Engineering photoluminescence using strain in two-dimensional transition-metal dichalcogenides

Atomically thin two-dimensional transition metal dichalcogenides have fascinated researchers due to their unique electronic and optical properties. The control of exciton-trion dynamics in two-dimensional semiconductors is critical for their application in optoelectronic devices. One way to engineer the exciton-trion dynamics is by applying strain in the monolayers of these two-dimensional materials using nanostructured substrates. Here we demonstrate a versatile route to engineering the exciton-trion dynamics in monolayer WSe₂ by applying biaxial strain. A polytetrafluoroethylene (PTFE) nanocone array decorated by thin gold film and fabricated via colloidal lithography is used to create the strain in the superposed monolayer. To distinguish the effect of strain and plasmonics, we compare our results on the nanocone surface with the one for monolayer WSe2 on a plane gold film.

Atomically thin two-dimensional (2D) semiconductors have received a lot of research attention owing to their high exciton binding energy and strong light-matter interaction. Still, the integration of these 2D semiconductors with nanostructures is required for practical photonics applications owing to their atomically thin nature. Here we present a versatile route to exciton-trion dynamics by producing biaxial strain using gold-decorated nanocone structures fabricated using the colloidal lithography technique. We use polystyrene beads as colloidal particles, and a self-assembled monolayer of these particles was deposited via convective self-assembly over a PTFE substrate. The self-assembled monolayer of these colloidal particles works as a mask, which is subsequently etched using reactive ion etching, which causes the formation of the PTFE nanocone array. A thin layer of gold is deposited on top of these nanocone structures, which creates the dual effect of strain and plasmonic enhancement. Finally, we integrate our CVD-grown monolayer WSe₂ with these nanocone structures using a wet transfer method. We demonstrate the strain engineering of exciton-trion dynamics using photoluminescence and Raman spectroscopy. We further compare exciton-trion interconversion due to the funneling effect in strained WSe₂ with a control experiment in unstrained WSe₂.