The role of momentum-dark excitons in the elementary optical response of bilayer WSe2

Jessica Lindlau,Victor Funk,Malte Selig,Alexander Högele,Andre Neumann,Gunnar Berghäuser,Michael Förg,Léo Colombier,Jonathan Förste,Kenji Watanabe,Ermin Malic,Feng Wang,Takashi Taniguchi,Jonghwan Kim

doi:10.1038/s41467-018-04877-3

Abstract

Monolayer transition metal dichalcogenides (TMDs) undergo substantial changes in the single-particle band structure and excitonic optical response upon the addition of just one layer. As opposed to the single-layer limit, the bandgap of bilayer (BL) TMD semiconductors is indirect which results in reduced photoluminescence with richly structured spectra that have eluded a detailed understanding to date. Here, we provide a closed interpretation of cryogenic emission from BL WSe2 as a representative material for the wider class of TMD semiconductors. By combining theoretical calculations with comprehensive spectroscopy experiments, we identify the crucial role of momentum-indirect excitons for the understanding of BL TMD emission. Our results shed light on the origin of quantum dot formation in BL crystals and will facilitate further advances directed at opto-electronic applications of layered TMD semiconductors in van der Waals heterostructures and devices.

Highlights

Monolayer transition metal dichalcogenides (TMDs) undergo substantial changes in the single-particle band structure and excitonic optical response upon the addition of just one layer
In the specific case of BL WSe2 crystals, the lowest conduction band (CB) minimum is located at Q, while the valence band (VB) maximum at K exceeds the one at Γ only by 40 ± 30 meV according to angle-resolved photoemission spectroscopy[33]
Using cryogenic spectroscopy of BL regions subjected to strain at unintentional disorder, we identify brightening of momentumindirect excitons that in some cases is accompanied by the formation of quantum dots (QDs) with intense emission of nonclassical light

Summary

Introduction

Monolayer transition metal dichalcogenides (TMDs) undergo substantial changes in the single-particle band structure and excitonic optical response upon the addition of just one layer.

Results

Conclusion