Figure 2

(a) and (c): Raman spectra of WS₂ (a) and WSe₂ (c) ultrathin layers; (b) and (d): The frequency difference (red) and the peak intensity ratio (blue) between E¹_2g and A_1g modes as a function of film thickness in WS₂ (b) and WSe₂ (d) respectively.