Single-photon emitters in two-dimensional hexagonal boron nitride: properties, preparations, and applications

Single-photon emitters play an essential role in quantum technologies, including quantum computing and quantum communications. Atomic defects in hexagonal boron nitride ( h-BN) have recently emerged as new room-temperature single-photon emitters in solid-state systems, but the development of scalable and tunable h-BN single-photon emitters requires external methods that can control the emission energy of individual defects. Here, by fabricating van der Waals heterostructures of h-BN and graphene, we demonstrate the electrical control of single-photon emission from atomic defects in h-BN via the Stark effect. By applying an out-of-plane electric field through graphene gates, we observed Stark shifts as large as 5.4 nm per GV/m. The Stark shift generated upon a vertical electric field suggests the existence of out-of-plane dipole moments associated with atomic defect emitters, which is supported by first-principles theoretical calculations. Furthermore, we found field-induced discrete modification and stabilization of emission intensity, which were reversibly controllable with an external electric field.

The solid-state single-photon source is the core of applications such as quantum cryptography, quantum sensing, and quantum computing. Recently, the point defects in two-dimensional (2D) hexagonal boron nitride (h-BN) have become excellent candidates for next-generation single-photon sources due to their chemical and physical stability and ultra-high brightness at room temperature. The 2D layered structure of h-BN allows the single-photon emitters (SPEs) in it to have high photon extraction efficiency and be integrated into photonic circuits easily. However, most of the SPEs found in h-BN flakes are present at the edges or wrinkles. Here, we report on the room-temperature SPEs in h-BN film grown by molecular beam epitaxy followed by a high-temperature post-annealing process and their deterministic transfer. Using the all-dry viscoelastic stamping method, the h-BN film grown on the Al2O3 substrate can be transferred to other substrates. The transferred SPEs are discretely distributed among the continuous h-BN flakes, and the SPE density is as high as ∼0.17 μm−2. After identification, the determined SPE can be deterministically transferred to other structures by the all-dry transfer method. The deterministic transfer of SPEs distributed on the h-BN flakes promises the potential to integrate SPEs into many quantum technology applications.

Quantum technology grown out of quantum information theory, including quantum communication, quantum computation and quantum sensing, not only provides powerful research tools for numerous fields, but also is expected to go to civilian use in the future. Solid-state spin-active defects are one of promising platforms for quantum technology, and the host materials include three-dimensional diamond and silicon carbide, and the emerging two-dimensional hexagonal boron nitride (hBN) and transition-metal dichalcogenides. In this review, we will focus on the spin defects in hBN, and summarize theoretical and experimental progresses made in understanding properties of these spin defects. In particular, the combination of theoretical prediction and experimental verification is highlighted. We also discuss the future advantages and challenges of solid-state spins in hBN on the path towards quantum information applications.

Optically active spin defects in wide-bandgap materials have many potential applications in quantum information and quantum sensing. Spin defects in two-dimensional layered van der Waals materials are just emerging to be investigated. Here we demonstrate that optically-addressable spin ensembles in hexagonal boron nitride (hBN) can be generated by femtosecond laser irradiation. We observe optically detected magnetic resonance (ODMR) of hBN spin defects created by laser irradiation. We show that the creation of spin defects in hBN is strongly affected by the pulse energy of the femtosecond laser. When the laser pulse number is less than a few thousand, the pulse number only affects the density of defects but not the type of defects. With proper laser parameters, spin defects can be generated with a high probability of success. Our work provides a convenient way to create spin defects in hBN by femtosecond laser writing, which shows promising prospects for quantum technologies.

We investigate defect properties in hexagonal boron nitride (hBN) which is attracting much attention as a single photon emitter. Using first-principles calculations, we find that nitrogen-vacancy defect V N has a lower energy structure in C 1 h symmetry in 1− charge state than the previously known D 3 h symmetry structure. Noting that carbon has one more valence electron than boron species, our finding naturally points to the correspondence between V N and V N C B defects with one charge state difference between them, which is indeed confirmed by the similarity of atomic symmetries, density of states, and excitation energies. Since V N C B is considered as a promising candidate for the source of single photon emission, our study suggests V N as another important candidate worth attention, with its simpler form without the incorporation of foreign elements into the host material.

Spin-active defects in layered hexagonal boron nitride (hBN) crystals have attracted increased attention in quantum sensing. Notably, the recently discovered negatively charged boron vacancy (V B −) center stands out due to its optical addressability and coherent controllability. Among the various methods reported for generating such defects, ion implantation is notable as a readily accessible technique. In this paper, the properties of V B − defects in hBN generated via ion implantation are extensively studied. We achieve a ubiquitous distribution of highly stable defects across the crystal sample, and find that the ion beam current density, rather than fluence, plays a critical role in determining the uniformity and density of defects. The generated defects display bright and stable photoluminescence, and we explicitly investigate the dependence of spin properties on factors such as laser, microwave power, and duration. An intriguing phenomenon is observed wherein the peak contrast exceeds 20% without any enhancing techniques in the optically detected magnetic resonance spectrum for some special defects. Our results provide valuable insights and suggestions for the controlled generation of V B − defects in hBN through ion implantation.

Recently, Stone–Wales (SW) defects gradually attracted people’s research interest because of their unique properties. The theoretical research indicated that the SW defect in hexagonal boron nitride (h-BN) can lead to new defect levels in bandgap, making h-BN apply in ultraviolet emitters. However, the SW defect is always observed in graphene and rarely observed in h-BN in the experiments. Here, we confirmed the SW defects are not easily formed in h-BN under thermodynamic conditions by first-principles calculations. Specifically, the monolayer h-BN with SW defect (h-BN-SW) has the weak bond strength, dynamic stability and high-temperature thermal stability, facilitating the healing of SW defects under high-temperature conditions and the role of hydrogen. Additionally, we found the SW defect in AB stacked h-BN (AB-h-BN) have good mechanical stability, dynamic stability and thermodynamic stability than h-BN-SW, especially for AB-h-BN-2SW (2SW defects formed in upper and lower layer of AB-h-BN, respectively), which can meet the requirements for its application in electronic devices. Even under thermodynamic conditions, the formation of SW defects is extremely challenging. Electron beam irradiation technology provides a window for the generation of SW defects in h-BN. This offers opportunities for the introduction and control of SW defects, while also creating potential for their application in electronic devices. Moreover, we found that the absorption peak broadens, and a new absorption peak appears with the generation of SW defects, which is mainly induced by the decrease of bandgap and the generation of defect levels. Our research can provide theoretical guidance at atomic scale for designing and applying h-BN with SW defect in the experiments.

Hexagonal boron nitride (hBN) is an emerging two-dimensional material for quantum photonics owing to its large bandgap and hyperbolic properties. Here we report two approaches for engineering quantum emitters in hBN multilayers using either electron beam irradiation or annealing and characterize their photophysical properties. The defects exhibit a broad range of multicolor room-temperature single photon emissions across the visible and the near-infrared spectral ranges, narrow line widths of sub-10 nm at room temperature, and a short excited-state lifetime, and high brightness. We show that the emitters can be categorized into two general groups, but most likely possess similar crystallographic structure. Remarkably, the emitters are extremely robust and withstand aggressive annealing treatments in oxidizing and reducing environments. Our results constitute a step toward deterministic engineering of single emitters in 2D materials and hold great promise for the use of defects in boron nitride as sources for quantum information processing and nanophotonics.

The atomic structure, stability, and dynamics of defects in hexagonal boron nitride (h-BN) are investigated using an aberration-corrected transmission electron microscope operated at 80 kV between room temperature and 1000 °C. At temperatures above 700 °C, parallelogram- and hexagon-shaped defects with zigzag edges become prominent, in contrast to the triangular defects typically observed at lower temperatures. The appearance of 120° corners at defect vertices indicates the coexistence of both N- and B-terminated zigzag edges in the same defect. In situ dynamics studies show that the hexagonal holes grow by electron-induced sputtering of B-N chains, and that at high temperatures these chains can migrate from one defect corner to another. We complement the experiments with first-principles calculation which consider the thermal equilibrium formation energy of different defect configurations. It is shown that, below a critical defect size, hexagonal defects have the lowest formation energy and therefore are the more-stable configuration, and triangular defects are energetically metastable but can be "frozen in" under experimental conditions. We also discuss the possible contributions of several dynamic processes to the temperature-dependent defect formation.

Understanding and engineering atomic defects in hexagonal boron nitride (hBN) provides a powerful platform for realizing solid-state quantum emitters and spin qubits, advancing the field of quantum information science and technologies. However, the full potential of such quantum defects remains locked by the critical lack of a deterministic structure-property relationship at the atomic scale. Here, we demonstrate a strategy to atomically engineer and decipher quantum defects in hBN by integrating scanning tunneling microscopy/spectroscopy (STM/STS) and noncontact atomic force-microscopy with a CO-functionalized tip. We implemented controllable argon ion bombardment to create both boron vacancies (VB) and nitrogen vacancies (VN) in submonolayer hBN grown on Cu(111). Simultaneously, encapsulated Ar species trapped between hBN and Cu(111) locally lift the hBN to form nanobubbles, thereby decoupling atomic vacancies from the metal substrate and enabling direct probing of their electronic states. For the on-bubble VN, STS measurement reveals a prominent in-gap state with a phonon replica. Furthermore, with aid of STM tip-assisted manipulation, we demonstrate that the tuning of nanobubble sizes modulates their strain profile, thereby modulating the energetic positions of electronic states in on-bubble defects, corroborated by density functional calculations. Our studies offer insight into the intrinsic defect structures in hBN and quantum defect engineering via local strain engineering.

Two-dimensional (2D) Van der Waals materials have emerged as a very attractive class of optoelectronic material due to the unprecedented strength in its interaction with light. In this talk I will discuss approaches to enhance and control this interaction by integrating these 2D materials with microcavities, and metamaterials. I will first discuss the formation of strongly coupled half-light half-matter quasiparticles (microcavity excitonpolaritons) [1], their spin-optic [2], and electrical control/excitation (Fig. 1) [3], in the 2D transition metal dichalcogenide (TMD) systems. Prospects for enhanced nonlinear optical interaction using Rydberg states in TMDs will be discussed. Use of photonic hypercrystals and hyperbolic media to enhance the spontaneous emission from the TMDs will also be presented. Following this, I will discuss the routing of valley excitons in 2D TMDs using chiral metasurfaces (Fig. 2) [4]. Finally, time permitting, I will briefly talk about developing deterministic quantum emitters at room temperature using defects in hexagonal boron nitride [5] and prospects of coupling these emitters to high Q resonators.

An efficient single-photon emitter (SPE) should emit photons at a high rate into a well-defined spatio-temporal mode along with an accessible numerical aperture (NA) to increase the light extraction efficiency that is required for effective coupling into optical waveguides. Based on a previously developed experimental approach to fabricate hybrid Fabry–Perot microcavities (Ortiz-Huerta et al. Opt. Express2018, 26, 33245), we managed to find analytical and finite-difference time-domain (FDTD) values for the, experimentally achievable, geometrical parameters of a hybrid plano-concave microcavity that enhances the spontaneous emission (i.e., Purcell enhancement) of color centers in two-dimensional (2D) hexagonal boron nitride (hBN) while simultaneously limiting the NA of the emitter. Paraxial approximation and a transfer matrix model are used to find the spotsize of the fundamental Gaussian mode and the resonant modes of our microcavity, respectively. A Purcell enhancement of 6 is found for a SPE (i.e., in-plane dipole) hosted by a 2D hBN layer inside the hybrid plano-concave microcavity.

Quantum emitters in two-dimensional hexagonal boron nitride (hBN) are of significant interest because of their unique photophysical properties, such as single-photon emission at room temperature, and promising applications in quantum computing and communications. The photoemission from hBN defects covers a wide range of emission energies but identifying and modulating the properties of specific emitters remain challenging due to uncontrolled formation of hBN defects. In this study, more than 2000 spectra are collected consisting of single, isolated zero-phonon lines (ZPLs) between 1.59 and 2.25 eV from diverse sample types. Most of ZPLs are organized into seven discretized emission energies. All emitters exhibit a range of lifetimes from 1 to 6 ns, and phonon sidebands offset by the dominant lattice phonon in hBN near 1370 cm−1. Two chemical processing schemes are developed based on water and boric acid etching that generate or preferentially interconvert specific emitters, respectively. The identification and chemical interconversion of these discretized emitters should significantly advance the understanding of solid-state chemistry and photophysics of hBN quantum emission.

The 2D layered hexagonal boron nitride (hBN) material is a promising platform for making single photon emitters (SPEs). Integrate SPEs with photonic cavities or waveguides, requires the deterministic positioning of these emitters as accurately as possible. Among all of the alternative techniques, He+ focused ion beam (FIB) theoretically has sub‐nanometer precision, but its practical positioning accuracy in processing SPEs is limited by the inevitable detrimental byproduct defects. So far, it remains challenging to achieve an accuracy below 100 nm for the positioning of SPEs in hBN. Here, we present a two‐step method for creating SPEs at the predetermined positions in hBN that combines the He+ FIB irradiation and thermal annealing under an oxygen atmosphere. With this method, we have realized the positioning accuracy of less than 50 nm, which, to the best of our knowledge, stands as the highest among the currently available hBN SPEs preparation methods. Moreover, the SPE conversion yield was over 35% and the emission brightness of individual SPE achieved up to 3 × 105 counts/s at room temperature. These SPEs fabricated precisely with nanoscale accuracy in hBN are expected to be good candidates for making large‐scale integrated SPEs in photonic circuits.

Spin defects in hexagonal boron nitride, and specifically the negatively charged boron vacancy (VB - ) centers, are emerging candidates for quantum sensing. However, the VB - defects suffer from low quantum efficiency and, as a result, exhibit weak photoluminescence. In this work, a scalable approach is demonstrated to dramatically enhance the VB - emission by coupling to a plasmonic gap cavity. The plasmonic cavity is composed of a flat gold surface and a silver cube, with few-layer hBN flakes positioned in between. Employing these plasmonic cavities, two orders of magnitude are extracted in photoluminescence enhancement associated with a corresponding twofold enhancement in optically detected magnetic resonance contrast. The work will be pivotal to progress in quantum sensing employing 2D materials, and in realization of nanophotonic devices with spin defects in hexagonal boron nitride.