Hexagonal Boron Nitride Films Research Articles

Enhanced Long-Term Stability of Crystalline Nickel-Boride (Ni23B6) Electrocatalyst by Encapsulation with Hexagonal Boron Nitride.

Nickel boride catalysts show great potential as low-cost and efficient alternatives to noble-metal catalysts in acidic media; however, synthesizing and isolating a specific phase and composition of nickel boride is nontrivial, and issues persist in their long-term stability as electrocatalysts. Here, a single-crystal nickel boride, Ni23B6, is reported which exhibits high electrocatalytic activity for the hydrogen evolution reaction (HER) in an acidic solution, and that its poor long-term stability can be overcome via encapsulation by single-crystal trilayer hexagonal boron nitride (hBN) film. Interestingly, hBN-covered Ni23B6 on a Ni substrate shows an identical overpotential of 52mV at a current density of 10mA cm-2 to that of bare Ni23B6. This phenomenon indicates that the single-crystalline hBN layer is catalytically transparent and does not obstruct HER activation. The hBN/Ni23B6/Ni has remarkable long-term stability with negligible changes to its polarization curves for 2000 cycles, whereas the Ni23B6/Ni shows significant degradation after 650 cycles. Furthermore, chronoamperometric measurements indicate that stability is preserved for >20h. Long-term stability tests also reveal that the surface morphology and chemical structure of the hBN/Ni23B6/Ni electrode remain preserved. This work provides a model for the practical design of robust and durable electrochemical catalysts through the use of hBN encapsulation.

Atomic sawtooth-like metal films for vdW-layered single-crystal growth

Atomic sawtooth surfaces have emerged as a versatile platform for growth of single-crystal van der Waals layered materials. However, the mechanism governing the formation of single-crystal atomic sawtooth metal (copper or gold) films on hard substrates (tungsten or molybdenum) remains a puzzle. In this study, we aim to elucidate the formation mechanism of atomic sawtooth metal films during melting–solidification process. Utilizing molecular dynamics, we unveil that the solidification of the liquid copper initiates at a high-index tungsten facet with higher interfacial energy. Subsequent tungsten facets follow energetically favourable pathways of forming single-crystal atomic sawtooth copper film during the solidification process near melting temperature. Formation of atomic sawtooth copper film is guaranteed with a film thickness exceeding the grain size of polycrystalline tungsten substrate. We further demonstrate the successful growth of centimeter-scale single-crystal monolayer hexagonal boron nitride films on atomic sawtooth copper films and explore their potential as efficient oxygen barrier.

Synthesis of multilayered h-BN film on the molten Ni–B and the influence of underneath W substrate

High-quality two-dimensional hexagonal boron nitride (h-BN) film with tens of layers has been used as a universal substrate and capping layer for the van der Waals devices. Various approaches have been carried out for the synthesis of multilayered h-BN. Among them, the metal flux method is reliable in yielding h-BN crystals with high crystalline quality. However, this time- and energy-demanding method hinders its scale application. Herein, inspired by the metal flux method, we reported the time-effective growth of high-quality multilayered h-BN film (ca. 20 nm) on a molten Ni–B layer wetting on W substrate with chemical vapor deposition method. The film exhibits an excellent stacking sequence and a full-width at half maximum of the Raman E2g peak narrow to 9.5 cm−1. Cross-sectional high-resolution transmission electron microscopy and in-situ x-ray diffraction spectroscopy were carried out to investigate the crystal structure evolution of Ni–B layer wetting on W substrate. It is found that the low surface tension caused by the spreading of Ni–B alloy on the W substrate and the presence of the Ni subsurface may be responsible for the formation of multilayered h-BN with excellent crystalline quality. Meanwhile, the W diffusion in Ni–B-based melt can hinder the formation of h-BN under certain growth conditions. The approach demonstrates the feasibility of large-scale growth of multilayered h-BN, paving the way to future applications in van der Waals electronic and optoelectronic devices.

Ultra-High-Efficiency Corrosion Protection of Metallic Surface Based on Thin and Dense Hexagonal Boron Nitride Coating Films.

Quest for ultrathin and highly effective anticorrosion coating films is critical for both fundamental community of materials science and industrial economics. A two-dimensional hexagonal boron nitride (h-BN) film is a newly developed effective anticorrosion material due to its superior impermeability, thermal stability, and chemical stability. However, research in growth and anticorrosion properties of large-area dense h-BN coating films still lies in its infancy. Here, we report on the synthesis of a large-area and continuous dense few-layer (∼4) h-BN coating film onto a metal surface by chemical vapor deposition (CVD) and its anticorrosion properties both in air and seawater environments. Cu coated in h-BN, which functions as an anticorrosive coating, exhibits an impressively reduced corrosion rate (CR) in a 3.5% NaCl solution (which mimics a seawater environment) when compared to bare Cu (approximately 27 times). At 200 °C, the h-BN coating can prevent Cu foil's surface from oxidizing, although doing so will cause a significant amount of oxide particles to simultaneously form on the bare Cu surface. In the meantime, the performance of the h-BN film remains unaltered after 100 days in an atmospheric environment, demonstrating the ultrahigh stability and corrosion resistance of the as-grown h-BN film.

Transfer-Free Analog and Digital Flexible Memristors Based on Boron Nitride Films.

The traditional von Neumann architecture of computers, constrained by the inherent separation of processing and memory units, faces challenges, for instance, memory wall issue. Neuromorphic computing and in-memory computing offer promising paradigms to overcome the limitations of additional data movement and to enhance computational efficiency. In this work, transfer-free flexible memristors based on hexagonal boron nitride films were proposed for analog neuromorphic and digital memcomputing. Analog memristors were prepared; they exhibited synaptic behaviors, including paired-pulse facilitation and long-term potentiation/depression. The resistive switching mechanism of the analog memristors were investigated through transmission electron microscopy. Digital memristors were prepared by altering the electrode materials, and they exhibited reliable device performance, including a large on/off ratio (up to 106), reproducible switching endurance (>100 cycles), non-volatile characteristic (>60 min), and effective operating under bending conditions (>100 times).

Large-area single-crystal hexagonal boron nitride: From growth mechanism to potential applications

To date, chemical vapor deposition has been employed to grow large-area polycrystalline (PC) films of hexagonal boron nitride (hBN). However, PC hBN thin films exhibit abundant grain boundaries, small grain sizes, and structural imperfections, which collectively degrade the performance as well as hinder the scalability and potential applications of hBN films. Recently, demonstrated deposition methods for growing large-area single-crystalline (SC) hBN films can break through these bottlenecks and have opened avenues for new opportunities. Large-area SC hBN films outperform their PC counterparts owing to the presence of fewer grain boundaries and a more homogeneous surface morphology. This review article presents a consolidated overview of the growth mechanisms of SC hBN films and role of metal catalysts (substrates) in the growth process. Applications, in which SC hBN outperforms PC hBN as well as the potential applications of SC hBN, are also discussed.

Hardware Implementation of Network Connectivity Relationships Using 2D hBN‐Based Artificial Neuron and Synaptic Devices

AbstractBrain‐inspired neuromorphic computing has been developed as a potential candidate for solving the von Neumann bottleneck of traditional computing systems. 2D materials‐based memristors have been exponentially investigated as promising building blocks of neuromorphic computing because of their excellent electrical performance, simple structure, and small device scale. However, while many researchers have focused on looking into individual artificial neuromorphic devices based on memristors, only few studies on the integration of artificial neuron and synaptic devices have been reported. In this work, both volatile and nonvolatile memristors are fabricated by using a 2D hexagonal boron nitride film for artificial neuron and synaptic devices, respectively. The leaky‐integrate‐and‐fire neuron performance and synaptic functions (e.g., synaptic weight plasticity and spike‐timing‐dependent plasticity) are well emulated with the fabricated volatile and nonvolatile devices. The MNIST image classification is conducted based on the experimental data. For the first time, an artificial neuron‐synapse‐neuron neural network is physically constructed using the artificial neuron and synaptic devices to mimic the biological neural networks. The synaptic connection strength modulation is experimentally demonstrated between the neurons depending on the conductance state of the synapse, paving the way for the development of large‐scale neural network hardware.

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

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

High-Crystallinity and High-Temperature Stability of the Hexagonal Boron Nitride Film Grown on Sapphire

High-Crystallinity and High-Temperature Stability of the Hexagonal Boron Nitride Film Grown on Sapphire

Thermal stability, work function and Fermi level analysis of 2D multi-layered hexagonal boron nitride films

Thermal stability and Fermi level analysis of commercial multilayer (ML) and as-grown 10 nm thick hexagonal boron nitride (hBN) films on Cu foil by atmospheric chemical vapor deposition (APCVD) by heating in N2 and air at temperatures ≤900 °C are studied. For all samples, progressive surface oxidation was observed correlating with higher hBN nucleation density. Thermal stability was measured in air from 200 °C to 400 °C (relative humidity ∼65%) where commercial ML and APCVD material degraded; BN peaks begin to be replaced by BO peaks at temperatures >300 °C. It is shown that hBN thermal stability is sensitive to Tgrowth and Cu foil surface preparation methods such as electropolishing (EP). We evaluate as-grown material for different Tgrowth conditions (1000 °C, 1030 °C, 1050 °C and 1060 °C) by ultraviolet photoelectron spectroscopy (UPS) to locate and understand subsequent Fermi level and work function variation, confirming our material is in general n-type, which becomes increasingly so as thermal stability regresses. To this effect, we observe a work function increase of 0.45 eV as BO increases with in-situ anneal temperature. An average electron affinity of 2.13 eV and 1.7 eV is determined for commercial hBN and APCVD material, respectively. Further work is required by material scientists to optimize the material properties of CVD hBN.

Armor for Steel: Facile Synthesis of Hexagonal Boron Nitride Films on Various Substrates

AbstractWhile hexagonal boron nitride (hBN) has been widely used as a buffer or encapsulation layer for emerging electronic devices, interest in utilizing it for large‐area chemical barrier coating has somewhat faded. A chemical vapor deposition process is reported here for the conformal growth of hBN on large surfaces of various alloys and steels, regardless of their complex shapes. In contrast to the previously reported very limited protection by hBN against corrosion and oxidation, protection of steels against 10% HCl and oxidation resistance at 850 °C in air is demonstrated. Furthermore, an order of magnitude reduction in the friction coefficient of the hBN coated steels is shown. The growth mechanism is revealed in experiments on thin metal films, where the tunable growth of single‐crystal hBN with a selected number of layers is demonstrated. The key distinction of the process is the use of N2 gas, which gets activated exclusively on the catalyst's surface and eliminates adverse gas‐phase reactions. This rate‐limiting step allowed independent control of activated nitrogen along with boron coming from a solid source (like elemental boron). Using abundant and benign precursors, this approach can be readily adopted for large‐scale hBN synthesis in applications where cost, production volume, and process safety are essential.

Decoupling the roles of defects/impurities and wrinkles in thermal conductivity of wafer-scale hBN films

We demonstrate a non-monotonic evolution of in-plane thermal conductivity of large-area hexagonal boron nitride films with thickness. Wrinkles and defects/impurities are present in these films. Raman spectroscopy, an optothermal non-contact technique, is employed to probe the temperature and laser power dependence property of the Raman active E2ghigh phonon mode, which, in turn, is used to estimate the rise in the temperature of the films under different laser powers. As the conventional Fourier law of heat diffusion cannot be directly employed analytically to evaluate the thermal conductivity of these films with defects and wrinkles, finite-element modeling is used instead. In the model, average heat resistance is used to incorporate an overall near-surface defect structure, and Voronoi cells with contact resistance at the cell boundaries are constructed to mimic the wrinkled domains. The effective in-plane thermal conductivity is estimated to be 87, 55, and 117 W/m K for the 2, 10, and 30 nm-thick films, respectively. We also present a quantitative estimation of the thermal resistance by defects and wrinkles individually to the heat flow. Our study reveals that the defects/impurities render a much higher resistance to heat transfer in the films than wrinkles.

Thickness-Controlled Synthesis of Uniform Hexagonal Boron Nitride Films on Metal Alloys by Chemical Vapor Deposition

Two-dimensional (2D) multilayer hexagonal boron nitride (hBN) is a superior substrate for other 2D materials due to its insulating property, atomic-scale smoothness, and absence of dangling bonds, enhancing electrical or optical properties in high-performance 2D material-based devices. In most fabrication processes for 2D heterostructure devices, thick hBN flakes exfoliated from bulk have been used because of their high quality and simplicity in the process. However, exfoliated hBN flakes are limited in size and thickness control, impeding scale-up production for industrialization. Therefore, new synthesis strategies are required to overcome the drawback of the exfoliation method. In this work, we investigated the synthesis of large-area, multilayer hBN via atmospheric pressure chemical vapor deposition using single crystalline metal and single crystal alloy substrates. The synthesized results are characterized by optical microscopy, scanning electron microscopy, Raman spectroscopy, atomic force microscopy, etc. These studies provide a more comprehensive understanding of the growth mechanism of hexagonal boron nitride.

Enhanced intensity of Raman signals from hexagonal boron nitride films

Optical spectroscopy is commonly used to study the properties of 2D materials. In order to obtain the best signal-to-noise ratio, it is important to optimize the incoupling of the excitation laser and, at the same time, reduce spurious light reflection. We performed Raman spectroscopy on exfoliated hexagonal boron nitride (hBN) flakes of different thicknesses, placed on a 300 nm SiO2 on Si substrate. By changing the hBN layer thickness, we found a specific thickness, where the Raman signals from the substrate and the hBN showed maximum intensity, whereas the backscattered laser light was suppressed. To explain the increased emission, we calculated the reflectivity and transmissivity of the full layer system (air, hBN, SiO2, and Si) as a function of hBN layer thicknesses for different excitation wavelengths (457, 532, and 633 nm), using the transfer-matrix algorithm. To compare theory with the experiment, we performed Raman measurements with these three different excitation wavelengths on different flakes and determined their thicknesses with AFM measurements. The experimental results are in good agreement with the calculations, which shows the importance of thin film interference to obtain optimum spectroscopic conditions. Since interference colors are easily visible in an optical microscope, this facilitates the choice of optimum flakes for a wide range of optical characterization techniques, including Raman, photoluminescence, and single defect spectroscopy.

Electronic Structure and Surface Chemistry of Hexagonal Boron Nitride on HOPG and Nickel Substrates.

The effect of point defects and interactions with the substrate are shown by density functional theory calculations to be of significant importance for the structure and functional properties of hexagonal boron nitride (h-BN) films on highly ordered pyrolytic graphite (HOPG) and Ni(111) substrates. The structure, surface chemistry, and electronic properties are calculated for h-BN systems with selected intrinsic, oxygen, and carbon defects and with graphene hybrid structures. The electronic structure of a pristine monolayer of h-BN is dependent on the type of substrate, as h-BN is decoupled electronically from the HOPG surface and acts as bulk-like h-BN, whereas on a Ni(111) substrate, metallic-like behavior is predicted. These different film/substrate systems therefore show different reactivities and defect chemistries. The formation energies for substitutional defects are significantly lower than for intrinsic defects regardless of the substrate, and vacancies formed during film deposition are expected to be filled by either ambient oxygen or carbon from impurities. Significantly lower formation energies for intrinsic and oxygen and carbon substitutional defects were predicted for h-BN on Ni(111). In-plane h-BCN hybrid structures were predicted to be terminated by N-C bonding. Substitutional carbon on the boron site imposes n-type semiconductivity in h-BN, and the n-type character increases significantly for h-BN on HOPG. The h-BN film surface becomes electronically decoupled from the substrate when exceeding monolayer thickness, showing that the surface electronic properties and point defect chemistry for multilayer h-BN films should be comparable to those of a freestanding h-BN layer.

Synthesis of hexagonal boron nitride nanocoatings for corrosion prevention of iron substrates

Iron and its alloys have applications in diverse fields such as civil infrastructure, aerospace, and defense. Corrosion of these alloys in saline environments is a significant concern that causes huge environmental losses. The current study explores pulsed laser deposition technique to synthesize hexagonal boron nitride (hBN) films as protective coatings, directly on iron (Fe), to mitigate the corrosion. Microstructural, mechanical, wetting, and corrosion properties of hBN-coated Fe substrates were investigated at different deposition temperatures (25–800 °C) and varying thicknesses (35–115 nm). Raman spectra and transmission electron microscopy confirmed the presence of hBN in as-deposited films. Crystallinity and surface roughness of hBN coatings increased as deposition temperature increased. Electrochemical studies performed in 3.5 wt% NaCl solution showed that the 115 nm thick hBN coatings deposited at 600 °C resulted in lower corrosion rates and has ~6-fold higher corrosion resistance than bare Fe. Also, the corrosion rates decreased with an increase in hBN coating thickness. Overall results suggest that hBN nanocoatings reduced the corrosion activity and can potentially serve as a corrosion-resistant barrier coatings in saline environments.

High detectivity solar blind photodetector based on mechanical exfoliated hexagonal boron nitride films

As an ultra-wide bandgap semiconductor, hexagonal boron nitride (h-BN) has drawn great attention in solar-blind photodetection owing to its wide bandgap and high thermal conductivity. In this work, a metal-semiconductor-metal structural two-dimensional h-BN photodetector was fabricated by using mechanically exfoliated h-BN flakes. The device achieved an ultra-low dark current (16.4 fA), high rejection ratio (R 205nm/R 280nm = 235) and high detectivity up to 1.28 × 1011 Jones at room temperature. Moreover, due to the wide bandgap and high thermal conductivity, the h-BN photodetector showed good thermal stability up to 300 °C, which is hard to realize for common semiconductor materials. The high detectivity and thermal stability of h-BN photodetector in this work showed the potential applications of h-BN photodetectors working in solar-blind region at high temperature.

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

Protected ultrathin cuprous oxide film for photocatalysis: Excitation and relaxation dynamics

In the search for cost-effective ways to produce green hydrogen, harnessing the most abundant renewable energy source---sunlight---directly through photoelectrochemical water splitting is highly desirable. Cuprous oxide (${\mathrm{Cu}}_{2}\mathrm{O}$) has proven itself as a material with great potential for the hydrogen evolution reaction (HER) and stands out by its Earth abundance and low processing cost. In this photoelectron spectroscopy study we investigated the electron dynamics in a heterostructure comprised of a ${\mathrm{Cu}}_{2}\mathrm{O}$ surface oxide grown on Cu(111) underneath a hexagonal boron nitride ($h$-BN) film, which replaces a conventional capping layer for corrosion protection. Our results show that it can be a viable cuprous oxide-based photoelectrode material. The $h$-BN film stays intact during the oxidation, and the oxide layer forms an ordered structure with defined valence and conduction bands. The conduction band is at a suitable energy to drive the HER for water splitting, but the photoexcited electrons display short lifetimes. Electrons are mainly excited in the copper substrate and are then captured in long-lived defect states in the ${\mathrm{Cu}}_{2}\mathrm{O}$ layer. The water splitting efficiency of this photocathode may still be improved by reducing the defect density.

Substrate effects on the near-field radiative heat transfer between two hBN films

Near-field radiative heat transfer (NFRHT) could surpass the blackbody limit defined by Stefan-Bolzmann’s law by several orders of magnitude, which has potential applications in thermal switching, thermal management, and photovoltaics. To further develop the NFRHT from theory to application, the substrate, which could enhance the stability of the structure, is a critical factor not to be ignored. However, the substrate effect on the NFRHT is still rarely discussed. In this work, we investigate the NFRHT between hexagonal boron nitride (hBN) films with different permittivities of the substrate. Results demonstrate that when the thickness of the film is 1 nm, increasing the permittivity of the substrate will suppress the NFRHT. In contrast, when the thickness of the film is larger (>2 nm), the high-permittivity substrate could enhance the NFRHT. The spectral heat flux (SHF) corresponding to substrates with different permittivities was investigated. The SHF in Type I hyperbolic band of hBN increases with the increase in the permittivity of the substrate, while that in Type II hyperbolic band is completely opposite. This competitive relationship leads to the above-mentioned phenomenon of NFRHT. The underlying physics mechanism can also be explained by the hyperbolic phonon polaritons (HPPs), which are analyzed by the energy transmission coefficients and dispersion relations. The findings in this work will deepen the understanding of the substrate on HPPs and pave a novel way for near-field radiation devices based on hyperbolic materials.