Emergent Phenomena and Applications in Artificially Stacked 2D Materials

Abstract

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) possess broken inversion symmetry and strong spin-orbit coupling, leading to unique spin-valley locking effect. In TMD multilayers, rich excitonic responses are identified from the direct-to-indirect bandgap transition, where the coupling among spin, valley and layer pseudospin plays a crucial role in forming bright and dark exciton species and their complex hybridizations. Furthermore, structural engineering can be leveraged in building Rhombohedral (R) and Hexagonal (H) stacking orders and forming moiré superlattice that further modulate the band structure and give rise to exotic physics. In this thesis, we first demonstrate the photoluminescence and reflectance spectra under varying doping densities and electric fields while increasing TMD layer thickness, to explain the bandgap transition. We further show that the spin-valley locking in H-stacked multilayer TMD yields an electronic superlattice structure, where alternating layers correspond to barriers and quantum wells, respectively, depending on the spin-valley indices and that the spin-valley locked superlattice hosts a kind of dipolar excitons with the electron and hole constituents separated in an every-other-layer configuration. Such excitons become optically bright via hybridization with intralayer excitons. This effect is also manifested by the presence of multiple anti-crossing patterns in the reflectance spectra, as the dipolar exciton is tuned through the intralayer resonance by an electric field. As layer thickness keeps increasing, the dipolar exciton can form one-dimensional Bose-Hubbard chain displaying a layer number dependent fine spectroscopy structures. In the next chapter, we identify the interfacial ferroelectricity in R-stacked twisted TMD. We perform scanning probe imaging to directly visualize the alternating domain polarizations. Optical spectroscopy of ABBA-twisted double bilayer TMD under varying out-of-plane electric fields reveals rich excitonic responses, among which the inter-bilayer excitons are coupled with local domain polarizations and result in built-in electric fields. Weak hysteresis loop of the inter-bilayer excitons’ emissions is observed while sweeping the external electric field at opposite directions, and confirms the domain wall dynamics dictated by the interfacial ferroelectricity. Finally, in the last chapter, we report the observation of exciton hybridizations coupled with interfacial ferroelectricity in R-stacked twisted bilayer WSe2 systems, where dipolar excitons are allowed due to the matched spin-valley index and can hybridize with certain intralayer A exciton branches through an electron hopping process, which also makes them optically bright. Combining the built-in electric fields, we reconstruct the hybridization behaviors that are coupled with the interfacial ferroelectricity from the R-stacked moiré interface. Furthermore, ferromagnetism and correlated states are identified in the same system through magneto-optic effect. Our results demonstrate the delicate coupling between the excitonic responses and the artificially stacked 2D materials and reveal exciting and exotic physical phenomena.