Preparation and regulation of graphene on transition metal substrates

<p indent="0mm">Since the successful exfoliation of graphene in 2004, it has attracted much attention due to its high stability, exceptional mechanical properties, and excellent electronic and thermal conductivity. The unique two-dimensional structure makes graphene an ideal candidate for various nano-devices and quantum devices. The synthesis of graphene is a complicated chemical process determined by both thermodynamics and kinetics. To precisely control the epitaxial growth of graphene, a deep understanding of the interplay between graphene and substrates is necessary. In this review, the synthesis and formation mechanisms of graphene on different transition metal surfaces by chemical vapor deposition were discussed. During graphene growth, transition metals serve as catalysts that not only facilitate the decomposition of carbon sources but also promote the incorporation of carbon atoms into graphene islands. Meanwhile, they provide a crystalline template for regulating the orientation and alignment of graphene islands. By choosing appropriate substrate structures, the epitaxial growth of wafer-scale single-crystal graphene films has been achieved, by seamlessly stitching a large number of unidirectionally aligned graphene islands. Additionally, the morphology and properties of graphene are highly dependent on the lattice mismatch between graphene and transition metals. In general, graphene grown on small-mismatch Cu and Ni substrates shows a flat structure. These substrates have been widely used for synthesizing wafer-scale single-crystal graphene. To date, high-quality monolayer and multilayer graphene wafers have been successfully achieved on Cu and Ni substrates, respectively. Cu/Ni alloyed substrates are popular for controlling the thickness of graphene films. By contrast, graphene on large-mismatch metal substrates, such as Ru, Rh, Pt, Ir, Re, Pd, etc., typically exhibited height-fluctuated moiré superstructures. This is ascribed to the site-dependent interaction between graphene and substrates. Their non-uniform surface features enable graphene moiré superstructures’ new applications in templating the fabrication of metal clusters and the self-assembly of molecules. Besides, these large-mismatch metal substrates were also used to synthesize graphene quantum dots. So far, significant progress has been achieved in the preparation of graphene, but there are still many challenges to realizing its industrial application. For example, despite the high quality of graphene grown on transition metal substrates, these substrates are not suitable for the real applications of graphene. A transfer process that avoids introducing defects or affecting the properties of graphene is highly demanded. Meanwhile, more and more researchers have dedicated to the exploration and development of techniques for synthesizing graphene on insulated substrates. Additionally, bilayer graphene exhibits a tunable bandgap under an applied electric field, and adjusting the rotation angle between the two layers can introduce new physical properties. However, research on the synthesis of bilayer graphene and the regulation of their rotational angle is still in its early stages. A deep understanding of the underlying mechanisms for controlling the thickness and rotation angle of graphene layers is needed. These studies are expected to broaden the application field of graphene, establishing a solid foundation for future industrial applications.