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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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.

Etching-Assisted Crumpled Graphene Wrapped Spiky Iron Oxide Particles for High-Performance Li-Ion Hybrid Supercapacitor

From graphene oxide wrapped iron oxide particles with etching/reduction process, high-performance anode and cathode materials of lithium-ion hybrid supercapacitors are obtained in the same process with different etching conditions, which consist of partially etched crumpled graphene (CG) wrapped spiky iron oxide particles (CG@SF) for a battery-type anode, and fully etched CG for a capacitive-type cathode. The CG is formed along the shape of spikily etched particles, resulting in high specific surface area and electrical conductivity, thus the CG-based cathode exhibits remarkable capacitive performance of 210 F g^-1 and excellent rate capabilities. The CG@SF can also be ideal anode materials owing to spiky and porous morphology of the particles and tightly attached crumpled graphene onto the spiky particles, which provides structural stability and low contact resistance during repetitive lithiation/delithiation processes. The CG@SF anode shows a particularly high capacitive performance of 1420 mAh g^-1 after 270 cycles, continuously increases capacity beyond the 270th cycle, and also maintains a high capacity of 170 mAh g^-1 at extremely high speeds of 100 C. The full-cell exhibits a higher energy density up to 121 Wh kg^-1 and maintains high energy density of 60.1 Wh kg^-1 at 18.0 kW kg^-1 . This system could thus be a practical energy storage system to fill the gap between batteries and supercapacitors.

2D reentrant auxetic structures of graphene/CNT networks for omnidirectionally stretchable supercapacitors

Stretchable energy storage systems are essential for the realization of implantable and epidermal electronics. However, high-performance stretchable supercapacitors have received less attention because currently available processing techniques and material structures are too limited to overcome the trade-off relationship among electrical conductivity, ion-accessible surface area, and stretchability of electrodes. Herein, we introduce novel 2D reentrant cellular structures of porous graphene/CNT networks for omnidirectionally stretchable supercapacitor electrodes. Reentrant structures, with inwardly protruded frameworks in porous networks, were fabricated by the radial compression of vertically aligned honeycomb-like rGO/CNT networks, which were prepared by a directional crystallization method. Unlike typical porous graphene structures, the reentrant structure provided structure-assisted stretchability, such as accordion and origami structures, to otherwise unstretchable materials. The 2D reentrant structures of graphene/CNT networks maintained excellent electrical conductivities under biaxial stretching conditions and showed a slightly negative or near-zero Poisson's ratio over a wide strain range because of their structural uniqueness. For practical applications, we fabricated all-solid-state supercapacitors based on 2D auxetic structures. A radial compression process up to 1/10^th densified the electrode, significantly increasing the areal and volumetric capacitances of the electrodes. Additionally, vertically aligned graphene/CNT networks provided a plentiful surface area and induced sufficient ion transport pathways for the electrodes. Therefore, they exhibited high gravimetric and areal capacitance values of 152.4 F g^-1 and 2.9 F cm^-2, respectively, and had an excellent retention ratio of 88% under a biaxial strain of 100%. Auxetic cellular and vertically aligned structures provide a new strategy for the preparation of robust platforms for stretchable energy storage electrodes.

Flexible and Robust Thermoelectric Generators Based on All-Carbon Nanotube Yarn without Metal Electrodes

As practical interest in flexible/or wearable power-conversion devices increases, the demand for high-performance alternatives to thermoelectric (TE) generators based on brittle inorganic materials is growing. Herein, we propose a flexible and ultralight TE generator (TEG) based on carbon nanotube yarn (CNTY) with excellent TE performance. The as-prepared CNTY shows a superior electrical conductivity of 3147 S/cm due to increased longitudinal carrier mobility derived from a highly aligned structure. Our TEG is innovative in that the CNTY acts as multifunctions in the same device. The CNTY is alternatively doped into n- and p-types using polyethylenimine and FeCl₃, respectively. The highly conductive CNTY between the doped regions is used as electrodes to minimize the circuit resistance, thereby forming an all-carbon TEG without additional metal deposition. A flexible TEG based on 60 pairs of n- and p-doped CNTY shows the maximum power density of 10.85 and 697 μW/g at temperature differences of 5 and 40 K, respectively, which are the highest values among reported TEGs based on flexible materials. We believe that the strategy proposed here to improve the power density of flexible TEG by introducing highly aligned CNTY and designing a device without metal electrodes shows great potential for the flexible/or wearable power-conversion devices.

Interfacial Energy-Controlled Top Coats for Gyroid/Cylinder Phase Transitions of Polystyrene-block-polydimethylsiloxane Block Copolymer Thin Films

Block copolymers (BCPs) with a high Flory-Huggins interaction parameter (χ) can form well-defined sub-10 nm periodic structures and can be used as a template for fabrication of various functional nanostructures. However, the large difference of surface energy between the blocks commonly found in high-χ BCPs makes it challenging to stabilize a useful gyroid morphology in thin film form. Here, we used an interfacial-energy-tailored top-coat on a blended film of a polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP and a low-molecular-weight PDMS homopolymer with a hydrophilic end functional group. The top coat consisted of a random mixture of 40% hydrolyzed poly(vinyl acetate)-random-poly(vinly alcohol) (PVA-r-PVAc, PVA40) and PVAc homopolymer. At the optimized top-coat composition, gyroid nanostructures with sub-10 nm strut width were achieved down to ∼125 nm film thickness, which is only 3 times the lattice parameter of the gyroid structure. This is in marked contrast with a mixed morphology of gyroid and cylinders obtained for other compositions of the top coat. Self-consistent field theoretic simulations were used to understand the effect of the interfacial energy between the top coat and BCP/homopolymer blends on the phase transition behavior of the BCP/homopolymer films.

Highly Ordered Nanoconfinement Effect from Evaporation-Induced Self-Assembly of Block Copolymers on In Situ Polymerized PEDOT:Tos

Organic thermoelectric materials based on conducting polymers have focused on increasing electrical conductivity and optimizing thermoelectric properties via dedoping processes. To control the crystallinity and crystal alignment for enhanced electrical conductivity, a confinement geometry in nanostructures with grapho-epitaxial growth of conducting polymers during in situ polymerization could be a promising approach. We obtained highly ordered lamellar, cylindrical and disordered nanostructures from PEO-b-PPO-b-PEO block copolymer (BCP) and iron(III) tosylate (Fe(Tos)₃) oxidant blended films and solvent evaporation-induced self-assembly (EISA) processes. Then, in situ vapor phase polymerization of poly(3,4-ethylenedioxythiophene) (PEDOT):Tos on differently ordered oxidant/BCP films was performed. The effect of BCP nanostructures on the crystallinity, crystal orientation and electrical conductivity of the PEDOTs was confirmed by nanostructural and crystallographic analyses using grazing incidence small and wide-angle X-ray scattering (GISAXS and GIWAXS, respectively) experiments before and after polymerization and after a washing process. Different washing solvents also affected the electrical conductance and crystal structure. We achieved thermoelectric thermopowers up to 70 μW·m^-1·K^-2 by using an immersion dedoping process to reduce the carrier concentration and enhance the Seebeck coefficient, with little change of crystal structure.

Biaxial Stretchability and Transparency of Ag Nanowire 2D Mass-Spring Networks Prepared by Floating Compression

Networks of silver nanowires (Ag NWs) have been considered as promising materials for stretchable and transparent conductors. Despite various improvements of their optoelectronic and electromechanical properties over the past few years, Ag NW networks with a sufficient stretchability in multiple directions that is essential for the accommodation of the multidirectional strains of human movement have seldom been reported. For this paper, biaxially stretchable, transparent conductors were developed based on 2D mass-spring networks of wavy Ag NWs. Inspired by the traditional papermaking process, the 2D wavy networks were produced by floating Ag NW networks on the surface of water and subsequently applying biaxial compression to them. It was demonstrated that this floating-compression process can reduce the friction between the Ag NW-water interfaces, providing a uniform and isotropic in-plane waviness for the networks without buckling or cracking. The resulting Ag NW networks that were transferred onto elastomeric substrates successfully acted as conductors with an excellent transparency, conductivity, and electromechanical stability under a biaxial strain of 30%. The strain sensors that are based on the prepared conductors demonstrated a great potential for the enhanced performances of future wearable devices.

Nickel Nanofoam/Different Phases of Ordered Mesoporous Carbon Composite Electrodes for Superior Capacitive Energy Storage

Electrochemical energy storage devices based on electric double layer capacitors (EDLCs) have received considerable attention due to their high power density and potential for obtaining improved energy density in comparison to the lithium ion battery. Ordered mesoporous carbon (OMC) is a promising candidate for use as an EDLC electrode because it has a high specific surface area (SSA), providing a wider charge storage space and size-controllable mesopore structure with a long-range order, suppling high accessibility to the electrolyte ions. However, OMCs fabricated using conventional methods have several drawbacks including low electronic conductivity and long ionic diffusion paths in mesopores. We used nickel nanofoam, which has a relatively small pore (sub-100 nm to subμm) network structure, as a current collector. This provides a significantly shortened electronic/ionic current paths and plentiful surface area, enabling stable and close attachment of OMCs without the use of binders. Thus, we present hierarchical binder-free electrode structures based on OMC/Ni nanofoams. These structures give rise to enhanced specific capacitance and a superior rate capability. We also investigated the mesopore structural effect of OMCs on electrolyte transport by comparing the capacitive performances of collapsed lamellar, cylindrical, and spherical mesopore electrodes. The highly ordered and straightly aligned cylindrical OMCs exhibited the highest specific capacitance and the best rate capability.

Reversibly Stretchable, Optically Transparent Radio-Frequency Antennas Based on Wavy Ag Nanowire Networks

We report a facile approach for producing reversibly stretchable, optically transparent radio-frequency antennas based on wavy Ag nanowire (NW) networks. The wavy configuration of Ag NWs is obtained by floating the NW networks on the surface of water, followed by compression. Stretchable antennas are prepared by transferring the compressed NW networks onto elastomeric substrates. The resulting antennas show excellent performance under mechanical deformation due to the wavy configuration, which allows the release of stress applied to the NWs and an increase in the contact area between NWs. The antennas formed from the wavy NW networks exhibit a smaller return loss and a higher radiation efficiency when strained than the antennas formed from the straight NW networks, as well as an improved stability in cyclic deformation tests. Moreover, the wavy NW antennas require a relatively small quantity of NWs, which leads to low production costs and provides an optical transparency. These results demonstrate the potential of these wavy Ag NW antennas in applications of wireless communications for wearable systems.

Floating compression of Ag nanowire networks for effective strain release of stretchable transparent electrodes

Manipulation of the configuration of Ag nanowire (NW) networks has been pursued to enhance the performance of stretchable transparent electrodes. However, it has remained challenging due to the high Young's modulus and low yield strain of Ag NWs, which lead to their mechanical failure when subjected to structural deformation. We demonstrate that floating a Ag NW network on water and subsequent in-plane compression allows convenient development of a wavy configuration in the Ag NW network, which can release the applied strain. A greatly enhanced electromechanical stability of Ag NW networks can be achieved due to their wavy configuration, while the NW networks maintain the desirable optical and electrical properties. Moreover, the produced NW networks can be transferred to a variety of substrates, offering flexibility for device fabrication. The Ag NW networks with wavy configurations are used as compliant electrodes for dielectric elastomer actuators. The study demonstrates their promising potential to provide improved performance for soft electronic devices.

Combined epitaxial self-assembly of block copolymer lamellae on a hexagonal pre-pattern within microgrooves

The directed self-assembly (DSA) of block copolymers (BCPs) has emerged as an alternative method to replace or complement conventional photolithography as a result of the approximately 10 nm scale of microdomain ordering, the variety of microstructures that can be obtained and its compatibility with current lithographic processes. In DSA, BCP microdomains are controlled via guide patterns and two main techniques are popular: graphoepitaxy and chemoepitaxy assembly. We have demonstrated a simple and feasible technology for a DSA process by combining graphoepitaxy with "inexpensive" chemoepitaxial assembly to improve the alignment of the lamellar microdomains. For chemoepitaxial assembly, the hexagonal surface patterns from cross-linkable, cylinder-forming BCP were used to guide the graphoepitaxial assembly of the overlying BCP lamellar film. When the guiding patterns were prepared on the hexagonal patterns, it was found that the degree of lamellar alignment was significantly improved compared with the lamellar alignment on the homogeneous neutral layers. Simulation results suggested that the underlying hexagonal pattern can assist the lamellar alignment by reducing the large number of orientation states of the lamellar layers. This strategy is applicable to various nanofabrication processes that require a high degree of fidelity in controlling the nanopatterns over large areas with reduced costs.

Perpendicularly Oriented Block Copolymer Thin Films Induced by Neutral Star Copolymer Nanoparticles

By introducing neutral star copolymers consisting of poly(styrene-r-methyl methacrylate) (PS-r-PMMA) arms, a perpendicular orientation of PS-b-PMMA microdomains in thin films could be achieved without any surface treatment. The star copolymers were synthesized by arm-first method in which short chain arms are cross-linked by employing a multifunctional coupling reagent via atom transfer radical polymerization. To find the optimal neutral composition for the perpendicular orientation, we varied the composition of MMA in PS-r-PMMA arms from 40 mol % to 80 mol %. It was found that the star copolymer having an overall PS and PMMA composition of 59:41 exhibits the well-ordered perpendicular orientation of lamellar structures after thermal annealing. Furthermore, we also prepared the deuterated star copolymers to trace them within PS-b-PMMA films along vertical direction by neutron reflectivity. In this case, it was observed that star copolymers were mainly located at the top surface and bottom interface of the films, thereby effectively neutralizing the surface/interfacial energy differences.

Nitrogen-doped graphene nanosheets from bulk graphite using microwave irradiation

Using simple microwave irradiation under the presence of sodium amide as a nitrogen source, preparation of nitrogen-doped graphene nanosheets has been successfully demonstrated. It is notable that exfoliation and nitrogen doping of graphite to nitrogen-doped graphene simultaneously occurred during the microwave irradiation within a minute, and nitrogen content of the doped graphene could reach up to 8.1%. It was also found that the binding configuration of nitrogen atom on graphitic layer consisted of various nitrogen-containing moieties such as pyridine-N, pyrrolic-N, and quaternary-N, and their composition was changed as a function of irradiation power. Although formation of undoped reduced graphene oxide by microwave irradiation resulted in slight increase of electrical conductivity because of the reductive recovery of oxidized graphite to graphene, nitrogen doping involved during irradiation induced much more notable increase of electrical conductivity more than 300 S cm(-1). Furthermore, nitrogen-doped graphene showed highly enhanced capacitive performance than that of undoped reduced graphene oxide, the specific capacitance of 200 F/g (current density of 0.5 A/g), which ascribes the pseudocapacitive effect from the incorporation of nitrogen atom on graphitic layer.

Sub-10 nm Graphene Nanoribbon Array field-effect transistors fabricated by block copolymer lithography

Sub-10 nm Graphene Nanoribbon Arrays are fabricated over large areas by etching CVD-grown graphene. A mask is used made by the directed self-assembly of a cylindrical PS-b-PDMS block copolymer under solvent annealing guided by a removable template. The optimized solvent annealing process, surface-modified removable polymeric templates, and high Flory-Huggins interaction parameters of the block copolymer enable a highly aligned array of nanoribbons with low line edge roughness to be formed. This leads to a higher on/off ratio and stronger temperature dependence of the current for nanoribbon FETs, and a photocurrent which is 30 times larger compared to unpatterned graphene.

High-Aspect-Ratio Perpendicular Orientation of PS-b-PDMS Thin Films under Solvent Annealing

A perpendicular orientation of high-aspect-ratio polystyrene-block-polydimethylsiloxane (PS-b-PDMS) cylindrical and lamellar PDMS microdomains was achieved by solvent annealing and then slowly drying thick PS-b-PDMS films. Perpendicularly oriented microdomains occurred throughout the film thickness, except at the air interface, where a layer of in-plane microdomains formed due to the surface energy difference between PS and PDMS. In contrast, thermal annealing produced in-plane orientation throughout the film thickness. The solvent-annealed perpendicular orientation was observed for cylindrical morphology PS-b-PDMS of 16 and 45 kg/mol, where PDMS is the minority block, and lamellar PS-b-PDMS of 43 kg/mol. To obtain fully perpendicular microdomain patterns, a nonselective high-powered 450 W CF₄/O₂ reactive ion etching process was performed to remove the top layer of the films. Substrate patterning using electron beam lithography produced local registration of 17 nm period hexagonal cylinder patterns.

Morphology control in block copolymer films using mixed solvent vapors

Solvent vapor annealing of block copolymer thin films can produce a range of morphologies different from the equilibrium bulk morphology. By systematically varying the flow rate of two different solvent vapors (toluene and n-heptane) and an inert gas, phase maps showing the morphology versus vapor pressure of the solvents were constructed for 45 kg/mol polystyrene-block-polydimethylsiloxane diblock copolymer films of different thicknesses. The final morphology was correlated with the swelling of the block copolymer and homopolymer films and the solvent vapor annealing conditions. Self-consistent field theory is used to model the effects of solvent swelling. These results provide a framework for predicting the range of morphologies available under different solvent vapor conditions, which is important in lithographic applications where precise control of morphology and critical dimensions are essential.

Aligned sub-10-nm block copolymer patterns templated by post arrays

Self-assembly of block copolymer films can generate useful periodic nanopatterns, but the self-assembly needs to be templated to impose long-range order and to control pattern registration with other substrate features. We demonstrate here the fabrication of aligned sub-10-nm line width patterns with a controlled orientation by using lithographically formed post arrays as templates for a 16 kg/mol poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) diblock copolymer. The in-plane orientation of the block copolymer cylinders was controlled by varying the spacing and geometry of the posts, and the results were modeled using 3D self-consistent field theory. This work illustrates how arrays of narrow lines with specific in-plane orientation can be produced, and how the post height and diameter affect the self-assembly.

Assembly of sub-10-nm block copolymer patterns with mixed morphology and period using electron irradiation and solvent annealing

Block copolymer self-assembly generates patterns with periodicity in the ∼10-100 nm range and is increasingly recognized as a route to lithographic patterning beyond the resolution of photolithography. Block copolymers naturally produce periodic patterns with a morphology and length-scale determined by the molecular architecture, and considerable research has been carried out to extend the range of patterns that can be produced from a given block copolymer, but the ability to control the period of the pattern over a wide range and to achieve complex structures with mixed morphologies from a given block copolymer is limited. Here we show how patterns consisting of coexisting sub-10-nm spheres and cylinders and sphere patterns with a range of periods can be created using a combination of serial solvent anneal processes and electron-beam irradiation of selected areas of a film of poly(styrene-block-dimethylsiloxane). These techniques extend the capabilities of block copolymer lithography, enabling complex aperiodic nanoscale patterns to be formed from a single block copolymer thin film.

Highly ordered square arrays from a templated ABC triblock terpolymer

Square-symmetry patterns are of interest in nanolithography but are not easily obtained from self-assembly of a diblock copolymer. Instead, we demonstrate highly ordered 44 nm period square patterns formed in a thin film of polyisoprene-block-polystyrene-block-polyferrocenylsilane (PI-b-PS-b-PFS) triblock terpolymer blended with 15% PS homopolymer by controlling the film thickness, solvent anneal conditions, the surface chemistry and topography of the substrates. The square patterns consist of PFS pillars that remained after removal of the PI and PS with an oxygen plasma. On an unpatterned smooth substrate, the average grain size of the square pattern was increased dramatically to several micrometers by the use of brush layers and specific solvent anneal conditions. Templated self-assembly of well-ordered square patterns was demonstrated on substrates containing nanoscale topographical sidewalls and posts, written by electron beam lithography, in which the sidewalls and base of the substrate were independently chemically functionalized.

Hierarchical nanostructures by sequential self-assembly of styrene-dimethylsiloxane block copolymers of different periods

Poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) block copolymers with a period as low as 13 nm have been self-assembled on a template formed from PS-b-PDMS of a 34–40 nm period, which is itself templated by micron-scale substrate features prepared using conventional lithography. This hierarchical process provides a simple method for directing the self-assembly of sub-10 nm features and registering them on the substrate.

Placement control of nanomaterial arrays on the surface-reconstructed block copolymer thin films

We present a control strategy for the facile placement of densely packed nanomaterial arrays (i.e., nanoparticles and nanorods) on surface reconstructed polystyrene-block-poly(methyl methacrylate) thin film patterns. The surface reconstruction of perpendicularly oriented block copolymer thin films, which were produced by a treatment with selective solvent vapors for both blocks, created the topographical nanopatterns with enough height contrast for nanoparticle deposition without the need for additional selective etching of a single block domain. The deposition method of nanomaterials was also optimized, and densely packed one- and two-dimensional nanomaterials arrays in the grooves of the block copolymer film patterns were fabricated efficiently. Then, we demonstrated that height contrast of the surface reconstructed block copolymer films could be reversed by electron beam irradiation resulting in nanomaterial arrays placed at the mesa phase of the nanopatterns. On the basis of this nanomaterial placement control strategy, dual nanomaterial arrays on a single block copolymer pattern were also realized.