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- Materials Science; Engineering; Physics and Astronomy; Cu/Ni(111) alloy; folds; graphene islands; joining; monolayer graphene; single crystal; superstructure
Fast-growth of single crystal monolayer graphene by CVD using methane and hydrogen has been achieved on "homemade" single crystal Cu/Ni(111) alloy foils over large area. Full coverage was achieved in 5 min or less for a particular range of composition (1.3 at.% to 8.6 at.% Ni), as compared to 60 min for a pure Cu(111) foil under identical growth conditions. These are the bulk atomic percentages of Ni, as a superstructure at the surface of these foils with stoichiometry CuNi (for 1.3 to 7.8 bulk at.% Ni in the Cu/Ni(111) foil) was discovered by low energy electron diffraction (LEED). Complete large area monolayer graphene films are either single crystal or close to single crystal, and include folded regions that are essentially parallel and that were likely wrinkles that "fell over" to bind to the surface; these folds are separated by large, wrinkle-free regions. The folds occur due to the buildup of interfacial compressive stress (and its release) during cooling of the foils from 1075 °C to room temperature. The fold heights measured by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) prove them to all be 3 layers thick, and scanning electron microscopy (SEM) imaging shows them to be around 10 to 300 nm wide and separated by roughly 20 μm. These folds are always essentially perpendicular to the steps in this Cu/Ni(111) substrate. Joining of well-aligned graphene islands (in growths that were terminated prior to full film coverage) was investigated with high magnification SEM and aberration-corrected high-resolution transmission electron microscopy (TEM) as well as AFM, STM, and optical microscopy. These methods show that many of the "join regions" have folds, and these arise from interfacial adhesion mechanics (they are due to the buildup of compressive stress during cool-down, but these folds are different than for the continuous graphene films-they occur due to "weak links" in terms of the interface mechanics). Such Cu/Ni(111) alloy foils are promising substrates for the large-scale synthesis of single-crystal graphene film.