Interlayer Valley Excitons in Two-Dimensional Semiconductor Heterostructures

Abstract

The ability to isolate and manipulate two-dimensional crystals into carefully designed van der Waals heterostructures is an emerging technology that offers an unprecedented level of control over the optical and electronic properties of the resulting synthetic materials. As the library of two-dimensional materials increases in size, adding new types of material properties and geometries, the power of this new technology grows. Many examples of new electronic behavior have already being found by exploring the possible heterostructures. In the following, I present an exploration into the optical properties of the MoSe2-WSe2 heterobilayer, which is an atomically thin type-II heterointerface between two valley semiconductors. In particular, this thesis presents the discovery and characterization of the properties of the interlayer exciton – the Coulomb bound state from electron and hole localized in different layers. This unique type of exciton is shown to demonstrate extended population and valley polarization lifetimes compared to the excitons localized in the constituent monolayers alone. Furthermore, the electric dipole moment associated with the separated charges provides a simple means to control the interlayer exciton energy, as well as the population and valley lifetimes. By placing the heterobilayer on top of a photonic crystal cavity, the intensity of light emission from the interlayer exciton can also be enhanced by an order of magnitude. Additionally, magnetic field dependent photoluminescence from trapped interlayer excitons in heterobilayers of different twist angles between the layers provides the first evidence for the influence of a moiré pattern on the optical properties of these excitons. Finally, the outlook for future studies in this new system are considered.