Influence of substrate, precursor, flow field, and hydrogen etching on the flame synthesis of monolayer graphene films

Graphene films are grown in an open atmosphere on metal substrates using a modified multiple inverse-diffusion flame (m-IDF) burner with hydrogen as fuel and hydrocarbon as precursor (delivered by multiple distinct tubes staged above the m-IDF burner surface). The flame synthesis procedure uses three stages consisting of () pretreatment, () growth, and () hydrogen etching and produces monolayer graphene (MLG) films with methane as a precursor on copper substrates at 1000 °C. Substrate material (e.g., copper, nickel, silicon), purity, and smoothness (i.e., unpolished, electropolished); precursor composition (i.e., CH, CH, CH); substrate orientation (i.e., parallel, perpendicular, tilted 45˚) with respect to the post-flame flow field; and post-growth in-situ hydrogen etching are investigated for their impact on the quality (i.e., defect level, graphitic structure), uniformity, and number of layers of the as-synthesized graphene films. On low-purity Cu Substrates, defects are observed because of the impurity content. Interestingly, on smooth electropolished Cu substrates, high defect levels are produced presumably because of the high nucleation rates and density from the higher flow rates used in flame synthesis. When using CH and CH as precursors versus CH, growth rates are slower although the graphene film characteristics are similar. The small substrate sizes used are not in the boundary-layer regime, so the substrate orientation does not affect the characteristics of the film. The initial quality of graphene films and the substrate materials are the two key factors controlling the hydrogen etching effect and the healing effect because of thermal recrystallization. Hydrogen annealing can remove adlayers and improve the quality of graphene on Cu but has minimal or detrimental effects on films on other substrate materials.