Introduction
Single crystals of graphene, without grain boundaries and associated defect clusters, represent an ideal material in electronic fields. Nowadays, a large number of methods have been proposed to produce graphene monolayer. These methods can be categorized into two major classes, i.e. bottom-up method and top-down method. The former methods include chemical vapor deposition (CVD) and epitaxial growth, the latter methods relies on the mechanical exfoliation of graphite.
Fig 1 SEM images of few-layer graphene single crystals on Cu surface.
(Adv. Mater. 2015, 27, 2821–2837)
Chemical vapor deposition
CVD relied on the chemical reactions of molecular building blocks to form covalently linked 2D networks to yield graphenes. CVD-derived graphene films are polycrystalline, and composed of numerous grains separated by grain boundaries, which are detrimental to their application in electronics field. The quality for graphene grown by CVD is generally not as good as those exfoliated from graphite because of the small grain size and high amount of grain boundaries and defects. To overcome these problem, several metals have been identified as promising catalysts in CVD for growth of large size graphene single crystals, such as Cu, Pt, and Ru.
Mechanical exfoliation
Exfoliation of graphite to afford graphene is one of the most promising ways to large-scale production at extremely low cost, whereby graphene is produced through the direct exfoliation of graphite in the liquid phase. In this process, the ideal case is that graphene can be peeled from the bulk graphite layer by layer. There are two kinds of mechanical routes to exfoliate graphite into graphene flakes, i.e. normal force and lateral force (Fig 2). One can exert normal force to overcome the Van der Waals attraction or exert lateral force to promote the relative motion between two graphite layers.
Fig 2 Two kinds of mechanical routes
(Adv. Mater. 2015, 27, 2821–2837)
