Towards the perfect graphene membrane? -Improvement and limits during formation of high quality graphene grown on Cu-foils

摘要

We investigated the structure and crystalline quality of monolayer graphene grown by hydrogen and methane chemical vapor deposition (CVD) on polycrystalline Cu foils. Our data show that the high temperature hydrogen pretreatment of the Cu foil has to be performed at a sufficiently high H2 pressure in order to avoid graphene (g) formation already during the pretreatment, which limits the achievable domain size during subsequent growth in the CH4/H2 mixture. Methane-hydrogen CVD sustains g growth but induces the faceting of the Cu substrate. Characterization by low energy electron microscopy evidenced a staircase Cu substrate morphology of alternating (410) and (100) planes interrupted by (n11) type facets. The g flakes cover the staircase shaped support as a coherent layer. The polycrystalline film mostly contains rotational domains that are preferentially, but not strictly, aligned with respect to the stepped support surface. The substrate induced corrugated morphology occurs also underneath large single crystalline flakes and is transferred to suspended membranes, produced by etching the Cu underneath the graphene. Thus, membranes manufactured from g-Cu are non flat. This explains their reported softened elastic response and the formation of so called nanorippled graphene after transfer from the Cu support which deteriorates its electrical conductivity.