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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.

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.

Adhesion between a stamp with an elastomeric layer and various devices or substrates is crucial to successfully fabricate flexible electronics using a transfer process. Although various transfer processes using stamps with different adhesion strengths have been suggested, the controllable range of adhesion is still limited to a narrow range. To precisely transfer devices onto selected substrates, however, the difference in adhesion between the picking and placing processes should be large enough to achieve a high yield. Herein, we report a simple way to extend the controllable adhesion range of stamps, which can be achieved by adjusting the thickness of the elastomeric layer and the separation velocity. The adhesion strength increased with decreasing layer thickness on the stamp due to a magnification of the confinement and rate-dependent effects on the adhesion. This enabled the controllable range of the adhesion strength for a 15 μm-thick elastomeric layer to be extended up to 12 times that of the bulk under the same separation conditions. The strategy of designing stamps using simple adhesion tests is also introduced, and the reversible transfer of thin Si chips was successfully demonstrated. Tuning and optimizing the adhesion strength of a stamp according to the design process suggested here can be applied to various materials for the selective transfer and replacement of individual devices.

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.

Ultrathin silicon-based flexible 16 × 16 NAND flash memory (f-NAND) is demonstrated utilizing roll-to-plate packaging. The roll-based thermo-compression bonding of the anisotropic conductive film (ACF) transfers and simultaneously interconnects the f-NAND on a flexible printed circuit board. Reliable circuitry operation of the 16 × 16 f-NAND is confirmed with excellent flexibility and stable ACF interconnections.

This study extended the conventional JKR theory on adhesive contact between elastic cylinders to the frictionless adhesive contact between elastic films coated on rigid cylindrical rollers. The plane-strain elasticity problem of indentation on an elastic film perfectly bonded to a rigid half-plane was revisited, with which the force-depth relations for both flat and cylindrical indentations were obtained in a simple form. By following Hertz analysis and using the solution of the cylindrical indentation on an elastic film, the adhesionless Hertzian contact between elastic films coated on rigid cylinders was obtained. However, the obtained Hertzian contact state should be considered as an approximate solution, since the pressure distributions in the contact region of the two elastic films may not match perfectly each other in the case of different Poisson’s ratios or thicknesses of the elastic films. By properly superposing the linear elastic solutions of flat and cylindrical indentations, the approximate solution to the adhesive JKR contact state between elastic films coated on rigid cylinders was also obtained, of which the relations between applied force, penetration depth, and contact width were expressed in terms of the geometric parameters and elastic constants of the two elastic films. The pull-off force at the moment of debonding two elastic films was also obtained as a function of the adhesion energy and compared with the normal two-dimensional JKR result.

The out-of-plane coefficient of thermal expansion (CTE) measurement method, which is named the thermal bulge method, was used to measure the linear CTE values for freestanding nanocrystalline ultra-thin gold films with thicknesses ranging from 115 to 1200 nm. White light interferometry was used to determine the out-of-plane thermal deformation as a function of temperature. The thermal strain showed a linear behavior in the temperature range of 20 to 100°C, but varied with the thickness (or grain size) of the specimen. The grain sizes, which were measured by transmission electron microscopy (TEM), showed a strong correlation with the CTE: Specimens with larger grain sizes (44 to 98 nm) had greater CTE values (9.6 to 13.6 ppm/°C). Starting from the CTE values for fine-grained materials, the CTE of nanocrystalline gold films increased with increasing grain size and approached the CTE of bulk gold. The thermal bulge method, which was previously developed by the authors, allowed for direct, repetitive, reliable, and quick measurement of the CTE values of ultra-thin gold films.

The transfer-assembly process is a promising technique for fabricating high-performance flexible devices. An important element of the transfer assembly process is controlling the adhesion during the picking and placing steps. In this study, we developed an apparatus to measure the adhesive force between a spherical polydimethylsiloxane (PDMS) stamp and a flat silicon substrate. The apparatus consisted of a multi-axis stage for alignment, a vertical stage, and a loadcell. The adhesive force between the stamp and the substrate was measured for various unloading velocities during the picking step. The size of the contact area was simultaneously measured during the picking step by incorporating an optical microscope with a high-speed camera into the apparatus. The effect of the unloading velocity on the adhesion was carefully investigated. The actual transfer process was performed to demonstrate the effect of rate-dependent adhesion during the transfer assembly of single-crystal silicon devices. Practical gui...

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.

The surface of water provides an excellent environment for gliding movement, in both nature and modern technology, from surface living animals such as the water strider, to Langmuir–Blodgett films. The high surface tension of water keeps the contacting objects afloat, and its low viscosity enables almost frictionless sliding on the surface. Here we utilize the water surface as a nearly ideal underlying support for free-standing ultra-thin films and develop a novel tensile testing method for the precise measurement of mechanical properties of the films. In this method, namely, the pseudo free-standing tensile test, all specimen preparation and testing procedures are performed on the water surface, resulting in easy handling and almost frictionless sliding without specimen damage or substrate effects. We further utilize van der Waals adhesion for the damage-free gripping of an ultra-thin film specimen. Our approach can potentially be used to explore the mechanical properties of emerging two-dimensional materials. The mechanical testing of thin films is non-trivial, due to their very fine dimensions. Kim et al. use the inherent surface tension of water as a platform for the frictionless tensile testing of gold films, with a thickness as fine as 55 nm.

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.

In this study, we fabricate an electrode structure, in which a carbon nanotube (CNT) film is coated on the composite of carbon nanotubes (CNTs) and polydimethylsiloxane (PDMS). Electromechanical tests disclose that incorporation of a high density of well-dispersed CNTs (with the aid of UV/O3 treatment) in the PDMS substrate offers an alternative current path when the CNT film cracked, helping demonstrate much improved electromechanical characteristics. Further analysis using scanning electron microscopy (SEM) combined with the data from the electromechanical tests illuminates some interesting electromechanical features and the failure mechanism of the material.

Transfer printing, a promising method for fabricating multi-scale structures on various substrates such as semiconductors and polymers, has been used to fabricate flexible devices with performance superior to that of conventional organic flexible devices. Although thin films might be expected to suffer damage during the transfer printing process, no reports of the degradation of mechanical properties during transfer printing have been published. The change in mechanical properties before and after transfer printing should be evaluated in terms of reliability and design for transfer printing to be successful. We propose a method of fabricating freestanding 200-nm-thick single-crystal silicon (SCS) thin-film specimens using transfer printing in order to investigate the mechanical properties of the transferred SCS thin-film specimens. The fabrication method combines several techniques such as semiconductor manufacturing, liftoff, and transfer printing processes. The core technology in this method is the fabrication of freestanding SCS thin-film structures suspended between two fixed ends. The mechanical properties of the freestanding SCS thin-film structures were measured using a microtensile machine capable of optical strain measurement. The test results provide insight into device design and reliability evaluation of flexible electronics fabricated by nanotransfer printing.