Nucleation sites for multilayer graphene on nickel catalyst

The structural quality of graphene films is of immense importance both in improving growth procedures and understanding the resulting films´ electronic properties. The graphene film needs to be atomically smooth with low defects, low roughness, and high electronic mobility for high performance Field Effect Transistor (FET) devices. Because of graphene´s high optical transmittance and conductivity, it is also being considered as a transparent conductive electrode [1]. Multilayer graphene prepared by diluted methane-based CVD at 1 atm on nickel (Ni) films deposited over Si/SiO₂ wafers has been shown in various colours, sizes, and shapes [2]. Their preferred nucleation sites in relation to the Ni grain boundaries are not well understood. In this study, we prepared a variety of Ni templates having grain structures ranging from small to large and with mixed distribution across the surface. This was achieved through evaporation and sputter deposition methods. We found the greatest variety of Ni grains was achieved by changing the sputter deposition temperature (100 and 250°C), and pressure (2 and 20 mT). The ability to grow single- and few-layer graphene on top of the Ni catalyst depended on the highly diluted methane gas in CVD, as confirmed by micro-Raman spectroscopy. Methane ratios from 0.5 vol.% to 0.41 vol.% exhibited a strong presence of graphene, 0.36 vol. % produced mixed results, and 0.24 vol.% or less produced no evidence of graphene on a Ni template after the anneal and CVD. Annealing temperatures with as small as a 25°C difference had a great influence on final size of graphene. The grains grew almost double in size when annealed at 975°C as compared to 950°C. So it is advantageous to perform annealing followed by growth at the highest possible temperature and then combine it with an optimized cooling rate.