Stretchable conductors for stretchable field-effect transistors and functional circuits

Toward an AI Era: Advances in Electronic Skins

Electronic skins (e-skins) have seen intense research and rapid development in the past two decades. To mimic the capabilities of human skin, a multitude of flexible/stretchable sensors that detect physiological and environmental signals have been designed and integrated into functional systems. Recently, researchers have increasingly deployed machine learning and other artificial intelligence (AI) technologies to mimic the human neural system for the processing and analysis of sensory data collected by e-skins. Integrating AI has the potential to enable advanced applications in robotics, healthcare, and human-machine interfaces but also presents challenges such as data diversity and AI model robustness. In this review, we first summarize the functions and features of e-skins, followed by feature extraction of sensory data and different AI models. Next, we discuss the utilization of AI in the design of e-skin sensors and address the key topic of AI implementation in data processing and analysis of e-skins to accomplish a range of different tasks. Subsequently, we explore hardware-layer in-skin intelligence before concluding with an analysis of the challenges and opportunities in the various aspects of AI-enabled e-skins.

Molecular "backbone surgery" of electron-deficient heteroarenes based on dithienopyrrolobenzothiadiazole: conformation-dependent crystal structures and charge transport properties

Electron-deficient heteroarenes based on dithienopyrrolobenzothiadiazole (BTP) have been highly attractive due to their fascinating packing structures, broad absorption profiles, and promising applications in non-fullerene organic solar cells. However, the control of their crystal structures for superior charge transport still faces big challenges. Herein, a conformation engineering strategy is proposed to rationally manipulate the single crystal structure of BTP-series heteroarenes. The parent molecule BTPO-c has a 3D network crystal structure, which originates from its banana-shaped conformation. Subtracting one thiophene moiety from the central backbone leads to a looser brickwork crystal structure of the derivative BTPO-z because of its interrupted angular-shaped conformation. Further subtracting two thiophene moieties results in the derivative BTPO-l with a compact 2D-brickwork crystal structure due to its quasi-linear conformation with a unique dimer packing structure and short π-π stacking distance (3.30 Å). Further investigation of charge-transport properties via single-crystal organic transistors demonstrates that the compact 2D-brickwork crystal structure of BTPO-l leads to an excellent electron mobility of 3.5 cm2 V-1 s-1, much higher than that of BTPO-c with a 3D network (1.9 cm2 V-1 s-1) and BTPO-z with a looser brickwork structure (0.6 cm2 V-1 s-1). Notably, this study presents, for the first time, an elegant demonstration of the tunable single crystal structures of electron-deficient heteroarenes for efficient organic electronics.

Silver Nanowires-Based Flexible Gold Electrode Overcoming Interior Impedance of Nanomaterial Electrodes

In the development of nanomaterial electrodes for improved electrocatalytic activity, much attention is paid to the compositions, lattice, and surface morphologies. In this study, a new concept to enhance electrocatalytic activity is proposed by reducing impedance inside nanomaterial electrodes. Gold nanodendrites (AuNDs) are grown along silver nanowires (AgNWs) on flexible polydimethylsiloxane (PDMS) support. The AuNDs/AgNWs/PDMS electrode affords an oxidative peak current density of 50 mA cm-2 for ethanol electrooxidation, a value ≈20 times higher than those in the literature do. Electrochemical impedance spectroscopy (EIS) demonstrates the significant contribution of the AgNWs to reduce impedance. The peak current densities for ethanol electrooxidation are decreased 7.5-fold when the AgNWs are electrolytically corroded. By in situ surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) simulation, it is validated that the ethanol electrooxidation favors the production of acetic acid with undetectable CO, resulting in a more complete oxidation and long-term stability, while the AgNWs corrosion greatly decreases acetic acid production. This novel strategy for fabricating nanomaterial electrodes using AgNWs as a charge transfer conduit may stimulate insights into the design of nanomaterial electrodes.

Hydrothermal Synthesis and Characterization of WSe2 Nanosheets: A Promising Approach for Wearable Photodetector Applications

The two-dimensional (2D) WSe2 nanostructure was successfully synthesized via the hydrothermal method and subjected to comprehensive characterization using various spectroscopic techniques. X-ray diffraction (XRD) analysis confirmed the formation of nanosheets with a hexagonal crystal structure having a space symmetry of P63/mmc. Scanning electron microscopy (SEM) images showed irregular and nonuniform morphology. The size of the 2D nanosheets was determined using transmission electron microscopy (TEM) providing insights intotheir physical characteristics. The optical spectrum analysis yielded a discernible band gap value of 2.1 eV, as determined by the Tauc equation. Photoluminescence (PL) spectra display an emission at a wavelength of 610 nm, showing a broad emission associated with self-trapped excitons. Under excitation at λexc = 360 nm, PL emission spectra displayed a distinct peak at 610 nm, demonstrating the ability of the nanostructure to emit vivid red light. Photometric analysis underscored the potential of this nanostructure as a prominent red-light source for diverse display applications. The optimized photodetection performance of a device showcases a photoresponsivity of approximately 1.25 × 10-3 AW-1 and a detectivity of around 5.19 × 108 Jones at a wavelength of 390 nm. Additionally, the quantum efficiency is reported to be approximately 6.99 × 10-3 at a wavelength of 635 nm. These findings highlight the capability of the device for efficient photoconversion at specified wavelengths, indicating potential applications in sensing, imaging, and optical communication. The combination of structural, morphological, and optical characterizations highlights the suitability of 2D WSe2 nanostructure for practical optoelectronic applications, particularly in display technologies.

Stretchable Electronics with Strain-Resistive Performance

Stretchable electronics have attracted tremendous attention amongst academic and industrial communities due to their prospective applications in personal healthcare, human-activity monitoring, artificial skins, wearable displays, human-machine interfaces, etc. Other than mechanical robustness, stable performances under complex strains in these devices that are not for strain sensing are equally important for practical applications. Here, a comprehensive summarization of recent advances in stretchable electronics with strain-resistive performance is presented. First, detailed overviews of intrinsically strain-resistive stretchable materials, including conductors, semiconductors, and insulators, are given. Then, systematic representations of advanced structures, including helical, serpentine, meshy, wrinkled, and kirigami-based structures, for strain-resistive performance are summarized. Next, stretchable arrays and circuits with strain-resistive performance, that integrate multiple functionalities and enable complex behaviors, are introduced. This review presents a detailed overview of recent progress in stretchable electronics with strain-resistive performances and provides a guideline for the future development of stretchable electronics.

Room-Temperature Liquid Metals for Flexible Electronic Devices

Room-temperature gallium-based liquid metals (RT-GaLMs) have garnered significant interest recently owing to their extraordinary combination of fluidity, conductivity, stretchability, self-healing performance, and biocompatibility. They are ideal materials for the manufacture of flexible electronics. By changing the composition and oxidation of RT-GaLMs, physicochemical characteristics of the liquid metal can be adjusted, especially the regulation of rheological, wetting, and adhesion properties. This review highlights the advancements in the liquid metals used in flexible electronics. Meanwhile related characteristics of RT-GaLMs and underlying principles governing their processing and applications for flexible electronics are elucidated. Finally, the diverse applications of RT-GaLMs in self-healing circuits, flexible sensors, energy harvesting devices, and epidermal electronics, are explored. Additionally, the challenges hindering the progress of RT-GaLMs are discussed, while proposing future research directions and potential applications in this emerging field. By presenting a concise and critical analysis, this paper contributes to the advancement of RT-GaLMs as an advanced material applicable for the new generation of flexible electronics.

Molecular Design of Multifunctional Integrated Polymer Semiconductors with Intrinsic Stretchability, High Mobility, and Intense Luminescence

Multifunctional semiconductors integrating unique optical, electrical, mechanical, and chemical characteristics are critical to advanced and emerging manufacturing technologies. However, due to the trade-off challenges in design principles, fabrication difficulty, defects in existing materials, etc., realizing multiple functions through multistage manufacturing is quite taxing. Here, an effective molecular design strategy is established to prepare a class of multifunctional integrated polymer semiconductors. The pyridal[1,2,3]triazole-thiophene co-structured tetrapolymers with full-backbone coplanarity and considerable inter/intramolecular noncovalent interactions facilitate short-range order and excellent (re)organization capability of polymer chains, providing stress-dissipation sites in the film state. The regioregular multicomponent conjugated backbones contribute to dense packing, excellent crystallinity, high crack onset strain over 100%, efficient carrier transport with mobilities exceeding 1 cm2  V-1  s-1 , and controllable near-infrared luminescence. Furthermore, a homologous blending strategy is proposed to further enhance the color-tunable luminescent properties of polymers while effectively retaining mechanical and electrical properties. The blended system exhibits excellent field-effect mobility (µ) and quantum yield (Φ), reaching a record Φ · µ of 0.43 cm2  V-1  s-1 . Overall, the proposed strategy facilitates a rational design of regioregular semicrystalline intrinsically stretchable polymers with high mobility and color-tunable intense luminescence, providing unique possibilities for the development of multifunctional integrated semiconductors in organic optoelectronics.

EVA and SEBS-MA copolymers incorporated silicone rubber/SEBS blends: improvement of mechanical and thermal properties

Enhancing the mechanical and thermal properties of Silicone rubber (SR)/SEBS blends using various compatibilizers opens the opportunity for such new blends to meet the market desire. For this purpose, blends with a 1:1 ratio of SR and SEBS are prepared with different amounts of EVA or SEBS-MA copolymers as compatibilizer. Mechanical properties of the blend are enhanced by adding EVA and SEBS-MA. Addition of 6 phr EVA profoundly improves the tensile strength from 7.70 to 10.06 MPa. Thermogravimetric analysis reveals that the presence of compatibilizer can improve the thermal stability of the blend, especially its initial degradation temperature (T5%). T5% of the blend increases from 376 to 390 °C when comprising 6 phr SEBS-MA. Morphology of the blends is investigated using SEM and AFM. Results of the relaxation modulus curves obtained by rubber process analyzer (RPA) demonstrate that the amount of relaxation in the uncured blends is higher than the cured ones. A comparison of the relaxation of the blends indicates that the relaxation modulus of the SEBS-MA compatibilized blends is enhanced more than other blends after curing. Further investigations indicate that the compatibilized blends exhibit higher tear energy and lower compression set.

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.

An In-Situ Cyanidation Strategy To Access Tetracyanodiacenaphthoanthracene Diimides with High Electron Mobilities Exceeding 10 cm2 V-1 s-1

A family of novel highly π-extended tetracyano-substituted acene diimides, named as tetracyanodiacenaphthoanthracene diimides (TCDADIs), have been synthesized using a facile four-fold Knoevenagel condensation strategy. Unlike conventional cyano substitution reactions, our approach enables access to a large π-conjugated backbone with the in-situ formation of four cyano substitutents at room temperature while avoiding extra cyano-functionalization reactions. TCDADIs decorated with different N-alkyl substituents present good solubility, near-coplanar backbones, good crystallinity, and low-lying lowest unoccupied molecular orbital energies of -4.33 eV, all of which contribute to desirable electron-transport performance when applied in organic field-effect transistors (OFET). The highest electron mobility of an OFET based on a 2-hexyldecyl-substituted TCDADI single crystal reaches 12.6 cm2  V-1  s-1 , which is not only among the highest values for the reported n-type organic semiconductor materials (OSMs) but also exceeds that of most n-type OSMs decorated with imide units.