Off-specular Neutron Reflectometry Research Articles

Synthesis and Characterization of Stimuli-Responsive Polymer Brushes in Nanofluidic Channels.

Nanochannels with controllable gating behavior are attractive features in a wide range of nanofluidic applications including viral detection, particle sorting, and flow regulation. Here, we use selective sidewall functionalization of nanochannels with a polyelectrolyte brush to investigate the channel gating response to variations in solution pH and ionic strength. The conformational and structural changes of the interfacial brush layer within the channels are interrogated by specular and off-specular neutron reflectometry. Simultaneous fits of the specular and off-specular signals, using a dynamical theory model and a fitting optimization protocol, enable detailed characterization of the brush conformations and corresponding channel geometry under different solution conditions. Our results indicate a collapsed brush state under basic pH, equivalent to an open gate, and an expanded brush state representing a partially closed gate upon decreasing the pH and salt concentration. These findings open new possibilities in noninvasive in situ characterization of tunable nanofluidics and lab-on-chip devices with advanced designs and improved functionality.

2D reflectometry for the investigation of polymer interfaces: off-specular neutron scattering

Specular and off-specular neutron reflectometry have been used in a combined approach to study thin polymer films. Our goal in this work is to illustrate the power of the off-specular scattering technique to probe the properties of the buried interface of immiscible polymer bilayers of deuterated polystyrene and protonated poly(methyl methacrylate) (h-PMMA). The diffuse scattering stemming from these systems is discussed in relation to thermal fluctuations at the polymer/polymer interface, providing a means to extract in-plane correlation lengths from buried interfaces. In addition the onset of hole formation in the top layer is evidenced by the diffuse scattering, not easily detectable by specular reflection alone.

Stability of Fluid Ultrathin Polymer Films in Contact with Solvent-Loaded Gels for Cultural Heritage.

The removal of ultrathin amorphous polymer films in contact with an aqueous gelled solution containing small amounts of good solvent is addressed by means of specular and off-specular neutron reflectometry. The distribution of heavy water and benzyl alcohol is revealed inside Laropal A81, often employed as a protective varnish layer for Culture Heritage in the restoration of easel paintings. The swelling kinetics, interface roughness, and film morphologies were recorded as a function of temperature and increasing benzyl alcohol concentration in the dispersion of Pemulen TR-2, a hydrophobically modified acrylic acid copolymer. The addition of small amounts of good solvent results in the appearance of water-filled cavities inside the varnish, which grow with time. It is shown that while increasing the solvent concentration greatly enhances the hole growth kinetics, an increase in temperature above the glass transition temperature does not have such a big effect on the kinetics.

Relationship between Structure and Fluctuations of Lipid Nonlamellar Phases Deposited at the Solid–Liquid Interface

The structure and dynamics of nanostructure films formed by mixtures of soy phosphatidylcholine and glycerol dioleate at the silicon-aqueous interface were studied by grazing incidence neutron spin echo spectroscopy (GINSES), specular and off-specular neutron reflectometry, and small-angle X-ray diffraction. Reverse hexagonal (HII) and micellar cubic phase (Fd3m) layers at the solid-liquid interface have been identified with neutron reflectometry measurements. A preferred orientation of the liquid crystalline (LC) domains was observed only for the anisotropic HII phase. The size of the LC domains was found to be about 1 micrometer as estimated from the width of the diffraction peaks. GINSES revealed that the cubic phase forms rather rigid films. In comparison, the HII film was more flexible, appearing as a modified undulation spectrum of the cylinders due to the interaction with the substrate.

The Search for Nanobubbles by Using Specular and Off-Specular Neutron Reflectometry.

We apply specular and off-specular neutron reflection at the hydrophobic silicon/water interface to check for evidence of nanoscopic air bubbles whose presence is claimed after an ad hoc procedure of solvent exchange. Nanobubbles and/or a depletion layer at the hydrophobic/water interface have long been discussed and generated a plethora of controversial scientific results. By combining neutron reflectometry (NR), off-specular reflectometry (OSS), and grazing incidence small angle neutron scattering (GISANS), we studied the interface between hydrophobized silicon and heavy water before and after saturation with nitrogen gas. Our specular reflectometry results can be interpreted by assuming a submolecular sized depletion layer and the off-specular measurements show no change with nitrogen super saturated water. This picture is consistent with the assumption that, following the solvent exchange, no additional nanobubbles are introduced at significant concentrations (if present at all). Furthermore, we discuss the results in terms of the maximum surface coverage of nanobubbles that could be present on the hydrophobic surface compatibly with the sensitivity limit of these techniques.

Lateral uniformity in chemical composition along a buried reaction front in polymers usingoff-specular reflectivity**Official contribution of the National Institute of Standards and Technology; not subject tocopyright in the United States.

Off-specular neutron reflectometry was applied to characterize the form and amplitude oflateral compositional variations at a buried reaction–diffusion front. In this work,off-specular neutron measurements were first calibrated using off-specular x-ray reflectivityand atomic force microscopy via a roughened glass surface, both as a free surface and as aburied interface that was prepared by spin coating thin polymer films upon the glasssurface. All three methods provided consistent roughness values despite the difference intheir detection mechanism. Our neutron results demonstrated, for the first time, that thecompositional heterogeneity at a buried reaction front can be measured; the modelsystem used in this study mimics the deprotection reaction that occurs duringthe photolithographic process necessary for manufacturing integrated circuits.

Separation and correlation of structural and magnetic roughness in a Ni thin film bypolarized off-specular neutron reflectometry

Diffuse (off-specular) neutron and x-ray reflectometry has been used extensively for thedetermination of interface morphology in solids and liquids. For neutrons, a novelpossibility is off-specular reflectometry with polarized neutrons to determine themorphology of a magnetic interface. There have been few such attempts due to the lowerbrilliance of neutron sources, though magnetic interaction of neutrons with atomicmagnetic moments is much easier to comprehend and easily tractable theoretically. Wehave obtained a simple and physically meaningful expression, under the Bornapproximation, for analyzing polarized diffuse (off-specular) neutron reflectivity(PDNR) data. For the first time PDNR data from a Ni film have been analyzed andseparate chemical and magnetic morphologies have been quantified. Also specularpolarized neutron reflectivity measurements have been carried out to measure themagnetic moment density profile of the Ni film. The fit to PDNR data results ina longer correlation length for in-plane magnetic roughness than for chemical(structural) roughness. The magnetic interface is smoother than the chemical interface.

Morphology of electrodeposited Ni/Cu multilayer: Specular and diffuse neutron reflectometry study

Structural studies of Ni/Cu multilayers grown by electro-deposition technique under different electro-chemical conditions have been carried out using specular and off-specular neutron reflectometry techniques at room temperature. The specular reflectivity measurements give values of layer thickness, density and interface roughness for these two films. The Off-specular reflectivity measurements indicate different interface morphology of the two films.

Magnetism at the interface between ferromagnetic and superconducting oxides

Carefully controlled interfaces between two materials can give rise to novel physical phenomena and functionalities not exhibited by either of the constituent materials alone. Modern synthesis methods have yielded high-quality heterostructures of oxide materials with competing order parameters. Although magnetic correlations at the interface are expected to be important in determining the macroscopic properties of such systems, a quantitative determination of the interfacial magnetization profile has thus far not been reported. Here we examine superlattices composed of the half-metallic ferromagnet La2/3Ca1/3MnO3 and the high-temperature superconductor YBa2Cu3O7 by absorption spectroscopy with circularly polarized X-rays and by off-specular neutron reflectometry. The resulting data yield microscopic insight into the interplay of spin and orbital degrees of freedom at the interface. The experiments also reveal an extensive rearrangement of the magnetic domain structure at the superconducting transition temperature. This methodology establishes an incisive probe of the interplay between competing electronic order parameters in oxide heterostructures.

Structural development of silicated films self-assembled at the air–water interface

The development of structure perpendicular to and in the plane of the interface has been studied for mesoporous silicate films self-assembled at the air/water interface. The use of constrained X-ray and neutron specular reflectometry has enabled a detailed study of the structural development perpendicular to the interface during the pre-growth phase. Off-specular neutron reflectometry and grazing incidence X-ray diffraction has enabled the in-plane structure to be probed with excellent time resolution. The growth mechanism under the surfactant to silicate source ratios used in this work is clearly due to the self-assembly of micellar and molecular species at the air/liquid interface, resulting in the formation of a planar mesoporous film that is tens of microns thick.

Polarization analysis of off-specular neutron scattering from domains and rough interfaces in a FeCoV/TiZr multilayer

Specular neutron reflectivity is commonly used as a powerful tool in determining the scattering length density (SLD) profile perpendicular to the surface of thin films. Information about the lateral fluctuations in the SLD, such as the presence of magnetic domains and rough interfaces between layers, can be obtained by studying the pattern of off-specular scattering. Advancements in neutron-source brilliance, beam-line optics and detector efficiency, as well as advances in the theoretical description of off-specular scattering, have contributed to the increasing importance of off-specular neutron-reflectometry studies. We present specular and off-specular polarized neutron reflectometry experiments on FeCoV/TiZr multilayers, with a detailed description of the orientations of the magnetic moments inside the FeCoV layers and at the FeCoV/TiZr interfaces.

Overview of new Larmor precession techniques

Larmor precession has been used in the past in neutron spin-echo and neutron depolarisation. In the last decade, interest has been revived in the inclined front and end faces of the precession regions combined with the neutron-resonance spin-echo (NRSE) technique. Various techniques based on these inclined faces have been developed, such as spin-echo, small-angle neutron scattering (SESANS), off-specular neutron reflectometry using SESANS and high-resolution diffraction using Larmor precession. An overview will be given of the various state-of-the-art techniques.

Mapping domain disorder in exchange-biased magnetic multilayers

Exchange anisotropy occurs at the interface between an anti ferromagnetic (AF) layer and a ferromagnetic layer, and results in a ferromagnet hysteresis loop displaced along the field axis. We have performed off-specular neutron reflectometry in order to characterize the domain structure in Co layers that are exchange biased by FeMn. This allows us to determine the domain direction distributions and lateral magnetic correlation lengths for the Co layers as a function of field with the exchange bias in two different thermally prepared states: high field cooled and zero field cooled. We find that a reversing exchange-biased layer is characterized by a very short (submicron) magnetic lengthscale, indicating domains much smaller than those in the ferro-magnet after ac demagnetization at temperatures above the blocking temperature. This indicates that the bias is not spatially uniform across the entire interface, underlining the complexity of the AF spin structure.

Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

The in-plane correlation lengths and angular dispersion of magnetic domains in a transition metal multilayer have been studied using off-specular neutron reflectometry techniques. A theoretical framework considering both structural and magnetic disorder has been developed, quantitatively connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The antiferromagnetic domain structure is highly vertically correlated throughout the multilayer. We are easily able to relate the neutron determined magnetic domain dispersion to magnetization and magnetoresistance experiments.