The Topology of Polymer Brushes Determines Their Nanoscale Hydration.

Time-of-flight neutron reflectometry (ToF-NR) performed under different relative humidity conditions demonstrates that polymer brushes constituted by hydrophilic, cyclic macromolecules exhibit a more compact conformation with lower roughness as compared to linear brush analogues, due to the absence of dangling chain ends extending at the polymer-vapor interface. In addition, cyclic brushes feature a larger swelling ratio and an increased solvent uptake with respect to their linear counterparts as a consequence of the increased interchain steric repulsions. It is proposed that differences in swelling ratios between linear and cyclic brushes come from differences in osmotic pressure experienced by each brush topology. These differences stem from entropic constraints. The findings suggest that to correlate the equilibrium swelling ratios at different relative humidity for different topologies a new form of the Flory-like expression for equilibrium thicknesses of grafted brushes is needed.

Effect of Nanodiamond Sizes on the Efficiency of the Quasi-Specular Reflection of Cold Neutrons.

Nanomaterials can intensively scatter and/or reflect radiation. Such processes and materials are of theoretical and practical interest. Here, we study the quasi-specular reflections (QSRs) of cold neutrons (CNs) and the reflections of very cold neutrons (VCNs) from nanodiamond (ND) powders. The fluorination of ND increased its efficiency by removing/replacing hydrogen, which is otherwise the dominant cause of neutron loss due to incoherent scattering. The probability of the diffuse reflection of VCNs increased for certain neutron wavelengths by using appropriate ND sizes. Based on model concepts of the interaction of CNs with ND, and in reference to our previous work, we assume that the angular distribution of quasi-specularly reflected CNs is narrower, and that the probability of QSRs of longer wavelength neutrons increases if we increase the characteristic sizes of NDs compared to standard detonation nanodiamonds (DNDs). However, the probability of QSRs of CNs with wavelengths below the cutoff of ~4.12 Å decreases due to diffraction scattering on the ND crystal lattice. We experimentally compared the QSRs of CNs from ~4.3 nm and ~15.0 nm ND. Our qualitative conclusions and numerical estimates can help optimize the parameters of ND for specific practical applications based on the QSRs of CNs.

Soliton-Mediated Magnetic Reversal in an All-Oxide-Based Synthetic Antiferromagnetic Superlattice.

All-oxide-based synthetic antiferromagnets (SAFs) are attracting intense research interest due to their superior tunability and great potentials for antiferromagnetic spintronic devices. In this work, using the La2/3Ca1/3MnO3/CaRu1/2Ti1/2O3 (LCMO/CRTO) superlattice as a model SAF, we investigated the layer-resolved magnetic reversal mechanism by polarized neutron reflectivity. We found that the reversal of LCMO layer moments is mediated by nucleation, expansion, and shrinkage of a magnetic soliton. This unique magnetic reversal process creates a reversed magnetic configuration of the SAF after a simple field cycling. Therefore, it can enable vertical data transfer from the bottom to the top of the superlattice. The physical origin of this intriguing magnetic reversal process could be attributed to the cooperation of the surface spin-flop effect and enhanced uniaxial magnetic anisotropy of the bottom LCMO layer. This work may pave a way to utilize all-oxide-based SAFs for three-dimensional spintronic devices with vertical data transfer and high-density data storage.

Oxidation of Polyunsaturated Lipid Membranes by Photocatalytic Titanium Dioxide Nanoparticles: Role of pH and Salinity.

In the present study, UV-induced membrane destabilization by TiO2 (anatase) nanoparticles was investigated by neutron reflectometry (NR), small-angle X-ray scattering (SAXS), quartz crystal microbalance with dissipation (QCM-D), dynamic light scattering (DLS), and ζ-potential measurements for phospholipid bilayers formed by zwitterionic palmitoyloleoylphosphatidylcholine (POPC) containing biologically relevant polyunsaturations. TiO2 nanoparticles displayed pH-dependent binding to such bilayers. Nanoparticle binding alone, however, has virtually no destabilizing effects on the lipid bilayers. In contrast, UV illumination in the presence of TiO2 nanoparticles activates membrane destabilization as a result of lipid oxidation caused by the generation of reactive oxygen species (ROS), primarily •OH radicals. Despite the short diffusion length characterizing these, the direct bilayer attachment of TiO2 nanoparticles was demonstrated to not be a sufficient criterion for an efficient UV-induced oxidation of bilayer lipids, the latter also depending on ROS generation in bulk solution. From SAXS and NR, minor structural changes were seen when TiO2 was added in the absence of UV exposure, or on UV exposure in the absence of TiO2 nanoparticles. In contrast, UV exposure in the presence of TiO2 nanoparticles caused large-scale structural transformations, especially at high ionic strength, including gradual bilayer thinning, lateral phase separation, increases in hydration, lipid removal, and potential solubilization into aggregates. Taken together, the results demonstrate that nanoparticle-membrane interactions ROS generation at different solution conditions act in concert to induce lipid membrane destabilization on UV exposure and that both of these need to be considered for understanding the performance of UV-triggered TiO2 nanoparticles in nanomedicine.

Magnetic Structure of Ion-Beam Imprinted Stripe Domains Determined by Neutron Scattering.

We present a detailed analysis of the in-plane magnetic vector configuration in head-to-head/tail-to-tail stripe domain patterns of nominal 5 µm width. The patterns have been created by He-ion bombardment induced magnetic patterning of a CoFe/IrMn3 exchange bias thin-film system. Quantitative information about the chemical and magnetic structure is obtained from polarized neutron reflectometry (PNR) and off-specular scattering (OSS). The technique provides information on the magnetic vector orientation and magnitude along the lateral coordinate of the sample, as well as the chemical and magnetic layer structure as a function of depth. Additional sensitivity to magnetic features is obtained through a neutron wave field resonance, which is fully accounted for in the presented analysis. The scattering reveals a domain width imbalance of 5.3 to 3.7 µm of virgin and bombarded stripes, respectively. Further, we report that the magnetization in the bombarded stripe significantly deviates from the head-to-head arrangement. A domain wall of 0.6 µm with homogeneous magnetization direction is found to separate the two neighboring domains. The results contain detailed information on length scales and magnetization vectors provided by PNR and OSS in absolute units. We illustrate the complementarity of the technique to microscopy techniques for obtaining a quantitative description of imprinted magnetic domain patterns and illustrate its applicability to different sample systems.

Unveiling structural, chemical and magnetic interfacial peculiarities in ε-Fe2O3/GaN (0001) epitaxial films.

The metastable ε-Fe2O3 is known to be the most intriguing ferrimagnetic and multiferroic iron oxide phase exhibiting a bunch of exciting physical properties both below and above room temperature. The present paper unveils the structural and magnetic peculiarities of a few nm thick interface layer discovered in these films by a number of techniques. The polarized neutron reflectometry data suggests that the interface layer resembles GaFeO3 in composition and density and is magnetically softer than the rest of the ε-Fe2O3 film. While the in-depth density variation is in agreement with the transmission electron microscopy measurements, the layer-resolved magnetization profiles are qualitatively consistent with the unusual wasp-waist magnetization curves observed by superconducting quantum interference device magnetometry. Interestingly a noticeable Ga diffusion into the ε-Fe2O3 films has been detected by secondary ion mass spectroscopy providing a clue to the mechanisms guiding the nucleation of exotic metastable epsilon ferrite phase on GaN at high growth temperature and influencing the interfacial properties of the studied films.

Magnetic and structural depth profiles of Heusler alloy Co2FeAl0.5Si0.5 epitaxial films on Si(1 1 1).

The depth-resolved chemical structure and magnetic moment of [Formula: see text], thin films grown on Si(1 1 1) have been determined using x-ray and polarized neutron reflectometry. Bulk-like magnetization is retained across the majority of the film, but reduced moments are observed within 45[Formula: see text] of the surface and in a 25[Formula: see text] substrate-interface region. The reduced moment is related to compositional changes due to oxidation and diffusion, which are further quantified by elemental profiling using electron microscopy with electron energy loss spectroscopy. The accuracy of structural and magnetic depth-profiles obtained from simultaneous modeling is discussed using different approaches with different degree of constraints on the parameters. Our approach illustrates the challenges in fitting reflectometry data from these multi-component quaternary Heusler alloy thin films.

The solid state conversion reaction of epitaxial FeF2(110) thin films with lithium studied by angle-resolved X-ray photoelectron spectroscopy.

The phase evolution and morphology of the solid state FeF2 conversion reaction with Li has been characterized using angle-resolved X-ray photoelectron spectroscopy (ARXPS). An epitaxial FeF2(110) film was grown on a MgF2(110) single crystal substrate and exposed to atomic lithium in an ultra-high vacuum chamber. A series of ARXPS spectra was taken after each Li exposure to obtain depth resolved chemical state information. The Li-FeF2 reaction initially proceeded in a layer-by-layer fashion to a depth of ∼1.2 nm. Beyond this depth, the reaction front became non-planar, and regions of unreacted FeF2 were observed in the near-surface region. This reaction progression is consistent with molecular dynamics simulations. Additionally, the composition of the reacted layer was similar to that of electrochemically reacted FeF2 electrodes. An intermediary compound FexLi2-2xF2, attributed to iron substituted in the LiF lattice, has been identified using XPS. These measurements provide insight into the atomistics and phase evolution of high purity FeF2 conversion electrodes without contamination from electrolytes and binders, and the results partially explain the capacity losses observed in cycled FeF2 electrodes.