Designing descriptors for multiple defects in two-dimensional materials is challenging due to the diverse local atomic environments created by different defect types and arrangements. Existing physics-informed descriptors struggle to distinguish distinct defect configurations with identical composition, while deep learning models, though powerful, require large data sets and are less interpretable. In this work, we address this limitation by engineering chemical descriptors and constructing structural features from nearest-neighbor distributions provided by the classical force-field-inspired descriptors (CFID). We show that our engineering method, combined with defect-aware structural features derived from the Hellinger distance, even excluding the full distribution features, improves data point discrimination in high-dimensional feature space while reducing the number of features by 50%. In predicting formation energy per defect site, this extended feature set balances reliance on a few dominant features, enhancing model interpretation and generalization at the cost of a marginal 10% increase in prediction error compared to baseline descriptors. This generalization capability is empirically validated on an external out-of-distribution data set of bulk hBN defects, where our model exhibits lower uncertainty and superior stability within the applicable physical domain (- 1 < E-f < 5 eV). However, predicting a highly complex and nonlinear target, such as the HOMO-LUMO gap, remains challenging, as none of our extensions outperform the baseline. This physics-informed approach offers an interpretable and computationally efficient alternative to deep learning models, providing new insights into defect representations in 2D materials and serving as a tool for the high-throughput prescreening of stable defect candidates prior to expensive first-principles calculations.

The potential of color centers in hexagonal boron nitride (hBN) for quantum technology applications has driven research to create emitters across a broad spectral range by using diverse techniques. Electron beam irradiation is one such approach that creates yellow emitters at room temperature,. however, their behavior at low temperatures remains unexplored. Here, we present a comprehensive photophysical characterization of these yellow emitters in hBN under cryogenic conditions. We identify a bright and photostable defect with a zero-phonon line (ZPL) at 547.5 nm and a phonon sideband (PSB) approximately 90 meV from the ZPL. Excitation through this PSB enhances the emission intensity by nearly 5-fold at 4.5 K. Temperature-dependent photoluminescence (PL) from 4.5 to 220 K shows a decreasing Debye-Waller (DW) factor with elevated temperature, reflecting enhanced phonon-assisted emission. Further analysis reveals the presence of an additional low-energy phonon mode, leading to a T 3 dependence of the ZPL line width and a T 2 dependence of the ZPL peak shift. These observations deepen our understanding of the nature of the emitters, opening new avenues for the precise tuning of quantum light sources.

Point defects in solid-state quantum systems are vital for enabling single-photon emission at specific wavelengths, making their precise identification essential for advancing applications in quantum technologies. However, pinpointing the microscopic origins of these defects remains a challenge. In this work, we propose Raman spectroscopy as a robust strategy for defect identification. Using density functional theory, we characterize the Raman signatures of 100 defects in hexagonal boron nitride (hBN) spanning periodic groups III to VI, encompassing around 30,000 phonon modes. Our findings reveal that the local atomic environment plays a pivotal role in shaping the Raman lineshape. Furthermore, we demonstrate that Raman spectroscopy can differentiate defects based on their spin and charge states as well as strain-induced variations. The ability to resolve spin configurations offers a pathway to identifying defects with spins suitable for quantum sensing. Finally, an experimental concept using tip-enhanced Raman spectroscopy has been proposed in this work. Therefore, this study not only provides a comprehensive theoretical database of Raman spectra for hBN defects but also establishes a novel experimental framework to identify point defects. More broadly, our approach offers a universal method for defect identification in any quantum materials with spin configurations specific to any quantum application.

Single photon emitters in hexagonal boron nitride are based on fluorescent point-like defects. These defects possess exceptional photophysical properties and have been the focus of research for their potential to advance photonic quantum technologies. However, achieving scalable integration of these emitters onto arbitrary platforms with high yield while retaining their characteristics remains challenging when the substrate is incompatible with the fabrication method. In this work, we introduce an all-dry transfer method that addresses these challenges more effectively than existing techniques. This polymer stamp-assisted transfer maintains high output and preserves emitter characteristics while eliminating wet chemical processes. Comprehensive post-transfer characterization verified retention of the defining single-photon characteristic, the second-order correlation function g((2))(0), and revealed an improvement of about 46%. This enhancement in photon purity may result from thermal desorption of weak surface contaminants during mild heating and from changes in local stress and strain induced by the transfer process. While g((2))(0) showed substantial improvement, other photophysical properties, such as emitter lifetime, emission spectrum, and photostability, remained nearly unchanged, indicating that the transfer preserves the overall optical quality. The technique achieves emitter survival probability of 81.8%, where success requires both the structural integrity of the transferred flake and the survival of the emitters with their defining optical properties. This high survival probability demonstrates the potential to scale the integration of single photon emitters across diverse photonic platforms. We expect that this process will contribute to applications of boron nitride defects in quantum technologies.

The negatively charged boron vacancy (VB-) defect in hexagonal boron nitride has recently emerged as a promising spin qubit for sensing due to its high-temperature spin control and versatile integration into van der Waals structures. While extensive experiments have explored their coherence properties, much less is known about the spin relaxation time (T1) and its control parameter dependence. In this work, we develop a parameter-free spin dynamics model based on the cluster expansion technique to investigate T1 relaxation mechanisms at low temperature. Our results reveal that the VB- center constitutes a strongly coupled electron spin-nuclear spin core, which necessitates the inclusion of the coherent dynamics and derived memory effects of the three nearest-neighbor nitrogen nuclear spins. Using this framework, this work closely reproduces the experimentally observed T1 time at B = 90 G and further predicts the T1 dependence on external magnetic field in the 0 <= B <= 2000 G interval, when the spin relaxation is predominantly driven by electron-nuclear and nuclear-nuclear flip-flop processes mediated by hyperfine and dipolar interactions. This study establishes a reliable and scalable approach for describing T1 relaxation in VB- centers and offers microscopic insights to support future developments in nuclear-spin-based quantum technologies.

Quantum emitters in hexagonal boron nitride (hBN) have gained significant attention due to a wide range of defects that offer high quantum efficiency and single-photon purity at room temperature. Most theoretical studies on hBN defects simulate monolayers, as this is computationally cheaper than calculating bulk structures. However, most experimental studies are carried out on multilayer to bulk hBN, which creates additional possibilities for discrepancies between theory and experiment. In this work, we present an extended database of hBN defects that includes a comprehensive set of bulk hBN defects along with their excited-state photophysical properties. The database features over 120 neutral defects, systematically evaluated across charge states ranging from -2 to +2 (600 defects in total). For each defect, the most stable charge and spin configurations are identified and used to compute the zero-phonon line, photoluminescence spectrum, absorption spectrum, Huang-Rhys (HR) factor, interactive radiative lifetimes, transition dipole moments, and polarization characteristics. Our analysis reveals that the electron-phonon coupling strength is primarily influenced by the presence of vacancies, which tend to induce stronger lattice distortions and broaden phonon sidebands. Additionally, correlation analysis shows that while most properties are independent, the HR factor strongly correlates with the configuration coordinates. All data are publicly available at https://h-bn.info, along with a new application programming interface (API) to facilitate integration with machine learning workflows. This database is therefore designed to bridge the gap between theory and experiment, aid in the reliable identification of quantum emitters, and support the development of machine-learning-driven approaches in quantum materials research.

Two-dimensional transition metal dichalcogenides (TMDs) are highly appealing for gas sensors, lab-on-a-chip devices, and biosensing applications because of their strong light-matter interaction and high surface-to-volume ratio. The ability to grow these van der Waals materials on different substrates and waveguide geometries opens a horizon toward scalable on-chip photonic nanodevices. Here we report on a versatile technique for remote optical gas sensing using two-dimensional TMDs. The adsorption of the gas molecules on the monolayer surface provides a gateway for gas sensing based on charge-transfer-induced photoluminescence variation. For gases that are weakly adsorbed on the surface of monolayer TMDs, purging the monolayers' surface by an inert gas like N2 can desorb gases from the monolayers at room temperature. We demonstrate CO, NO, and NO2 detection by monitoring photoluminescence from semiconducting MoS2 monolayers grown on SiO2/Si chips at a level of 10 ppm with fast response time. Observations are supported by our density functional theory calculations, which predict a significant interaction between these gases and MoS2 monolayers. These findings may lead to advances in remote sensing, surface-sensitive bioanalytics, and lab-on-a-chip sensors. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Hexagonal boron nitride (hBN) has emerged as a compelling platform for both classical and quantum technologies. In particular, the past decade has witnessed a surge of novel ideas and developments, which may be overwhelming for newcomers to the field. This review provides an overview of the fundamental concepts and key applications of hBN, including quantum sensing, quantum key distribution, quantum computing, and quantum memory. Additionally, critical experimental and theoretical advances that have expanded the capabilities of hBN are highlighted, in a cohesive and accessible manner. The objective is to equip readers with a comprehensive understanding of the diverse applications of hBN, and provide insights into ongoing research efforts.

Single photon emitters (SPEs) are a key component for their use as pure photon source in quantum technologies. In this study, we investigate the generation of SPEs from drop-casted hexagonal boron nitride (hBN) nanoflakes, examining the influence of the immersion solution and the source of hBN. We show that, depending on the utilized supplier and solution, the number and quality of the emitters change. We perform a comprehensive optical characterization of the deposited nanoflakes to assess the quality of the generated SPEs. Importantly, we provide quantitative data on SPE yields, highlighting significant variations among solvents and different sources of hBN. We find that hBN from Merck drop-casted in acetone provided the best quality emitters with a g((2)) < 0.1 and photoluminescence intensities above 300 kCounts/s. Their number of SPEs among all photon emitters was also the highest, with about 14%, rendering a total yield of about 1.25% of all drop-casted flakes. These numbers hold particular significance when evaluating drop-casting as a practical method for the generation of SPEs and their deposition and incorporation within existing nanophotonic systems. By choosing appropriate solvents and source materials' quality and yield of SPEs can be significantly increased, showcasing further optimization potential for the development of future quantum applications.