applications of 2D materials emerging at large strain levels. [8][9][10] Considering difficulties associated with building a microelectromechanical system for straining freestanding 2D materials, [11] 2D materials were often transferred onto a substrate such that the strain can be introduced to the 2D material by controlling the deformation of the bulk substrate. [12] Such fact has led to significant advances in the strategies for straining 2D materials with a film-substrate system, as well as in interface metrologies for the van der Waals (vdW) interaction between the 2D material and its substrate.Here, we first summarize recent experimental achievements on realizing mechanical strain to substrate-supported 2D materials by categorizing the deformation modes of the 2D material-substrate system. These deformation modes include in-plane modes caused by epitaxy, thermal-expansion mismatch, and stretching/compressing the substrate, as well as out-of-plane modes caused by wrinkling and buckling of 2D materials, bulging and poking 2D materials, and transferring 2D materials on a patterned substrate. We then review recent experimental characterizations of the mechanical response of 2D material-substrate interfaces to in-plane shear deformations and out-of-plane delamination. This is not meant to be an all-encompassing analysis of the broad-field topic of strain engineering, but instead, we point out how mechanical deformations are achieved into 2D materials within the film/ substrate system and how the mechanics of interfaces govern these deformation mechanisms. The goal is to deterministically apply both strain magnitude and strain distribution into these atomically thin films and ultimately achieve strain-coupled fundamental physics and chemistry, and exciting applications in a controllable manner. Considering the interdisciplinary nature of research in this field, we also refer the readers to comprehensive reviews from relevant perspectives, including synthesis of emerging 2D materials, characterizations of the strain in 2D materials (especially via the Raman spectroscopy), and applications of mechanically strained 2D materials. [6,13,14] 2D Materials

Flexible piezoresistive pressure sensors have been attracting wide attention for applications in health monitoring and human-machine interfaces because of their simple device structure and easy-readout signals. For practical applications, flexible pressure sensors with both high sensitivity and wide linearity range are highly desirable. Herein, a simple and low-cost method for the fabrication of a flexible piezoresistive pressure sensor with a hierarchical structure over large areas is presented. The piezoresistive pressure sensor consists of arrays of microscale papillae with nanoscale roughness produced by replicating the lotus leaf's surface and spray-coating of graphene ink. Finite element analysis (FEA) shows that the hierarchical structure governs the deformation behavior and pressure distribution at the contact interface, leading to a quick and steady increase in contact area with loads. As a result, the piezoresistive pressure sensor demonstrates a high sensitivity of 1.2 kPa and a wide linearity range from 0 to 25 kPa. The flexible pressure sensor is applied for sensitive monitoring of small vibrations, including wrist pulse and acoustic waves. Moreover, a piezoresistive pressure sensor array is fabricated for mapping the spatial distribution of pressure. These results highlight the potential applications of the flexible piezoresistive pressure sensor for health monitoring and electronic skin.

Transparent, stretchable films of carbon nanotubes (CNTs) have attracted significant attention for applications in flexible electronics, while the lack of structural strength in CNT networks leads to deformation and failure under high mechanical load. In this work, enhancement of the strength and load transfer capabilities of CNT networks by chemical vapor deposition of graphene in the nanotube voids is proposed. The graphene hybridization significantly strengthens the CNT networks, especially at nanotube joints, and enhances their resistance to buckling and bundling under large cyclic strain up to 20%. The hybridized films show linear and reproducible responses to tensile strains, which have been applied in strain sensors to detect human motions with fast response, high sensitivity, and durability.

Layered systems of 2D crystals and heterostructures are widely explored for new physics and devices. In many cases, monolayer or few-layer 2D crystals are transferred to a target substrate including other 2D crystals, and nanometer-scale blisters form spontaneously between the 2D crystal and its substrate. Such nanoblisters are often recognized as an indicator of good adhesion, but there is no consensus on the contents inside the blisters. While gas-filled blisters have been modeled and measured by bulge tests, applying such models to spontaneously formed nanoblisters yielded unrealistically low adhesion energy values between the 2D crystal and its substrate. Typically, gas-filled blisters are fully deflated within hours or days. In contrast, we found that the height of the spontaneously formed nanoblisters dropped only by 20-30% after 3 mo, indicating that probably liquid instead of gas is trapped in them. We therefore developed a simple scaling law and a rigorous theoretical model for liquid-filled nanoblisters, which predicts that the interfacial work of adhesion is related to the fourth power of the aspect ratio of the nanoblister and depends on the surface tension of the liquid. Our model was verified by molecular dynamics simulations, and the adhesion energy values obtained for the measured nanoblisters are in good agreement with those reported in the literature. This model can be applied to estimate the pressure inside the nanoblisters and the work of adhesion for a variety of 2D interfaces, which provides important implications for the fabrication and deformability of 2D heterostructures and devices.

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Monolayer two-dimensional (2D) crystals exhibit a host of intriguing properties, but the most exciting applications may come from stacking them into multilayer structures. Interlayer and interfacial shear interactions could play a crucial role in the performance and reliability of these applications, but little is known about the key parameters controlling shear deformation across the layers and interfaces between 2D materials. Herein, we report the first measurement of the interlayer shear stress of bilayer graphene based on pressurized microscale bubble loading devices. We demonstrate continuous growth of an interlayer shear zone outside the bubble edge and extract an interlayer shear stress of 40 kPa based on a membrane analysis for bilayer graphene bubbles. Meanwhile, a much higher interfacial shear stress of 1.64 MPa was determined for monolayer graphene on a silicon oxide substrate. Our results not only provide insights into the interfacial shear responses of the thinnest structures possible, but also establish an experimental method for characterizing the fundamental interlayer shear properties of the emerging 2D materials for potential applications in multilayer systems.

Transparent displays lie at the heart of next generation optoelectronics [1,2] in the era of augmented reality (AR), wearable electronics, and internet of things (IoTs). [3][4][5][6][7] Being transparent for light-emitting diodes (LEDs) significantly expands their applications by displaying visual information on objects without affecting their original appearance and functionality. However, there has been a large gap in the electroluminescence (EL) performance between transparent displays and nontransparent counterparts, [8] due in large part to imbalanced injection of charge carriers into the emitter, unoptimized energy band alignment of the top electrode, and vulnerability of organic and/or polymeric light emitting materials during the deposition of transparent conducting oxide electrodes. [9][10][11][12] The previous progresses and unmet requirements for transparent displays are described in Section S2.1, Figure S1, and Table S1 of the Supporting Information. In addition, there has been much need to develop novel device architectures [13][14][15][16] that consider the carrier dynamics for high-performance transparent quantum dot light-emitting diodes (Tr-QLEDs).For high-quality transparent displays, first of all, high transparency is an absolute requirement. [17] The effect of transparency on visibility of background is examined on the university logo and a leaf (Figure 1a). For transparency below 70% (semitransparency), the color and contrast of objects behind the display are significantly deteriorated. In contrast, Tr-QLEDs of 84% transparency present clear background view in both cases. Secondly, high brightness and color purity are particularly important for vividness of "see-through" displays. The maximum brightness of conventional displays (e.g., smart phones and monitors) is around 600 cd m −2 . For see-through displays, however, the displayed information becomes blurred at this brightness (i.e., 600 cd m −2 ) because of photointerference with ambient light (Figure 1b; Figure S2a, Supporting Information). Therefore, significantly higher brightness is required to ensure clear and vivid displays (Figure 1b). In addition, chromatic aberrations can be minimized by employing engineered quantum dots (QD) emitters [18,19] that exhibit better color purity than organic and/or polymer emitters ( Figure S2b, Supporting Information). Lastly, integration of highly deformable Displaying information on transparent screens offers new opportunities in next-generation electronics, such as augmented reality devices, smart surgical glasses, and smart windows. Outstanding luminance and transparency are essential for such "see-through" displays to show vivid images over clear background view. Here transparent quantum dot light-emitting diodes (Tr-QLEDs) are reported with high brightness (bottom: ≈43 000 cd m −2 , top: ≈30 000 cd m −2 , total: ≈73 000 cd m −2 at 9 V), excellent transmittance (90% at 550 nm, 84% over visible range), and an ultrathin form factor (≈2.7 µm thickness). These superb characteristics are accomplishe...

Electronic tattoos (e-tattoos), also known as epidermal electronics, are ultra-thin and ultra-soft noninvasive but skin-conformable devices with capabilities including physiological sensing and transdermal stimulation and therapeutics. The fabrication of e-tattoos out of conventional inorganic electronic materials used to be tedious and expensive. Recently developed cut-and-paste method has significantly simplified the process and lowered the cost. However, existing cut-and-paste method entails a medical tape on which the electronic tattoo sensors should be pasted, which increases tattoo thickness and degrades its breathability. To address this problem, here we report a slightly modified cut-and-paste method to fabricate low-cost, open-mesh e-tattoos with a total thickness of just 1.5 μm. E-tattoos of such thinness can be directly pasted on human skin and conforms to natural skin texture. We demonstrate that this ultra-thin, tape-free e-tattoo can confidently measure electrocardiogram (ECG), skin temperature, and skin hydration. Heart rate and even respiratory rate can be extracted from the ECG signals. A special advantage of such ultra-thin e-tattoo is that it is capable of high-fidelity sensing with minimized motion artifacts under various body movements. Effects of perspiration are found to be insignificant due to the breathability of such e-tattoos.