Mechanical properties characterization of two-dimensional materials via nanoindentation experiments

Nanoindentation has been widely adopted for mechanical properties characterization of two-dimensional (2D) materials, where one typically starts with measuring the indentation load displacement relationship of a selected 2D material, either free-standing or on a substrate, and then fits the result to an analytical model to extract the elastic modulus and strength of the material. However, the existing indentation models were not originally intended to be used for atomically thin materials, which has led to some controversies and confusion in the field. There is now an urgent need to develop new analytical models capable of describing the indentation response of 2D materials for accurate characterization of their mechanical properties. Here, we review recent progress in this field to identify existing issues and opportunities for future studies.