COMMUNICATION procedure as illustrated in Figure 1 a. Figure 3 a shows the parallel AgNW/PDMS elastic conductors with a linewidth of 800 μ m; the performance was found to be the same as that of the wileyonlinelibrary.com COMMUNICATION cycles of stretching/releasing in a large range of tensile strain (0-50%). This stable electrical response is due to buckling of the AgNW/PDMS layer. The physics/mechanics origin of the buckling is due to irreversible sliding of the AgNWs in the PDMS matrix. Following a parallel fabrication approach, line and cross patterns of AgNWs were fabricated. Furthermore, a stretchable LED circuit and a capacitive strain sensor were demonstrated using the AgNW/PDMS elastic conductors as interconnects or electrodes. With their superior conductivity, stretchability and compatibility with existing fabrication/patterning technology, the reported AgNW/PDMS elastic conductors may fi nd broad applications in stretchable electronics, skin sensors, wearable communication devices, photovoltaics and energy storage. Experimental SectionSample Preparation : AgNWs were provided by Blue Nano, Inc. PDMS were prepared using Sylgard 184 (Dow Corning) by mixing the "base" and the "curing agent" with a ratio of 10:1. After air bubbles disappeared, the liquid mixture was then thermally cured at 65 ° C for 12 hours to form cross-linked and solid PDMS.Fabrication and Measurement of Capacitive Strain Sensors : Liquid PDMS layer was fi rst casted onto a Si substrate patterned with rectangular AgNW fi lms. Before curing, we placed an already patterned and cured AgNW/PDMS fi lm (with the AgNW surface facing up) on the Si substrate as well as the wet PDMS and oriented it to make sure that both patterns are perfectly aligned. Following that, the whole piece was thermally cured and peeled off the Si substrate. This way, the top and bottom surfaces of the PDMS were symmetrically covered by the patterned AgNW stretchable electrodes. Cutting a strip off the PDMS substrate produced a capacitive strain sensor, as schematically shown in Figure 4 a. The strain sensor were repeatedly stretched and released by a tensile testing stage (Ernest F. Fullam), while the capacitance was measured at the same time by a LCR (inductance, capacitance, resistance) meter (Stanford Research Systems, SR715).

Considerable efforts have been made to achieve highly sensitive and wearable sensors that can simultaneously detect multiple stimuli such as stretch, pressure, temperature or touch. Here we develop highly stretchable multifunctional sensors that can detect strain (up to 50%), pressure (up to ∼1.2 MPa) and finger touch with high sensitivity, fast response time (∼40 ms) and good pressure mapping function. The reported sensors utilize the capacitive sensing mechanism, where silver nanowires are used as electrodes (conductors) and Ecoflex is used as a dielectric. The silver nanowire electrodes are screen printed. Our sensors have been demonstrated for several wearable applications including monitoring thumb movement, sensing the strain of the knee joint in patellar reflex (knee-jerk) and other human motions such as walking, running and jumping from squatting, illustrating the potential utilities of such sensors in robotic systems, prosthetics, healthcare and flexible touch panels.

Since the first successful synthesis of graphene just over a decade ago, a variety of twodimensional (2D) materials (e.g., transition metal-dichalcogenides, hexagonal boron-nitride, etc.) have been discovered. Among the many unique and attractive properties of 2D materials, mechanical properties play important roles in manufacturing, integration and performance for their potential applications. Mechanics is indispensable in the study of mechanical properties, both experimentally and theoretically. The coupling between the mechanical and other physical properties (thermal, electronic, optical) is also of great interest in exploring novel applications, where mechanics has to be combined with condensed matter physics to establish a scalable theoretical framework. Moreover, mechanical interactions between 2D materials and various substrate materials are essential for integrated device applications of 2D materials, for which the mechanics of interfaces (adhesion and friction) has to be developed for the 2D materials. Here we review recent theoretical and experimental works related to mechanics and mechanical properties of 2D materials. While graphene is the most studied 2D material to date, we expect continual growth of interest in the mechanics of other 2D materials beyond graphene.

Stretchable electronics are attracting intensive attention due to their promising applications in many areas where electronic devices undergo large deformation and/or form intimate contact with curvilinear surfaces. On the other hand, a plethora of nanomaterials with outstanding properties have emerged over the past decades. The understanding of nanoscale phenomena, materials, and devices has progressed to a point where substantial strides in nanomaterial-enabled applications become realistic. This review summarizes recent advances in one such application, nanomaterial-enabled stretchable conductors (one of the most important components for stretchable electronics) and related stretchable devices (e.g., capacitive sensors, supercapacitors and electroactive polymer actuators), over the past five years. Focusing on bottom-up synthesized carbon nanomaterials (e.g., carbon nanotubes and graphene) and metal nanomaterials (e.g., metal nanowires and nanoparticles), this review provides fundamental insights into the strategies for developing nanomaterial-enabled highly conductive and stretchable conductors. Finally, some of the challenges and important directions in the area of nanomaterial-enabled stretchable conductors and devices are discussed.

The transfer of synthesized 2D MoS2 films is important for fundamental and applied research. However, it is problematic to translate the well-established transfer processes for graphene to MoS2 due to different growth mechanisms and surface properties. Here we demonstrate a surface-energy-assisted process that can perfectly transfer centimeter-scale monolayer and few-layer MoS2 films from original growth substrates onto arbitrary substrates with no observable wrinkles, cracks, and polymer residues. The unique strategies used in this process include leveraging the penetration of water between hydrophobic MoS2 films and hydrophilic growth substrates to lift off the films and dry transferring the film after the lift off. This is in stark contrast with the previous transfer process for synthesized MoS2 films, which explores the etching of the growth substrate by hot base solutions to lift off the films. Our transfer process can effectively eliminate the mechanical force caused by bubble generations, the attacks from chemical etchants, and the capillary force induced when transferring the film outside solutions as in the previous transfer process, which consists of the major causes for the previous unsatisfactory transfer. Our transfer process also benefits from using polystyrene (PS), instead of poly(methyl methacrylate) (PMMA) that was widely used previously, as the carrier polymer. PS can form more intimate interaction with MoS2 films than PMMA and is important for maintaining the integrity of the film during the transfer process. This surface-energy-assisted approach can be generally applied to the transfer of other 2D materials, such as WS2.

PE has been explored for the manufacturing of flexible and stretchable electronic devices by printing functional inks containing soluble or dispersed materials, [14][15][16] which has enabled a wide variety of applications, such as transparent conductive films (TCFs), flexible energy harvesting and storage, thin film transistors (TFTs), electroluminescent devices, and wearable sensors. [17][18][19][20][21][22][23][24] The global PE market should reach $26.6 billion by 2022 from $14.0 billion in 2017 at a compound annual growth rate of 13.6%. [25] PE devices are manufactured by a variety of printing technologies. Typical printing technologies can be divided into two broad categories: noncontact patterning (or nozzle-based patterning) and contact-based patterning. The noncontact techniques include inkjet printing, electrohydrodynamic (EHD) printing, aerosol jet printing, and slot die coating, while screen printing, gravure printing, and flexographic printing are examples of the contact techniques. Each of these techniques has its own advantages and disadvantages, but they all rely on the principle of transferring inks to a substrate. Understanding the characteristics and recent advances of each printing technique is important to further the progress in PE. Moreover, to promote the lab-scale printing technologies to large-scale production process, roll-toroll (R2R) printing, which is one of the manufacturing methods to obtain large-area films with low cost and excellent durability, has drawn much attention from both industry and the research community.Nearly all of devices based on PE require conductive structures, interconnects, and contacts; therefore, highly conductive patterns, usually with high transparency and/or high resolution, fabricated by means of printing conductive materials are one of the most critical components in PE devices. Various printable conductive nanomaterials, such as metal nanomaterials (e.g., metal nanoparticles and metal nanowires) and carbon nanomaterials (e.g., graphene and carbon nanotubes (CNTs)), have been explored and used as major materials for PE. Applying printing technology to deposition of the conductive nanomaterials requires formulation of suitable inks. After depositing inks on different substrates, post-printing treatment, Printed electronics is attracting a great deal of attention in both research and commercialization as it enables fabrication of large-scale, low-cost electronic devices on a variety of substrates. Printed electronics plays a critical role infacilitating widespread flexible electronics and more recently stretchable electronics. Conductive nanomaterials, such as metal nanoparticles and nanowires, carbon nanotubes, and graphene, are promising building blocks for printed electronics. Nanomaterial-based printing technologies, formulation of printable inks, post-printing treatment, and integration of functional devices have progressed substantially in the recent years. This review summarizes basic principles and recent development of common printing technologie...

The Young's modulus and fracture strength of silicon nanowires with diameters between 15 and 60 nm and lengths between 1.5 and 4.3 µm were measured. The nanowires, grown by the vapor-liquid-solid process, were subjected to tensile tests in situ inside a scanning electron microscope. The Young's modulus decreased while the fracture strength increased up to 12.2 GPa, as the nanowire diameter decreased. The fracture strength also increased with the decrease of the side surface area; the increase rate for the chemically synthesized silicon nanowires was found to be much higher than that for the microfabricated silicon thin films. Repeated loading and unloading during tensile tests demonstrated that the nanowires are linear elastic until fracture without appreciable plasticity.Silicon (Si) nanowires (NWs) are one of the key building blocks for nanoelectronic and nanoelectromechanical devices. 1 They exhibit excellent mechanical, 2,3 electrical, 4 and optical 5 properties, in addition to interesting multifunctional properties such as piezoresistivity 6 and thermoelectricity. [7][8][9] As such, Si NWs have been used in a broad range of applications including nanoelectronics, 10-12 nanosensors, 13 nanoresonators, 14 light-emitting diodes, 15 and thermoelectric energy scavengers. 7,8 The operation and reliability of these nanodevices depend on the mechanical properties of Si NWs, which are expected to be different from their bulk counterparts due to their increasing surface-to-volume ratio.Existing techniques for measuring the mechanics of individual NWs include observing the vibration (or resonance) of cantilevered NWs inside a transmission or scanning electron microscope (TEM/SEM), [16][17][18] measuring the lateral bending of suspended NWs with an atomic force microscope (AFM), 3,19-21 measuring uniaxial tension of suspended NWs in SEM or TEM, 2,22-26 and nanoindentation of NWs on a substate. 27 Available experimental results on Si NWs exhibit significant scatter including the following: (1) some reported a decrease in Young's modulus with decreasing size, 2,9,24,28 while others showed an opposite trend; 20,21 (2) the reported strength values of vapor-liquid-solid (VLS) grown Si NWs ranged from 500 MPa to 12 GPa; 3,28 (3) Han et al. 2 observed pronounced plastic deformation of Si NWs by in situ TEM tensile tests at room temperature, while Gordon et al. 21 reported linear elastic behavior followed by brittle fracture using AFM bending tests.Moreover the experimental data show large discrepancy with the simulation results. 29 For instance, the experimentally measured Young's moduli started deviating from the bulk value at diameters of about 200 nm; 28 conversely, computational studies using both density functional theory (DFT) and classical molecular dynamics (MD) indicated that the transition diameter for Young's modulus of Si NWs is less than 10 nm. [30][31][32] The experimentally observed plasticity at room temperature occurred for Si NWs with diameter less than 60 nm, while MD simulations 33 predicted a simi...

The nonlinear mechanical response of monolayer graphene on polyethylene terephthalate (PET) is characterised using in‐situ Raman spectroscopy and atomic force microscopy. While interfacial stress transfer leads to tension in graphene as the PET substrate is stretched, retraction of the substrate during unloading imposes compression in the graphene. Two interfacial failure mechanisms, shear sliding under tension and buckling under compression, are identified. Using a nonlinear shear‐lag model, the interfacial shear strength is found to range between 0.46 and 0.69 MPa. The critical strain for onset of interfacial sliding is ∼0.3%, while the maximum strain that can be transferred to graphene ranges from 1.2% to 1.6% depending on the interfacial shear strength and graphene size. Beyond a critical compressive strain of around −0.7%, buckling ridges are observed after unloading. The results from this work provide valuable insight and design guidelines for a broad spectrum of applications of graphene and other 2D nanomaterials, such as flexible and stretchable electronics, strain sensing, and nanocomposites.