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.