The underlying signatures of the spin- and momentum-forbidden dark exciton states in the temperature-dependent photoluminescences from WSe2 monolayers

Abstract

With the extraordinary spin-, valley- and excitonic properties, atomically thin transition-metal dichalcogenide monolayers (TMD-MLs) have recently drawn extensive attention. [1], [2] Remarkably, TMD-MLs possess direct visible-light band gaps opened at distinctive K- and K' valleys, acting as a new degree of freedom and accessed selectively by polarized light. With the additional degree of freedom of valley, the energy spectra of the excitons in photo-excited TMD-MLs exhibit rich complex fine structures, composed of the bright exciton states, and spin-forbidden and momentum-forbidden dark states as well. The latter are optically invisible but known as of importance in the optical dynamics and optoelectronic properties of materials. Because of the optical invisibility, the two types of dark exciton are not easily measured and distinguished in conventional optical spectroscopies.[3] In this work, we will present a computational investigation of the exciton fine structures of WSe 2 monolayers by solving the Bethe-Salpeter equation (BSE) based on the density-functional-theory(DFT)-based tight binding theory. The DFT-based tight binding model is established via the transformation of maximally-localized Wannier function. The full low-lying fine structured exiton dispersions, including the bright exciton and the intra- and inter-band dark exciton states, are computed by solving the BSE with the consideration of the both electron-hole direct and exchange interactions. As the main result, we reveal and provide a model analysis to identify the distinctive signatures of the spin- and momentum-forbidden dark excions in the temperature-dependent photo-luminescence from WSe 2 monolayers, respectively.[4]