2016 Theses Doctoral

Excitonic Structure in Atomically-Thin Transition Metal Dichalcogenides

Zhang, Xiaoxiao

The strong and distinctive excitonic interactions are among one of the most interesting aspects of the newly discovered family of two-dimensional semiconductors, monolayers of transition metal dichalcogenides (TMDC). In this dissertation, we explore two types different types of excitonic states in these materials beyond the isolated exciton in its radiative ground state.
In the first part of this thesis, we examine higher-order excitonic states, involving correlations between more than a single electron and hole in the usual configuration of an exciton. In particular, we demonstrate the existence of four-body correlated or biexciton states in monolayer WSe₂. The biexciton is identified as a sharply defined state in photoluminescence spectra at high exciton density. The biexciton binding energy, i.e., the energy required to separate it into to isolated excitons, is found to be 52 meV , which is more than an order of magnitude greater than that in conventional quantum-well structures. Such high binding energy arises not only from the two-dimensional carrier confinement, but also from reduced and non-local dielectric screening. These results open the way for the creation of new correlated excitonic states linking the degenerate valleys in TMDC crystals, as well as more complex many-body states such as exciton condensates or the recently reported dropletons.
In the second part of this thesis, two chapters are devoted to the identification and characterization of intrinsic lower-energy dark excitonic states in monolayer WSe₂. These optically forbidden transitions arise from the conduction band spin splitting, which was previously neglected as it only arises from higher-order spin-orbit coupling terms. First, by examining light emission using temperature-dependent photoluminescence and time-resolved photoluminescence, we indirectly probe and identify the existence of dark states that lies ~30 meV below the optically bright states. The presence of the dark state is manifest in pronounced quenching of the bright exciton emission observed at reduced temperature. To extract exact energy levels and actually utilize these dark states, as the second step, we sought direct spectroscopic identification of these states. We achieve this by applying an in-plane magnetic field, which mixes the bright and spin forbidden dark excitons. Both neutral and charged dark excitonic states have been identified in this fashion, and their energy levels are in good agreement with ab-initio calculations using GW-BSE approach. Moreover, due to the protection from their spin structure, much enhanced emission and valley lifetime were observed for these dark states. These studies directly reveal the excitonic spin manifolds in this prototypical two-dimensional semiconductor and provide a new route to control the optical and valley properties of these systems.