Exciton-plasmon coupling and giant photoluminescence enhancement in monolayer MoS 2 through hierarchically designed TiO 2 /Au/MoS 2 ternary core-shell heterostructure.

Enhancing the light coupling efficiency of large-area monolayer molybdenum disulfide (1L-MoS ₂ ) is one of the major challenges for its successful applications in optoelectronics and photonics. Herein, we demonstrate a dramatically enhanced photoluminescence (PL) emission from direct chemical vapor deposited monolayer MoS ₂ on a fluorine-doped TiO ₂ /Au nanoparticle plasmonic substrate, where the PL intensity is enhanced by nearly three orders of magnitude, highest among the reported values. The formation of TiO ₂ /Au/1L-MoS ₂ ternary core-shell heterojunction is evidenced by the high-resolution transmission electron microscopy and Raman analyses. Localized surface plasmon resonance induced enhanced absorption and improved light coupling in the system was revealed from the UV-vis absorption and Raman spectroscopy analyzes. Our studies reveal that the observed giant PL enhancement in 1L-MoS ₂ results from two major aspects: firstly, the heavy p-doping of the MoS ₂ lattice is caused by the transfer of the excess electrons from the MoS ₂ to TiO ₂ at the interface, which enhances the neutral exciton emissions and restrains the trion formation. Secondly, the localized surface plasmon in Au NPs underneath the 1L-MoS ₂ film initiates exciton-plasmon coupling between excitons of the 1L-MoS ₂ and surface plasmons of the Au NPs at the MoS ₂ /Au interface. The PL and Raman analyses further confirm the p-doping effect. We isolate the contributions of plasmon enhancement from the theoretical calculation of the field enhancement factor using the effective medium approximation of plasmonic heterostructure, which is in excellent agreement with the experimental data. This work paves a way for the rational design of the plasmonic heterostructure for the effective improvement in the light emission efficiency of 1L-MoS ₂ , and may enable engineering the different contributions to enhance the optoelectronic performance of 2D heterostructures.
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