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.

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.

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.

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.

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.

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.

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.

Ga+ focused-ion-beam implantation-induced masking for H2 etching of ZnO films.

Gallium implantation of ZnO by a focused-ion beam is used to create a mask for ZnO dry etching with hydrogen. Effects of Ga(+) fluence on the etch stop properties and the associated mechanisms are investigated. The fluence of 2.8 × 10(16) cm(-2) is determined to be optimum to render the best mask quality. While lower fluences would cause less etching selectivity, higher fluences would cause erosion of the surface and particles to be precipitated on the surface after H(2) treatment at high temperature. In contrast to the commonly adopted gallium oxide formation on Si, transmission electron microscopy analysis reveals that, for the fluences ≤ 2.8 × 10(16) cm(-2), Ga(+) ions are incorporated as dopants into ZnO without any second phases or precipitates, indicating the Ga-doped ZnO layer behaves as a mask for H(2) etching due to the higher electronegativity of Ga(+) towards oxygen. However, for the fluences ≥ 4.6 × 10(16) cm(-2), the surface particles are responsible for the etch stop and are identified as ZnGa(2)O(4). We finally demonstrate a complicated pattern of 'NCKU' on ZnO by using this technique. The study not only helps clarify the related mechanisms, but also suggests a feasible extension of the etch stop process that can be applied to more functional material.

Growth of Ga-doped ZnS nanowires constructed by self-assembled hexagonal platelets with excellent photocatalytic properties.

A new process for making single crystalline undoped and Ga-doped ZnS nanowires with simple evaporation and condensation procedures on Si and GaN is introduced. The process does not need additional catalysts or precursors. The growth mechanism is studied using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. TEM images show that the undoped ZnS nanowires exhibit an ordinary straight morphology, whereas the Ga-doped nanowires are composed of aligned hexagonal platelets, connected in the center into nanowires to maximize surface area. The Ga 2p3 and S 2p peaks in the XPS results confirm the presence of Ga doping in the form of Ga-S bonding. Raman spectra show that the ZnS LO peak is red-shifted from 349 to 347 cm(-1), indicative of a tensile stress caused by the Ga dopants. The growth mechanism and photocatalytic activity of the Ga-doped ZnS nanowires are discussed. We also demonstrate the excellent photocatalytic activity of Ga-doped ZnS nanowires as compared to those of undoped ZnS nanowires and Ga-doped ZnS nanosheets.

Microstructural effects on the magnetic and magneto-transport properties of electrodeposited Ni nanowire arrays.

The magnetic and magneto-transport properties of Ni nanowire (NW) arrays, fabricated by electrodeposition in anodic-aluminum-oxide (AAO) templates, have been investigated. The AAO pores have diameters ranging from 35 to 75 nm, and the crystallinity of the Ni NW arrays could change from poly-crystalline to single-crystalline with the [111] and [110] orientations based on the electrodeposition potential. Notably, double switching magnetization loops and double-peaked magnetoresistance curves were observed in [110]-oriented NWs. The crystalline orientation of the Ni NW arrays is found to influence the corresponding magnetic and magneto-transport properties significantly. These magnetic behaviors are dominated by the competition between the magneto-crystalline and shape anisotropy.

Microstructure and formation of copper oxide in the Cu electro-polishing process.

In this study, the formation of Cu oxide on Cu film is studied during Cu electropolishing in a phosphoric acid-based electrolyte with various Cu ion concentrations, from 2.28% to 10.08%. In cyclic voltammetry measurement, the maximum current density of the anodic peak (Imax) decreases from 38.87 to 28.13 mA/cm2 with increasing Cu ion concentration, indicating that an oxide film forms on the Cu film surface and the thickness increases with Cu ion concentration. Microstructures and crystallography of the oxide film are examined by transmission electron microscopy, which confirms the increase of the oxide film thickness due to the high Cu ion concentration in a H3PO4 electrolyte. Three types of Cu oxide are detected using X-ray photoelectron spectroscopy, namely Cu2O, Cu(OH)2, and CuO. With a Cu-ion electrolyte concentration of less than 6.99%, Cu(OH)2 is dominant, while at higher Cu-ion concentrations, CuO predominates. The formation of CuO protects Cu from corrosion in the electrolyte with the Cu-ion concentration of over 6.99%.

Enhanced density control of Al:ZnO nanowires via one-by-one coupling of nanowires and pyramids.

Al doped ZnO nanowire arrays with controlled growth densities were fabricated on silicon without using catalysts via sputtering followed by thermal chemical vapor deposition (CVD). Scanning electron microscopy and high-resolution transmission electron microscopy results show that the Al:ZnO single-crystalline nanowires synthesized by CVD prefer growing epitaxially on the tips of the ZnO pyramids pre-synthesized by sputtering with the c-axis perpendicular to the substrate. Consequently, the densities of the as-grown Al:ZnO nanowires were controllable by changing the particle densities of the pre-grown ZnO seed layers. The Al concentration of the Al:ZnO nanowires were measured to be around 2.63 at.% by electron energy loss spectrum. Field-emission measurements show the turn-on fields of the Al:ZnO nanowire arrays with controllable area densities are tunable. Room-temperature cathodoluminescence spectra of the Al:ZnO nanowires show relatively strong and sharp ultraviolet emissions centered at 383 nm and broad green emissions at around 497 nm. This work provides a simple method to control the field emission and luminescence densities of Al doped ZnO nanowire arrays, which also shows good potential for developing nano-pixel optical devices.

Graphene/ZnO Nanowire/p-GaN Vertical Junction for a High-Performance Nanoscale Light Source.

We report on a high-brightness ultraviolet (UV) nanoscale light source. The light emission diodes are constructed with graphene/ZnO nanowire/p-GaN vertical junctions, which exhibit strong UV electroluminescence (EL) emissions centered at a wavelength of 397 nm at one end of the ZnO nanowire. Compared to the horizontal heterojunction, the vertical junction based on the ZnO nanowire increases the interface area of the heterojunction along with a high-quality interface, thus making the device robust under a large excitation current. In this structure, transparent flexible graphene is used as the top electrode, which can effectively improve performance by increasing the carrier injection area. Moreover, by analyzing the relationship between the integrated light intensity and applied bias, a superlinear dependency with a slope of 3.99 is observed, which means high electrical-to-optical conversion efficiency. Three electron-hole irradiation recombination processes are distinguished according to the EL emission spectra.

Oxide mediated liquid-solid growth of high aspect ratio aligned gold silicide nanowires on Si(110) substrates.

Silicon nanowires grown using the vapor-liquid-solid method are promising candidates for nanoelectronics applications. The nanowires grow from an Au-Si catalyst during silicon chemical vapor deposition. In this paper, the effect of temperature, oxide at the interface and substrate orientation on the nucleation and growth kinetics during formation of nanogold silicide structures is explained using an oxide mediated liquid-solid growth mechanism. Using real time in situ high temperature transmission electron microscopy (with 40 ms time resolution), we show the formation of high aspect ratio ( approximately 15.0) aligned gold silicide nanorods in the presence of native oxide at the interface during in situ annealing of gold thin films on Si(110) substrates. Steps observed in the growth rate and real time electron diffraction show the existence of liquid Au-Si nano-alloy structures on the surface besides the un-reacted gold nanostructures. These results might enable us to engineer the growth of nanowires and similar structures with an Au-Si alloy as a catalyst.

The synthesis and electrical characterization of Cu2O/Al:ZnO radial p-n junction nanowire arrays.

Vertically aligned large-area p-Cu(2)O/n-AZO (Al-doped ZnO) radial heterojunction nanowire arrays were synthesized on silicon without using catalysts in thermal chemical vapor deposition followed by e-beam evaporation. Scanning electron microscopy and high-resolution transmission electron microscopy results show that poly-crystalline Cu(2)O nano-shells with thicknesses around 10 nm conformably formed on the entire periphery of pre-grown Al:ZnO single-crystalline nanowires. The Al doping concentration in the Al:ZnO nanowires with diameters around 50 nm were determined to be around 1.19 at.% by electron energy loss spectroscopy. Room-temperature photoluminescence spectra show that the broad green bands of pristine ZnO nanowires were eliminated by capping with Cu(2)O nano-shells. The current-voltage (I-V) measurements show that the p-Cu(2)O/n-AZO nanodiodes have well-defined current rectifying behavior. This paper provides a simple method to fabricate superior p-n radial nanowire arrays for developing nano-pixel optoelectronic devices and solar cells.