Self-Healable Hydrogel–Liquid Metal Composite Platform Enabled by a 3D Printed Stamp for a Multimodular Sensor System

Hydrogels and liquid metals have been emerging as potential materials for use in self-healing electronics. This paper presents a simple fabrication procedure for a custom-designed hydrogel–liquid metal composite and its various applications. The hydrogel is patterned using three-dimensional printed molds for creating an electrical pathway, which is subsequently filled with liquid metal. The lifetime and self-healing property of the hydrogel improve drastically through coating of its surface with a moisture protectant layer and via the formation of an oxidized layer of liquid metal, respectively. Three joined units of the resulting hydrogel–liquid metal composite are successfully applied as self-healable electrodes in a customizable multimodular sensor system consisting of a photoresistor, a thermistor, and a tilt switch. The composite is also used as an electrode for biosignal (electromyogram, electrocardiogram, and electrodermal activity) detection, and its sensing ability is found to be comparable to that of a conventional Ag/AgCl electrode. The demonstrated hydrogel–liquid metal composite provides wide scope for researchers to achieve practical advances in self-healing electronics

Piezopotential-Programmed Multilevel Nonvolatile Memory As Triggered by Mechanical Stimuli

We report the development of a piezopotential-programmed nonvolatile memory array using a combination of ion gel-gated field-effect transistors (FETs) and piezoelectric nanogenerators (NGs). Piezopotentials produced from the NGs under external strains were able to replace the gate voltage inputs associated with the programming/erasing operation of the memory, which reduced the power consumption compared with conventional memory devices. Multilevel data storage in the memory device could be achieved by varying the external bending strain applied to the piezoelectric NGs. The resulting devices exhibited good memory performance, including a large programming/erasing current ratio that exceeded 10³, multilevel data storage of 2 bits (over 4 levels), performance stability over 100 cycles, and stable data retention over 3000 s. The piezopotential-programmed multilevel nonvolatile memory device described here is important for applications in data-storable electronic skin and advanced human-robot interface operations

Allrounder Strategy for Photopatterning Silver Nanowire Network Electrodes

Despite their high optical transparency and electrical conductivity, the commercialization of silver nanowire materials as transparent electrodes is challenging owing to the lack of a scalable micropatterning process. This paper proposes a versatile method for photopatterning silver nanowire networks, based on photoinduced nanowire–nanowire and nanowire–substrate cross-linking. Because the proposed method requires only a small loading of the photocross-linking agent, the intrinsic physical characteristics of the silver nanowire network can be preserved. Furthermore, through the roughness-assisted wetting phenomenon, the resulting patterns can be selectively hybridized to form bilayered nanowire/conducting polymer electrodes. The resulting hybrid transparent electrodes exhibit a low roughness, excellent tolerance to oxidation or electrochemical processes, and mechanical stability against bending without compromising the excellent optical/electrical characteristics achievable from the pristine silver nanowire network. These benefits are integrated to assemble an active-matrix-driven electrochromic display. The proposed method can thus facilitate the practical application of silver nanowire network based transparent electrodes

Petal-Inspired Diffractive Grating on a Wavy Surface: Deterministic Fabrications and Applications to Colorizations and LED Devices

Interestingly, the petals of flowering plants display unique hierarchical structures, in which surface relief gratings (SRGs) are conformably coated on a curved surface with a large radius of curvature (hereafter referred to as wavy surface). However, systematic studies on the interplay between the diffractive modes and the wavy surface have not yet been reported, due to the absence of deterministic nanofabrication methods capable of generating combinatorially diverse SRGs on a wavy surface. Here, by taking advantage of the recently developed nanofabrication composed of evaporative assembly and photofluidic holography inscription, we were able to achieve (i) combinatorially diverse petal-inspired SRGs with controlled curvatures, periodicities, and dimensionalities, and (ii) systematic optical studies of the relevant diffraction modes. Furthermore, the unique diffraction modes of the petal-inspired SRGs were found to be useful for the enhancement of the outcoupling efficiency of an organic light emitting diode (OLED). Thus, our systematic analysis of the interplay between the diffractive modes and the petal-inspired SRGs provides a basis for making more informed decisions in the design of petal-inspired diffractive grating and its applications to optoelectronics

Ultralightweight Strain-Responsive 3D Graphene Network

In this study, we fabricated a three-dimensionally assembled architecture made of reduced graphene oxide (rGO) and utilized it as an ultralightweight strain gauge. Building units for the assembly were prepared over the multiscale starting from functionalized GO nanosheets at the nanoscale to microfluidically processed solid-shelled bubbles at the microscale. These GO solid bubbles were elaborately assembled into close-packed 3D structures over the centimeter scale and then reduced by thermal treatment. Thermally reduced rGO assembly of which the internal structure was spontaneously transformed into a closed-cellular structure such as the 3D rhombic dodecahedral honeycomb lattice during thermal reduction could manifest superior elasticity against a strain of 30% by virtue of the hierarchically interconnected network while securing a low density of about 10 mg/cm3 and mechanical robustness, which was then applied as a strain gauge. The strain gauge with a thermally reduced 3D rGO structure exhibited a gauge factor of around 4 and excellent mechanical durability over 250 cycles, suggesting a new pathway for implementing ultralightweight strain-sensitive materials

Deterministic Multimodal Perturbation Enables Neuromorphic-Compatible Signal Multiplexing

Human multisensory neurons integrate multiple sensory information obtained from the external environment for precise interpretation of an event. Inspired by biological multisensory integration/multiplexing behavior, an artificial multimodal integration system capable of emulating the perception of discomfort based on the integration of multiple sensory signals is presented. The system utilizes a sensory ring oscillator that concisely and efficiently integrates thermosensory and hygrosensory signals from artificial receptors into voltage pulses whose amplitude and frequency reflect the two individual sensory signals. Subsequently, a synaptic transistor translates voltage pulses into a postsynaptic current, which exhibits a high correlation with the calculated humidex. Finally, the feasibility of the artificial multimodal integration system is successfully demonstrated using light-emitting diode discomfort indicators, suggesting that the proposed system can act as a foundation for future studies pertaining to neuromorphic perception and complex neurorobotics

Composition-Dependent Optoelectronic Properties of Mixed 2D/3D Metal Halide Perovskite Films for Light-Emitting Diodes

Low-dimensional perovskites with large organic cations have shown great potential for boosting the luminescence efficiency of metal halide perovskite light-emitting diodes (PeLEDs). Although numerous successful results have been obtained for mixed two-dimensional (2D)/three-dimensional (3D) perovskite films, the correlation of the optoelectronic properties with the crystallographic properties and film composition remains elusive. Herein, we investigated the optoelectronic quality of thin films and their impact on luminescence and transport behaviors in a mixed 2D/3D perovskite system containing 2D butylammonium lead bromide (BA2PbBr4) and 3D formamidinium lead bromide (FAPbBr3). Ultrafast transient absorption and temperature-dependent photoluminescence measurements revealed distinct changes in nonemissive decay of the excited states, including the vibrational coupling properties. These behaviors could then be closely correlated with the crystallographic evolution of the perovskite films. We rationalized the performance of PeLED devices and determined the possible limitations to further utilize the advantageous properties of mixed 2D/3D perovskite systems by examining both the luminescence and electrical properties of the perovskite films

Low-Voltage Organic Transistors with Carrier Mobilities over 10 cm²V^–2s^–1 Using Six-Branched Organic Azide

Organic thin-film transistors (OTFTs) are essential components for future flexible/wearable electronics. To fabricate OTFTs in an industrial level, following requirements should be met: high carrier mobility, low-voltage operation, compatibility with a reliable high-resolution patterning process, and high mechanical and electrical stability. Here, we report the synthesis of six-branched cross-linkers (6Bx) having an ultrahigh photo-cross-linking efficiency and its application to photo-patterning gate dielectric (GD) polymers and channel semiconducting (CS) polymers in polymer-based OTFTs. The use of 6Bx permits the generation of a high-resolution-patterned ultra-thin polymer gate dielectric with a low leakage current (7 × 10–9 A cm–2 at 1 MV cm–1). Moreover, cross-linking the GD polymer interfaced with p- or n-type CS polymer induces alignment of CS polymer chains at the interface. This yields excellent hole and electron mobilities of 12.42 and 10.11 cm2 V–1s–1, respectively, from p- and n-type OTFTs operated at <3 V, which are remarkably improved carrier mobilities at substantially low operation voltages compared to those by conventional test beds. Further, the fabrication of logic gates and ring oscillators demonstrates the reliability of polymer OTFTs cross-linked with 6Bx. This work presents a universal strategy for high mobility, reliable, and low-voltage operating OTFTs