Optimisation, Stabilisation, and Localisation of Quantum Emitters in Layer-Engineered Hexagonal Boron Nitride
For a long time, hexagonal boron nitride (hBN) has been considered the silent sister material of graphene, but recently an appreciation for it is starting to grow within the scientific community for the properties it possesses and not just its functionality as graphene’s supporting partner. The discovery of its ability to host quantum emitters from defect sites within its crystal structure has stimulated a recent increase in publications exploring their properties. The large bandgap hBN possesses, so juxtaposed to the conductive properties of graphene, and its ability to host energy levels at defects sites that are isolated from the band-edges, allows for single-photon emission (SPE) to occur at room temperature; thus, providing an exciting alternative to diamond for hosting quantum emission at these conditions. To date, most of the research on hBN and its emissive defects has revolved around thick, exfoliated hBN flakes. Theses samples are limited in area and combined with empirical SPE activation steps or from grown multi-layer hBN of typically, not well-defined structure and purity. The precise nature of such quantum emitters is still under debate because their generation or activation mechanisms remain poorly understood and their deterministic and scalable spatial positioning, both laterally and vertically, remains a scientific challenge. Spatial positioning is particularly important to enable effective and refined fundamental studies of the emissive defects, their stability and response to strain and coupling to e.g., external electric or magnetic fields, but also to open pathways to integrated device technology. Neither exfoliated hBN flakes or grown multilayers offer precise emitter position control (in zdirection) across the film thickness. Currently, investigations and characterisations of the material and emission sites are slow, utilising low-throughput techniques, which is why the development of fast analysis techniques combined with high-quality large-area material production are urgently required to better understand the material and emission sites. The work in this thesis begins by addressing the scalability issues of both characterisation and material size and it does it in two ways: first, by building upon previously developed chemical vapour deposition (CVD) growth procedures of monolayer (ML) hBN films, a large-area, continuous, and versatile SPE host material is created; second, to characterise it, a photoluminescence (PL) characterisation technique was newly applied that can simultaneously collect spatial, spectral, and temporal emitter information from 100s of emission sites at once. High-throughput scanning opens the possibility for comprehensive statistical behaviour analysis of emitters in hBN to be generated, elucidating in-depth information about their ensemble behaviour. Through the application of such a tool, the thesis next addresses salient issues that have plagued emission within ML hBN, namely its stability and contamination. First, the research presents pretreatment processes for CVD hBN MLs which are established to either fully suppress or activate emission, whilst removing the influence of process residues and environmental influences on emitter behaviour. Then, by being able to utilise such differently treated MLs as select building blocks and creating specific assembly protocols, the known emitter bleaching in air can be suppressed by sandwiching between two protecting (non-emissive) hBN MLs. The found stability lasts for months in air and allows for more sophisticated and intensive characterisation techniques to be used. Consequently, through second-order fluorescence intensity correlation measurements, the singlephoton nature of the emission sites are confirmed and comparisons of their structural origins to previous work can be commented on, which are based on the now-possible high-resolution spectral measurements. Finally, the thesis addresses the key technological challenge of scalable and deterministic SPE spatial position control, both laterally and vertically, whilst maintaining the 2D nature of the material. This is achieved through emitter localisation in hBN in all 3 dimensions via a ML engineering approach. The ML stacking process achieves vertical (z) emitter localisation at the atomic layer level, creating the thinnest, stable emissive structure. Such trilayer stacks and ultra-shallow z-localisation retains unique opportunities for external control of emission, quantum sensing and metrology, and efficient coupling to waveguides, fibres, or plasmonic and photonic cavities. Graphene is highly efficient in quenching fluorescence and by combining the trilayer hBN structure with a patterned CVD graphene mask, addressable emitter arrays and effective lateral (x-y) emitter localisation down to single emission sites is achieved. Such complete emitter site localisation is scalable and highly versatile, overcoming some of the major challenges hindering the materials advancement
Chemical Vapour Deposition, Single Photon Emitters, Hexagonal Boron Nitride, Quantum Emitters, 2d materials, Graphene, Layered Material, Nanofabrication, Super-resolution Microscopy
Networked Digital Library of Theses and Dissertations