Hierarchically Interpenetrated and Reentrant Microcellular Frameworks for Stretchable Lithium Metal Batteries.

With the rapid development of human-friendly wearable devices, energy storage components are required to have skin-like stretchability or free-form to fit closer and more comfortably to the human body. This study introduces a hierarchically interpenetrated reentrant microcellular structure combined with 2D cellular graphene/MXene/carbon nanotubes (CNTs) and 3D cellular melamine foam. This composite structure works as a stretchable framework of lithium metal composite electrodes to provide stretchability for lithium metal electrodes, which are promising as next-generation energy storage systems. The interpenetrated but independent cellular structures successfully obtain stable structural deformability from the nonconductive and deformable melamine foam, while at the same time, high electrical conductivity, lithiophilicity, and mechanical stability of the graphene/CNT/MXene network serve as a lithium deposition support during the electrodeposition of lithium. The reentrant structure is fabricated by radial compressing the hierarchical cellular structures to take advantage of the structural stretchability of the accordion-like reentrant frameworks. The lithium-deposited composite electrodes exhibit much lower overpotential during Li stripping and plating than lithium metal foil anodes and show stable electrochemical performances under 30% of mechanical strain. The reentrant microcellular electrodes offer great potential for advanced designs of lithium metal electrodes for stretchable batteries with high energy density.

Roll-to-plate 0.1-second shear-rolling process at elevated temperature for highly aligned nanopatterns.

The shear-rolling process is a promising directed self-assembly method that can produce high-quality sub-10 nm block copolymer line-space patterns cost-effectively and straightforwardly over a large area. This study presents a high temperature (280 °C) and rapid (~0.1 s) shear-rolling process that can achieve a high degree of orientation in a single process while effectively preventing film delamination, that can be applied to large-area continuous processes. By minimizing adhesion, normal forces, and ultimate shear strain of the polydimethylsiloxane pad, shearing was successfully performed without peeling up to 280 °C at which the chain mobility significantly increases. This method can be utilized for various high-χ block copolymers and surface neutralization processes. It enables the creation of block copolymer patterns with a half-pitch as small as 8 nm in a unidirectional way. Moreover, the 0.1-second rapid shear-rolling was successfully performed on long, 3-inch width polyimide flexible films to validate its potential for the roll-to-roll process.

Molecular dynamics study of shear-induced lamellar alignment of ABA triblock copolymer thin films.

In this study, the shear-induced lamellar alignment of a thin-film ABA triblock copolymer melt was achieved via a non-equilibrium coarse-grained molecular dynamics simulation. The ABA triblock copolymer system displayed a slightly different phase behavior under different shear conditions compared to the AB diblock copolymer system. Unlike previous studies that only considered the wall velocity, the Flory-Huggins parameter was considered in our study as a factor that determines lamellar alignment. Pre-aligned lamellae and randomly mixed polymers were used as the initial states for the shear simulation to compare the shear-induced lamellar alignment on each. The two initial conditions displayed different alignment behaviors; specifically, in the pre-aligned lamellae, a tilted structure was observed when the system was not aligned in the shear direction. To explain the difference between the tilted and realigned structures, the potential energy over the simulation time, polymer dynamics from the Van Hove correlation function, and the directional order parameter were investigated. It was inferred that a tilted structure is induced by the energy barrier of realignment originating from the restricted movement of the local polymer chains. Once they cross the energy barrier, block copolymers tend to align in the shear direction to attain energy stabilization through the polymer flow.

Sequentially Coated Wavy Nanowire Composite Transparent Electrode for Stretchable Solar Cells.

Recent advances in fabricating stretchable and transparent electrodes have led to various techniques for establishing next-generation form-factor optoelectronic devices. Wavy Ag nanowire networks with large curvature radii are promising platforms as stretchable and transparent electrodes due to their high electrical conductivity and stretchability even at very high transparency. However, there are disadvantages such as intrinsic nonregular conductivity, large surface roughness, and nanowire oxidation in air. Here, we introduce electrically synergistic but mechanically independent composite electrodes by sequentially introducing conducting polymers and ionic liquids into the wavy Ag nanowire network to maintain the superior performance of the stretchable transparent electrode while ensuring overall conductivity, lower roughness, and long-term stability. In particular, plenty of ionic liquids can be incorporated into the uniformly coated conducting polymer so that the elastic modulus can be significantly lowered and sliding can occur at the nanowire interface, thereby obtaining the high mechanical stretchability of the composite electrode. Finally, as a result of applying the composite film as the stretchable transparent electrode of stretchable organic solar cells, the organic solar cell exhibits a high power conversion efficiency of 11.3% and 89% compared to the initial efficiency even at 20% tensile strain, demonstrating excellent stretching stability.

Vibroacoustic Characteristics of a Specific Patterned Polymer with Graphene for an Electrostatic Speaker.

Graphene/polymer actuators were developed using bilayer graphene and various polymer substrates for use as transparent, flexible, and robust electrostatic speaker units. Additionally, a resonant frequency shift was induced using a polymer substrate on which various micropatterns were transferred to boost bass. The total sound pressure level (SPL) in the graphene/polymer actuator was measured by a sweep, and the frequency of the spectrum was confirmed to be one-third that of the octave band frequency. The change in the vibroacoustic characteristic with changes in Young's modulus and density was studied for the polymers of the same size and thickness. Particularly, the possibility of boosting bass was confirmed by inducing a resonant frequency shift and increasing the total SPL by adding micropatterns on a polymer substrate under the same conditions. The resonance frequency of 523 Hz and the SPL of 54 dBA in flat polymer film became 296 Hz and 69 dBA in a specific pattern, which produced a sound of >15 dB based on the same flat polymer. We expect that the design and information provided herein can provide the key parameters required to change the resonant frequency in small-size devices for the application of graphene/polymer thin-film actuators.

Phase Separation-Controlled Assembly of Hierarchically Porous Aramid Nanofiber Films for High-speed Lithium-Metal Batteries.

The growth of lithium (Li) dendrites reduces the lifespan of Li-metal batteries and causes safety issues. Herein, hierarchically porous aramid nanofiber separators capable of effectively suppressing the Li dendrite growth while maintaining highly stable cycle performances at high charge/discharge rates are reported. A two-step solvent exchange process combined with reprotonation-mediated self-assembly is utilized to control the bimodal porous structure of the separators. In particular, when ethanol and water are used sequentially, aramid nanofibers form hierarchical porous structures containing nanopores in macroporous polymer frameworks to yield a mechanically robust membrane with high porosity of 97% or more. The optimized samples exhibit high ionic conductivities of 1.87-4.04 mS cm-1 and high Li-ion transference numbers of 0.77-0.84 because of the ultrahigh porosity and selective affinity to anions. Li-metal symmetric cells do not show any noticeable presence of dendrites after 100 cycles, and they operate stably for more than 1500 cycles even under extreme conditions with a high current density of >20 mA cm-2 . In addition, the LiFePO4 /Li full cell retains 86.3% of its capacity after 1000 cycles at a charge rate of 30 C.

Anisotropic Alignment of Bacterial Nanocellulose Ionogels for Unconventionally High Combination of Stiffness and Damping.

Ionogels are emerging materials for advanced electrochemical devices; however, their mechanical instability to external stresses has raised concerns about their safety. This study reports aligned bacterial nanocellulose (BC) ionogel films swelled with the model ionic liquid (IL) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) for an unprecedented combination of high stiffness and high energy dissipation without significant loss of ionic conductivity. The aligned BC ionogel films are prepared through wet-state stretching methods, followed by drying and swelling by ILs. The aligned ionogel films exhibit significantly improved dynamic mechanical properties, overcoming the mechanical conventional limit of traditional materials by 2.0 times at 25 °C and by a maximum of 4.0 times at 0 °C. Additionally, the same samples exhibit relatively high ionic conductivities of 0.16 mS cm-1 at 20 °C and 0.45 mS cm-1 at 60 °C with storage moduli over 10 GPa. The synergistic effect of the mechanical reinforcements by alignment of the BC nanofibers and the plasticizing effects by ILs could be attributed to the significant enhancement of dynamic mechanical properties and the retention of ionic conductivities. These results will lead to a deeper understanding of the material design for mechanically superior ionogel systems with increasing demands for advanced electronic and electrochemical devices.

Intrinsically Stretchable and Printable Lithium-Ion Battery for Free-Form Configuration.

For next-generation wearable and implantable devices, energy storage devices should be soft and mechanically deformable and easily printable on any substrate or active devices. Herein, we introduce a fully stretchable lithium-ion battery system for free-form configurations in which all components, including electrodes, current collectors, separators, and encapsulants, are intrinsically stretchable and printable. The stretchable electrode acquires intrinsic stretchability and improved interfacial adhesion with the active materials via a functionalized physically cross-linked organogel as a stretchable binder and separator. Intrinsically stretchable current collectors are fabricated in the form of nanocomposites consisting of a matrix with excellent barrier properties without swelling in organic electrolytes and nanostructure-controlled multimodal conductive fillers. Due to structural and materials freedoms, we successfully fabricate several types of stretchable lithium-ion battery that reliably operates under various stretch deformations with capacity and rate capability comparable with a nonstretchable battery over 2.5 mWh cm-2 at 0.5 C, even under high mass loading conditions over 10 mg cm-2, including stacked configuration, direct integration on both sides of a stretch fabric, and application of various electrode materials and electrolytes. Especially, our stretchable battery printed on a stretch fabric also exhibits high performance and stretch/long-term stabilities in the air even with wearing and pulling.

Facile Achievement of Complementary Resistive Switching in Block Copolymer Micelle-Based Resistive Memories.

Interest in resistive random access memory (RRAM) has grown rapidly in recent years for realizing ultrahigh density data storage devices. However, sneak currents in these devices can result in misreading of the data, thus limiting the applicability of RRAM. Complementary resistive switching (CRS) memory consisting of two antiserial RRAMs can considerably reduce sneak currents; however, complicated device architectures and manufacturing processes still remain as challenges. Herein, an effective and simple approach for fabricating CRS memory devices using self-assembled block copolymer micelles is reported. Cu ions are selectively placed in the core of polystyrene-block-poly(2-vinylpyridine) spherical micelles, and a hexagonally packed micelle monolayer is prepared through spin-coating. The micelle monolayer can be a symmetrical resistive switching layer, because the micelles and Cu act as dielectric and active metals in memory devices, respectively. The locally enhanced electric field and Joule heating achieved by the structured Cu atoms inside the micelles promote metal ionization and ion migration in a controlled manner, thus allowing for position selectivity during resistive switching. The micelle-based memory device exhibits stable and reliable CRS behavior, with a nonoverlapping and narrow distribution of threshold voltages. Therefore, this approach is promising for fabricating CRS memory devices for high-performance and ultrahigh-density RRAM applications.

Chiral Magneto-Optical Properties of Supra-Assembled Fe3O4 Nanoparticles.

Research on the chiral magneto-optical properties of inorganic nanomaterials has enabled novel applications in advanced optical and electronic devices. However, the corresponding chiral magneto-optical responses have only been studied under strong magnetic fields of ≥1 T, which limits the wider application of these novel materials. In this paper, we report on the enhanced chiral magneto-optical activity of supra-assembled Fe3O4 magnetite nanoparticles in the visible range at weak magnetic fields of 1.5 mT. The spherical supra-assembled particles with a diameter of ∼90 nm prepared by solvothermal synthesis had single-crystal-like structures, which resulted from the oriented attachment of nanograins. They exhibited superparamagnetic behavior even with a relatively large supraparticle diameter that exceeded the size limit for superparamagnetism. This can be attributed to the small size of nanograins with a diameter of ∼12 nm that constitute the suprastructured particles. Magnetic circular dichroism (MCD) measurements at magnetic fields of 1.5 mT showed distinct chiral magneto-optical activity from charge transfer transitions of magnetite in the visible range. For the supraparticles with lower crystallinity, the MCD peaks in the 250-550 nm range assigned as the ligand-to-metal charge transfer (LMCT) and the inter-sublattice charge transfer (ISCT) show increased intensities in comparison to those with higher crystallinity samples. On the contrary, the higher crystallinity sample shows higher MCD intensities near 600-700 nm for the intervalence charge transfer (IVCT) transition. The differences in MCD responses can be attributed to the crystallinity determined by the reaction time, lattice distortion near grain boundaries of the constituent nanocrystals, and dipolar interactions in the supra-assembled structures.

Chiral Plasmonic Nanowaves by Tilted Assembly of Unidirectionally Aligned Block Copolymers with Buckling-Induced Microwrinkles.

Chiral-structured nanoscale materials exhibit chiroptical properties with preferential absorptions of circularly polarized light. The distinctive optical responses of chiral materials have great potential for advanced optical and biomedical applications. However, the fabrication of three-dimensional structures with mirrored nanoscale geometry is still challenging. This study introduces chiral plasmonic nanopatterns in wavy shapes based on the unidirectional alignment of block copolymer thin films and their tilted transfer, combined with buckling processes. The cylindrical nanodomains of polystyrene-block-poly(2-vinylpyridine) thin films were unidirectionally aligned over a large area by the shear-rolling process. The aligned block copolymer thin films were transferred onto uniaxially prestrained polydimethylsiloxane films at certain angles relative to the stretching directions. The strain was then released to induce buckling. The aligned nanopatterns across the axis of the formed microwrinkles were selectively infiltrated with gold ions. After reduction by plasma treatment, chiral plasmonic nanowave patterns were fabricated with the presence of mirror-reflected circular dichroism spectra. This fabrication method does not require any lithography processing or innately chiral biomaterials, which can be advantageous over other conventional fabrication methods for artificial nanoscale chiral materials.

The role of graphene patterning in field-effect transistor sensors to detect the tau protein for Alzheimer's disease: Simplifying the immobilization process and improving the performance of graphene-based immunosensors.

We report the improvement in the sensing performance of electrolyte-gated graphene field-effect transistor (FET) sensors capable of detecting tau protein through a simplified, linker-free, anti-tau antibody immobilization process. For most of the graphene-based immunosensor, linkers, such as pyrenebutanoic acid, succinimidyl ester (PSE) must be used to the graphene surface, while the other side of linkers serves to capture the antibodies that can specifically interact with the target biomarker. In this study, graphene was patterned into eight different types and linker-free patterned graphene FET sensors were fabricated to verify their detection performance. The linker-free antibody immobilization to patterned graphene exhibited that the antibody was immobilized to the edge defect and had a doping-like behaviors on graphene. As the tau protein concentration in the electrolyte increased from 10 fg/ml to 1 ng/ml, the performances, charge neutral point shift and current change rate of the patterned graphene sensors without linkers were enhanced 2-3 times compared to a pristine graphene sensor with the PSE linker. Moreover, tau protein in the plasma of five Alzheimer's disease patients was measured using a linker-free patterned graphene sensor. It shows a 3-4 times higher current change rate than that of pristine graphene sensor with the PSE linker. Since the antibody is immobilized directly without a linker, a patterned graphene sensor without a linker can operate more sensitively in higher ionic concentration electrolyte.

Shear-Rolling Process for Unidirectionally and Perpendicularly Oriented Sub-10-nm Block Copolymer Patterns on the 4 in Scale.

Shear alignment of the block copolymer (BCP) thin film is one of the promising directed self-assembly (DSA) methodologies for the unidirectional alignment of sub-10 nm microdomains of BCPs for next-generation nanolithography and nanowire-grid polarizers. However, because of the differences in the surface/interfacial energies at the top surface/bottom interface, the shear-induced ordering of BCP nanopatterns has been restricted to BCPs with spherical and cylindrical nanopatterns and cannot be realized for high-aspect-ratio perpendicular lamellar structures, which is essential for practical application to semiconductor pattern processes. It is still a difficult challenge to fabricate the unidirectional alignment in a short time over a large area. In this study, we propose an approach for combining the shear-rolling process with the filtered plasma treatment of BCP films for the fabrication of unidirectionally aligned and perpendicularly oriented lamellar nanostructures. This approach enables fabrication within 1 min on a 4 in scale. We treated filtered plasma on the BCP film for perpendicular orientation and executed the hot-rolling process with different roller and stage speeds. Large-scale shear was generated only at the location where the BCP film was in contact with both the roller and stage, effectively applying shear stress to a large area of the BCP film within a short time. The repeated application of this shear-rolling process can achieve a higher level of unidirectional alignment. Our aligned BCP vertical lamellae were used to fabricate a high-aspect-ratio sub-10-nm-wide metallic nanowire array via dry/wet processes. In addition, shear-rolling with chemoepitaxy patterns can achieve higher orientational order and lower defectivity.

Flexible/Stretchable Supercapacitors with Novel Functionality for Wearable Electronics.

With the miniaturization of personal wearable electronics, considerable effort has been expended to develop high-performance flexible/stretchable energy storage devices for powering integrated active devices. Supercapacitors can fulfill this role owing to their simple structures, high power density, and cyclic stability. Moreover, a high electrochemical performance can be achieved with flexible/stretchable supercapacitors, whose applications can be expanded through the introduction of additional novel functionalities. Here, recent advances in and future prospects for flexible/stretchable supercapacitors with innate functionalities are covered, including biodegradability, self-healing, shape memory, energy harvesting, and electrochromic and temperature tolerance, which can contribute to reducing e-waste, ensuring device integrity and performance, enabling device self-charging following exposure to surrounding stimuli, displaying the charge status, and maintaining the performance under a wide range of temperatures. Finally, the challenges and perspectives of high-performance all-in-one wearable systems with integrated functional supercapacitors for future practical application are discussed.

Stretchable Lithium-Ion Battery Based on Re-entrant Micro-honeycomb Electrodes and Cross-Linked Gel Electrolyte.

Stretchable energy storage devices are of great interest because of their potential applications in body-friendly, skin-like, wearable devices. However, stretchable batteries are very challenging to fabricate. The electrodes must have a degree of stretchability because the active materials occupy most of the volume, and the separator and packaging should also be stretchable. Here, an all-component stretchable lithium-ion battery was realized by leveraging the structural stretchability of re-entrant micro-honeycomb graphene-carbon nanotube (CNT)/active material composite electrodes and a physically cross-linked gel electrolyte, without using an inactive elastomeric substrate or matrix. Active materials interconnected via the entangled CNT and graphene sheets provided a mechanically stable porous network framework, and the inwardly protruding framework in the re-entrant honeycomb structure allowed for structural stretching during deformation. The composite network consisting solely of binder-free, highly conductive materials provided superior electron transfer, and vertically aligned microchannels enabled facile ion transport. Additionally, the physically cross-linked gel electrolyte increased the mechanical stability upon deformation of the electrodes and was effective as a stretchable separator. The resulting stretchable battery showed a high areal capacity of 5.05 mAh·cm-2, superior electrochemical performance up to 50% strain under repeated (up to 500) stretch-release cycles, and long-term stability of 95.7% after 100 cycles in air conditions.

Short-Chain Polyselenosulfide Copolymers as Cathode Materials for Lithium-Sulfur Batteries.

Copolymerization of sulfur, which forms sulfur-rich polymers, has recently opened a new era in the lithium-sulfur (Li-S) battery research as improved battery performances could be achieved compared to pure sulfur (S8). By means of organic chemistry, sulfur copolymers with desired features and chemical structures could be rationally designed and synthesized. In this study, sulfur-rich polymers consisting of short-chain tetrasulfide (R-S4-R) (PTS) and selenotrisulfide (R-SeS3-R) (PTSeS) bonds are suggested as cathode materials for Li-S batteries. Intrinsically short poly(seleno)sulfide bonds along with covalent anchoring effect effectively suppress the parasitic shuttle effect originating from soluble long-chain lithium polysulfides formed from pure S8. Furthermore, a comparative study demonstrates the indisputable advantage of the selenium doping, which enhances the electrical conductivity of the polymer and following battery performances. In terms of cycling performance, both PTSeS and PTS with ∼2 mg cm-2 polymer loading exhibit small capacity decays of 0.078 and 0.052% per cycle until 500 cycles at 0.5C, respectively. However, active material utilization and high rate performance are substantially superior in PTSeS due to the enhanced electron transfer kinetics. This work would provide useful design principles for fabrication of sulfur-based polymers with even greater applicability in future Li-S batteries.

Multiple Transfer of Layer-by-Layer Nanofunctional Films by Adhesion Controls.

Transfer methods to displace active functional layers onto desired surfaces have been developed for the fabrication of nanostructured thin film devices. However, multiple transfers with highly polar surfaces were not yet fully demonstrated presumably due to difficulty in the control of the competitive adhesions at interfaces. In this study, we present adhesion-assisted multiple transfer methods for the fabrication of highly ordered nanolaminated structures with layer-by-layer (LbL) assembled films composed of various functional nanomaterials. The interfacial adhesions were controlled with adhesive layers having a thickness of only 2.5 nm for the successful transfer of the LbL nanofunctional films from the donor substrates to the receiver substrates, which was determined mainly by the major functional moieties at the contact surfaces. The root-mean-square roughness should be lower than 200 nm for conformal contact in the transfer. The versatility of the proposed method was demonstrated with various functional Au, silica, ZnO, and TiO2 nanoparticles as constituent materials and various types of substrates including Si wafer, glass, and polyethylene terephthalate surfaces. The fabricated films with periodic depositions of two different materials could exhibit photoreflective properties with high-order reflection peaks, which were simply tunable by adjusting the order in the multiple transfer. This transfer method could effectively reduce the cost and time in the nanofabrication as it did not require costly equipment, harsh synthesis conditions, and hazardous solvents.

Universal perpendicular orientation of block copolymer microdomains using a filtered plasma.

Sub-10 nm patterns prepared by directed self-assembly (DSA) of block copolymer (BCP) thin films offer a breakthrough method to overcome the limitations of photolithography. Perpendicular orientation of the BCP nanostructures is essential for lithographic applications, but dissimilar surface/interfacial energies of two blocks generally favour parallel orientations, so that the perpendicular orientation could only be obtained under very limited conditions. Here, we introduce a generalized method for creating perpendicular orientations by filtered plasma treatment of the BCP films. By cross-linking the surface of disordered BCP films using only physical collisions of neutral species without ion bombardment or UV irradiation, neutral layers consistent with the BCP volume fraction are produced that promote the perpendicular orientations. This method works with BCPs of various types, volume fractions, and molecular weights individually at the top and bottom interfaces, so it was applied to orientation-controlled 3D multilayer structures and DSA processes for sub-10 nm line-spacing patterns.

Mechanical Fatigue Resistance of Piezoelectric PVDF Polymers.

The fatigue resistance of piezoelectric PVDF has been under question in recent years. While some report that a significant degradation occurs after 106 cycles of repeated voltage input, others report that the reported degradation originates from the degraded metal electrodes instead of the piezoelectric PVDF itself. Here, we report the piezoelectric response and remnant polarization of PVDF during 107 cycles of repeated compression and tension, with silver paste-based electrodes to eliminate any electrode effect. After applying repeated tension and compression of 1.8% for 107 times, we do not observe any notable decrease in the output voltage generated by PVDF layers. The results from tension experiments show stable remnant polarization of 5.5 µC/cm², however, the remnant polarization measured after repeated compression exhibits a 7% decrease as opposed to the tensed PVDF. These results suggest a possible anisotropic response to stress direction. The phase analyses by Raman spectroscopy reveals no significant change in the phase content, demonstrating the fatigue resistance of PVDF.

Coaxial struts and microfractured structures of compressible thermoelectric foams for self-powered pressure sensors.

Long-term operation of wearable pressure sensors to detect body movement requires self-powered human-based energy sources to minimize the need for recharging. Recently, pressure sensors with thermoelectric properties based on conducting polymers have been reported; however, these devices are limited in their ability to simultaneously achieve sufficient power generation and sensitivity of the sensor. In this article, we suggest a coaxial strut structure of poly(styrene-ethylene/butylene-styrene)(SEBS)-poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)-melamine foam (MF) with a fractured microstructure for a highly sensitive, efficient self-powered pressure sensor. In the coaxial struts, the MF core provides a compressible and elastic framework; the intermediate PEDOT:PSS acts as a conductor and a thermoelectric material; and the SEBS shell ensures mechanical stability and resilience to stabilize the brittle PEDOT:PSS layer under high loading conditions. Additionally, by compressing the coaxial foam to 1/20, partial microfracture of PEDOT:PSS occurs only in the SEBS shell; thus, the pressure sensitivity increases significantly while maintaining high conductivity and thermoelectric performance. The coaxial foam was assembled into a wearable TEG to generate 338 nW from the forearms and demonstrate the high sensitivity of pressure sensors without an external power supply.