Direct-Indirect Bandgap Transition in Monolayer MoS₂ under Hydrostatic Pressure

Monolayer MoS₂ is a promising material for optoelectronics applications thanks to its wide direct bandgap, strong spin-orbit coupling, large Coulomb interaction, and unique valley pseudospin degree of freedom, etc. It has a great potential for applications in spintronics, valleytronics and optoelectronics. The band structure of monolayer MoS₂ is well-known to have a direct gap at K (K’) point while the second lowest conduction band minimum is located at L point which may interact with the valence band maximum at K point to make an indirect optical bandgap transition. Here we experimentally demonstrate the direct-to-indirect bandgap transition by measuring the hydrostatic pressure dependent photoluminescence spectra at room temperature for monolayer MoS₂. With increasing pressure, the direct transition shifts at a rate of 49.4 meV/GPa while 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 prove that this transition is caused by the K-Λ crossover in the conduction band.