Different metal films (Co, Ni, Au, and Ag) were deposited on graphene and the interactions
between these metals and graphene were studied by Raman spectroscopy. The Raman
peaks were shifted after the deposition of metal films. The electron doping of graphene with
cobalt contacts and the hole doping with the nickel contacts are the main reasons for Raman
peak shift. However, for gold contacts and silver contacts with graphene, strain effect
dominates Raman peak shift instead of charge transfer.