Abstract
The strong light matter interaction and the valley selective optical selection rules make monolayer (ML) MoS2 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 homogenous and inhomogeneous contributions. This prevented in-depth studies, in contrast to the better-characterized ML materials MoSe2 and WSe2. In this work we show that encapsulation of ML MoS2 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 = 4K. 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.
Highlights
We show that encapsulation of ML MoS2 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 1⁄4 4 K
The first member of the transition metal dichalcogenides (TMDC) to be established as a direct gap semiconductor in monolayer (ML) form was MoS2 [1,2], which has resulted in a global research effort exploring this promising 2D semiconductor family [3,4,5,6,7,8,9,10,11,12,13,14,15,16]
We show that narrow linewidths can be achieved in other TMDC MLs via hexagonal boron nitride (hBN) encapsulation (3) The narrow linewidth allows us to distinguish spectrally the emission from the nonidentical valleys Kþ and K− in MoS2 as they are split by the valley Zeeman effect [43,44,45,46,47,48]
Summary
The first member of the transition metal dichalcogenides (TMDC) to be established as a direct gap semiconductor in monolayer (ML) form was MoS2 [1,2], which has resulted in a global research effort exploring this promising 2D semiconductor family [3,4,5,6,7,8,9,10,11,12,13,14,15,16]. (2) Remarkably, the emission linewidth and intensity in our hBN/ML MoS2/hBN structures are unchanged following several cool-down cycles to T 1⁄4 4 K and exposure to laser radiation This is in contrast to the variable optical response of uncapped MoS2 [40] and WS2 [41,42] MLs exfoliated onto Si=SiO2 substrates, which often depends on cool-down procedure and sample history. Reproducibility and stability are essential to better understand the unusual optical properties of these materials [6,24] and to prepare possible device applications We observe these narrow transition lines in different ML MoS2 samples exfoliated from different bulk material (i) grown by chemical vapor transport (CVT) (see Sec. V) and (ii) commercially available crystals. We show the generation and rotation of robust valley coherence, a coherent superposition of valley states using the chiral optical selection rules [26], an important step towards full optical control of valley states in these 2D materials [49,50,51]
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