Electron microscopy of hexagonal boron nitride powder

The morphology and crystalline state of h-BN powder after one-stage or two-stage baking process were investigated extensively. The particles are scale-shaped and the flat surfaces have a (001) habit plane of hexagonal close-packed structure. The side shape of particles after one-stage baking is round, while that after two-stage baking is dodecan, a twelve-faced prism, with the side habit of (100), (110) or their variants. Lattice image observation shows that the side surface of a one-stage baked particle is wavy and thin, while that of a two-stage baked particle is straight and thick. Many particles after one-stage baking are composed of overlapped grains contacting with each other at (001) flat surfaces forming a twist boundary. These grains have relative rotation angles ranging from 5 ° to 26 ° around the common [001] axis and have a coincidence lattice relation with respect to (001) flat planes. X-ray photoelectron spectroscopy analysis shows that both C and O segregate onto the surface of one-stage baked particles, while only C segregates onto the surface of two-stage baked particles. Formation of coincidence lattice grain boundary and impurity segregation both restrict the growth of diameter and thickness in scale-shaped particles. It is concluded that two-stage baked particles, having side surface habits, are stable against various environments.     

Oxygen-suppressed selective growth of monolayer hexagonal boron nitride on copper twin crystals

Controlled growth of hexagonal boron nitride (h-BN) with desired properties is essential for its wide range of applications.Here,we systematically carried out the chemical vapor deposition of monolayer h-BN on Cu twin crystals.It was found that h-BN nucleated and grew preferentially and simultaneously on the narrow twin crystal strips present in the Cu substrates.The density functional theory calculations revealed that the introduction of oxygen could efficiently tune the selectivity.This is because of the reduction in the dehydrogenation barrier of the precursor molecules by the introduction of oxygen.Our findings throw light on the direct growth of functional h-BN nanoribbons on nano-twinned crystal strips and switching of the growth behavior of h-BN films by oxygen.     

Scanning tunnelling microscopy and spectroscopy of ultra-flat graphene on hexagonal boron nitride

Graphene has demonstrated great promise for future electronics technology as well as fundamental physics applications because of its linear energy-momentum dispersion relations which cross at the Dirac point. However, accessing the physics of the low-density region at the Dirac point has been difficult because of disorder that leaves the graphene with local microscopic electron and hole puddles. Efforts have been made to reduce the disorder by suspending graphene, leading to fabrication challenges and delicate devices which make local spectroscopic measurements difficult. Recently, it has been shown that placing graphene on hexagonal boron nitride (hBN) yields improved device performance. Here we use scanning tunnelling microscopy to show that graphene conforms to hBN, as evidenced by the presence of Moiré patterns. However, contrary to predictions, this conformation does not lead to a sizeable band gap because of the misalignment of the lattices. Moreover, local spectroscopy measurements demonstrate that the electron-hole charge fluctuations are reduced by two orders of magnitude as compared with those on silicon oxide. This leads to charge fluctuations that are as small as in suspended graphene, opening up Dirac point physics to more diverse experiments.     

Synthesis of monolayer hexagonal boron nitride on Cu foil using chemical vapor deposition

Hexagonal boron nitride (h-BN) is very attractive for many applications, particularly, as protective coating, dielectric layer/substrate, transparent membrane, or deep ultraviolet emitter. In this work, we carried out a detailed investigation of h-BN synthesis on Cu substrate using chemical vapor deposition (CVD) with two heating zones under low pressure (LP). Previous atmospheric pressure (AP) CVD syntheses were only able to obtain few layer h-BN without a good control on the number of layers. In contrast, under LPCVD growth, monolayer h-BN was synthesized and time-dependent growth was investigated. It was also observed that the morphology of the Cu surface affects the location and density of the h-BN nucleation. Ammonia borane is used as a BN precursor, which is easily accessible and more stable under ambient conditions than borazine. The h-BN films are characterized by atomic force microscopy, transmission electron microscopy, and electron energy loss spectroscopy analyses. Our results suggest that the growth here occurs via surface-mediated growth, which is similar to graphene growth on Cu under low pressure. These atomically thin layers are particularly attractive for use as atomic membranes or dielectric layers/substrates for graphene devices.     

Few-atomic-layered hexagonal boron nitride: CVD growth,characterization, and applications

1. Download : Download high-res image (86KB)
2. Download : Download full-size image

     

Synthesis of boron carbide powder from hexagonal boron nitride

We report the synthesis of boron carbide powder via the reaction of hexagonal boron nitride with carbon black. The reaction between hexagonal boron nitride and carbon black completed at 1900 °C for 5 h in vacuum. The particle sizes of the synthesized boron carbide powder were about 100 nm from transmission electron microscopy. The possible reaction mechanism was that hexagonal boron nitride decomposed into elemental boron and nitrogen even when there was no carbon at a relatively low rate, and introduction of carbon into hexagonal boron nitride powder facilitated the decomposition process; the boron from the decomposition of boron nitride reacted with carbon to form boron carbide.     

Oxidation kinetics of hexagonal boron nitride powder

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900–1050°C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol^–1in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000 °C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B₂O₃ was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.     

High-resolution characterization of hexagonal boron nitride coatings exposed to aqueous and air oxidative environments

Hexagonal boron nitride (h-BN) is believed to offer better passivation to metallic surfaces than graphene owing to its insulating nature,which facilitates blocking the flow of electrons,thereby preventing the occurrence of galvanic reactions.Nevertheless,this may not be the case when an h-BN-protected material is exposed to aqueous environments.In this work,we analyzed the stability of mono and multilayer h-BN stacks exposed to H2O2 and atmospheric conditions.Our experiments revealed that monolayer h-BN is as inefficient as graphene as a protective coating when exposed to H2O2.Multilayer h-BN offered a good degree of protection.Monolayer h-BN was found to be ineffective in an air atmosphere as well.Even a 10-15 layers-thick h-BN stack could not completely protect the surface of the metal under consideration.By combining Auger electron spectroscopy and secondary ion mass spectrometry techniques,we observed that oxygen could diffuse through the grain boundaries of the h-BN stack to reach the metallic substrate.Fortunately,because of the diffusive nature of the process,the oxidized area did not increase with time once a saturated state was reached.This makes multilayer (not monolayer) h-BN a suitable long-term oxidation barrier.Oxygen infiltration could not be observed by X-ray photoelectron spectroscopy.This technique cannot assess the chemical composition of the deeper layers of a material.Hence,the previous reports,which relied on XPS to analyze the passivating properties of h-BN and graphene,may have ignored some important subsurface phenomena.The results obtained in this study provide new insights into the passivating properties of mono and multilayer h-BN in aqueous media and the degradation kinetics of h-BN-coated metals exposed to an air environment.     

Role of boron carbide in carbothermic formation of hexagonal boron nitride

Formation of hexagonal boron nitride by carbothermic reduction of boric oxide under nitrogen atmosphere at 1500 °C was investigated. Experiments were performed for durations in the range of 15 min to 3 h. Reaction products were subjected to powder X-ray diffraction analysis, chemical analysis and were examined by scanning electron microscope. Formation of hexagonal boron nitride was found to be complete in 3 h with most forming in the initial 2 h. Boron carbide was found to exist in the reaction products of the experiments in which hexagonal boron nitride formation was not complete. The aim of this study was to investigate the role of boron carbide in the carbothermic production of hexagonal boron nitride. For this purpose, conversion reaction of boron carbide into hexagonal boron nitride was studied. Boron carbide used in these experiments was produced in the same conditions that hexagonal boron nitride was formed, but under argon atmosphere. It was found that formation of hexagonal boron nitride from boron carbide—boric oxide mixtures was slower than activated carbon—boric oxide mixtures. It was concluded that boron carbide is not a necessary intermediate product in the carbothermic production of hexagonal boron nitride.     

Nitriding of iron boride to hexagonal boron nitride

A process for preparing hexagonal boron nitride (h-BN) from iron boride (FeB) has been described for the first time. FeB powder was nitrided in ammonia at 700 K for 24 hours yielding non-crystalline BN and -Fe₂N. Subsequent annealing in ammonia for 1 hour at 1273 K resulted in a mixture of crystalline h-BN together with -Fe, -Fe₃N and -Fe₄N. The morphology of the product grains is described. This comprises an iron nitride core loosely encapsulated in a foliated BN layer. The open nature of the product layer may account for the relatively low temperature (700 K) required for the nitriding reaction. These iron containing phases were removed with dilute mineral acid leaving h-BN and trace amounts of FeB₄₉.     

Effective mechanical properties of hexagonal boron nitride nanosheets

We propose an analytical formulation to extract from energy equivalence principles the equivalent thickness and in-plane mechanical properties (tensile and shear rigidity, and Poisson's ratio) of hexagonal boron nitride (h-BN) nanosheets. The model developed provides not only very good agreement with existing data available in the open literature from experimental, density functional theory (DFT) and molecular dynamics (MD) simulations, but also highlights the specific deformation mechanisms existing in boron nitride sheets, and their difference with carbon-based graphitic systems.     

Facile synthesis of hexagonal boron nitride fibers and flowers

Hexagonal boron nitride (h-BN) fibers and flowers were synthesized with a facile synthesis route. The precursor was obtained as an intermediate material by the reaction of KBH₄ and NH₄Cl at 120 °C for 48 h. h-BN was obtained by heating the intermediate material at 1250 °C for 10 h. The sample obtained was characterized by X-ray powder diffraction (XRD), Fourier transformation infrared spectroscopy (FT-IR), thermogravimetry (TG/DTA) and environment scanning electron microscopy (ESEM), which matched with h-BN. ESEM image indicated that the diameter of the BN fibers is mainly in the range of 1–2 μm.     

Third order elastic constants of hexagonal boron nitride

Third order elastic constants of hexagonal Boron Nitride have been evaluated using the Lannard-Jones potential. The results obtained are presented and compared with the only available measurement ofC ₃₃₃ for this material.     

Microstructural development with crystallization of hexagonal boron nitride

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Effective cleaning of hexagonal boron nitride for graphene devices

Hexagonal boron nitride (h-BN) films have attracted considerable interest as substrates for graphene. ( Dean, C. R. et al. Nat. Nanotechnol. 2010 , 5 , 722 - 6 ; Wang, H. et al. Electron Device Lett. 2011 , 32 , 1209 - 1211 ; Sanchez-Yamagishi, J. et al. Phys. Rev. Lett. 2012 , 108 , 1 - 5 .) We study the presence of organic contaminants introduced by standard lithography and substrate transfer processing on h-BN films exfoliated on silicon oxide substrates. Exposure to photoresist processing adds a large broad luminescence peak to the Raman spectrum of the h-BN flake. This signal persists through typical furnace annealing recipes (Ar/H(2)). A recipe that successfully removes organic contaminants and results in clean h-BN flakes involves treatment in Ar/O(2) at 500 °C.     

The effect of copper on the crystallization of hexagonal boron nitride

The crystallization process of hexagonal boron nitride in the presence of copper has been investigated. The positive effect of copper on the crystallinity of boron nitride was observed in the three studied systems of: nitrided boron, nitrided boron–carbon, and previously prepared turbostratic boron nitride. However, the presence of copper hindered the formation of boron carbonitride and produced graphite and boron nitride phases separately. Poor crystallinity was found as a conditio sine qua non for the existence of such a compound. Well-crystallized boron nitride had a very low spacing parameter 0.3328 nm and a regular hexagonal shape.