Recent progress of mechanical exfoliation and the application in the study of 2D materials

After the first exfoliation of graphene in 2004, more and more two-dimensional (2D) materials have been discovered, including insulators, semiconductors, semi-metals, metals, superconductors, and topological materials, which provide a huge playground for studying fascinating properties in 2D crystals. In the past 15 years, the fabrication ways of 2D materials can be summarized into two strategies: top-down and bottom-up. As a representative top-down strategy, mechanical exfoliation (ME) has been widely used for preparing high-quality 2D materials. Many pioneering works about 2D materials are discovered from exfoliated samples; however, the routine exfoliation method has drawbacks in low-yield ratios and small-size, which hindered the study progress of 2D materials. In this review, we first introduce some breakthroughs in the ME methods for preparing large-area 2D materials. The size and yield ratio of 2D materials have been greatly increased by modified ME methods, which will promote progress in 2D materials studies. Then, we discuss various remarkable physical properties discovered in exfoliated 2D materials, such as their mechanical, electrical, optical, and magnetic properties. Finally, the challenges and future perspectives of the ME method are discussed. There are more than 1500 kinds of cleavable layered materials according to theoretical predictions, therefore, the ME method will still play an important role in exploring new 2D materials. Furthermore, as the new ME method moves forward, exfoliation of wafer-scale, large-area 2D materials is also promising, which will show great potential in the future industry of information and electronic devices.