Nano-Si for On-Demand H2 Production: Optimization of Yield and Real-Time Visualization of Si─H2 O Reaction Using Liquid-Phase Transmission Electron Microscopy.

Hydrogen (H2 ), the most abundant element in the universe, has the potential to address the challenges of energy security and climate change. However, due to the lack of a safe and efficient method for storing and delivering hydrogen, its practical application is still in its infancy stages. To overcome this challenge, a promising solution is demonstrated in the form of on-demand production of H2 using nano-Silicon (Si) powders. The method offers instantaneous production of H2 , yielding a volume of 1.3 L per gram of Si at room temperature. Moreover, the H2 production yield and the rate can be effectively controlled by adjusting the reaction pH value and temperatures. Additionally, liquid-phase transmission electron microscopy (LPTEM) is utilized in situ to demonstrate the entire reaction in real-time, wherein H2 bubble formation is observed and illustrated the gradual conversion of crystalline Si particles into amorphous oxides. Moreover, it is confirmed that the purity of the generated gas is 99.5% using gas chromatography mass spectrometry (GC-MS). These findings suggest a viable option for instant H2 production in portable fuel cells using Si cartridges or pellets.

Sputter-Deposited High Entropy Alloy Thin Film Electrocatalyst for Enhanced Oxygen Evolution Reaction Performance.

Thin film catalysts, giving a different morphology, provide a significant advantage over catalyst particles for the gas evolution reaction. Taking the advantages of sputter deposition, a high entropy alloy (HEA) thin film electrocatalyst is hereby reported for the oxygen evolution reaction (OER). The catalyst characteristics are investigated not only in its as-deposited state, but also during and after the OER. For comparison, unary, binary, ternary, and quaternary thin film catalysts are prepared and characterized. The surface electronic structure modification due to the addition of a metal is studied experimentally and theoretically using density functional theory calculation. It is demonstrated that sputtered FeNiMoCrAl HEA thin film exhibits OER performance superior to all the reported HEA catalysts with robust electrocatalytic activity having a low overpotential of 220 mV at 10 mA cm-2 , and excellent electrochemical stability at different constant current densities of 10 and 100 mA cm-2 for 50 h. Furthermore, the microstructure transformation is investigated during the OER, which is important for the understanding of the OER mechanism provided by HEA electrocatalyst. Such a finding will contribute to future catalyst design.

Erratum: Chiral Spin-Wave Velocities Induced by All-Garnet Interfacial Dzyaloshinskii-Moriya Interaction in Ultrathin Yttrium Iron Garnet Films [Phys. Rev. Lett. 124, 027203 (2020)].

This corrects the article DOI: 10.1103/PhysRevLett.124.027203.

Reconfigurable Spin-Wave Interferometer at the Nanoscale.

Spin waves can transfer information free of electron transport and are promising for wave-based computing technologies with low-power consumption as a solution to severe energy losses in modern electronics. Logic circuits based on the spin-wave interference have been proposed for more than a decade, while it has yet been realized at the nanoscale. Here, we demonstrate the interference of spin waves with wavelengths down to 50 nm in a low-damping magnetic insulator. The constructive and destructive interference of spin waves is detected in the frequency domain using propagating spin-wave spectroscopy, which is further confirmed by the Brillouin light scattering. The interference pattern is found to be highly sensitive to the distance between two magnetic nanowires acting as spin-wave emitters. By controlling the magnetic configurations, one can switch the spin-wave interferometer on and off. Our demonstrations are thus key to the realization of spin-wave computing system based on nonvolatile nanomagnets.

Changes of Magnetism in a Magnetic Insulator due to Proximity to a Topological Insulator.

We report the modification of magnetism in a magnetic insulator Y_{3}Fe_{5}O_{12} thin film by topological surface states (TSS) in an adjacent topological insulator Bi_{2}Se_{3} thin film. Ferromagnetic resonance measurements show that the TSS in Bi_{2}Se_{3} produces a perpendicular magnetic anisotropy, results in a decrease in the gyromagnetic ratio, and enhances the damping in Y_{3}Fe_{5}O_{12}. Such TSS-induced changes become more pronounced as the temperature decreases from 300 to 50 K. These results suggest a completely new approach for control of magnetism in magnetic thin films.

Chiral Spin-Wave Velocities Induced by All-Garnet Interfacial Dzyaloshinskii-Moriya Interaction in Ultrathin Yttrium Iron Garnet Films.

Spin waves can probe the Dzyaloshinskii-Moriya interaction (DMI), which gives rise to topological spin textures, such as skyrmions. However, the DMI has not yet been reported in yttrium iron garnet (YIG) with arguably the lowest damping for spin waves. In this work, we experimentally evidence the interfacial DMI in a 7-nm-thick YIG film by measuring the nonreciprocal spin-wave propagation in terms of frequency, amplitude, and most importantly group velocities using all electrical spin-wave spectroscopy. The velocities of propagating spin waves show chirality among three vectors, i.e., the film normal direction, applied field, and spin-wave wave vector. By measuring the asymmetric group velocities, we extract a DMI constant of 16 µJ/m^{2}, which we independently confirm by Brillouin light scattering. Thickness-dependent measurements reveal that the DMI originates from the oxide interface between the YIG and garnet substrate. The interfacial DMI discovered in the ultrathin YIG films is of key importance for functional chiral magnonics as ultralow spin-wave damping can be achieved.

Strong Interlayer Magnon-Magnon Coupling in Magnetic Metal-Insulator Hybrid Nanostructures.

We observe strong interlayer magnon-magnon coupling in an on-chip nanomagnonic device at room temperature. Ferromagnetic nanowire arrays are integrated on a 20-nm-thick yttrium iron garnet (YIG) thin film strip. Large anticrossing gaps up to 1.58 GHz are observed between the ferromagnetic resonance of the nanowires and the in-plane standing spin waves of the YIG film. Control experiments and simulations reveal that both the interlayer exchange coupling and the dynamical dipolar coupling contribute to the observed anticrossings. The coupling strength is tunable by the magnetic configuration, allowing the coherent control of magnonic devices.

Effects of free carriers on piezoelectric nanogenerators and piezotronic devices made of GaN nanowire arrays.

This study investigates the role of carrier concentration in semiconducting piezoelectric single-nanowire nanogenerators (SNWNGs) and piezotronic devices. Unintentionally doped and Si-doped GaN nanowire arrays with various carrier concentrations, ranging from 10(17) (unintentionally doped) to 10(19) cm(-3) (heavily doped), are synthesized. For SNWNGs, the output current of individual nanowires starts from a negligible level and rises to the maximum of ≈50 nA at a doping concentration of 5.63 × 10(18) cm(-3) and then falls off with further increase in carrier concentration, due to the competition between the reduction of inner resistance and the screening effect on piezoelectric potential. For piezotronic applications, the force sensitivity based on the change of the Schottky barrier height works best for unintentionally doped nanowires, reaching 26.20 ± 1.82 meV nN(-1) and then decreasing with carrier concentration. Although both types of devices share the same Schottky diode, they involve different characteristics in that the slope of the current-voltage characteristics governs SNWNG devices, while the turn-on voltage determines piezotronic devices. It is demonstrated that free carriers in piezotronic materials can influence the slope and turn-on voltage of the diode characteristics concurrently when subjected to strain. This work offers a design guideline for the optimum doping concentration in semiconductors for obtaining the best performance in piezotronic devices and SNWNGs.

Crystal face-dependent nanopiezotronics of an obliquely aligned InN nanorod array.

This paper proposes an obliquely aligned InN nanorod array to maximize nanorod deformation in the application of nanopiezotronics. The surface-dependent piezotronic I-V characteristics of the InN nanorod array with exposed polar (0002) and semipolar ( ̅1102) planes were studied by conductive atomic force microscopy. The effects of the piezopotential, created in the InN under straining, and the surface quantum states on the transport behavior of charge carriers in different crystal planes of the InN nanorod were investigated. The crystal plane-dependent electron density in the electron surface accumulation layer and the strain-dependent piezopotential distribution modulate the interfacial contact of the Schottky characteristics for the (0002) plane and the quasi-ohmic behavior for the ( ̅1102) plane. Regarding the piezotronic properties under applied forces, the Schottky barrier height increases in conjunction with the deflection force with high current density at large biases because of tunneling. The strain-induced piezopotential can thus tune the transport process of the charge carriers inside the InN nanorod over a larger range than in ZnO. The quantized surface electron accumulation layer is demonstrated to modulate the piezopotential-dependent carrier transport at the metal/InN interfaces and become an important factor in the design of InN-based piezotronic devices and nanogenerators.

Ultra-Low-Power and Wide-Operating-Voltage-Window Capacitive Piezotronic Sensor through Coupling of Piezocharges and Depletion Widths for Tactile Sensing.

The rapid growth of Artificial Intelligence and Internet of Things (AIoT) demands the development of ultra-low-power devices for future advanced technology. In this study, we introduce a capacitive piezotronic sensor specifically designed for tactile sensing, which enables an ultra-low-voltage operation at nearly 0 reading bias conditions with a consistent response within a wide voltage range. This sensor directly detects capacitance changes induced by piezocharges, reflecting perturbation of the effective depletion width, and ensures ultralow power capability by eliminating the necessity of turning on the Schottky diode for the first time. The dynamic response of the sensor demonstrates ultralow power capability and immunity to triboelectric interference, making it particularly suitable for tactile sensing applications in robotics, prosthetics, and wearables. This study provides valuable insights and design guidelines for future ultra-low-power thin-film-based capacitive piezotronic/piezophototronic devices for tactile sensing.

Scalable Interlayer Nanostructure Design for High-Rate (10C) Submicron Silicon-Film Electrode by Incorporating Silver Nanoparticles.

High C-rate capability at 10C is a key performance indicator for the commercialization of the next-generation high-charging lithium microbattery. However, silicon (Si) anode satisfying the prerequisite high specific capacity suffers from poor electron/ionic conductivity, seriously limiting the 10C rate capability. Accordingly, we propose the strategy of inserting highly conductive silver nanoparticles (AgNPs) as an interlayer between two RF-sputtered amorphous Si thin films to form an Si/Ag/Si multilayered anode, with the density and spatial distribution of the AgNPs well-controlled by thermal evaporation. This strategy is exclusively beneficial to scale up film thickness for higher capacity. Without AgNPs, the 10C rate performance of the double-layer Si (D_Si) is worse than the single layer (S_Si) in the same total thickness, suggesting the adverse effect of the interface. However, this situation is progressively improved with the AgNPs density incorporated at the interface, where the densest AgNPs anode (D_SiAg3) demonstrated a noticeable improvement reaching 1250 mAh/g at 10 C with a 46% capacity retention rate. By scaling up to triple layers, T_SiAg3 performed the superior 10C rate capability to T_Si, testifying to the scalable potential of the unique design for boosting high-power batteries. Finally, with electrochemical impedance spectroscopy results, a possible mechanism to explain the enhancement in rate capability is subject to where Li-ion diffusion is accelerated by the charge-induced electric field condensing around the AgNPs. This design for a multilayered nanocomposite can contribute to the design and fabrication of high-charging batteries and battery-on-chip.

Giant optical anisotropy of oblique-aligned ZnO nanowire arrays.

A combined method of modified oblique-angle deposition and hydrothermal growth was adopted to grow an optically anisotropic nanomaterial based on single crystalline ZnO nanowire arrays (NWAs) with highly oblique angles (75°-85°), exhibiting giant in-plane birefringence and optical polarization degree in emission. The in-plane birefringence of oblique-aligned ZnO NWAs is almost one order of magnitude higher than that of natural quartz. The strong optical anisotropy in emission due to the optical confinement was observed. The oblique-aligned NWAs not only allow important technological applications in passive photonic components but also benefit the development of the optoelectronic devices in polarized light sensing and emission.

Noninvasive measurements of spin transport properties of an antiferromagnetic insulator.

Antiferromagnetic insulators (AFIs) are of substantial interest because of their potential in the development of next-generation spintronic devices. One major effort in this emerging field is to harness AFIs for long-range spin information communication and storage. Here, we report a noninvasive method to optically access the intrinsic spin transport properties of an archetypical AFI α-Fe2O3 via nitrogen-vacancy (NV) quantum spin sensors. By NV relaxometry measurements, we successfully detect the frequency-dependent dynamic fluctuations of the spin density of α-Fe2O3 along the Néel order parameter, from which an intrinsic spin diffusion constant of α-Fe2O3 is experimentally measured in the absence of external spin biases. Our results highlight the significant opportunity offered by NV centers in diagnosing the underlying spin transport properties in a broad range of high-frequency magnetic materials such as two-dimensional magnets, spin liquids, and magnetic Weyl semimetals, which are challenging to access by the conventional measurement techniques.

Large magnetoelectric resistance in the topological Dirac semimetal α-Sn.

The spin-momentum locking of surface states in topological materials can produce a resistance that scales linearly with magnetic and electric fields. Such a bilinear magnetoelectric resistance (BMER) effect offers a new approach for information reading and field sensing applications, but the effects demonstrated so far are too weak or for low temperatures. This article reports the first observation of BMER effects in topological Dirac semimetals; the BMER responses were measured at room temperature and were substantially stronger than those reported previously. The experiments used topological Dirac semimetal α-Sn thin films grown on silicon substrates. The films showed BMER responses that are 106 times larger than previously measured at room temperature and are also larger than those previously obtained at low temperatures. These results represent a major advance toward realistic BMER applications. Significantly, the data also yield the first characterization of three-dimensional Fermi-level spin texture of topological surface states in α-Sn.

Plasmonic coupling of silver nanoparticles covered by hydrogen-terminated graphene for surface-enhanced Raman spectroscopy.

We report on strong plasmonic coupling from silver nanoparticles covered by hydrogen-terminated chemically vapor deposited single-layer graphene, and its effects on the detection and identification of adenine molecules through surface-enhanced Raman spectroscopy (SERS). The high resistivity of the graphene after subjecting to remote plasma hydrogenation allows plasmonic coupling induced strong local electromagnetic fields among the silver nanoparticles to penetrate the graphene, and thus enhances the SERS efficiency of adenine molecules adsorbed on the film. The graphene layer protects the nanoparticles from reactive and harsh environments and provides a chemically inert and biocompatible carbon surface for SERS applications.

Growth and characteristics of self-assembly defect-free GaN surface islands by molecular beam epitaxy.

GaN surface nano-islands of high crystal quality, without any dislocations or other extended defects, are grown on a c-plane sapphire substrate by plasma-assisted molecular beam epitaxy. Nano-island growth requires special conditions in terms of V/III ratio and substrate temperature, distinct from either film or nanocolumn growth. The insertion of a nitrided Ga layer can effectively improve the uniformity of the nano-islands in both shape and size. The islands are well faced truncated pyramids with island size ranged from 30 to 110 nm, and height ranged from 30 to 55 nm. On, the other hand, the density and facet of the GaN surface islands would be affected by the growth conditions. An increase of the V/III ratio from 30 to 40 led to an increase in density from 1.4 x 10(9) to 4.3 x 10(9) cm(-2) and an evolution from {1-21-1} facets to {1-21-2} facets. The GaN layers containing the surface islands can moderate the compressive strain due to the lattice and thermal mismatch between GaN and c-sapphire. Conductive atomic force microscopy shows that the off-axis sidewall facets are more electrically active than those at the island center. The formation of the GaN surface islands is strongly induced by the Ehrlich-Schwoebel barrier effect of preexisting islands grown in the early growth stage. GaN surface islands are ideal templates for growing nano-devices.

Valence excitations and dopant distribution of Al doped ZnO nanowires analyzed by electron energy loss spectroscopy.

Valence electron energy loss spectroscopy (VEELS) with scanning transmission electron microscopy (STEM) has been employed to probe the valence excitations and dopant distribution of Al doped ZnO nanowires. The results reveal that while the typical Al concentration is on the order of 1020 1/cm3, Al tends to segregate at the surface leading to an Al-rich sheath. In VEEL spectra, O-2p, Zn-3d, Al-3p, O-2s, interband transitions as well as bulk plasmon have been identified. The bulk plasmon peak is blue-shifted, and the projected interband transition decreases from 2.14 to 1.88 eV as the doping concentration increases from 0.83 x 10(20) to 2.18 x 10(20) 1/cm3.

Synergistic effects of Ga doping and Mg alloying over the enhancement of the stress sensitivity of a Ga-doped MgZnO pressure sensor.

We demonstrate the synergistic effects of Ga doping and Mg alloying into ZnO on the large enhancement of the piezopotential and stress sensing performance of piezotronic pressure sensors made of Ga-doped MgZnO films. Piezopotential-induced pressure sensitivity was enhanced through the modulation of the Schottky barrier height. Doping with Ga (0.62 Å) of larger ionic radius and alloying with Mg (0.57 Å) of smaller ionic radius than Zn ions can synergistically affect the overall structural, optical and piezoelectric properties of the resulting thin films. The crystal quality of Ga-doped MgZnO films either improved (X Ga ≦ 0.041) or deteriorated (X Ga ≧ 0.041) depending on the Ga doping concentration. The band gap increased from 3.90 eV for pristine MgZnO to 3.93 eV at X Ga = 0.076, and the piezoelectric coefficient (d 33) improved from ∼23.25 pm V-1 to ∼33.17 pm V-1 at an optimum Ga concentration (X Ga = 0.027) by ∼2.65 times. The change in the Schottky barrier height ΔΦ b increased from -4.41 meV (MgZnO) to -4.81 meV (X Ga = 0.027) and decreased to -3.99 meV at a high Ga doping concentration (X Ga = 0.041). The stress sensitivity (0.2 kgf) enhanced from 28.50 MPa-1 for the pristine MgZnO to 31.36 MPa-1 (X Ga = 0.027) and decreased to 25.56 MPa-1 at higher Ga doping concentrations, indicating the synergistic effects of Ga doping and Mg alloying over the pressure sensing performance of Ga-doped MgZnO films.

Switching of a Magnet by Spin-Orbit Torque from a Topological Dirac Semimetal.

Recent experiments show that topological surface states (TSS) in topological insulators (TI) can be exploited to manipulate magnetic ordering in ferromagnets. In principle, TSS should also exist for other topological materials, but it remains unexplored as to whether such states can also be utilized to manipulate ferromagnets. Herein, current-induced magnetization switching enabled by TSS in a non-TI topological material, namely, a topological Dirac semimetal α-Sn, is reported. The experiments use an α-Sn/Ag/CoFeB trilayer structure. The magnetization in the CoFeB layer can be switched by a charge current at room temperature, without an external magnetic field. The data show that the switching is driven by the TSS of the α-Sn layer, rather than spin-orbit coupling in the bulk of the α-Sn layer or current-produced heating. The switching efficiency is as high as in TI systems. This shows that the topological Dirac semimetal α-Sn is as promising as TI materials in terms of spintronic applications.

Enhanced spontaneous light emission by multiple surface plasmon coupling.

Photoluminescence of polyfluoren copolymers, a white-light material, was demonstrated to be enhanced selectively by coupling with either localized or propagating modes of surface plasmon resonance (SPR). The silver sub-micron cylinders with 75nm height fabricated by e-beam lithography followed by e-beam evaporation and lift-off process. The enhanced light emissions at 500nm and 533nm are attributed to the low frequency branch of localized SPR. Furthermore, a 50nm silver thin film between these cylinders and the substrate provides propagating surface plasmons under excitation and enhances the blue emission band of the polyfluoren copolymer at 438nm. This delocalized SPR is sufficient for effective plasmon to light conversion. Moreover, by effectively coupling the localized and propagating SPR, we can experimentally demonstrate that the photoluminescence of polyfluoren copolymers is enhanced by 4 to 5.4 times at different wavelengths compared to enhancement by either single mode.