Growth and characterization of Graphene

Graphite can be obtained from carbon saturated molten Fe during the formation of steel. The fact that carbon can be incorporated in metals and led to the formation of graphitic layers was known since the 1940s. The amount of carbon that can be dissolved in most metals is up to a few atomic percent. In order to eliminate the competition of forming a carbide and graphite/Graphene growth non-carbide forming metals are used such as Ni, Au, Pt.
Carbon can be deposited on a metal surface by several techniques among which: flash evaporation, sputtering, PVD, CVD, ion implan.
tation and spin coating. The carbon source can be a solid, liquid or gas. In the case of a pure carbon source, flash evaporation, sputtering or PVD can be used to deposit carbon directly on the substrate of interest, before diffusion at high temperature followed by precipitation of graphite (Graphene) upon cooling. Zheng et al. reported the growth of Graphene on Ni using this technique. They also report that the number of Graphene layers can be controlled by the thickness of the initial amorphous carbon deposition. Baraton et al. reported the same growth using ion implantation of carbon in Ni thin films.
During high temperature annealing, carbon diffuses into the metal until it reaches the solubility limit (which is high for Ni). Upon cool.ing, carbon precipitates forming first Graphene, then graphite. The graphite film thickness depends on the metal, and the solubility of car.bon in that metal, the temperature at which the carbon is introduced, the thickness of the metal and the cooling rate.
Zheng et al. used e-Beam evaporated Ni films (100 . 300 nm) deposited on SiO2 substrates where a layer of a-C (2.5 . 40nm) was sputtered before Ni deposition. The annealing was performed in a tube furnace (similar to those used for CVD growth) with temperatures ranging from 650°C to 950°C under Ar flow at ∼ 1.7 Torr. Then the samples where cooled with a rate of ∼ 20 °C/s. Raman spectroscopy showed uniform Graphene growth on top of the Ni with number of layer depending on the amount of initial a-C and on the annealing temperature. Graphene formation and quality were found to be relatively independent of the annealing time, up to ∼ 60 min. Their results suggest that under these conditions, and assuming equal densities of the as-deposited a-C and Graphene, roughly half of the carbon source is crystallized into Graphene with the rest either outgassing from the system or remaining in the Ni film.
Growth of Graphene on Ni, Co, Ru, etc. was also reported by so.called CVD at high T, using various hydrocarbon precursors. However, the CVD process referred to in the aforementioned papers is a misnomer, since Graphene is not directly produced on the metal surface by the reaction and deposition of the precursor at the "growth T", but rather grows by carbon segregation from the metal bulk, as a result of carbon supersaturation in the solid, as discussed above.
We would like to stress the fact that we will refer to Graphene growth by carbon precipitation either if the source is a solid a-C deposit or a hydrocarbon gas cracked at high T, the key point is the use of a high C-solubility metal substrate and that the growth is achieved with rapid cooling of the substrate rather that at growth T.