Negligible Change In Resistance Research Articles

Photo‐Mediated Cascade Growth of Ag Nanocrystals in Flow Reactors for High‐Performance Flexible Transparent Electrodes

AbstractPhoto‐mediated synthesis wielding localized surface plasmon resonance (LSPR) of nanoscale metal is widely applied to sculpt silver (Ag) nanocrystals with controllable dimensions and morphologies. However, this photo‐mediated strategy remains underutilized owing to low efficiency and inferior quality in conventional batch reactors. Here, the effective synthesis of Ag nanocrystals is demonstrated via flow reactors and elucidates the photo‐mediated anisotropic growth mechanism. The investigation reveals that the intersection between excitation wavelengths and LSPR of Ag nanocrystals is a prerequisite for their growth. The final morphology is highly correlated with excitation wavelengths, which modulate the redox potential (reaction barrier) of Ag nanocrystals featuring distinct defective structures. Thus, by utilizing tandem‐connected flow reactors, size‐controllable Ag nanoplates can be yielded in a highly efficient cascade growth manner. In addition, composite conductive ink (submicron/nano‐Ag particles) is employed to create high‐performance flexible transparent electrodes. A sixfold conductivity enhancement especially under low sintering temperature (<80 °C), along with superior transmittance (over 90%), and distinguished cyclic durability (negligible resistance change over 1000 cycles) are achieved simultaneously. The study not only establishes comprehensive insights into the acquisition of well‐defined Ag nanocrystals in flow reactors but also ushers in enormous feasibility toward high‐performance and cost‐effective flexible electronics.

Highly Robust and Strain-Resilient Thin Film Conductors Featuring Brittle Materials.

Stretchable conductors with stable electrical conductivity under various deformations are essential for wearable electronics, soft robots, and biointegrated devices. However, brittle film-based conductors on elastomeric substrates often suffer from unexpected electrical disconnection due to the obvious mechanical incompatibility between stiff films and soft substrates. We proposed a novel out-of-plane crack control strategy to achieve the strain-insensitive electrical performance of thin-film-based conductors, featuring conductive brittle materials, including nanocrystalline metals (Cu, Ag, Mo) and transparent oxides (ITO). Our metal film-based conductors exhibit an ultrahigh initial conductivity (1.3 × 105 S cm-1) and negligible resistance change (R/R0 = 1.5) over wide strain range from 0 to 130%, enabled by film-induced substrate cracking and liquid metal-induced electrical self-repairing. They could function well under multimodal deformations (stretching, bending, and twisting) and severe mechanical damage (cutting and puncturing). We demonstrated the strain-resilient electrical functionality of metal film-based conductors in a flexible light-emitting diode display that shows high mechanical compliance.

Highly stable and strain-insensitive metal film conductors via manipulating strain distribution.

Metal film-based stretchable conductors are essential elements of flexible electronics for wearable, biomedical, and robotic applications, which require strain-insensitive high conductivity over a wide strain range and excellent cyclic stability. However, they suffer from serious electrical failure under monotonic and cyclic tensile loading at a small strain due to the uncontrolled film cracking behavior. Here, we propose a novel in-plane crack control strategy of engineering hierarchical microstructures to achieve outstanding electromechanical performance via harnessing the strain distribution in metal films. The wrinkles delay the crack initiation at undercuts which should be the most vulnerable sites during the stretching process. The surface protrusions/grooves/undercuts inhibit the crack propagation because of the effective strain redistribution. In addition, hierarchical microstructures significantly improve cyclic stability due to the strong interfacial adhesion and stable crack patterns. The metal film-based conductors exhibit ultrahigh strain-insensitive conductivity (1.7 × 107 S m-1), negligible resistance change (ΔR/R0 = 0.007) over an ultra-wide strain range (>200%), and excellent cyclic strain durability (>15 000 cycles at 100% strain). A range of metal films was explored to establish the universality of this strategy, including ductile copper and silver, as well as brittle molybdenum and high entropy alloy. We demonstrate the strain-insensitive electrical functionality of a metal film-based conductor in a flexible light-emitting diode circuit.

Ultrahigh Strain-Insensitive Integrated Hybrid Electronics Using Highly Stretchable Bilayer Liquid Metal Based Conductor.

Human-interfaced electronic systems require strain-resilient circuits. However, present integrated stretchable electronics easily suffer from electrical deterioration and face challenges in forming robust multilayered soft-rigid hybrid configurations. Here, a bilayer liquid-solid conductor (b-LSC) with amphiphilic properties is introduced to reliably interface with both rigid electronics and elastomeric substrates. The top liquid metal can self-solder its interface with rigid electronics at a resistance 30% lower than the traditional tin-soldered rigid interface. The bottom polar composite comprising liquid metal particles and polymers can not only reliably interface with elastomers but also help the b-LSC heal after breakage. The b-LSC can be scalably fabricated by printing and subsequent peeling strategies, showing ultra-high strain-insensitive conductivity (maximum 22532Scm-1 ), extreme stretchability (2260%), and negligible resistance change under ultra-high strain (0.34 times increase under 1000% strain). It can act as stretchable vertical interconnect access for connecting multilayered layouts and can be scalably and universally fabricated on various substrates with a resolution of ≈200µm. It is demonstrated that it can construct stretchable sensor arrays, multi-layered stretchable displays, highly integrated haptic user-interactive optoelectric E-skins, visualized heaters, robot touch sensing systems, and wireless powering for wearable electronics.

High Voltage and Low Leakage GaN-on-SiC MISHEMTs on a “Buffer-Free” Heterostructure

The performance of a novel ‘buffer-free’ AlGaN/GaN-on-SiC MISHEMTs for power switching applications is demonstrated in this letter. High voltage operation with exceptionally low gate and drain leakage currents is shown. A specific on-resistance of 3.61 m <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {\Omega } \cdot $ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and an abrupt breakdown voltage of 1622 V at a drain current of 22 nA/mm is achieved. Using two-terminal breakdown measurements, nitrogen-implanted GaN display breakdown fields of 0.96 MV/cm. The semi-insulating SiC substrate is capable of suppressing vertical leakage currents, ensuring that off-state operation is limited by lateral breakdown. The impact of electron trapping effects on dynamic on-resistance is small up to a drain quiescent voltage of at least 240 V. Drain current transient characteristics display a 14% increase in dynamic on-resistance with respect to quiescent drain bias, and a negligible change in resistance up to 100 ms. These types of ‘buffer-free’ heterostructures are of interest for power electronic applications above 1000 V and with potential for co-integration of power and RF-electronics.

Assessing the electrical property of carbon nanotube reinforced oxide ceramic matrix composites produced by ceramic injection moulding

Owing to its remarkable properties, multiwalled carbon nanotubes (MWCNTs) are attracting the interest for realization of sensors. The potential of MWCNTs for high temperature sensing applications was investigated by integration within reinforced aluminium oxide ceramic composite. MWCNTs up to 2 wt% were mixed with reinforced aluminum oxide ceramic composite using solution mixing method to ensure good homogeneilty of the oxide ceramic composite powders. Specimens were realized using ceramic injection moulding process (CIM) followed by debinding and sintering procedures. The topography of specimens were examined using atomic force microscopy (AFM). Electrical measurements were also carried out. The results show good demouldability at high MWCNTs concentration and homogenous distribution of the MWCNTs within the oxide ceramic matrix. AFM images illustrate the reduction of surface roughness by increasing the MWCNTs content which demonstrates the role of MWCNTs to improve the fracture resistance of the oxide ceramic matrix composite. As well, the electrical resistance of the feedstocks were reduced sharply. After sintering process, the resistance range drops enormously from M Ω to reach 12.1 Ω at low MWCNTs content; which make them suitable to be as electrode with high temperature capability. The electrical resistance temperature dependency shows a negative temperature coefficient behaviour with negligible resistance change.

3D Printed Spring‐Type Electronics with Liquid Metals for Highly Stretchable Conductors and Inductive Strain/Pressure Sensors

AbstractThe spring structure is stretchable and resilient, enabling electronic devices to be highly stretchable and durable. However, spring objects composed of only soft materials cannot be maintained without a supporting rod inside the coil owing to self‐weight deformation. Therefore, a design strategy should be investigated. In this study, self‐maintaining spring structures with soft materials are designed using physical simulations and soft spring‐type electronic devices (a stretchable conductor, strain sensor, and pressure sensor) with little hysteresis are developed. The spring‐type electronic devices consist of a core–sheath structure, which is a core channel filled with liquid metal and a 3D‐printed elastomeric sheath. The spring‐type conductor can be stretched with a small force (1.25 N) and has a negligible resistance change for up to 400% strain and ≈5.5% resistance increase at 640% strain. Moreover, the inductance induced by the coil structure monotonically decreases in the range of 200% stretching. The device also has high durability with little change in the electrical characteristics during 1000 cycles of 100% stretching. Furthermore, an inductive pressure sensor can measure &lt;250 Pa (50 mN) by compressing the spring. Spring‐type electronic devices composed of soft materials contribute to highly durable and stretchable sensors for robotic applications.

Evaluation of Ni-Based Flexible Resistance Temperature Detectors Fabricated by Laser Digital Pattering.

Temperature sensors are ubiquitous in every field of engineering application since temperature control is vital in operating, testing and monitoring various equipment systems. Herein, we introduce a facile and rapid laser digital patterning (LDP) process to fabricate low-cost, Ni-based flexible resistance temperature detectors (RTDs). Ni-based RTDs are directly generated on a thin flexible polyimide substrate (thickness: 50 µm) by laser-induced reductive sintering of a solution-processed nonstoichiometric nickel oxide (NiOx) nanoparticle thin film under ambient conditions. The shape of RTDs can be easily adjusted by controlling computer-aided design (CAD) data without using the physical patterning mask while the sensitivity (temperature coefficient of resistance (α) ~ 3.52 × 10−3 °C−1) of the sensors can be maintained regardless of shape and size of the sensor electrodes. The flexible Ni-based RTDs can operate over a wide temperature range up to 200 °C with excellent repeatability. Additionally, the Ni-based RTDs respond quickly to the temperature change and can operate in corrosive environments including water and seawater. Moreover, the Ni-based RTDs show a superior mechanical and electrical stability with a negligible resistance change up to a radius of curvature of 1.75 mm. Finally, a tape-pull test demonstrates the robust adhesion of Ni-based RTDs on the substrate.

Highly stretchable multilayer electronic circuits using biphasic gallium-indium.

Stretchable electronic circuits are critical for soft robots, wearable technologies and biomedical applications. Development of sophisticated stretchable circuits requires new materials with stable conductivity over large strains, and low-resistance interfaces between soft and conventional (rigid) electronic components. To address this need, we introduce biphasic Ga-In, a printable conductor with high conductivity (2.06 × 106 S m-1), extreme stretchability (>1,000%), negligible resistance change when strained, cyclic stability (consistent performance over 1,500 cycles) and a reliable interface with rigid electronics. We employ a scalable transfer-printing process to create various stretchable circuit board assemblies that maintain their performance when stretched, including a multilayer light-emitting diode display, an amplifier circuit and a signal conditioning board for wearable sensing applications. The compatibility of biphasic Ga-In with scalable manufacturing methods, robust interfaces with off-the-shelf electronic components and electrical/mechanical cyclic stability enable direct conversion of established circuit board assemblies to soft and stretchable forms.

Armadillo-inspired micro-foldable metal electrodes with a negligible resistance change under large stretchability

Inspired by the animal, armadillo, an elastic micro-foldable structure is firstly developed for constructing an Au electrode with a negligible resistance change and stability under large stretchability.

Highly air‐stabilized black phosphorus on disposable paper substrate as a tunnelling effect‐based highly sensitive piezoresistive strain sensor

AbstractThis report demonstrates synthesis of black phosphorus (BP) from red phosphorus using sonochemistry and the direct growth of highly air‐stable BP on cellulose paper using hydrothermal method with PDMS as passivation and its utilization in human motion monitoring system. PDMS not only hinders the oxidation of BP but also provides flexibility to the device, thereby allowing the use of paper‐based device as strain sensor. Fabricated device exhibits gauge factor of 6.1, which is comparable with the devices fabricated using sophisticated cleanroom techniques, and device is tested for 2,000 bending cycles revealing excellent reliability and repeatability. Device displayed excellent hysteresis suggesting that tunnelling effect is responsible for the current change of the device upon the external strain. Mathematical model is formulated to describe tunnelling conduction which matched the experimental results. Further, device exhibits excellent air stability for 98 days wherein negligible change in resistance and strain sensing performance is observed. Fabricated device is integrated onto different parts of the human body for its real‐time application in human motion monitoring. Successful demonstration of the direct growth of highly air‐stable BP on flexible and biodegradable cellulose paper opens up new avenues of research in optoelectronics, health care, security etc.

Ultrastretchable Hybrid Electrodes of Silver Nanowires and Multiwalled Carbon Nanotubes Realized by Capillary‐Force‐Induced Welding

AbstractThe paucity of soft materials with both high electrical conductivity and the capability to maintain their superior conductance under large and complex mechanical deformations is a crucial barrier hindering the development of stretchable electronics toward practical applications. A type of ultrastretchable hybrid electrodes composed of silver nanowires (AgNWs) and multiwalled carbon nanotubes (CNTs) are reported, which show sheet resistance as low as 7.1 Ω sq.−1 and a small resistance increase of only 42% when uniaxially stretched to a strain of 580%. The hybrid electrodes can also withstand equibiaxial stretching, showing a resistance increase of 100% at a strain of 200%, and survive at least 1500 stretching/releasing cycles with negligible resistance change. The key to such performance is welding the loosely stacked AgNWs and CNTs into one integrated network with the aid of capillary force. When coated onto both sides of a polyacrylate‐based elastomer, VHB, the AgNW/CNT hybrid films can serve as excellent compliant electrodes of a dielectric elastomer actuator, causing a maximum actuated areal strain of 190% with a relatively low starting electric field of 11.6 V µm−1. These ultrastretchable AgNW/CNT electrodes will pave the way for the rapid development of high‐end stretchable electronics.

Electromechanical Decoupling by Porous Aerogel Conducting Polymer

In the field of biointerface electronics, electromechanical decoupling—electrical performance independent of mechanical deformation—can ensure reliability of measured signals, which is critical in stable electrode. Herein, Chen et al. report such a decoupled and stretchable conductive polymer foam.

Enhanced flexibility and environmental durability of copper electrode produced with conductive ink containing silane coupling agents with diamine and ether spacer

Cu-based conductive inks are more cost effective than silver-based ones for flexible electronics. However, it is challenging to simultaneously ensure the mechanical flexibility, environmental oxidation durability, and low resistance of the sintered Cu electrodes. To achieve this goal, Cu-based conductive inks including silane coupling agents are developed and used to produce flexible Cu electrodes on cellulose paper after air–atmosphere sintering. The key factor to achieve the mechanical flexibility and environmental oxidation durability at the same time is the use of aminoethyl-aminopropyltrimethoxysilane (AEAPTMS) or 3-glycidoxypropyltrimethoxysilane (GPTMS). Negligible resistance change is observed in the sintered Cu electrode with AEAPTMS, even after 1000 repeated bending cycles with a bending radius of 5 mm and exposure at 60 °C/80% RH for 7 days. Especially, Cu electrodes produced using GPTMS show improved flexibility with negligible resistance change even after 10,000 bending cycles. To demonstrate the applicability of these Cu electrodes, a flexible paper-based movement sensor is successfully fabricated from the present Cu-based ink. This approach using silane coupling agents will contribute to the reliable operation of Cu-based flexible devices with high mechanical and environmental durability.

Sandwich-Structured Silver Nanowire Transparent Conductive Films with 3H Hardness and Robust Flexibility for Potential Applications in Curved Touch Screens.

A sandwich-structured bottom hard-coat/silver nanowire/top hard-coat (BHC/AgNW/THC) transparent conductive film (TCF) has been prepared by embedding the functional AgNW layer between two HC layers. The BHC/AgNW/THC TCFs show high scratch resistance with a hardness of 3H due to the enhanced adhesion to the substrate. In addition, the BHC/AgNW/THC TCFs exhibit a transmittance of 90.6% and a haze of 1% at 550 nm under a sheet resistance of 72 Ω/sq. Furthermore, highly enhanced long-term stability has been guaranteed by the HC layers due to their excellent gas barrier property. The amazing fact is that hard coating has little effect on the flexibility of AgNW films especially under extreme bending conditions and negligible resistance change could be observed after bending over thousands of times. Consequently, the greatly improved performance of BHC/AgNW/THC TCFs provided by employing hard coating layers paves the way for real-world applications of flexible AgNWs in vast areas that rigid indium tin oxide is not suitable.

PEDOT:PSS/Grafted-PDMS Electrodes for Fully Organic and Intrinsically Stretchable Skin-like Electronics.

Skin-like electronics require materials that are conducting, soft, intrinsically stretchable, and highly robust. However, electronic devices often consist of multilayers, and the failure of the electronic devices mostly starts from the debonding of the layers because of poor interfacial adhesion and large mechanical mismatch. Herein, we introduce a fully organic and intrinsically stretchable electrode that achieves high robustness by grafting the substrate to improve the interfacial adhesion and by introducing deep folds and wrinkles to improve the stretchability. The electrode exhibits a sheet resistance of 90 Ω/□ and negligible change in resistance at strains of up to 100% and shows no fatigue over 10 000 cyclic stretches to 100% strain. An iontronic skin with the electrodes is capable of detecting tiny objects exemplified by walking ants or fruit flies weighing less than 1 mg, and the device can be cyclically stretched to 30% for 1000 times without fatigue. The high robustness and stretchability of the fully organic iontronic skin lie in the wrinkle structures, small mechanical mismatch, and high interfacial strength among different layers. This work offers a general way to fabricate highly stretchable and robust devices.

Embedded PEDOT:PSS/AgNFs network flexible transparent electrode for solid-state supercapacitor

Energy storage devices that offer high energy storage capacity along with high electro-optical and mechanical performance is still a great challenge as a power source for transparent electronic gadgets. Here, we report the fabrication of an ultra-flexible, lightweight embedded PEDOT:PSS/AgNFs/NOA 63 hybrid transparent supercapacitor electrode (HTSE) film by using a peel-off transfer process technique. The HTSE film shows a good electro-optical performance, the sheet resistance of Rs ∼2.12 Ω/sq at 84.65% transmittance, along with excellent mechanical bending and flexibility characteristics. The HTSE film shows a negligible change in resistance, only a ∼1% increase in resistance is observed after 10,000 bending cycles at a bending radius of 2.0 mm. The HTSE film also exhibits good electrochemical performance with an areal capacitance of 3.64 mF/cm2 at 84.65% transmittance. The fabricated supercapacitor device also exhibits excellent mechanical flexibility, only <1.5% decrease in areal capacitance after 5000 repeated bending cycles at 2.0 mm bending radius. Owing to the high electro-optical, electrochemical and mechanical properties of the HTSE film, the fabricated solid-state supercapacitor device shows high transparency, good electrochemical performance, and long cyclic stability. The HTSE film will be a potential electrode for future transparent portable energy power source, and other flexible transparent electronic devices, an alternative to ITO.

Washable Smart Threads for Strain Sensing Fabrics

We present washable smart sensing threads for creation of smart wearable fabrics. More specifically, we report the fabrication of a facile stretchable and washable thread-based strain/force sensor, which can be sewn into wearable textiles and is capable of measuring human physiological motion. We use polybutylene terephthalate (PBT) puffy threads as our substrate which is made strain-sensitive using a simple reel-to-reel coating process. While PBT in itself is not very stretchable, threads made from them have coiled helical filaments. Stretching such threads uncoils these filaments imparting them stretchability. To convert them into strain sensor, we coat these PBT threads with resistive carbon ink. The straightening (uncoiling) of the helical filaments changes its resistance. There is a negligible change in resistance after repeated washing steps. Such washable immunity is achieved by coating thin layer of PDMS on them. The sensor exhibits a constant gauge factor of ~2.5 within its working range (50% strain) and exhibits a linear response to applied strain up to 50% of its initial length. Such strain sensing threads can form the basis of smart fabrics that can track human motion and for use in real-time physiological monitoring. We present promising results on the detection of the human form (seated versus standing) and human motion (running versus walking) using these threads sewn onto trousers.

Pressure Insensitive Strain Sensor with Facile Solution-Based Process for Tactile Sensing Applications.

Tactile sensors that can mechanically decouple, and therefore differentiate, various tactile inputs are highly important to properly mimic the sensing capabilities of human skin. Herein, we present an all-solution processable pressure insensitive strain sensor that utilizes the difference in structural change upon the application of pressure and tensile strain. Under the application of strain, microcracks occur within the multiwalled carbon nanotube (MWCNT) network, inducing a large change in resistance with gauge factor of ∼56 at 70% strain. On the other hand, under the application of pressure to as high as 140 kPa, negligible change in resistance is observed, which can be attributed to the pressure working primarily to close the pores, and hence minimally changing the MWCNT network conformation. Our sensor can easily be coated onto irregularly shaped three-dimensional objects (e.g., robotic hand) via spray coating, or be attached to human joints, to detect bending motion. Furthermore, our sensor can differentiate between shear stress and normal pressure, and the local strain can be spatially mapped without the use of patterned electrode array using electrical impedance tomography. These demonstrations make our sensor highly useful and important for the future development of high performance tactile sensors.

A Flexible Temperature Sensor Based on Reduced Graphene Oxide for Robot Skin Used in Internet of Things.

Flexible electronics, which can be distributed on any surface we need, are highly demanded in the development of Internet of Things (IoT), robot technology and electronic skins. Temperature is a fundamental physical parameter, and it is an important indicator in many applications. Therefore, a flexible temperature sensor is required. Here, we report a simple method to fabricate three lightweight, low-cost and flexible temperature sensors, whose sensitive materials are reduced graphene oxide (r-GO), single-walled carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs). By comparing linearity, sensitive and repeatability, we found that the r-GO temperature sensor had the most balanced performance. Furthermore, the r-GO temperature sensor showed good mechanical properties and it could be bent in different angles with negligible resistance change. In addition, the performance of the r-GO temperature sensor remained stable under different kinds of pressure and was unaffected by surrounding environments, like humidity or other gases, because of the insulating layer on its sensitive layer. The easy-fabricated process and economy, together with the remarkable performance of the r-GO temperature sensor, suggest that it is suitable for use as a robot skin or used in the environment of IoT.