Structure Of Hexagonal Boron Nitride Research Articles

Luminescent boron carbon oxynitride phosphors: a cost-efficient strategy to boost solar cell spectral responsiveness

The incorporation of a Luminescent down-shifting (LDS) layer has emerged as a compelling approach for augmenting the light absorption sensitivity and power conversion efficiency of solar cells, particularly in the short-wavelength light spectrum. In this investigation, we propose the utilization of low-cost, environmentally benign Boron carbon oxynitride (BCNO) phosphors as a viable material for the enhancement of solar radiation absorption in the ultraviolet-blue range. We synthesized BCNO phosphors through a combustion method and conducted a comprehensive analysis of the structural and spectral attributes concerning the impact of temperature. The synthesized boron carbon oxynitride phosphors exhibit a hexagonal boron nitride structure, with an irregular shape and an average particle size of 2447.9 nm. The analysis of photoluminescence spectra reveals that BCNO phosphors effectively capture photons within the 300–500 nm wavelength range and subsequently re-emit them at longer wavelengths. This phenomenon aligns with the overarching goal of optimizing solar cell performance, as it is in the longer wavelength range that solar cells exhibit enhanced efficiency. These findings support the promising potential of BCNO phosphors as a compelling choice for deployment as an LDS layer material on the periphery of solar cells. By facilitating increased photon absorption in the short-wavelength region, BCNO phosphors have the capacity to significantly enhance device performance.

Easy preparation of porous boron nitride with excellent cycle stability for water purification

The structure of hexagonal boron nitride (h-BN) is similar to that of graphite, which has strong reducibility and adsorbability. In this study, boron nitride precursor was made using melamine as a nitrogen source and boric acid as a source of boron under hydrothermal conditions, and the precursor was subsequently burned to create porous BN at a certain temperature. Effect of NH4HCO3 content on the porosity and structure of the materials was investigated to improve the adsorption performance of h-BN. The morphology, composition and structure of the materials were analyzed by scanning electron microscope, energy spectrum analyzer and X-ray diffraction. According to the findings, BN has a greater specific surface area of 169.29 m2/g under optimal preparation circumstances, which increases its capacity for adsorption. When the concentration of methylene blue was 200 mg/L, the adsorption capacity of the material reached to 377 mg/g in 120 min. In addition, the remarkable anti-oxidation and structural stability of the employed BN materials make it simple to regenerate them through burning or ethanol elution. In five cycles, the adsorption performance did not decrease.

Preparation and properties of AlN–BN composite ceramics through combined addition of c-BN and h-BN

“Lamellar” structure of hexagonal boron nitride (h-BN) is prone to the formation of pore defects and the establishment of directional alignment during sintering process, which lead to AlN/BN composite ceramics exhibiting low density and anisotropic flexural strength (σ≡, σ⊥, and σ∥) (Orientation Bias Index |IOP| ≫ 1); these factors greatly limit the application of AlN/BN composite ceramics. In this work, cubic boron nitride (c-BN) was introduced as a reinforcing phase into AlN/BN composite ceramics, which were prepared via hot pressing sintering, and the effects of c-BN addition on microstructure, mechanical properties, and anisotropy were systematically investigated. The results show that during hot pressing sintering, c-BN phase changes into h-BN with “onion” structure, which effectively fills pore defects and reduces the degree of alignment of h-BN. |IOP| of sample 0C25H (170) continues to decrease with the increase in c-BN amount up to 2.7%. and anisotropy is significantly reduced. With the increase in c-BN amount, σ≡ and σ⊥ first increase and then decrease, while σ∥ only increases. σ≡ and σ⊥ reach the highest values (σ≡ = 362 MPa and σ⊥ = 340 MPa) at a c-BN amount of 15% (sample 15C10H); these values are 20.5% and 15.6% higher than those of sample 0C25H (no c-BN added), respectively, and σ∥ reaches the highest value (σ∥ = 239 MPa) for sample 25C0H, which is 136.6% higher than that of sample 0C25H. Sample 25C0H exhibits the highest σ∥ value (239 MPa), which is 136.6% higher than that of sample 0C25H. The onion-like h-BN formed by phase transformation of c-BN not only provides significant toughening to AlN/BN composite ceramics but can also effectively reduce anisotropy in their mechanical properties.

Fully Recyclable Covalent Adaptable Network Composite with Segregated Hexagonal Boron Nitride Structure for Efficient Heat Dissipation

As electronics become smaller and denser in function, lighter polymer composites with high thermal conductivity (TC) have been increasingly developed as heat-dissipating materials. Since the polymer matrix exhibits a vanishingly low TC compared with that of the filler, the composite TC is determined by the heat conduction pathway formed along the interconnected filler networks. In this context, a high composite TC can be obtained by increasing the filler loading up to the maximum filler packing limit. However, a tradeoff between the composite weight and TC prohibits a constant increase in the filler loading. To this end, a highly networked but heat-processable poly(β-amino ester) covalent adaptable network (CAN) based on catalyst-free transesterification and a dynamic aza-Michael reaction is synthesized as a matrix to realize both a high composite TC and low density. Owing to the unique malleable characteristic of the CAN, conductive filler networks (or a segregated filler structure) are formed along the CAN domain interfaces upon simple heat-pressing a powder mixture of the CAN and hexagonal boron nitride (hBN). The resulting composite exhibits an exceptionally high TC of 13.5 W/mK at a low density of 1.75 g/cm3. The TC value corresponds to 197% of an identical CAN composite but with randomly dispersed hBN. To further highlight the versatility of the CAN matrix, ecofriendly composite recycling through reprocessing along with filler recovery by depolymerizing the matrix in heated water without using any external catalysts is also demonstrated.

Kagome network of miniband-edge states in double-aligned graphene–hexagonal boron nitride structures

Twistronic heterostructures have recently emerged as a new class of quantum electronic materials with properties determined by the twist angle between the adjacent two-dimensional materials. Here, we study moir\'e superlattice minibands in graphene (G) encapsulated in hexagonal boron nitride (hBN) with an almost perfect alignment with both the top and bottom hBN crystals. We show that, for such an orientation of the unit cells of the hBN layers that locally breaks inversion symmetry of the graphene lattice, the hBN/G/hBN structure features a kagome network of topologically protected states with energies near the miniband edge, propagating along the lines separating the areas with different miniband Chern numbers.

Simulations of spatially and angle-resolved vibrational electron energy loss spectroscopy for a system with a planar defect

Recent developments in experiments with vibrational electron energy loss spectroscopy (EELS) have revealed spectral shape variations at spatial resolutions down to sub-atomic scale. Interpretation in terms of local phonon density of states enables their qualitative understanding, yet a more detailed analysis is calling for advances in theoretical methods. In Zeiger and Rusz, Phys. Rev. Lett. 124, 025501 (2020) we have presented a frequency resolved frozen phonon multislice method for simulations of vibrational EELS. Detailed simulations for a plane wave electron beam scattering on a vibrating hexagonal boron nitride are presented in a companion manuscript (Zeiger and Rusz, arXiv:2104.03197). Here we present simulations of vibrational EELS assuming a convergent electron probe of nanometer size and atomic size on a hexagonal boron nitride structure model with a planar defect. With a nanometer beam we observe spectral shape modifications in the presence of the defect, which are correlated with local changes of the phonon density of states. With an atomic size electron beam, we observe the same, although with better contrast. In addition, we observe atomic level contrast and sub-atomic scale spectral shape modifications, which are particularly strong for small detector collection angles.

Controllable electronic effect via deep eutectic solvents modification for boosted aerobic oxidative desulfurization

Tuning electronic structure of metal catalysts by organic modification is an efficient strategy for domination of their catalytic performances. However, the electronic structure tuning over metal-free catalysts remains to be a challenge. In this work, an organic modification strategy with deep eutectic solvents (DES) as organic modifier over metal-free catalysts was proposed to tune the electronic structure of hexagonal boron nitride (h-BN). Detailed experiments showed that the electronic properties of h-BN was rationally tuned by the introduced DES, in which the B atoms in h-BN were positively charged. Meanwhile, the modulated electronic properties can lower the activation energy for the generation of hydroxyl radicals (•OH) from hydrogen peroxide (H2O2), thus leading to a promoted catalytic performance in oxidative desulfurization. This finding may pave a new strategy for tuning metal-free catalysts with highly-active catalytic performance.

Reliable and Tunable Elastic Interface States in Soft Metamaterials

Tunable elastic interface states in soft materials have been successfully observed in experiments recently, which provides great application potentials in dynamic vibration energy‐harvesting devices or mechanical control of soft robotics. However, the generation of the proposed tunable elastic interfacial states relies on two differently deformed soft periodic phononic crystals, and complicated mechanical loadings and external installations are inevitable, which are indeed difficult to generate and manipulate. Herein, the dynamically adjustable elastic localized states are implemented in a more reliable and convenient way, which is readily modulated simply through compressing or stretching a mirror‐symmetric metamaterial composite simultaneously. The primitive cell of the soft periodic structures is inspired by the symmetry‐broken hexagonal boron nitride structure. Repeatable and reversible frequency shift and switch on–off characteristics of the interface states are observed during the dynamical regulation process, which is used in the dynamic elastic filter or divider devices. The feasibility of utilizing mirror operation to produce the interface states is clearly demonstrated, which effectively reduces the geometry and configuration design during the manufacturing process. The mirror method is also generalized to other periodic photonic and phononic crystals for the generation of additional multiple tunable interface states.

Mechanical properties of hexagonal boron nitride monolayers: Finite element and analytical predictions

The mechanical response of two-dimensional nanostructures may be significantly affected by their size. In this work, a molecular structural mechanics model is developed and is implemented in order to predict the nanomechanical behavior and calculate the corresponding elastic properties of hexagonal boron nitride sheets and describe their size-dependence. The finite element approach utilizes appropriate spring-like elements for the modeling of interactions between atoms within the hexagonal boron nitride structure, the stiffness constants of which are obtained by the molecular mechanics theory. Adopting conventional finite element techniques, the global stiffness matrix of the structure of a desired sheet size can be assembled. Applying appropriate boundary conditions, the governing equilibrium static equation can be solved and the elastic mechanical properties as Young’s modulus, shear modulus, and Poisson’s ratio of the structure can be calculated. Fitting the results of the mechanical properties calculated by the finite element analysis, analytical–empirical equations are proposed for their direct prediction for an hexagonal boron nitride sheet having the size parameters of the structure as independent variables.

Forming the GaN Nanocrystals on the Graphene-Like g-AlN and g-Si3N3 Surface

The formation of GaN nanocrystals on the graphene-like AlN (g-AlN) modification and graphene-like (g-Si3N3) silicon nitride by ammonia molecular beam epitaxy has been investigated. It has been found that the GaN growth on the g-Si3N3 surface leads to the formation of misoriented nanocrystals. During the GaN growth on the g-AlN surface, the epitaxial growth of similarly oriented GaN quantum dots of the graphite-like modification has been observed. The lattice parameters and energy structure of two graphite-like GaN modifications with the alternating AB (graphite structure) and AA+ (hexagonal boron nitride structure) layers have been calculated.

Tunable color emission of transparent boron nitride nanophosphors towards anti-counterfeiting application

Water-dispersible transparent nanophosphors based on boron nitride nanoparticles were prepared by pyrolysis of boric acid-melamine molecular adduct. Intense blue-green emission was activated by concomitant incorporation of Ce3+ and Tb3+ when excited by near-UV light. Color emission from the nanophosphor was modified from blue to green by tuning atomic ratio of Ce3+ and Tb3+ without disturbance of crystal structure of hexagonal boron nitride. The intense green emission with high quantum efficiency (∼0.74) was realized in BN: Ce3+,Tb3+ phosphors at [Tb3+]/[Ce3+] = 0.62. However, further addition of Tb3+ ions induced formation of non-luminescent TbBO3 phase and mitigation of luminescence intensity. From the steady state and time-resolved photoluminescence spectra, the luminescence of Ce3+ was quenched with Tb3+ by energy transfer process of efficiency approaches to unity. Transparent thin films were deposited from aqueous colloidal solution of nanophosphors on various substrates to demonstrate high transparency of phosphor thin films feasible to anti-counterfeiting application which requires concealment and identification by naked eyes. Extreme sustainability of nanophosphors in high temperature environment was also confirmed from thermal treatment at 1200 °C for 5 h in air.

Формирование нанокристаллов GaN на поверхности графеноподобных g-AlN и g-Si-=SUB=-3-=/SUB=-N-=SUB=-3-=/SUB=-

In this work, the GaN nanocrystals formation on a graphene-like modification of AlN (g-AlN) and graphene-like silicon nitride (g-Si3N3) by ammonia molecular beam epitaxy was studied. The GaN growth on the g-Si3N3 surface was found to result in the misoriented nanocrystals formation. With the GaN growth on the g-AlN surface, epitaxial growth of the equally oriented GaN quantum dots with graphite-like modification was observed. The lattice parameters and the energy structure of two GaN graphite-like modifications with alternating layers AB (graphite structure) and AA’ (hexagonal boron nitride structure) were calculated. This work was supported by the Russian Foundation for Basic Research (№ 17-02-00947-Бел_а and 18-52-00008-а). This work was supported by the Belarusian Republican Foundation for Basic Research in the framework of the joint project Ф18Р-234.

Tuning the electronic structure of hexagonal boron nitride by carbon atom modification: a feasible strategy to reduce sliding friction

Essentially, the interlayer interaction of the two-dimensional (2D) layered material is the overlap of electron clouds outside the nucleus atomically. When the layered material slides between layers, the contribution of electrons to the interlayer interaction is dominant, so it is expected to modify the electronic configuration of the system to change the interlayer interaction for the purpose of reducing interlayer sliding friction. In this paper, using density functional theory (DFT) calculation, we report an effective method to reduce the sliding friction between the two-layer hexagonal boron nitride layers for changing the hexagonal boron nitride electronic structure via introducing carbon. Research results indicate that the increase of the potential energy fluctuation along the sliding path increases with the increase of the load, which is caused by the difference of the degree of interlayer interaction on the sliding path with the increase of the load; at the same time, we found that the appearance of C at the B or N position can promote the interlayer charge transfer to different extents (B position is better than N position) in the BN/BN bilayer, and then produce the effect of reducing the energy barrier on the sliding path between the layers.

Modifying the geometric and electronic structure of hexagonal boron nitride on Ir(111) by Cs adsorption and intercalation

Epitaxial hexagonal boron nitride on Ir(111) is significantly modified by adsorption and intercalation of alkali-metal atoms. Regarding geometry, intercalation lifts the two-dimensional layer from its substrate and reduces the characteristic corrugation imprinted by direct contact with the metal substrate. Moreover, the presence of charged species in close proximity to the hexagonal boron nitride (hBN) layer strongly shifts the electronic structure (valence bands and core levels). We used scanning tunneling microscopy, low-energy electron diffraction, x-ray photoelectron spectroscopy (XPS), and the x-ray standing wave technique to study changes in the atomic structure induced by Cs adsorption and intercalation. Depending on the preparation, the alkali-metal atoms can be found on top and underneath the hexagonal boron nitride in ordered and disordered arrangements. Adsorbed Cs does not change the morphology of hBN/Ir(111) significantly, whereas an intercalated layer of Cs decouples the two-dimensional sheet and irons out its corrugation. XPS and angle-resolved photoelectron spectroscopy reveal a shift of the electronic states to higher binding energies, which increases with increasing density of the adsorbed and intercalated Cs. In the densest phase, Cs both intercalates and adsorbs on hBN and shifts the electronic states of hexagonal boron nitride by 3.56 eV. As this shift is not sufficient to move the conduction band below the Fermi energy, the electronic band gap must be larger than 5.85 eV.

Implications of heterostructural alloying for enhanced piezoelectric performance of (Al,Sc)N

An understanding of the heterostructural implications on alloying in the aluminum nitride-scandium nitride system (${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$) can highlight opportunities and design principles for enhancing desired material properties by leveraging nonequilibrium states. The fundamental thermodynamics, and therefore composition- and structure-dependent mechanisms, underlying property evolution in this system have not been fully described, despite significant recent efforts driven by interest in enhanced piezoelectric performance. Practical realization of these enhanced properties, however, is hindered by the strong driving thermodynamic driving force for phase separation in the system, highlighting the need for increased study into the role of heterostructural alloying on the thermodynamics and composition-structure-property relationships in this system. With this need in mind, ab initio computed alloy thermodynamics and properties are compared to combinatorial thin-film synthesis and characterization to develop a more complete picture of the structure and property evolution across the ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$ composition space. The combination of structural frustration and a flattened free-energy landscape lead to substantial increases in electromechanical response. The energy scale of alloy metastability is found to be much larger than previously reported, helping to explain difficulties in achieving homogeneous materials with high scandium concentration. Scandium substitution for aluminum softens the wurtzite crystal lattice, and energetic proximity to the competing hexagonal boron-nitride structure enhances the piezoelectric stress coefficient. Overall, this work provides insight into the understanding of the structure-processing-property relationships in the ${\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Sc}}_{x}\mathrm{N}$ system, suggests material design strategies for even greater property enhancements, and demonstrates the increased property tunability and underexplored nature of nonequilibrium heterostructural alloys.

Resonant X-ray Emission of Hexagonal Boron Nitride.

The electronic structure of hexagonal boron nitride (h-BN) is explored using measurements of x-ray absorption and resonant inelastic x-ray scattering (RIXS) at the nitrogen K edge (1s) in tandem with calculations using many-body perturbation theory within the GW and Bethe-Salpeter equation (BSE) approximations. Our calculations include the effects of lattice disorder from phonons activated thermally and from zero point energy. They highlight the influence of disorder on near-edge x-ray spectra.

Influence of point defects on the near edge structure of hexagonal boron nitride

Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor with applications including gate insulation layers in graphene transistors, far-ultraviolet light emitting devices and as hydrogen storage media. Due to its complex microstructure, defects in hBN are challenging to identify. Here, we combine x-ray absorption near edge structure (XANES) spectroscopy with ab initio theoretical modeling to identify energetically favorable defects. Following annealing of hBN samples in vacuum and oxygen, the B and N $K$ edges exhibited angular-dependent peak modifications consistent with in-plane defects. Theoretical calculations showed that the energetically favorable defects all produce signature features in XANES. Comparing these calculations with experiments, the principle defects were attributed to substitutional oxygen at the nitrogen site, substitutional carbon at the boron site, and hydrogen passivated boron vacancies. Hydrogen passivation of defects was found to significantly affect the formation energies, electronic states, and XANES. In the B $K$ edge, multiple peaks above the major $1s$ to ${\ensuremath{\pi}}^{*}$ peak occur as a result of these defects and the hydrogen passivated boron vacancy produces the frequently observed doublet in the $1s$ to ${\ensuremath{\sigma}}^{*}$ transition. While the N $K$ edge is less sensitive to defects, features attributable to substitutional C at the B site were observed. This defect was also calculated to have mid-gap states in its band structure that may be responsible for the 4.1-eV ultraviolet emission frequently observed from this material.

A facile strategy to simultaneously exfoliate and functionalize boron nitride nanosheets via Lewis acid-base interaction

The structure of hexagonal boron nitride (h-BN) is similar to that of graphite and its practical application in polymer composites is blocked by incompatible surface nature. Here, we presented a facile method to simultaneously exfoliate and organically modify h-BN nanosheets with Lewis acid-base interaction. Due to the affinity of the nitrogen atoms, the synthesized nucleophilic flame retardant (Lewis bases) bonded with boron atoms of h-BN nanosheets (Lewis acids) and facilitated the direct exfoliation of bulk h-BN. The structure and morphology of flame retardant functionalized BN (f-BN) nanosheets were well characterized. Then performances of f-BN nanosheets as advanced nanofillers in TPU composites were investigated deeply. As confirmed by SEM, XRD, and TEM, f-BN nanosheets were well-dispersed in the TPU matrix and formed strong interfacial interaction with the polymer matrix. The dramatic barrier effect of f-BN nanosheets and intrinsic flame retardant of synthesized phosphonated polyethyleneimine (P-PEI) significantly improved thermal stability, fire safety, and smoke suppression of TPU composites. By adding 5.0wt% f-BN nanosheets, the maximum mass loss rate, peak heat release rate (pHRR), and smoke produce rate (SPR) were decreased by 20.8 %, 68.0%, and 53.6%, respectively. Moreover, the mechanical properties of TPU composites were enhanced. The self-stiffen effect of TPU composites was successfully achieved: after 108k dynamic compressive cycles, the storage modulus of TPU/f-BN-5.0 exhibits a 13.8% increase. Based on Lewis acid-base interaction, designing a nucleophilic flame retardant to simultaneously exfoliate and functionally modify h-BN nanosheets offers a promising avenue for the preparation of functionalized BN nanosheets, thus broadening its practical application in the polymer material field.

Structure and Growth of Hexagonal Boron Nitride on Ir(111).

Using the X-ray standing wave method, scanning tunneling microscopy, low energy electron diffraction, and density functional theory, we precisely determine the lateral and vertical structure of hexagonal boron nitride on Ir(111). The moiré superstructure leads to a periodic arrangement of strongly chemisorbed valleys in an otherwise rather flat, weakly physisorbed plane. The best commensurate approximation of the moiré unit cell is (12 × 12) boron nitride cells resting on (11 × 11) substrate cells, which is at variance with several earlier studies. We uncover the existence of two fundamentally different mechanisms of layer formation for hexagonal boron nitride, namely, nucleation and growth as opposed to network formation without nucleation. The different pathways are linked to different distributions of rotational domains, and the latter enables selection of a single orientation only.

Energetics and Electronic Structure of h-BN Nanoflakes.

We studied the energetics and electronic structure of hexagonal boron nitride (h-BN) nanoribbons with hydrogenated and clean edges with respect to the detailed edge shapes using density functional theory. Our calculations showed that the stability of h-BN edges strongly depends on the edge termination. In the case of hydrogenated edges, the formation energy is constant for all edge angles ranging from armchair to zigzag, indicating that h-BN may exhibit rich variation in their edge atomic arrangements under static conditions. The hydrogenated h-BN nanoribbons are insulators with an energy gap of 4 eV irrespective of edge shape, in which the lowest branch of the conduction band exhibits nearly free electron states nature distributed in the vacuum region outside the ribbons. In contrast, the formation energy of h-BN nanoribbons with clean edges monotonically increases as the edge angle is changed from armchair to zigzag. Our analysis reveals that the increase of density of states at the Fermi level arising from dangling bond states leads to this monotonic increase of edge formation energy in h-BN nanoribbons with clean edges.