Benzoin Radicals as Reducing Agent for Synthesizing Ultrathin Copper Nanowires

Transparent Electronics for Wearable Electronics Application

Recent advancements in wearable electronics offer seamless integration with the human body for extracting various biophysical and biochemical information for real-time health monitoring, clinical diagnostics, and augmented reality. Enormous efforts have been dedicated to imparting stretchability/flexibility and softness to electronic devices through materials science and structural modifications that enable stable and comfortable integration of these devices with the curvilinear and soft human body. However, the optical properties of these devices are still in the early stages of consideration. By incorporating transparency, visual information from interfacing biological systems can be preserved and utilized for comprehensive clinical diagnosis with image analysis techniques. Additionally, transparency provides optical imperceptibility, alleviating reluctance to wear the device on exposed skin. This review discusses the recent advancement of transparent wearable electronics in a comprehensive way that includes materials, processing, devices, and applications. Materials for transparent wearable electronics are discussed regarding their characteristics, synthesis, and engineering strategies for property enhancements. We also examine bridging techniques for stable integration with the soft human body. Building blocks for wearable electronic systems, including sensors, energy devices, actuators, and displays, are discussed with their mechanisms and performances. Lastly, we summarize the potential applications and conclude with the remaining challenges and prospects.

N,N-Dimethylformamide-Assisted Shape Evolution of Highly Uniform and Shape-Pure Colloidal Copper Nanocrystals

In this paper, the N,N-dimethylformamide (DMF)-assisted shape evolution of highly uniform and shape-pure copper nanocrystals (Cu NCs) is presented for the first time. Colloidal Cu NCs are synthesized via the disproportionation reaction of copper (I) bromide in the presence of a non-polar solvent mixture. It is observed that the shape of Cu NCs is systematically controlled by the addition of different amounts of DMF to the reaction mixture in high-temperature reaction conditions while maintaining a high size uniformity and shape purity. With increasing amount of DMF in the reaction mixture, the morphology of the Cu NCs change from a cube enclosed by six {100} facets, to a sphere with mixed surface facets, and finally, to an octahedron enclosed by eight {111} facets. The origin of this shape evolution is understood via first-principles density functional theory calculations, which allows the study of the change in the relative surface stability according to surface-coordinating adsorbates. Further, the shape-dependent plasmonic properties are systematically investigated with highly uniform and ligand-exchanged colloidal Cu NCs dispersed in acetonitrile. Finally, the facet-dependent electrocatalytic activities of the shape-controlled Cu NCs are investigated to reveal the activities of the highly uniform and shape-pure Cu NCs in the methanol oxidation reaction.

Cu Nanowires Passivated with Hexagonal Boron Nitride: An Ultrastable, Selectively Transparent Conductor

The copper nanowire (Cu NW) network is considered a promising alternative to indium tin oxide as transparent conductors for advanced optoelectronic devices. However, the fast degradation of copper in ambient conditions largely overshadows its practical applications. Here we demonstrate a facile method for epitaxial growth of hexagonal boron nitride (h-BN) of a few atomic layers on interlaced Cu NWs by low-pressure chemical vapor deposition, which exhibit excellent thermal and chemical stability under high temperature (900 °C in vacuum), high humidity (95% RH), and strong base/oxidizer solution (NaOH/H₂O₂). Meanwhile, their optical and electrical performances remain similar to those of the original Cu NWs (e.g., high optical transmittance (∼93%) and high conductivity (60.9 Ω/□)). A smart privacy glass is successfully fabricated based on a Cu@h-BN NW network and liquid crytal, which could rapidly control the visibility from transparent to opaque (0.26 s) and, at the same time, strongly block the mid-infrared light for energy saving by screening radiative heat. This precise engineering of epitaxial Cu@h-BN core-shell nanostructure offers broad applications in high-performance electronic and optoelectronic devices.

Synthesis of Ultrathin Metal Nanowires with Chemically Exfoliated Tungsten Disulfide Nanosheets

Transition metal dichalcogenides (TMDs) have attracted great interest owing to their fascinating properties with atomically thin nature. Although TMDs have been exploited for diverse applications, the effective role of TMDs in the synthesis of metal nanowires has not been explored. Here, we propose a new approach to synthesize ultrathin metal nanowires using TMDs for the first time. High-quality ultrathin nanowires with an average diameter of 11.3 nm are successfully synthesized for realizing high-performance transparent conductors that exhibit excellent conductivity and transparency with low haze. The growth mechanism is carefully investigated using high-resolution transmission electron microscopy, and growth of nanowires with tunable diameters is achieved by controlling the nanosheet dimension. Finally, we unravel the important role of TMDs acting as both reducing and nucleating agents. Therefore, our work provides a new strategy of the TMD as an innovative material for the growth of metal nanowires as a promising building block in next-generation optoelectronics.

Nanowire Genome: A Magic Toolbox for 1D Nanostructures

1D nanomaterials with high aspect ratio, i.e., nanowires and nanotubes, have inspired considerable research interest thanks to the fact that exotic physical and chemical properties emerge as their diameters approach or fall into certain length scales, such as the wavelength of light, the mean free path of phonons, the exciton Bohr radius, the critical size of magnetic domains, and the exciton diffusion length. On the basis of their components, aspect ratio, and properties, there may be imperceptible connections among hundreds of nanowires prepared by different strategies. Inspired by the heredity system in life, a new concept termed the "nanowire genome" is introduced here to clarify the relationships between hundreds of nanowires reported previously. As such, this approach will not only improve the tools incorporating the prior nanowires but also help to precisely synthesize new nanowires and even assist in the prediction on the properties of nanowires. Although the road from start-ups to maturity is long and fraught with challenges, the genetical syntheses of more than 200 kinds of nanostructures stemming from three mother nanowires (Te, Ag, and Cu) are summarized here to demonstrate the nanowire genome as a versatile toolbox. A summary and outlook on future challenges in this field are also presented.

Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes: Reaction Mechanism and Enhanced Sodium Storage Performance

One-dimensional nanotubes are very useful for achieving excellent performance in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to the fact that tubular structures can effectively alleviate the structural strain and shorten the ion diffusion length during repeated cycling. In this work, we report a Cu₂Sb-mediated growth strategy to controllably fabricate highly uniform Sb nanotubes (NTs), as well as Cu@Cu₂Sb and Cu₂Sb@Sb composite NTs, via a facile galvanic replacement reaction using Cu nanowires (NWs) as sacrificial templates. Benefiting from their structural merits, the Sb NTs manifest excellent sodium storage performance with superior rate performance (286 mAh g^-1 at 10 A g^-1) and extraordinary cycling stability (342 mAh g^-1 after 6000 cycles at 1 A g^-1). Furthermore, a full cell with Sb NTs as anode and Na₃(VOPO₄)₂F as cathode exhibits a high energy density (252 Wh kg^-1) and high output voltage (2.7 V), revealing their significant application promise in the next-generation SIBs.

Oxygen defect-induced localized surface plasmon resonance at the WO3-x quantum dot/silver nanowire interface: SERS and photocatalysis

Oxygen defects play a crucial role in a variety of functional transition metal oxides, ranging from photocatalytic materials to photoelectric devices. Tungsten oxide (WO3-x) is a type of transition metal oxide that has rich substoichiometric compositions and possesses oxygen defects. These oxygen defects determine the photon-electron interactions in the WO3-x structures. Therein, WO3-x quantum dots (QDs) exhibit fast carrier-transport for photon-electron interactions due to their strong quantum-size effects. Here, we report the use of non-stoichiometric WO3-x QDs, as a model material, in combination with silver nanowires (Ag NWs) to study photon-electron interactions on the nanoscale. We demonstrate that the incident photon-to-electron conversion efficiency can be increased by 8.5% and that the dye photodegradation performance was improved by 40% in a WO2.72 QD@Ag NW (WO2.72 QDs supported on AgNWs) composite compared to those of individual WO2.72 QDs under simulated AM 1.5G light. Furthermore, the WO3-x QD@Ag NW composite exhibits both photocatalytic activity and surface-enhanced Raman scattering (SERS) features, and the WO3-x QDs can be switched between a "photocatalytic state" and a "SERS state" by changing the stoichiometric ratio. The synergistic effects are ascribed to the "plasmonic state" of WO2.72 QDs upon light irradiation. This work provides new insight into the design of highly efficient transition metal oxide/plasmonic metal nanocomposites for photoelectric devices.

Size Distribution Control of Copper Nanoparticles and Oxides: Effect of Wet-Chemical Redox Cycling

In this work, we studied the effect of liquid-phase redox cycling on the size of Cu nanoparticles and oxides. The mixed solution of sodium hydroxide and ammonium persulfate was applied as the oxidation system at room temperature, and ascorbic acid was used as reduction agent at 80 °C in the cycling process. It was found that pristine copper particles with average size of around 800 nm and wide distribution from 300 to 1300 nm could be turned into the resulting particles with the average size of around 162.3 nm with the distribution from 75 to 250 nm after 5 redox cycles. It was also observed that uniform copper oxide nanowires formed after 5 oxidation cycles could be easily reduced into fine copper nanoparticles. The critical tuning factors including the precursor size, morphology, defects, reaction time, and the way of adding oxidant were investigated. It was suggested that the synergetic driving effect of chemical reduction and nanostructure thermodynamic instability in solution accounted for the size reformation of the copper nanoparticles. This proposed method of size-shrinking could be developed as a general strategy for large-scale tuning the properties of copper nanoparticles for wide applications and extended to other metal particles as well.

Copper Nanowire Dispersion through an Electrostatic Dispersion Mechanism for High-Performance Flexible Transparent Conducting Films and Optoelectronic Devices

Highly dispersed copper nanowire (CuNW) is an essential prerequisite for its practical application in various electronic devices. At present, the dispersion of CuNW is almost realized through the steric hindrance effect of polymers. However, the high post-treatment temperature of polymers makes this dispersion mechanism impractical for many actual applications. Here, after investigating the relationship between the electrostatic dispersion force and influence factors, an electrostatic dispersion mechanism is refined by us. Under the guidance of this mechanism, high dispersion of CuNW and a record low post-treatment temperature (80 °C) are realized simultaneously. The high dispersity endows CuNW with good stability (-45.66 mV) in water-based ink, high uniformity (65.7 ± 2.5 Ω sq^-1) in the prepared transparent conducting film (TCF) (23 cm × 23 cm), and industrial film preparation process, which are the issues that hinder the widespread application of CuNW-based TCF at present. The low post-treatment temperature makes the application of CuNW possible on any substrate. In addition, the charge modifier, 2-mercaptoethanol, enables CuNW to resist oxidation well. Finally, flexible optoelectronic devices employing the CuNW film as the electrode are fabricated and show efficiencies comparable to those of optoelectronic devices on indium tin oxide/glass.

Seeds screening aqueous synthesis, multiphase interfacial separation and in situ optical characterization of invisible ultrathin silver nanowires

We report a multi-step synthetic method to obtain ultrathin silver nanowires (Ag NWs) from an aqueous solution with a ∼17 nm diameter average, and where some of them decreased down to 9 nm. Carefully designed seed screening processes including LED irradiation at high temperature for a short time, and then continuous H2O2 etching, and relative growth mechanisms of high-yield five-twinned pentagonal seeds and ultrathin Ag NWs in aqueous environment are detailed. Then, a rapid and simple multiphase interfacial assembly method particularly suitable for the separation of ultrathin Ag NWs from various by-products was demonstrated with a clear mechanism explanation. Next, a unique optical interaction between light and individual AG NWs, as well as feature structures in the AG NWs film, was investigated by a micro-domain optical confocal microscope measurement in situ together with a theoretical explanation using modal transmission theory. That revealed that the haze problem of AG NWs films was not only arising from the interaction between light and individual or crossed Ag NWs but was also greatly dependent on a weak coupling effect of leaky modes supported by adjacent Ag NWs with large distances which had not been considered before. We then provided direct experimental evidence and concluded how to obtain haze-free films with 100% transparency in the whole visible range based on ultrathin Ag NWs. This breakthrough in diameter confinement and purification of Ag NWs is a highly expected step to overcome the well-focused light diffusion and absorption problems of Ag NWs-based devices applied in various fields such as flexible electronics, high-clarity displays, visible transparent heaters, photovoltaics and various optoelectronic technologies.

Synthesis of Silver Nanowires with Reduced Diameters Using Benzoin-Derived Radicals to Make Transparent Conductors with High Transparency and Low Haze

Reducing the diameter of silver nanowires has been proven to be an effective way to improve their optoelectronic performance by lessening light attenuation. The state-of-the-art silver nanowires are typically around 20 nm in diameter. Herein we report a modified polyol synthesis of silver nanowires with average diameters as thin as 13 nm and aspect ratios up to 3000. The success of this synthesis is based on the employment of benzoin-derived radicals in the polyol approach and does not require high-pressure conditions. The strong reducing power of radicals allows the reduction of silver precursors to occur at relatively low temperatures, wherein the lateral growth of silver nanowires is restrained because of efficient surface passivation. The optoelectronic performance of as-prepared 13 nm silver nanowires presents a sheet resistance of 28 Ω sq^-1 at a transmittance of 95% with a haze factor of ∼1.2%, comparable to that of commercial indium tin oxide (ITO).

Synthesis, Spray Deposition, and Hot-Press Transfer of Copper Nanowires for Flexible Transparent Electrodes

We report a solution-phase approach to the synthesis of crystalline copper nanowires (Cu NWs) with an aspect ratio >1000 via a new catalytic mechanism comprising copper ions. The synthesis involves the reaction between copper(II) chloride and copper(II) acetylacetonate in a mixture of oleylamine and octadecene. Reaction parameters such as the molar ratio of precursors as well as the volume ratio of solvents offer the possibility to tune the morphology of the final product. A simple low-cost spray deposition method was used to fabricate Cu NW films on a glass substrate. Post-treatment under reducing gas (5% H₂ + 95% N₂) atmosphere resulted in Cu NW films with a low sheet resistance of 24.5 Ω/sq, a transmittance of T = 71% at 550 nm (including the glass substrate), and a high oxidation resistance. Moreover, the conducting Cu NW networks on a glass substrate can easily be transferred onto a polycarbonate substrate using a simple hot-press transfer method without compromising on the electrical performance. The resulting flexible transparent electrodes show excellent flexibility ( R/ R_o < 1.28) upon bending to curvatures of 1 mm radius.

Advancements in Copper Nanowires: Synthesis, Purification, Assemblies, Surface Modification, and Applications

Copper nanowires (CuNWs) are attracting a myriad of attention due to their preponderant electric conductivity, optoelectronic and mechanical properties, high electrocatalytic efficiency, and large abundance. Recently, great endeavors are undertaken to develop controllable and facile approaches to synthesize CuNWs with high dispersibility, oxidation resistance, and zero defects for future large-scale nano-enabled materials. Herein, this work provides a comprehensive review of current remarkable advancements in CuNWs. The Review starts with a thorough overview of recently developed synthetic strategies and growth mechanisms to achieve single-crystalline CuNWs and fivefold twinned CuNWs by the reduction of Cu(I) and Cu(II) ions, respectively. Following is a discussion of CuNW purification and multidimensional assemblies comprising films, aerogels, and arrays. Next, several effective approaches to protect CuNWs from oxidation are highlighted. The emerging applications of CuNWs in diverse fields are then focused on, with particular emphasis on optoelectronics, energy storage/conversion, catalysis, wearable electronics, and thermal management, followed by a brief comment on the current challenges and future research directions. The central theme of the Review is to provide an intimate correlation among the synthesis, structure, properties, and applications of CuNWs.

Emerging Novel Metal Electrodes for Photovoltaic Applications

Emerging novel metal electrodes not only serve as the collector of free charge carriers, but also function as light trapping designs in photovoltaics. As a potential alternative to commercial indium tin oxide, transparent electrodes composed of metal nanowire, metal mesh, and ultrathin metal film are intensively investigated and developed for achieving high optical transmittance and electrical conductivity. Moreover, light trapping designs via patterning of the back thick metal electrode into different nanostructures, which can deliver a considerable efficiency improvement of photovoltaic devices, contribute by the plasmon-enhanced light-mattering interactions. Therefore, here the recent works of metal-based transparent electrodes and patterned back electrodes in photovoltaics are reviewed, which may push the future development of this exciting field.

Chemical formation of soft metal electrodes for flexible and wearable electronics

Efficient chemical approaches to fabricating soft metal electrodes aiming at wearable electronics are summarized and reviewed.

One-Pot Synthesis and Purification of Ultralong Silver Nanowires for Flexible Transparent Conductive Electrodes

Metal nanowires (NWs) have become the most promising candidates for the next generation of flexible transparent conductive electrodes (FTCEs), with high transmittance and low sheet resistance. In this work, ultralong silver NWs (Ag NWs), ∼220 μm (even larger than 400 μm) in length and ∼55 nm in diameter (aspect ratio: ∼4000), were synthesized via a one-pot polyol process using high molecular weight poly(vinylpyrrolidone) (M_w = 1 300 000) and an appropriate concentration of FeCl₃ (12.5 μM) through hydrothermal reaction. The prepared Ag NWs were purified by a filter cloth (pore size: about 30 × 50 μm²) to remove the Ag nanoparticles and short-length Ag NWs. The FTCE based on the ultralong Ag NWs without any post-treatments exhibits low sheet resistance of 155.0 Ω sq^-1 and transmittance of 97.70% at 550 nm. The outstanding performance of FTECs demonstrated that the ultralong Ag NWs are ideal materials for applications in flexible transparent optical devices.