Mechanism of Thin Layers Graphite Formation by 13C Implantation and Annealing

The mechanism of thin layers graphite (TLG) synthesis on a polycrystalline nickel film deposited on SiO2 (300 nm thick)/Si(100) has been investigated by 13C implantation of four equivalent graphene monolayers and annealing at moderate temperatures (450–600 °C). During this process, the implanted 13C segregates to the surface. Nuclear Reaction Analyses (NRA) are used for the first time in the topic of graphene synthesis to separate the isotopes and to determine the 12C and 13C concentrations at each step. Indeed, a significant part of carbon in the TLG also comes from residual 12C carbon absorbed into the metallic matrix. Raman spectroscopy and imaging are used to determine the main location of each carbon isotope in the TLG. The Raman mappings especially emphasize the role of 12C previously present at the surface that first diffuses along grain boundaries. They play the role of nucleation precursors. Around them the implanted 13C or a mixture of bulk 12C–13C aggregate and further precipitate into graphene-like fragments. Graphenization is effective at around 600 °C. These results point out the importance of controlling carbon incorporation, as well as the importance of preparing a uniform nickel surface, in order to avoid heterogeneous nucleation.