Charles F. Majkrzak

The morphology of symmetric diblock copolymers as revealed by neutron reflectivity

The specular reflectivity of neutrons has been used to characterize quantitatively the microphase separated morphology of symmetric, diblock copolymers of polystyrene (PS), and polymethylmethacrylate (PMMA), as a function of the total molecular weight of the copolymer where either block is perdeuterated. It is shown that the hyperbolic tangent function, as opposed to a linear or cosine‐squared function, most closely describes the concentration gradient at the interface between the lamellar copolymer microdomains. The effective width of the interface is found to be independent of the molecular weight of the copolymer blocks and has a value of 50±3 A. This interface is also found to be identical to that between PS and PMMA, homopolymers. However, using measured values of the Flory–Huggins interaction parameter for PS and PMMA, current theoretical treatments cannot describe the observed widths of the interface.

Hybrid bilayer membranes in air and water: infrared spectroscopy and neutron reflectivity studies.

In this report we describe the fabrication and characterization of a phospholipid/alkanethiol hybrid bilayer membrane in air. The bilayer is formed by the interaction of phospholipid with the hydrophobic surface of a self-assembled alkanethiol monolayer on gold. We have characterized the resulting hybrid bilayer membrane in air using atomic force microscopy, spectroscopic ellipsometry, and reflection-absorption infrared spectroscopy. These analyses indicate that the phospholipid added is one monolayer thick, is continuous, and exhibits molecular order which is similar to that observed for phospholipid/phospholipid model membranes. The hybrid bilayer prepared in air has also been re-introduced to water and characterized using neutron reflectivity and impedance spectroscopy. Impedance data indicate that when moved from air to water, hybrid bilayers exhibit a dielectric constant and thickness that is essentially equivalent to hybrid bilayers prepared in situ by adding phospholipid vesicles to alkanethiol monolayers in water. Neutron scattering from these samples was collected out to a wave vector transfer of 0.25 A(-1), and provided a sensitivity to changes in total layer thickness on the order of 1-2 A. The data confirm that the acyl chain region of the phospholipid layer is consistent with that observed for phospholipid-phospholipid bilayers, but suggest greater hydration of the phospholipid headgroups of HBMs than has been reported in studies of lipid multilayers.

AND/R: Advanced neutron diffractometer/reflectometer for investigation of thin films and multilayers for the life sciences

An elastic neutron scattering instrument, the advanced neutron diffractometer/reflectometer (AND/R), has recently been commissioned at the National Institute of Standards and Technology Center for Neutron Research. The AND/R is the centerpiece of the Cold Neutrons for Biology and Technology partnership, which is dedicated to the structural characterization of thin films and multilayers of biological interest. The instrument is capable of measuring both specular and nonspecular reflectivity, as well as crystalline or semicrystalline diffraction at wave-vector transfers up to approximately 2.20 Å(-1). A detailed description of this flexible instrument and its performance characteristics in various operating modes are given.

Neutron scattering studies of magnetic thin films and multilayers

Abstract The basic principles of the elastic scattering of polarized neutrons from magnetic films and thin film superlattices, both kinematically as well as dynamically in the continuum limit, are summarized first. The quantitative accuracy presently attainable in practice is discussed along with other relevant issues regarding experimental technique and data analysis. Investigations of interlayer coupling and the effects of strain and finite thickness in single crystalline, epitaxially grown superlattices are reviewed, focussing on specular neutron diffraction and reflectivity measurements. These superlattices include rare earth and semiconductor systems in addition to those that exhibit “giant” magnetoresistive effects and which are of particular current technological interest. A survey of some of the more recent studies of the enhancement/reduction of ferromagnetic moments at an interface with a nonmagnetic material is then presented. Finally, several future research directions, for example, magnetic nonspecular scattering and studies of the magnetic state of the materials which form the intervening layers between coherently coupled ferromagnetic layers in superlattices, are discussed.

Polarized neutron reflectometry

Abstract Polarized neutron reflectometry is a powerful technique for studying the microscopic magnetic structures in thin films and multilayers. The corresponding analysis of spin-dependent reflectivity data is discussed and illustrated with a number of specific examples. The measurement of the precession of the neutron moment within a nonmagnetic medium, but in the presence of an applied field, as a means of determining the nuclear density profile is also considered.

X-ray scattering studies of surface roughness of GaAs/A1As multilayers

We discuss the theory of X-ray scattering from multilayers with conformal roughness of the interfaces, and illustrate with an analysis of specular, diffuse and wide-angle scattering from a GaAs/A1As multilayer. This is a highly coherent multilayer structure deposited on a stepped, but otherwise smooth surface. The roughness due to the steps propagates through the layers and a distinct anisotropy is observed in the diffuse scattering. We discuss a method to treat diffuse scattering from such surfaces with slightly irregular steps.

Structure of symmetric polyolefin block copolymer thin films

The microstructure of thin films of nearly symmetric poly(ethylene–propylene)–poly(ethylethylene) (PEP‐PEE) diblock copolymers (f=0.55, where f is the volume fraction of PEP) was characterized by neutron reflectometry (NR). A symmetric film structure in which the PEE block segregates preferentially to both interfaces is observed above and below the bulk order–disorder transition (ODT). Measurements at room temperature for several chain lengths, N, provide a real‐space picture of the change in interdomain interfacial profiles associated with the crossover between the strong and weak segregation limits. The polymer/air and substrate/polymer interfaces are observed to induce an ordered microstructure even when the center of the film is disordered. The characteristic dimension of this near surface microstructure is larger than the corresponding bulk value for values of χN lying between those of the bulk Gaussian‐to‐stretched‐coil and order–disorder transitions, where χ is the segment–segment interaction param...

First-Principles Determination of Hybrid Bilayer Membrane Structure by Phase-Sensitive Neutron Reflectometry

The application of a new, phase-sensitive neutron reflectometry method to reveal the compositional depth profiles of biomimetic membranes is reported. Determination of the complex reflection amplitude allows the related scattering length density (SLD) profile to be obtained by a first-principles inversion without the need for fitting or adjustable parameters. The SLD profile so obtained is unique for most membranes and can therefore be directly compared with the SLD profile corresponding to the chemical compositional profile of the film, as predicted, for example, by a molecular dynamics simulation. Knowledge of the real part of the reflection amplitude, in addition to enabling the inversion, makes it possible to assign a spatial resolution to the profile for a given range of wavevector transfer over which the reflectivity data are collected. Furthermore, the imaginary part of the reflection amplitude can be used as a sensitive diagnostic tool for recognizing the existence of certain in-plane inhomogeneities in the sample. Measurements demonstrating the practical realization of this phase-sensitive technique were performed on a hybrid bilayer membrane (self-assembled monolayer of thiahexa (ethylene oxide) alkane on gold and a phospholipid layer) in intimate contact with an aqueous reservoir. Analysis of the experimental results shows that accurate compositional depth profiles can now be obtained with a spatial resolution in the subnanometer range, primarily limited by the background originating from the reservoir and the roughness of the films supporting substrate.

Magnetic-crystallographic phase diagram of the superconducting parent compound Fe1+xTe

Through neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector k= [0.4481(4)012] at base temperature in the superconducting parent compound Fe1+xTe. This critical concentration of interstitial iron corresponds to x?12% and leads to crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) A, and as the ordering temperature is approached its propagation vector decreases linearly in the H direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector k= [0.3855(2)012] at base temperature. For a sample with x?9(1)%, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the Neel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe1+xTe for 5%

An ion-channel-containing model membrane: structural determination by magnetic contrast neutron reflectometry

To many biophysical characterisation techniques, biological membranes appear as two-dimensional structures with details of their third dimension hidden within a 5 nm profile. Probing this structure requires methods able to discriminate multiple layers a few Ångströms thick. Given sufficient resolution, neutron methods can provide the required discrimination between different biochemical components, especially when selective deuteration is employed. We have used state-of-the-art neutron reflection methods, with resolution enhancement via magnetic contrast variation to study an oriented model membrane system. The model is based on the Escherichia coli outer membrane protein OmpF fixed to a gold surface via an engineered cysteine residue. Below the gold is buried a magnetic metal layer which, in a magnetic field, displays different scattering strengths to spin-up and spin-down neutrons. This provides two independent datasets from a single biological sample. Simultaneous fitting of the two datasets significantly refines the resulting model. A β-mercaptoethanol (βME) passivating surface, applied to the gold to prevent protein denaturation, is resolved for the first time as an 8.2 ± 0.6 Å thick layer, demonstrating the improved resolution and confirming that this layer remains after OmpF assembly. The thiolipid monolayer (35.3 ± 0.5 Å), assembled around the OmpF is determined and finally a fluid DMPC layer is added (total lipid thickness 58.7 ± 0.9 Å). The dimensions of trimeric OmpF in isolation (53.6 ± 2.5 Å), after assembly of lipid monolayer (57.5 ± 0.9 Å) and lipid bilayer (58.7 ± 0.9 Å), are precisely determined and show little variation.