Tailoring the Luminescence of Atomically Thin Semiconductors at the Sub-nanometer Scale

Moreover, due to the absence of dangling bonds in their surfaces and their relatively easy isolation, TMDs can be stacked onto other 2D materials [2]. These so called Van der Waals (vdW) heterostructures offer a new template that is appealing for opto-electronic applications. However, the optical properties of vdW heterostructures are governed by near field coupling mechanisms that arise at the heterointerface. These mechanisms sensitively depend on the sub-nm distance between the layers and therefore require a technique with sufficient resolution to properly characterize them. Here, I will present our results aimed at this goal. To investigate the TMDs, I used an approach that combines photoluminescence and reflectance spectroscopies together with scanning tunnelling microscopy (STM). To tailor the luminescence at the atomic scale, we exploit the ultimate in-plane resolution provided by an STM working at cryogenic temperatures and under ultra-high-vacuum conditions. The out-of-plane control is achieved using the van-der-Waals gap that separates the TMD from a graphene layer. I will demonstrate that when stacked on graphene, the TMD monolayer is virtually neutral, resulting in photoluminescence spectra composed of a single and narrow emission line at cryogenic temperatures [3]. In contrast to bare monolayers that display complex excitonic manifolds with emission spectra composed of a large number of features, arising from neutral excitons but also charged excitonic compounds, spin-dark excitons, localized defect-induced emission and possibly exciton-phonon replicas [4-5]. Finally, I will demonstrate for the first time the STM induced intrinsic luminescence of TMD-based heterostructures at low temperatures with atomic precision, which has remained elusive ever since the discovery of the direct bandgap transition of 1L-TMDs. Our results support monolayer TMDs as an interesting system to investigate light-matter interaction within an STM-junction and pave the way for novel architectures aimed at building low dimensional devices.

References
[1] K. F. Mak, J. Shan. Nature Nanotech. 10, 216 (2016).
[2] Geim, A., Grigorieva, I. Nature 499, 419–425 (2013).
[3] Lorchat, E., López, L.E.P. et al. Nat. Nanotechnol. 15, 283–288 (2020).
[4] D. Vaclavkova et al. Nanotechnology 29 (2018) 325705.
[5] E. Courtade et al. Phys. Rev. B 96, 085302 (2017).

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