Moiré excitons in a MoSe2/MoS2 Heterostructure

The vertical stacking of atomically thin planes of layered solids expands the properties of the constituting layers giving rise to new and attractive features. This approach has been successfully employed for transition metal dichalcogenides (TMDs). Stacking different semiconducting TMDs overcomes the limitations of isolated TMD monolayers, such as the very short exciton and valley polarization lifetimes. TMD heterostructures exhibit type II band alignment, which leads to the formation of interlayer excitons with radiative and valley lifetimes up to five orders of magnitude longer than for intralayer exitons.

Due to the weak van der Waals interactions in heterostructures, the lattice constant of each monolayer does not conform to that of the underlying substrate. If monolayers with different lattice parameters or with a nonzero  stacking angle are overlaid, a moiré pattern is formed.  The physics related to the moiré pattern has been studied in hBN/graphene heterostructures, where the induced periodic potential leads to the formation of new Dirac cones, opening of a band gap, and the appearance of Hofstadter butterfly states.

The moiré pattern formed in TMD heterostructures is also expected to have a significant influence on their properties.  The moiré pattern leads to a periodically modulated potential with minima for both intra and interlayer excitons. Moreover, spatially varying selection rules for interlayer excitons can result in an emission with helicity opposite  to that of the excitation laser. In addition, the variation of the confinement potential for different atomic registries should lead to an energy splitting of the intra- and interlayer transitions.

We have shown that the observed splitting of the intralayer exciton and trion in a monolayer MoSe2 assembled in a heterostructure with MoS2  is a direct consequence of the formed moiré pattern. The high quality of our heterostructure, with typical full width at half-maximum of the exciton and trion photoluminescence, PL, peaks of around 5 meV,  clearly reveals the splitting of the trion and exciton lines in PL and reflectivity spectra. The structure of the intralayer exciton transitions is observed consistently over the whole area where the two materials overlap, while no splitting is observed outside of the heterostructure (see FIG. 2).  Our results provide a clear optical fingerprint of the effect of the moiré potential on intra-layer excitons of a monolayer TMD.

FIG. 2 PL and reflectivity spectra of MoSe2 measured in (a) monolayer and (b) heterostructure. (c) Spatial map showing the presence of MoSe2 transition splitting, overlaid with the areas of most intense PL of MoS2 and MoSe2. (d) Energy of the observed MoSe2 transitions  extracted along the horizontal dashed line of panel (c). Open circles correspond to high energy peaks observed only in the heterostructure; closed circles represent low energy transitions observed in all the MoSe2 flake. Green diamonds: MoS2 PL intensity.

For more details please see Zhang et al. Nano Letters, 18, 7651 (2018).