Magnetic brightening and control of dark excitons in monolayer WSe2

Abstract

Monolayer transition metal dichalcogenide (TMDC) crystals, as direct-gap materials with unusually strong light-matter interaction, have attracted much recent attention. In contrast to the initial understanding, the minima of the conduction band are predicted to be spin split. Because of this splitting and the spin-polarized character of the valence bands, the lowest-lying excitonic states in WX 2 (X=S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark band-edge excitons, which strongly influence the light emission properties of the material. Here we show how an in-plane magnetic field can brighten the dark excitonic states and allow their properties to be revealed experimentally in monolayer WSe 2 . In particular, precise energy levels for both the neutral and charged dark excitons were obtained and compared with ab-initio calculations using the GW-BSE approach. Greatly increased emission and valley lifetimes were observed for the brightened dark states as a result of their spin configuration. These studies directly probe the excitonic spin manifold and provide a new route to tune the optical and valley properties of these prototypical twodimensional semiconductors.The electronic and optical properties of ultrathin TMDC crystals in the MX 2 (M = Mo, W, X = S, Se) family have attracted much recent attention. These 2D semiconductors exhibit a direct bandgap at monolayer thickness 1, 2 , have strong and anomalous excitonic interactions [3][4][5] , and offer the potential for highly efficient light emission. The materials also provide an ideal platform for access to the valley degree of freedom, since the optical selection rules provide a

Dark excitations in monolayer transition metal dichalcogenides

“…On SiO 2 we find a redshift of about Overall, the comparison of our calculations with the available results and particularly the experimental results [20] is good. Most importantly, we reproduce the experimentally observed ordering of D − , D, A − , and A excitations.…”

“…Due to the resulting mixture of dark and bright states, they become visible in photoluminescence. Recent theoretical studies have addressed dark excitons [20,22] using first-principles calculations, however, theory for dark and bright trions has so far exclusively been based on models [23][24][25]. We note that Feierabend et al [26] proposed further dark excitons with nonzero momenta.…”

“…However, basic optical properties such as photoluminescence intensities, exciton decay rates, valley scattering, etc., will depend strongly on the interplay between the bright and dark states in the low-energy part of the spectra [17][18][19]. In recent experiments by Zhang et al [20] and Molas et al [21], it has been shown that dark excitations can be experimentally probed by applying an in-plane magnetic field. Due to the resulting mixture of dark and bright states, they become visible in photoluminescence.…”

“…In contrast to this, TMDCs including W will show an inverted trend, i.e., the dark states should be more likely than bright states. However, the weight of dark states in photoluminescence will also depend on the mixing to bright states via the applied in-plane magnetic field [20,21].…”

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

“…Here, charge tunable samples [66] or experiments in transverse magnetic fields [67,68] are expected to give a better indication on the dark-bright order in future experiments on these narrow linewidth samples. The temperature dependence of the PL peak position follows a standard hyperbolic cotangent relation [69]:…”