Enhanced Photoluminescence of Monolayer WS2 on Ag Films and Nanowire–WS2–Film Composites

Monolayer transition metal dichalcogenides (TMDCs), due to their structural similarity to graphene, emerge as a promising alternative material of integrated optoelectronic devices. Recently, intense research efforts have been devoted to the combination of atomically thin TMDCs with metallic nanostructures to enhance the light–matter interaction in TMDCs. One crucial parameter for semiconductor–metallic nanostructure hybrids is the spacer thickness between the gain media and the plasmonic resonator, which needs to be optimized to balance radiation enhancement and radiation quenching. In current investigations of TMDCs–plamonic coupling, one often adopts a spacer thickness of ∼5 nm or larger, a typical value for transitional gain media–plasmonic composites. However, it is unclear whether this typical spacer thickness represents the optimal value for TMDCs–plasmonic hybrids. Here we address this critical issue by studying the spacer thickness dependence of the luminescent efficiency in the monolayer tungsten-disulfide (WS2)–Ag film hybrids. Surprisingly, we discovered that the optimal thickness occurs at ∼1 nm spacer, much smaller than the typical value used previously. In a WS2–Ag film system, at this optimal spacer thickness, the photoluminescence (PL) is increased by more than an order of magnitude due to exciton-coupled surface plasmon polaritons (SPPs), as compared to the as-grown WS2 on sapphire. We further explore a new composite system comprising Ag nanowires on top of a WS2–Ag film and observe additional enhancement of the PL (by a factor of 3) contributed by SPPs that are reflected from the end of the wires. Interestingly, in such a composite system, the additional improvement of the PL signal is observed only when the underlying Ag film is an epitaxial film instead of a commonly available thermal film. This is attributed to the reduction of propagation loss of the SPPs on atomically smooth, epitaxial films.