Tuning the Fröhlich exciton-phonon scattering in monolayer MoS2

Abstract

A direct band gap, remarkable light-matter coupling as well as strong spin-orbit and Coulomb interaction establish two-dimensional (2D) crystals of transition metal dichalcogenides (TMDs) as an emerging material class for fundamental studies as well as novel technological concepts. Valley selective optical excitation allows for optoelectronic applications based on the momentum of excitons. In addition to lattice imperfections and disorder, scattering by phonons is a significant mechanism for valley depolarization and decoherence in TMDs at elevated temperatures preventing high-temperature valley polarization required for realistic applications. Thus, a detailed knowledge about strength and nature of the interaction of excitons with phonons is vital. We directly access exciton-phonon coupling in charge tunable single layer MoS2 devices by polarization resolved Raman spectroscopy. We observe a strong defect mediated coupling between the long-range oscillating electric field induced by the longitudinal optical (LO) phonon in the dipolar medium and the exciton. We find that this so-called Fröhlich exciton LO-phonon interaction is suppressed by doping. This suppression correlates with a distinct increase of the degree of valley polarization of up to 20 % even at elevated temperatures of 220 K. Our result demonstrates a promising strategy to increase the degree of valley polarization towards room temperature valleytronic applications.

arXiv comment18 pages, 3 figures, SI

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• Engineering photonic environments for two-dimensional materials
• Exciton-phonon interaction calls for a revision of the "exciton" concept
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• Breakdown of Raman Selection Rules By Fröhlich Interaction in Few-Layer WS${}_2$
• Raman Polarization Switching in CrSBr
• Impact of indirect transitions on valley polarization in WS${}_2$ and WSe${}_2$
• Coupling of MoS${}_2$ Excitons with Lattice Phonons and Cavity Vibrational Phonons in Hybrid Nanobeam Cavities
• Raman spectroscopy of monolayer to bulk PtSe2 exfoliated crystals
• Phonon-induced exciton weak localization in two-dimensional semiconductors
• Electrical control of orbital and vibrational interlayer coupling in bi- and trilayer 2H-MoS${}_2$
• Multiphoton-pumped UV-Vis transient absorption spectroscopy of 2D materials: basic concepts and recent applications
• Ultrafast transient absorption spectroscopy of 2D semiconductors: a review
• Laser-patterned submicron Bi2Se3-WS2 pixels with tunable circular polarization at room temperature
• Exciton Mott Transition in Two-Dimensional Semiconductors
• Polarized and narrow excitonic emission from graphene-capped monolayer WS${}_2$ through resonant phonon relaxation
• Effect of spin-dependent tunneling and intervalley scattering in magnetic-semiconductor van der Waals heterostructures on exciton and trion polarization
• Polarization and driving force analysis of coherent optical shear phonons in graphite
• Photo-physics and electronic structure of lateral graphene/MoS2 and metal/MoS2 junctions
• Renormalization of excitonic properties by polar phonons