Intrinsic Line Shape of the Raman 2D-Mode in Freestanding Graphene Monolayers
Intrinsic Line Shape of the Raman 2D-Mode in Freestanding Graphene Monolayers
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DOI：
10.1021/nl400917e
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发表时间：
2013-08-01
期刊：
影响因子：
10.8
通讯作者：
Heinz, Tony F.
中科院分区：
文献类型：
--
作者：
Berciaud, Stephane;Li, Xianglong;Heinz, Tony F.
我们报道了对石墨烯单层中双声子谷间(2D)拉曼模式的综合研究，其动机是最近关于独立石墨烯中不对称2D模线型的报道。对于1.53-2.71 eV范围内的光子能量，独立样品的2D模拉曼响应表现为双峰，这与通常用于支撑单层的洛伦兹近似形成鲜明对比。在载流子密度大于2×10(11)cm(-2)的情况下，通过静电掺杂独立石墨烯来模拟独立和支撑情况之间的转变。这一结果定量地表明，低水平的电荷可以掩盖单层石墨烯固有的双峰2D线形。在原始的独立石墨烯中，我们观察到随着光子能量的减少，2D模式特征的展宽，这不能用基于共振内部和外部过程的简单一维模型来合理化。这表明远离布里渊区高对称线的声子波矢必然对2D模有贡献，因此需要全二维计算才能正确地描述多声子共振拉曼过程。
We report a comprehensive study of the two-phonon intervalley (2D) Raman mode in graphene monolayers, motivated by recent reports of asymmetric 2D-mode line shapes in freestanding graphene. For photon energies in the range 1.53-2.71 eV, the 2D-mode Raman response of freestanding samples appears as bimodal, in stark contrast with the Lorentzian approximation that is commonly used for supported monolayers. The transition between the freestanding and supported cases is mimicked by electrostatically doping freestanding graphene at carrier densities above 2 x 10(11) cm(-2). This result quantitatively demonstrates that low levels of charging can obscure the intrinsically bimodal 2D-mode line shape of monolayer graphene. In pristine freestanding graphene, we observe a broadening of the 2D-mode feature with decreasing photon energy that cannot be rationalized using a simple one-dimensional model based on resonant inner and outer processes. This indicates that phonon wavevectors away from the high-symmetry lines of the Brillouin zone must contribute to the 2D-mode, so that a full two-dimensional calculation is required to properly describe multiphonon-resonant Raman processes.