Xuechang Zhou

Stretchable Conductors with Ultrahigh Tensile Strain and Stable Metallic Conductance Enabled by Prestrained Polyelectrolyte Nanoplatforms

Stretchable conductors, including interconnects, fi lms, and circuits, are key and inevitable elements in the development of stretchable electronics, which have many potential applications in robotic skins, wearable displays, and solar cells, as well as medical implants for health monitoring and disease diagnostics, point of care treatment, and biological actuation. [ 1–5 ] One critical challenge in the realization of stretchable conductors is the simultaneous incorporation of excellent mechanical compliance (strain > > 1%) and stable metallic conductivity. Raised by the nature of materials, metals usually show high electronic performance and stability, but their mechanical stretchability is very poor. On the other hand, soft materials, such as polymers and elastomers, are typically mechanical soft yet exhibit poor conductivity. To advance this emerging fi eld, two major strategies, namely “materials that stretch” and “structures that stretch”, have been suggested to address the challenge. [ 1 ] In the former, conductive materials such as metals, carbon materials, and conducting polymers are fi lled into a rubbery matrix to form elastic conductive composites. [ 6–20 ] This strategy can readily yield highly elastic conductors with tensile strain larger than 100%, yet the conductivity is typically too low to be useful. Not until recently were single-walled-carbon-nanotube-based elastomers developed as promising stretchable conductors with conductivity as high as 5 × 10 3 S cm − 1 . [ 6 , 21 ] However, the conductivity decreased dramatically to 20 S cm − 1 when this elastic conductor was stretched to 140% tensile strain. In the latter strategy, thin fi lms or ribbons of highly conductive metal or alloy are confi gured into in-plane waves or out-of-plane buckles and then bonded onto elastomeric substrates. [ 22–37 ] When tensile force is applied on the substrate, the waves or buckles are straightened to achieve large strains, and they recover when the tensile is released. Therefore, the conductive path actually subjects the material to very little stress and maintains stable metallic conductance under reversible stretching and releasing. Currently, this strategy still requires

Three-dimensional compressible and stretchable conductive composites.

Three-dimensional (3D) conductive composites with remarkable flexibility, compressibility, and stretchability are fabricated by solution deposition of thin metal coatings on chemically modified, macroscopically continuous, 3D polyurethane sponges, followed by infiltration of the metallic sponges with polydimethylsiloxane (PDMS). These low-cost conductive composites are used as high-performance interconnects for flexible and stretchable light-emitting diode (LED) arrays, even with severe surface abrasion or cutting.

Matrix‐Assisted Catalytic Printing for the Fabrication of Multiscale, Flexible, Foldable, and Stretchable Metal Conductors

Matrix-assisted catalytic printing (MACP) is developed as a low-cost and versatile printing method for the fabrication of multiscale metal conductors on a wide variety of plastic, elastomeric, and textile substrates. Highly conductive Cu interconnects (2.0 × 10⁸ S/m) fabricated by MACP at room temperature display excellent flexibility, foldability, and stretchability.

Salt-assisted direct exfoliation of graphite into high-quality, large-size, few-layer graphene sheets

We report a facile and low-cost method to directly exfoliate graphite powders into large-size, high-quality, and solution-dispersible few-layer graphene sheets. In this method, aqueous mixtures of graphite and inorganic salts such as NaCl and CuCl2 are stirred, and subsequently dried by evaporation. Finally, the mixture powders are dispersed into an orthogonal organic solvent solution of the salt by low-power and short-time ultrasonication, which exfoliates graphite into few-layer graphene sheets. We find that the as-made graphene sheets contain little oxygen, and 86% of them are 1-5 layers with lateral sizes as large as 210 μm(2). Importantly, the as-made graphene can be readily dispersed into aqueous solution in the presence of surfactant and thus is compatible with various solution-processing techniques towards graphene-based thin film devices.

3D-patterned polymer brush surfaces

Polymer brush-based three-dimensional (3D) structures are emerging as a powerful platform to engineer a surface by providing abundant spatially distributed chemical and physical properties. In this feature article, we aim to give a summary of the recent progress on the fabrication of 3D structures with polymer brushes, with a particular focus on the micro- and nanoscale. We start with a brief introduction on polymer brushes and the challenges to prepare their 3D structures. Then, we highlight the recent advances of the fabrication approaches on the basis of traditional polymerization time and grafting density strategies, and a recently developed feature density strategy. Finally, we provide some perspective outlooks on the future directions of engineering the 3D structures with polymer brushes.

Polymer Pen Lithography Using Dual‐Elastomer Tip Arrays

Dual-elastomer tip arrays are developed as a simple and cost-effective approach to significantly improve the uniformity and precision of polymer pen lithography (PPL). Both experiment and mechanical simulation demonstrate that the hard-apex, soft-base tip structure of the dual-elastomer tip array leads to precise control of feature size and reduced variation among different tips over large areas through fine control of the tip deformation. The dual-elastomer tip array is believed to be readily applied to fabricate nano- and microstructures for fundamental study and applications such as bioassays, sensors, optical and electronic devices.

Liquid metal sponges for mechanically durable, all-soft, electrical conductors

Liquid metal sponges were developed by loading liquid metals (GaInSn) into elastomer sponges. The elasticity of 3D-interconnected networks and the fluidic nature of liquid metals led to the formation of all-soft structures for electrical conductors with high electrical conductivity and mechanical flexibility.

Surface‐Grafted Polymer‐Assisted Electroless Deposition of Metals for Flexible and Stretchable Electronics

Surface-grafted polymers, that is, ultrathin layers of polymer coating covalently tethered to a surface, can serve as a particularly promising nanoplatform for electroless deposition (ELD) of metal thin films and patterned structures. Such polymers consist of a large number of well-defined binding sites for highly efficient and selective uptake of ELD catalysts. Moreover, the polymer chains provide flexible 3D network structures to trap the electrolessly deposited metal particles, leading to strong metal-substrate adhesion. In the past decade, surface-grafted polymers have been demonstrated as efficient nanoplatforms for fabricating durable and high-performance metal coatings by ELD on plastic substrates for applications in flexible and stretchable electronics. This focus review summarizes these recent advances, with a particular focus on applications in polymeric flexible and stretchable substrates. An outlook on the future challenges and opportunities in this field is given at the end of this paper.

High‐Resolution, Large‐Area, Serial Fabrication of 3D Polymer Brush Structures by Parallel Dip‐Pen Nanodisplacement Lithography

Parallel dip-pen nanodisplacement lithography (p-DNL) is used for high resolution, serial fabrication of 3D structures of polymer brushes over millimeter length scales. With p-DNL, 2D initiator templates consisting of arrays of nanolines and nanodots with rationally designed lateral spacings are fabricated in parallel via a locally tip-induced nanodisplacement process, from which highly defined 3D polymer structures are grown via surface-initiated atom transfer radical polymerization.

Polymer nanostructures made by scanning probe lithography: recent progress in material applications.

Scanning probe lithography (SPL) is a series of techniques that utilizes a scanning probe or an array of probes for surface patterning. Recent developments of new material systems and patterning approaches have made SPL a promising, low-cost, bench-top, and versatile tool for fabricating various polymer nanostructures, with extraordinary importance in physical sciences, life sciences and nanotechnology. This feature article highlights the recent progress in four material applications: polymer resists, polymeric carriers for patterning functional materials, electronically active polymers and polymer brushes for tailoring surface morphology and functionality. An overview of future possibilities, with regard to challenges and opportunities in this field, is given at the end of the paper.