In this work, we uncover a mechanism initiating spontaneous nucleation of graphene flakes on copper foil during the annealing phase of chemical vapor deposition (CVD) process. We demonstrate that the carbon in the bulk of copper foil is the source of nucleation. Although carbon solubility in a pure copper bulk is very low, excess carbon can be embedded inside the copper foil during the foil production process. Using time-of-flight secondary ion mass spectrometry, we measured the distribution profile of carbon atoms inside the copper foils and its variation by thermal annealing. We also studied the role of hydrogen in the segregation of carbon from the bulk to the surface of copper during annealing by scanning electron microscopy and Raman analysis. We found that carbon atoms diffuse out from the copper foil and accumulate on its surface during annealing in the presence of hydrogen. Consequently, graphene crystals can be nucleated and grown while “any external” carbon precursor was entirely avoided. To our knowledge, this is the first time that such growth has been demonstrated to take place. We believe that this finding brings a new insight into the initial nucleation of graphene in the CVD process and helps to achieve reproducible growth recipes.

The growth of nearly full coverage of multilayer graphene on the surface of a 99.8% purity copper foil has been experimentally studied. It has been shown that the film thickness can be controlled by a single parameter, the growth time, and growth can be extended until nearly full coverage of more than one layer graphene over the copper surface. The results are supported by scanning electron microscopy and Raman analysis together with optical transmittance and sheet resistance measurements. It has been verified that silicon oxide impurity particles within the copper act as catalysts and the seeds of multilayer graphene islands. The linear increase of the average thickness of graphene to the growth time has been attributed to the interplay between the mean distance between the impurities on the surface and the molecular mean free path in the process gas. A qualitative model is proposed to explain the microscopic mechanism of the multilayer growth on copper. These results contribute to the understanding of the chemical vapour deposition growth kinetics towards the objective of large area high quality graphene production with tuneable layer thickness.

In chemical vapor deposition (CVD) growth of graphene, intrinsic carbon in copper has been shown to play a role, especially during the nucleation phase. Here, we report experimental results on depletion of carbon from the bulk of a Cu foil to its surface at different hydrogen pressures, which explain new aspects of the interplay between hydrogen and intrinsic carbon prior to growth. We observed that rising H 2 pressure boosts carbon depletion to the surface, but at the same time, at elevated H 2 pressures, the graphitic film formed on the Cu surface is etched away at a faster rate. This effect led us to practice annealing of copper under high hydrogen pressure as an approach to decrease the total content of carbon in the copper foil and consequently reducing the nucleation density of graphene flakes. These results enhance our understanding about the role of H 2 in the CVD process and explain some of the inconsistencies among the earlier reports.

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