Broken mirror symmetry in excitonic response of reconstructed domains in twisted MoSe2/MoSe2 bilayers

Abstract

Structural engineering of van der Waals heterostructures via stacking and twisting has recently been used to create moiré superlattices 1,2 , enabling the realization of new optical and electronic properties in solid-state systems. In particular, moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) have been shown to lead to exciton trapping 3-8 , host Mott insulating and superconducting states 9 , and act as unique Hubbard systems 10-13 whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures also feature atomic reconstruction and domain formation 14-20. Unfortunately, due to the nanoscale sizes (~10 nm) of typical moiré domains, the effects of atomic reconstruction on the electronic and excitonic properties of these heterostructures could not be investigated systematically and have often been ignored. Here, we use near-0 o twist angle MoSe2/MoSe2 bilayers with large rhombohedral AB/BA domains 21 to directly probe excitonic properties of individual domains with far-field optics. We show that this system features broken mirror/inversion symmetry, with the AB and BA domains supporting interlayer excitons with out-of-plane (z) electric dipole moments in opposite directions. The dipole orientation of ground-state -K interlayer excitons (XI,1) can be flipped with electric fields, while higher-energy K-K interlayer excitons (XI,2) undergo fieldasymmetric hybridization with intralayer K-K excitons (X0). Our study reveals the profound impacts of crystal symmetry on TMD excitons and points to new avenues for realizing topologically nontrivial systems 22,23 , exotic metasurfaces 24,25 , collective excitonic phases 26-28 , and quantum emitter arrays 29,30 via domain-pattern engineering. To date, most studies of twisted TMD bilayers have assumed a rigid rotation of layers without atomic-scale rearrangement 3-8. However, recent theoretical 14-16 and experimental 17-20 studies have

Twist-angle dependence of moiré excitons in WS2/MoSe2 heterobilayers

“…4b). In comparison, values between 17 meV and 100 meV have been reported based on first principle calculations [31][32][33][34] or measurements on homo-bilayers [35][36][37] . The relatively small values of 10-16 meV we measured may possibly be due to difference in the materials or a smaller interlayer exciton Bohr radius as a result of strong interlayer tunneling.…”

Guide to optical spectroscopy of layered semiconductors

“…In the indirect case, a phonon is needed in addition to a photon in the emission process 64,65 . The twist angle can also lead to the formation of moiré effects or to a local rearrangement of the stacking order (reconstructions), depending on the lattice mismatch between the stacked monolayers 14,[66][67][68] . Note that confinement in nanometre-sized moiré potentials results in a spreading in k space of the carrier wavefunction.…”

“…Absorption of interlayer excitons is also reported in homobilayers and homotrilayers of MoS 2 (reFS 116,117 ). In these systems, the transition energy of the absorption can be tuned through the application of an electric field perpendicular to the layers (Stark shift) over 120 meV, and the interaction between interlayer and intralayer excitons can be investigated 68,106,118 .…”

“…3a) offers new directions to explore and control exciton arrays in twisted TMD heterobilayers from potentials that trap individual excitons to the formation of minibands. This allows physics related to correlated states 4 , for potential applications in quantum optoelectronic devices 15,68 . Some key characteristics of interlayer excitons include a long lifetime (nanoseconds), a wide transition-energy tunability that ranges over several hundreds of millielectronvolts in applied electric fields (Fig.…”

Intercalated phases of transition metal dichalcogenides

“…3 Controlling the twist angle between bilayers or changing between odd-and even-layer thickness can change the structural symmetry and band structure, yielding interesting spin, valley, and excitonic physics. [5][6][7] The ability of TMDs to serve as an intercalation host originates from its van der Waals (vdW) gap. The interlayer distance, defined as the chalcogen (X)-to-chalcogen (X) distance, is approximately 3-3.5 Å in TMD.…”