Probing Correlation of Optical Emission and Defect Sites in Hexagonal Boron Nitride by High-Resolution STEM-EELS.

Optically bright emitters in hexagonal boron nitride (hBN) often acting as a source of a single-photon are mostly attributed to point-defect centers, featuring localized intra-bandgap electronic states. Although vacancies, anti-sites, and impurities have been proposed as candidates, the exact physical and chemical nature of most hBN single-photon emitters (SPEs) within the visible region are still up for debate. Combining site-specific high-angle annular dark-field imaging (HAADF) with electron energy loss spectroscopy (EELS), we resolve and identify a few carbon substitutions among neighboring hBN hexagons, all within the same sample region, from which typical defect emission is observed. Our experimental results are further supported by first-principles calculations, through which the stability and possible optical transitions of the proposed carbon-defect complex are assessed. The presented correlation between optical emission and defects provides valuable information toward the controlled creation of emitters in hBN, highlighting carbon complexes as another probable cause of its visible SPEs.

Alice (and Bob) in Flatland.

2D quantum materials have opened infinite doors, hosting intriguing phenomena and featuring incredible engineering potential. Whether these qualities can boost the use of 2D crystals for quantum applications remains an open field with yet unexplored paths.

Near-Unity Light-Matter Interaction in Mid-Infrared van der Waals Metasurfaces.

Accessing mid-infrared radiation is of great importance for a range of applications, including thermal imaging, sensing, and radiative cooling. Here, we study light interaction with hexagonal boron nitride (hBN) nanocavities and reveal strong and tunable resonances across its hyperbolic transition. In addition to conventional phonon-polariton excitations, we demonstrate that the high refractive index of hexagonal boron nitride outside the Reststrahlen band allows enhanced light-matter interactions in deep subwavelength (<λ/15) nanostructures across a broad 7-8 µm range. Emergence and interplay of Fabry-Perot and Mie-like resonances are examined experimentally and theoretically. Near-unity absorption and high quality (Q ≥ 80) resonance interaction in the vicinity of the hBN transverse optical phonon is further observed. Our study provides avenues to design highly efficient and ultracompact structures for controlling mid-infrared radiation and accessing strong light-matter interactions with hBN.

Room Temperature Negative Differential Resistance with High Peak Current in MoS2/WSe2 Heterostructures.

Two-dimensional transition metal dichalcogenide (2D TMD) semiconductors allow facile integration of p- and n-type materials without a lattice mismatch. Here, we demonstrate gate-tunable n- and p-type junctions based on vertical heterostructures of MoS2 and WSe2 using van der Waals (vdW) contacts. The p-n junction shows negative differential resistance (NDR) due to Fowler-Nordheim (F-N) tunneling through the triangular barrier formed by applying a global back-gate bias (VGS). We also show that the integration of hexagonal boron nitride (h-BN) as an insulating tunnel barrier between MoS2 and WSe2 leads to the formation of sharp band edges and unintentional inelastic tunnelling current. The devices based on vdW contacts, global VGS, and h-BN tunnel barriers exhibit NDR with a peak current (Ipeak) of 315 µA, suggesting that the approach may be useful for applications.

Highly Sustainable h-BN Encapsulated MoS2 Hydrogen Evolution Catalysts.

Despite the importance of the stability of the 2D catalysts in harsh electrolyte solutions, most studies have focused on improving the catalytic performance of molybdenum disulfide (MoS2) catalysts rather than the sustainability of hydrogen evolution. In previous studies, the vulnerability of MoS2 crystals is reported that the moisture and oxygen molecules can cause the oxidation of MoS2 crystals, accelerating the degradation of crystal structure. Therefore, optimization of catalytic stability is crucial for approaching practical applications in 2D catalysts. Here, it is proposed that monolayered MoS2 catalysts passivated with an atomically thin hexagonal boron nitride (h-BN) layer can effectively sustain hydrogen evolution reaction (HER) and demonstrate the ultra-high current density (500 mA cm⁻2 over 11 h) and super stable (64 h at 150 mA cm⁻2) catalytic performance. It is further confirmed with density functional theory (DFT) calculations that the atomically thin h-BN layer effectively prevents direct adsorption of water/acid molecules while allowing the protons to be adsorbed/penetrated. The selective penetration of protons and prevention of crystal structure degradation lead to maintained catalytic activity and maximized catalytic stability in the h-BN covered MoS2 catalysts. These findings propose a promising opportunity for approaching the practical application of 2D MoS2 catalysts having long-term stability at high-current operation.

Modification Strategies of Hexagonal Boron Nitride Nanomaterials for Photocatalysis.

Although hexagonal boron nitride (h-BN) was initially considered a less promising photocatalyst due to its large band gap and apparent chemical inertness, its unique two-dimensional lamellar structure coupled with high stability and environmental friendliness, as the second largest van der Waals material after graphene, provides a unique platform for photocatalytic innovation. This review not only highlights the intrinsic qualities of h-BN with photocatalytic potentials, such as high stability, environmental compatibility, and tunable bandgap through various modification strategies but also provides a comprehensive overview of the recent advances in h-BN-based nanomaterials for environmental and energy applications, as well as an in-depth description of the modification methods and fundamental properties for these applications. In addition, we discuss the challenges and prospects of h-BN-based nanomaterials for future photocatalysis.

Identification of exfoliated monolayer hexagonal boron nitride films with a digital color camera under white light illumination.

Optical microscopy with white light illumination has been employed when obtaining exfoliated monolayer hexagonal boron nitride (1L hBN) films from a large number of randomly placed films on a substrate. However, real-time observation of 1L hBN using a color camera under white light illumination remains challenging since hBN is transparent in the visible wavelength range. The poor optical constant of 1L hBN films in microphotographs is significantly improved using a Si substrate coated with a SiNxthin-film (SiNx/Si). When observing hBN thin films on SiNx/Si using a color digital camera in an optical microscope under white light illumination, the clarity of the captured color images depends on the thickness of the SiNxfilm (d). For real-time direct observation, thedwas optimized based on quantitative chromatic studies tailored to Bayer filters of a color image sensor. Through image simulation, it was determined that the color difference between 1L hBN and the bare substrate is maximized atd= 59 or 70 nm, which was experimentally verified. The SiNx/Si with optimizeddvalues visualized 1L hBN films without requiring significant contrast enhancement via image processing under white light illumination in real-time. Furthermore, the captured color photographs facilitate the reliable determination of the number of layers in few-layer hBN films using the contrast of the green channel of the images.

Self-powered semitransparent WS2/LaVO3vertical-heterostructure photodetectors by employing interfacial hexagonal boron nitride.

Two-dimensional (2D) semiconductor and LaVO3materials with high absorption coefficients in the visible light region are attractive structures for high-performance photodetector (PD) applications. Insulating 2D hexagonal boron nitride (h-BN) with a large band gap and excellent transmittance is a very attractive material as an interface between 2D/semiconductor heterostructures. We first introduce WS2/h-BN/LaVO3semitransparent PD. The photo-current/dark current ratio of the device exhibits a delta-function characteristic of 4 × 105at 0 V, meaning 'self-powered'. The WS2/h-BN/LaVO3PD shows up to 0.27 A W-1responsivity (R) and 4.6 × 1010cm Hz1/2W-1detectivity (D*) at 730 nm. Especially, it was confirmed that theD* performance improved by about 5 times compared to the WS2/LaVO3device at zero bias. Additionally, it is suggested that the PD maintains 87% of its initialRfor 2000 h under the atmosphere with a temperature of 25 °C and humidity of 30%. Based on the above results, we suggest that the WS2/h-BN/LaVO3heterojunction is promising as a self-powered optoelectronic device.

PdSe2/MoSe2: a promising van der Waals heterostructure for field effect transistor application.

The field-effect transistor (FET) is a fundamental component of semiconductors and the electronic industry. High on-current and mobility with layer-dependent features are required for outstanding FET channel material. Two-dimensional materials are advantageous over bulk materials owing to their higher mobility, high ON/OFF ratio, low tunneling current, and leakage problems. Moreover, two-dimensional heterostructures provide a better way to tune electrical properties. In this work, the two distinct possibilities of PdSe2/MoSe2heterostructure have been employed through mechanical exfoliation and analyzed their electrical response. These diffe approaches to heterostructure formation serve as crucial components of our investigation, allowing us to explore and evaluate the unique electronic properties arising from each design. This work demonstrates that the heterostructure possesses a better ON/OFF ratio of ∼5.78 × 105, essential in switching characteristics. Moreover, MoSe2provides a defect-free interface to PdSe2, resulting in a higher ON current of ∼10µA and mobility of ∼63.7 cm2V-1s-1, necessary for transistor applications. In addition, comprehending the process of charge transfer occurring at the interface between transition metal dichalcogenides is fundamental for advancing next-generation technologies. This work provides insights into the interface formed between the PdSe2and MoSe2that can be harnessed in transistor applications.

Exploring the properties of theVB-defect in hBN: optical spin polarization, Rabi oscillations, and coherent nuclei modulation.

Optically active point defects in semiconductors have received great attention in the field of solid-state quantum technologies. Hexagonal boron nitride, with an ultra-wide band gapEg= 6 eV, containing a negatively charged boron vacancy (VB-) with unique spin, optical, and coherent properties presents a new two-dimensional platform for the implementation of quantum technologies. This work establishes the value ofVB-spin polarization under optical pumping withλext= 532 nm laser using high-frequency (νmw= 94 GHz) electron paramagnetic resonance (EPR) spectroscopy. In optimal conditions polarization was found to beP≈ 38.4%. Our study reveals that Rabi oscillations induced on polarized spin states persist for up to 30-40µs, which is nearly two orders of magnitude longer than what was previously reported. Analysis of the coherent electron-nuclear interaction through the observed electron spin echo envelope modulation made it possible to detect signals from remote nitrogen and boron nuclei, and to establish a corresponding quadrupole coupling constantCq= 180 kHz related to nuclear quadrupole moment of14N. These results have fundamental importance for understanding the spin properties of boron vacancy.

Phonon transport in vacancy induced defective stanene/hBN van der Waals heterostructure.

In this study, Non-Equilibrium Molecular Dynamics (NEMD) simulation is employed to investigate the phonon thermal conductivity (PTC) of Sn/hBN van der Waals heterostructures with different vacancy-induced defects. We deliberately introduce three types of vacancies in Sn/hBN bilayer point vacancies, bivacancies, and edge vacancies at various concentrations ranging from 0.25% to 2%, to examine their effects on PTC across temperatures from 100 K to 600 K. The key findings of our work are (i) PTC declines monotonically with increasing vacancy concentration for all types of vacancies, with a maximum reduction of ∼62% observed at room temperature compared to its pristine form. (ii) The position of defects has an impact on PTC, with a larger decrease observed when defects are present in the hBN layer and a smaller decrease when defects are in the Sn layer. (iii) The type of vacancy also influences PTC, with point vacancies causing the most substantial reduction, followed by bivacancies, and edge vacancies having the least effect. A 2% defect concentration results in a ∼62% decrease in PTC for point vacancies, ∼51% for bivacancies, and ∼32% for edge vacancies. (iv) Finally, our results indicate that for a given defect concentration, PTC decreases as temperature increases. The impact of temperature on thermal conductivity is less pronounced compared to the effect of vacancies for the defective Sn/hBN bilayer. The presence of vacancies and elevated temperatures enhance phonon-defect and phonon-phonon scattering, leading to changes in the phonon density of states (PDOS) profile and the distribution of phonons across different frequencies of Sn/hBN bilayer, thus affecting its thermal conductivity. This work offers new insights into the thermal behavior of vacancy-filled Sn/hBN heterostructures, suggesting potential pathways for modulating thermal conductivity in bilayer van der Waals heterostructures for applications in thermoelectric, optoelectronics, and nanoelectronics in future.

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.

Greatly Enhanced Emission from Spin Defects in Hexagonal Boron Nitride Enabled by a Low-Loss Plasmonic Nanocavity.

The negatively charged boron vacancy (VB-) defect in hexagonal boron nitride (hBN) with optically addressable spin states has emerged due to its potential use in quantum sensing. Remarkably, VB- preserves its spin coherence when it is implanted at nanometer-scale distances from the hBN surface, potentially enabling ultrathin quantum sensors. However, its low quantum efficiency hinders its practical applications. Studies have reported improving the overall quantum efficiency of VB- defects with plasmonics; however, the overall enhancements of up to 17 times reported to date are relatively modest. Here, we demonstrate much higher emission enhancements of VB- with low-loss nanopatch antennas (NPAs). An overall intensity enhancement of up to 250 times is observed, corresponding to an actual emission enhancement of ∼1685 times by the NPA, along with preserved optically detected magnetic resonance contrast. Our results establish NPA-coupled VB- defects as high-resolution magnetic field sensors and provide a promising approach to obtaining single VB- defects.

Sandwiched Epitaxy Growth of 2D Single-Crystalline Hexagonal Bismuthene Nanoflakes for Electrocatalytic CO2 Reduction.

Two-dimensional (2D) bismuthene is predicted to possess intriguing physical properties, but its preparation remains challenging due to the high surface energy constraint. Herein, we report a sandwiched epitaxy growth strategy for the controllable preparation of 2D bismuthene between a Cu foil substrate and a h-BN covering layer. The top h-BN layer plays a crucial role in suppressing the structural transformation of bismuthene and compensating for the charge transfer from the bismuthene to the Cu(111) surface. The bismuthene nanoflakes present a superior thermal stability up to 500 °C in air, attributed to the passivation effect of the h-BN layer. Moreover, the bismuthene nanoflakes demonstrate an ultrahigh faradaic efficiency of 96.3% for formate production in the electrochemical CO2 reduction reaction, which is among the highest reported for Bi-based electrocatalysts. This study offers a promising approach to simultaneously synthesize and protect 2D bismuthene nanoflakes, which can be extended to other 2D materials with a high surface energy.

Spin Defects in hBN assisted by Metallic Nanotrenches for Quantum Sensing.

The omnipresence of hexagonal boron nitride (hBN) in devices embedding two-dimensional materials has prompted it as the most sought after platform to implement quantum sensing due to its testing while operating capability. The negatively charged boron vacancy (VB-) in hBN plays a prominent role, as it can be easily generated while its spin population can be initialized and read out by optical means at room-temperature. But the lower quantum yield hinders its widespread use as an integrated quantum sensor. Here, we demonstrate an emission enhancement amounting to 400 by nanotrench arrays compatible with coplanar waveguide (CPW) electrodes employed for spin-state detection. By monitoring the reflectance spectrum of the resonators as additional layers of hBN are transferred, we have optimized the overall hBN/nanotrench optical response, maximizing thereby the luminescence enhancement. Based on these finely tuned heterostructures, we achieved an enhanced DC magnetic field sensitivity as high as 6 × 10-5 T/Hz1/2.

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/Hz. Our results provide important suggestions for further applications of VB- centers in quantum information processing and ODMR-based quantum sensing.

Preparation of BN Nanoparticle with High Sintering Activity and Its Formation Mechanism.

Hexagonal boron nitride (h-BN) nanoparticles have attracted increasing attention due to their unique structure and properties. However, it is difficult to synthesize h-BN nanoparticles with uniform spherical morphology due to their crystal characteristic. The morphology control by tuning their precursor synthesis is a promising and effective strategy to solve this problem. Especially, the treatment temperature of precursors plays an important role in the morphology and surface area of h-BN nanoparticles. Herein, h-BN nanoparticles with different morphologies were synthesized via regulating the treatment temperature of precursors. The result shows that treatment temperature will affect the microstructure and state of precursor and further influence the morphology of h-BN products. Benefiting from the unique structure, the h-BN obtained using 250 °C precursors shows higher specific surface area (61.1 m2 g-1) than that of 85 °C (36.5 m2 g-1) and 145 °C (27.9 m2 g-1). h-BN products obtained using 250 °C precursors show higher specific surface area than that of 85 °C and 145 °C. The optimal condition for obtaining high-quality spherical h-BN is the pretreatment temperature of 250 °C and sintering temperature of 1300 °C. Importantly, compared with commercial h-BN nanoparticles, the synthesized h-BN nanoparticles show more uniform structure and larger specific surface area, indicating that sintering activity will be greatly improved. Furthermore, the reaction pathway and formation mechanism of h-BN was revealed by DFT calculations. The result shows that the five stationary states and five transition states exist in the reaction pathway, and the energy barrier can be overcome at high temperatures to form a ring h-BN. In view of its simplicity and efficiency, this work is promising for designing and guiding the synthesis of h-BN nanoparticles with uniform morphology.

The Synthesis of Three-Dimensional Hexagonal Boron Nitride as the Reinforcing Phase of Polymer-Based Electrolyte for All-Solid-State Li Metal Batteries.

Powdery hexagonal boron nitride (h-BN), as an important material for electrochemical energy storage, has been typically synthesized in bulk and one/two-dimensional (1/2D) nanostructured morphologies. However, until now, no method has been developed to synthesize powdery three-dimensional (3D) h-BN. This work introduces a novel NaCl-glucose-assisted strategy to synthesize micron-sized 3D h-BN with a honeycomb-like structure and its proposed formation mechanism. We propose that NaCl acts as the template of 3D structure and promotes the nitridation reaction by adsorbing NH3 . Glucose facilitates the homogeneous coating of boric acid onto the NaCl surface via functionalizing the NaCl surface. During the nitridation reaction, boron oxides (BO4 and BO3 ) form from a dehydration reaction of boric acid, which is then reduced to O2 -B-N and O-B-N2 intermediates before finally being reduced to BN3 by NH3 . When incorporated into polyethylene oxide-based electrolytes for Li metal batteries, 5 wt % of 3D h-BN significantly enhances ionic conductivity and mechanical strength. Consequently, this composite electrolyte demonstrates superior electrochemical stability. It delivers 300 h of stable cycles in the Li//Li cell at 0.1 mA cm-2 and retains 89 % of discharge capacity (138.9 mAh g-1 ) after 100 cycles at 1 C in the LFP//Li full cell.

High detectivity solar blind photodetector based on mechanical exfoliated hexagonal boron nitride films.

As an ultra-wide bandgap semiconductor, hexagonal boron nitride (h-BN) has drawn great attention in solar-blind photodetection owing to its wide bandgap and high thermal conductivity. In this work, a metal-semiconductor-metal structural two-dimensional h-BN photodetector was fabricated by using mechanically exfoliated h-BN flakes. The device achieved an ultra-low dark current (16.4 fA), high rejection ratio (R205nm/R280nm= 235) and high detectivity up to 1.28 × 1011Jones at room temperature. Moreover, due to the wide bandgap and high thermal conductivity, the h-BN photodetector showed good thermal stability up to 300 °C, which is hard to realize for common semiconductor materials. The high detectivity and thermal stability of h-BN photodetector in this work showed the potential applications of h-BN photodetectors working in solar-blind region at high temperature.

Identification of the monolayer thickness difference in a mechanically exfoliated thick flake of hexagonal boron nitride and graphite for van der Waals heterostructures.

Exfoliated flakes of layered materials, such as hexagonal boron nitride (hBN) and graphite with a thickness of several tens of nanometers, are used to construct van der Waals heterostructures. A flake with a desirable thickness, size, and shape is often selected from many exfoliated flakes placed randomly on a substrate using an optical microscope. This study examined the visualization of thick hBN and graphite flakes on SiO2/Si substrates through calculations and experiments. In particular, the study analyzed areas with different atomic layer thicknesses in a flake. For visualization, the SiO2thickness was optimized based on the calculation. As an experimental result, the area with different thicknesses in a hBN flake showed different brightness in the image obtained using an optical microscope with a narrow band-pass filter. The maximum contrast was 12% with respect to the difference of monolayer thickness. In addition, hBN and graphite flakes were observed by differential interference contrast (DIC) microscopy. In the observation, the area with different thicknesses exhibited different brightnesses and colors. Adjusting the DIC bias had a similar effect to selecting a wavelength using a narrow band-pass filter.