Direct observation of giant binding energy modulation of exciton complexes in monolayer MoSe2

Abstract

Screening due to the surrounding dielectric medium reshapes the electron-hole interaction potential and plays a pivotal role in deciding the binding energies of strongly bound exciton complexes in quantum confined monolayers of transition metal dichalcogenides (TMDs). However, owing to strong quasiparticle band-gap renormalization in such systems, a direct quantification of estimated shifts in binding energy in different dielectric media remains elusive using optical studies. In this work, by changing the dielectric environment, we show a conspicuous photoluminescence peak shift at low temperature for higher energy excitons $(2s,3s,4s,5s)$ in monolayer $\mathrm{MoS}{\mathrm{e}}_{2}$, while the $1s$ exciton peak position remains unaltered -- a direct evidence of varying compensation between screening induced exciton binding energy modulation and quasiparticle band-gap renormalization. The estimated modulation of binding energy for the $1s$ exciton is found to be $58.6%$ $(72.8%$ for $2s,75.85%$ for $3s$, and $85.6%$ for $4s)$ by coating an $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ layer on top, while the corresponding reduction in quasiparticle band-gap is estimated to be 246 meV. Such direct evidence of large tunability of the binding energy of exciton complexes as well as the band-gap in monolayer TMDs holds promise of novel device applications.