Radiative lifetimes of excitons and trions in monolayers of the metal dichalcogenideMoS2

Abstract

We present results on the radiative lifetimes of excitons and trions in a monolayer of metal dichalcogenide MoS2. The small exciton radius and the large exciton optical oscillator strength result in radiative lifetimes in the 0.18-0.30 ps range for excitons that have small in-plane momenta and couple to radiation. Average lifetimes of thermally distributed excitons depend linearly on the exciton temperature and can be in the few picoseconds range at small temperatures and more than a nanosecond near room temperature. Localized excitons exhibit lifetimes in the same range and the lifetime increases as the localization length decreases. The radiative lifetimes of trions are in the hundreds of picosecond range and increase with the increase in the trion momentum. Average lifetimes of thermally distributed trions increase with the trion temperature as the trions acquire thermal energy and larger momenta. We expect our theoretical results to be applicable to most other 2D transition metal dichalcogenides.

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

“…This demonstrates that the low temperature exciton dynamics in treated and non-treated MLs is not dominated by non-radiative recombination on the defects which are suppressed by the acid treatment. This exciton dynamics can be interpeted by a fast radiative decay at low temperatures as a result of the strong exciton oscillator strength in TMDC MLs, which is consistent with recent measurements performed on MoSe 2 and WSe 2 MLs [20], and theoretical predictions [22,23].…”

Optical tuning of exciton and trion emissions in monolayer phosphorene

“…But in our experiment, this energy difference decreases significantly from ,150 meV to ,100 meV when the power of excitation laser increases from 0.19 to 1.15 mW. Furthermore, localized excitons are expected to have a longer lifetime than free excitons because of the localization effect of squeezing an exciton in the zero-dimension-like state 28,30 . However, our TRPL measurements revealed that the carrier lifetime from X state is less than that from A state (discussed later).…”

Fast exciton annihilation by capture of electrons or holes by defects via Auger scattering in monolayer metal dichalcogenides

“…The result is |φ α ( r)| 2 where φ α ( r) is the Fourier transform of φ α ( k). Note that φ α ( r) is not the Fourier transform of ψ α, Q ( k), which also includes extra phase factors (see (10).…”

“…The exciton binding energies in 2D chalcogenides are almost an order of magnitude larger compared to other bulk semiconductors [1][2][3][4][5] . The strong Coulomb interactions and small exciton radii in 2D-TMDs result in large optical oscillator strengths 3,7,8 and short radiative lifetimes 10 . In this paper we show that the same factors also result in very fast capture of electrons and holes of excitons by defects from Auger processes leading to fast non-radiative recombination rates.…”

“…Therefore, most of the electrons and holes injected electrically or optically in TMDs recombine non-radiatively. Given that the average radiative lifetimes of excitons in TMDs are in the range of hundreds of picoseconds to a few nanoseconds 10 , the nonradiative recombination or capture times in TMDs are expected to be of the order of a few picoseconds. Several experimental results on the ultrafast carrier dynamics in photoexcited monolayer MoS 2 do indeed point to non-radiative recombinaton and/or capture times in the few picoseconds range [13][14][15]23 .…”