Thickness-dependent optical characteristics of WS₂ flakes prepared by Au- and Ag-assisted exfoliation

WS2 flakes with a number of layers (N (WS2)) ranging from 1 to 10 are exfoliated on ultraflat template-stripped Au and Ag layers. The apparent colors of WS2/Au and WS2/Ag strongly depend on the underlying metal layers as well as N (WS2). The measured and calculated optical reflectance spectra are in good agreement, confirming the identification of N (WS2) for each flake. The absorption in the WS2 flake for WS2/Au (A (WS2)-Au) and WS2/Ag (A (WS2)-Ag) is calculated: the maximum value of A (WS2)-Ag for N (WS2) = 10 (similar to 0.93) is much larger than that of A (WS2)-Au (similar to 0.5). As expected, the local maxima of A (WS2)-Au and A (WS2)-Ag for each N (WS2) are found near the exciton resonance wavelengths of WS2. The largest peak of A (WS2)-Ag is located close to the C exciton resonance wavelength, and the peak position shows a redshift with increasing N (WS2). Despite the extremely small flake thickness, the optical resonant modes can appear in WS2/Au and WS2/Ag, according to the anticipated phase shifts of light. These resonant modes can explain how N (WS2) and the metal layer affect the optical characteristics of the WS2/metal structures.