1 Two-Dimensional Synthetic Templates

The isolation of atomic-thin graphene sheets from bulk graphite has led to the discovery of a number of novel properties of graphene that have enormously benefited the development of nanotechnology [1], which has inspired the exploration of other 2D materials, such as transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), etc. [2]. To exfoliate a layered bulk crystal, the van der Waals interaction between the adjacent layers should be overcome, which can be realized by techniques like mechanical exfoliation [1] and solvent-assisted ultrasound exfoliation [3]. These two methods are relatively non-destructive so that the intrinsic properties of 2D materials could be preserved. However, the former method is mostly applied for fundamental studies, and the latter one is difficult to produce high-yield single-layer nanosheets. The intercalation-based methods, on the other hand, are more effective and reliable in mass production of single-layer nanosheets. It was demonstrated decades ago that many chemical species, like alkali metals, transition-metal halides and amine molecules can be intercalated into various layered materials [4]. As one of the most intensively studied intercalates, lithium, for example, can be inserted into the interlayer spaces of a layered crystal by soaking the crystal in n-butyl lithium solution at an elevated temperature (e.g. 100 oC) for a long time (e.g. 3 days)