Raman analysis of strained graphene grown on dewetted cobalt

Graphene grows onto cobalt by means of diffusion of carbon atoms during the isothermal stage of exposure to hydrocarbon precursor, followed by precipitation during cooling. This method, largely applied with nickel catalyst, is known to produce continuous, but not uniform, layers with the concurrent presence of mono- and poly-graphene areas. With the aid of Raman mapping of graphene still lying onto its catalyst, we are able to consider the possible origins for the observed distortions of the phonon modes with respect to the well-known picture of the monolayer material. Optical effects, doping, the presence of multilayered islands, and strain are kept into account. It is shown that some observations can be interpreted in terms of the occurrence of isotropic strain with the uniaxial component superimposed at the metal discontinuities. Strain is proposed to originate from the difference between the thermal expansion coefficients of graphene and cobalt. The present paper shows that inhomogeneities in graphene grown onto catalysts with high C solubility are not always directly related to excess of precipitation. The observation of strain in as-grown graphene opens the possibility of tailoring the electronic density of states via strain engineering directly during growth.