Evidence of indirect gap in monolayer WSe₂

Monolayer transition metal dichalcogenides, such as MoS₂ and WSe₂, have been known as direct gap semiconductors and emerged as new optically active materials for novel device applications. Here we reexamine their direct gap properties by investigating the strain effects on the photoluminescence of monolayer MoS₂ and WSe₂. Instead of applying stress, we investigate the strain effects by imaging the direct exciton populations in monolayer WSe₂-MoS₂ and MoSe₂-WSe₂ lateral heterojunctions with inherent strain inhomogeneity. We find that unstrained monolayer WSe₂ is actually an indirect gap material, as manifested in the observed photoluminescence intensity-energy correlation, from which the difference between the direct and indirect optical gaps can be extracted by analyzing the exciton thermal populations. Our findings combined with the estimated exciton binding energy further indicate that monolayer WSe₂ exhibits an indirect quasiparticle gap, which has to be reconsidered in further studies for its fundamental properties and device applications.Monolayer transition metal dichalcogenides have so far been thought to be direct bandgap semiconductors. Here, the authors revisit this assumption and find that unstrained monolayer WSe₂ is an indirect-gap material, as evidenced by the observed photoluminescence intensity-energy correlation.