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.