Identification of individual and few layers of WS2 using Raman Spectroscopy

TLDR

Raman scattering of single- and few-layer WS₂ is examined across visible excitation wavelengths (488, 514, 647 nm) to study layer-dependent spectral changes. First‑principles calculations of electronic and phonon band structures, combined with fourth‑order Fermi‑golden‑rule modeling of 2LA mode intensity versus laser wavelength, underpin the analysis. The A1g(Γ) phonon frequency decreases monotonically with layer number, a 514.5 nm‑induced second‑order LA(M) resonance appears in monolayer WS₂ due to electronic‑lattice coupling, and these results provide a clear, nondestructive Raman fingerprint for identifying single‑ and few‑layer WS₂ films.

Abstract

The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.