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This report summarizes progress made in understanding properties such as zero-phonon-line energies, emission and absorption polarizations, electron-phonon couplings, strain tuning and hyperfine coupling of single photon emitters in hexagonal boron nitride. The primary aims of this research are to discover the chemical nature of the emitting centres and to facilitate deployment in device applications. Critical analyses of the experimental literature and data interpretation, as well as theoretical approaches used to predict properties, are made. In particular, computational and theoretical limitations and challenges are discussed, with a range of suggestions made to overcome these limitations, striving to achieve realistic predictions concerning the nature of emitting centers. A symbiotic relationship is required in which calculations focus on properties that can easily be measured, whilst experiments deliver results in a form facilitating mass-produced calculations.

A fundamental building block for nanophotonics is the ability to achieve negative refraction of polaritons, because this could enable the demonstration of many unique nanoscale applications such as deep-subwavelength imaging, superlens, and novel guiding. However, to achieve negative refraction of highly squeezed polaritons, such as plasmon polaritons in graphene and phonon polaritons in boron nitride (BN) with their wavelengths squeezed by a factor over 100, requires the ability to flip the sign of their group velocity at will, which is challenging. Here we reveal that the strong coupling between plasmon and phonon polaritons in graphene–BN heterostructures can be used to flip the sign of the group velocity of the resulting hybrid (plasmon–phonon–polariton) modes. We predict all-angle negative refraction between plasmon and phonon polaritons and, even more surprisingly, between hybrid graphene plasmons and between hybrid phonon polaritons. Graphene–BN heterostructures thus provide a versatile platform for the design of nanometasurfaces and nanoimaging elements.

Boron nitride nanotubes (BNNTs) are tubular nanoparticles with a structure analogous to that of carbon nanotubes, but with B and N atoms that completely replace the C atoms. Many favorable results indicate BNNTs as safe nanomaterials; however, important concerns have recently been raised about ultra-pure, long (˜10 µm) BNNTs tested on several cell types. Here, we propose additional experiments with the same BNNTs, but shortened (˜1.5 µm) with a homogenization/sonication treatment that allows for their dispersion in gum Arabic aqueous solutions. Obtained BNNTs are tested on human endothelial and neuron-like cells with several independent biocompatibility assays. Moreover, for the first time, their strong sum-frequency generation signal is exploited to assess the cellular uptake. Our data demonstrate no toxic effects up to concentrations of 20 µg/ml, once more confirming biosafety of BNNTs, and again highlighting that nanoparticle aspect ratio plays a key role in the biocompatibility evaluation. Original submitted 3 December 2013; Revised submitted 28 January 2014; Published online 6 February 2014

HighlightsA core–sheath structured phase change nanocomposite (PCN) with aligned and overlapping interconnected BNNS networks were successfully fabricated.The PCN has an ultrahigh in-plane thermal conductivity (28.3 W m−1 K−1), excellent flexibility and high phase change enthalpy (101 J g−1).The PCN exhibits intensively potential applications in the thermal management of 5G base stations and thermoelectric generators.Thermal management has become a crucial problem for high-power-density equipment and devices. Phase change materials (PCMs) have great prospects in thermal management applications because of their large capacity of heat storage and isothermal behavior during phase transition. However, low intrinsic thermal conductivity, ease of leakage, and lack of flexibility severely limit their applications. Solving one of these problems often comes at the expense of other performance of the PCMs. In this work, we report core–sheath structured phase change nanocomposites (PCNs) with an aligned and interconnected boron nitride nanosheet network by combining coaxial electrospinning, electrostatic spraying, and hot-pressing. The advanced PCN films exhibit an ultrahigh thermal conductivity of 28.3 W m−1 K−1 at a low BNNS loading (i.e., 32 wt%), which thereby endows the PCNs with high enthalpy (> 101 J g−1), outstanding ductility (> 40%) and improved fire retardancy. Therefore, our core–sheath strategies successfully balance the trade-off between thermal conductivity, flexibility, and phase change enthalpy of PCMs. Further, the PCNs provide powerful cooling solutions on 5G base station chips and thermoelectric generators, displaying promising thermal management applications on high-power-density equipment and thermoelectric conversion devices.

The edge effects on the melting process of hexagonal boron nitride (h-BN) are studied using molecular dynamics simulation. First, the free-standing h-BN configuration containing 10,000 atoms is studied with different armchair/zigzag edge ratios to see the influence of armchair and zigzag edges of the initial configuration on the phase transition process from a crystal to a liquid state. Then, the number of atoms in this critical armchair/zigzag ratio configuration increases to find the standard number of atoms in the initial configuration. Next, the atomic melting mechanism and the phase transition temperature from crystal to liquid of the critical initial free-standing h-BN configuration are studied. Following, the armchair h-BN nanoribbon is created from the critical initial free-standing h-BN configuration to study the melting process and the atomic melting mechanism from crystal to liquid. Finally, the edge effects on the melting process are shown.

Color centers in hexagonal boron nitride (hBN) have recently emerged as promising candidates for a new wave of quantum applications. Thanks to hBN's high stability and two-dimensional (2D) layered structure, color centers in hBN can be readily integrated into nanophotonic and plasmonic structures on a chip. More importantly, the recently discovered optically addressable spin defects in hBN provide a quantum interface between photons and electron spins for quantum sensing applications. The most well-studied hBN spin defects, the negatively charged boron vacancy ( ) spin defects, have been used for quantum sensing of static magnetic fields, magnetic noise, temperature, strain, nuclear spins, paramagnetic spins in liquids, RF signals, and beyond. In particular, hBN nanosheets with spin defects can form van der Waals (vdW) heterostructures with other 2D materials for in situ quantum sensing and imaging. This review summarizes the rapidly evolving field of quantum sensing with spin defects in hBN. We introduce basic properties of hBN spin defects, quantum sensing protocols, and recent experimental demonstrations of quantum sensing and imaging with hBN spin defects. We also discuss methods to enhance their sensitivity. Finally, we envision some potential developments and applications of hBN spin defects.

When serving as a protection tissue and/or inducing a periodic lateral modulation for/in atomically thin crystals, hexagonal boron nitride (hBN) has revolutionized the research on van der Waals heterostructures. By itself, hBN appears as an emergent wide-bandgap material, which, importantly, can be optically bright in the far-ultraviolet range and which frequently displays midgap defect-related centers of yet-unclear origin, but, interestingly, acting as single-photon emitters. Controlling the hBN doping is of particular interest in view of the possible practical use of this material. Here, we demonstrate that enriching hBN with carbon (C) activates an optical response of this material in the form of a series of well-defined resonances in visible and near-infrared regions, which appear in the luminescence spectra measured under below-bandgap excitation. Two, qualitatively different, C-related radiative centers are identified: One follows the Franck–Condon principle that describes transitions between two defect states with emission/annihilation of optical phonons, and the other shows atomic-like resonances characteristic of intradefect transitions. With a detailed characterization of the energy structure and emission dynamics of these radiative centers, we contribute to the development of controlled doping of hBN with midgap centers.

Boron nitride nanosheets (BNNSs) have gained significant attraction in energy and environment fields because of their two-dimensional (2D) nature, large band gap and high thermal/mechanical performance. However, the current low production efficiency of high-quality BNNSs is still a bottleneck limiting their applications. Herein, based on sonication-assisted liquid-phase exfoliation, we demonstrated a rapid, high-efficient and scalable production strategy of BNNSs and documented the effects of a spectrum of exfoliation factors (e.g., ultrasonic condition, solvent and bulk material feeding) on the yield of BNNSs. A record of yield of 72.5% was achieved while the exfoliated BNNSs have few-layer and defect-free feature. Thanks to the Lewis acid sites of the boron atoms, the BNNSs can interact with the polysulfide anions in liquid electrolyte and also can facilitate the uniform lithium deposition, which finally endow a lithium-sulfur (Li-S) battery with long life. This work provides a facile and rapid strategy for large scale preparation of high-quality BNNSs, also contributes a long-life strategy for dendrite-free Li-S battery, opens new avenues of BNNSs in energy application.

In this work the fracture toughness of epoxy resin has been improved through the addition of low loading of single part and hybrid nanofiller materials. Functionalised multi-walled carbon nanotubes (f-MWCNTs) was used as single filler, increased the critical strain energy release rate, GIC, by 57% compared to the neat epoxy, at only 0.1 wt% filler content. Importantly, no degradation in the tensile or thermal properties of the nanocomposite was observed compared to the neat epoxy. When two-dimensional boron nitride nanosheets (BNNS) were added along with the one-dimensional f-MWCNTs, the fracture toughness increased further to 71.6% higher than that of the neat epoxy. Interestingly, when functionalised graphene nanoplatelets (f-GNPs) and boron nitride nanotubes (BNNTs) were used as hybrid filler, the fracture toughness of neat epoxy is improved by 91.9%. In neither of these hybrid filler systems the tensile properties were degraded, but the thermal properties of the nanocomposites containing boron nitride materials deteriorated slightly.

The hardness, toughness and chemical stability of the well-known superhard material cubic boron nitride have been improved by using a synthesis technique based on specially prepared ‘onion-like’ precursor materials. How to make superhard materials ultrahard Superhard polycrystalline cubic boron nitride, second only to diamond in hardness, is superior to diamond in terms of thermal and chemical stability and is used widely as an abrasive. The hardness of many materials can be improved by decreasing the grain size, and here Yongjun Tian and colleagues use this principle in a new synthesis technique — based on specially prepared 'onion-like' precursor materials — capable of increasing the hardness of cubic boron nitride. The structure of the resulting polycrystalline material is dominated by nanometre-scale twin domains, yielding a solid combining ultrahigh hardness (exceeding that of a synthetic diamond single crystal) with a high oxidization temperature and extreme fracture toughness. If nanotwins at similar scales can be reproduced in polycrystalline diamond, it may be possible to raise diamond itself to new levels of hardness and stability. Cubic boron nitride (cBN) is a well known superhard material that has a wide range of industrial applications. Nanostructuring of cBN is an effective way to improve its hardness by virtue of the Hall–Petch effect—the tendency for hardness to increase with decreasing grain size 1 , 2 . Polycrystalline cBN materials are often synthesized by using the martensitic transformation of a graphite-like BN precursor, in which high pressures and temperatures lead to puckering of the BN layers 3 . Such approaches have led to synthetic polycrystalline cBN having grain sizes as small as ∼14 nm (refs 1 , 2 , 4 , 5 ). Here we report the formation of cBN with a nanostructure dominated by fine twin domains of average thickness ∼3.8 nm. This nanotwinned cBN was synthesized from specially prepared BN precursor nanoparticles possessing onion-like nested structures with intrinsically puckered BN layers and numerous stacking faults. The resulting nanotwinned cBN bulk samples are optically transparent with a striking combination of physical properties: an extremely high Vickers hardness (exceeding 100 GPa, the optimal hardness of synthetic diamond), a high oxidization temperature (∼1,294 °C) and a large fracture toughness (>12 MPa m 1/2 , well beyond the toughness of commercial cemented tungsten carbide, ∼10 MPa m 1/2 ). We show that hardening of cBN is continuous with decreasing twin thickness down to the smallest sizes investigated, contrasting with the expected reverse Hall–Petch effect below a critical grain size or the twin thickness of ∼10–15 nm found in metals and alloys.