Exciton manifolds in highly ambipolar doped WS2

TL;DR

This study uses electrolyte gating and spectroscopic techniques to explore how high doping levels affect exciton properties in WS2 monolayers, revealing the nature of emission bands and screening effects in 2D semiconductors.

Contribution

It introduces a novel electrolyte gated WS2 device enabling detailed optical investigation of exciton and band gap evolution at high doping levels.

Findings

Recombination of spin- and momentum-forbidden excitons explains main emission bands.

Doping causes redshift and weakening of interband transitions.

The platform allows combined optical and transport studies at cryogenic temperatures.

Abstract

The disentanglement of single and many particle properties in 2D semiconductors and their dependencies on high carrier concentration is challenging to experimentally study by pure optical means. We establish an electrolyte gated WS2 monolayer field-effect structure capable to shift the Fermi level from the valence into the conduction band suitable to optically trace exciton binding as well as the single particle band gap energies in the weakly doped regime. Combined spectroscopic imaging ellipsometry and photoluminescence spectroscopies spanning large n- and p-type doping with charge carrier densities up to 10^14 cm-2 enable to study screening phenomena and doping dependent evolution of the rich exciton manifold whose origin is controversially discussed in literature. We show that the two most prominent emission bands in photoluminescence experiments are due to the recombination of spin-forbidden and momentum-forbidden charge neutral excitons activated by phonons. The observed interband transitions are redshifted and drastically weakened under electron or hole doping. This field-effect platform is not only suitable for studying exciton manifold but is also suitable for combined optical and transport measurements on degenerately doped atomically thin quantum materials at cryogenic temperatures.