Interface Properties of Metal/Graphene Heterostructures Studied by Micro-Raman Spectroscopy

Studies are conducted for the influence of the interface formation of graphene with various transition metals on its vibrational properties by using confocal micro-Raman spectroscopy. Micrometer-scale heterostructures consisting of patterned regions of the single layer and multilayer graphene (SLG and MLG, hereafter) covered with and without metals on the same graphene sheet were fabricated by thin-film deposition on the graphene surface through a shadow mask. Comparative analysis for these two different regions (SLG and MLG) fabricated within an identical graphene sheet enables us to investigate the interactions at and the doping effect from the metal/graphene interface as a function of the layers number of graphene without the influence of the unintentional doping. Confirmed dependences of the peaks shifts of the Raman bands (D, G, and 2D bands) on the graphene layers number and metal species (Co, Ni, and Au) reveal that the interfacial interactions are dramatically different between single layer and multilayer graphenes. In the metal/MLG heterostructures, the Raman band shifts are reasonably attributed to carrier doping from metals. It is found that the type of the doped carriers (electrons or holes) is different between Co/MLG and Au/MLG, irrespective of almost the same work functions of Co and Au. These analyses also provide the effective thickness of carrier doping (2−3 graphene layers) from the interfaces. In the metal/SLG heterostructures, significant differences from the metal/MLG heterostructures were observed for the Raman parameters of the G and 2D bands. It is suggested that there exist strong interactions at the metal/SLG interfaces different from those at the metal/MLG interfaces.