Manifold Coupling Mechanisms of Transition Metal Dichalcogenides to Plasmonic Gold Nanoparticle Arrays

We reveal the manifold interaction mechanisms between monolayers of MoS2 and single layers of plasmon-active gold nanoparticles. The MoS2 shows a 10- to 20-fold enhanced photoluminescence when covered with the gold nanoparticles. Surprisingly, we detect this enhancement also for excitation energies that are not resonant to the surface plasmon polaritons of the nanoparticles. Complementary Kelvin probe force measurements indicate a lowering of the work function when the MoS2 is decorated with the gold nanoparticles. This is in agreement with a reduced band gap for the decorated MoS2 as determined from absorbance measurements. We furthermore demonstrate a dielectric coupling between the two layers by spectroscopic imaging ellipsometry as well as Raman spectroscopy. Combining the various results, we discuss the enhanced photoluminescence in terms of a modified emission pattern of the radiative dipole in the MoS2 monolayers in the presence of the gold nanoparticles. In particular, the studied systems elucidate the underlying physical mechanisms of the enhanced photoluminescence for decorated MoS2 that stems predominantly from incoherent contributions including the far-field emission pattern, the dielectric coupling, and the electronic interaction mechanisms.