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

High mechanical performances of macroscopic graphene oxide (GO) papers are attracting great interest owing to their merits of lightweight and multiple functionalities. However, the loading role of individual nanosheets and its effect on the mechanical properties of the macroscopic GO papers are not yet well understood. Herein, we effectively tailored the interlayer adhesions of the GO papers by introducing small molecules, that is, glutaraldehyde (GA) and water molecules, into the gallery regions. With the help of in situ Raman spectroscopy, we compared the varied load-reinforcing roles of nanosheets, and further predicted the Young's moduli of the GO papers. Systematic mechanical tests have proven that the enhancement of the tensile modulus and strength of the GA-treated GO paper arose from the improved load-bearing capability of the nanosheets. On the basis of Raman and macroscopic mechanical tests, the influences of interlayer adhesions on the fracture mechanisms of the strained GO papers were inferred.

Carbon nanotubes have unprecedented mechanical properties as defect-free nanoscale building blocks, but their potential has not been fully realized in composite materials due to weakness at the interfaces. Here we demonstrate that through load-transfer-favored three-dimensional architecture and molecular level couplings with polymer chains, true potential of CNTs can be realized in composites as initially envisioned. Composite fibers with reticulate nanotube architectures show order of magnitude improvement in strength compared to randomly dispersed short CNT reinforced composites reported before. The molecular level couplings between nanotubes and polymer chains results in drastic differences in the properties of thermoset and thermoplastic composite fibers, which indicate that conventional macroscopic composite theory fails to explain the overall hybrid behavior at nanoscale.To build composites with superior strength and flawtolerance, nanoscale reinforcements have natural advantages than their micrometer-sized counterparts because of their paucity of structural defects and high aspect ratio.1 However, a huge challenge still lies in the manufacturing of a highperformance nanocomposite because of the agglomeration tendency of the nanometer-sized fillers and poor load transfer efficiency between the matrix and reinforcements. A good example is carbon nanotube (CNT) reinforced composites. Although individual CNTs have Young's modulus of 1 TPa and strength over 60 GPa, 2,3 to date CNT reinforced polymer composites fabricated by mixing polymers and nanotubes have shown only moderate enhancement in modulus and even more limited improvements in strength. 4 Even in the cases where CNTs are optimally dispersed at high volume fraction, their moduli and strengths are at least 2 orders of magnitude lower than what was theoretically predicted by composite theory. [5][6][7] Essentially, the mechanical performance of CNT reinforced composites relies on the load-bearing status of the CNTs in the matrix. However, two inherent problems of CNTs shadow their promise as efficient load-bearers. One is their waviness. A multiwalled carbon nanotube with a diameter of 10 nm is 10 12 times easier to be bent than a