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

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 MoSe2, 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 Al2O3 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.