A comprehensive comparison study on the vibrational and optical properties of CVD-grown and mechanically exfoliated few-layered WS₂

Tungsten disulfide (WS₂), a typical transition metal dichalcogenide (TMDC) material, transits from an indirect to direct bandgap when the thickness is thinned to a monolayer, thereby allowing for applications in transistors, photodetectors, and electroluminescent devices. The vibrational and optical properties of WS₂ strongly depend on the fabrication methods, such as mechanical exfoliation (ME) and chemical vapor deposition (CVD). Here we provide a comparative study on the vibrational and optical properties of CVD-grown and ME-prepared few-layered WS₂ by using several optical spectroscopic techniques. We observe dramatic differences in their Raman response, differential reflectance, photoluminescence (PL) and second-harmonic generation (SHG), which are all possibly related to their structural defects. Surprisingly, we find that the PL from a CVD-grown WS₂ bilayer is almost unpolarized and exhibits no significant difference in the two circularly polarized excitations, while these polarization dependences are much stronger for an ME-prepared bilayer. More interestingly, we observe that the CVD-grown bilayer has an unprecedentedly stronger SHG signal than both ME-prepared and CVD-grown monolayers. Our results not only point out the importance of improving current CVD-based growth methods to achieve large-area TMDC materials with optical, electrical and structural properties comparable to small-area few-layered flakes prepared by ME methods, but also demonstrate the promise of using CVD-grown bilayer TMDCs in general for nonlinear optoelectronic applications.