Quantum Emitters in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN), a two-dimensional van der Waals material, has drawn significant attention as a promising platform for hosting visible luminescent emitters, many of which exhibit quantum properties, functioning as single-photon emitters (SPEs) even at room temperature. The microscopic origin of these visible emissions remains a subject of debate, with two dominant hypotheses. The widely accepted one is based on the crystallographic defects introducing mid-gap states within the wide ∼6 eV hBN bandgap. Another recent hypothesis in 2023 suggests the presence of organic fluorophores, such as polycyclic aromatic hydrocarbon (PAH) molecules trapped between the hBN-substrate interface, as the source of ∼2 eV emitters. Understanding the emission mechanism is crucial for enabling on-demand, site-specific quantum emitter generation and integrating these emitters with photonic devices for quantum technologies.

In this PhD project, we first explore the site-specific creation of quantum emitters in exfoliated hBN flakes using strain engineering techniques. Localized strain was introduced by transferring hBN flakes onto dielectric nanopillar arrays, followed by oxygen irradiation and nitrogen annealing to activate or generate defects. Quantum emitters predominantly formed in strained regions, highlighting the interplay between strain and emitter localization. We suspect that this site-specificity may arise from lattice defects localized by strain. However, the accumulation of molecular residues at the hBN-pillar interface, possibly resulting from the wet-transfer fabrication process, might also play a role in site-specific emitter formation.

In the next part, we investigated the possible emission origin through systematic studies of emitter density. Higher emitter densities were observed in oxygen-irradiated samples subsequently annealed in nitrogen within a graphite box, suggesting a correlation between defect formation–potentially involving carbon-related centers–and luminescent emitter generation. Consistent results were observed in samples preannealed in an oxygen/argon flow environment, which minimized PAH contributions and thus reinforced the defect-based origin hypothesis. Additionally, hBN flakes of relatively lower crystal quality demonstrated higher emitter densities, further supporting the role of crystallographic defects. Furthermore, we investigated the effects of inert gas environments during activation annealing under continuous flow and found argon more effective than nitrogen in enhancing luminescent emitter densities. These findings emphasize the importance of crystallographic defects in emitter formation, although they do not entirely rule out contributions from molecular species. Whether defects act as the emitters themselves, serve as growth sites for carbon complexes such as nanographene, or facilitate PAH molecule incorporation remains unclear and demands further investigation.

Our work suggests the potential of combining strain and irradiation engineering for site-specific emitter fabrication in hBN. The results from our systematic studies support the explanation based on crystallographic defects for the emitter origin, while the molecular contributions cannot be entirely ruled out. Future studies incorporating rigorous cleaning procedures and comprehensive characterization techniques for both surface and bulk analysis are essential for a deeper understanding of emitter origin mechanisms. This will pave the way for deterministic and scalable quantum emitter fabrication in hBN for quantum technologies.