Layer breathing Raman mode in two-dimensional van der Waals material $\mathrm{Cr_2Ge_2Te_6}$

TL;DR

The study addresses how layer number influences lattice dynamics and interlayer coupling in the 2D ferromagnetic semiconductor Cr2Ge2Te6 (CGT). It uses Raman spectroscopy to detect the layer-breathing mode (LBM) and applies a finite linear chain model (LCM) to fit the layer-number dependence of the LBM frequency, extracting the interlayer force constant $K_c$. The B-mode appears around $v_B\approx158.73\ \mathrm{cm^{-1}}$ for a 6-layer sample, and its intensity grows as thickness decreases, indicating strong sensitivity to interlayer dynamics. The extracted $K_c=(1.33\pm0.09)\times10^{19}\ \mathrm{kg/m^{3}}$ shows vdW-driven, nearest-neighbor coupling comparable to that in MoS$_2$, WS$_2$, NbSe$_2$, establishing Raman spectroscopy as a robust probe of thickness-dependent interlayer dynamics in 2D magnetic semiconductors.

Abstract

Two-dimensional (2D) van der Waals (vdW) magnetic materials have emerged as key materials for next-generation magneto-electric and spintronic devices, where understanding the relationship between layer number, lattice dynamics, and magnetic interactions is very important. In this work, we report the observation of the layer breathing mode (LBM) in few-layer $\mathrm{Cr_2Ge_2Te_6}$, a ferromagnetic semiconductor with thickness dependent electronic, magnetic and optical properties, using Raman spectroscopy, which serves as a direct fingerprint of interlayer coupling and lattice symmetry. Group-theoretical symmetry analysis confirms that the CGT falls under the non-polar category of layered material. The evolution of the LBM-frequency with increasing layer number (N) reveals a distinct softening trend, characteristic of weakening restoring forces in thicker flakes. By fitting the experimental Raman data using the Linear Chain Model (LCM), we quantitatively extract the interlayer force constant ($\mathrm{K_c}$), providing a measure of the vdW coupling strength between layers.

Figures (5)

• Figure 1: a) The schematic of the crystal structure of CGT, b) XRD pattern of the bulk CGT crystal with inset showing an optical image of the crystal of size 3×2 mm, c) EDAX spectra for elemental confirmation. d) Raman spectra of bulk CGT crystal having out of plane Eg and in plane Ag vibrational modes.
• Figure 2: Symmetry operations in layered CGT: a) schematic side view illustrating different layer arrangement within a single unit cell of CGT. b) top view of CGT crystal structure, showing the in-plane atomic arrangement. c) side view of monolayer showing with stacking sequence of Te-Ge-Cr-Ge-Te respectively. The black line indicates the threefold ($\mathrm{C_3}$) rotational axis, whereas the horizontal ($\mathrm{\sigma_h}$) and the vertical ($\mathrm{\sigma_v}$) reflection operations are shown as orange and grey planes, respectively. d) side view of two stacked layers of CGT, with an inversion center located within the horizontal ($\mathrm{\sigma_h}$) reflection plane.
• Figure 3: a) Atomic force microscopy (AFM) image of a six layer CGT flake on $Si/SiO_2$ substrate, the step height profile of the flake is shown as an inset. The corresponding optical image is also shown. b) Raman spectra of CGT flakes with varying thickness at room temperature, showing all the six phonon vibrational modes, with a distinct evolution in their intensities as a function of layer number.
• Figure 4: a) The scattered points are experimental data, while the dashed curves are Lorentzian fittings to the data for all the samples (S1-S7). b) change in the intensity as a function of number of layers for the layer breathing mode B (dashed line is guide to the eye), c) frequency evolution of breathing modes as a function of number of layers (dashed line is guide to the eye).
• Figure 5: a) Schematic of linear chain model where each layer is considered as a ball connected by a spring with interlayer coupling constant $\mathrm{K_c}$, b) Side view of the six layers of CGT, c) The experimental data points are fitted with LCM as indicated by the red line.