ESE PhD Thesis Defense: “Engineering copper-vacancy color centers in zinc sulfide for quantum defect discovery”

Photoluminescent point defects, or color centers, in wide-bandgap semiconductors are important platforms for quantum information science because they can be operated as solid-state quantum spin-light interfaces. Implementing so-called defect qubits in an expanded variety of materials systems is beneficial for applications, since the host-defect material properties determine operating parameters such as emission wavelength, spin coherence time, and pathways for device integration. A key challenge is obtaining materials that contain defects of interest, and at sufficiently low concentrations to allow observation of quantum emission. This thesis concerns the creation of copper-vacancy complexes for quantum defect studies in zinc sulfide, a material in which there is no known defect qubit. Zinc sulfide, as the host material, possesses a wide bandgap and a low concentration of nuclear spins, enabling the operation of an electronically isolated spin-light interface with low magnetic background noise. The copper-vacancy center, as the point defect of interest, has been shown to exhibit favorable characteristics including radiative transitions between isolated states inside the zinc sulfide bandgap, a paramagnetic ground state, and a C3V-symmetric impurity-vacancy structure which results in favorable orbital and spin characteristics for several known defect qubits. We use both chemical synthesis and focused ion beam (FIB) implantation to obtain copper-vacancy color centers in zinc sulfide. FIB implantation of copper followed by annealing creates localized arrays of copper-vacancy color centers in single-crystal zinc sulfide. Studies of copper-vacancy center activation in bulk zinc sulfide reveal new evidence regarding the origins of the associated emission, and provide bright ensembles of centers sharing a single crystal lattice for field-dependent measurements. However, the background emission in commercially available zinc sulfide poses a barrier to observing quantum emission from copper-vacancy color centers. This barrier is overcome by the successful activation of copper-vacancy centers in colloidal nanocrystals of zinc sulfide, which we can sufficiently dilute using solution processing methods to the extent that we are able to measure photon antibunching from copper-vacancy centers. We discuss the templated assembly and isolation of colloidal nanocrystals of zinc sulfide containing copper-vacancy color centers, which can withstand liftoff and ligand-exchange procedures without quenching of the copper-vacancy luminescence. We further discuss techniques uniquely developed for the spin-optical characterization of these copper-vacancy centers as potential defect qubits. These include time-gating photoluminescence scans to improve the visibility of copper-vacancy centers based on the long-lived emission components we measure in ensemble studies, and 2D, room-temperature optically-detected magnetic resonance spectroscopy capabilities compatible with time-gating. Prior to the work presented here to gain access to red-emitting copper-vacancy color centers for their attractive properties as a defect qubit candidates, there has not been an intensive effort to create or understand red-emitting copper-vacancy color centers (R-Cu centers) in zinc sulfide since the mid-20th century. As a result, they have never been created using ion beam implantation, and there is only one report of copper-doped zinc sulfide nanocrystals which emit a red peak assigned to these color centers. In providing routes for obtaining arrays of localized emission from copper-vacancy color centers in both bulk and colloidal nanocrystal zinc sulfide, this thesis provides new understanding of the red emission from the copper-vacancy color centers and proposes a solution to inconsistencies in reports of their emission mechanism and peak energy. We find that the R-Cu emission arises from thermally activated carrier transfer between two radiative manifolds, producing an anomalous plateau in the thermal quenching profile and blueshifted luminescence upon increasing temperature. Understanding of these characteristics and their relationship to the charge and spin states of the R-Cu center can inform the development of protocols for operating the center as a quantum spin-light interface. We further demonstrate the powerful advantages of quantum defect exploration using colloidal nanocrystals in place of bulk single-crystals or powders.