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Substrate, environment and lattice imperfections have strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. Moreover, luminescent centers can be created on demand and with nm lateral precision using a focused helium ion beam [1,2]. We report on a combined optical and scanning tunneling spectroscopy (STS) study of MoS₂ on SiO₂ and hBN substrate. We demonstrate that apparent band gap for MoS₂ on SiO₂ is significantly reduced compared to MoS₂ what can be explained by a substantial amount of band tail states near the conduction band edge of MoS₂. The presence of those states in STS can be linked to a broad red-shifted PL peak that are all strongly diminished or even absent using high quality hBN substrates [3]. Suppression of the L peak by hBN encapsulation enables to spectrally isolate a very narrow emission potentially serving as single photon source that can be precisely engineered by the focused helium ion beam [4].

[1] J. Klein et al. Nature Commun. 10, 2755 (2019).
[2] J. Klein et al. 2D Materials 5, 11007 (2018).
[3] K. Klein et al. APL accepted (2019).
[4] J. Klein et al unpublished (2020).