Enhanced Neutral Exciton Diffusion in Monolayer WS 2 by Exciton–Exciton Annihilation

2  ABSTRACT:  Dominant  recombination  pathways  in  monolayer  transition  metal dichalcogenides  (TMDCs)  depend  primarily  on  background  carrier  concentration, generation rate and applied strain. Charged excitons formed in the presence of background carriers  mainly  recombine  nonradiatively.  Neutral  excitons  recombine  completely radiatively at low generation rates, but experience nonradiative exciton-exciton annihilation (EEA)  at  high  generation  rates.  Strain  can  suppress  EEA  resulting  in  near-unity photoluminescence  quantum  yield  (PL  QY)  at  all  exciton  densities.  Although  exciton diffusion is the primary channel of energy transport in excitonic  materials and a critical optoelectronic design consideration, the combined effects of these factors on exciton diffusion are not clearly understood. In this work, we decouple the diffusion of neutral and charged excitons with chemical counterdoping and explore the effect of strain and generation rate on exciton diffusion.  According to  the standard semiconductor paradigm a  shorter  carrier recombination lifetime should lead to a smaller diffusion length. Surprisingly, we find that increasing generation rate shortens the exciton lifetime but increases the diffusion length in unstrained monolayers  of TMDCs. When  we suppress EEA by strain, both  lifetime and diffusion  length  becomes  independent  of  generation  rate.  During  EEA  one  exciton nonradiatively recombines and kinetically energizes another exciton,  which then diffuses fast.  Our  results  probe  concentration  dependent  diffusion  of  pure  neutral  exciton  by counterdoping  and  elucidate  how  strain  controls  exciton  transport  and  many-body interactions in TMDC monolayers.