Ultrathin transparent conductive polyimide foil embedding silver nanowires

Ultrastrong, Ductile, Tear- and Folding-Resistant Polyimide Film Doubly Reinforced by an Aminated Rigid-Rod Macromolecule and Graphene Oxide

As a high-performance polymer with exceptional mechanical, thermal, and insulating properties, polyimide (PI) has been widely used as flexible circuit substrates for microelectronics, portable electronics, and wearable devices. Due to the growing demand for further thinning and lightweighting of electronic products, PI films need to have further enhanced mechanical properties. Traditional nanofiller-reinforced PI films often exhibit reduced ductility and limited improvements in strength. Therefore, it remains a challenge to simultaneously improve the strength and toughness of PI films while preserving their ductility. In this study, we report an exceptionally strong and ductile PI doubly reinforced by one-dimensional rigid-rod para-aramid, poly(p-aminophenylene aminoterephthalamide ((NH2)2-PPTA), and two-dimensional graphene oxide (GO) nanosheets. The amino side groups of (NH2)2-PPTA react with the anhydride end groups of PI, forming covalent bonds. At a (NH2)2-PPTA content of only 0.4 wt %, the (NH2)2-PPTA/PI film displays significantly enhanced mechanical properties. When 0.4 wt % of GO is added together with (NH2)2-PPTA, the tensile strength, tensile toughness, and strain at break reach 284.8 ± 5.3 MPa, 277.9 ± 7.6 MJ/m3, and 132.6 ± 3.8%, which are ∼178, ∼312, and ∼51% higher, respectively, than those of pure PI. Moreover, the doubly reinforced PI film also exhibits a 206% increase in tear strength and significantly enhanced folding resistance. The dual reinforcement of PI with (NH2)2-PPTA and GO improves the mechanical properties more efficiently than any single reinforcing agents previously reported and overcomes the disadvantage of most inorganic nanofillers that reduce ductility.

The Preparation of Thin Conductive Polyimide Foils for Nuclear Targets

Thin and conductive plastic foils are of great interest to the target preparation and nuclear physics communities as a backing support for neutron-induced reaction measurements. This paper describes the preparation and characterization of thin, freestanding conductive polyimide films with an areal density suitable for target preparation in nuclear chemistry applications. The films were fabricated by blending a variety of graphene-based nanoparticles, a custom-made graphene suspension, and carbon nanotubes within a polymer matrix. The fabrication of freestanding polyimide films with an areal density of 30 μg/cm2 (∼210 nm) was both time-consuming and difficult. Here, a novel approach is described that employs a sacrificial layer and graphene material to make thin (pure and conductive) polyimide foils readily available within 24 h.

Flexible Transparent Conductive Electrodes: Unveiling Growth Mechanisms, Material Dimensions, Fabrication Methods, and Design Strategies

Flexible transparent conductive electrodes (FTCEs) constitute an indispensable component in state-of-the-art electronic devices, such as wearable flexible sensors, flexible displays, artificial skin, and biomedical devices, etc. This review paper offers a comprehensive overview of the fabrication techniques, growth modes, material dimensions, design, and their impacts on FTCEs fabrication. The growth modes, such as the "Stranski-Krastanov growth," "Frank-van der Merwe growth," and "Volmer-Weber growth" modes provide flexibility in fabricating FTCEs. Application of different materials including 0D, 1D, 2D, polymer composites, conductive oxides, and hybrid materials in FTCE fabrication, emphasizing their suitability in flexible devices are discussed. This review also delves into the design strategies of FTCEs, including microgrids, nanotroughs, nanomesh, nanowires network, and "kirigami"-inspired patterns, etc. The pros and cons associated with these materials and designs are also addressed appropriately. Considerations such as trade-offs between electrical conductivity and optical transparency or "figure of merit (FoM)," "strain engineering," "work function," and "haze" are also discussed briefly. Finally, this review outlines the challenges and opportunities in the current and future development of FTCEs for flexible electronics, including the improved trade-offs between optoelectronic parameters, novel materials development, mechanical stability, reproducibility, scalability, and durability enhancement, safety, biocompatibility, etc.

Flexible, Transparent, and Hazy Composite Cellulosic Film with Interconnected Silver Nanowire Networks for EMI Shielding and Joule Heating

An optical transparent and hazy film with admirable flexibility, electromagnetic interference (EMI) shielding, and Joule heating performance meeting the requirements of optoelectronic devices is significantly desirable. Herein, a cellulose paper was infiltrated by epoxy resin to fabricate a transparent cellulose paper (TCP) with high transparency, optical haze, and favorable flexibility, owing to effective light scattering and mechanical enhancement of the cellulose network. Moreover, a highly connected silver nanowire (AgNW) network was constructed on the TCP substrate by the spray-coating method and appropriate thermal annealing technique to realize high electrical conductivity and favorable optical transmittance of the composite film at the same time, followed by coating of a polydimethylsiloxane (PDMS) layer for protection of the AgNW network. The obtained PDMS/AgNWs/TCP composite film features considerable optical transmittance (up to 86.8%) and haze (up to 97.7%), while satisfactory EMI shielding effectiveness (SE) (up to 39.1 dB, 8.2-12.4 GHz) as well as strong mechanical strength (higher than 41 MPa) were achieved. The coated PDMS layer prevented the AgNW network from falling off and ensured the long-term stability of the PDMS/AgNWs/TCP composite film under deformations. In addition, the multifunctional PDMS/AgNWs/TCP composite film also exhibited excellent Joule heating performance with low supplied voltages, rapid response, and sufficient stability. This work demonstrates a novel pathway to improve the performance of multifunctional transparent composite films for future advanced optoelectronic devices.

Heat-Resistant Black Insulative Thin Films for Flat-Panel Displays in Al-Doped Ag-Fe-O Systems

Multilayer antireflection (AR) coatings require a material with a large and constant absorption coefficient over the whole visible range and thermal stability. Coatings for use in touch panel displays are also required to be electrically insulative. In this study, 60 mol % Ag-40 mol % (Fe_1-xAl_x)-O (x = 0, 0.25, 0.50, 0.75, and 1.0) thin films are prepared by pulsed laser deposition, and their optical properties, electric resistance, and thermal stability are clarified by combining the experimental data and density functional theory (DFT) calculations. Over the visible range, large and constant absorption coefficients are obtained for all compositions. The standard deviations of the absorption coefficients of the x = 0.75 and 1.0 samples are found to be smaller than those of conventional materials like graphite and CrO_x. High sheet resistance (R_sheet > 10⁷ Ω·sq^-1) is also confirmed. It is determined that nanometer-sized Ag dispersed into a matrix, which was confirmed to be ionic Ag in the matrix phase, is responsible for the absorption at a shorter visible light range and insulative nature even at high Ag content. The films with high Al content are stable up to 500 °C. The potential of these black insulative Ag-Al-Fe-O thin films for use as black AR coatings is confirmed by optical simulations with multilayer stacks.

In Situ Growth of Silver Film on Polyimide with Tuned Morphologies for Flexible Electronics

In this work, the silver films with tuned morphologies have been fabricated on flexible polyimide substrate by in situ direct-ion-exchange technique. The morphology of Ag films with loose nanoparticles, dense polyhedrons, aggregated nanoparticle clouds, and dendrite structure can be obtained by a controlled reduced process as illustrated by scanning electron microscopy (SEM) and optical microscopy, respectively. All of the Ag films show good crystalline and high conductivity, which is confirmed by X-ray diffraction (XRD) and four-point probe resistance measurements. Infrared (IR) spectra demonstrate the occurrence of the polyimide surface metallization, which favors good adhesion between the Ag films and the flexible substrate. The adhesion test proves the strong adhesion of these Ag films, especially for the Ag films with the dendritic structure. Moreover, the mechanical properties of these Ag/PI films have been investigated as well. It can be found that all of the Ag/PI films exhibit low sensitivity to the bending test. However, the strain sensitivity strongly depends on the morphology of the Ag films, which can be applied for diverse flexible electronics.

Recent advances in printed flexible heaters for portable and wearable thermal management

Flexible resistive heaters (FRHs) with high heating performance, large-area thermal homogeneity, and excellent thermal stability are very desirable in modern life, owing to their tremendous potential for portable and wearable thermal management applications, such as body thermotherapy, on-demand drug delivery, and artificial intelligence. Printed electronic (PE) technologies, as emerging methods combining conventional printing techniques with solution-processable functional ink have been proposed to be promising strategies for the cost-effective, large-scale, and high-throughput fabrication of printed FRHs. This review summarizes recent progress in the main components of FRHs, including conductive materials and flexible or stretchable substrates, focusing on the formulation of conductive ink systems for making printed FRHs by a variety of PE technologies including screen printing, inkjet printing, roll-to-roll (R2R) printing and three-dimensional (3D) printing. Various challenges facing the commercialization of printed FRHs and improved methods for portable and wearable thermal management applications have been discussed in detail to overcome these problems.

Investigation of the Chemical Structure of Ultra-Thin Polyimide Substrate for the Xenon Flash Lamp Lift-off Technology

Lift-off is one of the last steps in the production of next-generation flexible electronics. It is important that this step is completed quickly to prevent damage to ultrathin manufactured electronics. This study investigated the chemical structure of polyimide most suitable for the Xe Flash lamp–Lift-Off process, a next-generation lift-off technology that will replace the current dominant laser lift-off process. Based on the characteristics of the peeled-off polyimide films, the Xe Flash lamp based lift-off mechanism was identified as photothermal decomposition. This occurs by thermal conduction via light-to-heat conversion. The synthesized polyimide films treated with the Xe Flash lamp–Lift-Off process exhibited various thermal, optical, dielectric, and surface characteristics depending on their chemical structures. The polyimide molecules with high concentrations of –CF₃ functional groups and kinked chemical structures demonstrated the most promising peeling properties, optical transparencies, and dielectric constants. In particular, an ultra-thin polyimide substrate (6 μm) was successfully fabricated and showed potential for use in next-generation flexible electronics.

Embedding Silver Nanowires into a Hydroxypropyl Methyl Cellulose Film for Flexible Electrochromic Devices with High Electromechanical Stability

Transparent conductive films (TCFs) based on silver nanowires (AgNWs) are becoming one of the best candidates in realizing flexible optoelectronic devices. The AgNW-based TCF is usually prepared by coating AgNWs on a transparent polymer film; however, the coated AgNWs easily detach from the polymer underneath because of the weak adhesion between them. Herein, a network of AgNWs is embedded in the transparent hydroxypropyl methyl cellulose film, which has a strong adhesion with the AgNWs. The obtained TCF shows high optical transmittance (>85%), low roughness (rms = 4.8 ± 0.5 nm), and low haze (<0.2%). More importantly, owing to the embedding structure and strong adhesion, this TCF also shows excellent electromechanical stability, which is superior to the reported ones. Employing this TCF in a flexible electrochromic device, the obtained device exhibits excellent cyclic electromechanical stability and high coloring efficiency. Our work demonstrates a promising TCF with superior electromechanical stability for future applications in flexible optoelectronics.

Optically Transparent Multiscale Composite Films for Flexible and Wearable Electronics

One of the key breakthroughs enabling flexible electronics with novel form factors is the deployment of flexible polymer films in place of brittle glass, which is one of the major structural materials for conventional electronic devices. Flexible electronics requires polymer films with the core properties of glass (i.e., dimensional stability and transparency) while retaining the pliability of the polymer, which, however, is fundamentally intractable due to the mutually exclusive nature of these characteristics. An overview of a transparent fiber-reinforced polymer, which is suggested as a potentially viable structural material for emerging flexible/wearable electronics, is provided. This includes material concept and fabrication and a brief review of recent research progress on its applications over the past decade.

Reduced Graphene Oxide Conformally Wrapped Silver Nanowire Networks for Flexible Transparent Heating and Electromagnetic Interference Shielding

Metal nanowire networks (MNNs) are promising as transparent electrode materials for a diverse range of optoelectronic devices and also work as active materials for electrical heating and electromagnetic interference (EMI) shielding applications. However, the relatively low performance and poor durability of MNNs are limiting the practical application of the resulting devices. Here, we report a controllable approach to enhance the conductivity and the stability of MNNs with their transmittance remaining unchanged, in which reduced graphene oxide conformally wrapped silver nanowire networks (AgNW@rGO networks) are synthesized by selective electrodeposition of GO nanosheets on AgNWs followed by a pulsed laser irradiation treatment. Experimental characterizations and finite-difference time-domain simulations indicate that pulsed laser irradiation at a specific wavelength not only reduces the GO but also welds the AgNWs together through a surface plasmon resonance process. As a result, the AgNW@rGO networks exhibit low sheet resistance of 3.3 Ω/□, average transmittance of 91.1%, and good flexibility. Wrapping with rGO improves the maximum electrical heating temperature of the AgNW network transparent heaters due to the effective suppression of the oxidation and the migration of surface silver atoms. In addition, excellent EMI shielding effectiveness of up to 35.5 dB in the 8.2-12.4 GHz frequency range is obtained as a consequence of the combined effects of dual reflection, conduction loss, and multiple dielectric polarization relaxation processes.

Wireless Monitoring Using a Stretchable and Transparent Sensor Sheet Containing Metal Nanowires

Mechanically and visually imperceptible sensor sheets integrated with lightweight wireless loggers are employed in ultimate flexible hybrid electronics (FHE) to reduce vital stress/nervousness and monitor natural biosignal responses. The key technologies and applications for conceptual sensor system fabrication are reported, as exemplified by the use of a stretchable sensor sheet completely conforming to an individual's body surface to realize a low-noise wireless monitoring system (<1 µV) that can be attached to the human forehead for recording electroencephalograms. The above system can discriminate between Alzheimer's disease and the healthy state, thus offering a rapid in-home brain diagnosis possibility. Moreover, the introduction of metal nanowires to improve the transparency of the biocompatible sensor sheet allows one to wirelessly acquire electrocorticograms of nonhuman primates and simultaneously offers optogenetic stimulation such as toward-the-brain-machine interface under free movement. Also discussed are effective methods of improving electrical reliability, biocompatibility, miniaturization, etc., for metal nanowire based tracks and exploring the use of an organic amplifier as an important component to realize a flexible active probe with a high signal-to-noise ratio. Overall, ultimate FHE technologies are demonstrated to achieve efficient closed-loop systems for healthcare management, medical diagnostics, and preclinical studies in neuroscience and neuroengineering.

Highly-robust, solution-processed flexible transparent electrodes with a junction-free electrospun nanofiber network

Flexible transparent electrodes (FTEs) are widely used in a variety of applications, including flexible displays and wearable devices. Important factors in FTE design include active control of electrical sheet resistance, optical transparency and mechanical flexibility. Because these factors are inversely proportional to one another, it is essential to develop a technique that maintains flexibility while actively controlling the sheet resistance and transparency for a variety of applications. This research presents a new method for fabricating transparent electrodes on flexible polyimide films using electrospinning and copper electroless deposition methods. A flat metal network-based electrode without contact resistance was fabricated by heat treatment and electroless deposition onto the electrospun seed layer. The fabricated FTEs exhibited a transparency exceeding 80% over the entire visible light range and a sheet resistance of less than 10.0 Ω sq⁻¹. Due to the heat treatment process, the adhesion between the metal network and the substrate was superior to other electrospinning-based transparent electrodes. Applicable to the large-area manufacturing process, the standard deviation of the network density of the fabricated large-area FTE was about 1%. This study does not require the polymer casting technique and has further advantages for mass production of electrodes and application to various fields.

The flexible transparent electrode of this study used electrospinning and electroless deposition, which is a fabrication method to remove contact resistance at the nanofiber intersection and fabricate large-area electrode.

Moiré-free fingerprint sensors based on multilayer oxide-metal-oxide electrodes

The display quality of touchscreen devices with on-screen fingerprint sensors is reduced by moiré patterns, interference phenomena caused by an overlap between the pixel pattern of the display, and the electrode pattern of the fingerprint sensor. A promising strategy for resolving this issue is to reduce the visibility of the moiré pattern, by including a filling layer with a transmittance similar to that of the electrodes, between the different patterns. We propose a moiré-free fingerprint sensor that uses an oxide-metal-oxide (IZO/Ag/IZO) multilayer as a highly transparent electrode. To verify the moiré reduction effect, we conducted a two-dimensional spectral analysis to calculate the spatial frequencies of the superimposed image of the display and the sensor patterns, and demonstrated experimentally that the proposed electrode greatly reduces the undesirable moiré phenomenon.

Effect of Graphene-EC on Ag NW-Based Transparent Film Heaters: Optimizing the Stability and Heat Dispersion of Films

To optimize the performance of silver nanowire (Ag NW) film heaters and explore the effect of graphene on a film, we introduced poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) and graphene modified with ethyl cellulose (graphene-EC) into the film. The high-quality and well-dispersed graphene-EC was synthesized from graphene obtained by electrochemical exfoliation as a precursor. The transparent film heaters were fabricated via spin-coating. With the assistance of graphene-EC, the stability of film heaters was greatly improved, and the conductivity was optimized by adjusting the Ag NW concentration. The film heaters exhibited a fast and accurate response to voltage, accompanied by excellent environmental endurance, and there was no significant performance degradation after being operated for a long period of time. These results indicate that graphene-EC plays a crucial role in optimizing film stability and heat dispersion in the film. The Ag NW/PEDOT:PSS-doped graphene-EC film heaters show a great potential in low-cost indium-tin-oxide-free flexible transparent electrodes, heating systems, and transparent film heaters.

Bio-inspired semi-transparent silver nanowire conductor based on a vein network with excellent electromechanical and photothermal properties

A bio-inspired conductive binary-network of vein–silver nanowires (AgNWs) was embedded in poly(dimethylsiloxane) (PDMS) to prepare a semi-transparent stretchable conductor (vein–AgNWs–PDMS) by a simple dipping process. The special conductive structure was constructed by using veins with a porous structure as an ideal skeleton to load AgNW networks, which allowed the vein–AgNWs–PDMS composite to show a low sheet resistance of 1 Ω sq⁻¹ with 74% transmittance. The figure of merit of vein–AgNWs–PDMS is as high as 2502 and can be adjusted easily by controlling the times of the dipping cycle. Furthermore, the vein–AgNWs–PDMS conductor can retain high conductivity after 150% mechanical elongation and exhibit excellent electromechanical stability in repeated stretch/release tests with 60% strain (500 cycles). As an example of an application, patterned light-emitting diode (LED) arrays using the vein–AgNWs–PDMS conductors have been fabricated, and worked well under deformation. Moreover, the photo-thermal properties of the vein–AgNWs–PDMS composite have been demonstrated by a position heating experiment using near-infrared (NIR) laser irradiation and the generated heat can be effectively dissipated through the vein network to avoid local overheating. Due to the high-performance and facile fabrication process, the vein–AgNWs–PDMS conductors will have multifunctional applications in stretchable electronic devices.

A bio-inspired binary-network conductive structure of vein–AgNWs was embedded in poly(dimethylsiloxane) to prepare a semi-transparent stretchable conductor (vein–AgNWs–PDMS) with a good photo-thermal effect for position heating and excellent electromechanical stability.

Transparent Electrode Based on Silver Nanowires and Polyimide for Film Heater and Flexible Solar Cell

Transparent, conductive, and flexible Ag nanowire (NW)-polyimide (PI) composite films were fabricated by a facile solution method. Well-dispersed Ag NWs result in percolation networks on the PI supporting layer. A series of films with transmittance values of 53–80% and sheet resistances of 2.8–16.5 Ω/sq were investigated. To further verify the practicability of the Ag NWs-PI film in optoelectronic devices, we utilized it in a film heater and a flexible solar cell. The film heater was able to generate a temperature of 58 °C at a driving voltage of 3.5 V within 20 s, indicating its potential application in heating devices that require low power consumption and fast response. The flexible solar cell based on the composite film with a transmittance value of 71% presented a power conversion efficiency of 3.53%. These successful applications proved that the fabricated Ag NWs-PI composite film is a good candidate for application in flexible optoelectronic devices.

Preparation of Palladium/Silver-Coated Polyimide Nanotubes: Flexible, Electrically Conductive Fibers

A simple and practical method for coating palladium/silver nanoparticles on polyimide (PI) nanotubes is developed. The key steps involved in the process are silver ion exchange/reduction and displacement reactions between silver and palladium ions. With the addition of silver, the conductivity of the PI nanotubes is greatly enhanced. Further, the polyimide nanotubes with a dense, homogeneous coating of palladium nanoparticles remain flexible after heat treatment and show the possibility for use as highly efficient catalysts. The approach developed here is applicable for coating various noble metals on a wide range of polymer matrices, and can be used for obtaining polyimide nanotubes with metal loaded on both the inner and outer surface.

Screen-Printed Fabrication of PEDOT:PSS/Silver Nanowire Composite Films for Transparent Heaters

A transparent and flexible film heater was fabricated; based on a hybrid structure of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) and silver nanowires (Ag NWs) using a screen printing; which is a scalable production technology. The resulting film integrates the advantages of the two conductive materials; easy film-forming and strong adhesion to the substrate of the polymer PEDOT:PSS; and high conductivity of the Ag NWs. The fabricated composite films with different NW densities exhibited the transmittance within the range from 82.3% to 74.1% at 550 nm. By applying 40 V potential on the films; a stable temperature from 49 °C to 99 °C was generated within 30 s to 50 s. However; the surface temperature of the pristine PEDOT:PSS film did not increase compared to the room temperature. The composite film with the transmittance of 74.1% could be heated to the temperatures from 41 °C to 99 °C at the driven voltages from 15 V to 40 V; indicating that the film heater exhibited uniform heating and rapid thermal response. Therefore; the PEDOT:PSS/Ag NW composite film is a promising candidate for the application of the transparent and large-scale film heaters.

Solution-processed multifunctional transparent conductive films based on long silver nanowires/polyimide structure with highly thermostable and antibacterial properties

The optical, electrical, thermal and antibacterial properties of AgNW/PI composite films.

Investigation of thermal and tensile properties of poly(benzimidazole-imide) composites incorporating salicylic acid–functionalized multiwalled carbon nanotubes

This work describes a novel aromatic poly(benzimidazole-imide) (PBII) with amino salicylic acid (ASA) segments in the main chain by melt/solid polymerization method under solvent-free conditions and its composites reinforced with ASA-functionalized multiwalled carbon nanotubes (MWCNTs-ASA). The polymer was obtained in high yield with an amorphous morphology, was soluble in various organic solvents, such as N,N′-dimethylacetamide, N,N′-dimethylformamide, N-methyl-2-pyrrolidone, and dimethyl sulfoxide, and could afford flexible and tough film via solution casting. MWCNT-ASA/PBII composite films were also prepared by casting a solution of precursor polymer containing different fractions of MWCNTs-ASA into a thin film (1, 2, and 5 wt%). The cast films exhibited good mechanical properties with tensile strengths of 90.00–128.3 MPa, elongation at break of 4.6–7.9%, and tensile modulus of 1.6–2.9 GPa. They were reasonably stable up to a temperature above 400°C for the PBII and above 450°C for the composites. Structural and morphological evaluation of the composites was carried out by Fourier transform infrared spectroscopy and X-ray diffraction. Dispersion of MWCNT-ASA in the polymer matrix was investigated by field emission scanning and transmission electron microscopy.

An antireflection transparent conductor with ultralow optical loss (<2 %) and electrical resistance (<6 Ω sq-1)

Transparent conductors are essential in many optoelectronic devices, such as displays, smart windows, light-emitting diodes and solar cells. Here we demonstrate a transparent conductor with optical loss of ∼1.6%, that is, even lower than that of single-layer graphene (2.3%), and transmission higher than 98% over the visible wavelength range. This was possible by an optimized antireflection design consisting in applying Al-doped ZnO and TiO₂ layers with precise thicknesses to a highly conductive Ag ultrathin film. The proposed multilayer structure also possesses a low electrical resistance (5.75 Ω sq⁻¹), a figure of merit four times larger than that of indium tin oxide, the most widely used transparent conductor today, and, contrary to it, is mechanically flexible and room temperature deposited. To assess the application potentials, transparent shielding of radiofrequency and microwave interference signals with ∼30 dB attenuation up to 18 GHz was achieved.

Transparent conductors are fundamental for optoelectronics. Using the transfer matrix method to optimise a multistructure of anti-reflection coatings containing an ultrathin metal film, Maniyara et al. achieve the highest transmittance of an antireflection transparent conductor combined with low resistance.

Highly Stretchable and Conductive Copper Nanowire Based Fibers with Hierarchical Structure for Wearable Heaters

Wearable heaters have been increasingly attracting researchers' great interest due to their efficient utility in maintaining warmth and in thermotherapy. Nowadays carbon nanomaterials and metallic nanowires tend to become the mainstream heating elements in wearable heaters considering their excellent electrical and mechanical properties. Though considerable progress has been made, there still exist challenging issues that need to be addressed in practical applications, including bad breathability and poor endurance to mechanical deformations. Here, we devise a copper nanowire based composite fiber with a unique hierarchical structure. This fiber possesses not only excellent heating performance, but also fantastic tolerance to mechanical impact, such as bending, twisting, and stretching. We further weave these fibers into a wearable heating fabric and realize smart personal heating management through an Android phone by integrating with a microcontroller unit. Two practical applications are demonstrated including a heating kneepad for articular thermotherapy and a heating coat on an infant model for maintaining warmth.

Degradation of Flexible, ITO-Free Oligothiophene Organic Solar Cells

We investigate the degradation of organic solar cells based on an oligothiophene (DCV5T-Me) small molecule donor and the acceptor C60. Two different flexible, transparent bottom electrode types are employed: a transparent metal electrode (TME) and silver nanowires (AgNWs). They exhibit high optical transparency up to 86% and a sheet resistance as low as 12Ω/□. Power conversion efficiencies of 7.0%, 5.7%, and 7.2% on TME, AgNWs, and indium tin oxide (ITO, reference) are reached, respectively. The solar cells are protected against moisture ingress utilizing a flexible alumina thin-film, exhibiting water vapor transmission rates down to 3 × 10(-5) g m(-2) day(-1) at 38 °C and 90% relative humidity (RH). Implementation of this ultrabarrier as top and bottom encapsulation enables fabrication of fully flexible devices. A decrease in PCE to 80% of initial values is observed after 1000 ± 50 h on flexible, encapsulated TME but only 20 ± 5 h on AgNWs in a climate of 38 °C/50% RH. Degradation in AgNW-based devices is attributed to electrode decomposition.

High-Performance, Solution-Processed, Embedded Multiscale Metallic Transparent Conductors

High-performance multiscale metallic transparent conductors (TCs) are demonstrated by incorporating Ag nanowire (NW) networks into microscale Ag grid structures. Highly conductive Ag grids are fabricated via direct imprinting of an Ag ion ink using a reservoir-assisted mold. In this mold, a macroscale cavity, called the "reservoir", is designed to connect to a grid-patterned cavity. The reservoir has a large cavity volume, which reduces unwanted residual layers within the grid spacings by introducing a thinner liquid film. The reservoir undergoes a large volume reduction during mold deformation, which improves ink filling within the grid-patterned cavity through deformation-induced ink injection. The multiscale metallic TCs show a sheet resistance (Rs) of <1.5 Ω/sq and a transmittance (T) of 86% at 550 nm, superior to the corresponding values of Ag NW networks (Rs of 15.6 Ω/sq at a similar T). We estimate the Rs-T performances of the Ag grids using geometrical calculations and demonstrate that their integration can enhance the opto-electrical properties of the Ag NW networks. Multiscale metallic TCs are successfully transferred and embedded into a transparent, flexible, and UV-curable polymer matrix. The embedded multiscale metallic TCs show reasonable electromechanical and chemical stability. The utility of these TCs is demonstrated by fabricating flexible organic solar cells.

Electrically robust silver nanowire patterns transferrable onto various substrates

We report a facile technique for patterning and transferring silver nanowires (AgNWs) onto various substrates. By employing only UV/O3 and vapor treatment of hexamethyldisilazane (HMDS), we are able to accurately manipulate the surface energy via alternating the terminal groups of a polydimethylsiloxane (PDMS) substrate, so as to assist selective formation and exfoliation of AgNW films. A simple UV/O3 treatment on PDMS enables uniform, well-defined, and highly conductive patterns of AgNWs after spin-coating. A vapor treatment of HMDS lowers the surface energy of the oxidized PDMS so that the patterned AgNWs embedded in an epoxy resin (EPR) are cleanly transferred from the PDMS to the target substrate. It is found that the AgNW-EPR composite on polyethylene glycol terephthalate (PET) exhibits remarkable durability under the bending test, tape test, ultrasonic treatment in water, and immersion of chemical solvents. In addition, we demonstrate that the AgNW-EPR composite work well as conductive patterns on the oxidized PDMS, polyvinyl alcohol (PVA), paper, and curved glass. The facile technique extends the applicability of AgNWs in the field of electronics, and it is potentially applicable to other nanomaterials.

Highly flexible transparent electrodes using a silver nanowires-embedded colorless polyimide film via chemical modification

AgNWs-embedded colorless polyimide encapsulated by a ZnO layer via chemical modification of a CPI film. The ZnO/AgNWs-embedded CPI substrate exhibited a low sheet resistance of 24 Ω sq⁻¹ while maintaining a high optical transparency of 81%.

Columnar grown copper films on polyimides strained beyond 100

Many flexible electronic devices contain metal films on polymer substrates to satisfy requirements for both electrical conductivity and mechanical durability. Despite numerous trials to date, the stretchability of metal interconnects remains an issue. In this paper, we have demonstrated a stretchable metal interconnect through control of the texture of a copper film with columnar grown grains on a polyimide (PI) substrate. The columnar grown copper films (CGC films) were deposited by regulating radio frequency (RF) sputtering powers. CGC films were able to sustain their electrical conductivity at strains above 100%. Instead of ultimate electrical discontinuity by channel crack propagation, CGC films maintained their conductivity by forming ligament structures, or a 'conductive net,' through trapped micro-cracks. XRD, AFM and in situ SEM analysis were used to investigate these stretchable conductors.

Growth Mechanism and Electrical and Magnetic Properties of Ag-Fe₃O₄ Core-Shell Nanowires

One-dimensional Ag-Fe3O4 core-shell heteronanowires have been synthesized by a facile and effective coprecipitation method, in which silver nanowires (AgNWs) were used as the nucleation site for growth of Fe3O4 in aqueous solution. The size and morphology control of the core-shell nanowires were achieved by simple adjustments of reaction conditions including FeCl3/FeCl2 concentration, poly(vinylpyrrolidone) (PVP) concentration, reaction temperature, and time. It was found that the Fe3O4 shell thickness could be tuned from 6 to 76 nm with the morphology variation between nanopheres and nanorods. A possible growth mechanism of Ag-Fe3O4 core-shell nanowires was proposed. First, the C═O derived from PVP on the surface of AgNWs provided nucleation points and in situ oxidation reaction between AgNWs and FeCl3/FeCl2 solution promoted the accumulation of Fe(3+) and Fe(2+) on the AgNWs surface. Second, Fe3O4 nanoparticles nucleated on the AgNWs surface. Lastly, Fe3O4 nanoparticles grew on the AgNWs surface by using up the reagents. Higher FeCl3/FeCl2 concentration or higher temperature led to faster nucleation and growth, resulting in the formation of Fe3O4 nanorods, whereas lower concentration or lower temperature resulted in slower nucleation and growth, leading to the formation of Fe3O4 nanospheres. Furthermore, the Ag-Fe3O4 core-shell nanowires exhibited good electrical properties and ferromagnetic properties at room temperature. Particularly, the magnetic saturation values (Ms) increased from 5.7 to 26.4 emu g(-1) with increasing Fe3O4 shell thickness from 9 to 76 nm. This growth of magnetic nanoparticles on 1D metal nanowires is meaningful from both fundamental and applied perspectives.

Nanopatterned graphene on a polymer substrate by a direct peel-off technique

A graphene (Gr) on a polyimide (PI) polymer film (Gr-PI film), obtained by a direct peel-off technique, is proposed and investigated. Thanks to its high transparency, electrical conductivity, mechanical strength, and chemical durability, the Gr-PI film is an ideal substrate for flexible electronic and optoelectronic devices, including transistors, light-emitting diodes, and plasmonic antennas. It is obtained using a straightforward method. After spin coating and curing a PI film on Gr previously grown on Cu, one can separate the Gr-PI film from the Cu foil thanks to the difference in the adhesive energy between the Gr-Cu and Gr-PI interfaces. The resulting Gr-PI film shows an average electrical sheet resistance ranging from 520 to 860 Ω/sq and a very high optical transmission (>90%), which have allowed the demonstration of a transparent heater. The surface morphology of the Gr-PI film follows that of the Cu foil, with the latter maintaining its surface properties and allowing in this way its reuse in subsequent chemical vapor deposition growth. The method can also be applied to patterned Gr, as is demonstrated for nanosize ribbons with a width of a few tens of nanometers.