Moiré excitons in MoSe2-WSe2 heterobilayers and heterotrilayers

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.

Interlayer Excitonic Spectra of Vertically Stacked MoSe2/WSe2 Heterobilayers

“…Due to the aforementioned complexities of the electronic band structure, the exact origin or these peaks is still a matter of debate. Depending on the experimental evidence and insight from theoretical calculations, the PL signals were attributed to 1) a mix of momentum direct () and momentum‐indirect () interlayer excitons, evidenced by the different temperature behaviors of the PL peak lifetimes; [ 36,43 ] 2) to a pair of momentum‐indirect excitons, with an energy separation due to the spin–orbit splitting of the conduction band valley at the Q point, evidenced by the observed opposite circular polarization; [ 47 ] 3) a singlet–triplet pair of neutral, momentum‐direct, excitons at the K and K ′ points, evidenced by recent measurements of the interlayer exciton PL under application of external magnetic fields; [ 48–51 ] and 4) a pair of spatially indirect neutral exciton and spatially indirect negatively charged trion, evidenced by an observed constant relative intensity of the two PL signals at low temperature, [ 40 ] but somewhat contradicted by the circular polarization response reported by other authors. For structurally similar / heterobilayers, experimental reports suggest the existence of three interlayer exciton peaks, which were attributed to , , and transitions based on a combination of optical measurements and theoretical simulations using the excitonic Bethe–Salpeter Equation (BSE).…”

“…[ 54 ] Based on magnetoluminescence experiments, a recent study suggested that the interlayer PL peaks in twisted / heterobi‐ and trilayer systems in R registry arise from moiré potential modulated momentum‐direct and phonon‐assisted momentum‐indirect interlayer exciton emissions. [ 51 ]…”

Stacking-dependent exciton multiplicity in WSe2 bilayers

“…To complement our theory analysis of excitons in 2H and 3R WSe 2 BLs, we determined the exciton g-factors from magneto-luminescence measurements and correlated them with theoretically calculated values in different spin, valley and layer configurations [28,65]…”

Interlayer Excitonic Spectra of Vertically Stacked MoSe2/WSe2 Heterobilayers

“…Q) interlayer excitons, evidenced by the different temperature behaviors of the PL peak lifetimes; [36,43] 2) to a pair of momentum-indirect K ! Q excitons, with an energy separation due to the spin-orbit splitting of the conduction band valley at the Q point, evidenced by the observed opposite circular polarization; [47] 3) a singlet-triplet pair of neutral, momentum-direct, excitons at the K and K 0 points, evidenced by recent measurements of the interlayer exciton PL under application of external magnetic fields; [48][49][50][51] and 4) a pair of spatially indirect neutral exciton and spatially indirect negatively charged trion, evidenced by an observed constant relative intensity of the two PL signals at low temperature, [40] but somewhat contradicted by the circular polarization response reported by other authors. For structurally similar MoS 2 /WS 2 heterobilayers, experimental reports suggest the existence of three interlayer exciton peaks, which were attributed to K !…”

“…[54] Based on magnetoluminescence experiments, a recent study suggested that the interlayer PL peaks in twisted MoSe 2 /WSe 2 heterobi-and trilayer systems in R registry arise from moiré potential modulated momentum-direct and phonon-assisted momentum-indirect interlayer exciton emissions. [51] On the theoretical side, accurate ab initio investigations of the excitonic spectra of van-der-Waals heterostructures are limited to rather small and symmetric systems due to the delicate interplay of quasiparticle band structure, effects from interlayer interaction and spin-orbit coupling and (computationally expensive) Coulomb interaction between electrons and holes. Despite these constraints, a number of studies using various methods have been reported recently, typically focusing on the properties of momentum-direct interlayer excitons [52,[54][55][56][57][58][59] and trions.…”