Boron Nitride Single Crystals Research Articles

The thermal oxidation of hexagonal boron nitride single crystals: Dry and ambient air compared

The thermal oxidation of hexagonal boron nitride single crystals: Dry and ambient air compared

Hardness of as-synthesized cubic boron nitride (111) bulk single crystals

The cubic boron nitride is the second most important superhard materials. To date, the effect of crystallographic anisotropy of the {111} plane on the Knoop microhardness of cubic boron nitride single crystals has not been experimentally obtained. Herein, we measure the detailed Knoop hardness and Vickers hardness for the {111} plane of the amber cubic boron nitride. We find that the crystal orientation dependence of Knoop hardness for amber samples is remarkable and has an obvious regular pattern. The Knoop hardness for amber samples shows a minima about 5° from ⟨110⟩ directions, which is different from the traditional viewpoint that the minimum hardness is on ⟨110⟩ directions. Raman spectra of cubic boron nitrides are measured and help us to explain the hardening of samples.

Process optimization of large-sized cubic boron nitride single crystal synthesis with Li3N as catalyst

Cubic boron nitride (cBN) was synthesized using cubic press with lithium nitride as the catalyst and hexagon boron nitride as the source material under high temperature and high pressure in this study. The high quality big-sized cBN single crystal can be synthesized with the optimized synthesis process. Comparing two different pressurization process, two different power process and three different heating times, the influence for the quality of cBN single crystal were studied. The results show that the process of gradually elevated pressure, the constant power and 15 minutes heating time are conducive to big-sized cBN crystals synthesis under high temperature and high pressure. Using the optimized synthesis process, the single block output reaches up to more than 120ct, the proportion of cBN crystal bigger than 50 mesh reaches to more than 16%, the static pressure strength of 60/70 mesh single crystal reaches to 30 N. Meanwhile, the cBN single crystals have fewer surface defects. The final result is high quality large-sized cBN single crystal was obtained.

Analysis of Cubic Boron Nitride Single Crystal Defects Growth under High Temperature and High Pressure

Cubic boron nitride (cBN) single crystals are synthesized under high temperature and high pressure in the Li-based system. The growth defects on hexagonal and triangular (111) surfaces of cBN single crystals after rapid cooling are discussed systemically for the first time using the atomic force microscope. Some impurity particles, triangle cone hole defects, lamellar-fault structures, and big steps are obvious on the surfaces of cBN single crystals. The formation mechanism of these defects is analyzed briefly at the synthetic process of cBN single crystals, and the growth mechanism of cBN single crystals transform from the two-dimensional growth to dislocation growth mechanism under high temperature and high pressure.

Hexagonal Boron Nitride Single Crystal Growth from Solution with a Temperature Gradient

Hexagonal boron nitride (hBN) is attracting much attention due to its tremendous applications including nanophotonic and electronic devices, substrates for two-dimensional (2D) materials, heat mana...

(Invited) Atomic Resolution, Coordinated Electron Microscopy Characterization of 2D Layers Synthesized By Confinement Heteroepitaxy (CHet) Technique

Since the discovery of graphene in 2004 [1] there has been a flurry of activity in investigating two dimensional (2D) materials, because of their superlative properties: spin-orbit coupling, massless Dirac fermions, mechanical strength, thermal and chemical stability [2]. Remarkably, the in-plane hexagonal symmetry of graphene is largely responsible for its behavior. However, the zero-band gap of graphene presents hurdles to its use as a semiconductor. This has spurred interest in synthesis other van der Waals 2D structures, such as h-BN [3] and more recently 2D Gallium Nitride (2D GaN) [4].The synthesis of 2D Nitrides was first reported in 2016 by applying Confinement Heteroepitaxy (CHet) process to form a 2D Gallium Nitride (GaN) layer with 1-2 atom thickness. In this technique, a silicon carbide (SiC) substrate is graphitized and deliberately damaged using oxygen plasma to provide a two-step intercalation path. First a metal (like Gallium, or other group V metal) through evaporation is intercalated between the graphene and SiC substrate. Then, to complete the half reaction, nitrogen is introduced by ammonia annealing at 700 °C to form a 1-layer thick III-nitride.In this study, the growth method is extended from 2D GaN to variety of other 2D metals (Indium), Nitrides (InN) and alloys (InGa alloys). We employ a coordinated approach to understand the surface in both plan and cross-section view – linking through a variety of electron microscopy techniques, including auger electron spectroscopy (AES), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Bright-field, Dark-field, aberration-corrected STEM-HAADF imaging, and atomic-scale electron energy loss spectroscopy (EELS) mapping using a Gatan K2 SI direct electron detector and other techniques mapping were all applied to characterize the SiC/2D-layer/graphene interface. These electron microscopy techniques allow a precise, local measurement of the atomic structure of these new materials, including such growth parameters as the effect of graphene on the 2D layer stability, the effect of the SiC step edges on the layers structure, and the effect of the plasma type and source used in damaging the graphene on the 2D layer chemistry. Plasmon mapping also describes the unique bonding in the materials and their electronic structure. Ultimately, the measurements presented here characterize a whole new class of nanomaterials and provide insight on the fundamentals of the intercalation growth process. 1. Novoselov, K.S., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.2. Soldano, C., A. Mahmood, and E. Dujardin, Production, properties and potential of graphene. Carbon, 2010. 48(8): p. 2127-2150.3. Watanabe, K., T. Taniguchi, and H. Kanda, Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal. Nature Materials, 2004. 3(6): p. 404-4094. Al Balushi, Z.Y., et al., Two-dimensional gallium nitride realized via graphene encapsulation. Nature Materials, 2016. 15(11): p. 1166-1171. Figure 1

Isotopic Disorder: The Prevailing Mechanism in Limiting the Phonon Lifetime in Hexagonal BN.

The phonon linewidth of isotopically controlled hexagonal boron nitride (h-BN) single crystals has been determined by Raman scattering. The scattering by isotopic mass disorder induces a phonon broadening that is largest for boron 11 fractions around 0.65. Lowest-order perturbation theory does not suffice to explain the dependence of the isotopic broadening on isotopic composition. A multiple-scattering theory based on the coherent potential approximation provides a good quantitative account of the phonon shift and broadening with isotopic composition observed in the experiments. Isotopic-disorder scattering is shown to have a prominent role in limiting the optical-phonon lifetime in h-BN.

Electron energy loss spectroscopy analysis for cubic boron nitride single crystals transition mechanism in Li3N-BN system

Cubic boron nitride (c-BN) single crystals were synthesized in Li3N-BN system. The variation of sp2:sp3 ratio from boron K-edges in different interfacial layers covering on as-grown c-BN single crystals was investigated by electron energy loss spectroscopy. The sp3 fractions from the outer to the inner layers exhibit an increasing trend, correspondingly, the sp2 fractions decrease gradually. The fractions of sp3-bonded BN from the outer to the inner are 10.36%, 11.55%, and 16.67%, respectively. The main phases in the synthesized interfacial powders are confined as c-BN, h-BN microcrystals. On the basis of our findings, it is concluded that c-BN could be transformed from h-BN directly and the catalysis of Li3BN2 was analyzed briefly.

Analysis of Transition Mechanism of Cubic Boron Nitride Single Crystals under High Pressure-High Temperature with Valence Electron Structure Calculation**Supported by the National Natural Science Foundation of China under Grant No 51272139.

The possibilities of hexagonal boron nitride (hBN) and lithium boron nitride (Li3BN2) transition into cubic boron nitride (cBN) under synthetic pressure 5.0 GPa and synthetic temperature 1700 K are analyzed with the use of the empirical electron theory of solids and molecules. The relative differences in electron density are calculated for dozens of bi-phase interfaces hBN/cBN, Li3BN2/cBN. These relative differences of hBN/cBN are in good agreement with the first order of approximation (), while those of Li3BN2/cBN are much greater than 10%. This analysis suggests that Li3BN2 is impossible to be intermediate phase but is a catalyst and cBN should be directly transformed by hBN.

Three-leaf dart-shaped single-crystal BN formation promoted by surface oxygen

Two-dimensional hexagonal boron nitride (h-BN) single crystals with various shapes have been synthesized by chemical vapor deposition over the past several years. Here, we report the formation of three-leaf dart (3LD)-shaped single crystals of h-BN on a Cu foil by atmospheric-pressure chemical vapor deposition. The leaves of the 3LD-shaped h-BN are as long as 18 μm, and their edges are smooth armchair on one side and stepped armchair on the other. Careful analysis revealed that surface oxygen plays an important role in the formation of the 3LD shape. Oxygen suppressed h-BN nucleation by passivating Cu surface active sites and lowered the edge attachment energy, which caused the growth kinetics to change to a diffusion-controlled mode.

Auger electron spectroscopy analysis for growth interface of cubic boron nitride single crystals synthesized under high pressure and high temperature

After rapid cooling, cubic boron nitride (c-BN) single crystals synthesized under high pressure and high temperature (HPHT) are wrapped in the white film powders which are defined as growth interface. In order to make clear that the transition mechanism of c-BN single crystals, the variation of B and N atomichybridstates in the growth interface is analyzed with the help of auger electron spectroscopy in the Li-based system. It is found that the sp2 fractions of B and N atoms decreases, and their sp3 fractions increases from the outer to the inner in the growth interface. In addition, Lithium nitride (Li3N) are not found in the growth interface by X-ray diffraction (XRD) experiment. It is suggested that lithium boron nitride (Li3BN2) is produced by the reaction of hexagonal boron nitride (h-BN) and Li3N at the first step, and then B and N atoms transform from sp2 into sp3 state with the catalysis of Li3BN2 in c-BN single crystals synthesis process.

Direct observation of the lowest indirect exciton state in the bulk of hexagonal boron nitride

We combine electron energy-loss spectroscopy and first-principles calculations based on density-functional theory (DFT) to identify the lowest indirect exciton state in the in-plane charge response of hexagonal boron nitride (h-BN) single crystals. This remarkably sharp mode forms a narrow pocket with a dispersion bandwidth of $\sim 100~\text{meV}$ and, as we argue based on a comparison to our DFT calculations, is predominantly polarized along the $\Gamma\text{K}$-direction of the hexagonal Brillouin zone. Our data support the recent report by Cassabois \emph{et al.} (Nat. Photon. \textbf{10}, 262) who indirectly inferred the existence of this mode from the photoluminescence signal, thereby establishing h-BN as an indirect semiconductor.

Defect sensitive etching of hexagonal boron nitride single crystals

Defect sensitive etching (DSE) was developed to estimate the density of non-basal plane dislocations in hexagonal boron nitride (hBN) single crystals. The crystals employed in this study were precipitated by slowly cooling (2–4 °C/h) a nickel-chromium flux saturated with hBN from 1500 °C under 1 bar of flowing nitrogen. On the (0001) planes, hexagonal-shaped etch pits were formed by etching the crystals in a eutectic mixture of NaOH and KOH between 450 °C and 525 °C for 1–2 min. There were three types of pits: pointed bottom, flat bottom, and mixed shape pits. Cross-sectional transmission electron microscopy revealed that the pointed bottom etch pits examined were associated with threading dislocations. All of these dislocations had an a-type burgers vector (i.e., they were edge dislocations, since the line direction is perpendicular to the [211¯0]-type direction). The pit widths were much wider than the pit depths as measured by atomic force microscopy, indicating the lateral etch rate was much faster than the vertical etch rate. From an Arrhenius plot of the log of the etch rate versus the inverse temperature, the activation energy was approximately 60 kJ/mol. This work demonstrates that DSE is an effective method for locating threading dislocations in hBN and estimating their densities.

Raman study of color-zoning cubic boron nitride single crystals

Unintentionally doped cubic boron nitride (cBN) single crystals with color zoning were investigated by Raman spectroscopy. Besides the TO mode at 1053cm−1 and the LO mode at 1304cm−1, some little vibrational bands at 925cm−1, 955cm−1and 1247cm−1 corresponding to the amber regions in the {1¯ 1¯ 1¯}N sectors of cBN crystals were observed. These vibrational bands probably result from the disorder-activated Raman scattering (DARS) due to the high defect densities. It is shown that these defects and impurities are nonuniform, easily formed and incorporated in {1¯ 1¯ 1¯}N growth sectors during the growth process of cBN crystals. These defects and impurities are color centers in cBN, therefore, the cBN crystal exhibits symmetrical color zoning relative to the {111} growth sectors.

Angle-resolved electron energy loss spectroscopy in hexagonal boron nitride

Electron energy loss spectra have been measured on hexagonal boron nitride single crystals employing a novel electron energy loss spectroscopic set-up composed by an electron microscope equipped with a monochromator and an in-column filter. This set-up provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes provide complementary pieces of information, offering a global view of excitations in reciprocal space. As an example of the capabilities of the method we show how easily the core loss spectra at the $K$ edges of boron and nitrogen can be measured and imaged. Low losses associated to interband and/or plasmon excitations are also measured. This energy range allows us to illustrate that our method provides results of quality comparable to those obtained from non resonant X-ray inelastic scattering, but with advantageous specificities such as an enhanced sensitivity at low q and a much higher simplicity and versatility that makes it well adapted to the study of two-dimensional materials and related heterostructures. Finally, by comparing theoretical calculations against our measures, we are able to relate the range of applicability of ab initio calculations to the anisotropy of the sample and assess the level of approximation required for a proper simulation of our acquisition method.

Synthesis of aligned symmetrical multifaceted monolayer hexagonal boron nitride single crystals on resolidified copper.

Atomically smooth hexagonal boron nitride (h-BN) films are considered as a nearly ideal dielectric interface for two-dimensional (2D) heterostructure devices. Reported mono- to few-layer 2D h-BN films, however, are mostly small grain-sized, polycrystalline and randomly oriented. Here we report the growth of centimetre-sized atomically thin h-BN films composed of aligned domains on resolidified Cu. The films consist of monolayer single crystalline triangular and hexagonal domains with size of up to ∼10 μm. The domains converge to symmetrical multifaceted shapes such as "butterfly" and "6-apex-star" and exhibit ∼75% grain alignment for over millimetre distances as verified through transmission electron microscopy. Scanning electron microscopy images reveal that these domains are aligned for over centimetre distances. Defect lines are generated along the grain boundaries of mirroring h-BN domains due to the two different polarities (BN and NB) and edges with the same termination. The observed triangular domains with truncated edges and alternatively hexagonal domains are in accordance with Wulff shapes that have minimum edge energy. This work provides an extensive study on the aligned growth of h-BN single crystals over large distances and highlights the obstacles that are needed to be overcome for a 2D material with a binary configuration.

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.

Thermodynamic Analysis about Nucleation and Growth of Cubic Boron Nitride Crystals in the hBN-Li3N System under High Pressure and High Temperature

The nucleation of cubic boron nitride (cBN) single crystals synthesized with lithium nitride (Li3N) as a catalyst under high pressure and high temperature (HPHT) was analyzed. Many nanometer-sized cubic boron nitride nuclei formed in the near surface layer, as detected by high resolution transmission electron microscopy. Based on the experiment results, the transformation kinetics is described by a nucleation and growth process in the thermodynamic stability region of cBN. A theoretical description is developed based on the heterogeneous nucleation and layer growth mechanism, and the relevant parameters are estimated and discussed. The critical crystal radius, r*, increases with the temperature under constant pressure; the change with temperature more pronounced at lower pressure (such as 4.5 GPa). The crystal growth velocity increased with the temperature, and it is parabolic with temperature under certain pressure. These results are consistent with experimental data.

XPS analysis for cubic boron nitride crystal synthesized under high pressure and high temperature using Li3N as catalysis

Cubic boron nitride (cBN) single crystals are synthesized with lithium nitride (Li3N) as catalyst under high pressure and high temperature. The variation of electronic structures from boron nitride of different layers in coating film on the cBN single crystal has been investigated by X-ray photoelectron spectroscopy. Combining the atomic concentration analysis, it was shown that from the film/cBN crystal interface to the inner, the sp2 fractions are decreasing, and the sp3 fractions are increasing in the film at the same time. Moreover, by transmission electron microscopy, a lot of cBN microparticles are found in the interface. For there is no Li3N in the film, it is possible that Li3N first reacts with hexagonal boron nitride to produce Li3BN2 during cBN crystals synthesis under high pressure and high temperature (HPHT). Boron and nitrogen atoms, required for cBN crystals growth, could come from the direct conversion from hexagonal boron nitride with the catalysis of Li3BN2 under high pressure and high temperature, but not directly from the decomposition of Li3BN2.

Crystal growth of hexagonal boron nitride (hBN) from Mg–B–N solvent system under high pressure

Transparent and colorless hexagonal boron nitride (hBN) single crystals were grown from the Mg–B–N system using high-pressure–high-temperature cubic anvil technique. By varying the synthesis conditions we could determine the sequence of phase transformations occurring in the Mg–B–N system, construct the pressure–temperature (P–T) phase diagram and discuss the possible growth mechanism. The largest plate-like-shaped hBN crystals with sizes up to 2.5mm in length and up to 10µm in thickness were grown at 30kbar and 1900–2100°C. The hBN crystals exhibited strong, narrow diffraction peaks typical of well-ordered stacking crystal planes, with the c-axis perpendicular to the crystal face. A characteristic Raman peak observed at 1367cm−1 with a full width at half maximum of 8cm−1 corresponds to the E2g vibration mode and indicates the high purity and order of hBN crystals grown by this method. From the practical point of view this work can stimulate further explorations of the Mg–B–N solvent system to obtain isotopically-enriched h10BN crystals, which can act as a key element in solid-state neutron detector devices.