Hongti Zhang

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.

Experimental study of a chemical reaction between LiF and Al

The interaction between LiF and Al has been studied by x-ray photoemission spectroscopy and high-resolution electron energy loss spectroscopy (HREELS). At room temperature, reaction between LiF and Al already occurred in the presence of Alq3, as revealed by the 0.25 eV shift of the Li 1s peak relative to the F 1s peak. Upon heating the LiF-coated Al substrate above 70 °C, reaction between LiF and Al also occurred in the absence of Alq3, suggested by the emergence of a peak 0.85 eV below the original Li 1s peak. The relatively large shift of 0.85 eV indicated that the emergent Li state possessed a metallic character. The reaction between LiF and Al upon heating was also corroborated by HREELS measurements. Heating of the Al/LiF/polymer device enhanced its electroluminescence, and may be associated with free Li atoms.

In situ mechanical characterization of CoCrCuFeNi high-entropy alloy micro/nano-pillars for their size-dependent mechanical behavior

High entropy alloys (HEAs), as a new kind of alloys with equi- or near equi-atomic alloy compositions, have recently received increased interest, but their mechanical properties at micro- and nanoscales are less studied, which could hinder their structural/functional applications in the small scales. In this work, the mechanical responses of single crystalline FCC-structured CoCrCuFeNi HEA micro- and nano-pillars were systematically investigated by an in situ SEM nanoindenter. The yield strengths of the HEA micro-/nano-pillars under uniaxial compression appear to be size-dependent (with the m value of ~0.46 in the Hall-Petch law relationship), but less sensitive when compared to typical metal/alloy micro- and nano-structures (e.g. with the m values of 0.6–0.9 for FCC metals). We also observed and analyzed the slip systems of the plastically deformed micro-/nano-pillars, and discussed their deformation mechanisms together with the Youngs modulus by multiple loading/unloading compressions experiments. Our results could provide useful insights in the design and application of HEA for functional micro- and nano-devices.

Seed-assisted smart construction of high mass loading Ni-Co-Mn hydroxide nanoflakes for supercapacitor applications

Smartly designed nanoarchitectures with effective hybridization of transition metal oxides/hydroxides are promising to realize high performance electrodes for energy storage devices. To promote the applications of high-power supercapacitors, a seed-assisted method is firstly applied to prepare mesoporous Ni–Co–Mn hydroxide nanoflakes (NCMH) on nickel foam with practical mass loadings (higher than 5 mg cm−2). Further mechanism study reveals that the Ni(OH)2 nanorod arrays, which are firstly prepared by a hydrothermal process, serve as seeds for the successful deposition of NCMH nanoflakes. Through this convenient and cost effective method, this design results in a more orderly spatial distribution, lower intrinsic resistance and shorter electron transport pathways. The proof-of-concept application of NCMH as a binder-free supercapacitor electrode reveals an impressive specific capacity of 1043.1 μA h cm−2 at a high mass loading of 5.2 mg cm−2. The NCMH//activated carbon asymmetric device delivered a maximum energy density of 55.42 W h kg−1 at a power density of 750 W kg−1, exhibiting great potential as an energy storage device and shedding light on the structural design of nanomaterials.

Ultralarge elastic deformation of nanoscale diamond

Small, smooth, and bendable diamonds If you manage to deform a diamond, it usually means you have broken it. Diamonds have very high hardness, but they do not deform elastically. This limits their usefulness for some applications. However, Banerjee et al. discovered that diamond nanoneedles can deform elastically after all (see the Perspective by LLorca). The key was in their small size (300 nm), which allowed for very smooth-surfaced, defect-free diamonds. The deformation was close to the theoretical limit for diamond, which opens up the potential for applications in microelectronics and drug delivery. Science, this issue p. 300; see also p. 264 Diamond nanoneedles have smooth surfaces and are defect-free, allowing them to deform elastically. Diamonds have substantial hardness and durability, but attempting to deform diamonds usually results in brittle fracture. We demonstrate ultralarge, fully reversible elastic deformation of nanoscale (~300 nanometers) single-crystalline and polycrystalline diamond needles. For single-crystalline diamond, the maximum tensile strains (up to 9%) approached the theoretical elastic limit, and the corresponding maximum tensile stress reached ~89 to 98 gigapascals. After combining systematic computational simulations and characterization of pre- and postdeformation structural features, we ascribe the concurrent high strength and large elastic strain to the paucity of defects in the small-volume diamond nanoneedles and to the relatively smooth surfaces compared with those of microscale and larger specimens. The discovery offers the potential for new applications through optimized design of diamond nanostructure, geometry, elastic strains, and physical properties.

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.

Rationally designed nickel oxide ravines@iron cobalt-hydroxides with largely enhanced capacitive performance for asymmetric supercapacitors

Prominent energy and power densities play crucial roles in supercapacitor devices because of their potential practical application in various electronic devices. Herein, an asymmetric supercapacitor (ASC) with high capacitive performance was manufactured by combining rationally designed NiO@FeCo-layered double hydroxide (LDH), which has enhanced areal capacitance and rate capability, with commercially available pen ink composites as the positive and negative electrodes, respectively. Strikingly, the FeCo-LDHs with ultra-stable rate capability (retaining 94% from 4 to 25 mA cm−2) reported for the first time in this study can be employed to modify other transition metal oxides/hydroxides to achieve balanced performance. The constructed ASC surprisingly delivers an ultrahigh energy density of 64.1 W h kg−1 and a power density of 15 kW kg−1 as well as a robust cyclability (90% capacitance retention after 3000 cycles). In addition, the ASC is capable of readily driving patterned commercial light-emitting diodes (LEDs), motor propellers, and even a toy car, demonstrating its application potential in future nano-energy storage devices.

Size-dependent fracture behavior of silver nanowires

Silver (Ag) nanowires have great potential to be used in the flexible electronics industry for their applications in flexible, transparent conductors due to high conductivity and light reflectivity. Those applications always involve mechanical loading and deformations, which requires an in-depth understanding of their mechanical behavior and performance under loadings. However, current understanding on the mechanical properties of Ag nanowires is limited, especially on their size-dependent fracture behavior. In this work, mechanical properties of Ag nanowires with diameters ranging from 50 to 300 nm were systematically studied by in situ TEM tensile testing for the first time. The size effect was clearly found, with the increasing of the diameter of Ag nanowires, the ultimate tensile stress decreased. More importantly, the fracture behavior of Ag nanowire was studied and a brittle-to-ductile transition in fracture behavior was observed at the diameters around 100 nm which could be attributed to the dislocation activities within the geometry confinement. This work could give insights for understanding nanosized Ag wires and the design of Ag nanowire-based flexible devices and touchable panels.

Mechanically stable ternary heterogeneous electrodes for energy storage and conversion

Recently, solid asymmetric supercapacitor (ASC) has been deemed as an emerging portable power storage or backup device for harvesting natural resources. Here we rationally engineered a hierarchical, mechanically stable heterostructured FeCo@NiCo layered double hydroxide (LDH) with superior capacitive performance by a simple two-step electrodeposition route for energy storage and conversion. In situ scanning electron microscope (SEM) nanoindentation and electrochemical tests demonstrated the mechanical robustness and good conductivity of FeCo-LDH. This serves as a reliable backbone for supporting the NiCo-LDH nanosheets. When employed as the positive electrode in the solid ASC, the assembly presents high energy density of 36.6 W h kg-1 at a corresponding power density of 783 W kg-1 and durable cycling stability (87.3% after 5000 cycles) as well as robust mechanical stability without obvious capacitance fading when subjected to bending deformation. To demonstrate its promising capability for practical energy storage applications, the ASC has been employed as a portable energy source to power a commercially available digital watch, mini motor car, or household lamp bulb as well as an energy storage reservoir, coupled with a wind energy harvester to power patterned light-emitting diodes (LEDs).