Harnessing quantum mechanics for real-world applications is a rapidly growing field. Single-photon emitters (SPEs) are crucial components in photonic quantum technologies, enabling quantum information processing and transmission. Various solid-state single-photon sources have been explored, including those embedded in three-dimensional crystals like diamond, silicon carbide, and zinc oxide. However, these often suffer from light trapping due to their bulky dielectric environment, resulting in reduced emission efficiency. Two-dimensional (2D) materials offer advantages, with reduced dielectric thickness leading to improved emission efficiency and easier integration with on-chip photonics. Transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN) are promising 2D hosts. While TMD-based SPEs only operate at low temperatures, hBN hosts bright, room-temperature SPEs, making it particularly attractive. hBN thin films can be prepared via various methods including high-temperature, high-pressure synthesis followed by mechanical or liquid exfoliation, or by direct growth using chemical vapor deposition (CVD) or metal organic vapor phase epitaxy (MOVPE). CVD is a promising approach for scalable fabrication, but CVD-grown hBN emitters often exhibit spectral diffusion, intermittency (blinking), and irreversible loss of emission (photobleaching). The mechanisms behind these instabilities remain unclear, and improving photostability is crucial for practical applications. Liquid exfoliated hBN offers sharp and stable emission after thermal annealing, but the resulting nanoflakes are too small for most applications. Understanding the varying photostability of hBN SPEs produced by different methods is critical for developing scalable fabrication strategies of photostable emitters.

Extensive research has been conducted on single-photon emitters (SPEs) and their applications in quantum technologies. Early work focused on SPEs in three-dimensional materials, with limitations due to light trapping in bulky dielectric environments. The exploration of two-dimensional (2D) materials like TMDs and hBN provided a path towards more efficient and easily integrable SPEs. Studies on TMD-based SPEs highlighted their low-temperature operation, while research on hBN-based SPEs revealed their potential for bright, room-temperature emission. Different synthesis methods for hBN, including CVD and liquid exfoliation, have been investigated, each with its own advantages and drawbacks regarding scalability, cost, and emitter properties. Existing literature has documented issues like spectral diffusion, blinking, and photobleaching in hBN SPEs, but a comprehensive understanding of the underlying mechanisms and effective mitigation strategies remained elusive. Previous work has explored annealing processes for improving stability but lacked a detailed mechanistic explanation. This gap in the understanding of photostability motivated the current study.

This study quantitatively evaluated the photobleaching lifetimes of various hBN samples: monolayer and multilayer CVD-grown hBN, and multilayer liquid-exfoliated hBN nanoflakes. Time-dependent photoluminescence (PL) measurements were performed using a custom-built confocal spectroscopic microscope. An environmental chamber allowed for controlling the atmosphere (N₂, O₂, water vapor). Laser power dependence measurements were conducted to investigate the bleaching mechanisms. Annular dark-field scanning transmission electron microscopy (ADF-STEM) at the emitter position provided high-resolution imaging of the hBN lattice, including vacancy defects and carbon substitutions. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition and bonding states of the hBN samples. Thermal annealing at 850 °C under Ar was performed to investigate its effect on the photostability. The bleaching lifetimes were analyzed using single and double exponential decay functions to fit the intensity decay curves. The ADF-STEM analysis focused on identifying monovacancies and heteroatom substitutions (C, O) with high crystallographic detail. XPS was used to determine the concentration and bonding states of C and O in different hBN samples. Finally, thermal annealing experiments examined the impact of high-temperature treatment on the photostability of emitters.

The study revealed that photobleaching in monolayer CVD hBN is dominated by photochemical reactions with O₂, resulting in a bleaching lifetime of approximately 5 s. This bleaching was significantly mitigated by stacking additional hBN layers, demonstrating a simple strategy for improving photostability. ADF-STEM imaging of photobleached monolayer CVD hBN revealed a rich variety of monovacancies, often with carbon and oxygen substitutions. A second, slower bleaching lifetime (150-220 s) was observed in multilayer samples in air and in monolayer samples under N₂, suggesting a mechanism independent of the external environment. XPS analysis confirmed the presence of carbon in CVD-grown hBN, with sp² C-C bonds indicating graphene-like domains. Thermal annealing at 850 °C in Ar eliminated both bleaching processes, yielding persistent photostability. In liquid exfoliated hBN, two emission modes were observed. Type-I exhibited blinking and bleaching similar to CVD hBN, while type-II showed remarkable photostability. Annealing eliminated type-I emitters but left type-II unchanged, indicating different structural origins. The second, slower bleaching mechanism was attributed to carbon-assisted defect migration, supported by theoretical predictions. The study demonstrated that optimizing carbon concentration and preventing graphene domain formation during synthesis is crucial for producing photostable and narrowband emitters.

The findings address the research question of improving photostability in hBN quantum emitters by identifying the key factors influencing their stability: oxygen exposure and carbon concentration. Shielding O₂ either by using N₂ atmosphere or stacking additional hBN layers significantly improves the lifetime, indicating that photochemical reactions with O₂ are a primary cause of fast bleaching. The discovery of a second bleaching mechanism, independent of environmental factors, highlights the importance of intrinsic material properties in determining the long-term stability of the emitters. This second mechanism was linked to carbon-assisted vacancy migration, providing a new avenue for material engineering. The combination of experimental and theoretical findings supports the hypothesis that carbon impurities near emitting defects facilitate structural changes under laser excitation, leading to non-emitting defects. The results demonstrate a clear correlation between carbon concentration, defect migration, and photostability, providing valuable insights for future material design and synthesis strategies. These findings are highly relevant for realizing practical applications of hBN SPEs in quantum technologies. The observed photostability improvements significantly enhance the potential of hBN SPEs for various quantum applications, offering a pathway towards robust and scalable devices.

This study successfully demonstrated that shielding O₂ and optimizing carbon substitution are key to enhancing the photostability of hBN quantum emitters. Two simple approaches, utilizing a N₂ atmosphere or multi-layer stacking, significantly increased emitter lifetime. A second, slower bleaching process was identified and linked to carbon-assisted vacancy migration. Atomic-scale characterization using ADF-STEM and XPS clarified the role of carbon impurities. Thermal annealing effectively eliminates both bleaching mechanisms. Future research should focus on further optimization of synthesis techniques to minimize carbon incorporation, creating more photostable hBN SPEs for quantum technologies.

The study primarily focused on commercially available hBN samples and a limited set of emission wavelengths. While ADF-STEM provided atomic-scale insights, co-localizing PL measurements with high-resolution STEM analysis remained challenging. The exact role of hydrogen in stabilizing vacancy structures is yet to be determined. The generalization of the observed carbon-assisted defect migration mechanism to other hBN synthesis methods requires further investigation.