Straightforward identification of monolayer WS2 structures by Raman spectroscopy

Tungsten disulfide (WS2) is a very promising material with great potential for optoelectronics applications. To grow WS2 monolayer samples, chemical vapor deposition (CVD) is very reliable when it comes to controlling the sample quality, but there is still a lot of debate on whether the method is controllable beyond research-only systems. Although monolayer crystals are prepared through CVD, the challenge is to efficiently characterize and differentiate monolayer crystals from bilayers and few-layers. In this work, we report a new straightforward Raman peak identification to discriminate monolayer WS2 crystals from layered and bulk samples. Our method is based on the A1g peak behavior. We show that through the comparison and analysis of other less convenient techniques that the strong splitting in A1g mode in close-to-resonance excitation energy condition is a much more convenient and fast identification method than either low-wavenumber Raman scattering characterization or atomic force microscopy.