Hot-electron cooling by acoustic and optical phonons in monolayers ofMoS2and other transition-metal dichalcogenides

Abstract

We study hot-electron cooling by acoustic and optical phonons in monolayer MoS2. The cooling power P (Pe = P/n) is investigated as a function of electron temperature Te (0-500 K) and carrier density n (10 10 -10 13 cm −2 ) taking into account all relevant electron-phonon (el-ph) couplings. We find that the cross over from acoustic phonon dominated cooling at low Te to optical phonon dominated cooling at higher Te takes place at Te ∼ 50 − 75 K. The unscreened deformation potential (DP) coupling to the TA phonon is shown to dominate P due to acoustic phonon scattering over the entire temperature and density range considered. The cooling power due to screened DP coupling to the LA phonon and screened piezoelectric (PE) coupling to the TA and LA phonons is orders of magnitude lower. In the Bloch-Grüneisen (BG) regime, P ∼ T 4 e (T 6 e ) and P ∼ n −1/2 (Pe ∼ n −3/2 ) are predicted for unscreened (screened) el-ph interaction. The cooling power due to optical phonons is dominated by zero-order DP couplings and the Fröhlich interaction, and is found to be significantly reduced by the hot-phonon effect when the phonon relaxation time due to phonon-phonon scattering is large compared to the relaxation time due to el-ph scattering. The Te and n dependence of the hot-phonon distribution function is also studied. Our results for monolayer MoS2 are compared with those in conventional two-dimensional electron gases (2DEGs) as well as monolayer and bilayer graphene.

Polarization analysis of excitons in monolayer and bilayer transition-metal dichalcogenides

“…The amplitude of this short-range interaction is governed by the deformation potential of the thickness fluctuations. 58,59 On the other hand, the exciton changes its polarization state due to the short-range interaction with E 2 modes since their atomic displacement complies with the transformation properties of off-diagonal Pauli matrices (σ x and σ y ).…”

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

“…The phonon Boltzmann equation [60] takes into account a common temperature that is achieved as a result of equilibrium reached between electrons and phonons. Hot-phonon effects is incorporated by replacing the temperature T l in Eq.11 by an effective lattice temperature T ph [60]. The charge carriers are assumed to be in quasi-thermal equilibrium during the exciton formation process.…”

Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2

“…10,11 Recently, signatures of a strong exciton-phonon interaction have been observed, 12,13 such as the preservation of valley coherence in double-resonant Raman scattering, 14 trion to exciton luminescence upconversion in monolayer WSe 2 assisted by A ′ 1 phonons, 15 and exciton enhanced anti-Stokes shifts in few layer MoTe 2 . 16 Despite a few theoretical proposals on the role of optical phonons in exciton dynamics, [17][18][19] and several experimental studies on phonon-limited exciton relaxation, [20][21][22] the details behind how and which phonons impact metrics such as the formation and relaxation of excitons remains largely unexplored. This knowledge is important for interpreting a wide range of 2D exciton phenomena and for exploring the potential of exciton-based 2D optoelectronics.…”