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 (∼0.93) is much larger than that of A _WS2-Au (∼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.