Pimenta Marcos A, Resende Geovani C, Ribeiro Henrique B, Carvalho Bruno R
Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais 30123-970, Brazil.
Department of Applied Physics, Stanford University, Stanford, California, 94305, USA.
Phys Chem Chem Phys. 2021 Dec 15;23(48):27103-27123. doi: 10.1039/d1cp03626b.
In this perspective review, we discuss the power of polarized Raman spectroscopy to study optically anisotropic 2D materials, belonging to the orthorhombic, monoclinic and triclinic crystal families. We start by showing that the polarization dependence of the peak intensities is described by the Raman tensor that is unique for each phonon mode, and then we discuss how to determine the tensor elements from the angle-resolved polarized measurements by analyzing the intensities in both the parallel- and cross-polarized scattering configurations. We present specific examples of orthorhombic black phosphorus and monoclinic 1T'-MoTe, where the Raman tensors have null elements and their principal axes coincide with the crystallographic ones, followed by a discussion on the results for triclinic ReS and ReSe, where the axes of the Raman tensor do not coincide with the crystallographic axes and all elements are non-zero. We show that the Raman tensor elements are, in general, given by complex numbers and that phase differences between tensor elements are needed to describe the experimental results. We discuss the dependence of the Raman tensors on the excitation laser energy and thickness of the sample within the framework of the quantum model for the Raman intensities. We show that the wavevector dependence of the electron-phonon interaction is essential for explaining the distinct Raman tensor for each phonon mode. Finally, we close with our concluding remarks and perspectives to be explored using angle-resolved polarized Raman spectroscopy in optically anisotropic 2D materials.
在这篇观点综述中，我们讨论了偏振拉曼光谱在研究属于正交晶系、单斜晶系和三斜晶系晶体家族的光学各向异性二维材料方面的作用。我们首先表明，峰强度的偏振依赖性由拉曼张量描述，而拉曼张量对于每个声子模式都是唯一的，然后我们讨论如何通过分析平行偏振和交叉偏振散射配置中的强度，从角度分辨偏振测量中确定张量元素。我们给出了正交晶系黑磷和单斜晶系1T'-MoTe的具体例子，其中拉曼张量具有零元素且其主轴与结晶轴重合，随后讨论了三斜晶系ReS和ReSe的结果，其中拉曼张量的轴与结晶轴不重合且所有元素均不为零。我们表明，拉曼张量元素通常由复数给出，并且需要张量元素之间的相位差来描述实验结果。我们在拉曼强度的量子模型框架内讨论了拉曼张量对激发激光能量和样品厚度的依赖性。我们表明，电子-声子相互作用的波矢依赖性对于解释每个声子模式独特的拉曼张量至关重要。最后，我们以结束语和展望结束，即在光学各向异性二维材料中使用角度分辨偏振拉曼光谱有待探索的方面。
