Controllable growth of single-crystal graphene

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Chemical vapor deposition (CVD) is one of the popular methods for the controllable synthesis of large-area high-quality graphene. CVD grown graphene over metals can be established over large area and this is important for applications, for example transparent conducting electrodes for solar cells, where a contiguous covering of graphene is required. However, due to the morphology of the catalytic substrates and the kinetic factors in the process of growth, the as-synthesized graphene is polycrystalline film merged by many small domains. The grain boundaries will make the physical and chemical properties far different from those of the intrinsic graphene. Instead, ideal single crystals have no grain boundaries, resulting in excellent properties with are close to the theoretical expectations. Accordingly, much effort has been devoted to the controllable synthesis of single-crystal graphene in terms of its size and morphology, which will have significant influence on its properties. It has been demonstrated that the crucial point for the growth of large single-crystal graphene is to reduce the nucleation densities. To achieve this, many strategies have been employed to optimize the morphology of the catalytic substrates, such as electropolishing, high-pressure annealing and resolidifying and to control the growth environment, such as equilibrating the catalyst vapor, using enclosure-like catalyst structures, maintaining inactive layer during the initial nucleation stage and so on. In spite of the size, the shape of graphene crystals can be modulated by the crystallographic orientation of substrates and the growth conditions. The competition and equilibrium between the intrinsic crystal form of graphene and the crystallographic orientation of substrates lead to the final shape of the graphene crystal. In addition, a deep insight into the mechanism of size and morphology will offer us an access to understand the growth process of graphene crystals. Here, the extreme importance of large-area graphene single crystals for electrical applications when comparing to polycrystalline graphene film have been highlighted and the representative accomplishments of single-crystal graphene using various metal catalyst or insulating substrate by CVD will be reviewed, including the derivations of its size and morphology and its applications in electronic devices. Further, the spatial structure of graphene crystals will be presented, which can offer more information about the growth mechanism of graphene crystals and open a new territory for graphene-based electronic and optical devices. The opportunities and challenges in the controllable growth of graphene single crystal will also be discussed.