Shang Xu

Approaching the ideal elastic strain limit in silicon nanowires

Single-crystalline silicon nanowires can be reversibly stretched above 10% elastic strain at room temperature. Achieving high elasticity for silicon (Si) nanowires, one of the most important and versatile building blocks in nanoelectronics, would enable their application in flexible electronics and bio-nano interfaces. We show that vapor-liquid-solid–grown single-crystalline Si nanowires with diameters of ~100 nm can be repeatedly stretched above 10% elastic strain at room temperature, approaching the theoretical elastic limit of silicon (17 to 20%). A few samples even reached ~16% tensile strain, with estimated fracture stress up to ~20 GPa. The deformations were fully reversible and hysteresis-free under loading-unloading tests with varied strain rates, and the failures still occurred in brittle fracture, with no visible sign of plasticity. The ability to achieve this “deep ultra-strength” for Si nanowires can be attributed mainly to their pristine, defect-scarce, nanosized single-crystalline structure and atomically smooth surfaces. This result indicates that semiconductor nanowires could have ultra-large elasticity with tunable band structures for promising “elastic strain engineering” applications.

Flexible Fiber-Shaped Supercapacitor Based on Nickel-Cobalt Double Hydroxide and Pen Ink Electrodes on Metallized Carbon Fiber

Flexible fiber-shaped supercapacitors (FSSCs) are recently of extensive interest for portable and wearable electronic gadgets. Yet the lack of industrial-scale flexible fibers with high conductivity and capacitance and low cost greatly limits its practical engineering applications. To this end, we here present pristine twisted carbon fibers (CFs) coated with a thin metallic layer via electroless deposition route, which exhibits exceptional conductivity with ∼300% enhancement and superior mechanical strength (∼1.8 GPa). Subsequently, the commercially available conductive pen ink modified high conductive composite fibers, on which uniformly covered ultrathin nickel-cobalt double hydroxides (Ni-Co DHs) were introduced to fabricate flexible FSSCs. The synthesized functionalized hierarchical flexible fibers exhibit high specific capacitance up to 1.39 F·cm-2 in KOH aqueous electrolyte. The asymmetric solid-state FSSCs show maximum specific capacitance of 28.67 mF·cm-2 and energy density of 9.57 μWh·cm-2 at corresponding power density as high as 492.17 μW·cm-2 in PVA/KOH gel electrolyte, with demonstrated high flexibility during stretching, demonstrating their potential in flexible electronic devices and wearable energy systems.

Facile fabrication of a novel nanoporous Au/AgO composite for electrochemical double-layer capacitor

In this work, a novel nanoporous gold/silver oxide (NP Au/AgO) composite (NPAAC) was fabricated by a simple two-step partial dealloying–stripping process. Microstructural and electrochemical characterizations suggested that the NPAAC has a bicontinuous, interpenetrating nanoporous structure with ultrafine ligament–channel sizes down to ∼10 nm, and it delivered a rectangle cyclic voltammetry (CV) curve with a wide electrochemical double-layer region from −0.3 to 0.7 V (vs. Ag/AgCl in sulfuric acid) and a specific capacitance of ∼80 F g−1. Moreover, the NPAAC revealed a superior cyclic performance as compared to metal-based pseudocapacitors, with stable charge–discharge cycling at ∼77 F g−1 for more than ten thousand times with no detectable degradation. This work demonstrates that the new systems of nanoporous noble metal/metal-oxide composites could be similarly fabricated by this approach and developed as potential double-layer capacitors with comparable cyclic performance to carbon-based supercapacitors.

In situ atomic-scale analysis of Rayleigh instability in ultrathin gold nanowires

Comprehensive understanding of the structural/morphology stability of ultrathin (diameter < 10 nm) gold nanowires under real service conditions (such as under Joule heating) is a prerequisite for the reliable implementation of these emerging building blocks into functional nanoelectronics and mechatronics systems. Here, by using the in situ transmission electron microscopy (TEM) technique, we discovered that the Rayleigh instability phenomenon exists in ultrathin gold nanowires upon moderate heating. Through the controlled electron beam irradiation-induced heating mechanism (with < 100 °C temperature rise), we further quantified the effect of electron beam intensity and its dependence on Rayleigh instability in altering the geometry and morphology of the ultrathin gold nanowires. Moreover, in situ high-resolution TEM (HRTEM) observations revealed surface atomic diffusion process to be the dominating mechanism for the morphology evolution processes. Our results, with unprecedented details on the atomic-scale picture of Rayleigh instability and its underlying physics, provide critical insights on the thermal/structural stability of gold nanostructures down to a sub-10 nm level, which may pave the way for their interconnect applications in future ultralarge- scale integrated circuits.

Controllable high-throughput fabrication of porous gold nanorods driven by Rayleigh instability

Nanoporous gold (NPG) nanorods have recently attracted tremendous interest and research effort due to their vast applications in biomedical engineering, catalysis and photonics areas. However, the rational fabrication of large volumes of low-aspect-ratio NPG nanoparticles with well-defined geometries remains a difficult challenge. Here we demonstrate a controllable fabrication of porous gold nanorods by a novel template-assisted electrodeposition and in situ fragmentation of Au–Ag alloy nanowires, followed by a dealloying process to convert the resulting Au–Ag nanorods into desired NPG nanorods. The thermal-induced fragmentation process was believed to be associated with Rayleigh instability of the alloy nanowires confined within anodic aluminum oxide (AAO) templates, which has been further confirmed by in situ TEM experiments for geometrically confined gold nanowires upon Joule heating. More importantly, the status of the nanowire-breakdown process can be monitored in situ by macroscopic current–voltage (I–V) measurements of the over-grown nanowires–AAO sandwich structure. Together with a one-step dealloying finishing process, our method could facilitate the mass production of high quality NPG nanorods with well-controlled diameters, which could open up many opportunities for low-cost, high-throughput fabrication of low-aspect-ratio porous metallic nanorods for biomedical (such as drug delivery) and other applications.

Mechanically Assisted Self‐Healing of Ultrathin Gold Nanowires

As the critical feature sizes of integrated circuits approaching sub-10 nm, ultrathin gold nanowires (diameter <10 nm) have emerged as one of the most promising candidates for next-generation interconnects in nanoelectronics. Also due to their ultrasmall dimensions, however, the structures and morphologies of ultrathin gold nanowires are more prone to be damaged during practical services, for example, Rayleigh instability can significantly alter their morphologies upon Joule heating, hindering their applications as interconnects. Here, it is shown that upon mechanical perturbations, predamaged, nonuniform ultrathin gold nanowires can quickly recover into uniform diameters and restore their smooth surfaces, via a simple mechanically assisted self-healing process. By examining the local self-healing process through in situ high-resolution transmission electron microscopy, the underlying mechanism is believed to be associated with surface atomic diffusion as evidenced by molecular dynamics simulations. In addition, mechanical manipulation can assist the atoms to overcome the diffusion barriers, as suggested by ab initio calculations, to activate more surface adatoms to diffuse and consequently speed up the self-healing process. This result can provide a facile method to repair ultrathin metallic nanowires directly in functional devices, and quickly restore their microstructures and morphologies by simple global mechanical perturbations.