Probing the interlayer mechanical coupling of 2D layered materials - A review

Two-dimensional (2D) layered materials are assembled through the intralayer covalent bonds and interlayer van der Waals (vdWs) interactions. The relatively weak interlayer vdWs interactions result in the weak interlayer mechanical coupling between layers, which strongly impacts the overall mechanical properties of multilayer 2D materials or heterostructures. Experimentally probing the interlayer mechanical coupling is of vital importance on the accumulation of fundamental parameters for their applications in flexible and stretchable devices, yet there are hardly comprehensive reviews in this research field. In this review, we firstly introduce the probing methods of interlayer mechanical coupling, including high-frequency and ultralow-frequency Raman characterizations, nanoindentation of multilayer 2D materials or heterostructures, surface indentation, pressurized blister test, characterization of spontaneously formed nanoblisters, and nano-friction tests. Based on the analysis and comparison of the existing methods and results, we also discuss the advantages and limitations of each method. Finally, the challenges and opportunities in this promising field are discussed. This review summarizes the recent progress in the probing of interlayer mechanical coupling of 2D layered materials and will provide important reference for the rational design of flexible and stretchable devices.