Progress has been made in the study of the interface mechanics of two-dimensional material van der Waals

Recently, Zhang Zhong, a researcher at the National Nanoscience Center of the Chinese Academy of Sciences, and Liu Luqi's team have made important progress in the study of the mechanical behavior of van der Waals interface. The research results are titled Elastocapillary cleaning of twisted bilayer graphene interfaces and published online in Nature Communications.

The two-dimensional materials represented by graphene have excellent physical properties such as force, electricity, light and heat. The van der Waal homogeneous/heterojunction system, which is assembled layer by layer stacking, can further expand its properties, such as double-layer angle graphene stacked at specific angles exhibiting physical and mechanical behaviors such as superconductivity and supersliptting. Due to the large specific surface area characteristics of two-dimensional materials, impurities such as water molecules in the air are inevitably mixed with impurities such as water molecules in the air during the construction of van der Waals homojunctions and aggregate to form micro- and nano-scale bubbles. On the one hand, the contaminated van der Waals interface is expected to significantly reduce the performance of micro and nano devices; on the other hand, this micro and nano scale bubbling has the characteristics of high pressure, limited domain, large deformation, etc., which provides new research opportunities for two-dimensional material strain engineering, high pressure chemistry, limited domain catalysis, liquid pool under electron microscopy and other fields. Therefore, how to overcome bubbling pollution to achieve van der Waals interface atomic-level cleanliness, bubbling strain size and distribution, pressure difference and other factors are the key issues in the preparation, transfer, physical properties measurement and application of two-dimensional materials.

It is difficult to measure and characterize the interface mechanical behavior of homogeneous/heterogeneous van der Waals materials. The research team proposed a new strategy for homogeneous/heterogeneous structures with controllable angles, and realized the preparation of angular double-layer graphene (ACS Appl. Mater. & Interfaces, 2020; 12(36): 40958-67）。 In this study, the research team used lateral force microscopy technology to characterize the curved graphene Moor cloud pattern, and achieved a visual representation of the cleanliness of the van der Waals interface. The study used capillary-assisted transfer technology to introduce water, ethanol and other media to construct nanoscale vacuoles. Nano-vacuoles exhibit geometric self-similarity under elastic properties and interfacial energy competition mechanisms, with specific elastic capillary parameters. Under the stimulation of the probe force, the graphene van der Waals interface exhibited self-cleaning phenomenon, and due to the instability of the edge of the vacuole, the "long-range" action between adjacent vacuoles induced spontaneous fusion of nanoveals. This study reveals the influence and contribution of the elastic energy of two-dimensional materials on the fusion process under different from the traditional Ostwald curing mechanism. Through theoretical analysis combined with microporous bubbling experimental techniques, the researchers further explored the influence and regulation of pre-tension on elastic capillary parameters and the "long-range" interaction between vacuoles, and the relevant mechanisms were supported and verified by molecular dynamics simulations.

Zhang Zhong's research group is committed to the study of low-dimensional micro-nano materials and structural mechanical behavior, and has successively realized the controllable shear deformation and interface shear stress measurement of the van der Waals interface between the double layers of graphene through the self-built micro-nano scale bubbling technology-atomic force microscopy-micro-Raman spectroscopy combined test and characterization technology platform (Phys. Rev. Lett.2017) Reveal the effects of interfacial strength differences on the strain distribution and size of micro- and nano-scale bubbling strains, and propose a theoretical solution for predicting the size and distribution of bubbling strains of different shapes at the nanoscale (Phys. Rev. Lett.2018, cover); experimental measurement of bending stiffness of nanoscale thickness two-dimensional materials. Due to the influence of shear deformation and slip at the interlayer van der Waals interface, the bending stiffness and Young's modulus of the intrinsic mechanical parameters of materials are independent mechanical parameters, and the relationship between bending stiffness and thickness in traditional thin plate theory is no longer applicable (Phys. Rev. Lett.2019, cover); commenting on the impact of the above research results in strain engineering, nanocomposites, etc., revealing the important impact of micro- and nano-scale interface mechanics in multidisciplinary research (Adv. Mater.2019, Compos. A 2021）。

The National Center for Nanoscience, the University of Science and Technology of China, the University of Texas at Austin, and Tsinghua University collaborated to complete the research. The research work has been funded by the National Natural Science Foundation of China, the Strategic Pilot Science and Technology Special Project of the Chinese Academy of Sciences (Category B), and the National Major Scientific Research Program.

Schematic diagram of corner bilayer graphene-encapsulated nanobicles, force analysis, and lateral force microscopy to characterize the fusion process of adjacent nanoveals

Source: National Center for Nanoscience, Chinese Academy of Sciences