Few-layer Hexagonal Boron Nitride Research Articles

Direct measurement of tensile mechanical properties of few-layer hexagonal boron nitride (h-BN)

Hexagonal boron nitride (h-BN) has excellent thermal conductivity and dielectric properties, which shows great potential for low-dimensional devices. However, mechanical properties of h-BN have not been comprehensively investigated through experiments. In this work, we conduct in situ direct tensile tests on freestanding single-crystal few-layer h-BN nanosheets with various layer numbers from 3 to 8, with an elaborate sample transfer and characterization protocol. Young's modulus of 573.8 ± 101.4 GPa and a tensile fracture strain up to 3.2% are revealed, which are comparable to its monolayer counterpart. Moreover, we find a tough-to-brittle transition in few-layer h-BN with the increase in layer number, which is attributed the interplay between the van der Waals interaction and in-plane covalent bonding. These findings could open up new possibilities in mechanical research of van der Waals materials and provide guidance for the design of h-BN-based devices and composites.

Synthesis of Few-Layer Hexagonal Boron Nitride for Magnetic Tunnel Junction Application.

Hexagonal boron nitride (hBN), a wide-gap two-dimensional (2D) insulator, is an ideal tunneling barrier for many applications because of the atomically flat surface, high crystalline quality, and high stability. Few-layer hBN with a thickness of 1-2 nm is an effective barrier for electron tunneling, but the preparation of few-layer hBN relies on mechanical exfoliation from bulk hBN crystals. Here, we report the large-area growth of few-layer hBN by chemical vapor deposition on ferromagnetic Ni-Fe thin films and its application to tunnel barriers of magnetic tunnel junction (MTJ) devices. Few-layer hBN sheets mainly consisting of two to three layers have been successfully synthesized on a Ni-Fe catalyst at a high growth temperature of 1200 °C. The MTJ devices were fabricated on as-grown hBN by using the Ni-Fe film as the bottom ferromagnetic electrode to avoid contamination and surface oxidation. We found that trilayer hBN gives a higher tunneling magnetoresistance (TMR) ratio than bilayer hBN, resulting in a high TMR ratio up to 10% at ∼10 K.

Functionalized boron nitride nanosheets and their polypropylene nanocomposites: influence on thermal, mechanical, rheological and wear behaviour

ABSTRACT The few-layer hexagonal boron nitride (hBN) nanosheets with hydroxyl (−OH) functional group at edges are synthesized using heat treatment of micron size hBN powder followed by sonication. These nanosheets are modified with a silane coupling agent, 3-Aminopropyl,triethoxysilane (APTES). The synthesized hBN nanosheets are mixed with the PP matrix using twin-screw extrusion followed by injection moulding. In case of APTES-modified hBN nanosheets incorporation in the PP matrix with PP-grafted maleic anhydride (PP-g-MA) shows the formation of an imide bond between the MA functional group of PP-g-MA and the amine group of APTES modified hBN nanosheets revealing compatibilization. The flow behaviour index values are lower for hBN nanosheets filled PP nanocomposites showing the improved processability from Rheological characterization. The few-layered – OH and APTES modified hBN nanosheet incorporation in the PP matrix improves the tensile and thermal properties. The incorporation of APTES modified hBN nanosheet in the PP matrix shows an increase in tensile modulus (18.1%) and tensile strength (8.1%) respectively. Wear studies show the incorporation of – OH and APTES modified hBN nanosheets in the PP matrix decreases the coefficient of friction and wear rate significantly. The wear rate at 4 min of wear testing for modified hBN nanosheet in the PP nanocomposites is ~ 96% less in comparison with neat PP. The wear mechanism is established from wear measurements and scanning electron microscopy (SEM) of worn-out wear surfaces of PP nanocomposites.

Study on the growth mechanism of monolayer and few-layer hexagonal boron nitride films on copper foil.

During the process of synthesizing h-BN on Cu foil via chemical vapor deposition (CVD), low-pressure CVD (LPCVD) typically synthesizes monolayer h-BN films, whereas atmospheric pressure CVD (APCVD) yields few-layer h-BN films. Herein, a growth mechanism for monolayer and few-layer h-BN on Cu foil is proposed using first-principles calculations: Cu(111) passivated h-BN hinders the diffusion of B and N atoms at the subsurface of Cu(111), preventing sufficient transportation of B and N atoms to the existing h-BN layer, thereby leading to the formation of monolayer h-BN in LPCVD. For APCVD, the edges of h-BN are passivated by H, which decreases the barrier energy for the diffusion of B and N atoms on the Cu(111) subsurface, and B and N atoms can easily migrate from the subsurface of Cu(111) to its surface, resulting in the nucleation of h-BN between the existing h-BN and Cu(111), and leading to the formation of few-layer h-BN films. This work provides a theoretical basis at the atomic scale for further understanding the growth of monolayer and few-layer h-BN films on Cu foil.

Antireflection Substrates for Determining the Number of Layers of Few-Layer Hexagonal Boron Nitride Films and for Visualizing Organic Monolayers

Antireflection Substrates for Determining the Number of Layers of Few-Layer Hexagonal Boron Nitride Films and for Visualizing Organic Monolayers

Phonon-Mediated Quasiparticle Lifetime Renormalizations in Few-Layer Hexagonal Boron Nitride.

Understanding the collective behavior of the quasiparticles in solid-state systems underpins the field of nonvolatile electronics, including the opportunity to control many-body effects for well-desired physical phenomena and their applications. Hexagonal boron nitride (hBN) is a wide-energy-bandgap semiconductor, showing immense potential as a platform for low-dimensional device heterostructures. It is an inert dielectric used for gated devices, having a negligible orbital hybridization when placed in contact with other systems. Despite its inertness, we discover a large electron mass enhancement in few-layer hBN affecting the lifetime of the π-band states. We show that the renormalization is phonon-mediated and consistent with both single- and multiple-phonon scattering events. Our findings thus unveil a so-far unknown many-body state in a wide-bandgap insulator, having important implications for devices using hBN as one of their building blocks.

Enhanced emission from hBN in sputtered microcavities.

In this observational study, we embed few-layer hexagonal boron nitride (hBN) inside a planar Fabry-Perot cavity fabricated using a pulsed DC magnetron sputtering system and show that the hBN retains its inherent visible range, defect-based luminescent properties following relatively energetic deposition processing. The observed surface-normal emission enhancement factor of ∼40 is in good agreement with theoretical predictions. We also found that embedded hBN subjected to a rapid thermal annealing treatment exhibits a cracking effect where the edges of the material glow distinctly brighter than adjacent regions. Our results might inform future efforts involving monolithic integration of hBN active layers.

Wet-jet milling exfoliated hexagonal boron nitride as industrial anticorrosive pigment for polymeric coatings

The incorporation of inorganic nanofillers into polymeric matrices represents an effective strategy for the development of smart coatings for corrosion protection of metallic substrates. In this work, wet-jet milling exfoliation was used to massively produce few-layer hexagonal boron nitride (h-BN) flakes as a corrosion-protection pigment in polyisobutylene (PIB)-based composite coatings for marine applications. This approach represents an innovative advance in the application of two-dimensional (2D) material-based composites as corrosion protection systems at the industrial scale. Although rarely used as an organic coating, PIB was selected as a ground-breaking polymeric matrix for our h-BN-based composite coating thanks to its excellent barrier properties. The optimization of the coating indicates that 5 wt.% is the most effective h-BN content, yielding a corrosion rate of the protected structural steel as low as 7.4 × 10−6 mm yr−1. The 2D morphology and hydrophobicity of the h-BN flakes, together with the capability of PIB to act as a physical barrier against corrosive species, are the main reasons behind the excellent anticorrosion performance of our composite coating.

High-performance flexible nanogenerators based on piezoelectric hBN- induced polyvinylidene fluoride nanofibers

Flexible piezoelectric nanogenerators (PENGs) are promising candidates for clean energy harvesting and powering electronics, especially for low-power modules. In this study, a PENG has been fabricated by incorporating functionalized mono-/few-layered hexagonal boron nitride nanosheets (F-hBNNS) in a polyvinylidene fluoride (PVDF) polymeric matrix. The synthesized 2D nanosheets are well characterized using different spectroscopic and microscopic methods. The analysis showed successful exfoliation of hBN, which indicated less than three layers of hBN after exfoliation and sulfonated and carboxylated functionalization on the surface. Also, the piezoresponse force microscopy (PFM) tests revealed a 3 times higher piezoelectric coefficient for F-hBNNS compared to that of the non-functionalized hBN with the same number of layers. The synthesized F-hBNNS with piezo-flexoelectric properties boosted the energy harvesting performance of the electrospun PVDF-based fibrous mats 5.5 times. The open-circuit output voltage test showed 23 V at 4 Hz with a power density of 1.93 μW/cm2 at a resistance loading of 16 MΩ in a simple finger tapping. The fabricated PENGs showed no decline in voltage output over 1000 cycles of bending-unbending of a robotic finger. It is proposed that the enhanced performance is due to (i) the synergistic effects of the flexoelectric and piezoelectric properties of synthesized F-hBNNS in the PVDF matrix and (ii) the manipulation of the crystalline structure of the PVDF chains, which is increased to 90% β-phase crystallinity with 49.03% total degree of crystallinity. This study shows the compatibility of the fabricated PENGs with the human body's biomechanics for being used as an efficient energy source for wearable devices with low power consumption.

Highly confined low-loss light transmission in linear array-enabled hybrid plasmonic waveguides

Deep subwavelength highly confined and long-range optical propagation is vital for photonics integration. However, the performance of the guided mode could be improved by the trade-off between light confinement and loss. Here, we demonstrate a high-performance hybrid waveguide consisting of a high-index nanowire separated from a linear array by a low-index dielectric gap. The array significantly achieves the plasmonic platform optimization based on a linear combination of few-layer graphene (FLG) and hexagonal boron nitride (hBN) layers. Through the hybridization of graphene plasmon polaritons and hyperbolic phonon polaritons mode, the resulting hybrid waveguide shows at least double times larger propagation distance and smaller mode area than the multilayer waveguide. Further, modulated by altering material configuration and geometric effects, the mode properties reveal that it is more flexible to adjust the optical transmission, along with a strong deep-subwavelength mode with low loss. Because of highly confined low-loss propagation, the hybrid waveguide is expected to be an excellent building block for various mid-infrared photonic integrated circuits. The present structure also has the potential to be extended to other FLGs, like magic-angle twisted bilayer graphene and trilayer graphene/hBN moiré superlattice.

The interplay of intra- and inter-layer interactions in bending rigidity of ultrathin 2D materials

Continuum mechanics break down in bending stiffness calculations of mono- and few-layered two-dimensional (2D) van der Waals crystal sheets, because their layered atomistic structures are uniquely characterized by strong in-plane bonding coupled with weak interlayer interactions. Here, we elucidate how the bending rigidities of pristine mono- and few-layered molybdenum disulfide (MoS2), graphene, and hexagonal boron nitride (hBN) are governed by their structural geometry and intra- and inter-layer bonding interactions. Atomic force microscopy experiments on the self-folded conformations of these 2D materials on flat substrates show that the bending rigidity of MoS2 significantly exceeds those of graphene or hBN of comparable layers, despite its much lower tensile modulus. Even on a per-thickness basis, MoS2 is found to possess similar bending stiffness to hBN and is much stiffer than graphene. Density functional theory calculations suggest that this high bending rigidity of MoS2 is due to its large interlayer thickness and strong interlayer shear, which prevail over its weak in-plane bonding.

Mobile Trions in Electrically Tunable 2D Hybrid Perovskites.

2D hybrid perovskites are currently in the spotlight of material research for light-harvesting and -emitting applications. It remains extremely challenging, however, to externally control their optical response due to the difficulties of introducing electrical doping. Here, an approach of interfacing ultrathin sheets of perovskites with few-layer graphene and hexagonal boron nitride into gate-tunable, hybrid heterostructures, is demonstrated. It allows for bipolar, continuous tuning of light emission and absorption in 2D perovskites by electrically injecting carriers to densities as high as 1012 cm-2 . This reveals the emergence of both negatively and positively charged excitons, or trions, with binding energies up to 46meV, among the highest measured for 2D systems. Trions are shown to dominate light emission and propagate with mobilities reaching 200cm2 V-1 s-1 at elevated temperatures. The findings introduce the physics of interacting mixtures of optical and electrical excitations to the broad family of 2D inorganic-organic nanostructures. The presented strategy to electrically control the optical response of 2D perovskites highlights it as a promising material platform toward electrically modulated light-emitters, externally guided charged exciton currents, and exciton transistors based on layered, hybridsemiconductors.

Few-layer hexagonal boron nitride / 3D printable polyurethane composite for neutron radiation shielding applications

Functional polymer composites can confer a range of benefits in practical applications that go beyond the individual properties of the constituent materials. Here we investigate and characterize the neutron absorbing capability of few-layer hexagonal boron nitride (h-BN) in composite with a 3D-printable thermoplastic polyurethane, and present experiment and simulation data to understand the processes and mechanisms in play. Shielding and protection from neutrons can be necessary in a range of terrestrial and space-based applications. The neutron absorption of composites with varying fractions of h-BN is strongly energy-dependent in the low-energy regime below 10 meV, and a composite containing 20 wt% h-BN shows a 70-fold reduction in the transmission relative to pure polyurethane at 0.5 meV neutron energies. This is attributed to the strong neutron capture cross-section of the naturally abundant boron-10 isotope, with energy-dependent measurements up to 100 meV confirming this point. Using inelastic neutron spectroscopy, we identify additional effects from the hydrogen in the polyurethane which both scatters diffusively and moderates neutrons inelastically via its phonon spectrum, enhancing the neutron absorption characteristics. Two models – based on analytic functions and Monte Carlo numerical techniques – are presented, and show excellent agreement with experiment results. The 3D-printability of the composite is demonstrated, and the opportunities and challenges for deploying these composites in neutron radiation protection applications are discussed.

Nanoscale infrared imaging and spectroscopy of few-layer hexagonal boron nitride

Nanoscale evaluation of the number of layers and boundaries in two-dimensional (2D) materials is crucial for understanding relationships between structure and property. Here, using scattering-type scanning near-field optical microscopy, we systematically studied on a nanoscale the infrared spectra and imaging of hexagonal boron nitride (h-BN), an ideal 2D insulating material. We revealed that the main factor determining the infrared amplitude changes at an optical frequency of about 1370 cm−1, corresponding to the in-plane phonon mode of h-BN. At lower frequencies, the amplitude is mainly determined by the local dielectric function of a sample and depends on the number of h-BN layers. At higher frequencies, it is affected by the phonon polariton waves of h-BN, and thus edges and grain boundaries of h-BN can be visualized due to the reflection of the waves at the boundary. The infrared spectra show a shoulder peak at higher frequencies, derived from the resonance with the phonon polaritons, in addition to a peak due to the in-plane phonon mode.

The behavior of thiacarbocyanine dyes on the surface of few-layered hexagonal boron nitride

The adsorption and self-assembly of several thiacarbocyanine dyes on hexagonal boron nitride (hBN) was investigated by combining steady-state spectroscopy and atomic force microscopy. The adsorption isotherms indicate that at saturation the density of the cationic TDC (5,5-dichloro-3-3′-diethyl-9-ethyl-thiacarbocyanine) molecules on hBN is similar to that of TD2 (3-3′-diethyl-9-ethyl-thiacarbocyanine) molecules, while the densities of TD0 (3-3′-diethyl-thiacarbocyanine) molecules and the zwitterionic THIATS (5,5-dichloro-3-3′-disulfopropyl-9-ethyl-thiacarbocyanine) molecules are significantly higher. The intermolecular distances between neighboring adsorbed dyes, calculated from these saturation densities indicate a flat-on adsorption for TDC, TD2 and TD0 and a partial edge-on adsorption for THIATS. AFM micrographs of the adsorbed TDC, TD0 and THIATS molecules indicate that already at low dye concentrations in the solution, where only a small fraction of the hBN surface is covered, the dye molecules already form upon adsorption to hBN aggregates of at least 10 nm, separated by areas where no adsorbed dye molecules can be detected. The resolution of the micrographs was however insufficient to show details of the packing of the adsorbed molecules. For THIATS, the thickness of the adsorbed layer is compatible with an edge-on adsorption, while for TDC and TD0, the thickness of the adsorbed layer is twice the thickness expected for flat-on adsorption. The exciton interaction extracted from the steady-state spectroscopy of the adsorbed dyes is much smaller than observed for H- or J-aggregates of the same dyes in solution or adsorbed to other surfaces such as Langmuir films or silver halides. For TDC, TD2 and TD0, the values of the exciton interaction are compatible with a close packed flat-on adsorption. Hence, optimizing the interaction between the adsorbed dye and hBN rather than between the adsorbed dye molecules governs the packing of the adsorbed dye molecules. The observation of the spectral shifts attributed to the exciton interaction at low average coverage of the hBN surface indicates that the exciton interaction already occurs at low coverage of the hBN surface. This observation is in agreement with the AFM micrographs, which show clustering of the adsorbed dye molecules already at low coverages.

Few-layered hexagonal boron nitride nanosheets stabilized Pt NPs for oxidation promoted adsorptive desulfurization of fuel oil

A few-layered hexagonal boron nitride nanosheets stabilized platinum nanoparticles (Pt/h-BNNS) is engineered for oxidation-promoted adsorptive desulfurization (OPADS) of fuel oil. It was found that the few-layered structure and the defective sites of h-BNNS not only are beneficial to the stabilization of Pt NPs but also favor the adsorption of aromatic sulfides. By employing Pt/h-BNNS with a Pt loading amount of 1.19 wt% as the active adsorbent and air as an oxidant, a 98.0% sulfur removal over dibenzothiophene (DBT) is achieved along with a total conversion of the DBT to the corresponding sulfones (DBTO2). Detailed experiments show that the excellent desulfurization activity originates from the few-layered structure of h-BNNS and the high catalytic activity of Pt NPs. In addition, the OPADS system with Pt/h-BNNS as the active adsorbent shows remarkable stability in desulfurization performance with the existence of different interferents such as olefin, and aromatic hydrocarbons. Besides, the Pt/h-BNNS can be recycled 12 times without a significant decrease in desulfurization performance. Also, a process flow diagram is proposed for deep desulfurization of fuel oil and recovery of high value-added products, which would promote the industrial application of such OPADS strategy.

Tuning the Thermal Transport of Hexagonal Boron Nitride/Reduced Graphene Oxide Heterostructures.

Tuning the thermal properties of materials is considered to be of crucial significance for improving the performance of electronic devices. Along these lines, the development of van der Waals (vdW) heterostructures becomes an effective solution to affect the thermal transport mechanisms. However, vdW interactions usually block phonon transport, which leads to a reduction in thermal conductivity. In this work, we experimentally demonstrate a large enhancement in the thermal conductivity of a vdW heterostructure composed of few-layer hexagonal boron nitride (h-BN) and reduced graphene oxide (RGO). By controlling the reduction temperature of RGO and changing the thickness of h-BN, the thermal conductivity of the RGO is increased by nearly 18 times, namely, from 91 to 1685 W m-1 K-1. Photothermal scanning imaging is used to reveal the changes in the heat transfer and temperature distribution of the h-BN/RGO heterostructure. Both photothermal scanning and Raman spectroscopy experiments show that the vdW interaction between h-BN and RGO can greatly increase the thermal conductivity of RGO, which is in contrast to the conventional understanding that vdW interaction reduces thermal conductivity. Our work paves the way for the manipulation of the thermal conductivity of two-dimensional (2D) heterostructures, which could be of great significance for future nanoelectronic circuits.

Few-layer hexagonal boron nitride as a shield of brittle materials for cryogenic s-SNOM exploration of phonon polaritons

Surface phonon polaritons (SPhPs) in van der Waals (vdW) materials are of great interest in fundamental and applied research fields. Probing the characteristics of vdW SPhPs at cryogenic temperatures is an essential task for their implementation in low-temperature physics. However, the most commonly used characterization technique of vdW SPhPs—scattering-type scanning near-field optical microscopy (s-SNOM) operating in a tapping mode (an intermittent-contact mode)—can be problematic at low temperatures because the sample being tested may become brittle and fragile. Therefore, high fracture toughness is desired for the samples under intermittent-contact s-SNOM scanning at low temperatures. In this work, by taking α-phase molybdenum trioxide (α-MoO3) as an example, we first confirm the potential surface deterioration induced by tip-sample interactions at low temperatures. Then, we propose to use few-layer hexagonal boron nitride as a mechanically tough yet optically passive cladding layer to enhance the surface stability of α-MoO3. Finally, we demonstrate the validity of our surface reinforcement strategy by probing the previously unexplored temperature dependence of SPhPs within the third Reststrahlen band of α-MoO3. Our method allows a sustained operation of tapping mode s-SNOM at cryogenic temperatures with negligible effect on intrinsic properties of SPhPs.

Laccase-Functionalized Hexagonal Boron Nitride-Coated Sponges for the Removal and Degradation of Anthracene

Superhydrophobic porous materials, for instance, sponges, membranes, and meshes, have attracted great attention due to their ability for adsorption of organic solvents, hydrocarbons, and oils while repelling water. In this work, we report an organic-solvent-free, environmentally benign, and cost-effective preparation method of a bifunctional adsorbent using a melamine sponge coated with few-layer hexagonal boron nitride nanosheets (hBNNs) functionalized with laccase (LA). The hBNNs are used as a building block for immobilizing and stabilizing LA for the removal and degradation of anthracene, a polycyclic aromatic hydrocarbon in crude oil, and to convert them to lighter and less-toxic substances. The physiochemical properties and performance of the sponges for the removal and degradation of anthracene were investigated thoroughly. The highest hydrocarbon degradation of 89% was obtained at an LA concentration of 2.72 mg/mL and pH 7, after 72 h. The degradation increased to 91% after 7 days of functionalized sponge exposure to the medium. Moreover, the functionalized sponges’ reusability studies revealed that the anthracene degradation efficiency was still as high as 54% after the hBNN–polyethylene oxide–LA sponges were repeatedly used 5 times. The strategy proposed for the fabrication of these sponges is facile and easy to scale up, not requiring the use of a complicated process or expensive equipment. These nano-engineered sponges are promising candidates for the separation and degradation of oils and hydrocarbons in oil spill remediation applications.

High magnetoresistance of a hexagonal boron nitride-graphene heterostructure-based MTJ through excited-electron transmission.

This work presents an ab initio study of a few-layer hexagonal boron nitride (hBN) and hBN–graphene heterostructure sandwiched between Ni(111) layers. The aim of this study is to understand the electron transmission process through the interface. Spin-polarized density functional theory calculations and transmission probability calculations were conducted on Ni(111)/nhBN/Ni(111) with n = 2, 3, 4, and 5 as well as on Ni(111)/hBN–Gr–hBN/Ni(111). Slabs with magnetic alignment in an anti-parallel configuration (APC) and parallel configuration (PC) were considered. The pd-hybridizations at both the upper and lower interfaces between the Ni slabs and hBN were found to stabilize the system. The Ni/nhBN/Ni magnetic tunnel junction (MTJ) was found to exhibit a high tunneling magnetoresistance (TMR) ratio at ∼0.28 eV for n = 2 and 0.34 eV for n > 2, which are slightly higher than the Fermi energy. The observed shifting of this high TMR ratio originates from the transmission of electrons through the surface states of the dz2-orbital of Ni atoms at interfaces which are hybridized with the pz-orbital of N atoms. In the case of n > 2, the proximity effect causes an evanescent wave, contributing to decreasing transmission probability but increasing the TMR ratio. However, the TMR ratio, as well as transmission probability, was found to be increased upon replacing the unhybridized hBN layer of the Ni/3hBN/Ni MTJ with graphene, thus yielding Ni/hBN–Gr–hBN/Ni. A TMR ratio as high as ∼1200% was observed at an energy of 0.34 eV, which is higher than the Fermi energy. Furthermore, a design is proposed for a device based on a new reading mechanism using the high TMR ratio observed just above the Fermi energy level.