Properties Of Hexagonal Boron Nitride Research Articles

Optical properties of hexagonal boron nitride thin films deposited by radio frequency bias magnetron sputtering

The optical properties of hexagonal boron nitride (h-BN) thin films were studied in this paper. The films were characterized by Fourier transform infrared spectroscopy, UV—visible transmittance and reflection spectra. h-BN thin films with a wide optical band gap Eg (5.86 eV for the as-deposited film and 5.97 eV for the annealed film) approaching h-BN single crystal were successfully prepared by radio frequency (RF) bias magnetron sputtering and post-deposition annealing at 970 K. The optical absorption behaviour of h-BN films accords with the typical optical absorption characteristics of amorphous materials when fitting is made by the Urbach tail model. The annealed film shows satisfactory structure stability. However, high temperature still has a significant effect on the optical absorption properties, refractive index n, and optical conductivity σ of h-BN thin films. The blue-shift of the optical absorption edge and the increase of Eg probably result from stress relaxation in the film under high temperatures. In addition, it is found that the refractive index clearly exhibits different trends in the visible and ultraviolet regions. Previous calculational results of optical conductivity of h-BN films are confirmed in our experimental results.

First-Principles Calculations of Structural and Elastic Properties of Hexagonal Boron Nitride

The lattice parameters, five independent elastic constants and the bulk modulus B on the applied pressure of hexagonal boron nitride (h-BN) are calculated by using a first-principles pseudopotential method. The calculation results are in good agreement with the experimental and theoretical values. It is found that the most stable structure of h-BN corresponds to the axial ratio c/a of about 2.652.

High-temperature conductance loss dominated defect level in h-BN: Experiments and first principles calculations

The dielectric properties of hexagonal boron nitride are investigated in detail. The permittivities hold extremely low values ranging from room temperature to 1500 °C, however, the dielectric loss tangents increase rapidly above 1000 °C. At 1500 °C, the dielectric loss tangent is 20 times more than that at room temperature. The first principles calculations show that the boron vacancy (VB) that gives an acceptor energy level near the valence band presents the lowest ionization energy in the investigated defects, and the calculated VB ionization energy agrees with the experimental value. It indicates that the rapid increase in dielectric loss tangents at high temperature is contributed by electrical conductivity produced by VB ionization under thermal excitation.

Near band-gap photoluminescence properties of hexagonal boron nitride

Near band-gap luminescence (hν⩾5eV) of hexagonal boron nitride has been studied by means of the time- and energy-resolved photoluminescence spectroscopy method. Two emissions have been observed at 5.5 and 5.3eV. The high-energy emission at 5.5eV is composed of fixed subbands assigned to bound excitons at 5.47, 5.56, and 5.61eV. The nonstructured low-energy emission at 5.3eV undergoes a large blueshift (up to 120meV) with a linear slope ΔElum∕ΔEexc<1 with increasing excitation energy Eexc. At Eexc⩾5.7eV, the band position is fixed and marks the transition from the Raman to the photoluminescence regime. We assign the 5.3eV band to quasi-donor-acceptor pair (q-DAP) states due to electrostatic band fluctuations induced by charged defects. The shift is explained by photoinduced neutralization of charged defect states. The absence of contribution to the q-DAP luminescence from exciton suggests the existence of a large exciton binding energy, which is qualitatively consistent with theoretical predictions.

Hexagonal boron nitride as a tablet lubricant and a comparison with conventional lubricants

The objective of this study was to investigate the lubrication properties of hexagonal boron nitride (HBN) as a new tablet lubricant and compare it with conventional lubricants such as magnesium stearate (MGST), stearic acid (STAC), and glyceryl behenate (COMP). Tablets were manufactured on an instrumented single-station tablet press to monitor lower punch ejection force (LPEF) containing varied lubricants in different ratio (0.5, 1, 2%). Tablet crushing strength, disintegration time and thickness were measured. Tensile strength of compacted tablets were measured by applying a diametrical load across the edge of tablets to determine mechanical strength. The deformation mechanism of tablets was studied during compression from the Heckel plots with or without lubricants. MGST was found to be the most effective lubricant based on LPEF—lubrication concentration profile and LPEF of HBN was found very close to that of MGST. HBN was better than both STAC and COMP. A good lubrication was obtained at 0.5% for MGST and HBN (189 and 195N, respectively). Where COMP and STAC showed 20 and 35% more LPEF compare to that of MGST (239 and 288N, respectively). Even at the concentration of 2% COMP and STAC did not decrease LPEF as much as 0.5% of MGST and HBN. Like all conventional lubricants the higher the concentration of HBN the lower the mechanical properties of tablets because of its hydrophobic character. However, this deterioration was not as pronounced as MGST. HBN had no significant effect on tablet properties. Based on the Heckel plots, it was observed that after the addition of 1% lubricant granules showed less plastic deformation.

Structural properties of hexagonal boron nitride

The electronic and structural properties of hexagonal boron nitride (BN) were studied using density functional theory calculations. Three different approximations for the exchange—correlation energy (the local density and two forms of the generalized gradient)—were used to calculate properties such as the bulk modulus, cohesive energy and lattice constants to determine their relative predictive abilities for this system. In general, calculations using the local density approximation produced properties slightly closer to experimental values than calculations with either generalized gradient approximations. Different stackings, or arrangements of one basal plane with respect to another, were examined to determine the equilibrium stacking(s) and it was found that the different stackings have similar cohesive energies and bulk moduli. Energy versus volume curves were calculated for each stacking using two different methods to determine their relative efficacy. Bulk moduli values obtained assuming no pressure dependence were closer to experimental values than those obtained from three common equations of state. Comparisons between the cohesive energies of hexagonal BN and cubic BN show that the cubic phase is more stable. The pressure/volume dependence of the band structure was studied for several different stackings and all showed similar behaviour, specifically a 3–4.5 eV band gap that was nearly independent of pressure in the −500 to +500 kb regime. These calculated results of the pressure/volume dependence of the band structure are the first reports for this system.

Electronic structure and bonding in hexagonal boron nitride

The bonding, cohesive, and electronic properties of hexagonal boron nitride were studiedusing density functional theory calculations. The properties of this system were calculatedusing three different exchange–correlation functionals (local density approximation and twoforms of the generalized gradient approximation) to determine their relative predictiveabilities for this system. In-plane and interplanar bonding was examined using banddiagrams, the density of states, and the electron localization function. Different stackings,or arrangements of one basal plane with respect to another, were examined to determinehow the bonding and electronic structure changed between different stackings. Calculatedband gaps were in the 2.9–4.5 eV range and predominantly indirect, regardless of stackingor the exchange–correlation functional used. The calculated band gaps are in the low rangeof experimental band gap values, and do not explain the large range of experimental values.

Phonon structure and dynamics of boron nitride single wall nanotube

We report a detailed study of the phonon properties of hexagonal boron nitride (BN) monolayers as well as nanotubes by using De Launay type of angular force model. The force constants used for calculation of phonon dispersion relations of the nanotube are derived from those for the monolayers of hexagonal by employing force constant method. These force constants have been modified to include the effect of curvature of the tubule. The results are then used to derive the phonon dispersion relations for (10,10) BN nanotubes using ‘zone-folding’ method. Calculated phonon modes are in good agreement with the experimental values obtained so far, for (10,10) armchair BN nanotubes.

Ab initio study of BN nanoarches' surfaces

The electronic and structural properties of hexagonal boron nitride (0 0 0 2) surfaces closed by one and two nanoarches are investigated through first-principles density functional calculations. The surface with one arch is semiconductor whereas the double arch surface presents a metallic behaviour. For the surface with two nanoarches, the level crossing the Fermi energy shows a delocalization along the direction of the nanoarches' axes, defining a one-dimensional channel for electron conduction at the surface. Possible reactive sites on these surfaces, related to the cubic boron nitride growth, are suggested.

Ab initio study of BN nanoarches' surfaces

The electronic and structural properties of hexagonal boron nitride (0 0 0 2) surfaces closed by one and two nanoarches are investigated through first-principles density functional calculations. The surface with one arch is semiconductor whereas the double arch surface presents a metallic behaviour. For the surface with two nanoarches, the level crossing the Fermi energy shows a delocalization along the direction of the nanoarches' axes, defining a one-dimensional channel for electron conduction at the surface. Possible reactive sites on these surfaces, related to the cubic boron nitride growth, are suggested.

Mechanical properties of hexagonal boron nitride synthesized from film of Cu/BN mixture by surface segregation

Hexagonal boron nitride (h-BN) has a low friction coefficient and weak surface attractive force similar to graphite. Furthermore, while graphite is conductive, BN is a good insulator. These properties make it suitable for application like lubricating coating or as an insulator/buffer layer in electronic devices. The synthesize of h-BN layer by surface segregation phenomena and mechanical properties of the h-BN surface segregated on Cu substrate have been investigated. During in situ annealing, the surface segregation of BN occurred on Cu/BN film deposited by deposition process with a rf magnetron co-sputtering system. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis showed that though the h-BN layer synthesized was not covered whole area of substrate but the h-BN layers partially covered substrate. And the concentration of oxygen on the surface after exposure in air is decreased with increase of BN concentration. The topography of atomic forces microscopy (AFM) showed that h-BN phases surface segregated are discontinuous droplet shape. The force curves of AFM and friction force of lateral force microscopy (LFM) showed that the h-BN droplet surface segregated have very weak attractive force and low friction coefficient equal to h-BN sintered plate.

GRAPHITE AND BORON NITRIDE (“WHITE GRAPHITE”): ASPECTS OF STRUCTURE, POWDER SIZE, POWDER SHAPE, AND PURITY

Graphite can be characterized by atomic structure and powder size, shape, and purity. A similar characterization can be applied to hexagonal boron nitride. Three typical graphites are studied to illustrate quality variation and some practical consequences. The properties of hexagonal boron nitride are described to distinguish it from the cubic form.

Energy bands and optical properties of hexagonal boron nitride and graphite

The band structure of hexagonal boron nitride in the two-dimensional approximation is computed using the tight-binding approximation in a semi-empirical way. The results are compared with those of the isoelectronic compound graphite and are shown to be in agreement with the basic experimental data. Three-dimensional band structure calculations have also been performed and the effect of the different spatial structure of boron nitride and graphite lattices on the electronic levels at the saddle point is shown to produce a doublet in the optical excitation spectrum of BN.