Enhancement of Raman Scattering and Exciton/Trion Photoluminescence of Monolayer and Few-Layer MoS2 by Ag Nanoprisms and Nanoparticles: Shape and Size Effects

The metal nanoparticle size and shape impact the plasmonic enhancement of Raman and photoluminescence (PL) spectra of monolayer and few-layer MoS2 decorated with them. The plasmonic enhancement is investigated for Ag nanotriangles (NTs or nanoprisms) of different sizes in comparison to Ag nanospheres (NSs) at room temperature. After the decoration with Ag NTs, the intensity of both Raman modes of MoS2 increases up to 6.8 times. The u-PL spectra of bare MoS2 show the presence of the A and B exciton bands as well as of a weak trion component. After covering the flakes with 50 nm Ag NTs, the highest integrated PL enhancement factors are 2.9 and 2.1 under 532 and 405 nm excitations, respectively. The revealed shape effect is that Ag NTs provide much stronger Raman and exciton emission enhancement than Ag NSs, which is due to the generation of plasmonic hot spots near the sharp edges and tips of NTs. Another mechanism of enhancement is the plasmonic coupling between the neighboring Ag NTs that causes the generation of hot spots in the gap between NTs. The revealed size effect is a decrease of Raman and PL enhancement with an increase in size of Ag NTs or NSs, which is due to an increase in radiative damping of plasmon oscillation occurring with an increase in nanoparticle size. The important feature is a strong enhancement of the A-trion component after decorating MoS2 with Ag nanoparticles. The phenomenon may be explained by the surface-plasmon-mediated generation of hot electrons in Ag nanostructures, which then inject to MoS2 flakes. This work gives new fundamental insights into the physical mechanisms of light-matter coupling in hybrid twodimensional (2D) semiconductor/plasmonic nanoparticle structures, which are highly promising for next-generation optoelectronic and nanophotonic devices.