Two-dimensional Hexagonal Boron Nitride Research Articles

Optimization of h-BN/Ag/Ag<sub>2</sub>O heterostructure by high temperature annealing and its surface-enhanced Raman scattering performance

As a commonly used food additive, synthetic dyes are widely used in foods such as beverages, sweets and dairy products, and in the food processing process. Excessive use of synthetic dyes may induce cell mutations, children’s hyperactivity, genetic diseases and even cancer, which greatly threatens human health. As a fast, cheap, stable, ultra-sensitive and accurate detection method, SERS detection can reflect rich information about molecular fingerprint through the vibration of the analyte, and accurately determine the trace quantity of synthetic dyes in the actual environment. The metal/semiconductor heterostructure formed by the combination of plasmon metal and semiconductor is advantageous for the enhancement of photoinduced charge transfer (PICT) efficiency, and has obvious advantages in the study of surface-enhanced Raman scattering. In this work, the prepared hybrid substrate is annealed to further improve the surface-enhanced Raman scattering activity. Initially, high-density and monodisperse silver/silver oxide (Ag/Ag<sub>2</sub>O) nanoparticles are loaded onto two-dimensional hexagonal boron nitride (h-BN) nanosheets. On this basis, by annealing the system at high temperature, an efficient charge transfer channel is constructed, which greatly improves the PICT efficiency and chemical enhancement (CM). It is confirmed experimentally that the SERS signal intensity of h-BN/Ag/Ag<sub>2</sub>O material annealed at 320 ℃ with crystal violet (CV) as probe molecule is significantly increased 18 times compared with that of unannealed material, enhancement factor (EF) reaches 16.3145 ×10<sup>7</sup>. Finally, based on the excellent SERS property of h-BN/Ag/Ag<sub>2</sub>O annealed at 320 ℃, the SERS detection of food additive Patent Blue V (PBV) is conducted in this work. The results show that the lowest detection concentration can reach 10<sup>–12</sup> M, and the trace detection of synthetic dye PBV is realized. It has excellent spatial uniformity and anti-interference capability, which is of great significance in implementing the actual scene detection of PBV. The h-BN/Ag/Ag<sub>2</sub>O annealed at 320 ℃, the composite which is constructed in this paper, has both physical enhancement and chemical enhancement, and possess significant advantages in ultra-low sensitive SERS detection of food additives.

Surface engineering of two-dimensional hexagonal boron-nitride for optoelectronic devices.

Two-dimensional hexagonal boron nitride (2D h-BN) is being extensively studied in optoelectronic devices due to its electronic and photonic properties. However, the controlled optimization of h-BN's insulating properties is necessary to fully explore its potential in energy conversion and storage devices. In this work, we engineered the surface of h-BN nanoflakes via one-step in situ chemical functionalization using a liquid-phase exfoliation approach. The functionalized h-BN (F-h-BN) nanoflakes were subsequently dispersed on the surface of TiO2 to tune the TiO2/QDs interface of the optoelectronic device. The photoelectrochemical (PEC) devices based on TiO2/F-h-BN/QDs with optimized addition of carbon nanotubes (CNTs) and scattering layers showed 46% improvement compared to the control device (TiO2/QDs). This significant improvement is attributed to the reduced trap/carrier recombination and enhanced carrier injection rate of the TiO2-CNTs/F-h-BN/QDs photoanode. Furthermore, by employing an optimized TiO2-CNTs/F-h-BN/QDs photoanode, QDs-sensitized solar cells (QDSCs) yield an 18% improvement in photoconversion efficiency. This represents a potential and adaptability of our approach, and pathway to explore surface-engineered 2D materials to optimize the interface of solar energy conversion and other emerging optoelectronic devices.

Combining experiments on luminescent centres in hexagonal boron nitride with the polaron model and ab initio methods towards the identification of their microscopic origin.

The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent centres with emission energies of ∼2 eV which exhibit pronounced phonon sidebands. We investigate the microscopic origin of these luminescent centres by combining ab initio calculations with non-perturbative open quantum system theory to study the emission and absorption properties of 26 defect transitions. Comparing the calculated line shapes with experiments we narrow down the microscopic origin to three carbon-based defects: C2CB, C2CN, and VNCB. The theoretical method developed enables us to calculate so-called photoluminescence excitation (PLE) maps, which show excellent agreement with our experiments. The latter resolves higher-order phonon transitions, thereby confirming both the vibronic structure of the optical transition and the phonon-assisted excitation mechanism with a phonon energy ∼170 meV. We believe that the presented experiments and polaron-based method accurately describe luminescent centres in hBN and will help to identify their microscopic origin.

Generation of Circularly Polarized High Harmonics with Identical Helicity in Two-Dimensional Materials

Generation of circularly polarized high harmonics with the same helicity to all orders is indispensable for chiral-sensitive spectroscopy with attosecond temporal resolution. Solid-state samples have added a valuable asset in controlling the polarization of emitted harmonics. However, maintaining the identical helicity of the emitted harmonics to all orders is a daunting task. In this work, we demonstrate a robust recipe for efficient generation of circularly polarized harmonics with the same helicity. For this purpose, a nontrivial tailored driving field, consisting of two co-rotating laser pulses with frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$, is utilized to generate harmonics from graphene. The Lissajous figure of the total driving pulse exhibits an absence of rotational symmetry, which imposes no constraint on the helicity of the emitted harmonics. Our approach to generating circularly polarized harmonics with the same helicity is robust against various perturbations in the setup, such as variation in the subcycle phase difference or the intensity ratio of the $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ pulses, as rotational symmetry of the total driving pulse remains absent. Our approach is expected to be equally applicable to other two-dimensional materials, among others, transition-metal dichalcogenides and hexagonal boron nitride, as our approach is based on the absence of rotational symmetry of the driving pulse. Our work paves the way for establishing compact solid-state chiral extreme ultraviolet sources, opening a realm for chiral light-matter interaction on its intrinsic timescale.

Reconstruction of attosecond beating by interference of two-photon interband transitions in solids

The reconstruction of attosecond beating by interference of two-photon transitions is one of the most widely used techniques for obtaining both the relative phases of harmonics forming an attosecond pulse train and the phase of atomic radiative transitions. If the latter is computed by theory, it allows one to reconstruct the attosecond pulse train; if the former is known experimentally, it allows for reconstruction of the electronic dynamics of photoionization in atomic and molecular systems with attosecond temporal resolution. As it relies on the interference of photoelectrons in vacuum, similar interference has never been contemplated inside crystals. Here we explore the applicability of this scheme to solid-state systems using a one-dimensional model and a density-functional theory--calculated structure of two-dimensional hexagonal boron nitride. We discuss the possibility of (i) reconstructing the relative phases between harmonics with trivial influence of the ``atomic phase'' and (ii) retrieving the relative phases of two-photon transitions through different bands, which are generally challenging to obtain both experimentally and numerically. These phases are recorded in the beating of the population signal arising from interfering two-photon pathways, and can be read out with angle-resolved photoemission spectroscopy. Furthermore, the amplitude of the population beating decays as the pump and probe pulses are separated in time due to electron-hole decoherence, providing a simple interferometric method to extract dephasing times.

Synthesis of Two-Dimensional Hexagonal Boron Nitride and Mid-Infrared Nanophotonics

Two-dimensional hexagonal boron nitride (2D h-BN), a representative of 2D layered materials with unique structure and properties, is one of the most promising inorganic nanomaterials in recent years. The excellent properties of h-BN in the mid-infrared (MIR) region (∼2–20 μm) have also received much attention. At the same time, with the advancement of materials exploration and device-on-chip integrated systems, the synthesis of high-quality h-BN has encountered great challenges, which is a prerequisite for the application of h-BN in the MIR region. In this paper, we first review the recent advances in 2D h-BN synthesis by highlighting the research, advantages and disadvantages of various synthesis methods, and the critical issues encountered so far. Then, advances in the study and application of h-BN in the MIR region are explored, including perfect absorption, photodetectors, electro-optical modulators, phonon polaritons, and plasma excitons. Finally, we present our views on the challenges encountered in the synthesis and application of 2D h-BN in the MIR region in the near future in the context of the article’s discussion and the potential of h-BN development, with the hope this review will be of some help to relevant researchers.

Identification of an Oxygen Defect in Hexagonal Boron Nitride

Paramagnetic fluorescent defects in two-dimensional hexagonal boron nitride (hBN) are promising building blocks for quantum information processing. Although numerous defect-related single-photon sources and a few quantum bits have been found, except for the boron vacancy, their identification is still elusive. Here, we demonstrate that the comparison of experimental and first-principles simulated electron paramagnetic resonance (EPR) spectra is a powerful tool for defect identification in hBN, and first-principles modeling is inevitable in this process as a result of the dense nuclear spin environment of hBN. In particular, a recently observed EPR center is associated with the negatively charged oxygen vacancy complex by means of the many-body perturbation theory method on top of hybrid density functional calculations. To our surprise, the negatively charged oxygen vacancy complex produces a coherent emission around 2 eV with a well-reproducing previously recorded photoluminescence spectrum of some quantum emitters, according to our calculations.

Bi-nanofillers integrated into PEO-based electrolyte for high-performance solid-state Li metal batteries

Solid-state lithium metal batteries (SSLMBs) with high safety and high energy densities draw widespread attention in recent years. Nevertheless, the intrinsic defects of the solid-state electrolyte (SSE) such as the poor compatibility with electrodes and low ionic conductivity hamper the application of SSMLBs. Herein, we introduce rigid two-dimensional hexagonal boron nitride (BN) nanosheets and flexible One-dimensional aramid nanofibers (ANFs) as bi-nanofillers into the polyethylene oxide (PEO)-based SSEs to fabricate a composite solid-state electrolyte (CSE) ANF-PEO-BN with network structure boosting the comprehensive performance of SSMLBs. ANF and BN with special properties and structures are prone to be dispersed homogeneously in the PEO matrix to build the ANF-PEO-BN electrolyte, whose rigid and flexible network structure ensures the formation of Li dendrites inhibited. Furthermore, the dissociation of lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) is promoted and more ion transport channels are provided due to the strong affinity between Lewis acid centers of ANF/BN and Li+/TFSI−. As a result, the LiFePO4/Li batteries deliver high-capacity retention of 86% after 500 cycles and Li/Li cells stably cycle beyond 3300 h with small overpotential of 10 mV at 0.1 mA cm−2/0.2 mAh cm−2.

Analytical and numerical design of a hybrid Fabry-Perot plano-concave microcavity for hexagonal boron nitride.

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. Express 2018, 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.

2D boron-nitride featuring B4 tetrahedros: An efficient photocatalyst for water splitting

Two-dimensional hexagonal boron nitride (h-BN) is an insulator with polar covalent B−N bonds. Based on current research on boron-nitrogen 2D materials and the cluster assembling method, here, we designed five novel monolayers with square lattice formed by tetrahedral B4 units covalently linked to V-group atoms, namely the B4N2, B4P2, B4As2, B4Sb2, and B4Bi2 monolayers, via first-principles calculations. Both the B4N2 and B4P2 have good stability, as demonstrated by high cohesive energies, all positive phonon modes, high in-plane Young's modulus and structural integrity through 5 ps's molecular dynamics simulations at both 300 and 500 K. Moreover, the B4P2 monolayer exhibits metallic properties, whereas the B4N2 monolayer is an indirect bandgap semiconductor with the bandgap of 2.56 eV and appropriate band edge positions, the optical adsorption in UV light region, and high electron mobility for photocatalytic water splitting. Our theoretical study adds new members of 2D boron nitrides (B4N2), which can be used as an efficient photocatalyst for water splitting.

First-principles Study of Electronic and Magnetic Properties of Two-dimensional Hexagonal Boron Nitride Doped with Germanium and Tin Atoms

For the study of geometrical structure, stability, and electronic and magnetic properties of Germanium and tin-doped two-dimensional hexagonal boron nitride (h-BN), First-principles calculations have been carried out. Plane-wave pseudo-potential method in association with the density functional theory (DFT) framework used in Quantum ESPRESSO codes has been implemented to perform the calculations. A 3X3 supercell size substitutional doping of a single Boron or Nitrogen atom was carried out for the study. Pristine h-BN showed non-magnetic behavior with comprehensive gap material having an indirect band gap of 4.64eV. The doping effect of Ge and Sn atoms at the B-site was energetically more favorable than N-site. The defected h-BN sheet was found to be severely distorted with remarkable alteration in bond length and angles around the defected sites. Ge doped h-BN showed semiconducting properties with a reduced band gap in comparison to the insulating nature of pristine h-BN, whereas half metallicity was noticed in Sn doped h-BN system. Both the systems showed a magnetic moment of 1.0 µB.

The fabrication of a flexible and portable sensor based on home-made laser-induced porous graphene electrode for the rapid detection of sulfonamides

A smartphone-based wireless electrochemical sensor for portable and fast detection of sulfonamides (SAs) using home-made flexible integrated three-electrode of laser-induced porous graphene (LIPG) modified with two-dimensional hexagonal boron nitride (2D h-BN) as working electrode was developed. The flexible integrated three-electrode of LIPG was prepared by direct laser-writing technique on polyimide (PI) film. The reference electrode was handmade by casting a silver and silver chloride ink on the surface of the LIPG. With excellent electrochemical performance, the pocket, disposable, cost-effective, flexible three-electrode constructed a novel sensing platform to detect sulfamethoxazole (SMZ) at a lower electrochemical oxidation potential of 0.56 V. The developed sensor displayed a good linear range from 0.5 to 362.5 µM, a low limit of detection was 0.011 µM and satisfactory recovery in the range of 97.5% − 101.3% in milk and lake water. This sensor was also applied for the detection of four other SAs (sulfanilamide, sulfapyridine, sulfadimidine, sulfisoxazole). This work will be valuable to the development of a simple, fast sensing platforms for outdoor detection and online monitoring in food safety and environmental safety areas.

Anomalous ferroelectricity and double-negative effects in bilayer hexagonal boron nitride

Recently, the presence of an out-of-plane ferroelectric polarization was experimentally confirmed in bilayer two-dimensional (2D) hexagonal boron nitride (h-BN). In this work, we employ first-principles computations to reveal that (i) such slidetronic ferroelectricity is electronic in nature and arises from in-plane interlayer translation, which is distinctive among 2D slidetronic ferroelectric materials; and (ii) there is a coexistence of a negative longitudinal piezoelectric effect and an out-of-plane negative Poisson's ratio in the ferroelectric state of bilayer h-BN, which we refer to as anomalous double-negative effects.

Band alignment of h-BN/β-Ga2O3 heterostructure grown via ion beam sputtering deposition

Integrating two-dimensional ultra-wide band gap hexagonal boron nitride (h-BN) on β-Ga2O3 surface into van der Waals heterostructures is of great interest for developing novel high-power devices and optoelectronic devices because of their unique properties. The energy band alignment at the heterointerface is critical for device design, however, the band alignment of h-BN/β-Ga2O3 heterojunction has not been investigated to date. In this work, the h-BN/β-Ga2O3 heterostructure is constructed by directly growing h-BN few-layer on the β-Ga2O3 single crystal substrate using ion beam sputtering deposition method. The high-quality few-layer h-BN with the abrupt interface and smooth surface allow for the accurate determination of band alignment at the h-BN/β-Ga2O3 heterointerface by X-ray photoelectron spectroscopies. The valence and conduction band offsets are determined to be 0.47 and 1.42 eV for the h-BN/β-Ga2O3 heterostructure, respectively, with a type-Ⅱ staggered band alignment. Furthermore, the electronic structures of h-BN/β-Ga2O3 heterostructure are also investigated experimentally and theoretically. These results indicate that the h-BN/β-Ga2O3 heterostructure has great potential in (opto)electronic devices.

Magnetically tunable and stable deep-ultraviolet birefringent optics using two-dimensional hexagonal boron nitride.

Birefringence is a fundamental optical property that can induce phase retardation of polarized light. Tuning the birefringence of liquid crystals is a core technology for light manipulation in current applications in the visible and infrared spectral regions. Due to the strong absorption or instability of conventional liquid crystals in deep-ultraviolet light, tunable birefringence remains elusive in this region, notwithstanding its significance in diverse applications. Here we show a stable and birefringence-tunable deep-ultraviolet modulator based on two-dimensional hexagonal boron nitride. It has an extremely large optical anisotropy factor of 6.5 × 10-12 C2 J-1 m-1 that gives rise to a specific magneto-optical Cotton-Mouton coefficient of 8.0 × 106 T-2 m-1, which is about five orders of magnitude higher than other potential deep-ultraviolet-transparent media. The large coefficient, high stability (retention rate of 99.7% after 270 cycles) and wide bandgap of boron nitride collectively enable the fabrication of stable deep-ultraviolet modulators with magnetically tunable birefringence.

Silver nanoparticles bridging segregated hexagonal boron nitride networks for enhancing the thermal conductivity of polystyrene composites

The segregated heat conductive networks constructed from two-dimensional (2D) hexagonal boron nitride (hBN) are promising in improve the thermal conductivity of polymer-based thermal management materials (TMMs), however, the high interfacial thermal resistance between adjacent hBN sheets remains challenging. Hence, high-performance polystyrene (PS)-based composites with continuous segregated hBN@AgNP networks were designed and prepared by hot-pressing method. Benefited from the modification and bridge effect of AgNP, the interconnected hBN exhibited significantly reduced contact thermal resistance, which provide the effective pathways for phonon conduction. The as-fabricated PS/hBN@AgNP composite exhibited high thermal conductivity (1.17 and 1.53 W/mK, 30 wt%) corresponding to enhancements of 738.4% and 989.9% compared with neat PS, significantly higher than random PS/hBN and composites without AgNP in the segregated structure. Therefore, this work provides a strategy for the preparation of high-performance polymer-based TMMs by reducing the interfacial thermal resistance in interconnected filler network.

Super-hydrophilic SERS sensor with both ultrahigh activity and exceptional 3D spatial uniformity for sensitive detection of toxic pollutants

The powerful surface-enhanced Raman scattering spectroscopy (SERS) practical technique is not only related to signal enhancement but also heavily dependent on spatial uniformity. Different from the routine hydrophobic substrates, an extraordinary super-hydrophilic SERS sensor is established by loading plasmonic Au nanoparticles (NPs) on two-dimensional (2D) hexagonal boron nitride (h-BN) then the hybrids uniformly are grafted into 3D bacterial nanocelluloses (BNCs). The Au NPs@h-BN/BNCs exhibit a remarkably high SERS activity with a limit of detection (LOD) of dyes at 0.1 femtomole (fM) level (∼10-16 M). It is attributed to the strong synergistic coupling effect between highly dense Au NPs and modified h-BN as well as multiple light-scattering effects in 3D porous supports. More importantly, the distinctive advantage is further highlighted by the exceptional 3D spatial SERS uniformity with relative standard deviation (RSD) less than 3.4% along both horizontal and vertical directions. The unique feature is due to the 3D homogeneous distribution of the injected colloidal analytes via ultra-rapid liquid diffusion throughout the entire body with superior wettability. Besides, the flexible SERS substrates with excellent easy-tailorable mechanical properties can be robust enough to withstand the portable operations in real-world scenarios. These competitive merits are particularly beneficial for reliable SERS quantitative surveillance.

Large-Scale Synthesis h-BN Films on Copper-Nickel Alloy by Atmospheric Pressure Chemical Vapor Deposition

Due to its high thermal and chemical stability, excellent dielectric properties, unique optical properties, corrosion resistance, and oxidation resistance, the two-dimensional hexagonal boron nitride (h-BN) is often used in a thermal conductor protective layer in deep ultraviolet light-emitting detector fields. However, due to the complicated growth conditions of h-BN, it is often necessary to prepare h-BN by the CVD method in a high vacuum environment, which is limited to a certain extent in terms of film size and production cost. In order to solve this problem, we proposed a method to prepare h-BN thin films by atmospheric CVD (APCVD). This method does not need a vacuum environment, which reduces energy consumption and cost, and makes the operation simpler and the experimental environment safer. The preparation of high-quality h-BN film was carried out using a Cu-Ni alloy as the growth substrate. The growth process of h-BN film was studied, and the influence of growth parameters on the structure of the h-BN film was explored. The morphological features and elemental composition pairs of the samples were characterized and analyzed, which confirmed that the high-quality h-BN film could be successfully grown on the Cu-Ni alloy substrate by APCVD. The UV detection device prepared by using the prepared h-BN film as the photoresponse material had good photoresponse characteristics and performance stability. It provides a new idea for the low-cost preparation of large-scale h-BN.

Tailoring the Emission Wavelength of Color Centers in Hexagonal Boron Nitride for Quantum Applications.

Optical quantum technologies promise to revolutionize today’s information processing and sensors. Crucial to many quantum applications are efficient sources of pure single photons. For a quantum emitter to be used in such application, or for different quantum systems to be coupled to each other, the optical emission wavelength of the quantum emitter needs to be tailored. Here, we use density functional theory to calculate and manipulate the transition energy of fluorescent defects in the two-dimensional material hexagonal boron nitride. Our calculations feature the HSE06 functional which allows us to accurately predict the electronic band structures of 267 different defects. Moreover, using strain-tuning we can tailor the optical transition energy of suitable quantum emitters to match precisely that of quantum technology applications. We therefore not only provide a guide to make emitters for a specific application, but also have a promising pathway of tailoring quantum emitters that can couple to other solid-state qubit systems such as color centers in diamond.

Enhancing polysulfone nanocomposite membrane heavy-metal-removal performance using an amine-functionalized separation layer with 3D nanonetworks

Two-dimensional (2D) hexagonal boron nitride (h-BN) nanosheet is a promising nanomaterial that may be used in the removal of heavy metals from wastewater. However, its large-scale production presents challenges related to time consumption, low yield and high cost. Herein, a novel and simple strategy is proposed to achieve the effective exfoliation of bulk h-BN during the membrane fabrication by intercalating with amino group-functionalized carbon nanotubes (CNT-NH) and an amphiphilic sodium styrene-maleic anhydride copolymer (SMANa). Significantly, an organic/inorganic 3D nanonetwork was formed in the dope solution, which stemmed from combined intercalation and hydrogen bonding, and then segregated onto the surface during the phase inversion. Subsequently, an amine-functionalized separation layer with 3D nanonetworks was constructed on the membrane surface by crosslinking polyethyleneimine (PEI) and glutaraldehyde (GA). The optimized membrane exhibited extremely high rejections of Cu2+ (99.6%), Pd2+ (98.6%), Ni2+ (98.4%) and Cd2+ (98.5%), together with a high permeate flux (30–40 L/m2·h·bar) at 0.1 MPa, during the simulated filtration of wastewater containing various heavy metal ions. Results on membrane surface chemical composition, zeta potential and molecular weight cut off (MWCO) indicate that the separation mechanism stems from a synergistic effect between electrostatic repulsion and size sieving.