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

Abstract

Screening due to 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 quasi-particle bandgap 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 (PL) 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 bandgap renormalization. The estimated modulation of binding energy for the 1s exciton is found to be . % ( . % for 2s, . % for 3s, . Table I. * Model predictedIn Table I, we have also shown the measured A 1 − trion binding energy: Δ 1 − = 1 0 − 1 − , which also exhibits 15.1% reduction in sample M2. Note that, Δ 1 − being estimated from the separation between two PL peaks, does not involve the quasi-particle bandgap, and provides an independent direct evidence of the modulation of binding energy in presence of larger dielectric screening.In conclusion, we explored new perspective of environment screening on two-dimensional monolayers by exploiting the idea of increasing mismatch between quasiparticle bandgap renormalization and modification in exciton binding energy for increasing quantum number of the exciton state. The proposed technique allows us to unambiguously estimate all the necessary information about the excitonic series and quasi-particle bandgap change in twodimensional monolayer embedded in different dielectric media. Our results clearly demonstrate the prominent effect of substrate and environment induced screening in two-dimensional system, making this effect crucial to be taken into account while analyzing results in existing devices based on 2D materials. For example, the band structure of the 2D material in the region underneath the contact material or in the presence of a gate dielectric is expected to be modified locally as a result of this effect, and is expected to play an important role in determining the device performance. Similar revisit will also be required in analyzing the performance of 2D material based photodetectors as this screening induced unintentionally created built-in field at the source junction will support efficient electron-hole separation. Finally, planar heterojunctions in two-dimensional crystals are generally difficult to achieve due to stringent growth conditions. The results described in this work open up the possibility of a new class of twodimensional planar heterostructure devices by only spatially modifying the substrate or top dielectric constant.ACKNOWLEDGMENT

Fundamental exciton linewidth broadening in monolayer transition metal dichalcogenides

“…Monolayers of transition metal dichalcogenides (TMDs, for example, MoS 2 , MoSe 2 , WS 2 and WSe 2 ) host a unique class of strongly bound two-dimensional excitons [1][2][3] with a large binding energy of about 500 meV [1,[4][5][6][7]. This has attracted significant interest from the researchers, and a wide variety of excitonic complexes and their manipulations have been reported in the recent past [8][9][10].…”

Temporally Resolving Synchronous Degenerate and Nondegenerate Two‐Photon Absorption in 2D Semiconducting Monolayers

“…We notice in a very recent work that giant binding energy modulation of exciton complexes has been realized by varying dielectric medium around monolayer MoSe 2 . About 60% reduction of A exciton binding energy in monolayer MoSe 2 was observed . Noticing that our monolayer MoSe 2 sample was fabricated on a 0.48 mm thick BK7 glass substrate instead of a SiO 2 /Si substrate employed by Wang et al., a different 1 s ‐2 p separation can be reasonably expected.…”

“…Different screening effects of sample dielectric environment, reshaping the electron‐hole interaction potentials, might be a possible reason. We notice in a very recent work that giant binding energy modulation of exciton complexes has been realized by varying dielectric medium around monolayer MoSe 2 . About 60% reduction of A exciton binding energy in monolayer MoSe 2 was observed .…”

Origin of selective enhancement of sharp defect emission lines in monolayer WSe2 on rough metal substrate

“…Apart from a quenching of the free exciton peak, we repeatedly observe a small blue-shift of the free exciton peak on the Au film. This shift arises from an interplay between the dielectric screening and the bandgap renormalization effect on the Au substrate [34][35][36]. We also consistently observe a large number of enhanced and sharp peaks, mostly in the range of 1.6 to 1.7 eV, when 1L-WSe 2 is on Au.…”