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 ${\mathrm{E}}_{2\mathrm{g}}^1$ red-shifted while the out-plane vibration mode ${\mathrm{A}}_{1\mathrm{g}}$ 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.

在这项研究中，由于层间相互作用，拉曼光谱和光致发光 (PL) 光谱测量揭示了单层 (1 L) 和双层 (2 L) WS ₂之间的不同光学特性。首先，超低频 (ULF) 拉曼光谱表明，与 1 L WS ₂相比，2 L WS ₂显示出额外的剪切模式 (S) 和层呼吸 (LB) 模式。2 L WS ₂具有面内振动模式${\mathrm{乙}}_{2\mathrm{G}}^1$ 面外振动模式发生红移 ${\mathrm{一种}}_{1\mathrm{G}}$蓝移。然后，在 2 L WS ₂的 PL 光谱中观察到直接跃迁 (DT) 和间接跃迁 (IT) 发射，而只有 1 L WS _{2 的}DT 。1 L WS ₂在7 K的低温下，与局部激子相对应的PL信号，即与DT发射的低能量侧尾测量，更为突出。DT发射和IT发射显示出与温度相反的变化趋势从 7 K 增加到 300 K，而 2 L WS ₂的平均声子能量估计几乎是 1 L WS _{2 的}两倍。这些观察结果说明层间相互作用不仅改变了 WS ₂的声子特性，但也会影响能带结构和激子动力学，包括激子辐射复合和激子-声子相互作用。