Recent advances in the fundamentals and in situ characterizations for mechanics in 2D materials

The growing need for integrating two-dimensional materials in electronic and functional devices requires the flexibility of the material. This necessitates the in situ characterization of their mechanical properties to understand their structure under stress loading in working devices. However, it is still challenging to directly characterize the mechanical behaviours of two-dimensional materials due to difficulties in handling these naturally fragile materials. In this review, we summarize the recent studies of mechanical properties in two-dimensional materials and their characterization using various microscopy techniques. This involves advances in fundamentals including the measurements of elastic properties, and the basic understanding of how structural parameters like defects and interfaces influence the deformation and failure process of two-dimensional materials. We also discuss the developed handling techniques for transferring two-dimensional materials to the characterization platforms, with the recent advances in in situ characterization studies based on atomic force microscopy and scanning/transmission electron microscopy. The above developments allowed the direct observation of unconventional mechanisms behind the deformation behaviour of two-dimensional materials, including plastic deformation, interlayer slip, phase transition and nanosized cracking. We then discuss the applications related to the mechanics of two-dimensional materials, including structural materials, electronic and optoelectronic properties, and further conclude with the opportunities and challenges in this field.