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Packaging insulation materials with high thermal conductivity and excellent dielectric properties are favorable to meet the high demand and rapid development of third generation power semiconductors. In this study, we propose to improve the thermal conductivity of epoxy resin (EP) by incorporating a three-dimensional boron nitride thermally conductive network. Detailedly, polyurethane foam (PU) was used as a supporter, and boron nitride nanosheets (BNNSs) were loaded onto the PU supporter through chemical bonding (BNNS@PU). After immersing BNNS@PU into the EP resin, EP-based thermally conductive composites were prepared by vacuum-assisted impregnation. Fourier transform infrared spectrometer and scanning electron microscope were used to characterize the chemical bonding and morphological structure of BNNS@PU, respectively. The content of BNNS in BNNS@PU/EP composites was quantitatively analyzed by TGA. The results show that the thermal conductivity of the BNNS@PU/EP composites reaches 0.521 W/m K with an enhancement rate η of 30.89 at an ultra-low BNNS filler content (5.93 wt. %). Additionally, the BNNS@PU/EP composites have excellent dielectric properties with the frequency range from 101 to 106 Hz. This paper provides an interesting idea for developing high thermal conductivity insulating materials used for power semiconductor packaging. [ABSTRACT FROM AUTHOR]

Two-dimensional magnetic materials have been increasingly studied and discussed in the field of spintronics due to their unique electronic properties, high spin polarizability, and a variety of magnetic properties. In this paper, we report a new two-dimensional bilayer hexagonal monolayer material bilayer hexagonal structure (BHS)-CrN2 by first-principles calculations. The BHS-CrN2 nanosheet is an intrinsic ferromagnetic semiconductor material, and the Curie temperature obtained by Monte Carlo simulation is 343 K. The absence of a significant imaginary frequency in the phonon spectrum indicates the dynamic stability of BHS-CrN2. After ab initio molecular dynamics simulation, the supercell of BHS-CrN2 remains a complete structure, indicating its thermal stability. The calculated elastic moduli satisfy the Born–Huang criterion, indicating that the BHS-CrN2 system has good mechanical stability. Interestingly, the compressive strain and O atom doping can transform the electronic structure of BHS-CrN2 from a semiconductor to a half-metal, and the Curie temperature of BHS-CrN2 can be further increased to 1059 K when a 5% tensile strain is applied. Furthermore, the BHS-CrN2 in the ferromagnetic state shows a significant in-plane magnetic anisotropy energy of 0.01 meV per Cr, and the CrP2 and CrAs2 show a large out-of-plane magnetic anisotropy energy of 0.207 and 0.988 meV per Cr, respectively. The results show that the intrinsic ferromagnetic semiconductor BHS-CrN2 has good stability, high Curie temperature, and tunable magnetic properties, which is a promising material for room-temperature spintronic devices. [ABSTRACT FROM AUTHOR]

In the first part of this paper, we present a model that explains and determines quantitatively the twists between nucleation islands in the case of a Volmer–Weber heteroepitaxial growth of tetrahedrally coordinated semiconductors along hexagonal orientations. These twists are caused by the network of the screw components of the 60° misfit dislocations. The orientations of the screw components are distributed randomly, and the maximum twist is obtained when all the screw components have the same orientation. The maximum twists are related to the density of misfit dislocations and, therefore, increase with the mismatch between the deposited materials and their substrate. In the second part of the paper, we study five systems having a large distribution of mismatches from 4% to 19%. For the four systems fulfilling the conditions necessary for the application of the model (plastic relaxation of grown islands), the measured maximum twists fit with the calculated values, thereby validating the model. The twists of nucleation islands are related to the mismatch and are, therefore, intrinsic to the material systems. The defects created at the coalescence of twisted islands determine the initial microstructure/defect distribution of the nucleation layer. [ABSTRACT FROM AUTHOR]

Edge kinetics in 2D structures has been a key to understanding the growth. In this paper, the effect of hydrogen passivation on the growth of hexagonal boron nitride (h-BN) was studied. Without hydrogen, the filling process of the gap on bare edges of h-BN is difficult because of the formation of dimers that distorts the edge. With hydrogen passivation, such difficulty can be largely reduced. In addition, hydrogen passivation can reduce the edge bending to the substrate. In summary, the amount of hydrogen passivation during the growth is the long-ignored parameter and can be the key to a good crystal quality. [ABSTRACT FROM AUTHOR]

This paper presents the development of an all-in-one probe to simultaneously measure all components of the generalized Ohm's law in reversed-field pinch plasmas and tokamaks. The polyhedral configuration of the Mach probe is achieved through the specific arrangement, angle, and depth of the collimator channel apertures drilled into the surface of a hollow boron nitride cylinder encasing it. This probe includes a central Mach probe to assess the ion velocity field in three dimensions. Initial tests at the RELAX and Madison Symmetric Torus machines have confirmed the probe's effectiveness, revealing an octahedron form similar to a tetrahedron. The probe seems to function correctly and is expected to facilitate the empirical validation of two-fluid equilibria at the periphery of toroidal plasmas. [ABSTRACT FROM AUTHOR]

With the increasing integration levels of modern electronic products, effective thermal management has become a critical concern to ensure the operational efficiency and longevity of electronic devices. Developing thermal interface materials that possess both high thermal conductivity (κ) and reliable insulation properties has presented a significant challenge. In this study, boron nitride fibers (BNF) with a substantial length-to-diameter ratio were fabricated to serve as vertically aligned thermal conduction channels within a composite pad containing a silicone gel matrix.Under typical packing pressure conditions, the resulting BNF pad exhibits an exceptionally high through-plane thermal conductivity (40 vol. %: κ⊥ = 16.77 W/mK). Additionally, the BNF pad demonstrates favorable mechanical and dielectric properties, enhancing its versatility and applicability. The results of actual heat dissipation tests further validate the excellent thermal management capabilities of these composites in high-power electronic applications. [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of the thermal conductivity behaviors exhibited by a diverse array of nanostructured materials, ranging from multilayer graphene nanocomposites to semiconductor-based nanostructures such as Bi0.5Sb1.5Te3 and In0.53Ga0.47As composites. The investigation extends to superlattices, nanowires, and hybrid nanostructures, encompassing materials like hexagonal boron nitride flakes, iron oxide nanoporous films, and organic-inorganic hybrid materials. The thermal conductivity of these materials is characterized by distinct trends, with some showcasing crystal-like behavior and others demonstrating glass-like characteristics. The analysis employs empirical expressions to discern the contributions of phonons and diffusons in crystal-like materials and incorporates Peierls contributions and Arrhenius-type terms for glass-like behavior. Noteworthy observations include deviations in fitting certain materials at lower temperatures and the identification of negative diffuson contributions in specific cases. These findings contribute to a nuanced understanding of thermal transport in nanostructured materials and have implications for applications in advanced thermal management systems and thermoelectric devices. The extracted parameters provide valuable insights for researchers exploring the thermal conductivity of diverse nanostructured materials. [ABSTRACT FROM AUTHOR]

This paper reveals the existence of a critical separation distance (d c) beyond which the elastic interactions between a pair of monovacancies in graphene or hexagonal boron nitride become inconsequential for the strength and toughness of the defective lattice. This distance is independent of the chirality of the lattice. For any inter-defect distance higher than d c , the lattice behaves mechanically as if there is a single defect. For a distance less than d c , the defect–defect elastic interactions produce distinctive mechanical behavior depending on the orientation (θ) of the defect pair relative to the loading direction. Both strength and toughness of the lattice containing a pair of "interacting monovacancies (iMVs)" are either higher or smaller than that of the lattice containing a pair of "non-interacting monovacancies (nMVs)," suggesting the existence of a critical orientation angle θ c. For θ < θ c , the smaller the distance between the iMVs, the higher the toughness and strength compared to the lattice containing nMVs, whereas, for θ ≥ θ c , the smaller the separation distance between the iMVs, the smaller the toughness and strength compared to the lattice containing nMVs. The transitional behavior has a negligible dependence on the chirality of the lattice, which indicates that the crystallographic anisotropy has a much weaker influence on toughness and strength compared to the anisotropy induced by the orientation angle itself. These observations underline an important point that the elastic fields emanating from vacancy defects are highly localized and fully contained within a small region of around 1.5 nm radius. [ABSTRACT FROM AUTHOR]

Thin hexagonal boron nitride layers have been shown to support highly confined hyperbolic phonon-polaritons, which are of interest for light guiding applications. Localized plasmon resonances in nanopatterned metal films can exhibit subwavelength-scale confinement as well as a high local field strength that is of importance to imaging and sensor applications. In this work, the interaction between hyperbolic phonon-polaritons in a hexagonal boron nitride thin film and plasmon-polaritons in a nanopatterned gold thin film is investigated by means of finite-difference time-domain simulations of a series of coupled and uncoupled layered devices. Both far-field and near-field properties are calculated and analyzed, enabling the features due to plasmon-polaritons and phonon-polaritons, individually, to be distinguished and the coupling between these excitations to be explored and characterized. [ABSTRACT FROM AUTHOR]

Two-dimensional lateral heterostructures exhibit novel electronic and optical properties that are induced by their in-plane interface for which the mechanical properties of the interface are important for the stability of the lateral heterostructure. Therefore, we performed molecular dynamics simulations and developed a convolutional neural network-based machine learning model to study the fracture properties of the interface in a graphene/hexagonal boron nitride lateral heterostructure. The molecular dynamics (MD) simulations show that the shape of the interface can cause an 80% difference in the fracture stress and the fracture strain for the interface. By using 11 500 training samples obtained with help of high-cost MD simulation, the machine learning model is able to search out the strongest interfaces with the largest fracture strain and fracture stress in a large sample space with over 150 000 structures. By analyzing the atomic configuration of these strongest interfaces, we disclose two major factors dominating the interface strength, including the interface roughness and the strength of the chemical bond across the interface. We also explore the correlation between the fracture properties and the thermal conductivity for these lateral heterostructures by examining the bond type and the shape of the graphene/hexagonal boron nitride interface. We find that interfaces comprised of stronger bonds and smoother zigzag interfaces can relieve the abrupt change of the acoustic velocity, leading to the enhancement of the interface thermal conductivity. These findings will be valuable for the application of the two-dimensional lateral heterostructure in electronic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional halide perovskite Cs2PbI2Cl2 with the Ruddlesden–Popper structure has attracted much interest in both experiment and theory, owing to its excellent structural stability and electronic and optical properties. Here, we design the graphene/Cs2PbI2Cl2 van der Waals (vdW) heterostructure (HS) and comprehensively investigate its structural, electronic, and contact properties by using first principle calculations. Four types of graphene/Cs2PbI2Cl2 HSs are considered, and the most stable one is identified. Because the composed system has weak vdW interaction, the intrinsic band structures of both graphene and Cs2PbI2Cl2 are well maintained. Meanwhile, the graphene opens a minute energy gap of about 68 meV, which may have resulted from a broken sublattice inversion symmetry and tiny structure distortion. Moreover, it is found that graphene/Cs2PbI2Cl2 forms a p-type Schottky contact. The HS undergoes a contact-type transition to p-type Ohmic contact and n-type Ohmic contact from the original p-type Schottky contact under positive and negative electric fields, respectively. When interlayer coupling strength increases or decreases, a contact-type transition to the p-type Ohmic contact from the original p-type Schottky contact occurs. These findings provide a meaningful guidance for tuning the electronic properties and constructing high-performance graphene/Cs2PbI2Cl2 HS-based Schottky devices. [ABSTRACT FROM AUTHOR]

Nanosecond pulsed electric fields over 200 kV/mm have been used for filler alignment in polymer-based thin film (120–250 μm) composites to improve their thermal conductivity, but little research has been performed on bulk composites because of the requirement of an extremely high nanosecond pulsed voltage. In this paper, a microsecond pulsed voltage with a pulse width of 1 μs and an electric field of only 11.76 kV/mm was adopted for the first time to prepare bulk composites of 1.7 mm. The effects of frequency on nanosheets' orientation and the thermal conductivity of epoxy composites were studied. The orientation degree of BN nanosheets was determined based on cross-sectional scanning electron microscopy images and X-ray diffraction peaks. The orientation degree and thermal conductivity of epoxy composites increased with the increase of the frequency, but the increase rate was lower under higher frequencies. The thermal conductivity of composites containing 10 wt. % BN nanosheets prepared under 100 Hz was 0.588 W/mK, which was more than twice that of composites without an applied electric field. Additionally, an effective medium theory model was adopted for thermal conductivity modeling. In addition, thermogravimetric and differential scanning calorimetry analyses were performed to evaluate the thermal properties of the composites. The results showed that after BN orientation in epoxy resin, the composites had higher glass transition temperature and better thermal stability. Microsecond pulsed electric fields are promising for preparing bulk insulation composites with higher thermal conductivity. [ABSTRACT FROM AUTHOR]

We theoretically prove in this paper that using planar multilayer graphene-hexagonal boron nitride (hBN) metamaterials (GhMMs) can yield ultrabroadband and high-contrast asymmetric transmission (AT) and asymmetric absorption (AA) of light. The AA and AT features are obtained in the far-infrared (FIR) and mid-infrared (MIR) regions for normally incident light with transverse magnetic polarization. Here, the GhMMs are integrated with two asymmetric gratings of Ge and are composed of alternating multilayers of graphene (11 multilayers) and hBN layers (10 layers). Moreover, the total subwavelength thickness of the hybrid structures is about 3 μm, being less than half of the free-space wavelength up to nearly 50 THz. This approach—which is similar to the one introduced by Xu and Lezec [Nat. Commun. 5, 4141 (2014)] for a passive hyperbolic metamaterial operating in the visible range—is based on the excitation of high- β modes of the GhMM with different transmission characteristics. In addition to being ultrabroadband and high-contrast, AT and AA features of the proposed GhMMs can be actively tuned by varying the chemical potential of graphene. Furthermore, it is shown that an on-off switching of AT factor at FIR and selective tunability at MIR frequencies can be obtained via varying μ. Due to its subwavelength and planar configuration and active operation, these multilayer graphene-hBN metamaterials with AT and AA characteristics hold promise for integration with compact optical systems operating in the MIR and FIR ranges and are suitable for applications such as optical diodes, sensors, and thermal emitters. [ABSTRACT FROM AUTHOR]

The vibration behaviors of van der Waals (vdW) heterostructures are studied based on molecular dynamics (MD) simulations and continuum mechanics modelling in this paper. Graphene/hexagonal boron nitride and graphene/silicene systems are considered as two typical examples of heterostructures studied here. Our MD results show that the resonance frequency of vdW heterostructures grows as their layer number increases and tends to be saturated when the layer number is relatively large. These findings deviate from results of the conventional composite beam (CB) model of vdW heterostructures. By abandoning the assumptions in the CB model, we propose a novel multiple beam (MB) model giving a result that agrees well with MD results. We find from the MB model that compared to other factors the interlayer shearing effect plays the key role in determining the resonance behaviors of vdW heterostructures. Considering this fact, we further simplify the MB model to a much simpler form which gives a simple but precise description of the vibration behaviors of vdW heterostructures. This simplified MB model suggests that the resonance frequency of vdW heterostructures can be optimized by changing their total mass, the sum of bending stiffness of their component layers, and the sum of interlayer shear modulus of their vdW layers. [ABSTRACT FROM AUTHOR]

Aluminum alloy and its allied composites are gaining popularity and find their exclusive application in all industries for more than two decades. Due to their increased usage, Al metal matrix composites were experimented by reinforcing them with variety of particles. The current study attempted the use of titanium carbide (TiC) and boron nitride (BN) as reinforcing materials for the Al7050 alloy through the stir casting technique. The percentage of reinforcement of TiC was kept constant as 8% by wt., and the BN was varied as 2% and 3% by wt. Mechanical properties, such as compressive strength, tensile strength, flexural strength, impact energy, microhardness (Vickers), the microstructure were studied using an optical microscope and by corrosion tests using polarization techniques. The results conclude that the Al hybrid composite with 8% TiC and 3% BN provides superior performance. [ABSTRACT FROM AUTHOR]

Many techniques to fabricate complex nanostructures and quantum emitting defects in low dimensional materials for quantum information technologies rely on the patterning capabilities of focused ion beam (FIB) systems. In particular, the ability to pattern arrays of bright and stable room temperature single-photon emitters (SPEs) in 2D wide-bandgap insulator hexagonal boron nitride (hBN) via high-energy heavy-ion FIB allows for direct placement of SPEs without structured substrates or polymer-reliant lithography steps. However, the process parameters needed to create hBN SPEs with this technique are dependent on the growth method of the material chosen. Moreover, morphological damage induced by high-energy heavy-ion exposure may further influence the successful creation of SPEs. In this work, we perform atomic force microscopy to characterize the surface morphology of hBN regions patterned by Ga+ FIB to create SPEs at a range of ion doses and find that material swelling, and not milling as expected, is most strongly and positively correlated with the onset of non-zero SPE yields. Furthermore, we simulate vacancy concentration profiles at each of the tested doses and propose a qualitative model to elucidate how Ga+ FIB patterning creates isolated SPEs that is consistent with observed optical and morphological characteristics and is dependent on the consideration of void nucleation and growth from vacancy clusters. Our results provide novel insight into the formation of hBN SPEs created by high-energy heavy-ion milling that can be leveraged for monolithic hBN photonic devices and could be applied to a wide range of low-dimensional solid-state SPE hosts. [ABSTRACT FROM AUTHOR]

The realization of fusion energy requires materials that can withstand high heat and particle fluxes at the plasma material interface. In this work, an electrothermal (ET) plasma source has been designed as a transient heat flux source for a linear plasma material interaction device. An ET plasma source operates in the ablative arc regime driven by a DC capacitive discharge. The current channel width is defined by the 4mm bore of a boron nitride liner. At large plasma currents, the arc impacts the liner wall, leading to high particle and heat fluxes to the liner material, which subsequently ablates and ionizes. This results in a high density plasma with a large unidirectional bulk flow out of the source exit. The pulse length for the ET source has been optimized using a pulse forming network to have durations of 1 and 2 ms. The peak currents and maximum source energies seen in this system are 1.9 kA and 1.2 kJ for the 2ms pulse and 3.2kA and 2.1 kJ for the 1ms pulse, respectively. This work is a proof of the principal project to show that an ET source produces electron densities and heat fluxes comparable to those anticipated in transient events in large future magnetic confinement fusion devices. Heat flux, plasma temperature, and plasma density were determined for each shot using infrared imaging and optical spectroscopy techniques. This paper will discuss the assumptions, methods, and results of the experiments. [ABSTRACT FROM AUTHOR]

Phonon polaritons (PHPs) in freestanding and supported multilayers of hexagonal boron nitride are systematically studied using a macroscopic optical-phonon model. The PHP properties such as confinement, group velocity, propagation quality factor (PQF), and wavelength scaling are studied. Owing to high-frequency screening, there is an upper frequency limit making the two-dimensional (2D) PHPs have a frequency band and also a maximum PQF occurs near the center frequency. The substrate's dielectric response should be included to accurately calculate the PHP properties. While the simple electrostatic approximation (ESA) is valid for PHPs with frequencies ω above ω 0 (e.g., ω > 1.03 ω 0 for the 30-layers; ω 0 is the Γ point optical-phonon frequency), it fails to describe the PHP properties near ω 0 and the effect of retardation should be included for a proper description. The PHP wavelength vs layer thickness near ω 0 deviates significantly from a linear scaling law given by the ESA due to strong coupling of photons and longitudinal optical phonons. The calculated PHP dispersion and scaling are compared with experimental data of a number of spectroscopic studies and found to be in good agreement for most of the results. While the frequency of incident light should be near the center frequency to maximize the PQF, the PHP wavelength, confinement, and propagation length can be engineered by varying the multilayer thickness and its dielectric environment. [ABSTRACT FROM AUTHOR]

We theoretically investigate the high-order harmonic generation (HHG) of the monolayer hexagonal boron nitride by two-color laser pulses, based on the ab initio time-dependent density-functional theory. We find that the waveform of the two-color laser field can dramatically control the harmonic spectrum. The two-color laser field can enhance the harmonic radiation more efficiently than the monochromatic pulse laser with the same incident energy. We investigate the influence of incident laser pulse parameters on the harmonic radiation, such as the relative phase of the two-color field, the amplitude ratio between component electric fields, and the laser orientation. We show that the HHG spectrum is controlled by both the electric field and the vector potential. The electronic band structure and the laser-matter energy transfer play an important role in determining the laser polarization for optimal HHG in the hBN crystal. Our work supplies a scheme to manipulate HHGs in two-dimensional materials and provides a potential methodology for the generation of intense extreme-ultraviolet pulses. [ABSTRACT FROM AUTHOR]

Four new boron nitride polymorphs hP24 BN, hP18-I BN, mP36 BN, and hP18-II BN with sp2 hybridization are investigated in this study by first-principles calculations, including the structural properties, stability, elastic properties, anisotropy, and electronic properties. Predicted by the electronic band structure, all the BN polymorphs in hP24, hP18-I, mP36, and hP18-II phase are wide bandgap semiconductor materials with a bandgap of 2.97–4.72 eV. Meanwhile, the bandgap of hP24 BN is larger than that of ZnO, and the bandgaps of hP18-I BN, mP36 BN, and hP18-II BN are larger than those of GaN and SiC. The new boron nitride polymorphs have mechanical stability, dynamic stability, and thermodynamic stability by analyzing the elastic constants, phonon spectra, and related enthalpies. The values of B/G of hP24 BN, hP18-I BN, mP36 BN, and hP18-II BN are all larger than 1.75, which confirms that they are ductile materials. Their densities are around 2.100 g/cm3, which are smaller than that of the c-BN (3.466 g/cm3). BN polymorphs in this study show different degrees of anisotropy in Young's modulus, and hP24 BN has the largest anisotropy in Young's modulus, while mP36 BN displays the smallest Young's modulus anisotropy. [ABSTRACT FROM AUTHOR]

In this work, we consider film/substrate semiconductor heterostructures with a hexagonal (wurtzite) and trigonal (corundum) crystal structure. We show that the differences between the stress level in the α-Ga2O3/α-Al2O3 heterostructure with the corundum crystal structure and the stress level in the GaN/AlN heterostructure with the wurtzite crystal structure do not exceed 50%. We study the effect of Al composition x and growth direction of the heterostructure on the critical film thickness for misfit dislocation formation in α-(AlxGa1−x)2O3/α-Al2O3 heterostructures. We provide a comparison between theoretical calculations of the critical film thickness and experimental data on the film thickness, at which the misfit dislocations were observed in α-(AlxGa1−x)2O3/α-Al2O3 heterostructures. [ABSTRACT FROM AUTHOR]

Photoblinking and photobleaching are commonly encountered problems for single-photon sources. Numerous methods have been devised to suppress these two impediments; however, either the preparation procedures or the operating conditions are relatively harsh, making them difficult to apply to practical applications. Here, we reported giant suppression of both photoblinking and photobleaching of a single fluorescent molecule, terrylene, via the utilization of hexagonal boron nitride (h-BN) flakes as substrates. Experimentally, a much-prolonged survival time of terrylene has been determined, which can have a photostable emission over 2 h at room temperature under ambient atmospheres. Compared with single molecules on a SiO2/Si substrate or glass coverslip, a more than 100-fold increase in the total number of photons collected from each terrylene on h-BN flakes has been demonstrated. We also proved that the photostability of terrylene molecules can be well maintained for more than 6 months even under ambient conditions without any further protection. Our results demonstrate that the utilization of h-BN flakes to suppress photoblinking and photobleaching of fluorescent molecules has promising applications in the production of high-quality single-photon sources at room temperature. [ABSTRACT FROM AUTHOR]

We study the interfacial thermal conductance of grain boundaries (GBs) between monolayer graphene and hexagonal boron nitride (h-BN) sheets using a combined atomistic approach. First, realistic samples containing graphene/h-BN GBs with different tilt angles are generated using the phase-field crystal model developed recently [P. Hirvonen et al., Phys. Rev. B 100, 165412 (2019)] that captures slow diffusive relaxation inaccessible to molecular dynamics (MD) simulations. Then, large-scale MD simulations using the efficient GPUMD package are performed to assess heat transport and rectification properties across the GBs. We find that lattice mismatch between the graphene and h-BN sheets plays a less important role in determining the interfacial thermal conductance as compared to the tilt angle. In addition, we find no significant thermal rectification effects for these GBs. [ABSTRACT FROM AUTHOR]

We report the growth of large-scale, single-oriented zinc oxide (ZnO) nanowall networks on epitaxial hexagonal boron nitride (h-BN) films and their application to flexible inorganic ultraviolet (UV) light sensors. Using catalyst-free metal-organic vapor phase epitaxy, ZnO nanowall networks with good vertical alignment are grown on epitaxial h-BN films. The single-oriented crystal structure of the ZnO nanostructures on h-BN is investigated using x-ray diffraction (XRD) spectroscopy, and the heteroepitaxial relationship between ZnO and h-BN is revealed through synchrotron radiation XRD. Interestingly, when utilizing the grown ZnO nanostructure as a channel for UV sensors, better performance merits such as a high IUV/Idark ratio, faster recovery time, and low dark current are achieved if h-BN is employed as a growth template. As an example of inorganic flexible optoelectronic device applications, flexible UV sensors are fabricated using ZnO/h-BN heterostructures owing to the insulating and transferrable nature of h-BN substrates. The sensor maintained an excellent performance, even under highly bent conditions. [ABSTRACT FROM AUTHOR]

Femtosecond optically excited coherent acoustic phonon modes (CAPs) are investigated in a free-standing van der Waals heterostructure composed of a 20-nm transparent hexagonal boron nitride (hBN) and a 42-nm opaque graphite layer. Employing ultrafast electron diffraction, which allows for the independent evaluation of strain dynamics in the constituent material layers, three different CAP modes are identified within the bilayer stack after the optical excitation of the graphite layer. An analytical model is used to discuss the creation of individual CAP modes. Furthermore, their excitation mechanisms in the heterostructure are inferred from the relative phases of these modes by comparison with a numerical linear-chain model. The results support an ultrafast heat transfer mechanism from graphite to the hBN lattice system, which is important to consider when using this material combination in devices. [ABSTRACT FROM AUTHOR]

The discovery of graphene and its fascinating capabilities has triggered an unprecedented interest in inorganic two-dimensional (2D) materials. van der Waals layered materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides, and the more recently re-discovered black phosphorus (BP) indeed display an exceptional technological potential for engineering nano-electronic and nano-photonic devices and components "by design," offering a unique platform for developing new devices with a variety of "ad hoc" properties. In this Perspective article, we provide a vision on the key transformative applications of 2D nanomaterials for the development of nanoelectronic, nanophotonic, optical, and plasmonic devices at terahertz frequencies, highlighting how the rich physical phenomena enabled by their unique band structure engineering can allow them to boost the vibrant field of quantum science and quantum technologies. [ABSTRACT FROM AUTHOR]

Ab initio pseudopotential calculations have made for the structural, electronic, elastic, mechanical, and electron–phonon interaction properties of molybdenum borocarbide ( Mo 2 BC) and niobium boronitride ( Nb 2 BN) superconductors. Analysis of the structural and electronic properties reveals that the nature of bonding in both these compounds is a combination of covalent, ionic, and metallic. The near-Fermi electronic states in both compounds are occupied by the d states of transition metal atoms. The electronic density of states at the Fermi level in Mo 2 BC is significantly higher than that in Nb 2 BN. Lattice dynamical calculations verify their dynamical stability in the base-centered orthorhombic Mo 2 BC-type crystal structure. We find that the total electron–phonon coupling constant is equal to 0.745 for Mo 2 BC and 0.539 for Nb 2 BN. The calculated superconducting transition temperature of 7.41 K for Mo 2 BC and 3.50 K for Nb 2 BN is comparable with their experimental values of 7.2 and 4.4 K, respectively. [ABSTRACT FROM AUTHOR]

Graphene-based van der Waals (vdW) heterostructures have attracted extensive attention for applications like nanoelectronics. The graphene and group-III monochalcogenide (MX, M = In and Ga and X = S and Se) heterostructures were established herein, and the electronic properties were investigated by the first-principles calculation. These heterostructures form an n-type Schottky contact at the interface and the Schottky barrier height can be modulated by the external strain. With graphene as electrodes, the device performances of the 9 nm MX metal-oxide-semiconductor FETs (MOSFETs) are investigated. Based on the computed tunneling probability and transfer characteristics, the GaSe MOSFET with a graphene electrode stands out with the highest tunneling probability and largest on-off ratio. We believe these results can provide physical insights into designing and fabricating devices based on the graphene and group-III monochalcogenide heterostructures. [ABSTRACT FROM AUTHOR]

The interlayer coupling, which has a strong influence on the properties of van der Waals heterostructures, strongly depends on the interlayer distance. Although considerable theoretical interest has been demonstrated, experiments exploiting a variable interlayer coupling on nanocircuits are scarce due to the experimental difficulties. Here, we demonstrate a novel method to tune the interlayer coupling using hydrostatic pressure by incorporating van der Waals heterostructure based nanocircuits in piston-cylinder hydrostatic pressure cells with a dedicated sample holder design. This technique opens the way to conduct transport measurements on nanodevices under pressure using up to 12 contacts without constraints on the sample at the fabrication level. Using transport measurements, we demonstrate that a hexagonal boron nitride capping layer provides a good protection of van der Waals heterostructures from the influence of the pressure medium, and we show experimental evidence of the influence of pressure on the interlayer coupling using weak localization measurements on a transitional metal dichalcogenide/graphene heterostructure. [ABSTRACT FROM AUTHOR]

The discovery of atom-like spin emitters associated with defects in two-dimensional (2D) wide-bandgap (WBG) semiconductors presents new opportunities for highly tunable and versatile qubits. So far, the study of such spin emitters has focused on defects in hexagonal boron nitride (hBN). However, hBN necessarily contains a high density of nuclear spins, which are expected to create a strong incoherent spin-bath that leads to poor coherence properties of spins hosted in the material. Therefore, identification of new qubit candidates in other 2DWBG materials is necessary. Given the time demands of ab initio methods, new approaches for rapid screening and calculations of identifying properties of suitable atom-like qubits are required. In this work, we present two new methods for rapid estimation of the zero-phonon line (ZPL), a key property of atomic qubits in WBG materials. First, the ZPL is calculated by exploiting Janak's theorem. For finite changes in occupation, we provide the leading-order estimate of the correction to the ZPL obtained using Janak's theorem, which is more rapid than the standard method (Δ SCF). Next, we demonstrate an approach to converging excited states that is faster for systems with small strain than the standard approach used in the Δ SCF method. We illustrate these methods using the case of the singly negatively charged calcium vacancy in SiS 2 , which we are the first to propose as a qubit candidate. This work has the potential to assist in accelerating the high-throughput search for quantum defects in materials, with applications in quantum sensing and quantum computing. [ABSTRACT FROM AUTHOR]

Efficient thermal transport control is a fundamental issue for electronic device applications such as information, communication, and energy storage technologies in modern electronics in order to achieve desired thermal conditions. Structural defects in materials provide a mechanism to adjust the thermal transport properties of these materials on demand. In this context, the effect of structural defects on lattice thermal conductivities of two-dimensional hexagonal binary group-III nitride (XN, X = B , Al, and Ga) semiconductors is systematically investigated by means of classical molecular dynamics simulations performed with recently developed transferable inter-atomic potentials accurately describing defect energies. Here, two different Green–Kubo based approaches and another approach based on non-equilibrium molecular dynamics are compared in order to get an overall understanding. Our investigation clearly shows that defect concentrations of 3 % decrease the thermal conductivity of systems containing these nitrites up to 95 %. Results hint that structural defects can be used as effective adjustment parameters in controlling thermal transport properties in device applications associated with these materials. [ABSTRACT FROM AUTHOR]

This paper proposes a molybdenum disulfide (MoS2)/Germanium (Ge) heterojunction device with a top gate composed of hexagonal boron nitride and graphene. First, the rectification direction of the device is reversed, and the rectification ratio is modulated from 0.09 to 7.3 by varying the gate voltage from −20 to 20 V. Optoelectronic characterization reveals that the device photoresponsivity and speed can improve by several orders of magnitude when graphene is used as the MoS2 contact electrode. There is an ambipolar photoresponse behavior in which the photocurrent polarity can be reversed depending on the wavelength of light and gate voltage applied. A conventional positive responsivity of 13.9 A/W is obtained with a 532 nm visible light. In contrast, the device responsivity changes from 33.7 A/W to −128 mA/W as the gate voltage decreases when a 1550 nm infrared light is used. This ambipolar infrared photocurrent is attributed to the competition between the bolometric effect in MoS2 and photocarrier effect in Ge. In addition, because it has opposite photoresponses at different wavelengths, this device has the potential to be used as a wavelength-distinguishing photodetector. These results offer a strategy for the development of two- and three-dimensional optoelectronic heterostructures. [ABSTRACT FROM AUTHOR]

Aiming at overpressure measurement, this paper presents a large-range graphene/hexagonal boron nitride (h-BN) heterostructure-based pressure sensor with a poly(methyl methacrylate) (PMMA) substrate. Graphene and h-BN are chosen as sensitive materials because they both have large Young's modulus, high intrinsic strength, high natural frequency, and atomic thickness at the same time. These characteristics provide favorable conditions for the application of the sensor in the high pressure and high frequency dynamic environment. Moreover, the photoresist-assisted transfer technology is proposed for transferring graphene from the growth substrate to the PMMA substrate and the lift-off method with exposure and development is developed to achieve metal patterning on the PMMA substrate. The sensor characterization results suggest that the graphene and h-BN films have good transfer qualities and the heterojunction possesses excellent electrical performance. The static pressure loading experiments confirm that the sensor has a pressure range of up to 85 MPa and its piezoresistive coefficient is 0.7 GPa−1, which indicates that the designed sensor is suitable for overpressure fields. This study provides a novel method for determining overpressure and lays a foundation for the fabrication of graphene-based electronic devices with an organic substrate. [ABSTRACT FROM AUTHOR]

The low thermal expansion coefficient, low dielectric constant, high thermal conductivity, high electrical resistivity, and excellent thermal stability of hexagonal boron nitride (hBN) make it an ideal filler for insulating and thermally conductive composite films. Its use has been suggested in insulating packaging systems to solve the heat dissipation problems in modern electronic equipment. Here, a novel strategy was proposed to construct superior thermally conductive networks via the vacuum-assisted filtration of hydroxyl-modified hexagonal boron nitride (BN-OH) and lignocellulosic fiber (LCF). The π–π interactions between the benzene ring of LCF and the hBN plane assist in the dispersal of BN-OH in aqueous solution. The abundance of functional groups such as hydroxyl and phenolic hydroxyl groups in the LCF structure allow for hydrogen bonding with the hydroxyl groups on the BN-OH surface, increasing interfacial interactions between BN-OH and LCF and reducing interfacial thermal resistance. In addition, the long-range one-dimensional structure of LCF can assist in the formation of a self-supporting, high-thermal-conductivity film with a high content of BN-OH. The through-plane thermal conductivity of BN-OH/LCF reached 5.34 W/(m⋅K) at 90 wt.% BN loading. This work provides inspiration for the green preparation of hBN thermally conductive composite films with high filler loadings. [ABSTRACT FROM AUTHOR]

Film capacitors have widely been used in modern electrical/electronic equipment because of their high insulation property and high-power density. However, the traditional dielectric films suffered a high conduction loss under high temperature and electric field conditions, which leads to the depression of high temperature energy storage properties. Herein, sandwich-structured composite films using biaxially oriented polyethylene terephthalate (BOPET) as the outer layers were prepared to construct the interfacial barrier, which was beneficial for hindering the propagation of breakdown path. In addition, epoxy resin was applied as the adhesive inner layer to bind the outer layers up tightly, while the boron nitride nanosheets (BNNSs) in the inner layer further improved the breakdown strength of the film and inhibited the conduction loss, especially under high temperature and electric field conditions. The composite film with optimized BNNSs content had a high discharged energy density of 9.11 J/cm3 and a ultrahigh charge–discharge efficiency of 95% at 25 °C, while the values were 6.45 J/cm3 and 70% at 150 °C, respectively, of which both were much higher than those of pure BOPET. Thus, the sandwich-structure method points out a promising way of preparing dielectric films with excellent energy storage performances. [ABSTRACT FROM AUTHOR]

Neutrons are encountered in many different fields, including condensed matter physics, space exploration, nuclear power, and healthcare. Neutrons interacting with a biological target produce secondary charged particles that are damaging to human health. The most effective way to shield neutrons is to slow them to thermal energies and then capture the thermalized neutrons. These factors lead us to consider potential materials solutions for neutron shields that maximize the protection of humans while minimizing the shield mass and adapt well to modern additive manufacturing techniques. Using hexagonal boron nitride (hBN) as a capture medium and high-density polyethylene (HDPE) as a thermalization medium, we aim to design the optimal internal structure of h10BN/HDPE composites by minimizing the effective dose, which is a measure of the estimated radiation damage exposure for a human. Through Monte Carlo simulations in Geant4, we find that the optimal structure reduces the effective dose up to a factor of 72 over aluminum (Al) and up to a factor of 4 over HDPE; this is a significant improvement in shielding effectiveness that could dramatically reduce the radiation exposure of occupational workers. [ABSTRACT FROM AUTHOR]

We present the fabrication and characterization of a flash memory device based on a van der Waals (vdW) heterostructure comprising graphene (Gr), hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2). The device exhibits versatile functionalities and demonstrates memtransistor-like behavior, making it a promising candidate for synaptic devices and neuromorphic computing. The Gr/h-BN/MoS2 heterostructure allows for the manipulation of charge concentration in the floating gate (FG) through control gate voltage (VBG), resulting in conductance switching. Moreover, the overlap structure of the FG and source-drain electrodes enables memtransistor-like operations, where the writing processes rely on the source-drain voltage (VDS). By varying carrier storage concentrations in the Gr-FG, multiple memory states can be achieved, offering analog information processing capabilities. In addition, the combination of dual-input functionality allows for the identification of a substantial on/off ratio even when utilizing a lower reading VDS, consequently strengthening the dependability of the memory state. Our work showcases the great potential of the Gr/h-BN/MoS2 heterostructure for advanced nanoelectronics, providing a platform for the development of computing architectures and neuromorphic systems. [ABSTRACT FROM AUTHOR]

The study of hexagonal boron nitride (hBN) in microfluidic and nanofluidic applications at the atomic level requires accurate force field parameters to describe the water-hBN interaction. In this work, we begin with benchmark quality first principles quantum Monte Carlo calculations on the interaction energy between water and hBN, which are used to validate random phase approximation (RPA) calculations. We then proceed with RPA to derive force field parameters, which are used to simulate water contact angle on bulk hBN, attaining a value within the experimental uncertainties. This paper demonstrates that end-to-end multiscale modeling, starting at detailed many-body quantum mechanics and ending with macroscopic properties, with the approximations controlled along the way, is feasible for these systems. [ABSTRACT FROM AUTHOR]

BN nanotubes (BNNTs) are structurally similar to their carbon counterparts, though much less investigated. New synthesis methods have been recently reported, enabling the production of industrial quantities and stimulating the search of new applications for the BNNTs. In this paper, we investigate the luminescence of multiwall BNNTs. By performing cathodoluminescence experiments on single tubes at 10K, we show that the tube luminescence is highly heterogeneous (i) from tube to tube and (ii) spatially along a single tube. By combining cathodoluminescence measurements with a nanometer excitation and transmission electron microscopy on the same tube, we correlate luminescence and structural features. We conclude that the near-band-edge luminescence of BNNTs (≈5.4 eV) is related to the presence of extended structural defects, such as dislocations or ruptures in the wall stacking. [ABSTRACT FROM AUTHOR]

Pressure-induced phase transition of boron nitride nanotubes (BNNTs) provides an effective approach to develop new boron nitride nanostructures with more desirable functions than those of carbon nanotubes, owing to the unique polar B–N bonds. However, the synthetic BNNTs usually comprise double- or multi-walls, whose structural evolution under pressure is complicated and remains largely elusive. Here, we unveil the complete phase transition behavior of hexagonal bundles of double-walled (DW) BNNTs of different chirality and diameters under hydrostatic pressures of up to 60 GPa. A series of new monolith phases are obtained from the compressed DW-BNNT bundles, whose structures can be well retained even after releasing the pressure. The bonding characters; electronic, optical, and mechanical properties; and Raman signature of these monolith phases are elucidated, which provide essential guidance for synthesis of new boron nitride materials with unprecedented properties for technological applications. [ABSTRACT FROM AUTHOR]

Boron dangling bonds have been proposed as the likely source of the 2-eV single-photon emission observed in hexagonal boron nitride (h-BN). Here, we examine the effect of dimensionality on this defect by studying its properties in a single monolayer. Based on first-principles calculations, we find that the qualitative features of the boron dangling bond are similar to those in bulk h-BN, with only minor shifts to the energetics and optical properties. We discuss the importance of correctly handling the compensating background charge for the properties of charged defects in reduced dimensions. Our results highlight the potential for harnessing single-photon emitters in a monolayer of h-BN for applications in quantum information science. [ABSTRACT FROM AUTHOR]

Zeolite-supported carbon nanotube (CNT) synthesis provides a route for its mass production due to the porous surfaces accommodating a large number of catalytic particles and maintaining the fine particle sizes during high-temperature growth. Coaxial single-walled carbon nanotubes (SWCNTs) and boron nitride nanotube (BNNT) van der Waals heterostructures have been produced recently on the CNT thin-film template. To achieve a high yield of the one-dimensional heterostructure, BNNT-wrapped SWCNTs were synthesized on zeolite support coated with catalytic nanoparticles. They were then dispersed in solution and examined by absorption, photoluminescence, and Raman spectroscopy. A robust thermal stability enhancement was observed, and optical characterizations revealed the composition of dispersed SWCNTs wrapped by BNNTs before air annealing includes individual SWCNTs, BNNT-wrapped SWCNT bundles, and BNNT-wrapped individual SWCNTs. Furthermore, the outside wrapping by BNNTs caused a significant down-shift of the photoluminescence spectrum in semiconducting SWCNTs, suggesting that BNNT-wrapped SWCNTs can be preserved after harsh dispersion treatments. [ABSTRACT FROM AUTHOR]

Applying classical molecular dynamics simulations, we report the effects of length (λ) and orientation (θ) of a line-defect on strength and toughness in defective 2D hexagonal boron nitride. Results reveal the existence of a "transition angle," θ t = 2.47 ° , at which both toughness and strength are insensitive to the finite length of the defect in an infinite domain. For θ < θ t , both toughness and strength increase with an increase in defect-length; whereas, for θ > --> θ t , they show the opposite behavior. Examination of the stress-fields shows that θ -dependent variation in stress-localization at the edges of the line-defect and symmetry-breaking of the stress-fields with respect to the defect-axis govern the disparate θ -dependent behavior. For θ < θ t , the intensity of elastic fields at the edges of the line-defect is substantially weakened by the elastic interactions originating from the atoms on the sides of the line-defect. For θ > --> θ t , the stress-intensity at the edges is strongly localized at the opposite sides of the line-defect. The stress-intensity increases asymptotically with the increasing defect-length and reduces the strength and toughness of the defective lattice. The stress-localization, however, saturates at a "saturation angle" of around 60 ° for strength and 30 ° for toughness. Additionally, there exists a critical defect-length λ c = 60 Å, below which there is a strong θ -dependent variation in elastic interactions between the edges, affecting strength and toughness substantially. For λ > --> λ c , the elastic interactions saturate and make both strength and toughness insensitive to the change in the length of the defect. [ABSTRACT FROM AUTHOR]

This study uses non-equilibrium molecular dynamics simulations to investigate the impact of antisite substitution on thermal conductivity. The phonon-dispersion curve and predicted thermal conductivity of pristine hexagonal boron nitride nanosheets (hBNNSs) show good agreement with the experiment results, indicating the reliability of the extep potential. It is clear that both neighboring substitution (NS) and random substitution (RS) drastically reduce the thermal conductivity of hBNNSs, of which RS has a larger effect. Calculations for the participation ratio and relaxation time show that the localization is the primary cause for the reduction in thermal conductivity when the defect concentration is low. When the defect concentration is higher, the primary cause is phonon-defect scattering in all phonon modes. RS has a larger effect on the phonon modes with long mean free paths, while NS has a larger effect on phonon modes with various lengths of mean free paths. [ABSTRACT FROM AUTHOR]

Newly synthesized nanocars have shown great potential to transport molecular payloads. Since wheels of nanocars dominate their motion, the study of the wheels helps us to design a suitable surface for them. We investigated C60 thermal diffusion on the hexagonal boron-nitride (h-BN) monolayer as the wheel of nanocars. We calculated C60 potential energy variation during the translational and rotational motions at different points on the substrate. The study of the energy barriers and diffusion coefficients of the molecule at different temperatures indicated three noticeable changes in the C60 motion regime. C60 starts to slide on the surface at 30 K–40 K, slides freely on the boron-nitride monolayer at 100 K–150 K, and shows rolling motions at temperatures higher than 500 K. The anomaly parameter of the motion reveals that C60 has a diffusive motion on the boron-nitride substrate at low temperatures and experiences superdiffusion with Levy flight motions at higher temperatures. A comparison of the fullerene motion on the boron-nitride and graphene surfaces demonstrated that the analogous structure of the graphene and hexagonal boron-nitride led to similar characteristics such as anomaly parameters and the temperatures at which the motion regime changes. The results of this study empower us to predict that fullerene prefers to move on boron-nitride sections on a hybrid substrate composed of graphene and boron-nitride. This property can be utilized to design pathways or regions on a surface to steer or trap the C60 or other molecular machines, which is a step toward directional transportation at the molecular scale. [ABSTRACT FROM AUTHOR]