Transparent Electrodes Printed with Nanocrystal Inks for Flexible Smart Devices

Transparent electrodes (TEs) are crucial in a wide range of modern electronic and optoelectronic devices. However, traditional TEs cannot meet the requirements of smart devices under development in unique fields, such as electronic skins, wearable electronics, robotic skins, flexible and stretchable displays, and solar cells. Emerging TEs printed with nanocrystal (NC) inks are inexpensive and compatible with solution processes, and have huge potential in flexible, stretchable, and wearable devices. Every development in ink‐based electrodes makes them more competitive for practical applications in various smart devices. Herein, we provide an overview of emergent ink‐based electrodes, such as transparent conducting oxides, metal nanowires, graphene, and carbon nanotubes, and their application in solution‐based flexible and stretchable devices.     

Organic‐Stabilizer‐Free Polyol Synthesis of Silver Nanowires for Electrode Applications

The polyol reduction of a Ag precursor in the presence of an organic stabilizer, such as poly(vinylpyrrolidone), is a widely used method for the production of Ag nanowires (NWs). However, organic capping molecules introduce insulating layers around each NW. Herein we demonstrate that Ag NWs can be produced in high yield without any organic stabilizers simply by introducing trace amounts of NaCl and Fe(NO₃)₃ during low‐temperature polyol synthesis. The heterogeneous nucleation and growth of Ag NWs on initially formed AgCl particles, combined with oxidative etching of unwanted Ag nanoparticles, resulted in the selective formation of long NWs with an average length of about 40 μm in the absence of a capping or stabilizing effect provided by surface‐adsorbing molecules. These organic‐stabilizer‐free Ag NWs were directly used for the fabrication of high‐performance transparent or stretchable electrodes without a complicated process for the removal of capping molecules from the NW surface.     

Flexible transparent PES/silver nanowires/PET sandwich-structured film for high-efficiency electromagnetic interference shielding

We have developed a kind of high-yield synthesis strategy for silver nanowires by a two-step injection polyol method. Silver nanowires and polyethylene oxide (PEO) (M(w) = 900,000) were prepared in a homogeneous-coating ink. Wet composite films with different thicknesses were fabricated on a PET substrate by drawn-down rod-coating technology. Silver nanowires on PET substrates present a homogeneous distribution under the assistance of PEO. Then PEO was thermally removed in situ at a relatively low temperature attributed to its special thermal behavior under atmospheric conditions. As-prepared metallic nanowire films on PET substrates show excellent stability and a good combination of conductivity and light transmission. A layer of transparent poly(ethersulfones) (PESs) was further coated on silver nanowire networks by the same coating method to prevent the shedding and corrosion of silver nanowires. Sandwich-structured flexible transparent films were obtained and displayed excellent electromagnetic interference (EMI) shielding effectiveness.     

Magnetically Assembled Multisegmented Nanowires and Their Applications

Here we report a cost effective and versatile way of synthesizing and assembling multi‐functional (e.g., gold‐polypyrrole‐nickel‐gold) nanowires. Multisegmented nanowires were synthesized using electrodeposition method for precise control over segment dimensions for proper expression of material functionality. The nanowires were integrated on microfabricated electrodes using magnetic dipole interactions between the ferromagnetic segment in the nanowire and the ferromagnetic electrodes. The electrical properties of multisegmented nanowires showed semiconducting behavior with the activation energy of ca. 0.27 eV for the conducting polymer segment of the nanowire. These nanowire devices showed response towards light and exposure to ammonia, demonstrating their potential use as photonic device and gas sensor, respectively.     

Length‐Dependent Charge Generation from Vertical Arrays of High‐Aspect‐Ratio ZnO Nanowires

Aqueous chemical growth of zinc oxide nanowires is a flexible and effective approach to obtain dense arrays of vertically oriented nanostructures with high aspect ratio. Herein we present a systematic study of the different synthesis parameters that influence the ZnO seed layer and thus the resulting morphological features of the free‐standing vertically oriented ZnO nanowires. We obtained a homogeneous coverage of transparent conductive substrates with high‐aspect‐ratio nanowire arrays (length/diameter ratio of up to 52). Such nanostructured vertical arrays were examined to assess their electric and piezoelectric properties, and showed an electric charge generation upon mechanical compressive stress. The principle of energy harvesting with these nanostructured ZnO arrays was demonstrated by connecting them to an electronic charge amplifier and storing the generated charge in a series of capacitors. We found that the generated charge and the electrical behavior of the ZnO nanowires are strictly dependent on the nanowire length. We have shown the importance of controlling the morphological properties of such ZnO nanostructures for optimizing a nanogenerator device.     

A Surfactant‐Encapsulating Polyoxometalate Nanowire Assembly as a New Carrier for Nanoscale Noble‐Metal Catalysts

The loading of noble‐metal nanoparticles (NMNPs) onto various carriers to obtain stable and highly efficient catalysts is currently an important strategy in the development of noble metal (NM)‐based catalytic reactions and their applications. We herein report a nanowire supramolecular assembly constructed from the surfactant‐encapsulating polyoxometalates (SEPs) CTAB‐PW₁₂, which can act as new carriers for NMNPs. In this case, the Ag NPs are loaded onto the SEP nanowire assembly with a narrow size distribution from 5 to 20 nm in diameter; the average size is approximately 10 nm. The Ag NPs on the nanowire assemblies are well stabilized and the over agglomeration of Ag NPs is avoided owing to the existence of well‐arranged polyoxometalate (POM) units in the SEP assembly and the hydrophobic surfactant on the surface of the nanowire assembly. Furthermore, the loading amount of the Ag NPs can be adjusted by controlling the concentration of the AgNO₃ aqueous solution. The resultant Ag/CTAB‐PW₁₂ composite materials exhibit high activity and good stability for the catalytic reduction of 4‐nitrophenol (4‐NP) with NaBH₄ in isopropanol/H₂O solution. The NMNPs‐loaded SEP nanoassembly may represent a new composite catalyst system for application in NM‐based catalysis.     

Synthesis of ultralong copper nanowires for high-performance transparent electrodes

Cu nanowires hold great promise for the fabrication of low-cost transparent electrodes. However, their current synthesis is mainly performed in aqueous media with poor nanowire dispersibility. We report herein the novel synthesis of ultralong single-crystalline Cu nanowires with excellent dispersibility, providing an excellent candidate material for high-performance transparent electrode fabrication.     

Low-temperature synthesis of single crystalline Ag2S nanowires on silver substrates

We report on the successful synthesis of silver sulfide (Ag(2)S) nanowires by a simple and mild gas-solid reaction approach. For the nanowire synthesis, a preoxidized silver substrate is exposed to an atmosphere of an O(2)/H(2)S mixture at room temperature or slightly above. The resulting Ag(2)S nanowires are phase pure with a monoclinic crystal structure and have diameters of a few tens of nanometers and lengths up to 100 mum. The influence of reaction conditions on the diameter, length, and morphology of the Ag(2)S nanowires has been studied by a number of structural and spectroscopic techniques. The nanowire growth mechanism on the Ag substrate has been discussed, which is likely characterized by continuous deposition at the tip. Additionally, we demonstrate thinning and cutting of individual Ag(2)S nanowires with electron beams and laser beams, which are potentially useful for nanowire manipulation and engineering.     

Formation of Ag2Se nanotubes and dendrite-like structures from UV irradiation of a CSe2/Ag colloidal solution

Ag2Se nanotubes have been successfully synthesized by UV photodissociation of adsorbed CSe2 on the surface of Ag nanowires under ambient conditions. Transmission electron microscopy was used to trace the growth of hollow interiors, allowing a detail study of the Kirkendall effect in 1-dimensional nanosystems. Voids were observed to grow from both ends of the nanowires along the longitudinal axis and ultimately merged to form hollow nanotubes. This phenomenon is attributed to the crystallographic selective adsorption of poly(vinylpyrolidone) on Ag nanowire templates. In addition, we observed the formation of dendrites on aging of the reaction mixture and explained its growth based on a diffusion-limited aggregation model.     

Controlled Self‐Assembly of Functional Metal Octaethylporphyrin 1 D Nanowires by Solution‐Phase Precipitative Method

Metal octaethylporphyrin M(OEP) (M=Ni, Cu, Zn, Pd, Ag, and Pt) nanowires are fabricated by a simple solution‐phase precipitative method. By controlling the composition of solvent mixtures, the diameters and lengths of the nanowires can be varied from 20 to 70 nm and 0.4 to 10 μm, respectively. The Ag(OEP) nanowires have lengths up to 10 μm and diameters of 20–70 nm. For the M(OEP) nanowires, the growth orientation and packing of M(OEP) molecules are examined by powder XRD and SAED measurements, revealing that these M(OEP) nanowires are formed by the self‐assembly of M(OEP) molecules through intermolecular π???π interactions along the π???π stacking axis, and the M²⁺ ion plays a key role in the nanowire formation. Using the bottom contact field effect transistor structure and a simple drop‐cast method, a single‐crystal M(OEP) nanowires‐based field effect transistor can be readily prepared with prominent hole transporting behaviour and charge‐carrier mobility up to 10^?3–10^?2 cm² V^?1 s^?1 for holes, which are 10 times higher than that of vacuum‐deposited M(OEP) organic thin‐film transistors (OTFTs).