Xiaofan Niu

Highly Flexible Silver Nanowire Electrodes for Shape‐Memory Polymer Light‐Emitting Diodes

Shape-memory polymer light-emitting diodes (PLEDs) using a new silver nanowire/polymer electrode are reported. The electrode can be stretched by up to 16% with only a small increase in sheet resistance. Large deformation shape change and recovery of the PLEDs to various bistable curvatures result in minimal loss of electroluminescence performance.

Intrinsically Stretchable Polymer Light‐Emitting Devices Using Carbon Nanotube‐Polymer Composite Electrodes

IO N Stretchable electronics are evolving from a conceptual curiosity into an important branch of modern electronics. Such devices can be potentially useful for a wide range of applications including wearable electronics, “smart skins”, and minimally invasive biomedical devices. [ 1–10 ] Conventional inorganic electronic devices are brittle and certain fl exibility may be obtained by using ultrathin layers of the inorganic materials. Buckled device confi gurations have been reported to introduce stretchability in integrated devices consisting of rigid functional components, however they lack intrinsic stretchability. [ 3 , 7 ] Here we report the fabrication of polymer light-emitting devices using single-walled carbon nanotube (SWNT)-polymer composite electrodes as both the electronand hole-injection electrodes. The devices are metal-free and can be linearly stretched up to 45% strain. This represents a proof-of-concept, highly stretchable semiconductor device wherein every part of the device is intrinsically stretchable. Stretchable devices reported so far generally employ a soft rubbery polymer to embed or bond active electronic components that are rigid. [ 1–3 ] Buckled interconnects can be made using prestrained poly(dimethylsiloxane) (PDMS) with evaporated metallic fi lms. [ 11 ] Rogers et al. also reported buckled active devices using prestrained PDMS; [ 12 , 13 ] the buckled interconnects or devices can be elongated until the vertical displacement has all been converted into planar strain, and after that the rigid components prevent further elongation. Someya et al. reported an elastic conductor formed by coating PDMS substrates with a composite consisting of carbon nanotubes, an ionic liquid salt, and a fl uorinated copolymer. [ 14–16 ] These elastic conductors have been used to wire various rigid active devices including organic light-emitting diodes (OLEDs) and sensors. [ 15 , 16 ] The interconnected devices can be made stretchable wherein the deformation occurs essentially at the elastic interconnects. The recent advancement in polymer electronics opens up new opportunities to achieve intrinsically stretchable devices. Compared with their inorganic counterparts, conjugated polymers are much more compliant. When fabricated on fl exible substrates such as polyethylene terephthlate (PET) or metal foils, highly bendable polymer devices have been demonstrated with fl exed radii as small as several millimeters. [ 17–19 ] Nonetheless,

Silver Nanowire Percolation Network Soldered with Graphene Oxide at Room Temperature and Its Application for Fully Stretchable Polymer Light-Emitting Diodes

Transparent conductive electrodes with high surface conductivity, high transmittance in the visible wavelength range, and mechanical compliance are one of the major challenges in the fabrication of stretchable optoelectronic devices. We report the preparation of a transparent conductive electrode (TCE) based on a silver nanowire (AgNW) percolation network modified with graphene oxide (GO). The monatomic thickness, mechanical flexibility, and strong bonding with AgNWs enable the GO sheets to wrap around and solder the AgNW junctions and thus dramatically reduce the inter-nanowire contact resistance without heat treatment or high force pressing. The GO-soldered AgNW network has a figure-of-merit sheet resistance of 14 ohm/sq with 88% transmittance at 550 nm. Its storage stability is improved compared to a conventional high-temperature annealed AgNW network. The GO-soldered AgNW network on polyethylene terephthalate films was processed from solutions using a drawdown machine at room temperature. When bent to 4 mm radius, its sheet resistance was increased by only 2-3% after 12,000 bending cycles. GO solder can also improve the stretchability of the AgNW network. Composite TCE fabricated by inlaying a GO-soldered AgNW network in the surface layer of polyurethane acrylate films is stretchable, by greater than 100% linear strain without losing electrical conductivity. Fully stretchable white polymer light-emitting diodes (PLEDs) were fabricated for the first time, employing the stretchable TCE as both the anode and cathode. The PLED can survive after 100 stretching cycles between 0 and 40% strain and can be stretched up to 130% linear strain at room temperature.

Elastomeric transparent capacitive sensors based on an interpenetrating composite of silver nanowires and polyurethane

Highly flexible transparent capacitive sensors have been demonstrated for the detection of deformation and pressure. The elastomeric sensors employ a pair of compliant electrodes comprising silver nanowire networks embedded in the surface layer of polyurethane matrix, and a highly compliant dielectric spacer sandwiched between the electrodes. The capacitance of the sensor sheets increases linearly with strains up to 60% during uniaxial stretching, and linearly with externally applied transverse pressure from 1 MPa down to 1 kPa. Stretchable sensor arrays consisting of 10 × 10 pixels have also been fabricated by patterning the composite electrodes into X-Y addressable passive matrix.

Compliant Silver Nanowire-Polymer Composite Electrodes for Bistable Large Strain Actuation

A new compliant electrode-based on silver nanowire-polymer composite has been developed. The composite electrode has low sheet resistance (as low as 10 Ω/sq), remains conductive (10(2) -10(3) Ω/sq) at strains as high as 140%, and can support Joule heating. The combination of the composite and a bistable electroactive polymer produces electrically-induced, large-strain actuation and relaxation, reversibly without the need of mechanical programming.

Intrinsically stretchable transparent electrodes based on silver-nanowire?crosslinked-polyacrylate composites

Stretchable transparent composites have been synthesized consisting of a silver nanowire (AgNW) network embedded in the surface layer of a crosslinked poly(acrylate) matrix. The interpenetrating networks of AgNWs and the crosslinked polymer matrix lead to high surface conductivity, high transparency, and rubbery elasticity. The presence of carboxylic acid groups on the polymer chains enhances the bonding between AgNWs and the polymer matrix, and further increases the stretchability of the composites. The sheet resistance of the composite electrode increases by only 2.3 times at 50% strain. Repeated stretching to 50% strain and relaxation only causes a small increase of the sheet resistance after 600 cycles. The morphology of the composites during reversible stretching and relaxation has been investigated to expound the conductivity changes.

A Healable, Semitransparent Silver Nanowire‐Polymer Composite Conductor

A quick recovery: A semitransparent composite conductor comprising a layer of silver nanowire percolation network inlaid in the surface layer of a Diels-Alder-based healable polymer film is fabricated. The composite is flexible and highly conductive, and is capable of both structural and electrical healing via heating. Cut samples that completely lose their conductivity can recover 97% of it within 5 minutes of heating at 110 °C. The cutting and healing can be repeated at the same location for multiple cycles.

A Solution Processed Flexible Nanocomposite Electrode with Efficient Light Extraction for Organic Light Emitting Diodes

Highly efficient organic light emitting diodes (OLEDs) based on multiple layers of vapor evaporated small molecules, indium tin oxide transparent electrode, and glass substrate have been extensively investigated and are being commercialized. The light extraction from the exciton radiative decay is limited to less than 30% due to plasmonic quenching on the metallic cathode and the waveguide in the multi-layer sandwich structure. Here we report a flexible nanocomposite electrode comprising single-walled carbon nanotubes and silver nanowires stacked and embedded in the surface of a polymer substrate. Nanoparticles of barium strontium titanate are dispersed within the substrate to enhance light extraction efficiency. Green polymer OLED (PLEDs) fabricated on the nanocomposite electrode exhibit a maximum current efficiency of 118 cd/A at 10,000 cd/m2 with the calculated external quantum efficiency being 38.9%. The efficiencies of white PLEDs are 46.7 cd/A and 30.5%, respectively. The devices can be bent to 3 mm radius repeatedly without significant loss of electroluminescent performance. The nanocomposite electrode could pave the way to high-efficiency flexible OLEDs with simplified device structure and low fabrication cost.

Bistable Large-Strain Actuation of Interpenetrating Polymer Networks

The bistable electroactive polymer is a new smart material capable of large strain, rigid-to-rigid actuation. At the rubbery state of the polymer heated to above its glass transition, stable electrically-induced actuation is obtained at strains as large as 150%. Electromechanical instability can be effectively overcome by the formation of interpenetrating polymer networks. An application as a refreshable braille display is demonstrated.

Long lifetime, fault-tolerant freestanding actuators based on a silicone dielectric elastomer and self-clearing carbon nanotube compliant electrodes

We explore the effect of pre-stretch and application of mechanical loads on a soft polydimethylsiloxane (PDMS) elastomer to obtain high linear strain freestanding dielectric elastomer actuators. It is shown that when the mechanical loads are properly applied, large linear actuation strains of 120% and work density of 0.5 J cm−3 can be obtained due to a transition from pure-biaxial to pure-uniaxial actuation conditions. Furthermore, we demonstrate that when coupled with single wall carbon nanotube (SWNT) compliant electrodes, fault-tolerance is introduced via self-clearing leading to significantly improved operational reliability. Cycling actuation tests reveal that even after more than 30 self-cleared electrical breakdown events the actuators maintain a high level of performance. Driven at moderate electric fields, the actuators display relatively high linear actuation strain (25%) without degradation of the electromechanical performance even after 85000 cycles.