Search

Materials that exhibit both strong covalent and weak van der Waals interactions pose a considerable challenge to many computational methods, such as DFT. This makes assessing the accuracy of calculated properties, such as exfoliation energies in layered materials like hexagonal boron nitride (h‐BN) problematic, when experimental data are not available. In this paper, we investigate the accuracy of equilibrium lattice constants and exfoliation energy calculation for various DFT‐based computational approaches in bulk h‐BN. We contrast these results with available experiments and reference fixed‐node diffusion quantum Monte Carlo (QMC) results. From our reference QMC calculation, we obtained an exfoliation energy of −33±2 meV/atom (‐0.38±0.02 J/m2). [ABSTRACT FROM AUTHOR]

Synaptic device is an important component in artificial neural networks. Electrically‐controlled long‐term depression (LTD) is demonstrated in three‐terminal photonic synaptic devices, which is useful for improving the efficiency of artificial neural networks. The mechanism of the three‐terminal photonic synaptic device is similar to that of the photo‐induced doping effect. Photo‐assisted carrier transport and recombination are expected to accelerate the LTD process. However, the effects of photon illumination on LTD are not investigated. Here, the effects of electrical stimulation and photo‐stimulation on the long‐term plasticity (LTP) and LTD process of photonic synaptic devices based on MoS2/h‐BN field‐effect transistors (FETs) are systemically investigated. The influences of gate dielectric layer on the photo‐induced doping are explored. The non‐volatile and electrically‐erasable properties of photo‐induced doping in MoS2/h‐BN FETs are due to the van der Waals energy barrier at the MoS2/h‐BN interface. The synaptic functions of LTP and LTD can be mimicked by the photo‐induced doping effect. The LTD process will be accelerated with the applications of positive gate voltage and laser illumination, which improves the speed of information processing. These results presented in this paper are useful for realizing high‐performance synaptic devices based on the photo‐induced doping effect. [ABSTRACT FROM AUTHOR]

Previous work that studied hexagonal boron nitride (h‐BN) memristor DC resistive‐switching characteristics is extended to include an experimental understanding of their dynamic behavior upon programming or synaptic weight update. The focus is on the temporal resistive switching response to driving stimulus (programming voltage pulses) effecting conductance updates during training in neural network crossbar implementations. Test arrays are fabricated at the wafer level, enabled by the transfer of CVD‐grown few‐layer (8 layer) or multi‐layer (18 layer) h‐BN films. A comprehensive study of their temporal response under various conditions–voltage pulse amplitude, edge rate (pulse rise/fall times), and temperature–provides new insights into the resistive switching process toward optimized devices and improvements in their implementation of artificial neural networks. The h‐BN memristors can achieve multi‐state operation through ultrafast pulsed switching (< 25 ns) with high energy efficiency (≈10 pJ pulse−1). [ABSTRACT FROM AUTHOR]

Hexagonal boron nitride (h‐BN) has received considerable attention, due to its high thermal conductivity and electrical insulation. However, the intrinsic platelike structure with the strong anisotropic property restricts its applications, and it is necessary to synthesize isotropic spherical h‐BN particles (SP‐BNs) with submicron size. Till now, methods to prepare (SP‐BN) still exist problems, such as high oxygen impurities and pollution, generated by the ammonia and pyrolysis of precursors. Here, a relatively green reaction between the restricted template of carbon nanospheres and boron trioxide (B2O3) under elevated temperature is conducted, and the SP‐BNs with an average diameter of 200–300 nm (named Nano‐BN‐s) have been successfully synthesized. Comprehensive scanning electron microscopy, transmission electron microscopy, and X‐ray diffraction characterizations confirm the obtained products are spherical boron nitride. With the analysis of X‐ray photoelectron spectroscopy and Fourier transform infrared, the reaction mechanism is briefly discussed. These results indicate the reaction occurs on the restricted template of carbon nanospheres, and the C atoms are substituted by B and N atoms as the reaction progress, forming the Nano‐BN‐s. What is more, the restricted template method plays a key role in the design and improves h‐BN‐based materials in the future and may also be extended to form other novel materials. [ABSTRACT FROM AUTHOR]

Here, we studied two different 2D monolayer systems (i.e., h‐BN and SiC), which exhibit unique electronic and magnetic properties. We analyzed the effects of Transition Methods (TM) doped atoms (i.e., Mn, Ni, and Sc) on the single vacancy (SV) h‐BN and SiC systems using first principles calculations. Through comparison we found that Mn substitution in SV h‐BN and SiC can modify their electronic and magnetic properties having larger magnetic moment as compared to other dopants (i.e., Ni and Sc.). Band structure and PDOS plots confirmed that TM doping in SV h‐BN can convert the pure h‐BN (insulator) to semiconductor, metal, or semi‐metal, it is also observed from the results that Mn‐doped h‐BN has higher band gap approximately equal to Eg ~ 2.7 eV during the negative spin and has smaller band gap, that is, (Eg ~ 1.1 eV) during the positive spin. Similarly doping with the TM atoms on the SV SiC monolayer system can convert pure SiC to metal or half metal. In addition, Mn‐doped SiC showed semimetal property having 0 eV band gap. These finding will help us to vary the electronic and magnetic properties of the pure h‐BN and SiC sheets which can be used for the opto‐electronic and spintronic applications. [ABSTRACT FROM AUTHOR]

First principle calculations of the phonons of graphene- h-BN heterostructures are presented and compared to those of the constituents. It is shown that AA and AB′ stacking are not only energetically less favoured than AB but also dynamically unstable. Low energy flat phonon branches of h-BN character with out of plane displacement have been identified and their coupling to electrons in graphene has been evaluated. [ABSTRACT FROM AUTHOR]

In the modern age of nanotechnology, the discovery of graphene has opened up the way to study and develop of several novel 2D materials. The unique physical and chemical properties of 2D materials have enhanced their research, making them superior to the commercial bulk materials used in various applications. Efforts have been made in the current study to present an overview of the intrinsic properties of these materials. Furthermore, synthesis and applications are also reviewed and discussed. Finally, the future outlook of 2D materials is discussed to enhance the research and performance of these materials, which can result in broader applications benefitting the electrical and electronics industries and society. Intensive research into 2D materials is expected to lead to the discovery of new materials with enhanced properties that will benefit the industry and society at large. [ABSTRACT FROM AUTHOR]

Bonding strength between fibers and matrices of ceramic matrix composites (CMCs) is of great importance for the fracture behavior and can be adjusted by appropriate fiber coatings. In this study, the continuous coating of SiBNC fibers with hexagonal boron nitride (h-BN) by CVD from TMB in an atmospheric pressure reactor is presented. Prior to fiber coating, the effects of deposition parameters, e.g., deposition temperature, concentration of the precursor and ammonia addition, and gas-flow rate are investigated on flat Si substrates, and the results are utilized for the fiber coating. Ammonia concentration has no influence on the deposition rate. A maximum deposition rate of 0.044 kg m−2 h−1 for static conditions and 0.040 kg m−2 h−1 for continuous conditions can be achieved, which is comparable to the literature. The depositions are characterized for their composition and structure. Smooth homogeneous coatings with a thickness of 180-250 nm are found on the fibers. High-resolution transmission electron microscopy (HRTEM) of the coated SiBNC fibers reveals that the hexagonal BN with 5 nm basal planes is mainly randomly oriented. [ABSTRACT FROM AUTHOR]

A thermal conductive plate in a device like a power module for an automobile requires materials with high thermal conductivity, acceptable electric breakdown strength, and high thermal stability. Thermoplastic polyimide (tpPI)/oriented hexagonal boron nitride (h‐BN) composite materials are produced by an electrostatic adsorption method. The tpPI/oriented h‐BN composite shows high thermal properties (glass transition temperature of about 240 °C and a thermal conductivity of 10 W/mK or more, so on.) In addition, the direct current breakdown strength exceeds 150 kV/mm. It is cleared that the tpPI/oriented h‐BN composite exhibits an acceptable breakdown strength, high thermal conductivity, and high thermal stability. © 2020 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Strong in-plane bonding (covalent) and weak van der Waals (vdW) interplanar interactions characterize a number of layered solids, as epitomized by graphite. The advent of graphene (Gr), individual atomic two-dimensional (2D) layers, isolated from mineral graphite via micromechanical exfoliation enabled the ability to pick, place and stack of arbitrary compositions. Moreover, this discovery implicated an access to other 2D vdW solids beyond graphene and artificially stacking atomic layers forming heterostructures/superlattices. Raman spectroscopy (RS) is a fast reliable non-destructive analytical tool and an integral part for lattice dynamical structural characterization of crystalline solids at nanoscale, revealing not only the collective atomic/molecular motions but also localized vibrations/modes and specifically used to determine the number of graphene layers and of other 2D vdW solids. We present Raman spectroscopy in first-, second- and higher-order vibrational modes involving 3 and 4 phonons (overtones and combination) and mapping of graphene (mono-, bi-, tri- and few-) layers, semiconducting transition metal dichalcogenides (TMDs) [molybdenum disulfide (MoS2) and tungsten disulfide (WS2)] and wide bandgap hexagonal boron nitride (h-BN) dispersed monolayers, revealing various molecular vibrations and structural quality/disorder. First- and higher-order phonon modes are observed and analyzed in terms of Raman intensity (spatial inhomogeneity or thickness variation), band position (intrinsic mechanical strain) and intensity ratio (structural disorder) as a function of graphene layer ( n). An empirical relation for G band position with n is corroborated. All of the higher order modes are observed to upshift almost linearly with n, betraying the underlying interlayer vdW interactions. These findings exemplify the evolution of structural parameters in layered materials in changing from 3- to 2- or low-dimensional regime. The results are presented in view of applications of graphene by itself and in combination that help better understanding of physical and electronic properties for nano-/optoelectronics. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The two-dimensional sp2-bonded material hexagonal boron nitride (h-BN) has unique electronic, thermal, mechanical, and chemical properties. It has recently found use as an ideal substrate for graphene-based electronic devices. We here describe synthesis of mono- to few-layer h-BN films using low pressure chemical vapor deposition (LPCVD) from borazine, with nickel, copper and platinum employed as catalytic substrates, and transfer of some of these films using a non-polymer method. Characterization of the films via Raman spectroscopy and transmission electron microscopy (TEM) is performed. Chemical vapor deposition synthesis of hexagonal boron nitride from borazine using metallic substrates. [ABSTRACT FROM AUTHOR]

The synthesis of lateral heterostructures of graphene and hexagonal boron nitride (h‐BN) has attracted attention due to the ability to broadly tune the electronic properties of the hybrid monolayer. Conventional synthesis methods for lateral heterostructures rely on chemical vapor deposition on metal surfaces, and application of the heterostructure in devices requires transfer of the material onto a suitable substrate during which the sample is susceptible to damage or contamination. Herein, a transfer‐free synthesis method to produce lateral heterostructures of graphene and h‐BN by chemical conversion of epitaxial graphene on 6H‐SiC(0001) is demonstrated. X‐ray photoelectron spectroscopy measurements confirm the substitution of graphene with h‐BN, while scanning tunneling microscopy reveals that the h‐BN domains reside in‐plane with graphene forming an interface along the zig‐zag direction. Raman spectroscopy measurements provide insight into the reaction mechanism in which h‐BN is substituted for graphene at defect sites thereby reducing the defect density in the lateral heterostructure. The development of scalable, transfer‐free synthesis methods for lateral heterostructures of graphene and h‐BN, as demonstrated in this work, is expected to facilitate a route toward large‐scale synthesis of 2D semiconductors and atomically thin circuitry directly on‐chip. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) layered materials are drawing considerable attention across multiple scientific and engineering disciplines owing to their exquisite electronic, optoelectronic, mechanical and chemical properties. In this article, a short review of our recent work on 2D-electronics will be presented. [ABSTRACT FROM AUTHOR]