Progress on Raman spectroscopy of two-dimensional materials

Abstract

Two-dimensional (2D) materials have attracted intense interest due to the planar structure, unique properties and broad applications. Raman spectroscopy is a quick and nondestructive finger-print method in the material science and has been used to study the material properties for many years. It plays a very important role in the structural characterization of 2D materials, which can be used to probe the electronic band structure, phonon dispersion and electron-phonon interaction in 2D materials. This review summarizes the recent progress of structural studying of 2D materials using Raman spectroscopy. Firstly, the typical Raman features of 2D materials are introduced on the basis of systematically analyzing the structures and Raman selection rules. Secondly, Raman spectroscopy can be used to quantify the structural parameters of 2D materials. The electronic band structure of 2D materials can be modified due to the interlayer interaction as the thickness increases, leading to the change of typical Raman features of 2D materials. Therefore, according to the position, width and intensity of Raman peaks, the layer number of 2D materials can be quickly and precisely distinguished. At the edge of 2D materials, breakdown of the Raman selection rules caused by the decrease of structural symmetry results in the anomalous Raman signals (additional phonon modes), and then the edge chirality of 2D materials can be determined. For 2D materials with in-plane anisotropic structure, the anisotropic polarized Raman intensity can be used to identify the crystalline orientation 2D materials. The change of composition in 2D alloy materials can induce the shift of Raman peaks. Combining with the MREI model analysis, the alloy composition quantification of unknown 2D alloy materials and the force parameters of atomic vibration can be both obtained using Raman spectroscopy. Thirdly, external environment, i.e. defect, doping, strain, and thermal effect, will strongly modify the electronic band structure, and then affect the Raman signals. Existence of defects can lead to the new Raman vibrational modes in 2D materials, so the defect density can be monitored according to the intensity ratio of this new Raman peak and the typical Raman features in the 2D materials. Based on the shift of Raman peaks and influence of doping, strain, and temperature on the electronic structure of 2D materials, we also investigate the doping density, strain, and the thermal conductivity of 2D materials. At last, the change of structure during the phase transition of 2D charge density wave materials can also induce the change of Raman selection rules, so the Raman spectroscopy has been applied to on the research of phase transition characteristics according to the change of Raman features.