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

Abstract

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.