Defects In Hexagonal Boron Nitride Research Articles

Unzipping hBN with ultrashort mid-infrared pulses.

Manipulating the nanostructure of materials is critical for numerous applications in electronics, magnetics, and photonics. However, conventional methods such as lithography and laser writing require cleanroom facilities or leave residue. We describe an approach to creating atomically sharp line defects in hexagonal boron nitride (hBN) at room temperature by direct optical phonon excitation with a mid-infrared pulsed laser from free space. We term this phenomenon "unzipping" to describe the rapid formation and growth of a crack tens of nanometers wide from a point within the laser-driven region. Formation of these features is attributed to the large atomic displacement and high local bond strain produced by strongly driving the crystal at a natural resonance. This process occurs only via coherent phonon excitation and is highly sensitive to the relative orientation of the crystal axes and the laser polarization. Its cleanliness, directionality, and sharpness enable applications such as polariton cavities, phonon-wave coupling, and in situ flake cleaving.

Single-Photon Emission from Point Defects in Hexagonal Boron Nitride Induced by Plasma Treatment.

Solid-state quantum emitters are gaining significant attention for many quantum information applications. Hexagonal boron nitride (h-BN) is an emerging host material for generating bright, stable, and tunable single-photon emission with narrow line widths at room temperature. In this work, we present a facile and efficient approach to generate high-density single-photon emitters (SPEs) in mechanically exfoliated h-BN through H- or Ar-plasma treatment followed by high-temperature annealing in air. It is notable that the postannealing is essential to suppress the fluorescence background in photoluminescence spectra and enhance emitter stability. These quantum emitters exhibit excellent optical properties, including high purity, brightness, stability, polarization degree, monochromaticity, and saturation intensity. The effects of process parameters on the quality of quantum emitters were systematic investigated. We find that there exists an optimal plasma power and h-BN thickness to achieve a high SPE density. This work offers a practical avenue for generating SPEs in h-BN and holds promise for future research and applications in quantum photonics.

Proximity-Induced Exchange Interaction: A New Pathway for Quantum Sensing Using Spin Centers in Hexagonal Boron Nitride.

Defects in hexagonal boron nitride (hBN), a two-dimensional van der Waals material, have attracted a great deal of interest because of its potential in various quantum applications. Due to hBN's two-dimensional nature, the spin center in hBN can be engineered in the proximity of the target material, providing advantages over its three-dimensional counterparts, such as the nitrogen-vacancy center in diamond. Here we propose a novel quantum sensing protocol driven by exchange interaction between the spin center in hBN and the underlying magnetic substrate induced by the magnetic proximity effect. By first-principles calculation, we demonstrate that the induced exchange interaction dominates over the dipole-dipole interaction by orders of magnitude when in the proximity. The interaction remains antiferromagnetic across all stacking configurations between the spin center in hBN and the target van der Waals magnets. Additionally, we explored the scaling behavior of the exchange field as a function of the spatial separation between the spin center and the targets.

Framework for Engineering of Spin Defects in Hexagonal Boron Nitride by Focused Ion Beams

AbstractHexagonal boron nitride (hBN) is gaining interest as a wide bandgap van der Waals host of optically active spin defects for quantum technologies. Most studies of the spin‐photon interface in hBN focus on the negatively charged boron vacancy (VB−) defect, which is typically fabricated by ion irradiation. However, the applicability and wide deployment of VB− defects is limited by VB− fabrication methods which lack robustness and reproducibility, particularly when applied to thin flakes (≲10 nm) of hBN. Here, two key factors are elucidated that underpin the formation and quenching of VB− centers by ion irradiation—density of defects generated in the hBN lattice and recoil‐implantation of foreign atoms into hBN. Critically, it is shown that the latter is extremely efficient at inhibiting the generation of optically‐active VB− centers. This is significant because foreign atoms such as carbon are commonplace on both the top and bottom surfaces of hBN during ion irradiation, in the form of hydrocarbon contaminants, polymer residues from hBN transfer methods, protective capping layers and substrates. Recoil implantation must be accounted for when selecting ion beam parameters such as ion mass, energy, fluence, incidence angle, and sputter/span yield, which are discussed in the context of a framework for VB− generation by high‐resolution focused ion beam (FIB) systems.

Selective Generation of Luminescent Defects in Hexagonal Boron Nitride

AbstractSingle photon emitters from atomic defects in crystals like hexagonal boron nitride (hBN) are vital for quantum technologies. Although various techniques are devised to obtain defects emission in hBN, simultaneous control over defects position, type, and emission spectrum has not been achieved yet. Here, ion implantation with 12C, 20Ne, and 69Ga are used to create a composite defects population with emission ≈820 nm. The correlation of Raman and photoluminescence (PL) spectroscopy helps to identify the defects’ type. After selecting Ga as the ion species yielding the maximum emitter brightness, a strategy based on thermal annealing is developed to modify the composition of the induced defects. This results in an emitter ensemble with selected spectral properties, even when starting from different implantation conditions. Specifically, thermal annealing induces a defect transmutation from one type to another, shifting the emission wavelength from 820 to 625 nm. Moreover, sample patterning is combined with focused ion beam implantation and subsequent annealing in an efficient method to deterministically set the defects position as well as the PL spectral composition. These results offer a practical avenue to achieve in situ positioning and tuning of ensembles of emitters in hBN, promising for quantum information and sensing applications.

Identifying Electronic Transitions of Defects in Hexagonal Boron Nitride for Quantum Memories

AbstractA quantum memory is a crucial keystone for enabling large‐scale quantum networks. Applicable to the practical implementation, specific properties, i.e., long storage time, selective efficient coupling with other systems, and a high memory efficiency are desirable. Though many quantum memory systems are developed thus far, none of them can perfectly meet all requirements. This work herein proposes a quantum memory based on color centers in hexagonal boron nitride (hBN), where its performance is evaluated based on a simple theoretical model of suitable defects in a cavity. Employing density functional theory calculations, 257 triplet and 211 singlet spin electronic transitions are investigated. Among these defects, it is found that some defects inherit the Λ electronic structures desirable for a Raman‐type quantum memory and optical transitions can couple with other quantum systems. Further, the required quality factor and bandwidth are examined for each defect to achieve a 95% writing efficiency. Both parameters are influenced by the radiative transition rate in the defect state. In addition, inheriting triplet‐singlet spin multiplicity indicates the possibility of being a quantum sensing, in particular, optically detected magnetic resonance. This work therefore demonstrates the potential usage of hBN defects as a quantum memory in future quantum networks.

Ab-Initio Investigations into Frenkel Defects in Hexagonal Boron Nitride for Quantum Optoelectronic Applications

Ab-Initio Investigations into Frenkel Defects in Hexagonal Boron Nitride for Quantum Optoelectronic Applications

Optically Active Spin Defects in Few-Layer Thick Hexagonal Boron Nitride.

Optically active spin defects in hexagonal boron nitride (hBN) are promising quantum systems for the design of two-dimensional quantum sensing units offering optimal proximity to the sample being probed. In this Letter, we first demonstrate that the electron spin resonance frequencies of boron vacancy centers (V_{B}^{-}) can be detected optically in the limit of few-atomic-layer thick hBN flakes despite the nanoscale proximity of the crystal surface that often leads to a degradation of the stability of solid-state spin defects. We then analyze the variations of the electronic spin properties of V_{B}^{-} centers with the hBN thickness with a focus on (i)the zero-field splitting parameters, (ii)the optically induced spin polarization rate and (iii)the longitudinal spin relaxation time. This Letter provides important insights into the properties of V_{B}^{-} centers embedded in ultrathin hBN flakes, which are valuable for future developments of foil-based quantum sensing technologies.

Arbitrarily structured quantum emission with a multifunctional metalens

Structuring light emission from single-photon emitters (SPEs) in multiple degrees of freedom is of great importance for quantum information processing towards higher dimensions. However, traditional control of emission from quantum light sources relies on the use of multiple bulky optical elements or nanostructured resonators with limited functionalities, constraining the potential of multi-dimensional tailoring. Here we introduce the use of an ultrathin polarisation-beam-splitting metalens for the arbitrary structuring of quantum emission at room temperature. Owing to the complete and independent polarisation and phase control at the single meta-atom level, the designed metalens enables simultaneous mapping of quantum emission from ultra-bright defects in hexagonal boron nitride and imprinting of an arbitrary wavefront onto orthogonal polarisation states of the sources. The hybrid quantum metalens enables simultaneous manipulation of multiple degrees of freedom of a quantum light source, including directionality, polarisation, and orbital angular momentum. This could unleash the full potential of solid-state SPEs for their use as high-dimensional quantum sources for advanced quantum photonic applications.

Coupling spin defects in hexagonal boron nitride to a microwave cavity

Optically addressable spin defects in hexagonal boron nitride (hBN) have become a promising platform for quantum sensing. While sensitivity of these defects is limited by their interactions with the spin environment in hBN, inefficient microwave delivery can further reduce their sensitivity. Here, we design and fabricate a microwave double arc resonator for efficient transferring of the microwave field at 3.8 GHz. The spin transitions in the ground state of VB− are coupled to the frequency of the microwave cavity, which result in enhanced optically detected magnetic resonance (ODMR) contrast. In addition, the linewidth of the ODMR signal further reduces, achieving a magnetic field sensitivity as low as 42.4 μT/√Hz. Our robust and scalable device engineering is promising for future employment of spin defects in hBN for quantum sensing.

Magnetic Field Sensitivity Optimization of Negatively Charged Boron Vacancy Defects in hBN.

AbstractOptically active spin defects in hexagonal boron nitride (hBN) have recently emerged as compelling quantum sensors hosted by a two dimensional (2D) material. The photodynamics and sensitivity of spin defects are governed by their level structure and associated transition rates. These are, however, poorly understood for spin defects in hBN. Here, optical and microwave pump‐probe measurements are used to characterize the relaxation dynamics of the negatively charged boron vacancy (VB−)—the most widely‐studied spin defect in hBN. A 5‐level model is used to deduce transition rates that give rise to spin‐dependent VB− photoluminescence, and the lifetime of the VB− intersystem crossing metastable state. The obtained rates are used to simulate the magnetic field sensitivity of VB− defects and demonstrate high resolution imaging of the magnetic field generated by a single magnetic particle using optimal sensing parameters predicted by the model. The results reveal the rates that underpin VB− photodynamics, which is important for both a fundamental understanding of the VB− as a spin‐photon interface and for achieving optimal sensitivity in quantum sensing applications.

Effect of environmental screening and strain on optoelectronic properties of two-dimensional quantum defects

Point defects in hexagonal boron nitride (hBN) are promising candidates as single-photon emitters (SPEs) in nanophotonics and quantum information applications. The precise control of SPEs requires in-depth understanding of their optoelectronic properties. However, how the surrounding environment of host materials, including the number of layers, substrates, and strain, influences SPEs has not been fully understood. In this work, we study the dielectric screening effect due to the number of layers and substrates, and the strain effect on the optical properties of carbon dimer and nitrogen vacancy defects in hBN from first-principles many-body perturbation theory. We report that environmental screening causes a lowering of the quasiparticle gap and exciton binding energy, leading to nearly constant optical excitation energy and exciton radiative lifetime. We explain the results with an analytical model starting from the Bethe–Salpeter equation Hamiltonian with Wannier basis. We also show that optical properties of quantum defects are largely tunable by strain with highly anisotropic response, in good agreement with experimental measurements. Our work clarifies the effect of environmental screening and strain on optoelectronic properties of quantum defects in two-dimensional insulators, facilitating future applications of SPEs and spin qubits in low-dimensional systems.

DC Magnetic Field Sensitivity Optimization of Spin Defects in Hexagonal Boron Nitride.

Spin defects existing in van der Waals materials attract wide attention thanks to their natural advantages for in situ quantum sensing, especially the negatively charged boron vacancy (VB-) centers in hexagonal boron nitride (h-BN). Here we systematically investigate the laser and microwave power broadening in continuous-wave optically detected magnetic resonance (ODMR) of the VB- ensemble in h-BN, by revealing the behaviors of ODMR contrast and line width as a function of the laser and microwave powers. The experimental results are well explained by employing a two-level simplified model of ODMR dynamics. Furthermore, with optimized power, the DC magnetic field sensitivity of VB- ensemble is significantly improved up to 2.87 ± 0.07 μT/. Our results provide important suggestions for further applications of VB- centers in quantum information processing and ODMR-based quantum sensing.

Manipulating the Charge State of Spin Defects in Hexagonal Boron Nitride.

Negatively charged boron vacancies (VB-) in hexagonal boron nitride (hBN) have recently gained interest as spin defects for quantum information processing and quantum sensing by a layered material. However, the boron vacancy can exist in a number of charge states in the hBN lattice, but only the -1 state has spin-dependent photoluminescence and acts as a spin-photon interface. Here, we investigate the charge state switching of VB defects under laser and electron beam excitation. We demonstrate deterministic, reversible switching between the -1 and 0 states (VB- ⇌ VB0 + e-), occurring at rates controlled by excess electrons or holes injected into hBN by a layered heterostructure device. Our work provides a means to monitor and manipulate the VB charge state, and to stabilize the -1 state which is a prerequisite for spin manipulation and optical readout of the defect.

A DFT study of electron–phonon interactions for the and defects in hexagonal boron nitride: investigating the role of the transition dipole direction

Quantum emitters in two-dimensional hexagonal boron nitride (h-BN) have generated significant interest due to observations of ultra-bright emission made at room temperature. The expectation that solid-state emitters exhibit broad zero-phonon lines at elevated temperatures has been put in question by recent observations of Fourier transform (FT) limited photons emitted from h-BN flakes at room temperature. All decoupled emitters produce photons that are directed in-plane, suggesting that the dipoles are perpendicular to the h-BN plane. Motivated by the promise of an efficient and scalable source of indistinguishable photons that can operate at room temperature, we have developed an approach using density functional theory (DFT) to determine the electron-phonon coupling for defects that have in- and out-of-plane transition dipole moments. Our DFT calculations reveal that the transition dipole for the defect is parallel to the h-BN plane, and for the defect is perpendicular to the plane. We calculate both the phonon density of states and the electron–phonon matrix elements associated with the h-BN defective structures. We find no indication that an out-of-plane transition dipole by itself will result in the low electron–phonon coupling that is expected to produce FT-limited photons at room temperature. Our work provides direction to future DFT software developments and adds to the growing list of calculations relevant to researchers in the field of solid-state quantum information processing.

Coherent dynamics of strongly interacting electronic spin defects in hexagonal boron nitride

Optically active spin defects in van der Waals materials are promising platforms for modern quantum technologies. Here we investigate the coherent dynamics of strongly interacting ensembles of negatively charged boron-vacancy ({{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}) centers in hexagonal boron nitride (hBN) with varying defect density. By employing advanced dynamical decoupling sequences to selectively isolate different dephasing sources, we observe more than 5-fold improvement in the measured coherence times across all hBN samples. Crucially, we identify that the many-body interaction within the {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} ensemble plays a substantial role in the coherent dynamics, which is then used to directly estimate the concentration of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}. We find that at high ion implantation dosage, only a small portion of the created boron vacancy defects are in the desired negatively charged state. Finally, we investigate the spin response of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-} to the local charged defects induced electric field signals, and estimate its ground state transverse electric field susceptibility. Our results provide new insights on the spin and charge properties of {{{{{{{{rm{V}}}}}}}}}_{{{{{{{{rm{B}}}}}}}}}^{-}, which are important for future use of defects in hBN as quantum sensors and simulators.

Creation and repair of luminescence defects in hexagonal boron nitride by irradiation and annealing for optical neutron detection

Hexagonal boron nitride (hBN) is promising for applications in neutron detection and quantum sensing. Here we created spin-dependent luminescence defects in hBN using 50 keV helium ion beam. The irradiated hBN flakes with a dose around 2 × 1014–1 × 1015/cm2 have the maximum photoluminescence (PL) intensity. The PL spectra are in the range of 700–1000 nm with a peak around 818 nm at 297 K after irradiation with a dose of less than 2 × 1014/cm2, which can be eliminated by high temperature annealing at 1097 K due to defects repair according to Raman spectra. However, the PL peaks exhibit a redshift from 818 nm to 830 nm when the irradiation dose is no less than 1 × 1015/cm2. Combining with constrained density functional theory calculations, it is found that high dose irradiation can cause local lattice swelling and further induce the redshift of PL spectra by 12 nm/0.33% strain. Furthermore, we proposal a novel optical neutron detection method by recording the intensity and position of PL signal of luminescence defects induced by neutron-10B nuclear reaction daughter particles. This work would be helpful to create luminescence defects in hBN for quantum sensing, and develop a novel neutron detector.

Laser Direct Writing of Visible Spin Defects in Hexagonal Boron Nitride for Applications in Spin-Based Technologies

Optically addressable spins in two-dimensional hexagonal boron nitride (hBN) attract widespread attention for their potential advantages in on-chip quantum devices, such as quantum sensors and quantum networks. A variety of spin defects have been found in hBN, but no convenient and deterministic generation methods have been reported for other defects except the negatively charged boron vacancy (VB−). Here we report that by using femtosecond laser direct writing technology, we can deterministically create spin defect ensembles with spectral range from 550 to 800 nm on nanoscale hBN flakes. Positive single-peak optically detected magnetic resonance (ODMR) signals are detected in the presence of a magnetic field perpendicular to the substrate, and the contrast can reach 0.8%. With the appropriate thickness of hBN flakes, substrate, and femtosecond laser pulse energy, we can deterministically and efficiently generate a bright spin defect array. Our results provide a convenient deterministic method to create spin defects in hBN, which will motivate more endeavors for future research and applications of spin-based technologies such as a quantum magnetometer array.

Substrate-Induced Modulation of Quantum Emitter Properties in 2D Hexagonal Boron Nitride: Implications for Defect-Based Single Photon Sources in 2D Layers

Quantum emitters (QEs) based on deep-level defects in hexagonal boron nitride (hBN) layers are promising alternatives to other qubit-candidates in three-dimensional wide bandgap semiconductors. The two-dimensional (2D) form factor of hBN allows the possibility of near-deterministic placement of quantum emitters and an ease of property-tuning via different means, such as application of strain. However, the 2D nature of hBN also results in a unique set of challenges, including a sensitivity of the QEs to their environment that can influence their different properties, such as their emission frequencies and brightness. In particular, although observed experimentally, theoretical works thus far have ignored substrate-induced modulation of hBN’s QE properties. As a result, to date, the magnitude of substrate effects and the underlying mechanism(s) involved in the modulation of QE properties remain unknown. In our density functional theory-based work, we use silicon dioxide as a prototype substrate to demonstrate that the substrate effects can indeed have a significant impact on ground- and excited-state properties of defects responsible for quantum emission. Our analysis shows large structural distortions at the defect sites due to substrate interactions, resulting in significant changes in quantum emission frequencies. These calculations reveal that accounting for substrate effects is critical to the successful use of hBN in quantum sensing and quantum computing.

Locally strained hexagonal boron nitride nanosheets quantified by nanoscale infrared spectroscopy.

Defect engineering in two-dimensional materials expands the realm of their applications in catalysis, nanoelectronics, sensing, and beyond. As limited tools are available to explore nanoscale functional properties in non-vacuum environments, theoretical modeling provides some invaluable insight into the effect of local deformations to deepen the understanding of experimental signals acquired by nanoscale chemical imaging. We demonstrate the controlled creation of nanoscale strained defects in hexagonal boron nitride (h-BN) using atomic force microscopy and infrared (IR) light under an inert environment. Nanoscale IR spectroscopy reveals the broadening of the in-plane phonon (E1u) mode of h-BN during defect formation while density functional theory-based calculations and molecular dynamics provide quantification of the tensile and compressive strain in the deformation.