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Wearable electronic devices have become familiar to people and have been expanded to various functions via development in the field of the flexible and stretchable electronic devices. These wearable devices, such as displays, motion sensors, electromyography sensors, and electrocardiogram sensors, require input and power systems to command information and supply energy, respectively. The triboelectric nanogenerator (TENG) has attracted attention as an eco-friendly device that provides sustainable power without an external power supply. Here, we report a self-powered stretchable TENG (S-TENG) touch sensor suitable for a wearable device that adapts to the skin's motion because of its stretchability. The S-TENG with a single-electrode structure was fabricated using atomically thin graphene (

Owing to a finite and single-atom-thick two-dimensional structure, graphene nanostructures such as nanoribbons possess outstanding physical properties and unique size-dependent characteristics due to nanoscale defects, especially for mechanical properties. Graphene nanostructures characteristically exhibit strong nonlinearity in deformation and the defect brings about an extremely localized singular stress field of only a few nanometers, which might lead to unique fracture properties. Fundamental understanding of their fracture properties and criteria is, however, seriously underdeveloped and limited to the level of continuum mechanics and linear elasticity. Here, we demonstrate the breakdown of continuum-based fracture criteria for graphene nanoribbons due to the strong nonlinearity and discreteness of atoms emerging with decreasing size and identify the critical sizes for these conventional criteria. We further propose an energy-based criterion considering atomic discrete nature, and show that it can successfully describe the fracture beyond the critical sizes. The complete clarification of fracture criterion for nonlinear graphene with nanoscale singularity contributes not only to the reliable design of graphene-based nanodevices but also to the elucidation of the extreme dimensional limit in fracture mechanics.

The development of input device technology in a conformal and stretchable format is important for the advancement of various wearable electronics. Herein, we report a capacitive touch sensor with good sensing capabilities in both contact and noncontact modes, enabled by the use of graphene and a thin device geometry. This device can be integrated with highly deformable areas of the human body, such as the forearms and palms. This touch sensor detects multiple touch signals in acute recordings and recognizes the distance and shape of the approaching objects before direct contact is made. This technology offers a convenient and immersive human-machine interface and additional potential utility as a multifunctional sensor for emerging wearable electronics and robotics.

Due to the atomistic thinness of graphene, it is non-trivial to measure fracture behaviors of freestanding graphene without any underlying materials. In this study, we present a methodology for measuring the fracture behavior of freestanding natural graphene with single crystallinity using an in situ tensile tester under a scanning electron microscope. A pre-crack was introduced in a freestanding graphene specimen using focused ion beam. Crystallographic information, geometric dimensions, and layer numbers of the graphene specimen were characterized before fracture testing using transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy. Taking the advantages of the in situ fracture test, we measured load–displacement data of single-crystalline bilayer graphene and observed an exciting fracture behavior during the crack extension along its zig-zag direction. Young’s modulus and the stress field of the specimen at the moment of fracture were evaluated from the measured data and finite element analysis. The present study provides a direct pathway to fracture mechanics tests of natural graphene under uniaxial tension, and could be an insightful cornerstone for fracture mechanics tests of other two-dimensional materials or atomically thin film.

Multilayer graphene consists of a stack of single-atomic-thick monolayer graphene sheets bound with π-π interactions and is a fascinating model material opening up a new field of fracture mechanics. In this study, fracture behavior of single-crystalline multilayer graphene was investigated using an in situ mode I fracture test under a scanning electron microscope, and abnormal crack propagation in multilayer graphene was identified for the first time. The fracture toughness of graphene was determined from the measured load-displacement curves and the realistic finite element modelling of specimen geometries. Nonlinear fracture behavior of the multilayer graphene is discussed based on nonlinear elastic fracture mechanics. In situ scanning electron microscope images obtained during the fracture test showed asynchronous crack propagation along independent paths, causing interlayer shear stress and slippages. We also found that energy dissipation by interlayer slippages between the graphene layers is the reason for the enhanced fracture toughness of multilayer graphene. The asynchronous cracking with independent paths is a unique cracking and toughening mechanism for single-crystalline multilayer graphene, which is not observed for the monolayer graphene. This could provide a useful insight for the design and development of graphene-based composite materials for structural applications.

Stretchable integrated circuits consisting of ultrathin Si transistors connected by multilayer graphene are demonstrated. Graphene interconnects act as an effective countervailing component to maintain the electrical performance of Si integrated circuits against external strain. Concentration of the applied strain on the graphene interconnect parts can stably protect the Si active devices against applied strains over 10%.

Graphene is considered as a promising material for flexible and transparent electrodes due to its outstanding electrical, optical, and mechanical properties. Efforts to mass-produce graphene electrodes led to the development of roll-to-roll chemical vapor deposition (CVD) graphene growth and transfer, and the only remaining obstacle to the mass-production of CVD graphene electrodes is a cost-effective patterning technique that is compatible with the roll-to-roll manufacturing. Herein, we propose a mechanics-driven technique for patterning graphene synthesized on copper foil (commonly used in roll-to-roll manufacturing). The copper foil is exposed to high temperature for a prolonged period during the CVD growth of graphene, and thus can result in recrystallization and grain growth of the copper foil and thereby reducing to the yield strength. This softening behavior of the copper was carefully controlled to allow simple stamp patterning of the graphene. The strength of the underlying substrate was controlled for the accuracy of the residual patterns. The proposed stamp patterning technique is mask-less and photoresist-free, and can be performed at room temperature without high-energy sources such as lasers or plasma. To demonstrate the capability of this process to produce a continuous electrode, a transparent in-plane supercapacitor was fabricated using the proposed patterning technique.

We have observed and analyzed the fracture characteristics of the monolayer CVD-graphene using pressure bulge testing setup. The monolayer CVD-graphene has appeared to undergo environmentally assisted subcritical crack growth in room condition, i.e. stress corrosion cracking arising from the adsorption of water vapor on the graphene and the subsequent chemical reactions. The crack propagation in graphene has appeared to be able to be reasonably tamed by adjusting applied humidity and stress. The fracture toughness, describing the ability of a material containing inherent flaws to resist catastrophic failure, of the CVD-graphene has turned out to be exceptionally high, as compared to other carbon based 3D materials. These results imply that the CVD-graphene could be an ideal candidate as a structural material notwithstanding environmental susceptibility. In addition, the measurements reported here suggest that specific non-continuum fracture behaviors occurring in 2D monoatomic structures can be macroscopically well visualized and characterized.

Roll-to-roll (R2R) transfer of a chemical vapor deposition (CVD) graphene is an inevitable step for large scale and high throughput manufacturing of graphene transparent electrodes on flexible substrates. The damages on graphene induced by high contact pressure of nip rolls during the roll transfer degrade the electrical properties of the transferred graphene on flexible substrates. In this study, we developed a damage mitigation method for the roll transfer of graphene. By analyzing scanning electron microscope (SEM) images of the damages on the transferred graphene, three types of failure modes were classified, and the corresponding failure mechanisms were revealed using the surface morphology and the finite element analyses. Based on the understanding of the failure mechanisms, the graphene transfer with a width of 400 mm was realized at a speed of 1000 mm min−1 using an R2R transfer machine with the capability of nip force control. The high electrical conductivity and uniformity of the roll-transferred graphene demonstrates the scalability and the productivity of the developed roll transfer technology.