Recent advances in free-standing two-dimensional crystals with atomic thickness: design, assembly and transfer strategies

Free-standing two-dimensional (2D) crystals with atomic thickness have attracted extensive attention because of their novel electronic, optical, mechanical and biocompatible properties, and so on. In recent years, the study of atomically thick 2D crystals has mainly focused on the layered materials with weak van der Waals forces between the layers. For the lack of executable synthetic strategies, preparation of atomically thick 2D crystals with a nonlayered structure or quasi-layered structure with relatively strong bonds between the layers is still a great challenge. This review mainly focuses on recent advances in synthetic strategies for atomically thick 2D crystals with a nonlayered structure as well as the quasi-layered structure with relatively strong bonds between the layers. Furthermore, methods for the modulation of the electronic structures of 2D crystals along with assembly and transfer techniques of the 2D crystals are discussed. The key points of each strategy in preparation, electronic structure modulation, assembly and transfer processes are also presented.