Development of Cu Substrate Preparation Techniques for Graphene Synthesis

The most common technique for producing large area graphene films is by chemical vapor deposition on Cu foil substrates. Cu is used as a substrate because the solubility of carbon in Cu at the temperatures at which the chemical vapor deposition is performed is very low. This ensures a surface-mediated growth that is self-limited to a single monolayer of graphene if low precursor pressures are used. Before performing chemical vapor deposition, the surface oxide and carbon that is on the unprocessed foil need to be removed to achieve uniform graphene growth. In addition, the roughness of the surface of the Cu foil should be reduced to help prevent defects from forming in the graphene film during growth. The goal of this research project is to determine the optimal procedure for preparation of the Cu foil substrate to produce high quality graphene. Cu foils with 99.8% and 99.999% purity were used for the experiment. The Cu substrate preparation procedure involves annealing in 1 × 10
^-5
Torr of H
₂
at 850 °C to remove the native oxide and to reduce surface roughness. This is followed by annealing in 1 × 10
^-6
Torr of O
₂
at 500 °C to remove carbon from the surface of the foil by conversion to CO
₂
and CO. At this temperature, the solubility of oxygen in Cu is negligible, thus preventing dissolution of oxygen into the bulk. After the oxygen anneal, the foil is annealed in 1 × 10
^-5
Torr of H
₂
at 850 °C to remove chemisorbed oxygen from the Cu surface that has formed during the anneal in O
₂
. The anneal durations in this study were varied to determine the optimal technique for graphene synthesis for each foil purity. The samples were characterized using X-ray photoelectron spectroscopy, scanning electron microscopy, and optical microscopy.