Catalytic effect of metal alloy sublayers on graphene formation in thermally annealed amorphous carbon ultrathin films

Abstract

The exceptional properties of graphene, such as mechanical strength, flexibility, thermal conductivity, electron mobility, biocompatibility, and transparency, make it an ideal material for numerous applications. The objective of this study was to develop a one-step process of graphene formation that uses inert thermal annealing of amorphous carbon (a-C) ultrathin films deposited onto different metal catalyst sublayers to transform highly sp3 hybridized a-C nanostructures within the film to sp2 hybridized graphene nanostructures. To elucidate the mechanism of graphene formation and the catalytic effect of the sublayers on sp3-to-sp2 transformation, Si/FeNi/a-C and Si/FeCo/a-C stacks were fabricated by physical vapor deposition techniques. X-ray photoelectron and Raman spectroscopies were used to analyze the structure of as-deposited and thermally annealed a-C films. Moreover, molecular dynamics simulations provided insight into graphenization at the atomic level. The sp3 fraction of as-deposited a-C films was found to play a pivotal role in the formation of graphene during thermal annealing. The carbon concentration in the catalyst sublayers and the compressive stress in the a-C films strongly affected atomic carbon hybridization and, consequently, the graphene quality. Differences in the catalytic effectiveness of the sublayers in producing graphene are interpreted in terms of the sublayer carbon concentration, atomic packing, and electronic structure of the valence band of elements affecting the interaction between carbon and transition metals, ultimately influencing graphene nucleation.