Freestanding Lamellar Porous Carbon Stacks for Low‐Temperature‐Foldable Supercapacitors

High‐performance supercapacitors (SCs) are important energy storage components for emerging wearable electronics. Rendering low‐temperature foldability to SCs is critically important when wearable devices are used in a cold environment. However, currently reported foldable SCs do not have a stable electrochemical performance at subzero temperatures, while those that are performing are not foldable. Herein, a freestanding pure‐carbon‐based porous electrode, namely, lamellar porous carbon stack (LPCS), is reported, which enables stable low‐temperature‐foldable SCs. The LPCS, which is fabricated with a simple vacuum filtration of a mixture of carbon fibers (CFs), holey reduced graphene oxides (HRGOs), and carbon nanotubes (CNTs), possesses a lamellar stacking of porous carbon thin sheets, in which the CFs act as the skeleton and the HRGOs and CNTs act as binders. The unique structure leads to excellent compression resilience, high foldability, and high electronic and ionic conductivity. SCs made with the LPCS electrodes and ionic liquid electrolyte show a high energy density (2.1 mWh cm^?2 at 2 mA cm^?2), low‐temperature long lifetime (95% capacity after 10 000 cycles at ?30 °C), and excellent low‐temperature foldability (86% capacity after 1000 folding cycles at ?30 °C).     

Ultrahigh Thermal Conductive yet Superflexible Graphene Films

Electrical devices generate heat at work. The heat should be transferred away immediately by a thermal manager to keep proper functions, especially for high‐frequency apparatuses. Besides high thermal conductivity (K ), the thermal manager material requires good foldability for the next generation flexible electronics. Unfortunately, metals have satisfactory ductility but inferior K (≤429 W m^?1 K^?1), and highly thermal‐conductive nonmetallic materials are generally brittle. Therefore, fabricating a foldable macroscopic material with a prominent K is still under challenge. This study solves the problem by folding atomic thin graphene into microfolds. The debris‐free giant graphene sheets endow graphene film (GF) with a high K of 1940 ± 113 W m^?1 K^?1. Simultaneously, the microfolds render GF superflexible with a high fracture elongation up to 16%, enabling it more than 6000 cycles of ultimate folding. The large‐area multifunctional GFs can be easily integrated into high‐power flexible devices for highly efficient thermal management.     

Folding Large Graphene‐on‐Polymer Films Yields Laminated Composites with Enhanced Mechanical Performance

A folding technique is reported to incorporate large‐area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water–air‐interface‐facilitated procedure, an A5‐size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a ≈0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three‐point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties.     

Assembly of Foldable 3D Microstructures Using Graphene Hinges

Origami/kirigami-inspired 3D assembly approaches have recently attracted attention for a variety of applications, such as advanced optoelectronic devices and biomedical sensors. The results reported here describe an approach to construct classes of multiple foldable 3D microstructures that involve deformations that typical conductive materials, such as conventional metal films, cannot tolerate. Atomically thin graphene sheets serve as folding hinges during a process of 2D to 3D conversion via a deterministic buckling process. The exceptional mechanical properties of graphene enable the controlled, geometric transformation of a 2D precursor bonded at selective sites on a prestretched elastomer into folded 3D microstructures, in a reversible manner without adverse effects on the electrical properties. Experimental and computational investigations of the folding mechanisms for such types of 3D objects reveal the underlying physics and the dependence of the process on the thickness of the graphene/supporting films that define the hinges.     

High‐Performance,Ultrathin, Ultraflexible Organic Thin‐Film Transistor Array Via Solution Process

Ultrathin organic thin‐film transistors (OTFTs) have received extensive attention due to their outstanding advantages, such as extreme flexibility, good conformability, ultralight weight, and compatibility with low‐cost and large‐area solution‐processed techniques. However, compared with the rigid substrates, it still remains a challenge to fabricate high‐performance ultrathin OTFTs. In this study, a high‐performance ultrathin 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C8‐BTBT) OTFT array is demonstrated via a simple spin‐coating method, with mobility as high as 11 cm² V⁻¹ s⁻¹ (average mobility: 7.22 cm² V⁻¹ s⁻¹), on/off current ratio of over 10⁶, switching current of >1 mA, and a good yield ratio as high as 100%. The ultrathin thickness at ≈380 nm and the ultralight weight at ≈0.89 g m⁻² enable the free‐standing OTFTs to imperceptibly adhere onto human skin, and even a damselfly wing without affecting its flying. More importantly, the OTFTs show good electrical characteristics and mechanical stability when conformed onto the curved surfaces and even folded in a book after 100 folding cycles. These results illustrate the broad application potential of this simply fabricated ultrathin OTFT in next‐generation electronics such as foldable displays and wearable devices.     

Highly Transparent Conducting Nanopaper for Solid State Foldable Electrochromic Devices

It is of great challenge to develop a transparent solid state electrochromic device which is foldable at the device level. Such devices require delicate designs of every component to meet the stringent requirements for transparency, foldability, and deformation stability. Meanwhile, nanocellulose, a ubiquitous natural resource, is attracting escalating attention recently for foldable electronics due to its extreme flexibility, excellent mechanical strength, and outstanding transparency. In this article, transparent conductive nanopaper delivering the state‐of‐the‐art electro‐optical performance is achieved with a versatile nanopaper transfer method that facilitates junction fusing for high‐quality electrodes. The highly compliant nanopaper electrode with excellent electrode quality, foldability, and mechanical robustness suits well for the solid state electrochromic device that maintains good performance through repeated folding, which is impossible for conventional flexible electrodes. A concept of camouflage wearables is demonstrated using gloves with embedded electrochromics. The discussed strategies here for foldable electrochromics serve as a platform technology for futuristic deformable electronics.     

Laser‐Induced Molybdenum Carbide–Graphene Composites for 3D Foldable Paper Electronics

Versatile and low‐cost manufacturing processes/materials are essential for the development of paper electronics. Here, a direct‐write laser patterning process is developed to make conductive molybdenum carbide–graphene (MCG) composites directly on paper substrates. The hierarchically porous MCG structures are converted from fibrous paper soaked with the gelatin‐mediated inks containing molybdenum ions. The resulting Mo₃C₂ and graphene composites are mechanically stable and electrochemically active for various potential applications, such as electrochemical ion detectors and gas sensors, energy harvesters, and supercapacitors. Experimentally, the electrical conductivity of the composite is resilient to mechanical deformation with less than 5% degradation after 750 cycles of 180° repeated folding tests. As such, the direct laser conversion of MCGs on papers can be applicable for paper‐based electronics, including the 3D origami folding structures.     

Tailored Crumpling and Unfolding of Spray‐Dried Pristine Graphene and Graphene Oxide Sheets

For the first time, pristine graphene can be controllably crumpled and unfolded. The mechanism for graphene is radically different than that observed for graphene oxide; a multifaced crumpled, dimpled particle morphology is seen for pristine graphene in contrast to the wrinkled, compressed surface of graphene oxide particles, showing that surface chemistry dictates nanosheet interactions during the crumpling process. The process demonstrated here utilizes a spray‐drying technique to produce droplets of aqueous graphene dispersions and induce crumpling through rapid droplet evaporation. For the first time, the gradual dimensional transition of 2D graphene nanosheets to a 3D crumpled morphology in droplets is directly observed; this is imaged by a novel sample collection device inside the spray dryer itself. The degree of folding can be tailored by altering the capillary forces on the dispersed sheets during evaporation. It is also shown that the morphology of redispersed crumpled graphene powder can be controlled by solvent selection. This process is scalable, with the ability to rapidly process graphene dispersions into powders suitable for a variety of engineering applications.     

Highly flexible and sensitive wearable strain and pressure sensor based on porous graphene paper for human motion

The demand for high-performance multifunctional wearable devices drives the rapid development of sensors with flexibility, sensitivity and easy preparation. Here, we report an efficient preparation method to fabricate a wearable strain and pressure sensor based on porous graphene paper (PGP), which is prepared by polymethylmethacrylate (PMMA) microsphere as a template. The prepared PGP-based strain and pressure sensor can detect multi-dimensional deformation and shows good flexibility even after more than 1000 s of repeated deformation cycles, while the rapid response time can be up to approximately 60 ms. Moreover, the obtained PGP-based sensor exhibits a good sensitivity that the gauge factor (GF) is up to 77 when the strain is in the range of 4–8%, much higher than other graphene materials. Importantly, the porous microstructure created by the PMMA microsphere in the PGP plays a vital role in the good comprehensive performance of the PGP-based sensor. The device shows potential applications in smart wearable devices to detect or monitor the posture and movement information of human beings.

     

Graphene‐Based Biosensors: Going Simple

The main properties of graphene derivatives facilitating optical and electrical biosensing platforms are discussed, along with how the integration of graphene derivatives, plastic, and paper can lead to innovative devices in order to simplify biosensing technology and manufacture easy‐to‐use, yet powerful electrical or optical biosensors. Some crucial issues to be overcome in order to bring graphene‐based biosensors to the market are also underscored.     

Etching‐Assisted Crumpled Graphene Wrapped Spiky Iron Oxide Particles for High‐Performance Li‐Ion Hybrid Supercapacitor

From graphene oxide wrapped iron oxide particles with etching/reduction process, high‐performance anode and cathode materials of lithium‐ion hybrid supercapacitors are obtained in the same process with different etching conditions, which consist of partially etched crumpled graphene (CG) wrapped spiky iron oxide particles (CG@SF) for a battery‐type anode, and fully etched CG for a capacitive‐type cathode. The CG is formed along the shape of spikily etched particles, resulting in high specific surface area and electrical conductivity, thus the CG‐based cathode exhibits remarkable capacitive performance of 210 F g^?1 and excellent rate capabilities. The CG@SF can also be ideal anode materials owing to spiky and porous morphology of the particles and tightly attached crumpled graphene onto the spiky particles, which provides structural stability and low contact resistance during repetitive lithiation/delithiation processes. The CG@SF anode shows a particularly high capacitive performance of 1420 mAh g^?1 after 270 cycles, continuously increases capacity beyond the 270th cycle, and also maintains a high capacity of 170 mAh g^?1 at extremely high speeds of 100 C. The full‐cell exhibits a higher energy density up to 121 Wh kg^?1 and maintains high energy density of 60.1 Wh kg^?1 at 18.0 kW kg^?1. This system could thus be a practical energy storage system to fill the gap between batteries and supercapacitors.     

Controlling Long‐Distance Photoactuation with Protein Additives

Long‐distance wireless actuation indicates precise remote control over materials, sensors, and devices that are widely utilized in biomedical, defence, disaster relief, deep ocean, and outer space applications to replace human work. Unlike radio frequency (RF) control, which has low tolerance toward electromagnetic interference (EMI), light control represents a promising method to overcome EMI. Nonetheless, long‐distance light‐controlled wireless actuation able to compete with RF control has not been achieved until now due to the lack of highly light‐sensitive actuator designs. Here, it is demonstrate that amyloid‐like protein aggregates can organize photomodule single‐layer reduced graphene oxide (rGO) into a well‐defined multilayer stack to display long‐distance photoactuation. The amyloid‐like proteinaceous component docks the rGO layers together to form a hybrid film, which can reliably adhere onto various material surfaces with robust interfacial adhesion. The sensitive photothermal effect and a fast bending in 1 s to switch a circuit are achieved after forming the film on a plastic substrate and irradiating the bilayer film with a blue laser from 100 m away. A photoactuation distance of 50 km can be further extrapolated based on a commercial high‐power laser. This study reveals the great potential of amyloid‐like aggregates in remote light control of robots and devices.     

Electrically Controlled Nano and Micro Actuation in Memristive Switching Devices with On‐Chip Gas Encapsulation

Nanoactuators are a key component for developing nanomachinery. Here, an electrically driven device yielding actuation stresses exceeding 1 MPa withintegrated optical readout is demonstrated. 10 nm thick Al₂O₃ electrolyte films are sandwiched between graphene and Au electrodes. These allow reversible room‐temperature solid‐state redox reactions, producing Al metal and O₂ gas in a memristive‐type switching device. The resulting high‐pressure oxygen micro‐fuel reservoirs are encapsulated under the graphene, swelling to heights of up to 1 µm, which can be dynamically tracked by plasmonic rulers. Unlike standard memristors where the memristive redox reaction occurs in single or few conductive filaments, the mechanical deformation forces the creation of new filaments over the whole area of the inflated film. The resulting on–off resistance ratios reach 10⁸ in some cycles. The synchronization of nanoactuation and memristive switching in these devices is compatible with large‐scale fabrication and has potential for precise and electrically monitored actuation technology.     

Mechanical properties of grafold: a demonstration of strengthened graphene

Morphological patterns and structural features play crucial roles in the physical properties of functional materials. In this paper, the mechanical properties of grafold, an architecture of folded graphene nanoribbon, are investigated via molecular dynamics simulations and intriguing features are discovered. In contrast to graphene, grafold is found to develop large deformations upon both tensile and compressive loading along the longitudinal direction. The tensile deformation is plastic, whereas the compressive deformation is elastic and reversible within the strain range investigated. The calculated Young's modulus, tensile strength, and fracture strain are comparable to those of graphene, while the compressive strength and strain are much higher than those of graphene. The length, width, and folding number of grafold have distinctive impacts on the mechanical performance. These unique behaviors render grafold a promising material for advanced mechanical applications.     

From Flatland to Spaceland: Higher Dimensional Patterning with Two‐Dimensional Materials

The creation of three‐dimensional (3D) structures from two‐dimensional (2D) nanomaterial building blocks enables novel chemical, mechanical or physical functionalities that cannot be realized with planar thin films or in bulk materials. Here, we review the use of emerging 2D materials to create complex out‐of‐plane surface topographies and 3D material architectures. We focus on recent approaches that yield periodic textures or patterns, and present four techniques as case studies: (i) wrinkling and crumpling of planar sheets, (ii) encapsulation by crumpled nanosheet shells, (iii) origami folding and kirigami cutting to create programmed curvature, and (iv) 3D printing of 2D material suspensions. Work to date in this field has primarily used graphene and graphene oxide as the 2D building blocks, and we consider how these unconventional approaches may be extended to alternative 2D materials and their heterostructures. Taken together, these emerging patterning and texturing techniques represent an intriguing alternative to conventional materials synthesis and processing methods, and are expected to contribute to the development of new composites, stretchable electronics, energy storage devices, chemical barriers, and biomaterials.     

Scalable 2D Hierarchical Porous Carbon Nanosheets for Flexible Supercapacitors with Ultrahigh Energy Density

2D carbon nanomaterials such as graphene and its derivatives, have gained tremendous research interests in energy storage because of their high capacitance and chemical stability. However, scalable synthesis of ultrathin carbon nanosheets with well‐defined pore architectures remains a great challenge. Herein, the first synthesis of 2D hierarchical porous carbon nanosheets (2D‐HPCs) with rich nitrogen dopants is reported, which is prepared with high scalability through a rapid polymerization of a nitrogen‐containing thermoset and a subsequent one‐step pyrolysis and activation into 2D porous nanosheets. 2D‐HPCs, which are typically 1.5 nm thick and 1–3 µm wide, show a high surface area (2406 m² g^?1) and with hierarchical micro‐, meso‐, and macropores. This 2D and hierarchical porous structure leads to robust flexibility and good energy‐storage capability, being 139 Wh kg^?1 for a symmetric supercapacitor. Flexible supercapacitor devices fabricated by these 2D‐HPCs also present an ultrahigh volumetric energy density of 8.4 mWh cm^?3 at a power density of 24.9 mW cm^?3, which is retained at 80% even when the power density is increased by 20‐fold. The devices show very high electrochemical life (96% retention after 10000 charge/discharge cycles) and excellent mechanical flexibility.     

Highly Stretchable and Reliable,Transparent and Conductive Entangled Graphene Mesh Networks

A highly stretchable and reliable, transparent and conductive entangled graphene mesh network (EGMN) exhibits an interconnected percolation network, as usually shown in 1D nanowires, but with the electrical, mechanical, and thermal properties of 2D graphene. The unique combination of the 2D material properties and the network structure of wrinkled, waved, and crumpled graphene enables the EGMN to demonstrate excellent electrical reliability, mechanical durability, and thermal stability, even under harsh environmental and external conditions such as very high temperature, humidity, bending, and stretching. Specifically, after 100 000 cycles of bending with radius of 2 mm, the EGMN maintains its resistance similar to its initial value. The EGMN shows a steady monotonic response in resistance to strain cycles of 50 000 times with nearly constant gauge factors of 0.76, 1.67, and 2.55 at 10%, 40%, and 70% strains, respectively. Moreover, the EGMN shows very little change in resistance with the temperature increasing up to 1000 °C, by in situ thermal analysis with transmission electron microscopy and also by long‐term stability testing at 70 °C and 70% relative humidity for 30 d. These results demonstrate that this novel entangled graphene mesh network can significantly broaden the application areas for various types of wearable and stretchable devices.     

Heterogeneous Integration of Carbon‐Nanotube–Graphene for High‐Performance,Flexible, and Transparent Photodetectors

Low‐dimensional carbon materials, such as semiconducting carbon nanotubes (CNTs), conducting graphene, and their hybrids, are of great interest as promising candidates for flexible, foldable, and transparent electronics. However, the development of highly photoresponsive, flexible, and transparent optoelectronics still remains limited due to their low absorbance and fast recombination rate of photoexcited charges, despite the considerable potential of photodetectors for future wearable and foldable devices. This work demonstrates a heterogeneous, all‐carbon photodetector composed of graphene electrodes and porphyrin‐interfaced single‐walled CNTs (SWNTs) channel, exhibiting high photoresponse, flexibility, and full transparency across the device. The porphyrin molecules generate and transfer photoexcited holes to the SWNTs even under weak white light, resulting in significant improvement of photoresponsivity from negligible to 1.6 × 10^?2 A W^?1. Simultaneously, the photodetector exhibits high flexibility allowing stable light detection under ≈50% strain (i.e., a bending radius of ≈350 µm), and retaining a sufficient transparency of ≈80% at 550 nm. Experimental demonstrations as a wearable sunlight sensor highlight the utility of the photodetector that can be conformally mounted on human skin and other curved surfaces without any mechanical and optical constraints. The heterogeneous integration of porphyrin–SWNT–graphene may provide a viable route to produce invisible, high‐performance optoelectronic systems.     

Improved Charge Extraction Beyond Diffusion Length by Layer‐by‐Layer Multistacking Intercalation of Graphene Layers inside Quantum Dots Films

Charge collection is critical in any photodetector or photovoltaic device. Novel materials such as quantum dots (QDs) have extraordinary light absorption properties, but their poor mobility and short diffusion length limit efficient charge collection using conventional top/bottom contacts. In this work, a novel architecture based on multiple intercalated chemical vapor deposition graphene monolayers distributed in an orderly manner inside a QD film is studied. The intercalated graphene layers ensure that at any point in the absorbing material, photocarriers will be efficiently collected and transported. The devices with intercalated graphene layers have superior quantum efficiency over single‐bottom graphene/QD devices, overcoming the known restriction that the diffusion length imposes on film thickness. QD film with increased thickness shows efficient charge collection over the entire λ ≈ 500–1000 nm spectrum. This architecture could be applied to boost the performance of other low‐cost materials with poor mobility, allowing efficient collection for films thicker than their diffusion length.     

Plasma Treatment for Nitrogen‐Doped 3D Graphene Framework by a Conductive Matrix with Sulfur for High‐Performance Li–S Batteries

Carbon materials have received considerable attention as host cathode materials for sulfur in lithium–sulfur batteries; N‐doped carbon materials show particularly high electrocatalytic activity. Efforts are made to synthesize N‐doped carbon materials by introducing nitrogen‐rich sources followed by sintering or hydrothermal processes. In the present work, an in situ hollow cathode discharge plasma treatment method is used to prepare 3D porous frameworks based on N‐doped graphene as a potential conductive matrix material. The resulting N‐doped graphene is used to prepare a 3D porous framework with a S content of 90 wt% as a cathode in lithium–sulfur cells, which delivers a specific discharge capacity of 1186 mAh g^?1 at 0.1 C, a coulombic efficiency of 96% after 200 cycles, and a capacity retention of 578 mAh g^?1 at 1.0 C after 1000 cycles. The performance is attributed to the flexible 3D structure and clustering of pyridinic N‐dopants in graphene. The N‐doped graphene shows high electrochemical performance and the flexible 3D porous stable structure accommodates the considerable volume change of the active material during lithium insertion and extraction processes, improving the long‐term electrochemical performance.