Spectroscopic visualization of exciton distribution in monolayer transition metal dichalcogenides and van der Waals heterostructures.

Gesuele F., Maddalena P.

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II - Fisica della materia
GSSI Ex ISEF - Aula C - Venerdì 27 h 09:00 - 12:00
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Monolayers transition metal dichalcogenides (TMD) are direct-gap semiconductor exhibiting strong room-temperature photoluminescence (PL) emission and formation of stable exciton states. Nowadays mechanical transfer techniques allow layers of different TMDs to be stacked in a tailored sequence to form van der Waals (vdW) heterostructures. Large area monolayers of TMD can be nowadays grown by means of chemical vapor deposition (CVD), however, spatial nonuniformities in the intensity and emission wavelength are commonly observed. Here we employ high-resolution confocal and near-field optical microscopy coupled to PL and Raman spectroscopy on CVD-grown $WS_{2}$ and $MoS_{2}$ monolayers of different sizes and shape. Our results show an increase of the integrated intensity and a red shift of peak wavelength at the edge of the individual grains and at the grain boundaries between individual grains. We attributed this behavior to the local excess of charge carriers at the edge of the individual grains leading to the formation of multiexciton states (charged trions and biexcitons). We discuss the effect of these spatial inhomogeneities on the formation of charge transfer excitons in the stacked vdW $WS_{2}$ - $MoS_{2}$ heterostructure. Our results pave the way to a better understanding of the formation of exciton species in monolayer TMD and vdW heterostructures in view of their application in nanophotonic devices.

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