Different optical characteristics between monolayer and bilayer WS2 due to interlayer interaction

Abstract

In this research Raman spectroscopy and photoluminescence (PL) spectroscopy measurements reveal different optical characteristics between monolayer (1 L) and bilayer (2 L) WS2 due to the interlayer interaction. First, ultralow frequency (ULF) Raman spectra indicate that 2 L WS2 shows additional shear mode (S) and layer-breathing (LB) mode in comparison with 1 L WS2. The 2 L WS2 has the in-plane vibration mode E2g1 red-shifted while the out-plane vibration mode A1g blue-shifted. Then, both direct transition (DT) and indirect transition (IT) emission are observed in the PL spectra for 2 L WS2 while only DT for 1 L WS2. The PL signal corresponding to localized excitons, that is measured as a low energy side tail with the DT emission, is more prominent for 1 L WS2 at low temperature of 7 K. The DT emission and IT emission show opposite variation trend as the temperature increases from 7 K to 300 K, while the average phonon energy in 2 L WS2 is estimated to be almost twice of that for 1 L WS2. These observations illustrate that the interlayer interaction not only change the phonon characteristics for WS2, but also impact the energy band structure and the exciton dynamics, including exciton radiative recombination and exciton-phonon interaction.