with thin film plasmonic cavity*

Interactions of tightly bound electron-hole pairs (excitons) and the lattice vibration modes (photon) in two-dimension (2D) transition metal dichalcogenide (TMD) crystal is a subject of considerable current interest. Here, we report investigations of exciton resonance Raman spectroscopy assisted by plasmonic thin film cavity from single monolayer to tens of monolayers in WS2. Without the plasmonic cavity and under A exciton resonant excitation condition, the Raman response indeed shows strong enhancement relative to the no-resonance condition, as reported by others previously [1]. With plasmonic cavity, the Raman signal is enhanced further as anticipated [2]. However, we find the enhancement factor show dramatic dependence on the TMD layer thickness with an optimal thickness in the range of few nanometer. More specifically, we find that at this range of thickness, the Raman enhancement is more than 2 orders of magnitude higher than that for the monolayer case. Possible mechanisms will be discussed.

[1] Xin Zhang,et al. Chem. Soc. Rev., 44, 2757 (2015)
[2] Yexin Zhang, et al. Nano Lett. 19, 6284−6291 (2019)