Phase Transformation on Multilayer 2M-WS 2 for Improved Surface-enhanced Raman Scattering.

In recent years, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been widely recognized as an ideal platform for surface-enhanced Raman scattering (SERS). Given their rich structural phases, phase transformation in 2D TMDCs is an efficient strategy to tailor their SERS performance. In this paper, we present the great SERS performance of multilayer 2M-WS 2 and then investigate the effect of its phase transformation on SERS performance. It is observed that multilayer 2M-WS 2 nanosheets undergo a thermally induced single-crystal phase transition from 2M-WS 2 to 2H-WS 2 upon thermal annealing or laser treatment. Distinguishing from the commercially available pure 2H-WS 2 (P-2H-WS 2 ), 2H-WS 2 obtained by annealing and laser treatment still retain SERS properties comparable to those of 2M-WS 2 , among which the detection limits for CV molecules (10 -8 M) are 3 orders of magnitude lower than that of P-2H-WS 2 and the Raman intensity enhancements are ∼10-37 times higher. In contrast to the charge transfer (CT) mechanism governed by the Fermi level in metallic-phase 2M-WS 2 , 2H-WS 2 obtained by phase transition exhibits accelerated CT facilitated by the bandgap reduction and reorganization resulting from the abundance of vacancies. This study introduces an interesting perspective and potential avenue for enhancing SERS through metal-to-semiconductor phase transitions in 2D TMDCs materials.