Controlling exciton dimensionality in novel transition-metal dichalcogenide platforms

Abstract

In this thesis, I control excitons in two-dimensional materials and explore their confinement across different dimensionality. I focus on transition-metal dichalcogenides (TMD), few-atom thick, layered materials, which are direct-bandgap semiconductors in the monolayer limit. These materials host excitons, bound electron-hole pairs, with large binding energies due to their two-dimensional nature. Broken inversion symmetry, along with strong spin-orbit interaction, leads to coupling of the spin and valley degrees of freedom, allowing for valley-selective optical transitions. First, I confine long-lived interlayer excitons in WSe2/WS2 heterobilayers to zero dimensions. Interlayer excitons have their electron and hole localised in separate TMD layers, providing extended optical lifetimes and a permanent electric dipole, opening the prospect of investigating long-ranged interactions in a many-body system. I find lifetimes approaching 0.2 ms for the unconfined excitons, longer by an order of magnitude than previous reports. I use an array of nanopillars to deterministically strain confine these interlayer excitons to zero dimensions and show them to have sufficiently long lifetime, up to ∼4 μs, to enable the investigation of quantum spin models. Next, I demonstrate one-dimensional electrical confinement of excitons in monolayer MoSe2. An in-plane electric field, combined with the creation of a lateral p-i-n junction, defines the confinement potential along an electrode edge. This leads to the emergence of quantised excitonic sub-bands, with magneto-optical measurements revealing the exchange-split linearly polarised fine structure of the exciton states. This system offers the possibility of electrically tuneable quantum confinement of excitons in an arbitrary geometry. Finally, I study excitonic complexes within two-dimensional Janus TMD monolayers. Here, an artificially altered atomic ordering breaks the out-of-plane mirror symmetry and creates an intrinsic electric field, promising a Rashba splitting and dipolar excitons within a single monolayer. I create an electrically gated device, allowing for excitonic charge-state control and identification of the charged exciton complexes in Janus WSeS for the first time. This provides the basis for future optoelectronic applications with Janus materials. The controlled excitonic confinement across zero-, one- and two-dimensions that I have demonstrated lays the groundwork for studying many-body phenomena in both the classical and quantum regime, and pursuing applications in quantum information technologies.