Neutron Reflectivity Techniques Research Articles

Iron self-diffusion in B2-FeRh thin film

Iron-rhodium thin films are in the center of scientific interest due to their great potential in spintronic, energy harvesting and other forthcoming applications. The structural stability of these materials in industrial applications relies heavily on their ability to withstand temperature variations, therefore it is crucial to gain a thorough understanding of the diffusion processes induced by temperature. To investigate the self-diffusion of iron in FeRh, an isotope-periodic multilayer structure of [nFe50.5Rh49.5(63 Å)/57Fe51.5Rh48.5(46 Å)]10 was prepared by molecular beam epitaxy. By using neutron reflectivity technique, which is capable of detecting atomic-scale diffusion lengths, we determined the pre-exponent factor and activation energy as D0 = (1.19 ± 0.5) × 10−16 m2s−1 and Ea = (1.00 ± 0.03) eV respectively.

Salt-Induced Diffusion of Star and Linear Polyelectrolytes within Multilayer Films.

This study explores the effect of salt on the diffusivity of polyelectrolytes of varied molecular architecture in layer-by-layer (LbL) films in directions parallel and perpendicular to the substrate using fluorescence recovery after photobleaching (FRAP) and neutron reflectivity (NR) techniques, respectively. A family of linear, 4-arm, 6-arm, and 8-arm poly(methacrylic acids) (LPMAA, 4PMAA, 6PMAA, and 8PMAA, respectively) of matched molecular weights were synthesized using atom transfer radical polymerization and assembled with a linear polycation, poly[2-(trimethylammonium)ethyl methacrylate chloride] (QPC). NR studies involving deuterated QPC revealed ∼10-fold higher polycation mobility for the 8PMAA/QPC system compared to all-linear LbL films upon exposure to 0.25 M NaCl solutions at pH 6. FRAP experiments showed, however, that lateral diffusion of star PMAAs was lower than LPMAA at NaCl concentrations below ∼0.22 M NaCl, with a crossover to higher mobility of star polymers in more concentrated salt solutions. The stronger response of diffusion of star PMAA to salt is discussed in the context of several theories previously suggested for diffusivity of polyelectrolyte chains in multilayer films and coacervates.

Polyamide thin films with nanochannel networks synthesized at the liquid–gas interface for water purification

An important challenge concerning thin-film composite (TFC) membranes in brine treatment is to increase their water permeance while maintaining salt rejection for desalination performance. Structural control of polyamide (PA) thin films is a key factor in improving the desalination performance. In this study, we employed a Langmuir–Schaefer technique to fabricate PA Langmuir monolayers by interfacial polymerization of m-phenylenediamine and trimesoyl chloride at the liquid–gas interface and deposition on polysulfone supports. This technique allows the fabrication of nanoscale-controlled PA multilayers (2.3 nm per layer) with a nano-channeled structure through repeated deposition. The nanochannels of PA thin-film multilayers were clarified using the neutron reflectivity technique. UV treatment facilitates the maintenance of water-permeable sites in the nanochannels, which increases the water permeance up to 203% without loss of salt rejection performance.

Thermal stability of interfacial mixed layers in c-Ni/a-Zr multilayer during annealing: Structural and magnetic properties

Annealing of crystalline multilayers composed of two miscible elements usually causes interfacial mixing of the constituent atoms, possibly leading to the formation of binary alloys at the interfaces. Magnetron sputtered c-Ni/a-Zr multilayers deposited at room temperature were vacuum annealed isochronally at 200 °C and 400 °C to observe thermal stability of the interfaces constituting the crystalline/amorphous multilayers. Control over the interfacial behaviour can aid the formation of materials with novel properties. The resultant changes in structural and magnetic properties of the multilayers were investigated in detail through X-ray Reflectivity, Polarized Neutron Reflectivity, SIMS and SQUID-based magnetization measurement techniques. The compositional variations in the amorphous mixed layers formed at the Ni-on-Zr and Zr-on-Ni interfaces due to atomic transport, were carefully observed as a function of annealing. Interface widths proceeded to increase with annealing at the expense of the Ni and Zr layers. The Ni-on-Zr interface was seen to be unstable at both temperatures; but the overall response of the interfaces to atomic diffusion was more pronounced when the multilayers were annealed at 400 °C. Under conditions of supersaturation of atoms within restricted spaces of the interfacial layers and the limited availability of components within the multilayer, unexpected demixing effects were observed at both the interfaces. A large increase in magnetic moment obtained after annealing at 400 °C, was attributed to the high densification of the Ni layers as well as to the incorporation of Ni crystallites into the Zr-on-Ni interface layers.

Antibiotic-in-Cyclodextrin-in-Liposomes: Formulation Development and Interactions with Model Bacterial Membranes.

Gram-negative bacteria possess numerous defenses against antibiotics, due to the intrinsic permeability barrier of their outer membrane (OM), explaining the recalcitrance of some common and life-threatening infections. We report the formulation of a new drug, PPA148, which shows promising activity against all Gram-negative bacteria included in the ESKAPEE pathogens. PPA148 was solubilized by inclusion complexation with cyclodextrin followed by encapsulation in liposomes. The complex and liposomal formulation presented increased activity against E. coli compared to the pure drug when assessed with the Kirby Bauer assay. The novel formulation containing 1 μg PPA148 reached similar efficacy levels equivalent to those of 30 μg of pure rifampicin. A range of biophysical techniques was used to explore the mechanism of drug uptake. Langmuir trough (LT) and neutron reflectivity (NR) techniques were employed to monitor the interactions between the drug and the formulation with model membranes. We found evidence for liposome fusion with the model Gram-negative outer membrane and for cyclodextrins acting as inner membrane (IM) permeation enhancers without presenting intrinsic antimicrobial activity. An antibiotic-in-cyclodextrin-in-liposomes (ACL) formulation was developed, which targets both the bacterial OM and IM, and offers promise as a means to breach the Gram-negative cell envelope.

Interfacial Behaviors of Transmembrane Peptides in Lipid Bilayer Studied by X-Ray and Neutron Reflectivity

Lipids and proteins can influence each other in so many different ways. Lipids may affect the structure of membrane proteins by influencing their backbone conformation, the tilt, rotation angles of their transmembrane (TM) segments, and the orientation of their side chains. The membrane-spanning parts in integral membrane proteins are predominantly hydrophobic, and most often helical. At the lipid-protein interface, the TM part of the protein and the hydrocarbon chains of the lipid molecules have to coexist to maintain the integrity of the membrane. Lipids are important components of lipid membrane are used in various experiments reported in this thesis and can act as model lipid bilayers. Once they support on solid substrate like silicon wafers, their structural properties can investigate by X-ray and neutron reflectivity and by other useful techniques. Reflectivity technique can provide detailed information such as their thickness and interaction between lipids and peptides. The thesis reports a detailed investigation of these lipids and peptides by X-ray and neutron reflection techniques

Free-Standing Janus Graphene Oxide with Anisotropic Properties for 2D Materials as Surfactant.

We present a simple and facile approach to creating asymmetrically modified graphene oxide sheets by grafting polymers with different polarities. Single-layered Janus graphene derivatives were prepared by grafting polymers with different polarities at the liquid-gas interface through one step functionalization. This approach allows obtaining free-standing monolayers of Janus graphene oxide sheets for large area, and also controlling the morphology (i.e., wrinkled Janus graphene oxide sheets) by a compression monolayer. A neutron reflectivity technique is used to check the functionalization on each side of the monolayer, and the results are compared with contact angles to determine its amphiphilic nature. The free-standing Janus monolayers become robust after UV-irradiation, and are able to withstand various solvents. Because these robust Janus graphene films can maintain their anisotropic functionalities over time, this technique provides a new strategy for fabricating functional materials that require amphiphilic properties (i.e., oil-water separation membranes and chemical compatibilizers functioning as a 2D surfactant) and different electrical functionalities (i.e., flexible lightweight p-n junction semiconductors and stimuli-driven actuators).

Iron self-diffusion in Fe5Ge3 thin film

Iron-germanides are widely investigated due to their possible application in nanoelectronics and spintronics applications. The feasibility of these materials in industrial application is highly dependent on their structural stability against temperature, therefore it is indispensable to understand temperature induced diffusion processes in this system. In order to study iron self-diffusion, [57Fe5Ge3(36 Å)/nFe5Ge3(62 Å)]10 isotope-periodic multilayer was prepared by molecular beam epitaxy. By using neutron reflectivity technique, which is sensitive to atomic scale diffusion lengths, we have determined the pre-exponent factor and activation energy as D0 = (8.22 ± 3.8) × 10−18 m2 s−1 and Ea = (0.28 ± 0.02) eV respectively.

Controlled deuterium labelling of imidazolium ionic liquids to probe the fine structure of the electrical double layer using neutron reflectometry.

We combined the deuterium labeling and neutron reflectivity techniques to determine the fine structure of the electric double layer structure in an imidazolium ionic liquid (IL). For this, a simple and large scale deuteration method for imidazolium ILs was developed, where the deuteration level can be systematically controlled.

Study on the Structure of the Electrical Double Layer Formed in Ionic Liquids Using Neutron Reflectivity

In recent times, ionic liquids (IL) have been studied as a new solvent because of their unique features, in particular, tunability and designability. The electrochemical window, viscosity, conductivity, and hydrophobicity of ILs can be tuned by altering their molecular structures. One of the applications of ILs is an electrolyte solution for electrochemical devices. In the last two decades, many researchers have investigated the IL/electrode interfaces, where the electrochemical reactions exactly occur. It has been well known that both the electrode surface and the electrical double layer (EDL) structure affect the kinetics of electrochemical reactions. However, the IL/electrode interface, especially, the EDL structure has not been well understood. The EDL formed in ILs is assumed to have different structure from that formed in aqueous electrolytes due to IL's high ionic strength. In this study, the EDL structure formed in the deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BMIM-d15]TFSA) was elucidated using neutron reflectivity (NR) technique. To analyze the EDL structure, it is important that the technique must not the disturb the EDL structure during measurements and the measurement should be carried out under the ideal electrochemical condition. The NR technique satisfies the both conditions. Further, neutron does not decompose organic compounds, including ILs. Therefore, the NR is suitable for the analysis of EDL structure. [BMIM-d15]TFSA was synthesized at the National Deuteration Facility (NDF), Australian Nuclear Science and Technology Organization (ANSTO). Before use, [BMIM-d15]TFSA was dehydrated at 80°C under vacuum for 24 h. Si(100) electrode was used as the working electrode. The working electrode was cleaned with acetone, ethanol, and Milli-Q. Pt wires were used as the reference and counter electrodes. The electrode potential was controlled by IviumStat. The specially designed electrochemical cell for NR measurements made from Kel-F was used and it was assembled in a glove box filled with Ar gas. NR measurements were carried out at BL-17 SHARAKU, Materials and Life Science Experimental Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC). NR profiles were analyzed using Motofit. Firstly, an impedance measurement was carried out to elucidate the flat band potential of Si electrode in [BMIM]TFSA. The flat band potential was estimated at –0.88 V vs Pt. In NR measurements, NR profiles were taken at open circuit voltage (OCV), –1.1 V (negative than the flat band potential) and +0.5 V (positive than the flat band potential). The analysis reveal that at any electrode potential, where we measured in this study, the EDL consists of IL molecule layers and a bulk IL layer. At the electrode potential, E = –1.1 V, [BMIM]+ molecules adsorb on the electrode surface and a TFSA– molecules adsorb on the [BMIM]+ layer. On the other hand, at E = +0.5 V, the stacking order was opposite, i.e., the first and second layer consist of TFSA– and [BMIM]+ molecules, respectively. The electrode surface is negatively charged when the electrode potential is negative than the flat band potential and vice versa. Therefore, these results show that the EDL structure reflects the charge of electrode surface. The detail measurements and analysis are under way.

Processes of silver photodiffusion into Ge‐chalcogenide probed by neutron reflectivity technique

We performed time‐resolved neutron reflectivity measurement for stacks of Ag 500 Å/Ge25S75 1500 Å/Si substrate and Ge33S67 1500 Å/Ag 500 Å/Si substrate to clarify silver photodiffusion process into Ge‐chalcogenide layer. For Ag 500 Å/Ge25S75 1500 Å/Si substrate stack, it was found that the silver layer dissolves into Ge‐chalcogenide layer within 2 min by the light exposure, and Ag‐doped reaction layer forms. However, two‐layer structure with thicknesses of 800 and 1100 Å was established by a prolonged light exposure for 70 min and it did not change to form one homogeneous layer. For Ge33S67 1500 Å/Ag 500 Å/Si substrate stack, silver rapidly dissolves into Ge33S67layer leaving a thin silver layer in the first 2 min, and then, silver slowly dissolves from the silver layer as the next reaction process. At approximately 25 min light exposure, an anomalous decrease in the neutron reflectivity, suggesting a formation of macroscopic surface roughness, was observed.

Effect of CO2 on a Mobility Gradient of Polymer Chains near an Impenetrable Solid

We report a mobility gradient of polymer chains in close proximity of a planar solid substrate in compressed carbon dioxide (CO2) gas. A series of bilayers composed of bottom hydrogenated polystyrene (h-PS) and top deuterated PS (d-PS) layers were prepared on Si substrates. A high-pressure neutron reflectivity (NR) technique was used to study the diffusive motion at the h-PS/d-PS interface as a function of the distance from the substrate interface. The results reveal that the interdiffusive chain dynamics gets strongly hindered compared to the bulk when the distance from the substrate is less than 3Rg (Rg is the radius of polymer gyration of the h-PS). At the same time, by utilizing rapid quench of CO2 and subsequent solvent leaching, we reveal the presence of the CO2-induced polymer adsorbed layer on the substrate. We postulate that loop components in the adsorbed polymer chains provide a structure that can trap the neighboring polymer chains effectively, hence reducing the chain mobility in the close vi...

Effect of divalent cation removal on the structure of gram-negative bacterial outer membrane models.

The Gram-negative bacterial outer membrane (GNB-OM) is asymmetric in its lipid composition with a phospholipid-rich inner leaflet and an outer leaflet predominantly composed of lipopolysaccharides (LPS). LPS are polyanionic molecules, with numerous phosphate groups present in the lipid A and core oligosaccharide regions. The repulsive forces due to accumulation of the negative charges are screened and bridged by the divalent cations (Mg2+ and Ca2+) that are known to be crucial for the integrity of the bacterial OM. Indeed, chelation of divalent cations is a well-established method to permeabilize Gram-negative bacteria such as Escherichia coli. Here, we use X-ray and neutron reflectivity (XRR and NR, respectively) techniques to examine the role of calcium ions in the stability of a model GNB-OM. Using XRR we show that Ca2+ binds to the core region of the rough mutant LPS (RaLPS) films, producing more ordered structures in comparison to divalent cation free monolayers. Using recently developed solid-supported models of the GNB-OM, we study the effect of calcium removal on the asymmetry of DPPC:RaLPS bilayers. We show that without the charge screening effect of divalent cations, the LPS is forced to overcome the thermodynamically unfavorable energy barrier and flip across the hydrophobic bilayer to minimize the repulsive electrostatic forces, resulting in about 20% mixing of LPS and DPPC between the inner and outer bilayer leaflets. These results reveal for the first time the molecular details behind the well-known mechanism of outer membrane stabilization by divalent cations. This confirms the relevance of the asymmetric models for future studies of outer membrane stability and antibiotic penetration.

Electrokinetics as an alternative to neutron reflectivity for evaluation of segment density distribution in PEO brushes.

Unravelling details of charge, structure and molecular interactions of functional polymer coatings defines an important analytical challenge that requires the extension of current methodologies. In this article we demonstrate how streaming current measurements interpreted with combined self consistent field (SCF) and soft surface electrokinetic theories allow the evaluation of the segment distribution within poly(ethylene oxide) (PEO) brushes beyond the resolution limits of neutron reflectivity technique.

Twisted magnetic structure in ferromagnetic ultrathin Ni films induced by magnetic anisotropy interaction with antiferromagnetic FeMn

A twisted magnetic structure in Ni ultrathin films attached to antiferromagnetic FeMn is revealed by a combination of the depth-resolved x-ray magnetic circular dichroism (XMCD) and the polarized neutron reflectivity (PNR) techniques. The depth-resolved XMCD at remanent magnetization shows that the perpendicular magnetization component in the Ni film decreases around the interface to FeMn when the film exhibits perpendicular magnetization, whereas the in-plane component is kept constant through the whole film in the case of in-plane magnetization. Moreover, the PNR data shows that when a weak in-plane magnetic field is applied to a film, which exhibits perpendicular magnetization at the remanent state, an in-plane magnetization component is induced in Ni around the interface to FeMn. These results are reasonably interpreted by assuming that the magnetic moment in the Ni layer is twisted from the perpendicular to the in-plane directions towards the interface to FeMn. Such a magnetic structure is supposed to be induced by a magnetic anisotropy interaction at the interface between ferromagnetic Ni and antiferromagnetic FeMn, which makes the Ni magnetic moment around the interface rotate towards the in-plane direction.

Stoner enhanced paramagnetic influence on superconductivity in a superconductor/metallic heterostructure

The unusual magnetic behaviour of a Pd/Nb67Ti33/Pd heterostructure was investigated using polarised neutron reflectivity technique. On application of a large in-plane magnetic field, Stoner enhanced paramagnet Pd was found to influence the magnetic state of the Nb67Ti33 layer both above and below the superconducting transition temperature Tc significantly. Unlike the case of a conventional proximity effect for a superconductor/metallic heterostructure, the pair correlation in the superconducting state has been found to be more stable in the higher field limit compared to its low field counterpart, possibly signaturing a ‘novel’ coupled state in the system. The superconducting state in Nb67Ti33 has been found to be diamagnetic in nature at all the fields that can be fitted using a Meissner kind of behaviour in the high field limit. The magnetic properties of Pd and Nb67Ti33 are in excellent agreement with those measured using bulk magnetisation measurements.

A neutron reflection study of adsorbed deuterated myoglobin layers on hydrophobic surfaces

The structure of adsorbed globular protein layers on hydrophobic surfaces is elucidated in detail by combining the use of a fully deuterated protein, myoglobin, and the neutron reflectivity technique. The hydrophobic surfaces consist of grafted self-assembled monolayer of octadecyltrichlorosilane (OTS) and polystyrene (PS) layer on silicon substrates. Different protein concentrations ranging from 1mg/ml to 0.01mg/ml are used. On the OTS surface and for low protein concentration, the adsorbed protein layer consists of a dense layer of thickness around 13Å indicating that proteins are denaturated when adsorbed on the hydrophobic interface – myoglobin being a globular protein with an average diameter of about 40Å. At high protein concentration, an additional layer is observed on the top of this first denaturated layer. The thickness of this layer corresponds roughly to the dimensions of the myoglobin suggesting that additional proteins in their bulk conformation are adsorbed on the top. In the case of PS, the protein is significantly less flattened at the interface, PS being a less hydrophobic surface.

Silicon supported lipid–DNA thin film structures at varying temperature studied by energy dispersive X-ray diffraction and neutron reflectivity

Non-viral gene transfection by means of lipid-based nanosystems, such as solid supported lipid assemblies, is often limited due to their lack of stability and the consequent loss of efficiency. Therefore not only a detailed thermo-lyotropic study of these DNA–lipid complexes is necessary to understand their interaction mechanisms, but it can also be considered as a first step in conceiving and developing new transfection biosystems. The aim of our study is a structural characterization of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC)–dimethyl-dioctadecyl-ammonium bromide (DDAB)–DNA complex at varying temperature using the energy dispersive X-ray diffraction (EDXD) and neutron reflectivity (NR) techniques. We have shown the formation of a novel thermo-lyotropic structure of DOPC/DDAB thin film self-organized in multi-lamellar planes on (1 0 0)-oriented silicon support by spin coating, thus enlightening its ability to include DNA strands. Our NR measurements indicate that the DOPC/DDAB/DNA complex forms temperature-dependent structures. At 65 °C and relative humidity of 100% DNA fragments are buried between single lamellar leaflets constituting the hydrocarbon core of the lipid bilayers. This finding supports the consistency of the hydrophobic interaction model, which implies that the coupling between lipid tails and hypo-hydrated DNA single strands could be the driving force of DNA–lipid complexation. Upon cooling to 25 °C, EDXD analysis points out that full-hydrated DOPC–DDAB–DNA can switch in a different metastable complex supposed to be driven by lipid heads–DNA electrostatic interaction. Thermotropic response analysis also clarifies that DOPC has a pivotal role in promoting the formation of our observed thermophylic silicon supported lipids–DNA assembly.

Polymer brushes in restricted geometries

The structure of polymer brushes has been the subject of considerable theoretical and experimental activity. Experimental measurements of the steric forces between grafted brushes and the structure of a single brush at the solid-solution interface are in good agreement with theoretical predictions. However, the physical structure brush layers adopt under confinement has remained elusive. Here, we report measurements of polymer brush density distributions in good solvent as a function of compression. The extension and interpenetration of the brushes are unambiguously determined using neutron reflectivity and contrast matching techniques. The measurements reveal a significant increase in brush concentration at the grafting surface with compression. This is in contrast to theoretical predictions of uniform density distributions as polymer brushes are compressed. The low interpenetration and brush compaction at the anchor surface provides new insight into why such layers are so effective at reducing friction and wear between surfaces.

Structure and dynamics of phospholipid bilayer films under electrochemical control

Vesicle fusion was used to deposit mixed dimyristoyl phosphatidylethanolamine–dimyristoyl phosphatidylserine (DMPE–DMPS) phospholipid bilayers on Au electrodes. Bilayer structure and composition, when exposed to aqueous NaF and subject to an applied electrochemical potential, were studied using electrochemical, spectroscopic and neutron reflectivity (NR) techniques. Interfacial capacitance data indicate the formation of compact films. Chronocolometric data show that surface charge is significantly altered by the presence of lipid in the potential range −0.75 < E/V (Ag|AgCl) < 0.35. NR measurements were made on lipid films in which the hydrocarbon tails were either fully hydrogenous (h-DMPE–h-DMPS) or perdeuterated (d-DMPE–d-DMPS), in each case serially exposed to D2O and H2O electrolytes and subject to different applied potentials. Guided by simulations of candidate interfacial structures, these yield the spatial distributions of lipid and solvent within the layers. Adjacent to the electrode, a compact inner leaflet is formed, with potential-dependent solvent volume fraction in the range 0.09 < ϕS < 0.19; there was no evidence of an intervening water layer. The outer leaflet contains rather more solvent, 0.52 < ϕS < 0.55. NR-derived film thickness and PM-IRRAS intensity data show that the lipid molecules are tilted from the surface normal by ca. 26°. Bilayer solvation and charge data show a strong correlation for the inner leaflet and very little for the outer leaflet.