Dark trions and biexcitons in WS2 and WSe2 made bright by e-e scattering

Mark Danovich,Vladimir I Fal’Ko,Viktor Zólyomi

doi:10.1038/srep45998

Mark Danovich, Vladimir I Fal’Ko + Show 1 more

Open Access

https://doi.org/10.1038/srep45998

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Journal: Scientific Reports	Publication Date: Apr 6, 2017
Citations: 50	License type: open-access

Affiliation: Henry Royce Institute, University of Manchester

Abstract

The direct band gap character and large spin-orbit splitting of the valence band edges (at the K and K’ valleys) in monolayer transition metal dichalcogenides have put these two-dimensional materials under the spot-light of intense experimental and theoretical studies. In particular, for Tungsten dichalcogenides it has been found that the sign of spin splitting of conduction band edges makes ground state excitons radiatively inactive (dark) due to spin and momentum mismatch between the constituent electron and hole. One might similarly assume that the ground states of charged excitons and biexcitons in these monolayers are also dark. Here, we show that the intervalley (K ⇆ K′) electron-electron scattering mixes bright and dark states of these complexes, and estimate the radiative lifetimes in the ground states of these “semi-dark” trions and biexcitons to be ~10 ps, and analyse how these complexes appear in the temperature-dependent photoluminescence spectra of WS2 and WSe2 monolayers.

Highlights

The direct band gap character and large spin-orbit splitting of the valence band edges in monolayer transition metal dichalcogenides have put these two-dimensional materials under the spot-light of intense experimental and theoretical studies
The truly 2D nature of TMDCs1–7 enhances the effects of Coulomb interaction[8,9], resulting in charge complexes such as excitons[10,11,12,13], trions[13] and biexcitons[14] with binding energies that are orders of magnitude larger compared to conventional semiconductors such as GaAs
These complexes, which dominate the optical response of these materials, are comprised of spin/valley polarised electrons and holes residing at the corners K and K′of the hexagonal Brillouin zone (BZ), where the selection rules of optical transitions require the same spin and valley states of the involved electrons at the conduction and valence band edges

Summary

Introduction

The direct band gap character and large spin-orbit splitting of the valence band edges (at the K and K’ valleys) in monolayer transition metal dichalcogenides have put these two-dimensional materials under the spot-light of intense experimental and theoretical studies. The truly 2D nature of TMDCs1–7 enhances the effects of Coulomb interaction[8,9], resulting in charge complexes such as excitons[10,11,12,13], trions[13] and biexcitons[14] with binding energies that are orders of magnitude larger compared to conventional semiconductors such as GaAs. These complexes, which dominate the optical response of these materials, are comprised of spin/valley polarised electrons and holes residing at the corners K and K′of the hexagonal Brillouin zone (BZ), where the selection rules of optical transitions require the same spin and valley states of the involved electrons at the conduction and valence band edges.

Results

Conclusion