K-Λ crossover transition in the conduction band of monolayer MoS under hydrostatic pressure.

Monolayer MoS is a promising material for optoelectronics applications owing to its direct bandgap, enhanced Coulomb interaction, strong spin-orbit coupling, unique valley pseudospin degree of freedom, etc. It can also be implemented for novel spintronics and valleytronics devices at atomic scale. The band structure of monolayer MoS is well known to have a direct gap at K (K') point, whereas the second lowest conduction band minimum is located at Λ point, which may interact with the valence band maximum at K point, to make an indirect optical bandgap transition. We experimentally demonstrate the direct-to-indirect bandgap transition by measuring the photoluminescence spectra of monolayer MoS under hydrostatic pressure at room temperature. With increasing pressure, the direct transition shifts at a rate of 49.4 meV/GPa, whereas the indirect transition shifts at a rate of -15.3 meV/GPa. We experimentally extract the critical transition point at the pressure of 1.9 GPa, in agreement with first-principles calculations. Combining our experimental observation with first-principles calculations, we confirm that this transition is caused by the K-Λ crossover in the conduction band.