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.

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.

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.

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.

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.

Tunable Work Function of Mg xZn1- xO as a Viable Friction Material for a Triboelectric Nanogenerator.

Since the invention of triboelectric nanogenerators (TENGs), their output performance has been improved through various approaches such as material surface modification, device structure optimization, and so on, but rarely through the development of new friction materials. In this work, a magnetron sputtered Mg xZn1- xO film is developed as a viable friction material that rubs against polydimethylsiloxane in a TENG. The work function, measured by Kelvin probe microscopy, of the Mg xZn1- xO films can be effectively tuned by varying Mg composition, x, and exposed surface facets, which are shown to dominate the charge-transfer behavior. In addition, film thickness also plays an important role, affecting the output performance. The output voltage and total charge of a TENG with a Mg xZn1- xO film are demonstrated to be tremendously enhanced by 55 and 90 times, respectively, compared to that of a TENG with a ZnO film. Even more intriguingly, the tribo-output polarity can be reversed by adjusting the relative work function through varying the preferred growth orientation of the Mg xZn1- xO film, for a given value of Mg content.

Freestanding Three-Dimensional CuO/NiO Core-Shell Nanowire Arrays as High-Performance Lithium-Ion Battery Anode.

We demonstrate significant improvement of CuO nanowire arrays as anode materials for lithium ion batteries by coating with thin NiO nanosheets conformally. The NiO nanosheets were designed two kinds of morphologies, which are porous and non-porous. By the NiO nanosheets coating, the major active CuO nanowires were protected from direct contact with the electrolyte to improve the surface chemical stability. Simultaneously, through the observation and comparison of TEM results of crystalline non-porous NiO nanosheets, before and after lithiation process, we clearly prove the effect of expected protection of CuO, and clarify the differences of phase transition, crystallinity change, ionic conduction and the mechanisms of the capacity decay further. Subsequently, the electrochemical performances exhibit lithiation and delithiation differences of the porous and non-porous NiO nanosheets, and confirm that the presence of the non-porous NiO coating can still effectively assist the diffusion of Li+ ions into the CuO nanowires, maintaining the advantage of high surface area, and improves the cycle performance of CuO nanowires, leading to enhanced battery capacity. Optimally, the best structure is validated to be non-porous NiO nanosheets, in contrary to the anticipated porous NiO nanosheets. In addition, considering the low cost and facile fabrication process can be realized further for practical applications.

Author Correction: High-quality AlN grown with a single substrate temperature below 1200 °C.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Optimal geometrical design of inertial vibration DC piezoelectric nanogenerators based on obliquely aligned InN nanowire arrays.

Piezoelectric nanogenerators have been investigated to generate electricity from environmental vibrations due to their energy conversion capabilities. In this study, we demonstrate an optimal geometrical design of inertial vibration direct-current piezoelectric nanogenerators based on obliquely aligned InN nanowire (NW) arrays with an optimized oblique angle of ∼58°, and driven by the inertial force of their own weight, using a mechanical shaker without any AC/DC converters. The nanogenerator device manifests potential applications not only as a unique energy harvesting device capable of scavenging energy from weak mechanical vibrations, but also as a sensitive strain sensor. The maximum output power density of the nanogenerator is estimated to be 2.9 nW cm-2, leading to an improvement of about 3-12 times that of vertically aligned ZnO NW DC nanogenerators. Integration of two nanogenerators also exhibits a linear increase in the output power, offering an enormous potential for the creation of self-powered sustainable nanosystems utilizing incessantly natural ambient energy sources.

High-quality AlN grown with a single substrate temperature below 1200 °C.

1.5-µm AlN grown by metal-organic chemical vapor deposition (MOCVD), with a single substrate temperature of 1180 °C, exhibits atomically flat surface and the XRD (102) peak width of 427 arcsec. The results are achieved with a pulsed NH3-flow condition, serving as an alternative for the commonly used temperature-varied buffer structure, which is often complicated and time-consuming. Inserting two pulsed-NH3-flow AlN layers in the epitaxial structure not only releases the lattice strain via the formation of three-dimensional nano-islands, but also smoothens the surface with prolonged lateral migration of Al adatoms. This effective growth technique substantially simplifies the manufacture of device-quality AlN.

Ultrathin (Bi1-xSbx)2Se3 Field Effect Transistor with Large ON/OFF Ratio.

Ultrathin three-dimensional topological insulator films are promising for use in field effect devices. (Bi1-xSbx)2Se3 ultrathin films were fabricated on SrTiO3 substrate, where large resistance changes of ∼25 000% could be achieved using the back gate voltage. We suggest that the large ON/OFF ratio was caused by the combined effect of Sb-doping and the reduction of film thickness down to the ultrathin regime. The crossover of different quantum transport under an electric field may form the basis for topological insulators (TI)-based spin transistors with large ON/OFF ratios in the future.

Derivation of the surface free energy of ZnO and GaN using in situ electron beam hole drilling.

Surface free energy, as an intrinsic property, is essential in determining the morphology of materials, but it is extremely difficult to determine experimentally. We report on the derivation of the SE of different facets of ZnO and GaN experimentally from the holes developed using electron beam drilling with transmission electron microscopy. Inverse Wullf's construction is employed to obtain polar maps of the SE of different facets to study different nanomaterials (ZnO and GaN) in different morphologies (nanorod, nanobelt and thin film) to prove its versatility and capability. The results show that the SE of ZnO{10-13} is derived to be 0.99 J m(-2), and the SE of ZnO{10-10} is found to be less than {0002} and {11-20}. A GaN thin film also exhibits a similar trend in the SE of different facets as ZnO and the SE of GaN{10-13} is determined to be 1.36 J m(-2).

Ultrasensitive Thin-Film-Based Alx Ga1-x N Piezotronic Strain Sensors via Alloying-Enhanced Piezoelectric Potential.

Alx Ga1-x N thin-film-based piezotronic strain sensors with ultrahigh strain sensitivity are fabricated through alloying of AlN with GaN. The strain sensitivity of the ternary compound Alx Ga1-x N is higher than those of the individual binary compounds GaN and AlN. Such a high performance can be attributed to the piezoelectric constant enhancement via intercalation of Al atoms into the GaN matrix, the effect of residual strain, and a suppressed screening effect.

Crystal Orientation Dynamics of Collective Zn dots before Preferential Nucleation.

The island nucleation in the context of heterogeneous thin film growth is often complicated by the growth kinetics involved in the subsequent thermodynamics. We show how the evolution of sputtered Zn island nucleation on Si(111) by magnetron sputtering in a large area can be completely understood as a model system by combining reflective second harmonic generation (RSHG), a 2D pole figure with synchrotron X-ray diffraction. Zn dots are then oxidized on the surfaces when exposed to the atmosphere as Zn/ZnO dots. Derived from the RSHG patterns of Zn dots at different growth times, the Zn dots grow following a unique transition from kinetic to thermodynamic control. Under kinetic-favoring growth, tiny Zn dots prefer arranging themselves with a tilted c-axis to the Si(111) substrate toward any of the sixfold in-plane Si<110> directions. Upon growth, the Zn dots subsequently evolve themselves to a metastable state with a smaller tilting angle toward selective <110> directions. As the Zn dots grow over a critical size, they become most thermodynamically stable with the c-axis vertical to the Si(111) substrate. For a system with large lattice mismatch, small volume dots take kinetic pathways with insignificant deviations in energy barriers.

Bottom-Up Nano-heteroepitaxy of Wafer-Scale Semipolar GaN on (001) Si.

Semipolar {101¯1} InGaN quantum wells are grown on (001) Si substrates with an Al-free buffer and wafer-scale uniformity. The novel structure is achieved by a bottom-up nano-heteroepitaxy employing self-organized ZnO nanorods as the strain-relieving layer. This ZnO nanostructure unlocks the problems encountered by the conventional AlN-based buffer, which grows slowly and contaminates the growth chamber.

Synthesis and characterization of bis (acetylacetonato κ-O, O') [zinc(ii)/copper(ii)] hybrid organic-inorganic complexes as solid metal organic precursors.

We have synthesized novel metal organic hybrid mixed compounds of bis (acetylacetonato κ-O, O') [zinc(ii)/copper(ii)]. Taking C10H14O4Zn0.7Cu0.3 (Z0.7C0.3AA) as an example, the crystals are composed of Z0.7C0.3AA units and uncoordinated water molecules. Single-crystal X-ray diffraction results show that the complex Z0.7C0.3AA crystallizes in the monoclinic system, space group P21/n. The unit cell dimensions are a = 10.329(4) Å, b = 4.6947(18) Å, and c = 11.369(4) Å; the angles are α = 90°, ß = 91.881(6)°, and γ = 90°, the volume is 551.0(4) Å(3), and Z = 2. In this process, the M(ii) ions of Zn and Cu mix and occupy the centers of symmetrical structural units, which are coordinated to two ligands. The measured bond lengths and angles of O-M-O vary with the ratio of metal species over the entire series of the complexes synthesized. The chemistry of the as-synthesized compounds has been characterized using infrared spectroscopy, mass spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy analysis, and the morphology of the products has been characterized using scanning electron microscopy. The thermal decomposition of the Z0.7C0.3AA composites measured by thermogravimetric analysis suggests that these complexes are volatile. The thermal characteristics of these complexes make them attractive precursors for metal organic chemical vapor deposition.

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.