Exciton formation assisted by longitudinal optical phonons in monolayer transition metal dichalcogenides

Abstract

We examine a mechanism by which excitons are generated via the LO (longitudinal optical) phonon-assisted scattering process after optical excitation of monolayer transition metal dichalcogenides. The exciton formation time is computed as a function of the exciton center-of-mass wavevector, electron and hole temperatures, and carrier densities for known values of the Fröhlich coupling constant, LO phonon energy, lattice temperature, and the exciton binding energy in layered structures. For the monolayer MoS2, we obtain ultrafast exciton formation times on the sub-picosecond time scale at charge densities of 5 × 10 11 cm −2 and carrier temperatures less than 300 K, in good agreement with recent experimental findings (≈ 0.3 ps). While excitons are dominantly created at zero center-of-mass wavevectors at low charge carrier temperatures (≈ 30 K), the exciton formation time is most rapid at non-zero wavevectors at higher temperatures (≥ 120 K) of charge carriers. The results show the inverse square-law dependence of the exciton formation times on the carrier density, consistent with a squarelaw dependence of photoluminescence on the excitation density. Our results show that excitons are formed more rapidly in exemplary monolayer selenide-based dichalcogenides (MoSe2 and WSe2) than sulphide-based dichalcogenides (MoS2 and WS2).

Ultrafast Laser Spectroscopy of Two‐Dimensional Materials Beyond Graphene

“…It was found that the exciton formation times in MLs of MoS 2 , MoSe 2 , WS 2 , and WSe 2 are in the range of 0.3 to 0.5 ps. These experimental results are consistent with a theoretical study showing that the exciton formation time can be shorter than 1 ps, achieved by a longitudinal optical phonon‐assisted process.…”

Thermal Transport in 2D Semiconductors—Considerations for Device Applications

“…A model within the framework of Fermi's Golden rule was established to derive the formation dynamics of excitons from free carriers in TMDs. [189] Depending on the density of the electrons and holes, nonzero momentum excitons could be generated from free electron-hole pairs in layered materials via the longitudinal optical phonons coupling of charge carriers at high temperature (≈120 K). On the other hand, in monolayer TMDs, excitons can be relaxed by phonons by means of the deformation potential or in the process of piezoelectric coupling.…”

Observation of exciton-phonon coupling in MoSe2 monolayers

“…Under non-resonant optical excitation high energy excitons are generated that subsequently loose energy by phonon emission, as recently discussed for MoSe 2 MLs [29]. Interactions of carriers with phonons play a key role in the exciton formation process [30][31][32][33]. Other important signatures of the exciton-phonon interaction are single resonant and double resonant Raman scattering processes [34][35][36][37][38][39].…”