The transformation of a gold film on few-layer graphene to produce either hexagonal or triangular nanoparticles during annealing

The shape transformation of gold directly on graphene has been well studied by thermally annealing gold-deposited graphene samples at the temperature range from 600 to 800 � C. We find that few-layer graphene can be served as a platform to transform a gold film into mainly hexagonal gold nanoparticles (AuNPs) at 600 or 700 � C, or coexistence of hexagonal and triangular AuNPs at 800 � C. Especially, the size and density of these AuNPs are dependent on the number of graphene layers, indicating the strong relationship between gold shape transformation and the number of graphene layers on the substrate. We propose that annealing-induced growth of gold islands and the layer-dependent interactions among Au and n-layer graphene are the two main causes for this shape transformation. Meanwhile, Raman enhancing effects of these AuNPs are also investigated. These faceted AuNPs exhibit excellent SERS effects on Raman spectra of few-layer graphene with the enhancement factors up to several hundreds. Combined with n-layer graphenes, these faceted AuNPs can be used as graphene-based SERS substrates for increasing Raman signals of adsorbed rhodamine 6G molecules with a larger scale than those based on fresh graphenes.