Graphene was grown on Cu foil using low-pressure chemical vapor deposition and then transferred using polymethylmethacrylate (molecular weight ~ 950 K; dissolved 1% w/v in anisole) onto a Si substrate with a ~285 nm thick SiO2 layer as described in an earlier work. The graphene-coated Si/SiO2 substrates were then subjected to the following radiolysis procedure.