Nanogap Plasmonics for Tailored Properties of 2D Materials

Tailoring light–matter interactions in monolayer MoS2 is critical for its use in optoelectronic and nanophotonic devices. Optical cavities with multiple tunable resonances have the potential to provide unique electromagnetic environments at two or more distinct wavelengths—critical for control of optical processes such as nonlinear generation, entangled photon generation, or photoluminescence (PL) enhancement. Here, we show a plasmonic nanocavity based on a nanopatch antenna design that has two tunable resonant modes in the visible spectrum. The importance of utilizing two resonances simultaneously is demonstrated by integrating monolayer MoS2, a two-dimensional semiconductor, into the colloidally synthesized nanocavities. We observe a 2000-fold enhancement in the PL intensity of MoS2—which has intrinsically low absorption and small quantum yield—at room temperature, enabled by the combination of tailored absorption enhancement at the first harmonic and PL quantum-yield enhancement at the fundamental resonance. Next, we explore how the emission spectrum can be tailored, including complex excitonic states. We demonstrate that the peak emission wavelengths of the A and B excitons can be tuned up to 40 and 25 nm, respectively, by integrating monolayer MoS2 into a plasmonic nanocavity with tunable plasmon resonances. Contrary to the intrinsic photoluminescence spectrum of monolayer MoS2, we are also able to create a dominant B exciton peak when the nanocavity is resonant with its emission. Additionally, we observe a 1200-fold enhancement of the A exciton emission and a 6100-fold enhancement of the B exciton emission when normalized to the area under a single nanocavity and compared to a control sample on thermal oxide.