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Martensite is a needle-shaped microstructure formed by a rapid, diffusionless transformation and significantly affects the mechanical properties of materials. Here, in two-dimensional ReS2 we show that martensite-like domain structures can form via a diffusionless transformation, involving small lattice deformations. By analyzing the strain distribution and topology of the as-grown chemical vapor deposition samples, we find that cooling-induced strain at the ReS2/substrate interface is responsible for the mechanical loading and is essential for martensite-like domain formation. Meanwhile, the effect of cooling rate, flake size and substrate on the microstructures revealed the mechanical origin of the transformation. The strain-induced lattice reconstructions are rationalized and possibly lead to ferroelastic effects. In view of the strong anisotropy in electronic and optical properties in two dimensional materials like ReS2, opportunities exist for strain-correlated micro/nanostructure engineering, which has potential use in next-generation strain-tunable devices. Two-dimensional materials have well-defined atomic-scale structure, which has the potential to be tuned by processing. Here, substrate-induced straining during the growth of ReS2 causes the formation of martensite-like domain structures.

Two-dimensional (2D) materials and soft materials are both susceptible to mechanical instabilities, such as buckling, wrinkling, folding and creasing, especially when located on surfaces. Here, we report that weak van der Waals interactions cause the interface between 2D molybdenum disulphide (MoS2) and a soft poly(methyl methacrylate) coating to demonstrate mechanical instability and delamination. The resulting non-uniform and buckled interface greatly hampers the ability of the coating to protect the MoS2 substrate. Also, the corrosion rate of 2D MoS2 and quench rate of intrinsic luminescence in 2D MoS2 were significantly accelerated by the soft coating. Owing to the formation of corrosive cavities at the interface, the geometry and size of the flakes became the dominating factor, and a critical size of 2D flakes for such interfacial instability was determined based on elasticity theory. Such hazardous corrosion in a 2D material caused by a soft coating raises concern for their use in electronic packaging, and for the processing of van der Waals-layered materials for future applications. The interface between a 2D material and a soft polymer coating may suffer from mechanical instabilities and delamination. A team led by Thuc Hue Ly at City University of Hong Kong investigated the role of soft coating layers like Poly methyl methacrylate (PMMA) in the protection of atomically thin transition metal dichalcogenides. A combination of experimental techniques including atomic force microscopy, photoluminescence and Raman spectroscopy indicated formation of corrosive cavities at the interface between PMMA and the chalcogenide monolayers. The enhanced corrosion behaviour of 2D materials in the presence of soft polymer coatings resembles the corrosion under insulation (CUI) phenomenon known in industry. As the geometry and size of flakes dominate the corrosion rate, a rational design of such interfaces should be employed to avoid CUI events.

Metals have the best dielectric screening capability among all materials; however, it is usually difficult to fabricate continuous and uniform ultrathin (few-atomic-layer thickness) metal films. Conversely, high-quality atomic-thick semiconductor or semimetal materials (so called two-dimensional materials) such as graphene or MoS2 can be readily obtained and robust in ambient conditions; however, their dielectric screening capabilities are greatly reduced by their reduced dimensionality. Particularly, in the vertical direction, the dielectric screening of two-dimensional materials is insufficient; thus, the performances of devices by two-dimensional materials were easily affected by the coulomb-scattering or other kind of sources. Herein, we propose that with a screw dislocation connecting the van der Waals layers in two-dimensional MoS2 spiral structures, excellent dielectric screening in the vertical direction can be achieved. Our Kelvin force microscopy directly demonstrates that the external impurity ...

The emergent two-dimensional (2D) materials are atomically thin and ultraflexible, promising for a variety of miniaturized, high-performance, and flexible devices in applications. On one hand, the ultrahigh flexibility causes problems: the prevalent wrinkles in 2D materials may undermine the ideal properties and create barriers in fabrication, processing, and quality control of materials. On the other hand, in some cases the wrinkles are used for the architecturing of surface texture and the modulation of physical/chemical properties. Therefore, a thorough understanding of the mechanism and stability of wrinkles is highly needed. Herein, we report a critical length for stabilizing the wrinkles in 2D materials, observed in the wrinkling and wrinkle elimination processes upon thermal annealing as well as by our in situ TEM manipulations on individual wrinkles, which directly capture the evolving wrinkles with variable lengths. The experiments, mechanical modeling, and self-consistent charge density functional tight binding (SCC-DFTB) simulations reveal that a minimum critical length is required for stabilizing the wrinkles in 2D materials. Wrinkles with lengths below a critical value are unstable and removable by thermal annealing, while wrinkles with lengths above a critical value are self-stabilized by van der Waals interactions. It additionally confirms the pronounced frictional effects in wrinkles with lengths above critical value during dynamical movement or sliding.

Delamination of thin films from the supportive substrates is critical issues in thin film industry and technology. The emergent two-dimensional materials, atomic layered materials, such as transition metal dichalcogenides are highly flexible thus the buckles and wrinkles can be easily generated and play vital effects on the physical properties. Here we introduce one kind of patterned buckling behavior caused by the delamination from substrate initiated at the edges of the chemical vapor deposition synthesized monolayer transition metal dichalcogenides, mainly due to the thermal expansion mismatch. The atomic force microscopy and optical characterizations clearly showed the puckered structures associated with strains, whereas the transmission electron microscopy revealed the special sawtooth shaped edge structures which break the geometrical symmetry of the buckling behavior of hexagonal samples. The condition of this edge delamination is in accordance with the fracture theory. This edge delamination process and buckling upon synthesis is universal for most of the ultrathin two dimensional materials, and it is definitely noteworthy in their future applications., Comment: 18 pages, 6 figures