Spectrally narrow exciton luminescence from monolayer MoS2 and MoSe2 exfoliated onto epitaxially grown hexagonal BN.

The strong light-matter interaction in transition metal dichalcogenide (TMD) monolayers (MLs) is governed by robust excitons. Important progress has been made to control the dielectric environment surrounding the MLs, especially through hexagonal boron nitride (hBN) encapsulation which drastically reduces the inhomogeneous contribution to the exciton linewidth. Most studies use exfoliated hBN from high quality flakes grown under high pressure. In this work, we show that hBN grown by molecular beam epitaxy (MBE) over a large surface area substrate has a similarly positive impact on the optical emission from TMD MLs. We deposit MoS2 and MoSe2 MLs on ultrathin hBN films (few MLs thick) grown on Ni/MgO(111) by MBE. Then, we cover them with exfoliated hBN to finally obtain an encapsulated sample: exfoliated hBN/TMD ML/MBE hBN. We observe improved optical quality of our samples compared to TMD MLs exfoliated directly on SiO2 substrates. Our results suggest that hBN grown by MBE could be used as a flat and charge-free substrate for fabricating TMD-based heterostructures on a larger scale. [ABSTRACT FROM AUTHOR]