Abstract
Layered two-dimensional materials exhibit rich transport and optical phenomena in twisted or lattice-incommensurate heterostructures with spatial variations of interlayer hybridization arising from moiré interference effects. Here, we report experimental and theoretical studies of excitons in twisted heterobilayers and heterotrilayers of transition metal dichalcogenides. Using MoSe2-WSe2 stacks as representative realizations of twisted van der Waals bilayer and trilayer heterostructures, we observe contrasting optical signatures and interpret them in the theoretical framework of interlayer moiré excitons in different spin and valley configurations. We conclude that the photoluminescence of MoSe2-WSe2 heterobilayer is consistent with joint contributions from radiatively decaying valley-direct interlayer excitons and phonon-assisted emission from momentum-indirect reservoirs that reside in spatially distinct regions of moiré supercells, whereas the heterotrilayer emission is entirely due to momentum-dark interlayer excitons of hybrid-layer valleys. Our results highlight the profound role of interlayer hybridization for transition metal dichalcogenide heterostacks and other realizations of multi-layered semiconductor van der Waals heterostructures.
Highlights
Layered two-dimensional materials exhibit rich transport and optical phenomena in twisted or lattice-incommensurate heterostructures with spatial variations of interlayer hybridization arising from moiré interference effects
A MoSe2 crystal with monolayer and bilayer regions was stacked onto a WSe2 monolayer by dry viscoelastic stamping[38], encapsulated from both sides by hexagonal boron nitride and stamped into contact with a gold electrode with gate voltage referenced against a grounded layer of silver capped by SiO2 (Supplementary Note 1)
In the framework of moiré excitons, multi-peak PL has been ascribed to interlayer exciton states confined in moiré quantum wells[16,45]
Summary
Layered two-dimensional materials exhibit rich transport and optical phenomena in twisted or lattice-incommensurate heterostructures with spatial variations of interlayer hybridization arising from moiré interference effects. The formation of moiré superlattices has profound effects on the electronic band structure, as evidenced by the emergence of correlated transport phenomena in flat bands of twisted bilayer[7,8] and trilayer[9,10] graphene, or detected optically in twisted homobilayers[11] and aligned HBLs12,13 of transition metal dichalcogenides (TMDs) The latter exhibit rich moiré signatures in the optical spectra of intralayer[14] and interlayer[15,16,17,18] excitons formed by Coulomb attraction among layer-locked and layer-separated electrons and holes. For the explicit case of MoSe2-WSe2 HTLs, one would expect sizable hybridization effects between the MoSe2 bilayer band edge states at Q and their counterparts in monolayer WSe2, rendering the overall heterostructure an indirect band gap semiconductor
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