Institut de Physique et Chimie des Matériaux de Strasbourg and NIE, UMR 7504, Université de Strasbourg and CNRS, Strasbourg, France.
Nano Lett. 2013 Aug 14;13(8):3517-23. doi: 10.1021/nl400917e. Epub 2013 Jul 10.
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 × 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.
我们对石墨烯单层中的双声子谷间（2D）拉曼模式进行了全面研究，这是受到最近关于独立石墨烯中 2D 模式线形状不对称的报道的启发。对于光子能量在 1.53-2.71 eV 的范围内，独立样品的 2D 模式拉曼响应呈现双峰，与通常用于支撑单层的洛伦兹近似形成鲜明对比。通过在载流子密度高于 2×10(11) cm(-2)时对独立石墨烯进行静电掺杂，可以模拟独立和支撑情况之间的转变。这一结果定量地证明了低水平的充电可以掩盖单层石墨烯本征的双峰 2D 模式线形状。在原始的独立石墨烯中，我们观察到 2D 模式特征随光子能量的降低而展宽，这不能用基于共振内和外过程的简单一维模型来合理解释。这表明声子波矢必须来自于布里渊区的高对称线之外，才能对 2D 模式做出贡献，因此需要进行全二维计算才能正确描述多声子共振拉曼过程。
