Simultaneous generation of direct- and indirect-gap photoluminescence in multilayer MoS2 bubbles

Transition metal dichalcogenide (TMD) materials have received enormous attention due to their extraordinary optical and electrical properties, with MoS₂ being one of the most representative examples. As the thickness increases from monolayer to multilayer, the photoluminescence (PL) of MoS₂ is gradually quenched due to the direct-to-indirect band gap transition. How to enhance PL response and decrease the layer dependence in multilayer MoS₂ remains a challenge. In this work, we report simultaneous generation of three PL peaks at around 1.3, 1.4, and 1.8 eV on multilayer MoS₂ bubbles. The temperature dependent PL measurements indicate that the two peaks at 1.3 and 1.4 eV come from phonon-assisted indirect-gap transitions while the peak at 1.8 eV comes from the direct-gap transition. The weakening of interlayer coupling on multilayer MoS₂ bubbles, which may account for the emergence of PL peaks, is confirmed by the low-frequency Raman spectroscopy. Using first-principles calculations, the band structure evolution of multilayer MoS₂ under strain is studied, from which the origin of the three PL peaks of MoS₂ bubbles is further confirmed. Moreover, PL standing waves are observed in MoS₂ bubbles that create Newton-Ring-like patterns. This work demonstrates that the bubble structure may provide new opportunities for engineering the electronic structure and optical properties of layered materials.