Investigating the role of dark excitons on electron-hole liquid photoluminescence in monolayer transition-metal dichalcogenides

Recent theoretical and experimental work on monolayer transition-metal dichalcogenides show that optical excitation and strain leads to a transition from an excitonic to electron-hole liquid (EHL) phase. This phase transition is accompanied by a huge (23-fold) increase in photoluminescence (PL) but so far a mechanism has not been confirmed. Here, authors investigate how dark excitons beyond the light cone may influence the PL response of 1L-MoS2 in the excitonic vs EHL regime. They predict that in the excitonic to plasma transition, intraband collisions redefine the effective light cone of optically accessible carriers. Also, sample strain is shown to impact the spectral positions of bright and dark exciton transitions by way of altering the momentum space band positions of 1L-MoS2, increasing the ratio of bright carriers within the light cone.