Mechanical exfoliation of large area 2D materials from vdW crystals

Monolayer two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides (TMDCs), provide a versatile platform for exploring novel physical phenomena at the 2D limit, and show great promise for next-generation electronic, optoelectronic, and quantum devices. To overcome the weak van der Waals interaction in the bulk layered crystal and achieve high quality single-crystal monolayers is a crucial task in top-down mechanical exfoliation. Tape exfoliation has long been the dominant approach to obtain single-crystal monolayers with high quality. More recently, there has been a fast development of using metals as an intermediate to enhance monolayer area and exfoliation yield. This review will provide a survey of mechanical exfoliation strategies of tape and metal-assisted exfoliations, particularly for the most popular graphene and TMDC materials. The interfacial interaction and lateral strain between monolayer and other materials such as oxides and metals play a crucial role in monolayer selectivity and yield. The challenges and opportunities will be highlighted for future development of exfoliating procedures to achieve large-area and high-quality 2D material monolayers and artificial stacks.