Growth of Single-Layer and Multilayer Graphene on Cu/Ni Alloy Substrates.

ConspectusGraphene, a one-atom-thick layer of carbon with a honeycomb lattice, has drawn great attention due to its outstanding properties and its various applications in electronic and photonic devices. Mechanical exfoliation has been used for preparing graphene flakes (from monolayer to multilayer with thick pieces also typically present), but with sizes limited typically to less than millimeters, its usefulness is limited. Chemical vapor deposition (CVD) has been shown to be the most effective technique for the scalable preparation of graphene films with high quality and uniformity. To date, CVD growth of graphene on the most commonly used substrates (Cu and Ni foils) has been demonstrated and intensively studied. However, a survey of the existing literature and earlier work using Cu or Ni substrates for CVD growth indicates that the bilayer and multilayer graphene over a large area, particularly single crystals, have not been obtained.In this Account, we review current progress and development in the CVD growth of graphene and highlight the important challenges that need to be addressed, for example, how to achieve large single crystal graphene films with a controlled number of layers. A single-layer graphene film grown on polycrystalline Cu foil was first reported by our group, and since then various techniques have been devoted to achieving the fast growth of large-area graphene films with high quality. Commercially available Cu/Ni foils, sputtered Cu/Ni thin films, and polycrystalline Cu/Ni foils have been used for the CVD synthesis of bilayer, trilayer, and multilayer graphene. Cu/Ni alloy substrates are particularly interesting due to their greater carbon solubility than pure Cu substrates and this solubility can be finely controlled by changing the alloy composition. These substrates with controlled compositions have shown the potential for the growth of layer-tunable graphene films in addition to providing a much higher growth rate due to their stronger catalytic activity. However, the well-controlled preparation of single crystal graphene with a defined number of layers on Cu/Ni substrates is still challenging.Due to its small lattice mismatch with graphene, a single crystal Cu(111) foil has been shown to be an ideal substrate for the epitaxial growth of graphene. Our group has reported the synthesis of large-size single crystal Cu(111) foils by the contact-free annealing of commercial Cu foils, and single crystal Cu/Ni(111) alloy foils have also been obtained after the heat-treatment of Ni-coated Cu(111) foils. The use of these single crystal foils (especially the Cu/Ni alloy foils) as growth substrates has enabled the fast growth of single crystal single-layer graphene films. By increase of the Ni content, single crystal bilayer, trilayer, and even multilayer graphene films have been synthesized. In addition, we also discuss the wafer-scale growth of single-layer graphene on the single crystalline Cu/Ni(111) thin films.Recent research results on the large-scale preparation of single crystal graphene films with different numbers of layers on various types of Cu/Ni alloy substrates with different compositions are reviewed and discussed in detail. Despite the remarkable progress in this field, further challenges, such as the wafer-scale synthesis of single crystal graphene with a controlled number of layers and a deeper understanding of the growth mechanism of bilayer and multilayer graphene growth on Cu/Ni substrates, still need to be addressed.