Investigating spectral enhancement of monolayer MoS${}_2$ coupled with Ag nanowires gap-mode surface plasmons

The combination with surface plasmons (SPs) nanostructures is an effective method to improve the optical properties of two-dimensional (2D) materials such as molybdenum disulfide (MoS${}_2$). Silver nanowire (Ag NW) is one of the most important SPs materials because of its tunable SPs resonance characteristics from visible light to near-infrared light. SPs can effectively confine the light field to the nanometer scale for propagation and localization, breaking the limit of optical diffraction, and the distribution of its electromagnetic field often depends on the dielectric environment. The existence of the gap-mode SPs makes the electromagnetic field between the Ag NWs coupled, so the double Ag NWs can produce additional electromagnetic field enhancement than the single Ag NW. Here, the gap-mode SPs of two different positional relationships of Ag NWs were studied through theoretical simulation, and the increase in the field enhancement factor was obtained as a function of gap distance. It is experimentally confirmed that the Raman and photoluminescence intensities of monolayer MoS${}_2$ are enhanced by the coupling of gap-mode SPs, and the effects of doping and stress are excluded. The results are helpful to better understand the gain mechanism of gap-mode SPs on the optical properties of 2D materials, and provide a reference for the design of new SPs nanostructures.