Surface enhanced Raman scattering of monolayer MX₂ with metallic nano particles

Monolayer transition metal dichalcogenides MX₂ (M = Mo, W; X = S) exhibit remarkable electronic and optical properties, making them candidates for application within flexible nano-optoelectronics. The ability to achieve a high optical signal, while quantitatively monitoring strain in real-time is the key requirement for applications in flexible sensing and photonics devices. Surface-enhanced Raman scattering (SERS) allows us to achieve both simultaneously. However, the SERS depends crucially on the size and shape of the metallic nanoparticles (NPs), which have a large impact on its detection sensitivity. Here, we investigated the SERS of monolayer MX₂, with particular attention paid to the effect of the distribution of the metallic NPs. We show that the SERS depends crucially on the distribution of the metallic NPs and also the phonon mode of the MX₂. Moreover, strong coupling between MX₂ and metallic NPs, through surface plasmon excitation, results in splitting of the E'(Γ)and A₁'(Γ) modes and an additional peak becomes apparent. For a WS₂-Ag system the intensity of the additional peak increases exponentially with local strain, which opens another interesting window to quantitatively measure the local strain using SERS. Our experimental study may be useful for the application of monolayer MX₂ in flexible nano-optoelectronics.