Bulk-like Layer Research Articles

Elastic Properties of Low-Dimensional Single-Crystalline Dielectric Oxides through Controlled Large-Area Wrinkle Generation.

Freestanding single-crystalline SrTiO3 membranes, as high-κ dielectrics, hold significant promise as the gate dielectric in two-dimensional (2D) flexible electronics. Nevertheless, the mechanical properties of the SrTiO3 membranes, such as elasticity, remain a critical piece of the puzzle to adequately address the viability of their applications in flexible devices. Here, we report statistical analysis on plane-strain effective Young's modulus of large-area SrTiO3 membranes (5 × 5 mm2) over a series of thicknesses (from 6.5 to 32.2 nm), taking advantage of a highly efficient buckling-based method, which reveals its evident thickness-dependent behavior ranging from 46.01 to 227.17 GPa. Based on microscopic and theoretical results, we elucidate these thickness-dependent behaviors and statistical data deviation with a bilayer model, which consists of a surface layer and a bulk-like layer. The analytical results show that the ∼3.1 nm surface layer has a significant elastic softening compared to the bulk-like layer, while the extracted modulus of the bulk-like layer shows a variation of ∼40 GPa. This variation is considered as a combined contribution from oxygen deficiency presenting in SrTiO3 membranes, and the alignment between applied strain and the crystal orientation. Upon comparison of the extracted elastic properties and electrostatic control capability to those of other typical gate dielectrics, the superior performance of single-crystalline SrTiO3 membranes has been revealed in the context of flexible gate dielectrics, indicating the significant potential of their application in high-performance flexible 2D electronics.

The different types of magnetic coupling achieved via dimensionality control of interfacial charge transfer in cobaltite-cuprate superlattices

The interface effect in artificial oxide heterostructures provides opportunities to engineer and explore desired functionalities associated with spintronic devices. Understanding the basic mechanism behind the interface effect is crucial for possible applications. Herein, charge-transfer-mediated magnetic coupling in LaCoO3/SrCuO2 (LCO/SCO) superlattices is investigated by changing the thickness of the SCO layer, which exhibits dimensional control of the structure transformation from planar-type to chain-type configuration. We found that the LCO/SCO superlattices display obvious soft-hard magnetic coupling when the SCO layer is thin, but the exchange bias effect gradually becomes dominant as thickness increases. As suggested by the x-ray absorption spectrum, electronic charge transfer dependent on SCO thickness is present at the interface of LCO/SCO superlattices. As the SCO is thin, the interaction of interfacial Co4+ and Co3+ results in the soft magnetic layer strongly coupling with the intrinsic hard magnetic layer of the LCO layer. But as the SCO layer thickens, charge transfer reduces, and the soft magnetic properties of the interface are suppressed. Instead, the long-range antiferromagnetic order of the bulk-like planar-type SCO layer couples with the adjacent ferromagnetic layer, leading to the exchange bias effect. Our results reveal an extraordinary spectrum of possibilities to design and engineer multifunctionalities of advanced oxide electronics.

Presence of bicontinuous microemulsion-type domains and dielectrically inert interfacial water layers in lamellar gel-stabilized oil-in-water emulsions

HypothesisLamellar gels are widely formulated in household and cosmetic products because of their eminent ability to improve long-term stability of thermodynamically unstable oil-in-water emulsions. However, despite long study, how and why membrane internal structure and membrane-membrane interactions are modified by the presence of polar and nonpolar oils remains elusive. ExperimentsUsing small- and wide-angle X-ray scattering, dielectric spectroscopy, and field-emission transmission electron microscope, we investigate intermembrane interactions and water-mediated microscopic interfacial properties in lamellar gels and lamellar gel-stabilized oil-in-water emulsions based on cetyltrimethylammonium chloride and 1-hexadecanol. FindingsReducing membrane surface charge density enhances undulation fluctuation disorder, resulting in a crossover of dominant interactions from electrostatic double-layer repulsion to Helfrich interaction. Oil-emulsification induces similar structural impacts to the reduced 1-hexadecanol ratio, confirming preferential dissolution of higher-alcohol in oil phases. An emerging Teubner-Stray scattering component upon emulsification of nonpolar oil evidences that oil droplets and lamellar gels are indirectly connected via bicontinuous microemulsion-type domains. Dielectric spectra reveal strikingly small water permittivity in the lamellar gel and emulsion samples, which is quantitatively explained by a cumulative effect of a dielectrically inert interfacial thin water layer (<1nm) and a highly polarizable bulk-like water layer. This phenomenon appears to be intrinsic to diverse lamellar stack architectures.

Neutron reflectivity study on the nanostructure of PMMA chains near substrate interfaces based on contrast variation accompanied with small molecule sorption.

In the case of poly(methyl methacrylate) (PMMA) thin films on a Si substrate, thermal annealing induces the formation of a layer of PMMA chains tightly adsorbed near the substrate interface, and the strongly adsorbed PMMA remains on the substrate, even after washing with toluene (hereinafter called adsorbed sample). Neutron reflectometry revealed that the concerned structure consists of three layers: an inner layer (tightly bound on the substrate), a middle layer (bulk-like), and an outer layer (surface) in the adsorbed sample. When an adsorbed sample was exposed to toluene vapor, it became clear that, between the solid adsorption layer (which does not swell) and bulk-like swollen layer, there was a "buffer layer" that could sorb more toluene molecules than the bulk-like layer. This buffer layer was found not only in the adsorbed sample but also in the standard spin-cast PMMA thin films on the substrate. When the polymer chains were firmly adsorbed and immobilized on the Si substrate, the freedom of the possible structure right next to the tightly bound layer was reduced, which restricted the relaxation of the conformation of the polymer chain strongly. The "buffer layer" was manifested by the sorption of toluene with different scattering length density contrasts.

Peculiarities in optical response of hybrid-barrier GaSb/InAs/AlSb resonant tunneling diode structure

We present comprehensive investigation of the optical properties of hybrid-barrier GaSb-based resonant tunneling structures, containing a bulk-like GaInAsSb absorption layer and two asymmetric type II GaSb/InAs/AlSb quantum wells. Methods of optical spectroscopy by means of Fourier-transformed photoluminescence and photoreflectance are employed to probe optical transitions in this complex multilayer system. Based on the comparison between the absorption-like and emission-like spectra (also in function of temperature) confronted with band structure calculations four main transitions could be resolved and identified. For one of them, there has been observed unusually strong linear polarization dependence never reported in structures of that kind. It has been interpreted as related to a transition at the GaSb/GaInAsSb interface, for which various scenarios causing the polarization selectivity are discussed.

Do Interfacial Layers in Thin Films Act as an Independent Layer within Thin Films?

The thermodynamic behavior of thin Poly(vinyl methyl ether) (PVME) films including the irreversible adsorbed layer on the substrate is investigated. In the first step, the growth kinetics of the adsorbed layer was studied combining a leaching technique and atomic force microscopy. Furthermore, it was shown that there is a critical initial film thickness for the formation of a surface-filling adsorbed layer. Additionally, spectroscopic ellipsometry measurements were carried out to investigate the influence of the adsorbed layer on the glass transition temperature of the thin films. For 30 nm films and films with lower thicknesses, the influence of the adsorbed layer percolates strongly to the bulk-like layer of the film. Finally, the molecular dynamics of the adsorbed layer was studied by broadband dielectric spectroscopy, employing a nanostructured-electrode system. One process was revealed, which was assigned either to molecular fluctuations taking place in the loosely bounded part of the adsorbed layer or to the desorption/adsorption of segments at the substrate.

Elucidation of a Heterogeneous Layered Structure in the Thickness Direction of Poly(vinyl alcohol) Films with Solvent Vapor-Induced Swelling.

We investigated the swelling behaviors of poly(vinyl alcohol) (PVA) films deposited on Si wafers with water vapor, which is a good solvent for PVA for elucidating structural and dynamical heterogeneities in the film thickness direction. Using deuterated water vapor, structural and dynamical differences in the thickness direction can be detected easily as different degrees of swelling in the thickness direction by neutron reflectivity. Consequently, the PVA film with a degree of saponification exceeding 98 mol % exhibits a three-layered structure in the thickness direction. It is considered that an adsorption layer consisting of molecular chains that are strongly adsorbed onto the solid substrate is formed at the interface with the substrate, which is not swollen with water vapor compared with the bulk-like layer above it. The adsorption layer is considered to exhibit significantly slower dynamics than the bulk. Furthermore, a surface layer that swells excessively compared with the underneath bulk-like layer is found. This excess swelling of the surface layer may be related to a higher mobility of the molecular chains or lower crystallinity at the surface region compared to the underneath bulk-like layer. Meanwhile, for the PVA film with a much lower degree of saponification, a thin layer with a slightly lower degree of swelling than the bulk-like layer above it can be detected at the interface between the film and substrate only under a high humidity condition. This layer is considered to be the adsorption layer composed of molecular chains loosely adsorbed onto the Si substrate.

Selective Control of Ion Transport by Nanoconfinement: Ionic Liquid in Mesoporous Resorcinol-Formaldehyde Monolith.

Thermal and dynamic properties of ionic liquid (IL)-based electrolytic solution (Li+TFSI- in Pyr13+TFSI-; 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide = Pyr13+TFSI-) confined in nanoporous polymer hosts were investigated with respect to the pore size/porosity and the surface chemistry of the polymer host. As host material, mesoporous resorcinol-formaldehyde (RF) polymer monoliths with three-dimensionally connected pore structure were prepared, with precise control of the pore size ranging from ca. 7 to 60 nm. Thermal analysis of RF polymer-ionic liquid composites showed stability up to almost 400 °C and a melting point depression proportional to the inverse of the pore diameter. Good ionic conductivity comparable to that of a commercial separator is obtained, which is dependent on the porosity (i.e., pore volume) of the confining host material (i.e., the number of charge carriers available in the system). Further pulsed field gradient (PFG) NMR experiments revealed that the diffusion coefficient of Pyr13+ cation becomes smaller than that of TFSI- anion inside RF pores, which is contradictory to the bulk IL system. This change in the ionic motion is due to electrostatic attraction between the pore walls and Pyr13+ cations, resulting in a layer structure composed of a Pyr13+ cation-rich layer adsorbed at the pore wall surface and a TFSI- anion-enriched bulklike layer at the pore center. Our study suggests that transport characteristics of the ions of interest can be controlled by optimizing the surface chemistry of the host framework and their motion can be separately monitored by PFG NMR spectroscopy.

First-principles study of CO and OH adsorption on in-doped ZnO surfaces

We present a first-principles computational study of CO and OH adsorption on non-polar ZnO (101¯0) surfaces doped with indium. The calculations were performed using a model ZnO slab. The position of the In dopants was varied from deep bulk-like layers to the surface layers. It was established that the preferential location of the In atoms is at the surface by examining the dependence of the defect formation energy as well as the surface energy on In location. The adsorption sites on the surface of ZnO and the energy of adsorption of CO molecules and OH-species were determined in connection to In doping. It was found that OH has higher bonding energy to the surface than CO. The presence of In atoms at the surface of ZnO is favorable for CO adsorption, resulting in an elongation of the C-O bond and in charge transfer to the surface. The effect of CO and OH adsorption on the electronic and conduction properties of surfaces was assessed. We conclude that In-doped ZnO surfaces should present a higher electronic response upon adsorption of CO.

Temperature and Thickness Dependence of Statistical Fluctuations of the Gilbert Damping in Co - Fe - B / MgO Bilayers

We theoretically investigate the temperature and thickness dependence of the effective Gilbert damping constant (α) in the Co-Fe-B/MgO system using atomistic spin dynamics. We consider a high damping constant at the interface layer and a low damping constant for the bulklike layers due to large interfacial spin-orbit coupling. We find a strong dependence of the effective Gilbert damping with the film thickness, in quantitative agreement with experimental data. The temperature dependence of the effective damping arising from thermal-spin fluctuations up to temperatures of 400 K is weak, with no apparent change over the studied temperature range. Interestingly, we find that the temperature produces a different effect: a statistical fluctuation of the Gilbert damping parameter for a given relaxation induced solely from the finite size of the system. This statistical variation of the Gilbert damping is an intrinsic effect and is important for spintronic devices operating at gigahertz frequencies, where the dynamic response must be carefully controlled.

Accurate chemical analysis of oxygenated graphene-based materials using X-ray photoelectron spectroscopy

A simple, fast and general protocol for quantitative analysis of X-ray photoelectron spectroscopy (XPS) data provides accurate estimations of chemical species in graphene and related materials (GRMs). XPS data are commonly used to estimate the quality of and defects in graphene and graphene oxide (GO), by comparing carbon and oxygen 1s XPS peaks, obtaining an O/C ratio. This approach, however, cannot be used in the presence of extraneous oxygen contamination.The protocol, based on quantitative line-shape analysis of C 1s signals, uses asymmetric pseudo-Voigt line-shapes (APV), in contrast to Gaussian-based approaches conventionally used in fitting XPS spectra, thus allowing better accuracy in quantifying C 1s contributions from graphitic carbon (sp2), defects (sp3 carbon), carbons bonded to hydroxyl and epoxy groups, and from carbonyl and carboxyl groups. The APV protocol was evaluated on GRMs with O/C ratios ranging from 0.02 to 0.30 with film thicknesses from monolayers to bulk-like (>30 nm) layers and also applied to previously published data, showing better results compared to those from conventional XPS fitting protocols.Based uniquely on C 1s data, the APV protocol can quantify O/C ratio and the presence of specific functional groups in GRMs even on SiOx, substrates, or in samples containing water.

Saturated aqueous NaCl solution and pure water in Na-montmorillonite clay at thermodynamic conditions of hydraulic fracturing: Thermodynamics, structure and diffusion from molecular simulations

We present a Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation study of an aqueous NaCl solution in Na-montmorillonite (Na-MMT) clay pores, including the thermodynamic, structural and dynamical properties. We represent the clay pores by Na-MMT slits of an interlayer separation ranging from about 10 to 32 Å, covering clay pores from dry clay to clay pores with a bulk-like water layer in the middle of the pore. We consider the clay in equilibrium with the saturated salt solution at a typical shale gas reservoir condition of a temperature of 365 K and a pressure of 275 bar. The equilibrium amount of the adsorbed ions and water molecules obtained from the GCMC simulations are employed in MD to simulate the disjoining pressure and in turn the swelling free energy curve which determines the number of stable states in the clay, their composition, and the corresponding equilibrium interlayer separations. In addition, using MD simulations, we evaluate the hydration energy, atomic and charge density profiles, orientation distribution, hydrogen-bond network, and in-plane self-diffusivity of the water and ions to provide insight into the microscopic behaviour of the aqueous NaCl solution in the interlayer galleries of the hydrophilic Na-MMT pores. We compare the simulation results for the confined aqueous NaCl solutions with those obtained for the Na-MMT in equilibrium with a pure water reservoir at the same temperature and pressure.

Site-Resolved Contributions to the Magnetic-Anisotropy Energy and Complex Spin Structure of Fe/MgO Sandwiches

Fe/MgO-based Magnetic Tunnel Junctions (MTJs) are among the most promising candidates for spintronic devices due to their high thermal stability and high tunneling magnetoresistance. Despite its apparent simplicity, the nature of the interactions between the Fe and MgO layers leads to complex finite size effects and temperature dependent magnetic properties which must be carefully controlled for practical applications. In this letter, we investigate the electronic, structural and magnetic properties of MgO/Fe/MgO sandwiches using first principles calculations and atomistic spin modeling based on a fully parameterized spin Hamiltonian. We find a large contribution to the effective interfacial magnetic anisotropy from the two-ion exchange energy. Minimization of the total energy using atomistic simulations shows a surprising spin spiral ground state structure at the interface owing to frustrated ferromagnetic and antiferromagnetic interactions, leading to a reduced Curie temperature and strong layer-wise temperature dependence of the magnetization. The different temperature dependences of the interface and bulk-like layers results in an unexpected non-monotonic temperature variation of the effective magnetic anisotropy energy and temperature-induced spin-reorientation transition to an in-plane magnetization at low temperatures. Our results demonstrate the intrinsic physical complexity of the pure Fe/MgO interface and the role of elevated temperatures providing new insight when interpreting experimental data of nanoscale MTJs.

Epitaxial growth of Al9Ir2 intermetallic compound on Al(100): Mechanism and interface structure

The adsorption of Ir adatoms on Al(100) has been investigated under various exposures and temperature conditions. The experimental and theoretical results reveal a diffusion of Ir adatoms within the Al(100) surface selvedge already at 300 K. Above 593 K, two domains of a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R26.{6}^{\ensuremath{\circ}}$ phase are identified by low energy electron diffraction (LEED) and scanning tunneling microscopy measurements. This phase corresponds to the initial growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound at the Al(100) surface. The ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic domains are terminated by bulklike pure Al layers. The structural stability of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) grown on Al(100) has been analyzed by density functional theory based calculations. Dynamical LEED analysis is consistent with an Ir adsorption leading to the growth of an ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ intermetallic compound. We propose that the epitaxial relationship ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}(001)\ensuremath{\parallel}\mathrm{Al}$(100) and ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}[100]\ensuremath{\parallel}\mathrm{Al}[031]/[013]$ originates from a matching of Al atomic arrangements present both on Al(100) and on pure Al(001) layers present in the ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ compound. Finally, the interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ precipitates and the Al matrix has been characterized by transmission electron microscopy measurements. The cross-sectional observations are consistent with the formation of ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$(001) compounds. These measurements indicate an important Ir diffusion within Al(100) near the surface region. The coherent interface between ${\mathrm{Al}}_{9}{\mathrm{Ir}}_{2}$ and the Al matrix is sharp.

Structure-property relationships at Nafion thin-film interfaces: Thickness effects on hydration and anisotropic ion transport

The effect of film thickness on water uptake and structure in ultra-thin Nafion is probed via in situ neutron reflectometry for a series of 10 films with thicknesses ranging from 5 to 153nm. Observed interfacial lamellae are used to understand anomalous transport limitations in such films. Results show three distinct thickness regimes: (i) in the truncated regime (< 12nm), the entire film consists of lamellae; (ii) in the thin-film regime (12–42nm), a non-lamellar bulk-like layer forms between the lamellae and vapor; (iii) in the thick-film regime (≥ 60nm), the bulk-like layer thickness exceeds the radius of gyration for thin-film Nafion. The water uptake in the sample varies non-monotonically with thickness, and can be ordered as: thin-film < truncated < thick-film. In the thin-film regime, the water uptake in the bulk-like layer and lamellae are equal, and both increase with thickness, except for a well-hydrated layer adjacent to the SiO2 substrate. In the thick-film regime, the bulk-like layer water uptake equals that in macroscopic Nafion membranes, and is invariant with film thickness, while the lamellar water uptake greatly exceeds this. Composition depth profiles are used to predict the anisotropic ionic conductivities. These are fitted to previously published experimental results to demonstrate that the lamellar structure is required for accurate conductivity predictions. These results provide key insights for minimizing transport losses in fuel cell catalyst layers.

Shubnikov–de Haas Effect in InGaSb/InAs superlattices

Magneto-transport measurements have been made on strain-free In0.25Ga0.75Sb/InAs superlattices grown on GaSb substrates. The superior structural quality of these superlattices compared to older materials in the literature resulted in a much higher resolution of the Shubnikov–de Haas oscillations. Three separate conduction channels were observed. Measurements with the magnetic field in the plane of the sample indicated that the channel with the longer period was due to three dimensional-like conduction within the superlattice while the other two channels showed two dimensional behavior most likely associated with conduction at the surface or interfaces between the superlattice and bulk-like layers. Comparison with multi-carrier analysis indicated that the channel with intermediate period was due to hole conduction while the others were due to electron conduction. Oscillations were observed at temperatures up to 20 K. The concentration of the superlattice channel at low temperature was 5.93 × 1015 cm−3 while those of the two dimensional hole and electron channels were 1.64 × 1011 cm−2 and 5.20 × 1011 cm−2.

Unraveling the Dynamics of Nanoscopically Confined PVME in Thin Films of a Miscible PVME/PS Blend

Broadband dielectric spectroscopy (BDS) was employed to investigate the glassy dynamics of thin films (7-200 nm) of a poly(vinyl methyl ether) (PVME)/polystyrene (PS) blend (50:50 wt %). For BDS measurements, nanostructured capacitors (NSCs) were employed, where films are allowed a free surface. This method was applied for film thicknesses up to 36 nm. For thicker films, samples were prepared between crossed electrode capacitors (CECs). The relaxation spectra of the films showed multiple processes. The first process was assigned to the α-relaxation of a bulklike layer. For films measured by NSCs, the rates of α-relaxation were higher compared to those of the bulk blend. This behavior was related to the PVME-rich free surface layer at the polymer/air interface. The second process was observed for all films measured by CECs (process X) and the 36 nm film measured by NSCs (process X2). This process was assigned to fluctuations of constraint PVME segments by PS. Its activation energy was found to be thickness-dependent because of the evidenced thickness dependency of the compositional heterogeneity. Finally, a third process with an activated temperature dependence was observed for all films measured by NSCs (process X1). It resembled the molecular fluctuations in an adsorbed layer found for thin films of pure PVME, and thus, it is assigned accordingly. This process undergoes an extra confinement because of frozen adsorbed PS segments at the polymer/substrate interface. To our knowledge, this is the first example where confinement-induced changes were observed by BDS for blend thin films.

Finite Thickness Effects on Nafion Water Uptake and Ionic Conductivity at Hydrophilic Substrate Interfaces, and Implications for PEMFC Performance

Nafion has long been the state-of-the-art solid polymer electrolyte material for a range of next-generation electrochemical energy and chemical separation devices, such as polymer electrolyte membrane fuel cells (PEMFCs). While significant attention over several decades has focused on the polymer structure, proton transport mechanism, and structure-function relationships for bulk membranes, only recently has attention turned to Nafion properties in the catalyst layer (CL), the complex heterogeneous layer where key limiting charge-transfer processes in the PEMFC occur. Recent work suggests that high transport resistance in the CL Nafion contributes to poor PEMFC performance with low Pt loading, but the specific causes of the transport limitations and their implications for PEMFC design are not fully understood. Understanding and quantifying the role of CL Nafion in limiting PEMFC performance is hindered by the complex nano- and micro-structure of the CL – where thin Nafion sheets coat ionomer-flooded agglomerates of carbon-supported Pt catalyst nanoparticles – and by the complex influence of multiple material interfaces and operating conditions on the thin-film Nafion properties. We report here the of dependence of the polymer ionic domain structure and water uptake on the thickness of ultrathin Nafion films (ranging from 5 – 153 nm) coated onto hydrophilic SiO2 substrates, using in situ neutron reflectometry (NR) as a probe. NR is capable of determining structure with sub-Angstrom precision in idealized planar interfaces. Because of isotopic variations in neutron scattering cross sections, it is particularly sensitive to certain elements such as H (and thus water) as well as Li, V, and numerous others. Our results herein show that ionic domains form as sheet-like lamellae at the SiO2 interface, with a thick, bulk-like layer forming at the lamellae/vapor interface for films thicker than 20 nm. The bulk-like layer water uptake increases with increasing thickness for films with equivalent Nafion thickness t Naf < 60 nm, but these bulk-like layers have constant water uptake, similar to bulk 1100 eq. wt. nafion (l = 10) for t Naf ≥ 60 nm. We also show that while the lamellae form due to substrate interactions, their structure and water uptake are also influenced by the presence and composition of the bulk-like layer. Finally, these results indicate that inferring water content from whole-sample measurements (such as swelling or mass uptake measurements) can, in some cases, give an insufficient or even inaccurate measure of the Nafion water uptake. These structural measurements from NR are then used to predict the ionic conductivity of ultrathin Nafion films, using previously established relationships for s ion based on l. Fitting these predictions to experimental data provides evidence of transport limitations in the Nafion film, which increase with increasing proximity to the substrate, and helps to explain previously counter-intuitive trends in ionic conductivity vs. thickness for these films. These effective conductivities are then implemented in numerical simulations of the PEMFC anode and cathode catalyst layers, and demonstrate that both finite thickness effects and sheet-like ionic domain structures at film interfaces can have significant implications for PEMFC performance.

Finite Thickness Effects on Nafion Water Uptake and Ionic Conductivity at Hydrophilic Substrate Interfaces, and Implications for PEMFC Performance

The effect of film thickness on water uptake in Nafion films is probed via neutron reflectometry for a series of 10 samples with thicknesses ranging from 7–103 nm, and the resulting effects on ionic conductivity. The water uptake in the lamellae and the bulk-like layer varies non-monotonically, decreasing as the equivalent Nafion thickness increases from 7 nm to 12 nm, increasing with increasing film thickness for equivalent Nafion thicknesses ranging from 12 nm—60 nm, and then constant for films with equivalent thickness ≥ 60 nm. Composition depth profiles are used to predict the anisotropic ionic conductivities, which are used in a flooded-agglomerate model of PEMFC catalyst layers to predict the effect of lamellar structure transport properties on PEMFC performance.

Atomically Thin Interfacial Suboxide Key to Hydrogen Storage Performance Enhancements of Magnesium Nanoparticles Encapsulated in Reduced Graphene Oxide.

As a model system for hydrogen storage, magnesium hydride exhibits high hydrogen storage density, yet its practical usage is hindered by necessarily high temperatures and slow kinetics for hydrogenation-dehydrogenation cycling. Decreasing particle size has been proposed to simultaneously improve the kinetics and decrease the sorption enthalpies. However, the associated increase in surface reactivity due to increased active surface area makes the material more susceptible to surface oxidation or other side reactions, which would hinder the overall hydrogenation-dehydrogenation process and diminish the capacity. Previous work has shown that the chemical stability of Mg nanoparticles can be greatly enhanced by using reduced graphene oxide as a protecting agent. Although no bulklike crystalline MgO layer has been clearly identified in this graphene-encapsulated/Mg nanocomposite, we propose that an atomically thin layer of honeycomb suboxide exists, based on first-principles interpretation of Mg K-edge X-ray absorption spectra. Density functional theory calculations reveal that in contrast to conventional expectations for thick oxides this interfacial oxidation layer permits H2 dissociation to the same degree as pristine Mg metal with the added benefit of enhancing the binding between reduced graphene oxide and the Mg nanoparticle, contributing to improved mechanical and chemical stability of the functioning nanocomposite.