Mechanical Properties of 2D Materials Studied by In Situ Microscopy Techniques

Two-dimensional (2D) materials have been demonstrated as promising building blocks in future electronic and their mechanical properties are quite important for various applications. Due to their atomic thickness and planar nature, the investigation of the mechanical properties and related atomic mechanism are quite challenging. This review focuses on the recently developed in situ techniques based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) in characterization of the mechanical properties of 2D materials. Insitu methods used for studying their elastic properties, fracture behavior, andsurface/interface energy are introduced in detail. Specifically, the AFM indentation test and microelectromechanical systems (MEMS) device are generally used to investigate the elastic properties; the manipulator based methods show their flexibility in studying the fracture, adhesion, cleavage, and friction properties; atomic level fracture mechanism can be revealed with in situ high resolution TEM (HRTEM); the pressurized blister test and the buckle/wrinkle based methods are widely used to measure the surface/interface properties. Moreover, the influence of sample preparation process, defects and layer numbers to their mechanical properties are also discussed. Finally, the extensions of above methods to investigate the strain-modulated physical properties of 2D materials are introduced.