XPS and Raman measurements for the samples before and after Ge intercalation. (a) Ge 3d core-level spectra of Gr/Ir (black curve) and Gr/Ge/Ir (red curve) samples. (b) C 1s core-level spectra of Gr/Ir (black curve) and Gr/Ge/Ir (red curve) samples. (c) Raman spectra of graphene before (black curve) and after (red curve) Ge intercalation. Inset is the 2D band of the graphene after intercalation, well fitted by a single Lorentzian peak. (d)–(f) Raman maps of the intensity ratio of 2D to G bands ( I 2 D / I G ) , intensity ratio of D to G bands ( I D / I G ) , and positions of G band for Ge intercalated graphene, respectively.