title: Optical reflectivity and Raman scattering in few-layer-thick graphene highly doped by K and Rb.
abstract: We report the optical reflectivity and Raman scattering of few layer (L) graphene exposed to K and Rb vapors. Samples many tens of layers thick show the reflectivity and Raman spectra of the stage 1 bulk alkali intercalation compounds (GICs) KC(8) and RbC(8). However, these bulk optical and Raman properties only begin to appear in samples more than about 15 graphene layers thick. The 1 L to 4 L alkali exposed graphene Raman spectra are profoundly different than the Breit-Wigner-Fano (BWF) spectra of the bulk stage 1 compounds. Samples less than 10 layers thick show Drude-like plasma edge reflectivity dip in the visible; alkali exposed few layer graphenes are significantly more transparent than intrinsic graphene. Simulations show the in-plane free electron density is lower than in the bulk stage 1 GICs. In few layer graphenes, alkalis both intercalate between layers and adsorb on the graphene surfaces. Charge transfer electrically dopes the graphene sheets to densities near and above 10(+14) electrons/cm(2). New intrinsic Raman modes at 1128 and 1264 cm(-1) are activated by in-plane graphene zone folding caused by strongly interacting, locally crystalline alkali adlayers. The K Raman spectra are independent of thickness for L = 1-4, indicating that charge transfer from adsorbed and intercalated K layers are similar. The Raman G mode is downshifted and significantly broadened from intrinsic graphene. In contrast, the Rb spectra vary strongly with L and show increased doping by intercalated alkali as L increases. Rb adlayers appear to be disordered liquids, while intercalated layers are locally crystalline solids. A significant intramolecular G mode electronic resonance Raman enhancement is observed in K exposed graphene, as compared with intrinsic graphene.
authors.affiliation: Department of Chemistry, Columbia University, New York, New York 10027, USA. nj2153@columbia.edu
authors.forename: Naeyoung
authors.initials: N
authors.lastname: Jung
authors.forename: Bumjung
authors.initials: B
authors.lastname: Kim
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Crowther
authors.forename: Namdong
authors.initials: N
authors.lastname: Kim
authors.forename: Colin
authors.initials: C
authors.lastname: Nuckolls
authors.forename: Louis
authors.initials: L
authors.lastname: Brus