Photophysics of interlayer excitons in TMDC heterobilayers

Abstract

Since the first isolation of graphene (a single sheet of graphite) in 2004 and the remarkable discoveries achieved in the following years, there has been an ongoing growth in studies and interest in two-dimensional (2D) materials and their heterostructures. With the constant discovery of new 2D materials over time, their range of material properties has expanded significantly as well, opening up a greater variety of applications and corresponding theoretical and experimental research inquiries for their implementation. The majority of them are classified as layered van der Waals (vdW) materials, and the development of different transfer techniques has made it possible to fabricate vdW heterostructures, which introduced exciting possibilities for the development of quantum technologies. The unique characteristics of the constituent layers, in conjunction with the high carrier mobility characteristic of 2D materials, provide a wealth of opportunities for the development of devices with remarkable properties for various applications. Interest in semiconducting transition metal dichalcogenides (TMDCs) among other 2D materials has grown significantly owing to their exceptional optical, mechanical, and electrical properties, particularly as they exhibit direct bandgaps in atomic layer thicknesses (i.e., monolayers). The vdW heterostructures of monolayer TMDCs have been growing in popularity among researchers over the last decade since they typically exhibit staggered type-II band alignment, which promotes ultra-fast charge transfer between the constituent layers, in turn, leading to the formation of strongly Coulomb-bound electron-hole pairs (i.e., interlayer excitons, IXs) located in different layers. The emission characteristics of the IXs can be regulated by varying the twist angle between the constituent layers of the heterostructures, and a superlattice of single-photon quantum emitters of IXs through their localization to periodic quantum dot-like Moiré potentials can be achieved. The main focus of this dissertation is investigating and understanding the photophysics of IXs of hBN-encapsulated WSe2-MoSe2 heterobilayers, which will allow one to grasp the importance and capabilities they hold for the development of quantum applications. The heterobilayer samples used in this study were fabricated by first isolating the 2D layers using the micromechanical exfoliation method and then vertically stacking them via the dry transfer technique. Low-temperature photoluminescence (PL) spectroscopy methods have been performed on the IX species of the fabricated heterobilayers, including magneto-PL, excitation pump power-dependent PL, temperature-dependent PL, and time-resolved PL (TRPL). Finally, first-order correlation g(1)(τ) measurements in the time domain were performed using a home-built free-space Michelson interferometer. Our results on the effect of Moiré-localization of IXs demonstrate that the well-protection of the localized emitters can lead to prolonged dephasing times (T2 ~ 730 fs). Remarkably, we have also successfully shown the presence of coherent coupling for the first time between the two spin states (spin-singlet and spin-triplet) of IXs of hBN-encapsulated WSe2-MoSe2 heterobilayers by utilizing the quantum beat interferometry in the time domain with resulting dephasing times up to T2 ~ 400 fs. Our results on the dephasing characteristics of IXs can provide important insights into the future of exciton-based device development in quantum photonic and valleytronic applications.