A stretchable crumpled graphene photodetector with plasmonically enhanced photoresponsivity

Wearable sensor supports in-situ and continuous monitoring of plant health in precision agriculture era

Plant health is intricately linked to crop quality, food security and agricultural productivity. Obtaining accurate plant health information is of paramount importance in the realm of precision agriculture. Wearable sensors offer an exceptional avenue for investigating plant health status and fundamental plant science, as they enable real-time and continuous in-situ monitoring of physiological biomarkers. However, a comprehensive overview that integrates and critically assesses wearable plant sensors across various facets, including their fundamental elements, classification, design, sensing mechanism, fabrication, characterization and application, remains elusive. In this study, we provide a meticulous description and systematic synthesis of recent research progress in wearable sensor properties, technology and their application in monitoring plant health information. This work endeavours to serve as a guiding resource for the utilization of wearable plant sensors, empowering the advancement of plant health within the precision agriculture paradigm.

Layer-by-layer thinning of two-dimensional materials

Etching technology - one of the representative modern semiconductor device makers - serves as a broad descriptor for the process of removing material from the surfaces of various materials, whether partially or entirely. Meanwhile, thinning technology represents a novel and highly specialized approach within the realm of etching technology. It indicates the importance of achieving an exceptionally sophisticated and precise removal of material, layer-by-layer, at the nanoscale. Notably, thinning technology has gained substantial momentum, particularly in top-down strategies aimed at pushing the frontiers of nano-worlds. This rapid development in thinning technology has generated substantial interest among researchers from diverse backgrounds, including those in the fields of chemistry, physics, and engineering. Precisely and expertly controlling the layer numbers of 2D materials through the thinning procedure has been considered as a crucial step. This is because the thinning processes lead to variations in the electrical and optical characteristics. In this comprehensive review, the strategies for top-down thinning of representative 2D materials (e.g., graphene, black phosphorus, MoS2, h-BN, WS2, MoSe2, and WSe2) based on conventional plasma-assisted thinning, integrated cyclic plasma-assisted thinning, laser-assisted thinning, metal-assisted splitting, and layer-resolved splitting are covered in detail, along with their mechanisms and benefits. Additionally, this review further explores the latest advancements in terms of the potential advantages of semiconductor devices achieved by top-down 2D material thinning procedures.

Broadband Photodetectors and Imagers in Stretchable Electronics Packaging

The integration of flexible electronics with optics can help realize a powerful tool that facilitates the creation of a smart society wherein internal evaluations can be easily performed nondestructively from the surface of various objects that is used or encountered in daily lives. Here, organic-material-based stretchable optical sensors and imagers that possess both bending capability and rubber-like elasticity are reviewed. The latest trends in nondestructive evaluation equipment that enable simple on-site evaluations of health conditions and abnormalities are discussed without subjecting the targeted living bodies and various objects to mechanical stress. Real-time performance under real-life conditions is becoming increasingly important for creating smart societies interwoven with optical technologies. In particular, the terahertz (THz)-wave region offers a substance- and state-specific fingerprint spectrum that enables instantaneous analyses. However, to make THz sensors accessible, the following issues must be addressed: broadband and high-sensitivity at room temperature, stretchability to follow the surface movements of targets, and digital transformation compatibility. The materials, electronics packaging, and remote imaging systems used to overcome these issues are discussed in detail. Ultimately, stretchable optical sensors and imagers with highly sensitive and broadband THz sensors can facilitate the multifaceted on-site evaluation of solids, liquids, and gases.

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Stretchable Thin-Film Transistors Based on Wrinkled Graphene and MoS2 Heterostructures

Two-dimensional (2D) materials are outstanding candidates for stretchable electronics, but a significant challenge is their heterogeneous integration into stretchable geometries on soft substrates. Here, we demonstrate a strategy for stretchable thin film transistors (2D S-TFT) based on wrinkled heterostructures on elastomer substrates where 2D materials formed the gate, source, drain, and channel and characterized them with Raman spectroscopy and transport measurements. The 2D S-TFTs had initial mobility of 4.9 ± 0.7 cm2/(V s). The wrinkling reduced the strain transferred into the 2D materials by a factor of 50, allowing a substrate stretch of up to 23% that could be cycled thousands of times without electrical degradation. The stretch did not alter the mobility but did lead to strain-induced threshold voltage shifts by ΔVT = -1.9 V. These 2D S-TFTs form the foundation for stretchable integrated circuits and enable investigations of the impact of heterogeneous strain on electron transport.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Latest Innovations in 2D Flexible Nanoelectronics

2D materials with dangling-bond-free surfaces and atomically thin layers have been shown to be capable of being incorporated into flexible electronic devices. The electronic and optical properties of 2D materials can be tuned or controlled in other ways by using the intriguing strain engineering method. The latest and encouraging techniques in regard to creating flexible 2D nanoelectronics are condensed in this review. These techniques have the potential to be used in a wider range of applications in the near and long term. It is possible to use ultrathin 2D materials (graphene, BP, WTe2 , VSe2 etc.) and 2D transition metal dichalcogenides (2D TMDs) in order to enable the electrical behavior of the devices to be studied. A category of materials is produced on smaller scales by exfoliating bulk materials, whereas chemical vapor deposition (CVD) and epitaxial growth are employed on larger scales. This overview highlights two distinct requirements, which include from a single semiconductor or with van der Waals heterostructures of various nanomaterials. They include where strain must be avoided and where it is required, such as solutions to produce strain-insensitive devices, and such as pressure-sensitive outcomes, respectively. Finally, points-of-view about the current difficulties and possibilities in regard to using 2D materials in flexible electronics are provided.

Area-Specific, Hierarchical Nanowrinkling of Two-Dimensional Materials

This paper describes an approach to generate hierarchical wrinkles in two-dimensional (2D) electronic materials with spatial control over adjacent wavelengths. A rigid fluoropolymer mold was used to pattern a sacrificial polymer skin layer on monolayer graphene, molybdenum disulfide, and hexagonal boron nitride on prestrained thermoplastic sheets. Strain relief and removal of the polymer layer resulted in 2D-material wrinkles whose wavelengths scaled linearly with the local skin thickness. A second generation of wrinkles could be created on top of the first generation by applying a subsequent cycle of polymer skin coating, strain relief, and polymer removal. This area-specific hierarchical wrinkling is general and will facilitate the engineering of the local properties of various 2D materials and their heterostructures.

Advanced Stretchable Photodetectors: Strategies, Materials and Devices

Past decades have witnessed the generation of new stretchable photodetectors in electronic eyes, health sensing, wearable devices, intelligent monitoring and other fields. Stretchable devices require not only outstanding performance but also excellent flexibility, adaptability and stability. Innovative strategies have been proposed to realize the stretchability of devices. In addition, novel functional materials including zero-dimensional nanomaterials, one-dimensional inorganic nanomaterials, two-dimensional layered materials, organic materials, and organic-inorganic composite materials with excellent properties are emerging to continuously improve the performance of devices. Here, the recent research progress of stretchable photodetectors in terms of both various design methods and functional materials is outlined. The optical performance and stretchable properties are also comprehensively reviewed. Finally, a summary and the challenges associated with the application of stretchable photodetectors are presented.

Nanomaterial-Based Synaptic Optoelectronic Devices for In-Sensor Preprocessing of Image Data

With the advance in information technologies involving machine vision applications, the demand for energy- and time-efficient acquisition, transfer, and processing of a large amount of image data has rapidly increased. However, current architectures of the machine vision system have inherent limitations in terms of power consumption and data latency owing to the physical isolation of image sensors and processors. Meanwhile, synaptic optoelectronic devices that exhibit photoresponse similar to the behaviors of the human synapse enable in-sensor preprocessing, which makes the front-end part of the image recognition process more efficient. Herein, we review recent progress in the development of synaptic optoelectronic devices using functional nanomaterials and their unique interfacial characteristics. First, we provide an overview of representative functional nanomaterials and device configurations for the synaptic optoelectronic devices. Then, we discuss the underlying physics of each nanomaterial in the synaptic optoelectronic device and explain related device characteristics that allow for the in-sensor preprocessing. We also discuss advantages achieved by the application of the synaptic optoelectronic devices to image preprocessing, such as contrast enhancement and image filtering. Finally, we conclude this review and present a short prospect.

Plasmonic sensors based on graphene and graphene hybrid materials

The past decade has witnessed a rapid growth of graphene plasmonics and their applications in different fields. Compared with conventional plasmonic materials, graphene enables highly confined plasmons with much longer lifetimes. Moreover, graphene plasmons work in an extended wavelength range, i.e., mid-infrared and terahertz regime, overlapping with the fingerprints of most organic and biomolecules, and have broadened their applications towards plasmonic biological and chemical sensors. In this review, we discuss intrinsic plasmonic properties of graphene and strategies both for tuning graphene plasmons as well as achieving higher performance by integrating graphene with plasmonic nanostructures. Next, we survey applications of graphene and graphene-hybrid materials in biosensors, chemical sensors, optical sensors, and sensors in other fields. Lastly, we conclude this review by providing a brief outlook and challenges of the field. Through this review, we aim to provide an overall picture of graphene plasmonic sensing and to suggest future trends of development of graphene plasmonics.

Stretchable broadband photo-sensor sheets for nonsampling, source-free, and label-free chemical monitoring by simple deformable wrapping

Chemical monitoring communicates diverse environmental information from industrial and biological processes. However, promising and sustainable systems and associated inspection devices that dynamically enable on-site quality monitoring of target chemicals confined inside transformable and opaque channels are yet to be investigated. This paper designs stretchable photo-sensor patch sheets for nonsampling, source-free, and label-free on-site dynamic chemical monitoring of liquids flowing inside soft tubes via simple deformable surface wrapping. The device integrates carbon nanotube-based broadband photo-absorbent thin films with multilayer-laminated stretchable electrodes and substrates. The patterned rigid-soft structure of the proposed device provides durability and optical stability against mechanical deformations with a stretchability range of 70 to 280%, enabling shape-conformable attachments to transformable objects. The effective use of omnidirectional and transparent blackbody radiation from free-form targets themselves allows compact measurement configuration and enhances the functionality and simplicity of this scheme, while the presenting technology monitors concentrations of arbitrary water-soluble chemicals.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

Photothermochemical Nanoassembly of 3D Porous Graphene and Palladium Nanoparticles for High-Performance Hydrogen Detection

Hybrid materials comprising graphene and palladium nanoparticles (PdNPs) are desirable for high-performance hydrogen detection because of the high specific surface area, electron mobility, and flexibility of graphene and the high electrochemical responsivity and reversibility of PdNPs. However, obtaining hybrid materials is energy-intensive and time-consuming. Here, a facile and rapid laser photothermochemical single-step processing method to synchronously produce a nanoassembly of three-dimensional porous graphene and PdNPs from polymer films is reported. Polymers with intrinsic microporosity show high solubility in volatile solvents and miscibility with inorganic materials, allowing the fabrication of homogeneous polymer films containing Pd ligands. The films are photothermally processed using a laser to generate a nanohybrid via photoinduced thermal and chemical processes. The nanohybrid exhibits four-times-enhanced electrical conductivity compared to plain porous graphene, high crystallinity, and coherent covalent metal bonds with a homogeneous size and distribution of PdNPs in hierarchical micro/meso/macroporous graphene structures, allowing high-performance hydrogen sensing (1 ppm) with outstanding mechanical reliability, flexibility, and durability upon bending and twisting. The nanoassembly is integrated with a wireless sensing platform, and hydrogen leakage (1 ppm) is detected using a smart phone. This laser-based nanomanufacturing of the nanoassembly can potentially be applied to wearable detector production platforms in the military and industry.

Mechanism, Material, Design, and Implementation Principle of Two-Dimensional Material Photodetectors

Two-dimensional (2D) materials may play an important role in future photodetectors due to their natural atom-thin body thickness, unique quantum confinement, and excellent electronic and photoelectric properties. Semimetallic graphene, semiconductor black phosphorus, and transition metal dichalcogenides possess flexible and adjustable bandgaps, which correspond to a wide interaction spectrum ranging from ultraviolet to terahertz. Nevertheless, their absorbance is relatively low, and it is difficult for a single material to cover a wide spectrum. Therefore, the combination of phototransistors based on 2D hybrid structures with other material platforms, such as quantum dots, organic materials, or plasma nanostructures, exhibit ultra-sensitive and broadband optical detection capabilities that cannot be ascribed to the individual constituents of the assembly. This article provides a comprehensive and systematic review of the recent research progress of 2D material photodetectors. First, the fundamental detection mechanism and key metrics of the 2D material photodetectors are introduced. Then, the latest developments in 2D material photodetectors are reviewed based on the strategies of photocurrent enhancement. Finally, a design and implementation principle for high-performance 2D material photodetectors is provided, together with the current challenges and future outlooks.

Stretchable Inorganic GaN-Nanowire Photosensor with High Photocurrent and Photoresponsivity

To effectively implement wearable systems, their constituent components should be made stretchable. We successfully fabricated highly efficient stretchable photosensors made of inorganic GaN nanowires (NWs) as light-absorbing media and graphene as a carrier channel on polyurethane substrates using the pre-strain method. When a GaN-NW photosensor was stretched at a strain level of 50%, the photocurrent was measured to be 0.91 mA, corresponding to 87.5% of that (1.04 mA) obtained in the released state, and the photoresponsivity was calculated to be 11.38 A/W. These photosensors showed photocurrent and photoresponsivity levels much higher than those previously reported for any stretchable semiconductor-containing photosensor. To explain the superior performances of the stretchable GaN-NW photosensor, it was approximated as an equivalent circuit with resistances and capacitances, and in this way, we analyzed the behavior of the photogenerated carriers, particularly at the NW-graphene interface. In addition, the buckling phenomenon typically observed in organic-based stretchable devices fabricated using the pre-strain method was not observed in our photosensors. After a 1000-cycle stretching test with a strain level of 50%, the photocurrent and photoresponsivity of the GaN-NW photosensor were measured to be 0.96 mA and 11.96 A/W, respectively, comparable to those measured before the stretching test. To evaluate the potential of our stretchable devices in practical applications, the GaN-NW photosensors were attached to the proximal interphalangeal joint of the index finger and to the back of the wrist. Photocurrents of these photosensors were monitored during movements made about these joints.

Metal Halide Perovskite/2D Material Heterostructures: Syntheses and Applications

The past decade has witnessed the great success achieved by metal halide perovskites (MHPs) in photovoltaic and related fields. However, challenges still remain in further improving their performance, as well as, settling the stability issue for future commercialization. Recently, MHP/2D material heterostructures that combining MHPs with the low-cost and solution-processable 2D materials have demonstrated unprecedented improvement in both performance and stability due to the distinctive features at hetero-interface. The diverse fabrication techniques of MHPs and 2D materials allow them to be assembled as heterostructures with different configurations in a variety of ways. Moreover, the large families of MHPs and 2D materials provide the opportunity for the rational design and modification on compositions and functionalities of MHP/2D materials heterostructures. Herein, a comprehensive review of MHP/2D material heterostructures from syntheses to applications is presented. First, various fabrication techniques for MHP/2D material heterostructures are introduced by classifying them into solid-state methods and solution-processed methods. Then the applications of MHP/2D heterostructures in various fields including photodetectors, solar cells, and photocatalysis are summarized in detail. Finally, current challenges for the development of MHP/2D material heterostructures are highlighted, and future opportunities for the advancements in this research field are also provided.

High-Performance Phototransistors Based on MnPSe3 and Its Hybrid Structures with Au Nanoparticles

Layered metal thiophosphates with a general formula MPX₃ (M is a group VIIB or VIII element and X is a chalcogen) have emerged as a novel member in a two-dimensional (2D) family with fascinating physical and chemical properties. Herein, the photoelectric performance of the few-layer MnPSe₃ was studied for the first time. The multilayer MnPSe₃ shows p-type conductivity and its field-effect transistor delivers an ultralow dark current of about 0.1 pA. The photoswitching ratio reaches ∼10³ at a wavelength of 375 nm, superior to that of other thiophosphates. A responsivity and detectivity of 392.78 mA/W and 2.19 × 10⁹ Jones, respectively, have been demonstrated under irradiation of 375 nm laser with a power intensity of 0.1 mW/cm². In particular, the photocurrent can be remarkably increased up to 30 times by integrating a layer of Au nanoparticle array at the bottom of the MnPSe₃ layer. The metal-semiconductor interfacial electric field and the strain-induced flexoelectric polarization field caused by the underlying nanorugged Au nanoparticles are proposed to contribute together to the significant current improvement.

Strain Engineering in 2D Material-Based Flexible Optoelectronics

Flexible optoelectronics, as promising components hold shape-adaptive features and dynamic strain response under strain engineering for various intelligent applications. 2D materials with atomically thin layers are ideal for flexible optoelectronics because of their high flexibility and strain sensitivity. However, how the strain affects the performance of 2D materials-based flexible optoelectronics is confused due to their hypersensitive features to external strain changes. It is necessary to establish an evaluation system to comprehend the influence of the external strain on the intrinsic properties of 2D materials and the photoresponse performance of their flexible optoelectronics. Here, a focused review of strain engineering in 2D materials-based flexible optoelectronics is provided. The first attention is on the mechanical properties and the strain-engineered electronic properties of 2D semiconductors. An evaluation system with relatively comprehensive parameters in functionality and service capability is summarized to develop 2D materials-based flexible optoelectronics in practical application. Based on the parameters, some strategies to improve the functionality and service capability are proposed. Finally, combining with strain engineering in future intelligence devices, the challenges and future perspective developing 2D materials-based flexible optoelectronics are expounded.

Fractal Web Design of a Hemispherical Photodetector Array with Organic-Dye-Sensitized Graphene Hybrid Composites

The vision system of arthropods consists of a dense array of individual photodetecting elements across a curvilinear surface. This compound-eye architecture could be a useful model for optoelectronic sensing devices that require a large field of view and high sensitivity to motion. Strategies that aim to mimic the compound-eye architecture involve integrating photodetector pixels with a curved microlens, but their fabrication on a curvilinear surface is challenged by the use of standard microfabrication processes that are traditionally designed for planar, rigid substrates (e.g., Si wafers). Here, a fractal web design of a hemispherical photodetector array that contains an organic-dye-sensitized graphene hybrid composite is reported to serve as an effective photoactive component with enhanced light-absorbing capabilities. The device is first fabricated on a planar Si wafer at the microscale and then transferred to transparent hemispherical domes with different curvatures in a deterministic manner. The unique structural property of the fractal web design provides protection of the device from damage by effectively tolerating various external loads. Comprehensive experimental and computational studies reveal the essential design features and optoelectronic properties of the device, followed by the evaluation of its utility in the measurement of both the direction and intensity of incident light.

Graphene Hybrid Structures for Integrated and Flexible Optoelectronics

Graphene (Gr) has many unique properties including gapless band structure, ultrafast carrier dynamics, high carrier mobility, and flexibility, making it appealing for ultrafast, broadband, and flexible optoelectronics. To overcome its intrinsic limit of low absorption, hybrid structures are exploited to improve the device performance. Particularly, van der Waals heterostructures with different photosensitive materials and photonic structures are very effective for improving photodetection and modulation efficiency. With such hybrid structures, Gr hybrid photodetectors can operate from ultraviolet to terahertz, with significantly improved R (up to 10⁹ A W^-1 ) and bandwidth (up to 128 GHz). Furthermore, integration of Gr with silicon (Si) complementary metal-oxide-semiconductor (CMOS) circuits, the human body, and soft tissues is successfully demonstrated, opening promising opportunities for wearable sensors and biomedical electronics. Here, the recent progress in using Gr hybrid structures toward high-performance photodetectors and integrated optoelectronic applications is reviewed.

Heterogeneous deformation of two-dimensional materials for emerging functionalities

Atomically thin 2D materials exhibit strong intralayer covalent bonding and weak interlayer van der Waals interactions, offering unique high in-plane strength and out-of-plane flexibility. While atom-thick nature of 2D materials may cause uncontrolled intrinsic/extrinsic deformation in multiple length scales, it also provides new opportunities for exploring coupling between heterogeneous deformations and emerging functionalities in controllable and scalable ways for electronic, optical, and optoelectronic applications. In this review, we discuss (i) the mechanical characteristics of 2D materials, (ii) uncontrolled inherent deformation and extrinsic heterogeneity present in 2D materials, (iii) experimental strategies for controlled heterogeneous deformation of 2D materials, (iv) 3D structure-induced novel functionalities via crumple/wrinkle structure or kirigami structures, and (v) heterogeneous strain-induced emerging functionalities in exciton and phase engineering. Overall, heterogeneous deformation offers unique advantages for 2D materials research by enabling spatial tunability of 2D materials' interactions with photons, electrons, and molecules in a programmable and controlled manner.

Photonic crystal-based smart contact lens for continuous intraocular pressure monitoring

Glaucoma is a very common disease after cataracts and is dangerous enough to cause irreversible blindness. However, often the main symptom of glaucoma is difficult to recognize because it may be absent or appear late, so the risk of blindness is greater. Intraocular pressure (IOP) is a well-known primary factor indicating glaucoma. In this study, we demonstrate a smart IOP sensor embedded in a contact lens that works through visual color changes without an external power source such as a battery or RF-based wireless power transfer. A microhydraulic amplification mechanism is adopted to enhance the range of color change from a photonic crystal (PC)-based flexible membrane whose lattice distance between nanostructures varies according to the morphology changes of an eyeball caused by IOP. The performance of the sensor is quantitatively demonstrated using an artificial silicone eye model for in vitro evaluation and a porcine eyeball for ex vivo verification. It has a limit of detection (LOD) of 3.2 and 5.12 mmHg, which was measured and evaluated using a spectrometer and a smartphone camera, respectively. The results prove that our sensor embedded in the contact lens can continuously monitor the IOP change using color change, and a smartphone camera can be used as a quantitative IOP measurement system in a noninvasive manner without an expensive optical spectrometer.

Homecare Robotic Systems for Healthcare 4.0: Visions and Enabling Technologies

Powered by the technologies that have originated from manufacturing, the fourth revolution of healthcare technologies is happening (Healthcare 4.0). As an example of such revolution, new generation homecare robotic systems (HRS) based on the cyber-physical systems (CPS) with higher speed and more intelligent execution are emerging. In this article, the new visions and features of the CPS-based HRS are proposed. The latest progress in related enabling technologies is reviewed, including artificial intelligence, sensing fundamentals, materials and machines, cloud computing and communication, as well as motion capture and mapping. Finally, the future perspectives of the CPS-based HRS and the technical challenges faced in each technical area are discussed.

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

Tunable Fabrication of Conductive Ti3C2Tx MXenes via Inflating a Polyurethane Balloon for Acute Force Sensing

Conductive microwrinkles present a superior performance in ultrasensitive sensing, smart controlling, as well as energy conservation because of their unique structures. These wrinkles are usually prepared by the deposition of a thin conductive stiff layer on a soft substrate under a certain strain. However, traditional conductive materials may encounter some deficiencies, such as fragility or poor dispersity, in any solvent. To promote the applicability of conductive microwrinkles, here, we adopt a new two-dimensional nanomaterial Ti₃C₂T_x MXene as the conductive stiff layer to construct the microwrinkles. By combining the spraying and inflating techniques, the hierarchical complex and delicate Ti₃C₂T_x-polyurethane (Ti₃C₂T_x-PU) microwrinkles have become facilely available. The characteristic wavelength and amplitude of the microwrinkles could be easily adjusted by altering the inflating height of the PU film or the spraying volume of the Ti₃C₂T_x solution. Because the as-prepared Ti₃C₂T_x wrinkles could sensitively generate deformation inducing a resistance change under a force, these structures are also assembled to detect the applied force. The Ti₃C₂T_x force sensors showed quick response to a tiny force and stable reliability over hundreds of cycles, which hold a promising potential to monitor or employ the microforce.

Quantitative Understanding of Charge-Transfer-Mediated Fe3+ Sensing and Fast Photoresponse by N-Doped Graphene Quantum Dots Decorated on Plasmonic Au Nanoparticles

The formation of a heterostructure with plasmonic nanoparticles drastically alters the optoelectronic properties of graphene quantum dots (GQDs), resulting in exceptional properties. In the present work, we prepare nitrogen-doped GQDs decorated on gold nanoparticles (Au@N-GQDs) by a one-step green reduction method and study its extraordinary fluorescence and photoresponse characteristics. The as-prepared Au@N-GQDs show more than one order of magnitude enhancement in the fluorescence intensity as compared to the bare N-GQDs, which is attributed to hot electron generation and improved absorption in N-GQDs by local field enhancement and the modification of the edge functional groups. Because of the selective coordination to Fe³⁺ ions, the Au@N-GQDs exhibit extraordinary quenching of fluorescence, with ultrahigh sensitivity for the detection of Fe³⁺ (<1 nM). A new model for the charge-transfer dynamics is developed involving the Langmuir's law of adsorption to explain the unusual quenching, which strongly deviates from the known models of static/dynamic quenching. The proposed sensor is successfully implemented for the ultrasensitive detection of Fe³⁺ ions in human serum and Brahmaputra river water samples, representing its high potential applications in clinical as well as environmental diagnosis. Additionally, because of its high absorption in the UV-vis-NIR region and high charge density with long life excitons, the Au@N-GQDs are utilized as photodetectors with ∼10⁴ times faster response than that of bare N-GQDs. The Au@N-GQD-based photodetector possesses a high responsivity of ∼1.36 A/W and a remarkably high external quantum efficiency of ∼292.2%, which is much superior to the GQD-based photodetectors reported till date. The underlying mechanism of ultrafast photoresponse is ascribed to the transfer of hot electrons along with the tunneling of the electrons from Au NPs to N-GQDs as well as the defect reduction of N-GQDs by the incorporation of Au NPs. Without the use of any charge transporting layer, the outstanding performance of N-GQD-based plasmonic photodetector opens up unique opportunities for future high-speed optoelectronic devices.

Two-dimensional layered materials: from mechanical and coupling properties towards applications in electronics

With the increasing interest in nanodevices based on two-dimensional layered materials (2DLMs) after the birth of graphene, the mechanical and coupling properties of these materials, which play an important role in determining the performance and life of nanodevices, have drawn increasingly more attention. In this review, both experimental and simulation methods investigating the mechanical properties and behaviour of 2DLMs have been summarized, which is followed by the discussion of their elastic properties and failure mechanisms. For further understanding and tuning of their mechanical properties and behaviour, the influence factors on the mechanical properties and behaviour have been taken into consideration. In addition, the coupling properties between mechanical properties and other physical properties are summarized to help set up the theoretical blocks for their novel applications. Thus, the understanding of the mechanical and coupling properties paves the way to their applications in flexible electronics and novel electronics, which is demonstrated in the last part. This review is expected to provide in-depth and comprehensive understanding of mechanical and coupling properties of 2DLMs as well as direct guidance for obtaining satisfactory nanodevices from the aspects of material selection, fabrication processes and device design.

Photonic crystallization of two-dimensional MoS2 for stretchable photodetectors

Low temperature synthesis of high quality two-dimensional (2D) materials directly on flexible substrates remains a fundamental limitation towards scalable realization of robust flexible electronics possessing the unique physical properties of atomically thin structures. Herein, we describe room temperature sputtering of uniform, stoichiometric amorphous MoS2 and subsequent large area (>6.25 cm2) photonic crystallization of 5 nm 2H-MoS2 films in air to enable direct, scalable fabrication of ultrathin 2D photodetectors on stretchable polydimethylsiloxane (PDMS) substrates. The lateral photodetector devices demonstrate an average responsivity of 2.52 μW A-1 and a minimum response time of 120 ms under 515.6 nm illumination. Additionally, the surface wrinkled, or buckled, PDMS substrate with conformal MoS2 retained the photoconductive behavior at tensile strains as high as 5.72% and over 1000 stretching cycles. The results indicate that the photonic crystallization method provides a significant advancement in incorporating high quality semiconducting 2D materials applied directly on polymer substrates for wearable and flexible electronic systems.

Materials and Designs for Wearable Photodetectors

Photodetectors (PDs), as an indispensable component in electronics, are highly desired to be flexible to meet the trend of next-generation wearable electronics. Unfortunately, no in-depth reviews on the design strategies, material exploration, and potential applications of wearable photodetectors are found in literature to date. Thus, this progress report first summarizes the fundamental design principles of turning "hard" photodetectors "soft," including 2D (polymer and paper substrate-based devices) and 1D PDs (fiber shaped devices). In short, the flexibility of PDs is realized through elaborate substrate modification, material selection, and device layout. More importantly, this report presents the current progress and specific requirements for wearable PDs according to the application: monitoring, imaging, and optical communication. Challenges and future research directions in these fields are proposed at the end. The purpose of this progress report is not only to shed light on the basic design principles of wearable PDs, but also serve as the roadmap for future exploration in wearable PDs in various applications, including health monitoring and Internet of Things.

Wrinkling of two-dimensional materials: methods, properties and applications

Recently, two-dimensional (2D) materials, including graphene, its derivatives, metal films, MXenes and transition metal dichalcogenides (TMDs), have been widely studied because of their tunable electronic structures and special electrical and optical properties. However, during the fabrication of these 2D materials with atomic thickness, formation of wrinkles or folds is unavoidable to enable their stable existence. Meaningfully, it is found that wrinkled structures simultaneously impose positive changes on the 2D materials. Specifically, the architecture of wrinkled structures in 2D materials additionally induces excellent properties, which are of great importance for their practical applications. In this review, we provide an overview of categories of 2D materials, which contains formation and fabrication methods of wrinkled patterns and relevant mechanisms, as well as the induced mechanical, electrical, thermal and optical properties. Furthermore, these properties are modifiable by controlling the surface topography or even by dynamically stretching the 2D materials. Wrinkling offers a platform for 2D materials to be applied in some promising fields such as field emitters, energy containers and suppliers, field effect transistors, hydrophobic surfaces, sensors for flexible electronics and artificial intelligence. Finally, the opportunities and challenges of wrinkled 2D materials in the near future are discussed.

Graphene-Supported Thin Metal Films for Nanophotonics and Optoelectronics

Graphene-metal hybrid nanostructures have attracted considerable attention due to their potential applications in nanophotonics and optoelectronics. The output characteristics of devices based on such nanostructures largely depend on the properties of the metals. Here, we study the optical, electrical and structural properties of continuous thin gold and copper films grown by electron beam evaporation on monolayer graphene transferred onto silicon dioxide substrates. We find that the presence of graphene has a significant effect on optical losses and electrical resistance, both for thin gold and copper films. Furthermore, the growth kinetics of gold and copper films vary greatly; in particular, we found here a significant dependence of the properties of thin copper films on the deposition rate, unlike gold films. Our work provides new data on the optical properties of gold and copper, which should be considered in modeling and designing devices with graphene-metal nanolayers.

White-Light Photosensors Based on Ag Nanoparticle-Reduced Graphene Oxide Hybrid Materials

The unique and outstanding electrical and optical properties of graphene make it a potential material to be used in the construction of high-performance photosensors. However, the fabrication process of a graphene photosensor is usually complicated and the size of the device also is restricted to micrometer scale. In this work, we report large-area photosensors based on reduced graphene oxide (rGO) implemented with Ag nanoparticles (AgNPs) via a simple and cost-effective method. To further optimize the performance of photosensors, the absorbance and distribution of the electrical field intensity of graphene with AgNPs was simulated using the finite-difference time-domain (FDTD) method through use of the surface plasmon resonance effect. Based on the simulated results, we constructed photosensors using rGO with 60–80 nm AgNPs and analyzed the characteristics at room temperature under white-light illumination for outdoor environment applications. The on/off ratio of the photosensor with AgNPs was improved from 1.166 to 9.699 at the bias voltage of −1.5 V, which was compared as a sample without AgNPs. The proposed photosensor affords a new strategy to construct cost-effective and large-area graphene films which raises opportunities in the field of next-generation optoelectronic devices operated in an outdoor environment.

Centimeter-Scale Periodically Corrugated Few-Layer 2D MoS2 with Tensile Stretch-Driven Tunable Multifunctionalities

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers exhibit superior optical, electrical, and structural properties unattainable in any traditional materials. Many of these properties are known to be controllable via external mechanical inputs, benefiting from their extremely small thickness coupled with large in-plane strain limits. However, realization of such mechanically driven tunability often demands highly complicated engineering of 2D TMD layer structures, which is difficult to achieve on a large wafer scale in a controlled manner. Herein, we explore centimeter-scale periodically corrugated 2D TMDs, particularly 2D molybdenum disulfide (MoS₂), and report their mechanically tunable multifunctionalities. We developed a water-assisted process to homogeneously integrate few layers of 2D MoS₂ on three-dimensionally corrugated elastomeric substrates on a large area (>2 cm²). The evolution of electrical, optical, and structural properties in these three-dimensionally corrugated 2D MoS₂ layers was systematically studied under controlled tensile stretch. We identified that they present excellent electrical conductivity and photoresponsiveness as well as systematically tunable surface wettability and optical absorbance even under significant mechanical deformation. These novel three-dimensionally structured 2D materials are believed to offer exciting opportunities for large-scale, mechanically deformable devices of various form factors and unprecedented multifunctionalities.

High-performance stretchable photodetector based on CH3NH3PbI3 microwires and graphene

A stretchable photodetector was fabricated by releasing a prestrained 3 M very high bond (VHB) substrate coated with perovskite CH₃NH₃PbI₃ microwires and graphene. The light harvesting CH₃NH₃PbI₃ microwires were realized through a transformation from CH₃NH₃PbI₃ bulk single crystals. Graphene served as an expressway for photoinduced carriers in the perovskite. Under a very low working voltage bias of 0.01 V and irradiance power of 13.5 mW cm^-2 under 785 nm laser illumination, the responsivity could be as high as 2.2 mA W^-1. When the device was stretched up to 30%, 50%, and 80% strain, the responsivity was maintained at 0.96, 0.60, and 0.21 mA W^-1, respectively. It also showed a fast photoresponse (<0.12 s) after stretching to 10%, 30%, 50%, 80%, and even to 100%. The device performed well after 100 cycles of stretching to 50% strain.

Hierarchical Multicomponent Inorganic Metamaterials: Intrinsically Driven Self-Assembly at the Nanoscale

Increasingly intricate in their composition and structural organization, hierarchical multicomponent metamaterials with nonlinear spatially reconfigurable functionalities challenge the intrinsic constraints of natural materials, revealing tremendous potential for the advancement of biochemistry, nanophotonics, and medicine. Recent breakthroughs in high-resolution nanofabrication utilizing ultranarrow, precisely controlled ion or laser beams have enabled assembly of architectures of unprecedented structural and functional complexity, yet costly, time- and energy-consuming high-resolution sequential techniques do not operate effectively at industry-required scale. Inspired by the fictional Baron Munchausen's fruitless attempt to pull himself up, it is demonstrated that metamaterials can undergo intrinsically driven self-assembly, metaphorically pulling themselves up into existence. These internal drivers hold a key to unlocking the potential of metamaterials and mapping a new direction for the large-area, cost-efficient self-organized fabrication of practical devices. A systematic exploration of these efforts is presently missing, and the driving forces governing the intrinsically driven self-assembly are yet to be fully understood. Here, recent progress in the self-organized formation and self-propelled growth of complex hierarchical multicomponent metamaterials is reviewed, with emphasis on key principles, salient features, and potential limitations of this family of approaches. Special stress is placed on self-assembly driven by plasma, current in liquid, ultrasonic, and similar highly energetic effects, which enable self-directed formation of metamaterials with unique properties and structures.

Nanomaterial-Enabled Wearable Sensors for Healthcare

Highly sensitive wearable sensors that can be conformably attached to human skin or integrated with textiles to monitor the physiological parameters of human body or the surrounding environment have garnered tremendous interest. Owing to the large surface area and outstanding material properties, nanomaterials are promising building blocks for wearable sensors. Recent advances in the nanomaterial-enabled wearable sensors including temperature, electrophysiological, strain, tactile, electrochemical, and environmental sensors are presented in this review. Integration of multiple sensors for multimodal sensing and integration with other components into wearable systems are summarized. Representative applications of nanomaterial-enabled wearable sensors for healthcare, including continuous health monitoring, daily and sports activity tracking, and multifunctional electronic skin are highlighted. Finally, challenges, opportunities, and future perspectives in the field of nanomaterial-enabled wearable sensors are discussed.

An Omnidirectionally Stretchable Photodetector Based on Organic-Inorganic Heterojunctions

Omnidirectionally stretchable photodetectors are limited by difficulties in designing material and fabrication processes that enable stretchability in multiaxial directions. Here, we propose a new approach involving an organic-inorganic p-n heterojunction photodetector comprised of free-standing ZnO nanorods grown on a poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate transport layer coated on a three-dimensional micropatterned stretchable substrate containing bumps and valleys. This structure allows for efficient absorption of stretching strain. This approach allows the device to accommodate large tensile strain in all of the directions. The device behaves as a photogated p-n heterojunction photodetector in which current modulation was obtained by sensing the mechanisms that rely on photovoltage and photogating effects. The device exhibits a high photoresponse to UV light and reliable electrical performance under applied stretching in uniaxial and omniaxial directions. Furthermore, the device can be easily and conformally attached to a human wrist. This allowed us to investigate the response of the device to UV light during human activity.

Flexible Graphene-Based Wearable Gas and Chemical Sensors

Wearable electronics is expected to be one of the most active research areas in the next decade; therefore, nanomaterials possessing high carrier mobility, optical transparency, mechanical robustness and flexibility, lightweight, and environmental stability will be in immense demand. Graphene is one of the nanomaterials that fulfill all these requirements, along with other inherently unique properties and convenience to fabricate into different morphological nanostructures, from atomically thin single layers to nanoribbons. Graphene-based materials have also been investigated in sensor technologies, from chemical sensing to detection of cancer biomarkers. The progress of graphene-based flexible gas and chemical sensors in terms of material preparation, sensor fabrication, and their performance are reviewed here. The article provides a brief introduction to graphene-based materials and their potential applications in flexible and stretchable wearable electronic devices. The role of graphene in fabricating flexible gas sensors for the detection of various hazardous gases, including nitrogen dioxide (NO₂), ammonia (NH₃), hydrogen (H₂), hydrogen sulfide (H₂S), carbon dioxide (CO₂), sulfur dioxide (SO₂), and humidity in wearable technology, is discussed. In addition, applications of graphene-based materials are also summarized in detecting toxic heavy metal ions (Cd, Hg, Pb, Cr, Fe, Ni, Co, Cu, Ag), and volatile organic compounds (VOCs) including nitrobenzene, toluene, acetone, formaldehyde, amines, phenols, bisphenol A (BPA), explosives, chemical warfare agents, and environmental pollutants. The sensitivity, selectivity and strategies for excluding interferents are also discussed for graphene-based gas and chemical sensors. The challenges for developing future generation of flexible and stretchable sensors for wearable technology that would be usable for the Internet of Things (IoT) are also highlighted.