Tuning and Identification of Interband Transitions in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure

Few-layer molybdenum disulfide (MoS2) is advantageous for application in next-generation electronic and optoelectronic devices. For monolayer MoS2, it has been established that both the conduction band minimum (CBM) and the valence band maximum (VBM) occur at the K point in the Brillouin zone. For bilayer MoS2, it is known that the VBM occurs at the Gamma point. However, whether the K valley or the Lambda valley forms the CBM and the energy difference between them remain disputable. Theoretical calculations have not provided a conclusive answer. In this paper, we demonstrate that a direct K-K to an indirect Lambda-K interband transition in bilayer MoS2 can be optically detected by tuning the hydrostatic pressure. A changeover of the CBM from the K valley to the Lambda valley is observed to occur under a pressure of approximately 1.5 GPa. The experimental results clearly indicate that the K valley forms the CBM under zero strain, while the Lambda valley is approximately 89 +/- 9 meV higher in energy.