hydrolysis. The amount of PLA 2, which resides in a 21 ± 1 Å thick layer at the water-bilayer interface, as well as its depth of penetration into the membrane, increase during the lag phase, the length of which is also proportional to the enzyme concentration. Hydrolysis of a single-chain deuterium labelled d 31-POPC reveals for the first time that there is a significant asymmetry in the distribution of the reaction products between the membrane and the aqueous environment. The lyso-lipid leaves the membrane while the number of PLA 2 molecules bound to the interface increases with increasing fatty acid content. These results constitute the first direct measurement of the membrane structure and composition, including the location and amount of the enzyme during hydrolysis. These are discussed in terms of a model of fatty-acid mediated activation of PLA 2.

Interfacial Structure in Conjugated Polymers: Characterization and Control of the Interface between Poly(9,9-dioctylfluorene) and Poly(9,9-dioctylfluorene-alt-benzothiadiazole)

Specular neutron reflectivity and nuclear reaction analysis (NRA) are used to measure the interfacial width between poly(9,9-dioctylfluorene) (F8) and poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) in a 500 nm bilayer. Annealing at temperatures up to 280 °C allows us to vary this width over a large range, from ∼1 nm to greater than 30 nm. Approximate calculations using the predictions of self-consistent field theory (SCFT) for both Gaussian and semiflexible chains suggest that in the liquid−liquid regime the Gaussian results of SCFT are valid for this system. We also demonstrate the ability to control the interfacial width in 100 nm bilayers, on both silicon and indium tin oxide (ITO). The procedure of “preannealing” the F8 and F8BT layers, before bringing them together to make a bilayer, allows us to independently control the interfacial width and the properties of the bulk of these 100 nm films.

Adsorption of Randomly Annealed Polyampholytes at the Silica−Water Interface

We have investigated the adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) with various molar compositions. The adsorption was performed from dilute aqueous solutions onto silicon substrates. The adsorbed layers were characterized by reflectivity techniques such as reflectometry, ellipsometry, and neutron specular reflection. As expected for annealed polyampholytes, the adsorption was found to depend strongly on the pH, with a maximum within the isoelectric domain of the polyampholyte. The monomer volume fraction profiles of the adsorbed layers were determined from neutron specular reflection measurements. In the isoelectric domain, the polyampholyte chains adopt a compact conformation, with a layer thickness of about 60 A. The polyampholyte layer is as dense as the adsorbed layer of fully charged polyelectrolyte but much thicker. Finally, we found that changing the ratio of neutral units along the polyampholyte chain in the isoelectric domain had no significant effect on the concentration profile of the adsorbed layer.

Fast swelling kinetics of thin polystyrene films

In situ the swelling of thin polystyrene (PSd) films by toluene is investigated with specular and off-specular neutron reflectivity. Due to the very high time resolution of 30 s, the initial stages of swelling as well as the fast swelling are detected. Followed by a slow swelling and final stages, in total four different regimes are identified until equilibrium with respect to a homogenously swollen PSd film is reached. Film instabilities are avoided in the absence of any motor movements in the time-of-flight mode of the neutron reflectometer D17.

Early-stage roughening of the polymer-polymer interface approaching the glass transition temperature by real-time neutron reflection

The early-stage roughening of the interface between thin deuterated poly(methyl methacrylate) (d-PMMA) layers on thick polystyrene (PS) films was studied as a function of the temperature using real-time specular neutron reflectivity. By measuring the growth of the interface roughness as a precursor of the dewetting, the characteristic time constant of the early stages of the process was studied as a function of the temperature approaching the glass transition temperature (T(g)) of the two polymers from above and compared with the prediction of the growth of the interface by the spinodal process. Both solid and liquid regimes were probed, in which the viscosity of the thin film or the substrate dominates respectively. The characteristic growth time of the process also depends on the upper film thickness to a power of 5 or 6 in the solid or liquid regimes, respectively, as predicted by the theory of spinodal dewetting.

Neutron Reflection Study of a Water-Soluble Biocompatible Diblock Copolymer Adsorbed at the Air/Water Interface: The Effects of pH and Polymer Concentration

The effect of varying both the solution pH and copolymer concentration on the structure of layers of poly[2-(methacryloyloxy)ethyl phosphorylcholine-block-2-(dimethylamino)ethyl methacrylate] copolymer (denoted as MPC(30)-DMA(60), M(n) = 18,000) adsorbed at the air/water interface is studied using surface tension and specular neutron reflection. The surface structure of the adsorbed diblock copolymer is represented by a dense layer of 10-15 A on the air side, accompanied by a loose layer of 20-30 A extending into the aqueous phase. Although the uniform layer model generally provided a reasonable description of the adsorbed copolymer chains, some deviations were observed. A more detailed analysis showed that the distribution of the copolymer across the interface required a minimum of three layers to take into account the structural inhomogeneities. Refinement of the structural distributions involved the combined fitting of partially deuterated copolymer in null reflecting water and D(2)O and the fully hydrogenated copolymer in D(2)O, leading to a substantial improvement in the reliability of the structural profiles obtained. The data analysis showed an increase in surface excess at higher copolymer concentrations and at more alkaline pH. However, the copolymer layer was fully immersed in water under all conditions studied. Because the surface excess showed a steady increase across the cmc over the high pH range, we speculate that copolymer adsorption above the cmc involves the formation of surface micellar aggregates under these conditions.

A new polarized neutron reflectometer at Dhruva for specular and off-specular neutron reflectivity studies

A polarized neutron reflectometer for vertical sample geometry has been installed in the guide tube laboratory of Dhruva reactor at Trombay, India. The instrument uses a monochromatic neutron beam of 2.5 Å in a step scan mode with a linear position s

Reversible Activation of Diblock Copolymer Monolayers at the Interface by pH Modulation, 2: Membrane Interactions at the Solid/Liquid Interface

A monolayer of the pH-responsive poly[2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate] diblock copolymer [PDMAEMA-PMMA] was transferred from the air/water interface to a silicon substrate for evaluation as a tunable interlayer between biological material and solid substrates. Specular neutron reflectivity experiments revealed that the weak polyelectrolyte PDMAEMA chains at the solid/liquid interface can be reversibly activated by pH modulation. The thickness, scattering length density, and surface roughness of the polymer film can be systematically controlled by pH titration. As a simple model of plasma membranes, a lipid bilayer was deposited onto the polymer film. The membrane-substrate interaction was characterized by neutron reflectivity experiments, demonstrating that the membrane-substrate distance could be reversibly regulated by pH titration. These results confirm the potential of stimuli-responsive polymers for precise control of cell-surface interactions.

Specular reflection of thermal neutrons from Gd-containing layers and optimization of antireflective underlayers for polarizing coatings

A detailed analysis of specular neutron reflection from an absorbing medium is given. The experimental studies revealed that oxidation and roughness of the surface are the main factors that determine the neutron reflectivity of Gd-containing layers. It is concluded that the empirical approach used at present does not guarantee optimization of underlayer parameters (composition and thickness) and technological regimes. An algorithm of optimization is proposed, in which account is taken of the substrate potential, the dependence of the underlayer potential on the thermal neutron wavelength, the polarizing coating imperfections that enhance reflection of neutrons with the undesired spin. The antireflective TiZrGd underlayer for CoFe/TiZr supermirrors produced at the magnetron facility DIOGEN (PNPI, Gatchina) is optimized.

Microscopic characterization of corrosion morphology: A study in specular and diffuse neutron reflectivity

Detailed morphologies of the exposed surface of a Ni film vis-à-vis a buried interface below it have been determined by diffuse (off-specular) neutron scattering (DNS) and specular neutron reflectometry (NR). The exposed surface shows distinct morphological changes with respect to the buried interface, due to corrosion. The results demonstrate the strength of DNS in obtaining morphology of hidden interfaces and exposed surfaces simultaneously.

Direct Measurement of the Counterion Distribution within Swollen Polyelectrolyte Films

The depth profile of the counterion concentration within thin polyelectrolyte films was measured in situ using contrast variant specular neutron reflectivity to characterize the initial swelling stage of the film dissolution. We find substantial counterion depletion near the substrate and enrichment near the periphery of the film extending into the solution. These observations challenge our understanding of the charge distribution in polyelectrolyte films and are important for understanding film dissolution in medical and technological applications.

Adsorption of Nonionic Surfactant Mixtures at the Hydrophilic Solid−Solution Interface

The adsorption of the mixed nonionic surfactants, monododecyl triethylene glycol (C2EO3) and monododecyl octaethylene glycol (C12EO8), at the hydrophilic silica-solution interface has been studied by specular neutron reflectivity. The adsorption at the solid-solution interface is compared with that previously measured at the air-solution interface. The marked differences that are observed are explained in terms of the different packing constraints or preferred curvature arising from the disparity in the respective headgroup dimensions.

Specular reflection of neutrons from potentials with smooth boundaries

An analytical expression for reflection of neutrons from potentials with smooth boundaries, when smoothness is described by the Eckart function, is obtained and used for fitting experimental data. The analytical fitting is shown to be 7 times faster than the numerical one, based on Parrat matrix approach.

Neutron spin echo scattering angle measurement (SESAME)

We describe experiments in which the neutron spin echo technique is used to measure neutron scattering angles. We have implemented the technique, dubbed spin echo scattering angle measurement (SESAME), using thin films of Permalloy electrodeposited on silicon wafers as sources of the magnetic fields within which neutron spins precess. With 30-μm-thick films we resolve neutron scattering angles to about 0.02° with neutrons of 4.66Å wavelength. This allows us to probe correlation lengths up to 200nm in an application to small angle neutron scattering. We also demonstrate that SESAME can be used to separate specular and diffuse neutron reflection from surfaces at grazing incidence. In both of these cases, SESAME can make measurements at higher neutron intensity than is available with conventional methods because the angular resolution achieved is independent of the divergence of the neutron beam. Finally, we discuss the conditions under which SESAME might be used to probe in-plane structure in thin films and show that the method has advantages for incident neutron angles close to the critical angle because multiple scattering is automatically accounted for.