Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

Abstract

The strong light-matter interaction and the valley selective optical selection rules make monolayer (ML) MoS 2 an exciting 2D material for fundamental physics and optoelectronics applications. But, so far, optical transition linewidths even at low temperature are typically as large as a few tens of meV and contain homogeneous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the bettercharacterized ML materials MoSe 2 and WSe 2 . In this work, we show that encapsulation of ML MoS 2 in hexagonal boron nitride can efficiently suppress the inhomogeneous contribution to the exciton linewidth, as we measure in photoluminescence and reflectivity a FWHM down to 2 meV at T ¼ 4 K. Narrow optical transition linewidths are also observed in encapsulated WS 2 , WSe 2 , and MoSe 2 MLs. This indicates that surface protection and substrate flatness are key ingredients for obtaining stable, high-quality samples. Among the new possibilities offered by the well-defined optical transitions, we measure the homogeneous broadening induced by the interaction with phonons in temperature-dependent experiments. We uncover new information on spin and valley physics and present the rotation of valley coherence in applied magnetic fields perpendicular to the ML.

Excitation energy dependence of Raman spectra of few-layer WS2

“…In the current case, there is no obvious candidate for the continuum excitation. Although the broad exciton lines may suggest that the exciton states themselves might serve as a 'continuum', recent reports that the homogeneous linewidth of the excitons in TMDs is very narrow [58][59][60], indicating that the commonly observed broad exciton linewidth is mainly due to inhomogeneous broadening that would not qualify as the 'continuum' for Fano resonance.…”

Engineering Valley Polarization of Monolayer WS2: A Physical Doping Approach

“…However, the reported values of P c in monolayer TMDs vary from 2% to nearly 100% among different TMD semiconductors, which are mainly attributed to intervalley scattering process from the selectively pumped valley to the opposite one . As for the PL measurements, various approaches have been adopted to obtain a sizable P c like applying magnetic field, cooling down to liquid helium cryogenic temperature, using near resonant light excitation or building van der Waals heterostructure . More practical and desirable methods could be electrical and optical control of valley polarization in TMDs at noncryogenic temperature and off‐resonance conditions.…”

Realization of an Electrically Tunable Narrow-Bandwidth Atomically Thin Mirror Using Monolayer MoSe2

“…In the absence of free carriers, the lowest energy elementary optical excitations in TMDs are excitons with an ultralarge binding energy of ∼0.5 eV [6]. Encapsulation of TMD monolayers with hexagonal boron nitride (h-BN) [7,8] dramatically improves the optical quality, yielding an exciton linewidth ∼2 meV [9,10]. These narrow linewidths have a dominant contribution from radiative decay of ∼1.5 meV for MoSe 2 [11,12].…”