High-quality Hexagonal Boron Nitride Research Articles

Pressureless-sintered boron nitride nanosheets/glass composite ceramics for excellent mechanical, dielectric and thermo-conductive performances

Because they have a high application potential in the thermal management of insulation environments, high-quality hexagonal boron nitride (h-BN)-based multiphase ceramics have been highly desired. However, so far, their synthesis is still full of challenges. Here, a kind of boron nitride nanosheets (BNNSs)/glass (GS) composite ceramics was prepared by a pressureless sintering method at a lower temperature of 900 °C. Due to a tightly bonded interaction between BNNSs and GS, the formed BNNSs/GS ceramics exhibit excellent multifunction performance. They have an outstanding compressive strength in the range of 19 ∼ 64 MPa and Vickers hardness ranging from 50 to 179 HV. For the BNNSs/GS ceramics with BNNS’s filling fraction of 90 wt%, their maximum side-surface TC values are 12.01 ± 0.18 W m−1 K−1 at 25 °C and 13.64 ± 0.37 W m−1 K−1 at 300 °C, respectively. In the ultra-high frequency range of 26.5 ∼ 40 GHz, the dielectric constant values of the BNNSs/GS ceramics are primarily between 2 and 3, and the corresponding loss tangent values are < 0.3. In addition, based on the remarkable integrity of their structure, these BNNSs/GS ceramics exhibit outstanding thermal-shock stability and prominent thermal management capacity during lots of heating/cooling-testing cycles. Therefore, we believe this kind of BNNSs/GS ceramic system will have great application potential in the new-generation thermal management and/or insulation packaging fields.

Controllable Synthesis of High-Quality Hexagonal Boron Nitride Films on Ni-B Alloy

Controllable Synthesis of High-Quality Hexagonal Boron Nitride Films on Ni-B Alloy

High-Throughput Growth of Hexagonal Boron Nitride Film Using Porous-Structure Isolation Layer

Chemical vapor deposition is considered as the most hopeful method for the synthesis of large-area high-quality hexagonal boron nitride on the substrate of catalytic metal. However, the size the hexagonal boron nitride films are limited to the size of growth chamber, which indicates a lower production efficiency. In this paper, the utilization efficiency of growth chamber is highly improved by alternately stacking multiple pieces of Cu foils and carbon fiber surface felt with porous structure. Uniform and continuous hexagonal boron nitride films are prepared on Cu foils through chemical vapor deposition utilizing ammonia borane as the precursor. This work develops a simple and practicable method for high-throughput preparation of hexagonal boron nitride films, which could contribute to the industrial application of hexagonal boron nitride.

Hysteretic behavior of all CVD h-BN/graphene/h-BN heterostructure field-effect transistors by interfacial charge trap

Superior electrical transport properties of graphene field-effect transistors (FETs) can be achieved when graphene is supported by a high-quality hexagonal boron nitride (h-BN) layer. The h-BN layer can serve as a high-performance insulator for a graphene channel to achieve high carrier mobility and to minimize doping effects on graphene by screening out the interaction with a gate oxide substrate. However, chemical vapor deposition (CVD) grown h-BN can have surface defects and impurities unlike mechanically exfoliated h-BN layers, which can result in completely different electrical transport properties of graphene FETs. In this study, we fabricated all CVD h-BN/graphene/h-BN FETs to explore how surface defects and impurities on the supporting h-BN layer can influence electrical transport properties of graphene. The presence of surface defects and impurities on the supporting h-BN layer, as revealed by Raman spectroscopy analysis, resulted in significant p-doping and hysteresis in the h-BN/graphene/h-BN FETs. Hysteresis in the FETs was induced by charge carrier trapping at the surface defects and impurities of the h-BN layer, which can be controlled by modulating a back-gate voltage. Based on the charge trapping mechanism on the surface of the h-BN, electrical charge states in all CVD h-BN/graphene/h-BN can be modulated dynamically by realizing two different resistance states (‘trap’ vs. ‘release’ states), which can be utilized as memristive devices or sensors.

The Winner Takes It All: Carbon Supersedes Hexagonal Boron Nitride with Graphene on Transition Metals at High Temperatures.

The production of high-quality hexagonal boron nitride (h-BN) is essential for the ultimate performance of 2D materials-based devices, since it is the key 2D encapsulation material. Here, a decisive guideline is reported for fabricating high-quality h-BN on transition metals. It is crucial to exclude carbon from the h-BN related process, otherwise carbon prevails over boron and nitrogen due to its larger binding energy, thereupon forming graphene on metals after high-temperature annealing. The surface reaction-assisted conversion from h-BN to graphene with high-temperature treatments is demonstrated. The pyrolysis temperature Tp is an important quality indicator for h-BN/metals. When the temperature is lower than Tp , the quality of the h-BN layer is improved upon annealing. While the annealing temperature is above Tp , in case of carbon-free conditions, the h-BN disintegrates and nitrogen desorbs from the surface more easily than boron, eventually leading to clean metal surfaces. However, once the h-BN layer is exposed to carbon, graphene forms on Pt(111) in the high-temperature regime. This not only provides an indispensable principle (avoid carbon) for fabricating high-quality h-BN materials on transition metals, but also offers a straightforward method for the surface reaction-assisted conversion from h-BN to graphene on Pt(111).

First-principles study of the growth and diffusion of B and N atoms on the sapphire surface with h-BN as the buffer layer

Currently, the preparation of large-size and high-quality hexagonal boron nitride is still an urgent problem. In this study, we investigated the growth and diffusion of boron and nitrogen atoms on the sapphire/h-BN buffer layer by first-principles calculations based on density functional theory. The surface of the single buffer layer provides several metastable adsorption sites for free B and N atoms due to exothermic reaction. The adsorption sites at the ideal growth point for B atoms have the lowest adsorption energy, but the N atoms are easily trapped by the N atoms on the surface to form N–N bonds. With the increasing buffer layers, the adsorption process of free atoms on the surface changes from exothermic to endothermic. The diffusion rate of B atoms is much higher than that of the N atoms thus the B atoms play a major role in the formation of B–N bonds. The introduction of buffer layers can effectively shield the negative effect of sapphire on the formation of B–N bonds. This makes the crystal growth on the buffer layer tends to two-dimensional growth, beneficial to the uniform distribution of B and N atoms. These findings provide an effective reference for the h-BN growth.

Effect of employing chromium as a buffer layer on the crystallinity of hexagonal boron nitride films grown by LPCVD

Using chromium (Cr), which was deposited by radio frequency magnetron sputtering, as a buffer layer for synthesizing high-quality hexagonal boron nitride (hBN) films by low-pressure chemical vapor deposition (LPCVD) was demonstrated. The effect of growth temperature and annealing process on the quality of the Cr buffer layer was investigated. The characterization of the dependence of hBN film quality on growth temperature, substrate, and annealing process was discussed. All evidence shows that using a Cr buffer layer can significantly improve the crystalline quality of hBN. A DUVPD based on hBN film using Cr as the bottom electrode was fabricated with a small leakage current. The photocurrent is 3.5 nA at a bias of 500 V, and it exhibits good switching characteristics.

Water Icing Triggered Scalable and Controllable Exfoliation of Hexagonal Boron Nitride Nanosheets

Herein, a scalable and controllable approach is proposed to exfoliate high quality hexagonal boron nitride nanosheets (h-BNNSs) from h-BN flakes, which relies on the efficient reduction of h-BNNS interlayer interaction by rapid volume expansion of water in icing. Water can enter into the h-BNNS interlayer through the “entry” caused by the hydroxyl induced local structure distortion as demonstrated by molecular dynamics simulations. The entered water could form fairly compact initial nuclei, and then ice and rapidly expand as temperature drops sharply, resulting in the interlayer space increase and interlayer force reduction between h-BNNSs and thus achieving efficient exfoliation of h-BNNSs in subsequent ultrasonication. The thicknesses of h-BNNSs are dominated by freezing and exfoliation cycles, and monolayer h-BNNSs can be obtained after five cycles of freezing and exfoliation currently. The as-obtained h-BNNSs have excellent dispersibility in water enabling their versatile applications as polymer additives, thermal conductive fillers, and flame-retardants, etc.

High-Quality Hexagonal Boron Nitride from 2D Distillation.

The production of high-quality two-dimensional (2D) materials is essential for the ultimate performance of single layers and their hybrids. Hexagonal boron nitride (h-BN) is foreseen to become the key 2D hybrid and packaging material since it is insulating, impermeable, flat, transparent, and chemically inert, though it is difficult to attain in ultimate quality. Here, a scheme is reported for producing single layer h-BN that shows higher quality in view of mosaicity and strain variations than material from chemical vapor deposition (CVD). We delaminate CVD h-BN from Rh(111) and transfer it to a clean metal surface. The twisting angle between BN and the second substrate yields metastable moiré structures. Annealing above 1000 K leads to 2D distillation, i.e., catalyst-assisted BN sublimation from the edges of the transferred layer and subsequent condensation into superior quality h-BN. This provides a way for 2D material production remote from CVD instrumentation.

Growth of High-Quality Hexagonal Boron Nitride Single-Layer Films on Carburized Ni Substrates for Metal-Insulator-Metal Tunneling Devices.

Two-dimensional (2D) hexagonal boron nitride (h-BN) plays a significant role in nanoscale electrical and optical devices because of its superior properties. However, the difficulties in the controllable growth of high-quality films hinder its applications. One of the crucial factors that influence the quality of the films obtained via epitaxy is the substrate property. Here, we report a study of 2D h-BN growth on carburized Ni substrates using molecular beam epitaxy. It was found that the carburization of Ni substrates with different surface orientations leads to different kinetics of h-BN growth. While the carburization of Ni(100) enhances the h-BN growth, the speed of the h-BN growth on carburized Ni(111) reduces. As-grown continuous single-layer h-BN films are used to fabricate Ni/h-BN/Ni metal-insulator-metal (MIM) devices, which demonstrate a high breakdown electric field of 12.9 MV/cm.

Deep ultraviolet hyperspectral cryomicroscopy in boron nitride: Photoluminescence in crystals with an ultra-low defect density

We report the development of a scanning confocal microscope dedicated to photoluminescence in the 200 nm-wavelength range for samples at cryogenic temperatures (5 K–300 K). We demonstrate the performances of our deep ultraviolet cryomicroscope in high-quality hexagonal boron nitride (hBN) crystals, although it can be utilized for biological studies in its range of operating wavelengths. From the mapping of photoluminescence, we bring evidence for the suppression of extrinsic recombination channels in regions free from defects. The observation of emission spectra dominated by intrinsic recombination processes was never reported before in hBN by means of photoluminescence spectroscopy. We show that photoluminescence tomography now competes with cathodoluminescence and that deep ultraviolet cryomicroscopy by photoluminescence is a novel powerful tool in materials science applications, with the great advantage of an efficient non-invasive photo-excitation of carriers.

Bluish emission of economical phosphor h-BN nanoparticle fabricated via mixing annealing route using non-toxic precursor

In the present work, a facile and economical mixing annealing approach was employed using a nontoxic precursor (boric acid and urea) and reactive atmosphere (Ar & N2) at 1000 ​°C to produce high quality hexagonal boron nitride (h-BN) nano-particles. The preferred (002) orientations confirmed the hexagonal phase and average crystallite size ​≤ ​5 ​nm. The reactive environments tune the surface microstructure into 50–60 ​nm diameter micro-flakes (NFs) and 2–4 ​μm length nano-sheets (NSs) for N2 & Ar atmosphere, respectively. The chemical characteristics of the prepared nanoparticles have also been investigated in term of composition, bonding and chemical states. Optical studies revealed the applied atmosphere dependent optical properties i.e. optical band gap ~5.48 ​eV & 5.25 ​eV, emission band ~451 & 454 ​nm and CIE coordinates (0.2550, 0.2562) & (0.2543, 0.2516) indicate to bluish emission. These results present prodigious prospective of novel h-BN nanoparticles for various optically active practical applications.

Blue-light-emitting color centers in high-quality hexagonal boron nitride

Light emitters in wide band gap semiconductors are of great fundamental interest and have potential as optically addressable qubits. Here we describe the discovery of a new color center in high-quality hexagonal boron nitride (h-BN) with a sharp emission line at 435 nm. The emitters are activated and deactivated by electron beam irradiation and have spectral and temporal characteristics consistent with atomic color centers weakly coupled to lattice vibrations. The emitters are conspicuously absent from commercially available h-BN and are only present in ultra-high-quality h-BN grown using a high-pressure, high-temperature Ba-B-N flux/solvent, suggesting that these emitters originate from impurities or related defects specific to this unique synthetic route. Our results imply that the light emission is activated and deactivated by electron beam manipulation of the charge state of an impurity-defect complex.

Wafer-scale and selective-area growth of high-quality hexagonal boron nitride on Ni(111) by metal-organic chemical vapor deposition

We demonstrate wafer-scale growth of high-quality hexagonal boron nitride (h-BN) film on Ni(111) template using metal-organic chemical vapor deposition (MOCVD). Compared with inert sapphire substrate, the catalytic Ni(111) template facilitates a fast growth of high-quality h-BN film at the relatively low temperature of 1000 °C. Wafer-scale growth of a high-quality h-BN film with Raman E2g peak full width at half maximum (FWHM) of 18~24 cm−1 is achieved, which is to the extent of our knowledge the best reported for MOCVD. Systematic investigation of the microstructural and chemical characteristics of the MOCVD-grown h-BN films reveals a substantial difference in catalytic capability between the Ni(111) and sapphire surfaces that enables the selective-area growth of h-BN at pre-defined locations over a whole 2-inch wafer. These achievement and findings have advanced our understanding of the growth mechanism of h-BN by MOCVD and will contribute an important step toward scalable and controllable production of high-quality h-BN films for practical integrated two-dimensional materials-based systems and devices.

Hexagonal Boron Nitride Growth on Cu-Si Alloy: Morphologies and Large Domains.

Controllable synthesis of high-quality hexagonal boron nitride (h-BN) is desired toward the industrial application of 2D devices based on van der Waals heterostructures. Substantial efforts are devoted to synthesize h-BN on copper through chemical vapor deposition, which has been successfully applied to grow graphene. However, the progress in synthesizing h-BN has been significantly retarded, and it is still challenging to realize millimeter-scale domains and control their morphologies reliably. Here, the nucleation density of h-BN on Cu is successfully reduced by over two orders of magnitude by simply introducing a small amount of silicon, giving rise to large triangular domains with maximum 0.25 mm lateral size. Moreover, the domain morphologies can be modified from needles, tree patterns, and leaf darts to triangles through controlling the growth temperature. The presence of silicon alters the growth mechanism from attachment-limited mode to diffusion-limited mode, leading to dendrite domains that are rarely observed on pure Cu. A phase-field model is utilized to reveal the growing dynamics regarding B-N diffusion, desorption, flux, and reactivity variables, and explain the morphology evolution. The work sheds lights on the h-BN growth toward large single crystals and morphology probabilities.

Electronic structure of graphene nanoribbons on hexagonal boron nitride

Hexagonal boron nitride is an ideal dielectric to form two-dimensional heterostructures due to the fact that it can be exfoliated to be just few atoms thick and its a very low density of defects. By placing graphene nanoribbons on high quality hexagonal boron nitride it is possible to create ideal quasi one dimensional (1D) systems with very high mobility. The availability of high quality one-dimensional electronic systems is of great interest also given that when in proximity to a superconductor they can be effectively engineered to realize Majorana bound states. In this work we study how a boron nitride substrate affects the electronic properties of graphene nanoribbons. We consider both armchair and zigzag nanoribbons. Our results show that for some stacking configurations the boron nitride can significantly affect the electronic structure of the ribbons. In particular, for zigzag nanoribbons, due to the lock between spin and sublattice degree of freedom at the edges, the hexagonal boron nitride can induce a very strong spin-splitting of the spin polarized, edge sates. We find that such spin-splitting can be as high as 40~meV.

Locally Gated SnS2/hBN Thin Film Transistors with a Broadband Photoresponse

Next-generation flexible and transparent electronics demand newer materials with superior characteristics. Tin dichalcogenides, Sn(S,Se)2, are layered crystal materials that show promise for implementation in flexible electronics and optoelectronics. They have band gap energies that are dependent on their atomic layer number and selenium content. A variety of studies has focused in particular on tin disulfide (SnS2) channel transistors with conventional silicon substrates. However, the effort of interchanging the gate dielectric by utilizing high-quality hexagonal boron nitride (hBN) still remains. In this work, the hBN coupled SnS2 thin film transistors are demonstrated with bottom-gated device configuration. The electrical transport characteristics of the SnS2 channel transistor present a high current on/off ratio, reaching as high as 105 and a ten-fold enhancement in subthreshold swing compared to a high-κ dielectric covered device. We also demonstrate the spectral photoresponsivity from ultraviolet to infrared in a multi-layered SnS2 phototransistor. The device architecture is suitable to promote diverse studied on flexible and transparent thin film transistors for further applications.

Ni foam assisted synthesis of high quality hexagonal boron nitride with large domain size and controllable thickness

The scalable synthesis of two-dimensional (2D) hexagonal boron nitride (h-BN) is of great interest for its numerous applications in novel electronic devices. Highly-crystalline h-BN films, with single-crystal sizes up to hundreds of microns, are demonstrated via a novel Ni foam assisted technique reported here for the first time. The nucleation density of h-BN domains can be significantly reduced due to the high boron solubility, as well as the large specific surface area of the Ni foam. The crystalline structure of the h-BN domains is found to be well aligned with, and therefore strongly dependent upon, the underlying Pt lattice orientation. Growth-time dependent experiments confirm the presence of a surface mediated self-limiting growth mechanism for monolayer h-BN on the Pt substrate. However, utilizing remote catalysis from the Ni foam, bilayer h-BN films can be synthesized breaking the self-limiting effect. This work provides further understanding of the mechanisms involved in the growth of h-BN and proposes a facile synthesis technique that may be applied to further applications in which control over the crystal alignment, and the numbers of layers is crucial.

Large-Scale Growth of High-Quality Hexagonal Boron Nitride Crystals at Atmospheric Pressure from an Fe–Cr Flux

Single crystals of hexagonal boron nitride (hBN) have recently been envisioned for electronic, optoelectronic, and nanophotonic applications. In this study, the production of large-scale, high-quality hBN single crystals via precipitation from a new solvent composed of Fe and Cr was demonstrated to be viable at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 2 mm and a thickness of around 200 μm. The Raman spectra and photoluminescence emission spectra demonstrate that the crystals produced with this solvent are pure hBN phase, and with low defect and residual impurity concentrations. The use of an Fe–Cr mixture provides a lower cost alternative to the more common Ni–Cr solvent for growing large hBN of comparable quality.

The synthesis of hexagonal boron nitride via chemical vapor deposition

Hexagonal boron nitride, a two dimensional wide bandgap insulator material which has a similar structure with graphene, has many excellent physical properties. For instance, hexagonal boron nitride has super mechanical properties, chemical and thermal stabilities, resulting in its wide potential applications in the areas of non-linear optics, ultraviolet laser and protection layers. In addition, due to its atomically smooth surface that is free of trapped charge and dangling bonds, hexagonal boron nitride is a super dielectric material for graphene electrical devices. Moreover, hexagonal boron nitride can be fabricated into planar or vertical hetero-structures with other two dimensional materials, which show unique properties and potential applications in novel electronic devices. How to synthesize large scale and high quality hexagonal boron nitride is currently key scientific issue. Up to now, many strategies, including mechanical exfoliation, deposition by ion beam sputtering, co-segregation, chemical vapor deposition and so on, have been developed for growing this material. Among these methods, chemical vapor deposition provides a versatile platform for modulating the growth and etching process of this material and producing a large scale material with high quality. This review summarizes a series of work regarding the synthesis of hexagonal boron nitride by chemical vapor deposition method, in which most recent work is also presented, and discusses the choices of reaction precursors and substrates in details. The use of solid borane ammonia complex leads to better controls over the properties of hexagonal boron nitride such as layer numbers and uniformity compared with the use of mixed B and N compounds as precursors. On the other hand, the types, surface structures and properties of the supporting substrates are critical for the growth of hexagonal boron nitride. For example, Cu substrate has advantages for growing single layered hexagonal boron nitride crystal or film, while the use of Ni or Fe substrates usually leads to multi-layered materials that are applicable to different applications. This difference largely results from two different growth mechanisms in two typical systems, i.e., surface diffusion and co-segregation mechanism. Moreover, owing to the nature of complex systems of chemical vapor deposition, kinetic controls over the growth and etching of hexagonal boron nitride can result in a more precise controls over size, shape and edge structures of this material, and the related progresses are presented. Finally, the key challenge and future directions are also presented. From the basic scientific research and large-scale application points of view, there are still plenty room for further studies of controllable synthesis of hexagonal boron nitride. How to optimize the growth process, further understand the growth mechanism, and realize the large sized layer number controlled growth of single crystal hexagonal boron nitride are important issues required to be solved in future. Further explorations of a better substrate such as an alloy possibly leads to combined advantages of components in an alloy, overcoming the difficulties associated with uniform production of high quality hexagonal boron nitride crystal or film. In addition, searching better reaction precursors also deserves attention, and working on this may be rewarding.