Ultralow-Frequency Raman Spectroscopy of Two-dimensional Materials

Abstract

In two-dimensional materials (2DMs), atoms within one layer (in-plane) are joined by covalent bonds, whereas van der Waals (vdW) interactions keep the layers together. Raman spectroscopy is a powerful tool for measuring the lattice vibrational modes in 2DMs, including the intralayer and interlayer vibrations, and has shown great potential for the characterizations of the layer number, interlayer coupling and layer-stacking configurations in 2DMs via the ultralow-frequency (ULF) interlayer vibrational modes. This chapter begins with an introduction of how the monolayer 2DMs stack to assemble a large family of two-dimensional systems (Section 10.1), which are likely to exhibit modified interlayer coupling and thus various ULF mode behaviours. In sequence, Section 10.2 provides a detailed description of the physical origins of the interlayer vibrations and the linear chain model (LCM) to depict their layer-number dependent frequencies. Subsequently, two popular Raman setups are introduced to perform the ULF modes measurements (Section 10.3). Then, we provide a review of the ULF Raman spectroscopy of various types of 2DMs, including: (1) layer-number dependent (Section 10.4.1) and (2) stacking-order dependent (Section 10.4.2) ULF Raman spectroscopy in isotropic 2DMs; (3) ULF Raman spectroscopy in anisotropic 2DMs(Section 10.4.3); and (4) ULF Raman modes in twisted 2DMs (Section 10.4.4) and heterostructures (Section 10.4.5).