Thin-Film Metal Hydrides

Room-Temperature Magnetism in 2D MnGa4 -H Induced by Hydrogen Insertion

2D room-temperature magnetic materials are of great importance in future spintronic devices while only very few are reported. Herein, a plasma-enhanced chemical vapor deposition approach is exploited to construct the 2D room-temperature magnetic MnGa₄ -H single crystal with a thickness down to 2.2 nm. The employment of H₂ plasma makes hydrogen atoms can be easily inserted into the MnGa₄ lattice to modulate the atomic distance and charge state, thereby ferrimagnetism can be achieved without destroying the structural configuration. The as-obtained 2D MnGa₄ -H crystal is high-quality, air-stable, and thermo-stable, demonstrating robust and stable room-temperature magnetism with a high Curie temperature above 620 K. This work enriches the 2D room-temperature magnetic family and opens up the possibility for the development of spintronic devices based on 2D magnetic alloys.

Chirality-Induced Noncollinear Magnetization and Asymmetric Domain-Wall Propagation in Hydrogenated CoPd Thin Films

Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits a strong vertical and lateral antiferromagnetic coupling in the perpendicular component between the areas with and without plasma pretreatment, and asymmetric domain-wall propagation in the plasma-pretreated areas during magnetization reversal. These phenomena exhibit evident magnetic chirality and can be interpreted with the Ruderman-Kittel-Kasuya-Yosida coupling and the Dzyaloshinskii-Moriya interaction (DMI). The sample processing demonstrated in this study allows easy incorporation of lithography techniques that can define areas with or without DMI to create intricate magnetic patterns on the sample, which provides an avenue toward more sophisticated control of canted spin textures in future spintronic devices.

Hydrogen Solubility and Atomic Structure of Graphene Supported Pd Nanoclusters

We investigated the atomic structure of graphene supported Pd nanoclusters and their interaction with hydrogen up to atmospheric pressures at room temperature by surface X-ray diffraction and scanning tunneling microscopy. We find that Ir seeded Pd nanocluster superlattices with 1.2 nm cluster diameters can be grown on the graphene/Ir(111) moiré template with high structural perfection. The superlattice clusters are anchored through the rehybridized graphene to the Ir support, which superimposes a 2.0% inplane compression onto the clusters. During hydrogen exposure at 10 mbar pressure and room temperature, a significant part of the clusters gets unpinned from the superlattice. The clusters in registry undergo an out-of-plane expansion only, whereas the detached clusters expand in in- and out-of-plane directions. The formation of a hydrogen rich PdH_x α' phase was not observed. After exposure to 1 bar, the majority of the clusters are unpinned from superlattice sites, due to their surface interaction with hydrogen and possible spill over to the graphene support. Only minor sintering was observed, which is more pronounced for the unpinned clusters. The results give evidence that ultrasmall Pd clusters on graphene are a stable hydrogen storage system with reduced hydrogen storage hysteresis and maintain a large surface area for hydrogen chemisorption.

High-resolution x-ray scattering from epitaxial thin films of Y/Nb on Al2O3

During the 1990s, Roger Cowley had a strong interest in the crystal and magnetic structures of rare-earth superlattices as a means to understand the rich and exotic magnetic properties of the rare-earth metals. High-quality samples can be grown by molecular beam epitaxy on sapphire substrates by first depositing a thin epitaxial layer of niobium, then a layer of yttrium or lutetium as a seed. High-resolution x-ray scattering is an excellent probe to characterise the crystal quality and was used to study the structure of the niobium layer. However, relatively little attention was paid to the seed layer. This article summarises some of the x-ray experiments performed by the Cowley group to study the structure of epitaxialniobium onsapphire, and extends the work to report some results on the structure of thinyttrium seed layers. The structure of the yttrium films is shown to have a strong dependence on the thickness of the niobium buffer, with the buffer needing to be thicker than a critical value of  ∼80for the formation of misfit dislocations at the Nb/Al2O3interface before highly coherent Y films can be grown. Yttrium films grown on Nb buffers thinner than  ∼500show a similar two-peak line shape inscans through their specular Bragg peaks to that seen in the specular Nb Bragg peaks, with a resolution-limited feature on a broader diffuse peak. The resolution-limited feature depends on the thickness of the yttrium film, becoming weaker and having a stronger decay with increasingas the film thickness increases, while the width of the yttrium broad peak evolves as the square root of the width of the niobium Bragg peak. The data are discussed within the context of theories describing the scattering from films with misfit dislocations.

A fiber-optic nanoplasmonic hydrogen sensor via pattern-transfer of nanofabricated PdAu alloy nanostructures

We demonstrate the transfer of arrays of nanofabricated noble metal and alloy nanostructures obtained by high-temperature annealing on a flat parent support onto optical fibers, to create a hysteresis-free fiber optic nanoplasmonic hydrogen sensor. This work enables the integration of complex nanofabricated structures and their arrangements in tailored arrays with fiber optics to realize optical sensors, which will find application in a wide range of disciplines.

Nanostructured Metal Hydrides for Hydrogen Storage

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

Note: Stability and lifetime of scandium deuteride film cathode in a vacuum arc ion source

This paper reports the properties of the plasma and gas produced in a vacuum arc discharge with scandium deuteride (ScD_1.8) film cathodes. The thickness of the ScD_1.8 film influences the quantity of the gases released from the cathode material. The deuterium gas releasing in the discharge process was in a depth range from the cathode surface to the cathode interior, that is, between 3 and 6 μm. Surprisingly, after discharge, the deuterium ion ratio remains the same in the film with different thicknesses. That indicates that the release of deuterium gas in a 3 μm-thick ScD_1.8 film is enough for ionization. In addition, as the number of discharge increases, the stability of atomic fraction ratio gets worse and the ratio of deuterium ions decreases.

A capacitive ultrasonic transducer based on parametric resonance

A capacitive ultrasonic transducer based on a parametric resonator structure is described and experimentally demonstrated. The transducer structure, which we call capacitive parametric ultrasonic transducer (CPUT), uses a parallel plate capacitor with a movable membrane as part of a degenerate parametric series RLC resonator circuit with a resonance frequency of f_o. When the capacitor plate is driven with an incident harmonic ultrasonic wave at the pump frequency of 2f_o with sufficient amplitude, the RLC circuit becomes unstable and ultrasonic energy can be efficiently converted to an electrical signal at f_o frequency in the RLC circuit. An important characteristic of the CPUT is that unlike other electrostatic transducers, it does not require DC bias or permanent charging to be used as a receiver. We describe the operation of the CPUT using an analytical model and numerical simulations, which shows drive amplitude dependent operation regimes including parametric resonance when a certain threshold is exceeded. We verify these predictions by experiments with a micromachined membrane based capacitor structure in immersion where ultrasonic waves incident at 4.28 MHz parametrically drive a signal with significant amplitude in the 2.14 MHz RLC circuit. With its unique features, the CPUT can be particularly advantageous for applications such as wireless power transfer for biomedical implants and acoustic sensing.

Solid-state reactions and hydrogen storage in magnesium mixed with various elements by high-pressure torsion: experiments and first-principles calculations

The HPT technique is effective in synthesizing Mg-based hydrogen storage materials and improving the air resistivity and hydrogenation properties.

Finite-size effects: hydrogen in Fe/V(001) superlattices

We investigate the effect of finite size on phase boundaries of hydride formation in ultrathin metallic films, using Fe/V(001) superlattices as a model system. The critical temperature is determined to scale linearly with the inverse thickness of the V layers. The decrease of the ordering temperature with decreasing layer thickness arises from the missing H neighbors at the interfaces, analogous to observed finite-size effects in magnetic layers and nanosized ice crystals.

Understanding the role of vanadium in enhancing the low-temperature hydrogenation kinetics of an Mg thin film

Mg100-xVx (x = 0 to 15) thin films capped with Pd were prepared by electron beam codeposition and studied for their hydrogenation/dehydrogenation kinetics and cycling properties at 140 °C under hydrogenation pressures of 0.1 MPa. It has been found that the Mg100-xVx thin films show significantly higher reversible hydrogen-storage capacity and faster kinetics in comparison with a pure Mg thin film; for instance, the maximum hydrogen absorption (3.7% mass fraction hydrogen) can be obtained in the fifth cycle for Mg90V10 in less than 5 min. The addition of V clearly plays a favorable role in improving the reversible hydrogen-storage capacity of an Mg film; however, with increasing hydrogenation/dehydrogenation cycles the hydrogen-storage capacity gradually deteriorates. To explore the origin of the effect of V on the improved hydrogenation of an Mg thin film, in this work we focused on studying the structural variations of the Mg90V10 thin film before and after hydrogenation at different stages of cycling; the films were investigated by X-ray diffraction as well as scanning and transmission electron microscopy. We concluded that (1) early in the absorption/desorption cycling the as-deposited structure of percolating layers of nanocrystalline V throughout a Mg matrix is preserved; (2) the percolating V layers envelope fine Mg grains and act as (a) dispersers that isolate small Mg grains, (b) fast diffusers of hydrogen, and (c) hydrogen catalysts at the Mg/V interface to form MgH2; and (3) with progressive cycling, the continuous layers of V aggregate to spherical nanoparticles, which interrupts the continuity of fast hydrogen diffusion through V.

Using light transmission to watch hydrogen diffuse

Because of its light weight and small size, hydrogen exhibits one of the fastest diffusion rates in solid materials, comparable to the diffusion rate of liquid water molecules at room temperature. The diffusion rate is determined by an intricate combination of quantum effects and dynamic interplay with the displacement of host atoms that is still only partially understood. Here we present direct observations of the spatial and temporal changes in the diffusion-induced concentration profiles in a vanadium single crystal and we show that the results represent the experimental counterpart of the full time and spatial solution of Fick's diffusion equation. We validate the approach by determining the diffusion rate of hydrogen in a single crystal vanadium (001) film, with net diffusion in the [110] direction.

Hydrogen electrosorption into Pd-Cd nanostructures

Hydrogen-absorbing materials are crucial for both the purification and storage of hydrogen. Pd and Pd-based alloys have been studied extensively for their use as both hydrogen dissociation catalysts and hydrogen selective membrane materials. It is known that incorporating metal atoms of different sizes into the Pd lattice has a major impact on the hydrogen absorption process. In this paper, hydrogen electrosorption into nanostructured Pd-Cd alloys has been studied for different compositions of Cd that varied from 0 to 15 at. %. The low cost of Cd makes it an attractive material to combine with Pd for hydrogen sorption. A combination of chronoamperometry and cyclic voltammetric experiments was used to determine the ratio of the H/(Pd + Cd) and the kinetics of hydrogen sorption into these Pd-Cd alloys at different potentials. It was found that the maximum H/(Pd + Cd) value was 0.66 for pure Pd, and this decreased with increasing the amount of Cd. Also, the alpha (solid solution) to beta phase (metal hydride) hydrogen transition was determined to be the slowest step in the absorption process and was practically eliminated when an optimum amount of Cd atoms was doped (i.e., Pd-Cd(15%)). With increasing the amount of Cd, more hydrogen was absorbed into the Pd-Cd nanostructures at the higher potentials (the alpha phase region). The faster kinetics, along with the decrease in the phase transition of hydrogen sorption into the Pd-Cd nanostructures when compared to pure Pd, makes the Pd-Cd nanostructures attractive for use as a hydrogen dissociation catalytic capping layer for other metal hydrides or as a hydrogen selective membrane.

Deducing 2D crystal structure at the liquid/solid interface with atomic resolution: a combined STM and SFG study

Sum frequency generation vibrational spectroscopy (SFG) has been applied to study two-dimensional (2D) crystals formed by an isophthalic acid diester on the surface of highly oriented pyrolytic graphite, providing complementary measurements to scanning tunneling microscopy (STM) and computational modeling. SFG results indicate that both aromatic and C=O groups in the 2D crystal tilt from the surface. This study demonstrates that a combination of SFG and STM techniques can be used to gain a more complete picture of 2D crystal structure, and it is necessary to consider solvent-2D crystal interactions and dynamics in the computer models to achieve an accurate representation of interfacial structure.