Crystalline Hexagonal Boron Nitride Research Articles

Micro- and Macrostructures of Aligned Boron Nitride Nanotube Arrays.

Boron nitride nanotubes (BNNTs) possess a broad range of applications because of several engineering-relevant properties, including high specific strength and stiffness, thermal stability, and transparency to visible light. The morphology of these nanoscale fibers must be controlled to maximize such properties, which can be achieved by synthesizing long aligned arrays of crystalline hexagonal boron nitride (hBN) nanotubes. Herein, we synthesize high-quality millimeter length, vertically aligned (VA-) BNNTs using free-standing carbon nanotube (CNT) arrays as scaffolds. In addition to high optical transparency of the VA-BNNTs, we also demonstrate several micro- and macrostructures of BNNTs via patterning and/or postprocessing of the arrays, including engineering of either disconnected or interconnected tubes in VA-, horizontally aligned (HA-), or coherently buckled BNNTs. The internanotube spacings and interconnections between aligned BNNT can thus be tailored to create BN macrostructures with complex shapes and advantaged morphologies for hierarchical materials and devices.

Mesoporous boron nitride in contact with water - Chemical stability and adsorption properties

In the context of adsorptive water treatment, exhaust gas treatment, gas storage, or catalysis, porous boron nitride is gaining in scientific attention due to its versatile properties. The material is above all characterized by its high chemical and thermal stability. However, it has recently been shown that porous boron nitride is highly water-susceptible, which severely limits its application in non-inert atmospheres. So far, solely the influence of chemical composition on water stability has been investigated in literature. In the present work, the influence of different pore sizes is considered. Two different boron nitrides, a mesoporous and a micro-mesoporous one, are investigated with regard to their stability in air, water vapor, and liquid water for multiple weeks. Using different analytical methods, it can be shown that the stability of porous boron nitride strongly depends on pore size and structure. We demonstrate that the mesoporous boron nitride synthesized in this work has much higher water stability than micro-mesoporous boron nitrides. The mesoporous material showed minor to no changes in its chemical structure and isothermal shape when exposed to water vapor and air. We suggest that a higher amount of crystalline hexagonal boron nitride leads to increased stability. This conclusion opens pathways to improve the water stability of porous boron nitride in order to make it suitable for non-inert gas phase adsorption applications.

Defect-Regulated Frustrated-Lewis-Pair Behavior of Boron Nitride in Ambient Pressure Hydrogen Activation.

The construction of heterogeneous frustrated Lewis pairs (FLPs) with performance comparable to or surpassing the homogeneous counterparts in H2 activation is a long-standing challenge. Herein, sterically hindered Lewis acid ("B" center) and Lewis base ("N" center) sites were anchored within the rigid lattice of highly crystalline hexagonal boron nitride (h-BN) scaffolds. The active sites were created via precision defect regulation during the molten-salt-involved (NaNH2 and NaBH4) h-BN construction procedure. The as-afforded h-BN scaffolds achieved highly efficient H2/D2 activation and dissociation under ambient pressure via FLP-like behavior, and attractive catalytic efficiency in hydrogenation reactions surpassing the current heterogeneous analogues.

Role of sublimation kinetics of ammonia borane in chemical vapor deposition of uniform, large-area hexagonal boron nitride

Hexagonal boron nitride (h-BN) is a critical 2D insulator used as a substrate, gate dielectric, or encapsulation layer for graphene and other 2D materials and their van der Waals heterostructures. It is also promising as an active layer in single-photon emitters and other photonic devices. With the chemical formula H3N-BH3, ammonia borane is the most attractive precursor for up-scalable growth of large-area h-BN, using chemical vapor deposition given its stoichiometric B:N ratio, high stability under ambient conditions, nontoxicity, and high solubility in common solvents. Here, the synthesis of large-area (100 × 150 mm2) crystalline hexagonal boron nitride layers by thermal activation and decomposition of the precursor ammonia borane is presented. We describe two different reaction pathways for h-BN synthesis, providing evidence for dissimilarities in the sublimation kinetics of ammonia borane and how these differences critically influence the growth of h-BN. This understanding helps us accelerate h-BN production, reuse precursors, and reduce machine runtime, paving the way for upscalability. Moreover, our work provides a consistent unified view explaining the diverse deposition conditions reported in the literature for h-BN grown by CVD using ammonia borane as a precursor.

Boron Nitride Nanotube Impurity Detection and Purity Verification

Interest in the boron nitride nanotube (BNNT) composites has grown exponentially because of BNNTs’ unique properties. The vast majority of papers have used a BNNT material without identifying its purity. Impurities are known to misrepresent the BNNT concentration effect on performance properties and can have their own property effect on performance. A recent paper reported a major improvement in a biological application after purification. This demonstrates the need for a discussion of the optimum procedures to detect each impurity and quantify its degree of removal after a purification process. Here we evaluate the ability of thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction, and nonresonant Raman to detect BNNT’s boron, boron oxide, and varying forms of boron nitride impurities. We discover that unstacked boron nitride impurities are present and detected in BNNT using nonresonant Raman. Stacked crystalline hexagonal boron nitride (h-BN) is separately detected using X-ray diffraction. We show that, in contrast to aggressive high temperature purification methods that attack the boron nitride tubes, a low temperature hydrocarbon purification procedure removes both unstacked boron nitride impurities as well as stacked h-BN. We demonstrated the ability of nonresonant Raman spectra to detect and quantify the degree of removal of both forms of unstacked and stacked boron nitride impurities, of the ability of TGA to detect and quantitively verify the absence of boron, and of FTIR techniques to detect and quantitavely verify the degree of removal of boron oxide.

Heteroepitaxial growth of sp2-hybridized boron nitride multilayer on nickel substrates by CVD: the key role of the substrate orientation

sp2-hybridized boron nitride is identified as a strategic material for many purposes related to the integration of graphene and two-dimensional materials in devices and the fabrication of van der Waals heterostructures. Thus, it becomes mandatory to have scalable synthesis and characterization procedures for providing suitable and reliable boron nitride material according to these identified needs. We report here on the growth of sp2-hybridized boron nitride film on polycrystalline nickel substrate by chemical vapor deposition with borazine as precursor. We propose a complete study of the influence of the underlying nickel grain orientation on the BN structure layers, in terms of thickness, crystallographic orientation, domain size and stacking. We show the heteroepitaxial growth of continuous, single crystalline hexagonal boron nitride multilayer film on nickel (111)-like grains. We highlight its ABC stacking sequence with AB stacking faults and show how it impacts the Raman and cathodoluminescence spectra.

Combustion synthesis of hexagonal boron nitride nanoplates with high aspect ratio

High crystalline hexagonal boron nitride nanoplates with high aspect ratio of ~400 have been synthesized by combustion synthesis method through magnesiothermic reduction reaction between B2O3 and Mg in N2 pressure. The synthesized hexagonal boron nitride nanoplates were about 50 nm in thickness and larger than 20 μm in lateral size. The six-fold symmetric spots electron diffraction pattern of transmission electron microscopy shows that the nanoplate is well crystallized. Hexagonal boron nitride nanoplates grow via an Oswald ripening process and have larger lateral size when it was prepared with larger magnesium particles. High temperature liquid magnesium provides an important environment for the growth and crystallization of boron nitride. This work provides an effective way to achieve low-cost and large-scale preparation of high-quality boron nitride nanoplates.

Heat-Stable and Moisture-Proof CsPbBr3 Quantum Dots Under Dual Protection: Coordination of Non-Crystalline SiO2 and Crystalline h-BN Nanosheets

It’s hard for pure perovskite quantum dots (QDs) to maintain long-term and stable optical properties in practical conditions. Here, CsPbBr3 QDs were first stabilized in amorphous SiO2 and crystalline hexagonal boron nitride (h-BN) nanosheets. The transmission electron microscopy (TEM) images display the laminated structure consisting of CsPbBr3/SiO2 nanospheres and h-BN nanosheets. Besides, the photoluminescence (PL) intensity of CsPbBr3/SiO2/h-BN nanohybrid only has a slight decrease than CsPbBr3/SiO2 nanocomposite. But, dual protection of SiO2 and h-BN give perovskite QDs better humidity stability and thermal stability. Besides, CsPbBr3/SiO2/h-BN block exhibits excellent water-resistance. This work provides a novel method to accelerate the application of perovskite QDs in optoelectronic devices.

Construction of a Nanoporous Highly Crystalline Hexagonal Boron Nitride from an Amorphous Precursor for Catalytic Dehydrogenation.

Hexagonal boron nitride (h-BN) is regarded as a graphene analogue and exhibits important characteristics and vast application potentials. However, discovering a facile method for the preparation of nanoporous crystalline h-BN nanosheets (h-BNNS) is still a challenge. Herein, a novel and simple route for the conversion of amorphous h-BN precursors into highly crystalline h-BNNS was achieved through a successive dissolution-precipitation/crystallization process in the presence of magnesium. The h-BNNS has high crystallinity, high porosity with a surface area of 347 m2 g-1 , high purity, and enhanced thermal stability. Improved catalytic performance of crystalline h-BNNS was evidenced by its much higher catalytic efficiency in the dehydrogenation of dodecahydro-N-ethylcarbazole, compared with its amorphous h-BN precursor, as well as other precious-metal-loaded heterogeneous catalysts.

Construction of a Nanoporous Highly Crystalline Hexagonal Boron Nitride from an Amorphous Precursor for Catalytic Dehydrogenation

AbstractHexagonal boron nitride (h‐BN) is regarded as a graphene analogue and exhibits important characteristics and vast application potentials. However, discovering a facile method for the preparation of nanoporous crystalline h‐BN nanosheets (h‐BNNS) is still a challenge. Herein, a novel and simple route for the conversion of amorphous h‐BN precursors into highly crystalline h‐BNNS was achieved through a successive dissolution–precipitation/crystallization process in the presence of magnesium. The h‐BNNS has high crystallinity, high porosity with a surface area of 347 m2 g−1, high purity, and enhanced thermal stability. Improved catalytic performance of crystalline h‐BNNS was evidenced by its much higher catalytic efficiency in the dehydrogenation of dodecahydro‐N‐ethylcarbazole, compared with its amorphous h‐BN precursor, as well as other precious‐metal‐loaded heterogeneous catalysts.

Enhanced tribo-chemical properties of oxygen functionalized mechanically exfoliated hexagonal boron nitride nanolubricant additives

Two dimensional (2D) materials with layered lattice structure as nanofluid additives are useful to improve the tribological properties of metallic sliding interfaces. To enhance the tribological efficiency, the bulk crystalline hexagonal boron nitride (h-BN) powder sample was mechanically exfoliated by ball milling, and further processed through ultrasonication for de-aggregation. High resolution X-ray diffraction (HR XRD) and high resolution transmission electron microscopy (HR TEM) results clearly indicate the exfoliation of bulk h-BN into thinner two-dimensional (2D) crystalline sheets without creating noticeable structural defects. The topography of exfoliated nanosheets is well confirmed by atomic force microscopy (AFM). Oxygen functionalization into the h-BN nanosheets after the mechanical exfoliation was investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Oxygen functionalization of h-BN improved the cohesive compatibility between h-BN and commercial 10W30 lubricant oil for stable dispersion. Friction coefficient and wear of sliding metallic interfaces were reduced significantly in the presence of few layered exfoliated h-BN nanofluid as compared to neat lubricated oil. Micro- XPS and energy-disperse X-ray spectroscopy (EDX) analysis demonstrated the presence of adsorbed h-BN tribofilm in the metallic wear track. Thin 2D sheets of h-BN nanofluid was effective as an additive for low shear resistance under the tribo stressed condition which is the main reason for significant reduction in friction coefficient. Moreover, the enhanced wear resistance of exfoliated h-BN additives was explained by low shear resistance and high compressive/tensile strength of planer sheets which restricted the mechanical damage and protected the metallic interfaces against deformation and wear.

Single-step synthesis of crystalline h-BN quantum- and nanodots embedded in boron carbon nitride films

Herein we report on the synthesis and characterization of novel crystalline hexagonal boron nitride (h-BN) quantum- and nanodots embedded in large-area boron carbon nitride (BCN) films. The films were grown on a Cu substrate by an atmospheric pressure chemical vapour deposition technique. Methane, ammonia, and boric acid were used as precursors for C, N and B to grow these few atomic layer thick uniform films. We observed that both the size of the h-BN quantum/nanodots and thickness of the BCN films were influenced by the vaporization temperature of boric acid as well as the H3BO3 (g) flux over the Cu substrate. These growth conditions were easily achieved by changing the position of the solid boric acid in the reactor with respect to the Cu substrate. Atomic force microscope (AFM) and TEM analyses show a variation in the h-BN dot size distribution, ranging from nanodots (∼224 nm) to quantum dots (∼11 nm) as the B-source is placed further away from the Cu foil. The distance between the B-source and the Cu foil gave an increase in the C atomic composition (42 at% C–65 at% C) and a decrease in both B and N contents (18 at% B and 14 at% N to 8 at% B and 7 at% N). UV–vis absorption spectra showed a higher band gap energy for the quantum dots (5.90 eV) in comparison with the nanodots (5.68 eV) due to a quantum confinement effect. The results indicated that the position of the B-source and its reaction with ammonia plays a significant role in controlling the nucleation of the h-BN quantum- and nanodots. The films are proposed to be used in solar cells. A mechanism to explain the growth of h-BN quantum/nanodots in BCN films is reported.

Van der Waals epitaxial growth of atomically thin Bi₂Se₃ and thickness-dependent topological phase transition.

Two-dimensional (2D) atomic-layered heterostructures stacked by van der Waals interactions recently introduced new research fields, which revealed novel phenomena and provided promising applications for electronic, optical, and optoelectronic devices. In this study, we report the van der Waals epitaxial growth of high-quality atomically thin Bi2Se3 on single crystalline hexagonal boron nitride (h-BN) by chemical vapor deposition. Although the in-plane lattice mismatch between Bi2Se3 and h-BN is approximately 65%, our transmission electron microscopy analysis revealed that Bi2Se3 single crystals epitaxially grew on h-BN with two commensurate states; that is, the (1̅21̅0) plane of Bi2Se3 was preferably parallel to the (1̅100) or (1̅21̅0) plane of h-BN. In the case of the Bi2Se3 (2̅110) ∥ h-BN (11̅00) state, the Moiré pattern wavelength in the Bi2Se3/h-BN superlattice can reach 5.47 nm. These naturally formed thin crystals facilitated the direct assembly of h-BN/Bi2Se3/h-BN sandwiched heterostructures without introducing any impurity at the interfaces for electronic property characterization. Our quantum capacitance (QC) measurements showed a compelling phenomenon of thickness-dependent topological phase transition, which was attributed to the coupling effects of two surface states from Dirac Fermions at/or above six quintuple layers (QLs) to gapped Dirac Fermions below six QLs. Moreover, in ultrathin Bi2Se3 (e.g., 3 QLs), we observed the midgap states induced by intrinsic defects at cryogenic temperatures. Our results demonstrated that QC measurements based on h-BN/Bi2Se3/h-BN sandwiched structures provided rich information regarding the density of states of Bi2Se3, such as quantum well states and Landau quantization. Our approach in fabricating h-BN/Bi2Se3/h-BN sandwiched device structures through the combination of bottom-up growth and top-down dry transferring techniques can be extended to other two-dimensional layered heterostructures.

Unipolar Switching Behavior in Highly Crystalline Hexagonal Boron Nitride

ABSTRACTWe observed resistive switching in highly crystalline layered insulator hexagonal boron nitride (h-BN) under electric field in a nano-device configuration. Two distinct resistive states were observed in the 2D material heterostack. Electrical characterization using capacitance-voltage approach further revealed the role of h-BN as the active switching element. The switching behavior could be attributed to substitutional doping of h-BN under electric field present in the active region, possibly resulting in the formation of multi-element complex in which electrical conductivity depends on the amount of substituted dopant in the boron nitride crystal lattice. Since switching is observed independent of the direction of electric field, it is unipolar in nature. The observed memristance phenomenon in layered insulator may be potentially used in the form of NVM, providing possible direction to implement information storage or reconfigurable logic applications.

Continuous Growth of Hexagonal Graphene and Boron Nitride In-Plane Heterostructures by Atmospheric Pressure Chemical Vapor Deposition

Graphene-boron nitride monolayer heterostructures contain adjacent electrically active and insulating regions in a continuous, single-atom thick layer. To date structures were grown at low pressure, resulting in irregular shapes and edge direction, so studies of the graphene-boron nitride interface were restricted to the microscopy of nanodomains. Here we report templated growth of single crystalline hexagonal boron nitride directly from the oriented edge of hexagonal graphene flakes by atmospheric pressure chemical vapor deposition, and physical property measurements that inform the design of in-plane hybrid electronics. Ribbons of boron nitride monolayer were grown from the edge of a graphene template and inherited its crystallographic orientation. The relative sharpness of the interface was tuned through control of growth conditions. Frequent tearing at the graphene-boron nitride interface was observed, so density functional theory was used to determine that the nitrogen-terminated interface was prone to instability during cool down. The electronic functionality of monolayer heterostructures was demonstrated through fabrication of field effect transistors with boron nitride as an in-plane gate dielectric.

Graphene Field-Effect Transistors Based on Boron–Nitride Dielectrics

Two-dimensional atomic sheets of graphene represent a new class of nanoscale materials with potential applications in electronics. However, exploiting the intrinsic characteristics of graphene devices has been problematic due to impurities and disorder in the surrounding dielectric and graphene/dielectric interfaces. Recent advancements in fabricating graphene heterostructures by alternately layering graphene with crystalline hexagonal boron nitride (hBN), its insulating isomorph, have led to an order of magnitude improvement in graphene device quality. Here, recent developments in graphene devices utilizing boron-nitride dielectrics are reviewed. Field-effect transistor (FET) characteristics of these systems at high bias are examined. Additionally, existing challenges in material synthesis and fabrication and the potential of graphene/BN heterostructures for novel electronic applications are discussed.

Facile Synthesis of Large‐Area Ultrathin Hexagonal BN Films via Self‐Limiting Growth at the Molten B2O3 Surface

A self-limiting growth process based on the interface-controlled reaction of molten boron oxide (B2 O3 ) with ammonia (NH3 ) is demonstrated for the facile and lost-cost synthesis of ultrathin (20-30 nm) crystalline hexagonal boron nitride (h-BN) films over large areas. The as-grown h-BN films are of high quality, being densely continuous, uniform and smooth, and highly transparent over a broad wavelength range.

Suppression of thermally activated carrier transport in atomically thin MoS2 on crystalline hexagonal boron nitride substrates

We present the temperature-dependent carrier mobility of atomically thin MoS2 field-effect transistors on crystalline hexagonal boron nitride (h-BN) and SiO2 substrates. Our results reveal distinct weak temperature dependence of the MoS2 devices on h-BN substrates. The room temperature mobility enhancement and reduced interface trap density of the single and bilayer MoS2 devices on h-BN substrates further indicate that reducing substrate traps is crucial for enhancing the mobility in atomically thin MoS2 devices.

Synthesis and anti-oxidation performance of nanoflake-decorated boron nitride hollow microspheres

In this study, a novel boron nitride composite structure composed of nanoflake-decorated hollow microspheres is demonstrated. A paste-like mixture is firstly prepared by mixing amorphous boron with ferric nitrate ethanol solution, followed by heating at 1300°C in a flowing ammonia atmosphere for 5h. Both the nanoflakes and microspheres are composed of crystalline hexagonal boron nitride. The hollow spheres have outer diameters of 0.8–3μm with thickness of about 300nm, while the nanoflakes have thicknesses of 2–7nm and lengths of 10–100nm. Heating temperature is found to be crucial for the formation of this composite structure. The growth process consists of a vapor–liquid–solid growth stage of smooth hollow microspheres at lower temperature and a vapor–solid growth stage of nanoflakes on the surface of the microspheres at higher temperature. The boron nitride composite structure exhibits excellent anti-oxidation performance up to 900°C.

A general route for the convenient synthesis of crystalline hexagonal boron nitride micromesh at mild temperature

In this study, hexagonal boron nitride (h-BN) micromeshes with high crystallinity and yield have been prepared by B2O3, Mg and NaN3 in a stainless steel autoclave at 450 °C, which have diameters of up to 100 µm, thickness of ∼210 nm and pore size of 2.8 µm on average. A near-band edge emission at 217 nm of these micromeshes has been observed through the photoluminescence (PL) spectra. Their formation mechanism was considered through an oriented aggregation process, in which the in-situ produced MgO microparticles served as template. As B2O3 was substituted by other boron sources (such as H3BO3, Na2B4O7, NH4HB4O7 or Mg(BO2)2·H2O), h-BN micromeshes with different pore sizes could also be produced; therefore, it is a general way for the convenient synthesis of crystalline h-BN micromeshes. The as-obtained product is promising as a catalyst support, in chemical filtration and in separations under severe operating conditions.