The nature of graphene-metal bonding probed by Raman spectroscopy: The special case of cobalt

Abstract

The nature of graphene-metal bonding is crucial for the performance of graphene-based electronic devices. Raman spectroscopy is a powerful technique for probing the electronic behaviour of graphene-metal interfaces. The changes in the Raman spectrum of pristine graphene upon contact with standard metal layers are reported here. In particular, the study is focused on metallization by electron-beam evaporation using chromium or titanium (commonly used as an adhesion layer to improve the bonding of other metals such as gold) and nickel or cobalt (ferromagnetic materials used for spintronics). The results obtained indicate that the main changes in the Raman spectra can be explained in terms of a biaxial strain generated by graphene trying to match the crystalline lattice of the metal. In the case of cobalt, we find that the strong binding of some cobalt atoms to graphene generates a spectrum with a duplication of the characteristic graphene peaks: those corresponding to cobalt physisorbed to graphene and those corresponding to cobalt chemisorbed to graphene, strongly redshifted. Such special behaviour of the graphene-cobalt interface is correlated to the low contact resistance and the enhanced perpendicular magnetic anisotropy of cobalt on graphene.