van der Waals Epitaxy of Organic Semiconductor Thin Films on Atomically Thin Graphene Templates for Optoelectronic Applications.

ConspectusOrganic semiconductors (OSCs) offer unique advantages with respect to mechanical flexibility, low-cost processing, and tunable properties. The optical and electrical properties of devices based on OSCs can be greatly improved when an OSC is coupled with graphene in a certain manner. Our research group has focused on using graphene as a growth template for OSCs and incorporating such high-quality heterostructures into optoelectronic devices. The idea is that graphene's atomically flat surface with a uniform sp2 carbon network can serve as a perfect quasi-epitaxial template for the growth of OSCs. In addition, OSC-graphene heterostructures benefit from graphene's unique characteristics, such as its high charge-carrier mobility, excellent optical transparency, and fascinating mechanical durability and flexibility.However, we have often found that OSC molecules assemble on graphene in unpredictable manners that vary from batch to batch. From observations of numerous research systems, we elucidated the mechanism underlying such poor repeatability and set out a framework to actually control the template effect of graphene on OSCs. In this Account, we not only present our scientific findings in this spectrum of areas but also convey our research scheme to the readers so that similar heterostructure complexes can be systematically studied.We began with experiments showing that the growth of OSCs on a graphene surface was driven by van der Waals interactions and is therefore sensitive to the cleanliness of the graphene surface. Nonetheless, we noted that, even on similarly clean graphene surfaces, the OSC thin film still varied with the underlying substrate. Thanks to the graphene-transfer method and in situ gating methods that we developed, we discovered that the decisive parameter for molecule-graphene interaction (and, hence, for the growth of OSCs on graphene) is the charge density in the graphene. Thus, to prepare a graphene template for high-quality graphene-OSC heterostructures, we controlled the charge density in the graphene to minimize the molecule-graphene interaction. Moreover, the possible charge transfer between OSC molecules and graphene, which induces additional molecule-graphene interactions, should also be taken into account. Eventually, we demonstrated a wide range of optoelectronic applications that benefitted from high-quality OSC-graphene heterostructures fabricated using our proof-of-concept systems.

Hexaazatriphenylene-Based Two-Dimensional Conductive Covalent Organic Framework with Anisotropic Charge Transfer.

The development of covalent organic frameworks (COFs) with efficient charge transport is of immense interest for applications in optoelectronic devices. To enhance COF charge transport properties, electroactive building blocks and dopants can be used to induce extended conduction channels. However, understanding their intricate interplay remains challenging. We designed and synthesized a tailor-made COF structure with electroactive hexaazatriphenylene (HAT) core units and planar dioxin (D) linkages, denoted as HD-COF. With the support of theoretical calculations, we found that the HAT units in the HD-COF induce strong, eclipsed π-π stacking. The unique stacking of HAT units and the weak in-plane conjugation of dioxin linkages leads to efficient anisotropic charge transport. We fabricated HD-COF films to minimize the grain boundary effect of bulk COFs, which resulted in enhanced conductivity. As a result, the HD-COF films showed an electrical conductivity as high as 1.25 S cm-1 after doping with tris(4-bromophenyl)ammoniumyl hexachloroantimonate.

Crocodile-Skin-Inspired Omnidirectionally Stretchable Pressure Sensor.

Stretchable pressure sensors are important components of multimodal electronic skin needed for potentializing numerous Internet of Things applications. In particular, to use pressure sensors in various wearable/skin-attachable electronics, both high deformability and strain-independent sensitivity must be realized. However, previously reported stretchable pressure sensors cannot meet these standards because they exhibit limited stretchability and nonuniform sensitivity under deformation. Herein, inspired by the unique sensory organ of a crocodile, an omnidirectionally stretchable piezoresistive pressure sensor made of polydimethylsiloxane (PDMS)/silver nanowires (AgNWs) composites with microdomes and wrinkled surfaces is developed. The stretchable pressure sensor exhibits high sensitivity that changes negligibly even under uniaxial and biaxial tensile strains of 100% and 50%, respectively. This behavior is attributed to the microdomes responsible for detecting applied pressures being weakly affected by tensile strains, while the isotropic wrinkles between the microdomes deform to effectively reduce the external stress. In addition, because the device comprises all PDMS-based structures, it exhibits outstanding robustness under repeated mechanical stimuli. The device shows strong potential as a wearable pressure sensor and an artificial crocodile sensing organ, successfully detecting applied pressures in various scenarios. Therefore, the pressure sensor is expected to find applications in electronic skin for prosthetics and human-machine interface systems.

π-Extended Thiazole-Containing Polymer Semiconductor for Balanced Charge-Carrier Mobilities.

A low-band gap semiconducting polymer with an acceptor-donor-acceptor architecture is newly designed and synthesized by incorporating a π-extended thiazole-vinylene-thiazole unit. The resulting thiazole-containing diketopyrrolopyrrole copolymer exhibits well-balanced ambipolar characteristics with hole mobility of up to 0.11 cm2 V-1 s-1 and electron mobility of up to 0.30 cm2 V-1 s-1 , which are suitable for applications in polymer electronics.

Electric-Field-Tunable Growth of Organic Semiconductor Crystals on Graphene.

Growth of organic semiconductor thin films on a two-dimensional template is affected by its properties and is not well understood. This growth process dictates a thin film's final morphology and crystal structure and is controlled by the interactions between ad-molecules and the template. Here, we report that the template's charge density determines the tuning of such interactions. We observe the dependence of pentacene nucleation on charge carrier density ng in graphene under an applied electric field and contact-doping and then deduce that the interaction energy EA between the ad-molecule and the graphene is related linearly to ng. This tunability of EA allows control of the pentacene crystals growth. We exploit these findings to demonstrate that graphene, in which ng is controlled, can be used to template pentacene thin films for improved optoelectronic properties, such as electrical conductivity and exciton diffusion length.

Ternary Blend Strategy for Achieving High-Efficiency Organic Photovoltaic Devices for Indoor Applications.

Monomeric perylene diimide (PDI) small molecules display a high absorption coefficient and crystallinity in solid-state thin films due to strong π-π interactions between the molecules. To take advantage of these exciting properties of PDIs, N,N'-bis(1-ethylpropyl)perylene-3,4,9,10-tetracarboxylic diimide (EP-PDI) was mixed with a binary blend of PTB7 and PC71 BM to fabricate an efficient ternary blend, which were in turn used to produce organic photovoltaic (OPV) devices well suited to indoor applications (PTB7=poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}), PC71 BM=[6,6]-phenyl-C71 -butyric acid methyl ester). We varied the PC71 BM/EP-PDI weight ratio to investigate the influence of EP-PDI on the optical, electrical, and morphological properties of the PTB7:PC71 BM:EP-PDI ternary blend. Compared with the reference PTB7:PC71 BM binary blend, the ternary blends showed strong optical absorption in the wavelength range in which the spectra of indoor LED lamps show their strongest peaks. The addition of EP-PDI to the binary blend was found to play an important role in altering the morphology of the blend in such a way as to facilitate charge transport in the resulting ternary blend. Apparently, as a result, the optimal PTB7:PC71 BM:EP-PDI-based inverted OPV device exhibited a power conversion efficiency (PCE) of 15.68 %, a fill factor (FF) of 68.5 %, and short-circuit current density (JSC ) of 56.7 µA cm-2 under 500 lx (ca. 0.17 mW cm-2 ) indoor LED light conditions.

Controllable Bipolar Doping of Graphene with 2D Molecular Dopants.

The fine control of graphene doping levels over a wide energy range remains a challenging issue for the electronic applications of graphene. Here, the controllable doping of chemical vapor deposited graphene, which provides a wide range of energy levels (shifts up to ± 0.5 eV), is demonstrated through physical contact with chemically versatile graphene oxide (GO) sheets, a 2D dopant that can be solution-processed. GO sheets are a p-type dopant due to their abundance of electron-withdrawing functional groups. To expand the energy window of GO-doped graphene, the GO surface is chemically modified with electron-donating ethylene diamine molecules. The amine-functionalized GO sheets exhibit strong n-type doping behaviors. In addition, the particular physicochemical characteristics of the GO sheets, namely their sheet sizes, number of layers, and degree of oxidation and amine functionality, are systematically varied to finely tune their energy levels. Finally, the tailor-made GO sheet dopants are applied into graphene-based electronic devices, which are found to exhibit improved device performances. These results demonstrate the potential of GO sheet dopants in many graphene-based electronics applications.

Copper-Vapor-Assisted Growth and Defect-Healing of Graphene on Copper Surfaces.

Although there is significant progress in the chemical vapor deposition (CVD) of graphene on Cu surfaces, the industrial application of graphene is not realized yet. One of the most critical obstacles that limit the commercialization of graphene is that CVD graphene contains too many vacancies or sp3 -type defects. Therefore, further investigation of the growth mechanism is still required to control the defects of graphene. During the growth of graphene, sublimation of the Cu catalyst to produce Cu vapor occurs inevitably because the process temperature is close to the melting point of Cu. However, to date few studies have investigated the effects of Cu vapor on graphene growth. In this study, how the Cu vapor produced by sublimation affects the chemical vapor deposition of graphene on Cu surfaces is investigated. It is found that the presence of Cu vapor enlarges the graphene grains and enhances the efficiency of the defect-healing of graphene by CH4 . It is elucidated that these effects are due to the removal by Cu vapor of carbon adatoms from the Cu surface and oxygen-functionalized carbons from graphene. Finally, these insights are used to develop a method for the synthesis of uniform and high-quality graphene.

Bistable Solid-State Fluorescence Switching in Photoluminescent, Infinite Coordination Polymers.

Photo-functional infinite coordinated polymers (ICPs) were synthesized that consist of the photochromic dithienylethene (DTE) and a luminescent bridging unit to give enhanced fluorescence in the solid state. We could fabricate well-ordered micropatterns of these ICPs by a soft-lithographic method, which repeatedly showed high contrast on-off fluorescence switching.

Side-Chain-Induced Rigid Backbone Organization of Polymer Semiconductors through Semifluoroalkyl Side Chains.

While high-mobility p-type conjugated polymers have been widely reported, high-mobility n-type conjugated polymers are still rare. In the present work, we designed semifluorinated alkyl side chains and introduced them into naphthalene diimide-based polymers (PNDIF-T2 and PNDIF-TVT). We found that the strong self-organization of these side chains induced a high degree of order in the attached polymer backbones by forming a superstructure composed of "backbone crystals" and "side-chain crystals". This phenomenon was shown to greatly enhance the ordering along the backbone direction, and the resulting polymers thus exhibited unipolar n-channel transport in field-effect transistors with remarkably high electron mobility values of up to 6.50 cm(2) V(-1) s(-1) and with a high on-off current ratio of 10(5).

Atomically thin epitaxial template for organic crystal growth using graphene with controlled surface wettability.

A two-dimensional epitaxial growth template for organic semiconductors was developed using a new method for transferring clean graphene sheets onto a substrate with controlled surface wettability. The introduction of a sacrificial graphene layer between a patterned polymeric supporting layer and a monolayer graphene sheet enabled the crack-free and residue-free transfer of free-standing monolayer graphene onto arbitrary substrates. The clean graphene template clearly induced the quasi-epitaxial growth of crystalline organic semiconductors with lying-down molecular orientation while maintaining the "wetting transparency", which allowed the transmission of the interaction between organic molecules and the underlying substrate. Consequently, the growth mode and corresponding morphology of the organic semiconductors on graphene templates exhibited distinctive dependence on the substrate hydrophobicity with clear transition from lateral to vertical growth mode on hydrophilic substrates, which originated from the high surface energy of the exposed crystallographic planes of the organic semiconductors on graphene. The optical properties of the pentacene layer, especially the diffusion of the exciton, also showed a strong dependency on the corresponding morphological evolution. Furthermore, the effect of pentacene-substrate interaction was systematically investigated by gradually increasing the number of graphene layers. These results suggested that the combination of a clean graphene surface and a suitable underlying substrate could serve as an atomically thin growth template to engineer the interaction between organic molecules and aromatic graphene network, thereby paving the way for effectively and conveniently tuning the semiconductor layer morphologies in devices prepared using graphene.

Age of Flexible Electronics: Emerging Trends in Soft Multifunctional Sensors.

With the commercialization of first-generation flexible mobiles and displays in the late 2010s, humanity has stepped into the age of flexible electronics. Inevitably, soft multifunctional sensors, as essential components of next-generation flexible electronics, have attracted tremendous research interest like never before. This review is dedicated to offering an overview of the latest emerging trends in soft multifunctional sensors and their accordant future research and development (R&D) directions for the coming decade. First, key characteristics and the predominant target stimuli for soft multifunctional sensors are highlighted. Second, important selection criteria for soft multifunctional sensors are introduced. Next, emerging materials/structures and trends for soft multifunctional sensors are identified. Specifically, the future R&D directions of these sensors are envisaged based on their emerging trends, namely i) decoupling of multiple stimuli, ii) data processing, iii) skin conformability, and iv) energy sources. Finally, the challenges and potential opportunities for these sensors in future are discussed, offering new insights into prospects in the fast-emerging technology.

Self-Assembly of Organic Semiconductors on Strained Graphene under Strain-Induced Pseudo-Electric Fields.

Graphene is used as a growth template for van der Waals epitaxy of organic semiconductor (OSC) thin films. During the synthesis and transfer of chemical-vapor-deposited graphene on a target substrate, local inhomogeneities in the graphene-in particular, a nonuniform strain field in the graphene template-can easily form, causing poor morphology and crystallinity of the OSC thin films. Moreover, a strain field in graphene introduces a pseudo-electric field in the graphene. Here, the study investigates how the strain and strain-induced pseudo-electric field of a graphene template affect the self-assembly of π-conjugated organic molecules on it. Periodically strained graphene templates are fabricated by transferring graphene onto an array of nanospheres and then analyzed the growth and nucleation behavior of C60 thin films on the strained graphene templates. Both experiments and a numerical simulation demonstrated that strained graphene reduced the desorption energy between the graphene and the C60 molecules and thereby suppressed both nucleation and growth of the C60. A mechanism is proposed in which the strain-induced pseudo-electric field in graphene modulates the binding energy of organic molecules on the graphene.

Mechanically Robust and Linearly Sensitive Soft Piezoresistive Pressure Sensor for a Wearable Human-Robot Interaction System.

Soft piezoresistive pressure sensors play an underpinning role in enabling a plethora of future Internet of Things (IoT) applications such as human-robot interaction (HRI) technologies, wearable devices, and metaverse ecosystems. Despite significant attempts to enhance the performance of these sensors, existing sensors still fall short of achieving high strain tolerance and linearity simultaneously. Herein, we present a low-cost, facile, and scalable approach to fabricating a highly strain-tolerant and linearly sensitive soft piezoresistive pressure sensor. Our design utilizes thin nanocracked gold films (NC-GFs) deposited on poly(dimethylsiloxane) (PDMS) as electrodes of the sensor. The large mismatch stress between gold (Au) and PDMS induces the formation of secondary wrinkles along the pyramidal-structured electrode under pressure; these wrinkles function as protuberances on the electrode and enable exceptional linear sensitivity of 4.2 kPa-1 over a wide pressure range. Additionally, our pressure sensor can maintain its performance even after severe mechanical deformations, including repeated stretching up to 30% strain, due to the outstanding strain tolerance of NC-GF. Our sensor's impressive sensing performance and mechanical robustness make it suitable for diverse IoT applications, as demonstrated by its use in wearable pulse monitoring devices and human-robot interaction systems.

Unveiling the Role of Side Chain for Improving Nonvolatile Characteristics of Conjugated Polymers-Based Artificial Synapse.

Interest has grown in services that consume a significant amount of energy, such as large language models (LLMs), and research is being conducted worldwide on synaptic devices for neuromorphic hardware. However, various complex processes are problematic for the implementation of synaptic properties. Here, synaptic characteristics are implemented through a novel method, namely side chain control of conjugated polymers. The developed devices exhibit the characteristics of the biological brain, especially spike-timing-dependent plasticity (STDP), high-pass filtering, and long-term potentiation/depression (LTP/D). Moreover, the fabricated synaptic devices show enhanced nonvolatile characteristics, such as long retention time (≈102 s), high ratio of Gmax/Gmin, high linearity, and reliable cyclic endurance (≈103 pulses). This study presents a new pathway for next-generation neuromorphic computing by modulating conjugated polymers with side chain control, thereby achieving high-performance synaptic properties.

Rational molecular and device design enables organic solar cells approaching 20% efficiency.

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

Proton-selective coating enables fast-kinetics high-mass-loading cathodes for sustainable zinc batteries.

The pressing demand for sustainable energy storage solutions has spurred the burgeoning development of aqueous zinc batteries. However, kinetics-sluggish Zn2+ as the dominant charge carriers in cathodes leads to suboptimal charge-storage capacity and durability of aqueous zinc batteries. Here, we discover that an ultrathin two-dimensional polyimine membrane, featured by dual ion-transport nanochannels and rich proton-conduction groups, facilitates rapid and selective proton passing. Subsequently, a distinctive electrochemistry transition shifting from sluggish Zn2+-dominated to fast-kinetics H+-dominated Faradic reactions is achieved for high-mass-loading cathodes by using the polyimine membrane as an interfacial coating. Notably, the NaV3O8·1.5H2O cathode (10 mg cm-2) with this interfacial coating exhibits an ultrahigh areal capacity of 4.5 mAh cm-2 and a state-of-the-art energy density of 33.8 Wh m-2, along with apparently enhanced cycling stability. Additionally, we showcase the applicability of the interfacial proton-selective coating to different cathodes and aqueous electrolytes, validating its universality for developing reliable aqueous batteries.

Transparent superhydrophobic/translucent superamphiphobic coatings based on silica-fluoropolymer hybrid nanoparticles.

This paper describes a simple approach to prepare a transparent superhydrophobic coating and a translucent superamphiphobic coating via spraying silica-fluoropolymer hybrid nanoparticles (SFNs) without any pre- or post-treatment of substrates; these nanoparticles create both microscale and nanoscale roughness, and fluoropolymer acts as a low surface energy binder. We also demonstrate the effects of varying the concentration of the SFN sol on the water and hexadecane repellency and on the transparency of the coated glass substrates. An increase in the concentration of the sol facilitates the transition between the superhydrophobic/transparent and superamphiphobic/translucent states. This transition results from an increase in the discontinuities in the three-phase (solid-liquid-gas) contact line and in the light scattering properties due to micropapillae tuned by varying the concentration of the sol. This versatile and controllable approach can be applied to a variety of substrates over large areas and may provide a wide range of applications for self-cleaning coatings of optoelectronics, liquid-repellent coatings, and microfluidic systems.

Advances in Biodegradable Electronic Skin: Material Progress and Recent Applications in Sensing, Robotics, and Human-Machine Interfaces.

The rapid growth of the electronics industry and proliferation of electronic materials and telecommunications technologies has led to the release of a massive amount of untreated electronic waste (e-waste) into the environment. Consequently, catastrophic environmental damage at the microbiome level and serious human health diseases threaten the natural fate of the planet. Currently, the demand for wearable electronics for applications in personalized medicine, electronic skins (e-skins), and health monitoring is substantial and growing. Therefore, "green" characteristics such as biodegradability, self-healing, and biocompatibility ensure the future application of wearable electronics and e-skins in biomedical engineering and bioanalytical sciences. Leveraging the biodegradability, sustainability, and biocompatibility of natural materials will dramatically influence the fabrication of environmentally friendly e-skins and wearable electronics. Here, the molecular and structural characteristics of biological skins and artificial e-skins are discussed. The focus then turns to the biodegradable materials, including natural and synthetic-polymer-based materials, and their recent applications in the development of biodegradable e-skin in wearable sensors, robotics, and human-machine interfaces (HMIs). Finally, the main challenges and outlook regarding the preparation and application of biodegradable e-skins are critically discussed in a near-future scenario, which is expected to lead to the next generation of biodegradable e-skins.