Transparent flexible graphene triboelectric nanogenerators

Advances of Strategies to Increase the Surface Charge Density of Triboelectric Nanogenerators: A Review

Triboelectric nanogenerators (TENGs) have manifested a remarkable potential for harvesting environmental energy and have the prospects to be utilized for various uses, for instance, self-powered sensing devices, flexible wearables, and marine corrosion protection. However, the potential for further development of TENGs is restricted on account of their low output power that in turn is determined by their surface charge density. The current review majorly focuses on the selection and optimization of triboelectric materials. Subsequently, various methods capable of enhancing the surface charge density of TENGs, including environmental regulation, charge excitation, charge pumping, electrostatic breakdown, charge trapping, and liquid-solid structure are comprehensively reviewed. Lastly, the review is concluded by highlighting the existing challenges in enhancing the surface charge density of TENGs and exploring potential opportunities for future research endeavors in this area.

Enhanced-Performance Triboelectric Nanogenerator Based on Polydimethylsiloxane/Barium Titanate/Graphene Quantum Dot Nanocomposites for Energy Harvesting

Triboelectric nanogenerators (TENGs) have been developed as promising energy-harvesting devices to effectively convert mechanical energy into electricity. TENGs use either organic or inorganic materials to initiate the triboelectrification process, followed by charge separation. In this study, a high-performance composite-based triboelectric nanogenerator (CTENG) device was fabricated, comprising polydimethylsiloxane (PDMS) as a polymeric matrix, barium titanite (BTO) nanopowders as dielectric fillers, and graphene quantum dots (GQDs) as conductive media. The PDMS/BTO/GQD composite film was prepared with GQDs doped into the mixture of PDMS/BTO and mechanically stirred. The composition of the GQD varied from 0 to 40 wt %. The composite was spin-coated onto flexible ITO on a PET sheet and dried in an oven at 80 °C for 24 h. The output performance of TENGs is enhanced by the increased concentration of 30 wt % GQD, which is 2 times higher than nanocomposite films without GQD. The PDMS/BTO/G30 TENG film depicted an increase in open-circuit voltage output (VOC), short-circuit current output (ISC), and power density reaching ∼310.0 V, ∼23.0 μA, and 1.6 W/m2, respectively. The simple and scalable process for the PDMS/BTO/GQD TENGs would benefit as a sustainable energy-harvesting system in small electronic devices.

Flexible and Ultra Low Weight Energy Harvesters Based on 2D Phosphorene or Black phosphorus (BP): Current and Futuristic Prospects

Phosphorene, or two-dimensional (2D) black phosphorus, has recently emerged as a competitor of graphene as it offers several advantages, including a tunable band gap, higher on/off current ratio, piezoelectric nature, and biocompatibility. Researchers have succeeded in obtaining several forms of phosphorene, such as nanosheets, nanorods, nanoribbons, and quantum dots, with satisfactory yields. Nanostructures with various controlled properties have been fabricated in multiple devices for energy production. These phosphorene-based devices are lightweight, flexible, and efficient, demonstrating great potential for energy-harvesting applications in sensors and nanogenerators. While ongoing exploration and advancements continue for these lightweight energy harvesters, it is essential to review the current progress in order to develop a future roadmap for the potential use of these phosphorene-based energy harvesters in space programs. They could be employed in applications such as wearable devices for astronauts, where ultralow weight is a vital component of any integrated device. This review also anticipates the growing significance of phosphorene in various emerging applications such as robots, information storage devices, and artificial intelligence.

Macroscopic assembly of 2D materials for energy storage and seawater desalination

Since the discovery of graphene in 2004, two-dimensional (2D) materials have attracted widespread attention due to their excellent physical and chemical properties in the fields of energy, environment, catalysis, and optoelectronics. However, there are still many key problems in the process of practical application. To further promote the potential of 2D materials for practical applications, macroscopic assembly of 2D materials is crucial for the continued development of 2D materials, especially in the fields of energy storage and seawater desalination. Therefore, this review focuses on the latest progress and current status related to the macroscopic assembly of 2D materials, including 1D fibers, 2D films, and 3D architectures. In addition, the application of macroscopic bodies assembled based on 2D materials in the fields of energy storage and seawater desalination is also introduced. Finally, future directions for the macroscopic assembly of 2D materials and their applications are prospected.

TMDC ternary alloy-based triboelectric nanogenerators with giant photo-induced enhancement

Multifunctional self-powered energy harvesting devices have attracted significant attention for wearable, portable, IoT and healthcare devices. In this study, we report transition metal dichalcogenide (TMDC) ternary alloy (Mo0.5W0.5S2)-based self-powered photosensitive vertical triboelectric nanogenerator (TENG) devices, where the ternary alloy functions both as a triboelectric layer and as a photoabsorbing material. The scalable synthesis of the highly crystalline Mo0.5W0.5S2 ternary alloy can overcome the limitations of binary TMDCs (MoS2, WS2) by utilizing its superior optical characteristics, enabling this semiconductor-based TENG device to simultaneously exhibit photoelectric and triboelectric properties. Benefiting from visible light absorption, this vertical TENG device generates higher triboelectric outputs and exhibits excellent power harvesting properties under visible light illumination. The open circuit voltage and short circuit currents of the devices under illumination (410 nm, 525 μW cm-2) are enhanced by 62% and 253%, respectively, while in the darkness, a very high photoresponsivity of ∼45.5 V mW-1 (voltage mode) is exhibited, indicating the superior energy harvesting potential under ultralow illumination. Furthermore, the energy harvesting ability from regular human activities and the operation as artificial e-skin expands the multi-functionality of this TENG device, paving a pathway for simultaneous mechanical and photonic energy harvesting with self-powered sensing.

Nano Groove and Prism-Structured Triboelectric Nanogenerators

Enhancing the output power of triboelectric nanogenerators (TENGs) requires the creation of micro or nano-features on polymeric triboelectric surfaces to increase the TENGs' effective contact area and, therefore, output power. We deploy a novel bench-top fabrication method called dynamic Scanning Probe Lithography (d-SPL) to fabricate massive arrays of uniform 1 cm long and 2.5 µm wide nano-features comprising a 600 nm deep groove (NG) and a 600 nm high triangular prism (NTP). The method creates both features simultaneously in the polymeric surface, thereby doubling the structured surface area. Six thousand pairs of NGs and NTPs were patterned on a 6×5 cm2 PMMA substrate. It was then used as a mold to structure the surface of a 200 µm thick Polydimethylsiloxane (PDMS) layer. We show that the output power of the nano-structured TENG is significantly more than that of a TENG using flat PDMS films, at 12.2 mW compared to 2.2 mW, under the same operating conditions (a base acceleration amplitude of 0.8 g).

Research Progress on the Application of Triboelectric Nanogenerators for Wind Energy Collection

The escalating global energy demand necessitates the exploration of renewable energy sources, with wind energy emerging as a crucial and widely available resource. With wind energy exhibiting a vast potential of approximately 1010 kw/a per year, about ten times that of global hydroelectric power generation, its efficient conversion and utilization hold the promise of mitigating the pressing energy crisis and replacing the dominant reliance on fossil fuels. In recent years, Triboelectric Nanogenerators (TENGs) have emerged as novel and efficient means of capturing wind energy. This paper provides a comprehensive summary of the fundamental principles governing four basic working modes of TENGs, elucidating the structures and operational mechanisms of various models employed in wind energy harvesting. Furthermore, it highlights the significance of two major TENG configurations, namely, the vertical touch-separation pattern structure and the independent layer pattern for wind energy collection, emphasizing their respective advantages. Furthermore, the study briefly discusses the current strengths of nano-friction power generation in wind energy harvesting while acknowledging the existing challenges pertaining to device design, durability, operation, and maintenance. The review concludes by presenting potential research directions and prospects for triboelectric nanogenerators generation in the realm of wind energy, offering valuable insights for researchers and scholars in the field.

Triboelectric Nanogenerators Based on 2D Materials: From Materials and Devices to Applications

Recently, there has been an increasing consumption of fossil fuels such as oil and natural gas in both industrial production and daily life. This high demand for non-renewable energy sources has prompted researchers to investigate sustainable and renewable energy alternatives. The development and production of nanogenerators provide a promising solution to address the energy crisis. Triboelectric nanogenerators, in particular, have attracted significant attention due to their portability, stability, high energy conversion efficiency, and compatibility with a wide range of materials. Triboelectric nanogenerators (TENGs) have many potential applications in various fields, such as artificial intelligence (AI) and the Internet of Things (IoT). Additionally, by virtue of their remarkable physical and chemical properties, two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), MXenes, and layered double hydroxides (LDHs), have played a crucial role in the advancement of TENGs. This review summarizes recent research progress on TENGs based on 2D materials, from materials to their practical applications, and provides suggestions and prospects for future research.

An Advanced Strategy to Enhance TENG Output: Reducing Triboelectric Charge Decay

The Internet of Things (IoT) is poised to accelerate the construction of smart cities. However, it requires more than 30 billion sensors to realize the IoT vision, posing great challenges and opportunities for industries of self-powered sensors. Triboelectric nanogenerator (TENG), an emerging new technology, is capable of easily converting energy from surrounding environment into electricity, thus TENG has tremendous application potential in self-powered IoT sensors. At present, TENG encounters a bottleneck to boost output for large-scale commercial use if just by promoting triboelectric charge generation, because the output is decided by the triboelectric charge dynamic equilibrium between generation and decay. To break this bottleneck, the strategy of reducing triboelectric charge decay to enhance TENG output is focused. First, multiple mechanisms of triboelectric charge decay are summarized in detail with basic theoretical principles for future research. Furthermore, recent advances in reducing triboelectric charge decay are thoroughly reviewed and outlined in three aspects: inhibition and application of air breakdown, simultaneous inhibition of air breakdown and triboelectric charge drift/diffusion, and inhibition of triboelectric charge drift/diffusion. Finally, challenges and future research focus are proposed. This review provides reference and guidance for enhancing TENG output.

Highly Stretchable Graphene Scrolls Transistors for Self-Powered Tribotronic Non-Mechanosensation Application

Owing to highly desired requirements in advanced disease diagnosis, therapy, and health monitoring, noncontact mechanosensation active matrix has drawn considerable attention. To satisfy the practical demands of high energy efficiency, in this report, combining the advantage of multiparameter monitoring, high sensitivity, and high resolution of active matrix field-effect transistor (FET) with triboelectric nanogenerators (TENG), we successfully developed the tribotronic mechanosensation active matrix based on tribotronic ion gel graphene scrolls field-effect transistors (GSFET). The tribopotential produced by TENG served as a gate voltage to modulate carrier transport along the semiconductor channel and realized self-powered ability with considerable decreased energy consumption. To achieve high spatial utilization and more pronounced responsivity of the dielectric of this transistor, ion gel was used to act as a triboelectric layer to conduct friction and contact electrification with external materials directly to produce triboelectric charges to power GFET. This tribopotential-driving device has excellent tactile sensing properties with high sensitivity (1.125 mm^-1), rapid response time (~16 ms), and a durability operation of thousands of cycles. Furthermore, the device was transparent and flexible with the capability of spatially mapping touch stimuli and monitoring real-time temperature. Due to all these unique characteristics, this novel noncontact mechanosensation GSFET active matrix provided a new method for self-powered E-skin with promising potential for self-powered wearable devices and intelligent robots.

Transfer-Free CVD Growth of High-Quality Wafer-Scale Graphene at 300 °C for Device Mass Fabrication

Direct chemical vapor deposition of graphene on semiconductors and insulators provides high feasibility for integration of graphene devices and semiconductor electronics. However, the current methods typically rely on high temperatures (>1000 °C), which can damage the substrates. Here, a growth method for high-quality large-area graphene at 300 °C is introduced. A multizone furnace with gradient temperature control was designed according to a computational fluid dynamics model. The crucial roles of the chamber pressure in the film continuity and hydrogen composition in the graphene defect density at low temperature were revealed. As a result, a uniform graphene film with the Raman ratio I_D/I_G = 0.08 was obtained. Furthermore, a technique of laminating single-crystal Cu foil as a sacrificial layer on the substrate was proposed to realize transfer-free growth, and a wafer-scale graphene transistor array was demonstrated with good performance consistency, which paves the way for mass fabrication of graphene devices.

Enhanced Performance of Triboelectric Nanogenerators and Sensors via Cold Spray Particle Deposition

In this study, a high-performance triboelectric nanogenerator (TENG) is developed based on cold spray (CS) deposition of composite material layers. Composite layers were fabricated by cold spraying of micron-scale tin (Sn) particles on aluminum (Al) and polytetrafluoroethylene (PTFE) films, which led to improved TENG performance owing to functionalized composite layers as friction layers and electrodes, respectively. As-sprayed tin composite layers not only enhanced the flow of charges by strong adhesion to the target layer but also formed a nano-microstructure on the surface of the layers, thereby increasing the surface area during friction. More importantly, the electricity generation performance was improved more than 6 times as compared to the TENG without CS deposition on it. From parametric studies, the TENG using the cold-sprayed composite layer produced an electrical potential of 1140 V for a simple structure with a 25.4 × 25.4 mm² contact area. We also optimize the geometry and fabrication process of the TENG to increase the manufacturing efficiency while reducing the processing cost. The resultant sprayed layers and structures exhibited sustainable robustness by showing consistent electrical performance after the mechanical adhesion test. The proposed manufacturing approach is also applicable for processing three-dimensional (3D) complex layers owing to the technological convergence of a cold spray gun attached to a robotic arm, which makes possible to fabricate the 3D TENG. To elaborate, a composite layer having the shape of a 3D ball is produced, and the exercise status of the ball is monitored in real-time. The fabricated 3D ball using the TENG transmitted a distinguishable signal in real-time according to the state of the ball. The proposed TENG sensing system can be utilized as a self-powered sensor without the need of a battery, amplifier, and rectifier. The results of this study can potentially provide insights for the practical material design and fabrication of self-powered TENG systems.

High-Performance Triboelectric Nanogenerators Based on Commercial Textiles: Electrospun Nylon 66 Nanofibers on Silk and PVDF on Polyester

A high-performance textile triboelectric nanogenerator is developed based on the common commercial fabrics silk and polyester (PET). Electrospun nylon 66 nanofibers were used to boost the tribo-positive performance of silk, and a poly(vinylidene difluoride) (PVDF) coating was deployed to increase the tribo-negativity of PET. The modifications confer a very significant boost in performance: output voltage and short-circuit current density increased ∼17 times (5.85 to 100 V) and ∼16 times (1.6 to 24.5 mA/m²), respectively, compared with the Silk/PET baseline. The maximum power density was 280 mW/m² at a 4 MΩ resistance. The performance boost likely results from enhancing the tribo-positivity (and tribo-negativity) of the contact layers and from increased contact area facilitated by the electrospun nanofibers. Excellent stability and durability were demonstrated: the nylon nanofibers and PVDF coating provide high output, while the silk and PET substrate fabrics confer strength and flexibility. Rapid capacitor charging rates of 0.045 V/s (2 μF), 0.031 V/s (10 μF), and 0.011 V/s (22 μF) were demonstrated. Advantages include high output, a fully textile structure with excellent flexibility, and construction based on cost-effective commercial fabrics. The device is ideal as a power source for wearable electronic devices, and the approach can easily be deployed for other textiles.

Bioinspired Freeze-Tolerant Soft Materials: Design, Properties, and Applications

In nature, many biological organisms have developed the exceptional antifreezing ability to survive in extremely cold environments. Inspired by the freeze resistance of these organisms, researchers have devoted extensive efforts to develop advanced freeze-tolerant soft materials and explore their potential applications in diverse areas such as electronic skin, soft robotics, flexible energy, and biological science. Herein, a comprehensive overview on the recent advancement of freeze-tolerant soft materials and their emerging applications from the perspective of bioinspiration and advanced material engineering is provided. First, the mechanisms underlying the freeze tolerance of cold-enduring biological organisms are introduced. Then, engineering strategies for developing antifreezing soft materials are summarized. Thereafter, recent advances in freeze-tolerant soft materials for different technological applications such as smart sensors and actuators, energy harvesting and storage, and cryogenic medical applications are presented. Finally, future challenges and opportunities for the rapid development of bioinspired freeze-tolerant soft materials are discussed.

Energy Interplay in Materials: Unlocking Next-Generation Synchronous Multisource Energy Conversion with Layered 2D Crystals

Layered 2D crystals have unique properties and rich chemical and electronic diversity, with over 6000 2D crystals known and, in principle, millions of different stacked hybrid 2D crystals accessible. This diversity provides unique combinations of properties that can profoundly affect the future of energy conversion and harvesting devices. Notably, this includes catalysts, photovoltaics, superconductors, solar-fuel generators, and piezoelectric devices that will receive broad commercial uptake in the near future. However, the unique properties of layered 2D crystals are not limited to individual applications and they can achieve exceptional performance in multiple energy conversion applications synchronously. This synchronous multisource energy conversion (SMEC) has yet to be fully realized but offers a real game-changer in how devices will be produced and utilized in the future. This perspective highlights the energy interplay in materials and its impact on energy conversion, how SMEC devices can be realized, particularly through layered 2D crystals, and provides a vision of the future of effective environmental energy harvesting devices with layered 2D crystals.

Recent Development of Morphology-Controlled Hybrid Nanomaterials for Triboelectric Nanogenerator: A Review

Being cognizant of modern electronic devices, the scientists are continuing to investigate renewable green-energy resources for a decade. Amid different energy harvesting systems, the triboelectric nanogenerators (TENGs) have been found to be the most promising mechanical harvesting technology and have drawn attention to generate electrical energy. Thanks to its instant output power, choice to opt for wide-ranging materials, low maintenance cost, easy fabrication process and environmentally friendly nature. Due to numerous working modes of TENGs, it is dedicated to desired application at ambient conditions. In this review, an advance correlation of TENGs have been explained based on the variety of nanostructures, including 0D, 1D, 2D, 3D, metal organic frameworks (MOFs), coordination polymers (CPs), covalent organic frameworks (COFs), and perovskite materials. Moreover, an overview of previous and current perspectives of various nanomaterials, synthesis, fabrication and their applications in potential fields have been discussed in detail.

Three-Dimensional Printable, Highly Conductive Ionic Elastomers for High-Sensitivity Iontronics

The development of hydrogels and ionic gels for applications in fields such as soft electronics and wearable sensors is limited by liquid evaporation or leakage. Ionic conductors without volatile liquids are better choices for flexible and transparent devices. Here, a liquid polymer electrolyte (LPE) is prepared from a mixture of lithium bis(trifluoromethane)sulfonimide and polyethylene glycol (PEG) above the melting point of PEG. A three-dimensional (3D) printable solvent-free ionic elastomer (IE) is introduced by photopolymerization of ethyl acrylate and hydroxyethyl acrylate in the prepared LPE. The conductivity is significantly improved by the presence of a high content of the lithium salt. Dynamic cross-linking networks improve the stretchability and resilience of the elastomer. The pattern design capability of the IE is provided by light-curing 3D printing. These features demonstrate that the IE has broad application prospects in flexible sensors, ion skins, and soft robots.

Electrode materials for stretchable triboelectric nanogenerator in wearable electronics

Stretchable Triboelectric Nanogenerators (TENGs) for wearable electronics are in significant demand in the area of self-powered energy harvesting and storage devices. Designing a suitable electrode is one of the major challenges in developing a fully wearable TENG device and requires research aimed at exploring new materials and methods to develop stretchable electrodes. This review article is dedicated to presenting recent developments in exploring new materials for flexible TENGs with special emphasis on electrode components for wearable devices. In addition, materials that can potentially deliver properties such as transparency, self-healability and water-resistance are also reviewed. Inherently stretchable materials and a combination of soft and rigid materials including polymers and their composites, inorganic and ceramic materials, 2D materials and carbonaceous nanomaterials are also addressed. Additionally, various fabrication strategies and geometrical patterning techniques employed for designing highly stretchable electrodes for wearable TENG devices are also explored. The challenges reflected in the present approaches as well as feasible suggestions for future advancements are discussed.

Schematic illustration of the general requirements of components of a wearable TENG.

Filling the gap between topological insulator nanomaterials and triboelectric nanogenerators

Reliable energy modules and higher-sensitivity, higher-density, lower-powered sensing systems are constantly required to develop wearable electronics and the Internet of Things technology. As an emerging technology, triboelectric nanogenerators have been potentially guiding the landscape of sustainable power units and energy-efficient sensors. However, the existing triboelectric series is primarily populated by polymers and rubbers, limiting triboelectric sensing plasticity to some extent owing to their stiff surface electronic structures. To enrich the current triboelectric group, we explore the triboelectric properties of the topological insulator nanofilm by Kelvin probe force microscopy and reveal its relatively positive electrification charging performance. Both the larger surface potential difference and the conductive surface states of the nanofilms synergistically improve the charge transfer behavior between the selected triboelectric media, endowing the topological insulator-based triboelectric nanogenerator with considerable output performance. Besides serving as a wearable power source, the ultra-compact device array demonstrates innovative system-level sensing capabilities, including precise monitoring of dynamic objects and real-time signal control at the human-machine interface. This work fills the blank between topological quantum matters and triboelectric nanogenerators and, more importantly, exploits the significant potential of topological insulator nanofilms for self-powered flexible/wearable electronics and scalable sensing technologies.

Advances in Inorganic Nanomaterials for Triboelectric Nanogenerators

Triboelectric nanogenerators (TENGs) that utilize triboelectrification and electrostatic induction to convert mechanical energy to electricity have attracted increasing interest in the last 10 years. As a universal physical phenomenon, triboelectrification can occur between any two surfaces that experience physical contact and separation regardless of the type of material. For this reason, many materials, including both organic and inorganic materials, have been studied in TENGs with different purposes. Although organic polymers are mainly used as triboelectric materials in TENGs, the application of inorganic nanomaterials has also been intensively studied because of their unique dielectric, electric, piezoelectric, and optical properties, which can improve the performance of TENGs. A review of how inorganic nanomaterials are used in TENGs would help researchers gain an overview of the progress in this area. Here, we present a review to summarize how inorganic nanomaterials are utilized in TENGs based on the roles, types, and characteristics of the nanomaterials.

Recent trends in 2D materials and their polymer composites for effectively harnessing mechanical energy

Self-powered wearable devices, with the energy harvester as a source of energy that can scavenge the energy from ambient sources present in our surroundings to cater to the energy needs of portable wearable electronics, are becoming more widespread because of their miniaturization and multifunctional characteristics. Triboelectric and piezoelectric nanogenerators are being explored to harvest electrical energy from the mechanical vibrations. Integration of these two effects to fabricate a hybrid nanogenerator can further enhance the output efficiency of the nanogenerator. Here, we have discussed the importance of 2D materials which plays an important role in the fabrication of nanogenerators because of their distinct characteristics, such as, flexibility, mechanical stability, nontoxicity, and biodegradability. This review mainly emphasizes the piezoelectric, triboelectric, and hybrid nanogenerator based on the 2D materials and their van der Waals heterostructure, as well as the effect of polymer-2D composite on the output performance of the nanogenerator.

Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

Self-powered bifunctional sensor based on tribotronic planar graphene transistors

With the development of material science, micro-nano-fabrication and microelectronics, the higher level requirements are posed on the electronic skins (E-skin). The lower energy consumption and multiple functions are the imperative requirements to spurred scientists and mechanists to make joint efforts to meet. To achieve lower energy consumption, a promising energy-harvesting style of triboelectric nanogenerators (TENG) is incorporated into the field effect transistors (FETs) to play the important role for sensor. For bifunctional sensor, to harness the difficult for reflecting the magnitude of frequency, we resorted to synaptic transistors to achieve more intelligentization. Furthermore, with regards to the configuration of FET, we continued previous work: using the electrolyte gate dielectrics of FET-ion gel as the electrification layer to achieve high efficient, compact and extensively adoption for mechanosensation. The working principle of the GFET was based on the coupling effects of the FET and the TENG. This newly emerged self-powered sensor would offer a new platform for lower power consumption sensor for human-machine interface and intelligent robot.

2D Materials for Skin-Mountable Electronic Devices

Skin-mountable devices that can directly measure various biosignals and external stimuli and communicate the information to the users have been actively studied owing to increasing demand for wearable electronics and newer healthcare systems. Research on skin-mountable devices is mainly focused on those materials and mechanical design aspects that satisfy the device fabrication requirements on unusual substrates like skin and also for achieving good sensing capabilities and stable device operation in high-strain conditions. 2D materials that are atomically thin and possess unique electrical and optical properties offer several important features that can address the challenging needs in wearable, skin-mountable electronic devices. Herein, recent research progress on skin-mountable devices based on 2D materials that exhibit a variety of device functions including information input and output and in vitro and in vivo healthcare and diagnosis is reviewed. The challenges, potential solutions, and perspectives on trends for future work are also discussed.

Triboelectric-optical responsive cholesteric liquid crystals for self-powered smart window, E-paper display and optical switch

Intelligent responsive devices are crucial for a variety of applications ranging from smart electronics to robotics. Electro-responsive cholesteric liquid crystals (CLC) have been widely applied in display panels, smart windows, and so on. In this work, we realize the mechanical stimuli-triggered optical responses of the CLC by integrating it with a triboelectric nanogenerator (TENG), which converts the mechanical motion into alternating current electricity and then tunes the different optical responses of the CLC. When the voltage applied on the CLC is relatively low (15-40 V), the TENG drives the switching between the bistable planar state and focal conic state of the CLC, which shows potential applications in self-powered smart windows or E-paper displays. When the voltage supplied by the TENG is larger than 60 V, a self-powered optical switch is demonstrated by utilizing the transformation between focal conic state and instantons homeotropic state of the CLC. This triboelectric-optical responsive device consumes no extra electric power and suggests a great potential for future smart electronics.

Film-Sponge-Coupled Triboelectric Nanogenerator with Enhanced Contact Area Based on Direct Ultraviolet Laser Ablation

Triboelectric nanogenerators (TENGs) recently have emerged as applicable and eco-friendly harvesting devices. Numerous studies have been actively conducted to fabricate a flexible and robust TENG with high-output performance. Herein, a film-sponge-coupled TENG (FS-TENG) is proposed using direct ultraviolet laser ablation, as a method for surface modification of a polyimide (PI) film. This state-of-the-art method has advantages of accuracy as well as time efficiency in creating the pattern on the surface; thus, the pre-designed patterns can be precisely constructed within only a minute. In the laser-ablated PI film, the structural design and chemical modification on the surface are investigated related to the triboelectric output performance. Thereafter, a sponge is fabricated based on non-woven polyamide and silicone rubber, which can fully contact with the micro-/nano-scaled structure on the surface of the PI film. After an optimization, the FS-TENG exhibits 48.19 V of open-circuit voltage and 1.243 μA of short-circuit current, which shows approximately 3 times enhanced electric performance compared to the FS-TENG using a pristine PI film. The FS-TENG device demonstrates its robustness through both mechanical stress and flexible stress by showing less than 5% degradation after 50,000 cycles. On the basis of the high flexibility and stability of the FS-TENG, a self-powered scoreboard is successfully developed for lighting a scoreboard in a soccer field. This feasible lighting system can be operated by harvesting the kinetic energy of a soccer player without an additional power source. The novel FS-TENG, thus, provides remarkable potential for a self-powered indoor harvesting system.

Self-Powered Smart Arm Training Band Sensor Based on Extremely Stretchable Hydrogel Conductors

The development of elastic electronic technology has promoted the application of triboelectric nanogenerators (TENGs) in flexible wearable electronics. However, most of the flexible electronics cannot achieve the requirements of being extremely stretchable, transparent, and highly conductive at the same time. Herein, we report a TENG constructed using a double-network polymer ionic conductor sodium alginate/zinc sulfate/poly acrylic-acrylamide (SA-Zn) hydrogel, which exhibited outstanding stretchability (>10,000%), high transparency (>95%), and good conductivity (0.34 S·m^-1). The SA-Zn hydrogel TENG (SH-TENG) could harvest energy from typical human movements, such as bending, stretching, and twisting, which could light up 234 green commercial LEDs easily. Additionally, the SH-TENG can be used to prepare a self-powered smart training band sensor for monitoring arm stretching motion. This work may provide an innovative platform for accessing the next generation of sustainable wearable and sports monitoring electronics.

Flexible Layered-Graphene Charge Modulation for Highly Stable Triboelectric Nanogenerator

The continuous quest to enhance the output performance of triboelectric nanogenerators (TENGs) based on the surface charge density of the tribolayer has motivated researchers to harvest mechanical energy efficiently. Most of the previous work focused on the enhancement of negative triboelectric charges. The enhancement of charge density over positive tribolayer has been less investigated. In this work, we developed a layer-by-layer assembled multilayer graphene-based TENG to enhance the charge density by creatively introducing a charge trapping layer (CTL) Al₂O₃ in between the positive triboelectric layer and conducting electrode to construct an attractive flexible TENG. Based on the experimental results, the optimized three layers of graphene TENG (3L-Gr-TENG) with CTL showed a 30-fold enhancement in output power compared to its counterpart, 3L-Gr-TENG without CTL. This remarkably enhanced performance can be ascribed to the synergistic effect between the optimized graphene layers with high dielectric CTL. Moreover, the device exhibited outstanding stability after continuous operation of >2000 cycles. Additionally, the device was capable of powering 20 green LEDs and sufficient to power an electronic timer with rectifying circuits. This research provides a new insight to improve the charge density of Gr-TENGs as energy harvesters for next-generation flexible electronics.

Structural and Chemical Modifications Towards High-Performance of Triboelectric Nanogenerators

Harvesting abundant mechanical energy has been considered one of the promising technologies for developing autonomous self-powered active sensors, power units, and Internet-of-Things devices. Among various energy harvesting technologies, the triboelectric harvesters based on contact electrification have recently attracted much attention because of their advantages such as high performance, light weight, and simple design. Since the first triboelectric energy-harvesting device was reported, the continuous investigations for improving the output power have been carried out. This review article covers various methods proposed for the performance enhancement of triboelectric nanogenerators (TENGs), such as a triboelectric material selection, surface modification through the introduction of micro-/nano-patterns, and surface chemical functionalization, injecting charges, and their trapping. The main purpose of this work is to highlight and summarize recent advancements towards enhancing the TENG technology performance through implementing different approaches along with their potential applications. This paper presents a comprehensive review of the TENG technology and its factors affecting the output power as material selection, surface physical and chemical modification, charge injection, and trapping techniques.

Volatile organic compounds sensing based on Bennet doubler-inspired triboelectric nanogenerator and machine learning-assisted ion mobility analysis

Ion mobility analysis is a well-known analytical technique for identifying gas-phase compounds in fast-response gas-monitoring systems. However, the conventional plasma discharge system is bulky, operates at a high temperature, and inappropriate for volatile organic compounds (VOCs) concentration detection. Therefore, we report a machine learning (ML)-enhanced ion mobility analyzer with a triboelectric-based ionizer, which offers good ion mobility selectivity and VOC recognition ability with a small-sized device and non-strict operating environment. Based on the charge accumulation mechanism, a multi-switched manipulation triboelectric nanogenerator (SM-TENG) can provide a direct current (DC) bias at the order of a few hundred, which can be further leveraged as the power source to obtain a unique and repeatable discharge characteristic of different VOCs, and their mixtures, with a special tip-plate electrode configuration. Aiming to tackle the grand challenge in the detection of multiple VOCs, the ML-enhanced ion mobility analysis method was successfully demonstrated by extracting specific features automatically from ion mobility spectrometry data with ML algorithms, which significantly enhance the detection ability of the SM-TENG based VOC analyzer, showing a portable real-time VOC monitoring solution with rapid response and low power consumption for future internet of things based environmental monitoring applications.

Tribo-Induced Color Tuner toward Smart Lighting and Self-Powered Wireless Sensing

The color-tuning capability of solid-state lighting (SSL) systems are highly demanded for smart lighting according to the environmental conditions, as well as wireless sensing of the environmental information. In the meanwhile, state-of-the-art triboelectric nanogenerator (TENG)-based sensing systems rely on bulky and expensive devices, which require cable connections and additional power consumptions. This work aims at solving these challenges, through developing a tribo-induced color tuner that can be integrated into the vastly distributed commercial SSL system. This tribo-induced color tuner includes a concentric color conversion plate consisting of (Sr,Ca)AlSiN3:Eu phosphor and TiO2, a tribo-induced liquid lens, and a rotary freestanding sliding TENG. The color oscillation between purple and pink is achieved upon the tribo-charging by the TENG, which reveals the input mechanical motion signals. The signal can be conveniently sent by everywhere-existed lamps and processed by everyone-owned smartphone cameras or closed-circuit televisions. Through this approach, the function of wireless sensing is achieved without the need of preamplification, with no additional power supply required, as demonstrated for wireless sensing of the rotation speed. The smart lighting for underwater photographing is also demonstrated by the color-tunable SSL system with the best imaging quality achieved.

Enhanced Triboelectric Nanogenerator Based on Tungsten Disulfide via Thiolated Ligand Conjugation

Two-dimensional transition-metal dichalcogenides (TMDs) are of particular interest as a new active material for future triboelectric nanogenerators (TENGs) owing to their excellent electrical properties, optical transparency, flexibility, ultrathin thickness, and biocompatibility. Here, we propose a new approach to engineer the surface of TMDs via conjugation with thiolated ligands having different alkane chain lengths and to develop TMD-based TENG devices that exhibit enhanced output performance for the first time. The triboelectric charging behaviors of ligand-conjugated TMDs are successfully investigated, and the electrical output performance of TMD TENGs based on TMD-to-polymer device geometries with a vertical contact-separation mode is dramatically improved, exhibiting an output voltage of 12.2 V and a power density of 138 mW/m². Furthermore, the ligand-conjugated TMD TENG device exhibits a highly stable operation under repeated contact and separation over 10 000 cycles, as well as high chemical stability, as a result of novel defect engineering via thiolated ligand conjugation. Detailed investigation reveals that the improved performance of the ligand-conjugated TMD TENG device originates from the synergistic effect of defect engineering and the p-type doping effect of TMDs, correlated with the increased electric potential difference between triboelectric layers. These findings provide a new potential of TMDs as a promising building block for the next-generation energy harvesting system.

Ultra-stretchable and healable hydrogel-based triboelectric nanogenerators for energy harvesting and self-powered sensing

The next-generation multifunctional soft electronic devices require the development of energy devices possessing comparable functions. In this work, an ultra-stretchable and healable hydrogel-based triboelectric nanogenerator (TENG) is prepared for mechanical energy harvesting and self-powered sensing. An ionic conductive hydrogel was developed with graphene oxide and Laponite. as the physical cross-linking points, exhibiting high stretchability (∼1356%) and healable capability. When using the hydrogel as the electrode, the TENG can operate normally at 900% tensile strain, while the electrical output of the TENG can fully recover to the initial value after healing the damage. This hydrogel-based TENG is demonstrated to power wearable electronics, and is used as a self-powered sensor for human motion monitoring and pressure sensing. Our work shows opportunities for multifunctional power sources and potential applications in wearable electronics.

An ultra-stretchable and self-healing hydrogel is developed with graphene oxide and Laponite as collaborative physical crosslinking points, which is utilized in triboelectric nanogenerators for mechanical energy harvesting and self-powered sensing.

Transparent Triboelectric Nanogenerator Based on Thermoplastic Polyurethane Films

This study aims at investigating flexible and transparent thermoplastic polyurethane (TPU) as a novel material for triboelectric nanogenerator (TENG) devices with a polyethylene terephthalate layer. Devices having TPU-either as a flat film or as electrospun micrometer-dimension fibers with varying concentrations of TPU-were tested. The best output performing device provided 21.4 V and 23 μA as open-circuit voltage and short-circuit current respectively, with the application of a small force of 0.33 N indicating the high efficiency of the device. Devices with flat films-obtained using the doctor-blade (DB) technique-have high transparency (80%) as well as high TENG output. The topography of the TPU layer, characterized by atomic force microscopy, reveals nanoscale roughness of the film surface. Finally, we demonstrate that gentle hand tapping on the TENG device can power upto 11 light-emitting diodes (LEDs). The high transparency, lightweight, simple fabrication, flexibility, and robust features of such device make it an added value for various optoelectronic applications.

Real-Time Monitoring System of Automobile Driver Status and Intelligent Fatigue Warning Based on Triboelectric Nanogenerator

To improve automobile safety, identifying driver fatigue is considerably crucial because it is one of the main causes of traffic accidents. In this research, smart systems based on a triboelectric nanogenerator are designed, which can provide driver status monitoring and fatigue warning in real time. The smart system consists of a self-powered steering-wheel angle sensor (SSAS) and a signal processing unit. The SSAS, which comprises a stator, a rotor, and a sleeve, is mounted on the steering rod, and the electrodes are designed with a phase difference to improve the resolution of the sensor. The turning angle of the steering wheel operated by the driver is recorded by the SSAS; meanwhile, the number of rotations, the average angle, and other parameters in the driver's recorded data are analyzed by the signal processing unit from which a warning threshold for each parameter is determined. The system assesses the status of the driver in real-time by comparing these parameters and threshold values, and experimental results demonstrate that driver status is accurately judged. This work has important potential applications in the fields of traffic safety and intelligent driving.

Triboelectric nanogenerators for human-health care

Since the world's first triboelectric nanogenerator (TENG) was proposed in 2012, numerous TENG-based devices and equipment have sprung up in various fields. In particular, TENG has great potential in the field of human-health care due to its small size, self-powered and low cost. With the continuous deepening of TENG research, its structure, function and technical concept are becoming more and more abundant. In order to summarize the progress and development status of TENG in health care, based on the different types of applications subdirection, this paper reviews the TENG-based research work of this field in recent eight years. The characteristics of various types of TENG-based applications and their corresponding technologies are introduced and analyzed, under the comparison of their structure and performance. This review is dedicated to provide reference and inspiration for the future development and innovation of TENG for health care.

Patchable and Implantable 2D Nanogenerator

With the development of technology, electronic devices are becoming more miniaturized and multifunctional. With the development of small electronic devices, they are changing from the conventional accessory type, which is portable, to the patchable type, which can be attached to a person's apparel or body, and the eatable/implantable type, which can be directly implanted into the human body. In this regard, it is necessary to address various technical issues, such as high-capacity/high-efficiency small-sized battery technology, component miniaturization, low power technology, flexible technology, and smart sensing technology. In addition, there is a demand for self-powered wireless systems in particular devices. A piezoelectric/triboelectric nanogenerator (PENG/TENG) can generate electric energy from small amounts of mechanical energy such as from blood flow and heartbeats in the human body as well as human movement, so it is expected that it will enable the development of self-powered wireless systems. Due to their unique properties, such as flexibility, transparency, mechanical stability, and nontoxicity, 2D materials are optimal materials for the development of implantable and patchable self-powered nanodevices in the human body. In this Review, the studies related to patchable and implantable devices for the human body using PENGs/TENGs based on 2D materials are discussed.

Extremely Foldable and Highly Porous Reduced Graphene Oxide Films for Shape-Adaptive Triboelectric Nanogenerators

Here, a thin and foldable porous reduced graphene oxide (rGO) fabricated by a solvent casting method (SC-rGO) is introduced. The SC-rGO is superior to aluminum as a positive triboelectric material in triboelectric nanogenerators (TENGs), significantly enhancing TENG output performance. The film shows extremely foldable features, where it could be folded by 1/16 size. The electrical properties and device performance of SC-rGO are optimized varying thicknesses from 5 to 30 µm. A 30 µm thick TENG with a non-annealed SC-rGO film (STENG) shows the highest output of about 255 µW cm^-2 due to its high carrier concentration, low work function, and high surface area. After annealing, STENG performance is optimized with a 10 µm thick SC-rGO because their work functions decreases, while the corresponding carrier concentrations decrease according to the thickness of the SC-rGO films. The SC-rGO films are highly durable and stable, where their output and conductivity show negligible changes after 100 000 cycles of mechanical deformation. A large SC-rGO with a size of 13 × 3 cm² is fabricated and is attached inside a person's arm to demonstrate the shape-adaptive characteristics. Consequently, 170 V is obtained and it turns on 19 green light emitting diodes by simply touching the STENG.

Unprecedented enhancement of wear resistance for epoxy-resin graphene composites

Epoxy resins (ERs) have extraordinary mechanical, electrical and chemical properties, and are widely used in the aerospace, electronics and marine industries. Nonetheless, solidified ERs have intrinsic brittleness and low wear resistance. Until now, the promotion of the wear resistance of ER is limited to 30 times, through blending from one to four reinforcing materials. Therefore, it has been a challenge to enhance the wear resistance of ER to over 30 times. Additionally, mechanisms to improve the tribological properties of polymer composites are elusive. In this study, novel ER/graphene composites (ECs) were developed, and the wear resistance of EC with 5 wt% graphene (EC5) was shown to be 628 times that of pure ER at 10 N. To the best of our knowledge, the unprecedented enhancement of wear resistance for ER is the highest reported. The enhancement mechanisms of graphene reinforcement to ER were determined by molecular dynamics simulations. When the content of graphene reaches 5 wt%, exfoliated graphene flakes adhere the most on the surface of a stainless-steel ball during sliding tests, reducing the wear most effectively. However, when the content of graphene is over 5 wt%, graphene flakes accumulate inside the composites, and less exfoliated graphene flakes adhere to the surface of the ball during sliding, increasing the wear. The developed binary ECs are light-weight and cost-effective and have minimal impact on the environment. This composite has many potential applications for high-performance components used in the aerospace, electronics and marine industries.

Self-Powered Room-Temperature Ethanol Sensor Based on Brush-Shaped Triboelectric Nanogenerator

Highly sensitive ethanol sensors have been widely utilized in environmental protection, industrial monitoring, and drink-driving tests. In this work, a fully self-powered ethanol detector operating at room temperature has been developed based on a triboelectric nanogenerator (TENG). The gas-sensitive oxide semiconductor is selected as the sensory component for the ethanol detection, while the resistance change of the oxide semiconductor can well match the "linear" region of the load characteristic curve of TENG. Hence, the output signal of TENG can directly reveal the concentration change of ethanol gas. An accelerator gearbox is applied to support the operation of the TENG, and the concentration change of ethanol gas can be visualized on the Liquid Crystal Display. This fully self-powered ethanol detector has excellent durability, low fabrication cost, and high selectivity of 5 ppm. Therefore, the ethanol detector based on TENG not only provides a different approach for the gas detection but also further demonstrates the application potential of TENG for various sensory devices.

Potential of graphene-based materials to combat COVID-19: properties, perspectives, and prospects

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a new virus in the coronavirus family that causes coronavirus disease (COVID-19), emerges as a big threat to the human race. To date, there is no medicine and vaccine available for COVID-19 treatment. While the development of medicines and vaccines are essentially and urgently required, what is also extremely important is the repurposing of smart materials to design effective systems for combating COVID-19. Graphene and graphene-related materials (GRMs) exhibit extraordinary physicochemical, electrical, optical, antiviral, antimicrobial, and other fascinating properties that warrant them as potential candidates for designing and development of high-performance components and devices required for COVID-19 pandemic and other futuristic calamities. In this article, we discuss the potential of graphene and GRMs for healthcare applications and how they may contribute to fighting against COVID-19.

Performance-Enhanced Triboelectric Nanogenerator Based on the Double-Layered Electrode Effect

Recently, studies on enhancing the performance of triboelectric nanogenerators (TENGs) by forming nanostructures at the contacting interface have been actively reported. In this study, a double-layered bottom electrode TENG (DE-TENG) was successfully fabricated using a metal deposition layer after the water-assisted oxidation (WAO) process. As previously reported, the WAO process for the enhancement of electrical performance increases the effective contact area with an inherent surface oxidation layer (Al₂O₃). As a new approach for modifying deficiencies in the WAO process, a metal deposition onto the oxidation layer was successfully developed with increased device output performance by restoring the surface conductivity. The proposed metal–dielectric–metal sandwich-structured DE-TENG generated approximately twice the electrical output generated by the WAO process alone (WAO-TENG). This dramatically improved electrical output was proven by a theoretical demonstration based on a double capacitance structure. In addition, the double capacitance structure was confirmed with the aid of a field emission scanning electron microscope. The optimal point at which the DE-TENG generates the highest electrical outputs was observed at a specific Cu layer sputtering time. The exceptional durability of the DE-TENG was proved by the 1 h endurance test under various relative humidity conditions. The potential of a self-powered force sensor using this DE-TENG is demonstrated, having a comparably high sensitivity of 0.82 V/N. Considering its structure, increased electrical energy, easy fabrication, and its durability, this novel DE-TENG is a promising candidate for the self-powered energy harvesting technology in our near future.

Two-dimensional graphitic carbon nitride nanosheets: a novel platform for flexible, robust and optically active triboelectric nanogenerators

We report on the characteristics of mechanically flexible, stable and photoactive triboelectric nanogenerators based on two-dimensional graphitic carbon nitride (g-C₃N₄) nanosheets. The performance of nanogenerator devices has been studied with varying frictional surfaces (such as polypropylene, aluminium oxide, Teflon and polyethylene terephthalate). Energy band diagrams have been used to explain the mechanism of triboelectric charge transfer in pristine and doped g-C₃N₄, with the former showing better characteristics. An optimized device has been found to be responsive to external stimuli to generate an output voltage of 10 V upon simple biomechanical impulses. To demonstrate the efficacy for practical applications of g-C₃N₄-based triboelectric nanogenerators, output voltages have been recorded for different common activities like walking, water showering, using as a writing/drawing pad, etc. Repetitive finger tapping on a device could charge a capacitor to as high as 55 V within ∼50 s, while that under UV illumination is found to be much faster (∼14 s) due to photoinduced carrier generations in g-C₃N₄. The exhibition of a superior photoresponsivity of ∼117 V W^-1 under UV illumination demonstrates the dual functionality of g-C₃N₄-based triboelectric devices as a nanogenerator as well as an active flexible photosensor, which is hitherto unreported. Excellent mechanical flexibility, stability and photoinduced enhancement of output characteristics make g-C₃N₄ an attractive candidate for nanogenerator devices for future applications.

Aim high energy conversion efficiency in triboelectric nanogenerators

Triboelectric nanogenerators (TENGs) that enable the conversion of a given mechanical energy into electrical energy at high efficiency have been very important in practice. Since the given mechanical energy is involuntarily converted to secondary energy sources (light, heat, and sound during triboelectrification), the significant amount of energy being converted is lost. Various studies have thus been continuously carried out to overcome this issue. Since the first TENGs found in 2012, various developments in TENGs have been made: (1) the mechanical-electrical energy conversion characteristics of potential organic/inorganic material groups have been introduced, (2) the integration into the device structure considering the diversity of mechanical energy, and (3) user friendly and industrial application platforms have been aggressively studied. Despite the remarkable progress and improvement of TENGs, their mechanical-electrical conversion efficiency is still quite low. We therefore need to discover and develop materials that can be converted to improve efficiency. Here, we outline the recent progress made in a group of high polarity triboelectric materials that exploit surface charge density and charge transfer properties. We also review the recent boosting powering TENGs. The aim of this work is to provide insight into the future direction and strategies for highly enhanced powering TENGs through material research.

A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites-MoS2 heterostructures, 2D-0D MoS2/quantum dots (QDs) and 2D-2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W-1 up to 1010 A W-1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10-9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

Multifunctional Water Drop Energy Harvesting and Human Motion Sensor Based on Flexible Dual-Mode Nanogenerator Incorporated with Polymer Nanotubes

In the world of increasing energy consumption, nanogenerators have shown great potential for energy harvesting and self-powered portable electronics. Herein, a flexible and dual-mode triboelectric nanogenerator (TENG) combining both vertical contact-separation and single electrical modes has been developed to convert environmental mechanical energy into electricity using highly encapsulated and multifunctional strategies. By introducing the polymer melt wetting technique, polymer nanotubes are fabricated on the surface of the TENG, which provides self-cleaning and hydrophobic features beneficial for water drop energy harvesting using the device. In such mechanical energy harvesting, the maximum output power of 0.025 mW and the open-circuit voltage of 41 V can be achieved. By designing the dimensions of the device, the dual-mode TENG is utilized as a self-powered sensor to detect human body motions such as phalanges' movement of fingers. The fabricated dual-mode TENG promotes the development of energy-harvesting and self-powered human motion sensors for artificial intelligent prosthetics, human kinematics, and human body recovery treatment.

Suppressed Interdiffusion and Degradation in Flexible and Transparent Metal Electrode-Based Perovskite Solar Cells with a Graphene Interlayer

Metal-based transparent conductive electrodes (TCEs) are attractive candidates for application in indium tin oxide (ITO)-free solar cells due to their excellent electrical conductivity and cost effectiveness. In perovskite solar cells (PSCs), metal-induced degradation with the perovskite layer leads to various detrimental effects, deteriorating the device performance and stability. Here, we introduce a novel flexible hybrid TCE consisting of a Cu grid-embedded polyimide film and a graphene capping layer, named GCEP, which exhibits excellent mechanical and chemical stability as well as desirable optoelectrical properties. We demonstrated the critical role of graphene as a protection layer to prevent metal-induced degradation and halide diffusion between the electrode and perovskite layer; the performance of the flexible PSCs fabricated with GCEP was comparable to that of their rigid ITO-based counterparts and also exhibited outstanding mechanical and chemical stability. This work provides an effective strategy to design mechanically and chemically robust ITO-free metal-assisted TCE platforms in PSCs.