Optical Reflectivity and Raman Scattering in Few-Layer-Thick Graphene Highly Doped by K and Rb.

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• 作者：Naeyoung Jung ; Bumjung Kim ; Andrew C. Crowther ; Namdong Kim ; Colin Nuckolls ; Louis Brus
• 刊名：ACS Nano
• 出版年：2011
• 出版时间：July 26, 2011
• 年：2011
• 卷：5
• 期：7
• 页码：5708-5716
• 全文大小：976K
• 年卷期：0
• ISSN：1936-086X

文摘

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) KC₈ and RbC₈. 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/cm². 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.