(a) The Raman spectra before and after the proposed treatment at the same position. Two distinct bands can be observed, which are designated as G band and 2D band. (b) The histogram of the G band position from the mapping data before and after the treatment. (c), (d) The Raman mapping of the band intensity ratio I(2D)/I(G) before and after the treatment, respectively. The mapping was performed on a graphene strip, where the black area represents the edge outside the graphene.