Role of carbon dissolution and recondensation in graphene epitaxial alignment on cobalt

The crystalline quality of the graphene lattice is a crucial parameter that not only rules the electronic and transport properties of the carbon film, but also its interaction with the substrate. Elucidating the effect of different growth pathways on the resulting graphene-substrate structural configurations and the microscopic mechanisms for their formation is, therefore, a goal of utmost importance. By using electron spectro-microscopy with high chemical and structural sensitivity, we image the structural transformation that graphene on cobalt undergoes at temperatures above 500 degrees C, from a rotationally-incoherent, defective layer to a high quality epitaxial one. We find that the transformation takes place via the growth and propagation of mesoscopic carbidic islands. We identify the underlying mechanism for the formation of epitaxial graphene to involve the dissolution and recondensation of carbon within these regions. The activation energy of the process is estimated to be 1.84 +/- 0.11 eV, indicating that the carbon detachment is the rate-limiting step. With the aid of theoretical calculations, we show that the martensitic phase transition occurring in cobalt above 420 degrees C does not affect the graphene transformation. These findings help to establish the optimal parameters to grow high-quality graphene epilayers on Co, opening viable routes towards usage in artificially fabricated magnetic heterostructures. (C) 2019 Elsevier Ltd. All rights reserved.