Flexible and Stretchable Smart Display: Materials,Fabrication, Device Design,and System Integration

Recent technological advances in nanomaterials have driven the development of high‐performance light‐emitting devices with flexible and stretchable form factors. Deformability in such devices is mainly achieved by replacing the rigid materials in the device components with flexible nanomaterials and their assemblies (e.g., carbon nanotubes, silver nanowires, graphene, and quantum dots) or with intrinsically soft materials and their composites (e.g., polymers and elastomers). Downscaling the dimensions of the functional materials to the nanometer range dramatically decreases their flexural rigidity, and production of polymer/elastomer composites with functional nanomaterials provides light‐emitting devices with flexibility and stretchability. Furthermore, monolithic integration of these light‐emitting devices with deformable sensors furnishes the resulting display with various smart functions such as force/capacitive touch‐based data input, personalized health monitoring, and interactive human–machine interfacing. These ultrathin, lightweight, and deformable smart optoelectronic devices have attracted widespread interest from materials scientists and device engineers. Here, a comprehensive review of recent progress concerning these flexible and stretchable smart displays is presented with a focus on materials development, fabrication techniques, and device designs. Brief overviews of an integrated system of advanced smart displays and cutting‐edge wearable sensors are also presented, and, to conclude, a discussion of the future research outlook is given.     

High‐Nanofiller‐Content Graphene Oxide–Polymer Nanocomposites via Vacuum‐Assisted Self‐Assembly

Highly ordered, homogeneous polymer nanocomposites of layered graphene oxide are prepared using a vacuum‐assisted self‐assembly (VASA) technique. In VASA, all components (nanofiller and polymer) are pre‐mixed prior to assembly under a flow, making it compatible with either hydrophilic poly(vinyl alcohol) (PVA) or hydrophobic poly(methyl methacrylate) (PMMA) for the preparation of composites with over 50 wt% filler. This process is complimentary to layer‐by‐layer assembly, where the assembling components are required to interact strongly (e.g., via Coulombic attraction). The nanosheets within the VASA‐assembled composites exhibit a high degree of order with tunable intersheet spacing, depending on the polymer content. Graphene oxide–PVA nanocomposites, prepared from water, exhibit greatly improved modulus values in comparison to films of either pure PVA or pure graphene oxide. Modulus values for graphene oxide–PMMA nanocomposites, prepared from dimethylformamide, are intermediate to those of the pure components. The differences in structure, modulus, and strength can be attributed to the gallery composition, specifically the hydrogen bonding ability of the intercalating species     

Highly Conductive Nanocomposites with Three‐Dimensional,Compactly Interconnected Graphene Networks via a Self‐Assembly Process

Polymer‐based materials with high electrical conductivity are of considerable interest because of their wide range of applications. The construction of a 3D, compactly interconnected graphene network can offer a huge increase in the electrical conductivity of polymer composites. However, it is still a great challenge to achieve desirable 3D architectures in the polymer matrix. Here, highly conductive polymer nanocomposites with 3D compactly interconnected graphene networks are obtained using a self‐assembly process. Polystyrene (PS) and ethylene vinyl acetate (EVA) are used as polymer matrixes. The obtained PS composite film with 4.8 vol% graphene shows a high electrical conductivity of 1083.3 S/m, which is superior to that of the graphene composite prepared by a solvent mixing method. The electrical conductivity of the composites is closely related to the compact contact between graphene sheets in the 3D structures and the high reduction level of graphene sheets. The obtained EVA composite films with the 3D graphene structure not only show high electrical conductivity but also exhibit high flexibility. Importantly, the method to fabricate 3D graphene structures in polymer matrix is facile, green, low‐cost, and scalable, providing a universal route for the rational design and engineering of highly conductive polymer composites.     

Inkjet Printed Large‐Area Flexible Few‐Layer Graphene Thermoelectrics

Graphene‐based organic nanocomposites have ascended as promising candidates for thermoelectric energy conversion. In order to adopt existing scalable printing methods for developing thermostable graphene‐based thermoelectric devices, optimization of both the material ink and the thermoelectric properties of the resulting films are required. Here, inkjet‐printed large‐area flexible graphene thin films with outstanding thermoelectric properties are reported. The thermal and electronic transport properties of the films reveal the so‐called phonon‐glass electron‐crystal character (i.e., electrical transport behavior akin to that of few‐layer graphene flakes with quenched thermal transport arising from the disordered nanoporous structure). As a result, the all‐graphene films show a room‐temperature thermoelectric power factor of 18.7 µW m^?1 K^?2, representing over a threefold improvement to previous solution‐processed all‐graphene structures. The demonstration of inkjet‐printed thermoelectric devices underscores the potential for future flexible, scalable, and low‐cost thermoelectric applications, such as harvesting energy from body heat in wearable applications.     

Kitchen Chemistry 101: Multigram Production of High Quality Biographene in a Blender with Edible Proteins

A high yielding aqueous phase exfoliation of graphite to high quality graphene using edible proteins and kitchen chemistry is reported here. Bovine serum albumin (BSA), β‐lactoglobulin, ovalbumin, lysozyme, and hemoglobin are used to exfoliate graphite and the exfoliation efficiency depended on the sign and magnitude of the protein charge. BSA showed maximum exfoliation rate, facilitated graphite exfoliation in water, at room temperature, by turbulence/shear force generated in a kitchen blender at exfoliation efficiencies exceeding 4 mg mL^?1 h^?1. Raman spectroscopy and transmission electron microscopy indicated 3–5 layer, defect‐free graphene of 0.5 μm size. Graphene dispersions loaded on a cellulose paper (650 μg cm^?2) showed the film conductivity of 32 000 S m^?1, which is much higher than graphene/polymer composites. Our method yielded ≈7 mg mL^?1, BSA‐coated graphene with controllable surface charge, which is stable under wide ranges of pH (3.0–11) and temperature (5.0–50 °C), and in fetal bovine serum, for more than two months.These findings may lead to the large scale production of graphene for biological applications.     

Stiff and Transparent Multilayer Thin Films Prepared Through Hydrogen‐Bonding Layer‐by‐Layer Assembly of Graphene and Polymer

Due to their exceptional orientation of 2D nanofillers, layer‐by‐layer (LbL) assembled polymer/graphene oxide thin films exhibit unmatched mechanical performance relative to any conventionally produced counterparts with similar composition. Unprecedented mechanical property improvement, by replacing graphene oxide with pristine graphene, is demonstrated in this work. Polyvinylpyrrolidone‐stabilized graphene platelets are alternately deposited with poly(acrylic acid) using hydrogen bonding assisted LbL assembly. Transmission electron microscopy imaging and the Halpin‐Tsai model are used to demonstrate, for the first time, that intact graphene can be processed from water to generate polymer nanocomposite thin films with simultaneous parallel‐alignment, high packing density, and exfoliation. A multilayer thin film with only 3.9 vol% of highly exfoliated, and structurally intact graphene, increases the elastic modulus (E) of a polymer multilayer thin film by 322% (from 1.41 to 4.81 GPa), while maintaining visible light transmittance of ≈90%. This is one of the greatest improvements in elastic modulus ever reported for a graphene‐filled polymer nanocomposite with a glassy (E > 1 GPa) matrix. The technique described here provides a powerful new tool to improve nanocomposite properties (mechanical, gas transport, etc.) that can be universally applied to a variety of polymer matrices and 2D nanoplatelets.     

Thermal Transport in Conductive Polymer–Based Materials

The demand for flexible conductive materials has motivated many recent studies on conductive polymer–based materials. However, the thermal conductivity of conductive polymers is relatively low, which may lead to serious heat dissipation problems for device applications. This review provides a summary of the fundamental principles for thermal transport in conductive polymers and their composites, and recent advancements in regulating their thermal conductivity. The thermal transport mechanisms in conductive polymer–based materials and up‐to‐date experimental approaches for measuring thermal conductivity are first summarized. Effective approaches for the regulation of thermal conductivity are then discussed. Finally, thermal‐related applications and future perspectives are given for conductive polymers and their composites.     

Single‐Crystal Hybrid Perovskite Platelets on Graphene: A Mixed‐Dimensional Van Der Waals Heterostructure with Strong Interface Coupling

Van der Waals (vdW) heterostructures open up excellent prospects in electronic and optoelectronic applications. In this work, mixed‐dimensional metal‐halide perovskite/graphene heterostructures are prepared through selective growth of CH₃NH₃PbBr₃ platelets on patterned single‐layer graphene using chemical vapor deposition. Preferred growth of single‐crystal CH₃NH₃PbBr₃ platelets on graphene surfaces is achieved, which is accompanied by significant photoluminescence quenching. Raman spectra reveal that perovskite platelets cause p‐type doping in the graphene layer. A significant Fermi level decrease of 272 meV in graphene is estimated, which corresponds to a high doping density of 7.5 × 10¹² cm^?2. Surface potentials measured by Kelvin probe force microscopy indicate a negatively charged perovskite surface under illumination, which is consistent with the upward band bending deduced from conducting atomic force microscopy measurements. Moreover, a field‐effect phototransistor is fabricated using the perovskite/graphene heterostructure channel, and the increased Dirac voltage under illumination confirms an enhanced p‐type character in graphene. These findings enrich the understanding of strong interface coupling in such mixed‐dimensional vdW heterostructures and pave the way toward novel perovskite‐based optoelectronic devices.     

Lithium–Sulfur Battery Cable Made from Ultralight,Flexible Graphene/Carbon Nanotube/Sulfur Composite Fibers

The emergence of flexible and wearable electronic devices with shape amenability and high mobility has stimulated the development of flexible power sources to bring revolutionary changes to daily lives. The conventional rechargeable batteries with fixed geometries and sizes have limited their functionalities in wearable applications. The first‐ever graphene‐based fibrous rechargeable batteries are reported in this work. Ultralight composite fibers consisting of reduced graphene oxide/carbon nanotube filled with a large amount of sulfur (rGO/CNT/S) are prepared by a facile, one‐pot wet‐spinning method. The liquid crystalline behavior of high concentration GO sheets facilitates the alignment of rGO/CNT/S composites, enabling rational assembly into flexible and conductive fibers as lithium–sulfur battery electrodes. The ultralight fiber electrodes with scalable linear densities ranging from 0.028 to 0.13 mg cm^?1 deliver a high initial capacity of 1255 mAh g^?1 and an areal capacity of 2.49 mAh cm^?2 at C /20. A shape‐conformable cable battery prototype demonstrates a stable discharge characteristic after 30 bending cycles.     

An Intrinsically Stretchable High‐Performance Polymer Semiconductor with Low Crystallinity

For wearable and implantable electronics applications, developing intrinsically stretchable polymer semiconductor is advantageous, especially in the manufacturing of large‐area and high‐density devices. A major challenge is to simultaneously achieve good electrical and mechanical properties for these semiconductor devices. While crystalline domains are generally needed to achieve high mobility, amorphous domains are necessary to impart stretchability. Recent progresses in the design of high‐performance donor–acceptor polymers that exhibit low degrees of energetic disorder, while having a high fraction of amorphous domains, appear promising for polymer semiconductors. Here, a low crystalline, i.e., near‐amorphous, indacenodithiophene‐co‐benzothiadiazole (IDTBT) polymer and a semicrystalline thieno[3,2‐b]thiophene‐diketopyrrolopyrrole (DPPTT) are compared, for mechanical properties and electrical performance under strain. It is observed that IDTBT is able to achieve both a high modulus and high fracture strain, and to preserve electrical functionality under high strain. Next, fully stretchable transistors are fabricated using the IDTBT polymer and observed mobility ≈0.6 cm² V^?1 s^?1 at 100% strain along stretching direction. In addition, the morphological evolution of the stretched IDTBT films is investigated by polarized UV–vis and grazing‐incidence X‐ray diffraction to elucidate the molecular origins of high ductility. In summary, the near‐amorphous IDTBT polymer signifies a promising direction regarding molecular design principles toward intrinsically stretchable high‐performance polymer semiconductor.     

Tunable Assembly of sp3 Cross‐Linked 3D Graphene Monoliths: A First‐Principles Prediction

One of the biggest challenges in graphene applications is how one can fabricate 3D architectures comprising graphene sheets in which the resulting architectures have inherited graphene's excellent intrinsic properties but have overcome its shortcomings. Two series of 3D graphene monoliths (GMs) using zigzag or armchair graphene nanoribbons as building blocks and sp³ carbon chains as junction nodes are constructued, and calculations based on first principles are performed in order to predict their mechanical and electronic properties. The perfect match between sp² nanoribbons and sp³ linkers results in favorable energy and mechanical/dynamic stability. Owing to their tailored motifs, wine‐rack‐like pores, and rigid sp³ linkers, these GMs possess high surface areas, appreciable mechanical strength, and tunable band gaps. Negative linear compressibilities in a wide range are found for the zigzag GMs. By solving the problems of zero gap and dimensionality of graphene sheets simultaneously, these GMs offer a viable strategy towards many applications, e.g., microelectronic devices, energy storage, molecular sieves, sensitive pressure detectors, and telecommunication line systems.     

A Facile Approach to Chemically Modified Graphene and its Polymer Nanocomposites

A scalable approach for the mass production of chemically modified graphene has yet to be developed, which holds the key to the large‐scale production of stable graphene colloids for optical electronics, energy conversion, and storage materials, catalysis, sensors, composites, etc. Here, a facile approach to fabricating covalently modified graphene and its polymer nanocomposites is presented. The method involves: i) employing a common furnace, rather than a furnace installed with a quartz tube and operated in inert gas as required in previous studies, to treat a commercial graphite intercalation compound with thermal shocking and ultrasonication and fabricate graphene platelets (GnPs) with a thickness of 2.51 ± 0.39 nm that contain only 7 at% oxygen; ii) grafting these GnPs with a commercial, long‐chain surfactant, which is able to create molecular entanglement with polymer matrixes by taking advantage of the reactions between the epoxide groups of the platelets and the end amine groups of the surfactant, to produce chemically modified graphene platelets (m‐ GnPs); and iii) solution‐mixing m‐GnPs with a commonly used polymer to fabricate nanocomposites. These m‐GnPs are well dispersed in a polymer with highly improved mechanical properties and a low percolation threshold of electrical conductivity at 0.25 vol%. This novel approach could lead to the future scalable production of graphene and its nanocomposites.     

Robust Fabrication of Nonstick,Noncorrosive, Conductive Graphene‐Coated Liquid Metal Droplets for Droplet‐Based,Floating Electrodes

Nontoxic liquid metals (conductive materials in a liquid state at room temperature) are an emerging class of materials for applications ranging from soft electronics and robotics to medical therapy and energy devices. Their sticky and corrosive properties, however, are becoming more of a critical concern for circuits and devices containing other metals as these are easily destroyed or contaminated by the liquid metals. Herein, a feasible method for fabricating highly conductive graphene‐coated liquid metal (GLM) droplets is reported and their application as nonstick, noncorrosive, movable, soft contacts for electrical circuits is demonstrated. The as‐prepared GLM droplets consist of a liquid‐phase soft core of liquid metal and a slippery outer layer of graphene sheets. These structures address the issue of simultaneous control of the wettability and conductivity of a soft electronic contact by combining extraordinary properties, i.e., nonstick, noncorrosive, yet exhibiting high electronic conductivity while in contact with metal substrates, e.g., Au, Cu, Ag, and Ni. As proof‐of‐concept, the as‐prepared GLM droplets are demonstrated as floating electrodes for movable, recyclable electronic soft contacts in electrical circuits.     

Ultrahigh Conductive Graphene Paper Based on Ball‐Milling Exfoliated Graphene

Due to low density, extremely high electrical and thermal conductivities, graphene has great potential to construct lightweight thermal conductive paper for high‐power electric devices. However, the remarkable properties of graphene are on a molecular level and difficult to achieve when processed into macroscopic paper. Here, an effective route to construct ultrahigh conductive graphene paper is developed. First, large‐volume, high‐concentration, plane‐defect‐free, few‐layer graphene dispersion is fast produced from graphite at high yield through ball milling. The exfoliated graphene dispersion is further processed into graphene paper through fast filtration, thermal treatment, and mechanical compression. The electrical and thermal conductivities of the resultant graphene paper are as high as 2231 S cm^?1 and 1529 W m^?1 K^?1, superior to previously reported graphene papers. Structural analyses confirm that the ultrahigh conductivities are attributed to high quality of graphene sheets, their compact ordered stacking, and large graphitic crystalline domain size, which improve electron and phonon transport within basal plane of graphene sheet and between graphene sheets.     

Unconventional Carbon: Alkaline Dehalogenation of Polymers Yields N‐Doped Carbon Electrode for High‐Performance Capacitive Energy Storage

Polymers are important precursors for the fabrication of carbon materials. Herein, halogenated polymers are explored as precursors for the synthesis of high‐quality carbon materials via alkaline dehalogenation. It is found that the halogen elements (F, Cl) connecting to vinylidene units are highly reactive so that dehalogenation can take place a few seconds at room temperature by simple hand grinding in the presence of strong inorganic alkaline. Furthermore, the halogen element‐leaving sites are shown to be susceptible to heteroatom doping (e.g., N doping) to become stable capacitive sites for charge storage (e.g., ions). By using a mixture of NaOEt and KOH as dehalogenation reagents, abundant hierarchical pores (macro/meso/micropores) in the resultant doped carbon matrix for fast mass transportation can be created. Very high capacitance (328 F g^?1 at 0.5 A g^?1) and rate capability (75.3% retention at 50 A g^?1 and 62.5% retention at 100 A g^?1) are observed for the newly developed halogenated polymer‐derived doped carbon materials.     

Novel Brush Polymers with Phosphorylcholine Bristle Ends: Synthesis,Structure, Properties,and Biocompatibility

N ew brush polymers with various numbers of bristle ends incorporating phosphorylcholine (PC) moieties are synthesized. The polymers are thermally stable up to 175 °C and form good‐quality films with conventional spin‐, roll‐, and dip‐coating, and subsequent drying processes. Interestingly, all these brush polymers, as a PC‐containing polymer, demonstrate a stable molecular multi‐bilayer structure in thin films that arise due to the efficient self‐assembly of the bristles for temperatures <55 °C and PC‐rich surfaces, and therefore successfully mimic natural cell‐membrane surfaces. These brush‐polymer films exhibit excellent water wettability and water sorption whilst retaining the remarkable molecular multi‐bilayer structure, and thus have hydrophilic surfaces. These novel multi‐bilayer structured films repel fibrinogen molecules and platelets from their surfaces but also have bactericidal effects on bacteria. Moreover, the brush‐polymer films are found to provide comfortable surface environments for the successful anchoring and growth of HEp‐2 cells, and to exhibit excellent biocompatibility in mice. These newly developed brush polymers are suitable for use in biomedical applications including medical devices and biosensors that require biocompatibility and the reduced possibility of post‐operative infection.     

Ultrathin,Organic, Semiconductor/Polymer Blends by Scanning Corona‐Discharge Coating for High‐Performance Organic Thin‐Film Transistors

A new thin‐film coating process, scanning corona‐discharge coating (SCDC), to fabricate ultrathin tri‐isopropylsilylethynyl pentacene (TIPS‐PEN)/amorphous‐polymer blend layers suitable for high‐performance, bottom‐gate, organic thin‐film transistors (OTFTs) is described. The method is based on utilizing the electrodynamic flow of gas molecules that are corona‐discharged at a sharp metallic tip under a high voltage and subsequently directed towards a bottom electrode. With the static movement of the bottom electrode, on which a blend solution of TIPS‐PEN and an amorphous polymer is deposited, SCDC provides an efficient route to produce uniform blend films with thicknesses of less than one hundred nanometers, in which the TIPS‐PEN and the amorphous polymer are vertically phase‐separated into a bilayered structure with a single‐crystalline nature of the TIPS‐PEN. A bottom‐gate field‐effect transistor with a blend layer of TIPS‐PEN/polystyrene (PS) (90/10 wt%) operated at ambient conditions, for example, indeed exhibits a highly reliable device performance with a field‐effect mobility of approximately 0.23 cm² V^?1 s^?1: two orders of magnitude greater than that of a spin‐coated blend film. SCDC also turns out to be applicable to other amorphous polymers, such as poly(α‐methyl styrene) and poly(methyl methacrylate) and, readily combined with the conventional transfer‐printing technique, gives rise to micropatterned arrays of TIPS‐PEN/polymer films.     

Synthesis and Applications of Stimuli‐Responsive DNA‐Based Nano‐ and Micro‐Sized Capsules

Stimuli‐responsive, drug‐loaded, DNA‐based nano‐ and micro‐capsules attract scientific interest as signal‐triggered carriers for controlled drug release. The methods to construct the nano‐/micro‐capsules involve i) the layer‐by‐layer deposition of signal‐reconfigurable DNA shells on drug‐loaded microparticles acting as templates, followed by dissolution of the core templates; ii) the assembly of three‐dimensional capsules composed of reconfigurable DNA origami units; and iii) the synthesis of stimuli‐responsive drug‐loaded capsules stabilized by DNA?polymer hydrogels. Triggers to unlock the nano‐/micro‐capsules include enzymes, pH, light, aptamer?ligand complexes, and redox agents. The capsules are loaded with fluorescent polymers, metal nanoparticles, proteins or semiconductor quantum dots as drug models, with anti‐cancer drugs, e.g., doxorubicin, or with antibodies inhibiting cellular networks or enzymes over‐expressed in cancer cells. The mechanisms for unlocking the nano‐/micro‐capsules and releasing the drugs are discussed, and the applications of the stimuli‐responsive nano‐/micro‐capsules as sense‐and‐treat systems are addressed. The scientific challenges and future perspectives of nano‐capsules and micro‐capsules in nanomedicine are highlighted.     

Seriography‐Guided Reduction of Graphene Oxide Biopapers for Wearable Sensory Electronics

Novel nacre‐mimic bio‐nanocomposites, such as graphene‐based laminates, are pushing the boundaries of strength and toughness as flexible engineering materials. Translating these material advances to functional flexible electronics requires methods for generating print‐scalable microcircuits (conductive elements surrounded by dielectric) into these strong, tough, lightweight bio‐nanocomposites. Here, a new paradigm for printing flexible electronics by employing facile, eco‐friendly seriography to confine the reduction of graphene oxide biopapers reinforced by silk interlayers is presented. Well‐defined, micropatterned regions on the biopaper are chemically reduced, generating a 10⁶ increase in conductivity (up to 10⁴ S m^?1). Flexible, robust graphene‐silk circuits are showcased in diverse applications such as resistive moisture sensors and capacitive proximity sensors. Unlike conductive (i.e., graphene‐ or Ag nanoparticle‐loaded) inks printed onto substrates, seriography‐guided reduction does not create mechanically weak interfaces between dissimilar materials and does not require the judicious formation of ink. The unimpaired functionality of printed‐in graphene‐silk microcircuits after thousands of punitive folding cycles and chemical attack by harsh solvents is demonstrated. This novel approach provides a low‐cost, portable solution for printing micrometer‐scale conductive features uniformly across large areas (>hundreds of cm²) in layered composites for applications including wearable health monitors, electronic skin, rollable antennas, and conformable displays.     

Nitrogen‐Doped Nanoporous Carbon/Graphene Nano‐Sandwiches: Synthesis and Application for Efficient Oxygen Reduction

A zeolitic‐imidazolate‐framework (ZIF) nanocrystal layer‐protected carbonization route is developed to prepare N‐doped nanoporous carbon/graphene nano‐sandwiches. The ZIF/graphene oxide/ZIF sandwich‐like structure with ultrasmall ZIF nanocrystals (i.e., ≈20 nm) fully covering the graphene oxide (GO) is prepared via a homogenous nucleation followed by a uniform deposition and confined growth process. The uniform coating of ZIF nanocrystals on the GO layer can effectively inhibit the agglomeration of GO during high‐temperature treatment (800 °C). After carbonization and acid etching, N‐doped nanoporous carbon/graphene nanosheets are formed, with a high specific surface area (1170 m² g^?1). These N‐doped nanoporous carbon/graphene nanosheets are used as the nonprecious metal electrocatalysts for oxygen reduction and exhibit a high onset potential (0.92 V vs reversible hydrogen electrode; RHE) and a large limiting current density (5.2 mA cm^?2 at 0.60 V). To further increase the oxygen reduction performance, nanoporous Co‐N_x/carbon nanosheets are also prepared by using cobalt nitrate and zinc nitrate as cometal sources, which reveal higher onset potential (0.96 V) than both commercial Pt/C (0.94 V) and N‐doped nanoporous carbon/graphene nanosheets. Such nanoporous Co‐N_x/carbon nanosheets also exhibit good performance such as high activity, stability, and methanol tolerance in acidic media.