Recently, several experiments and theoretical studies demonstrated the possibility of tuning or modulating band gap values of nanostructures composed of bi-layer graphene, bi-layer hexagonal boron-nitride (BN) and hetero-layer combinations. These triple layers systems present several possibilities of stacking. In this work we report an ab initio (within the formalism of density functional theory (DFT)) study of structural and electronic properties of some of these stacked configurations. We observe that an applied external electric field can alter the electronic and structural properties of these systems. With the same value of the applied electric field the band gap values can be increased or decreased, depending on the layer stacking sequences. Strong geometrical deformations were observed. These results show that the application of an external electric field perpendicular to the stacked layers can effectively be used to modulate their inter-layer distances and/or their band gap values.

The synthesis of boron nitride nanowires on silicon (Si) and nanorods on molybdenum (Mo) substrates at the same experimental conditions was composed. Fine tip nanowires with diameters around 50 nm were produced on Si substrates, whereas, nanorods with diameter around 100 nm were formed on Mo substrates. The change in length from 5 μm to 100 μm for nanowires and 0.2 μm to 0.8μm for nanorods following variation of substrate temperature were studied systematically.

Scanning Electron Microscopy was used to analyze the surface images of BN nanowires and nanorods. Energy Dispersive X-Ray spectroscopy (EDS) was used to analyze boron and nitrogen concentration in the samples. The crystal structures of BN samples were investigated using Raman spectroscopy and x-ray diffraction. The experimental results showed that the nanorods are hexagonal mixed with cubic, whereas the nanowires are hexagonal.

Elimination of degenerate epitaxy in the growth of icosahedral boron arsenide (B12As2, abbreviated as IBA) was achieved on m-plane 15R-SiC substrates and 4H-SiC substrates intentionally misoriented by 7 degrees from (0001) towards [1-100]. Synchrotron white beam x-ray topography (SWBXT) revealed that only single orientation IBA was present in the epitaxial layers demonstrating the absence of twin variants which dominantly constitute the effects of degenerate epitaxy. Additionally, low asterism in the IBA diffraction spots compared to those grown on other SiC substrates indicates a superior film quality. Cross-sectional high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) both confirmed the absence of twins in the IBA films and their high quality. The ease of nucleation on the ordered step structures present on these unique substrates overrides symmetry considerations that drive degenerate epitaxy and dominates the nucleation process of the IBA.

The crystallographic properties of bulk icosahedral boron arsenide (B12As2) crystals grown by precipitation from molten nickel solutions were characterized. Large crystals (5-8 mm) were produced by dissolving the boron in nickel at 1150°C for 48-72 hours, reacting with arsenic vapor, and slowly cooling to room temperature. The crystals varied in color from black and opaque to clear and transparent. Raman spectroscopy, x-ray topography (XRT), and defect selective etching revealed that the B12As2 single crystals were high quality with low dislocation densities. Furthermore, XRT results suggest that the major face of the plate-like crystals was (111) type, while (100), (010) and (001) type facets were also observed optically. The predominant defect in these crystals was edge character growth dislocations with a <001> Burgers vector, and <-110> line direction. In short, XRT characterization shows that solution growth is a viable method for producing good quality B12As2 crystals.

We report here molecular dynamics results for boron nitride nanoscroll structures (BNNSs) with relation to their stability and formation mechanisms. We show that, similarly to carbon nanoscrolls, BNNSs are stable due to van der Waals interactions among overlapping layers. The energy balance between losses and gains (due to elastic deformations and van der Waals interactions, respectively) when the structure is rolled up leads to the existence of a critical value of the internal scroll diameter where stable or metastable structures can be formed. The mechanisms of scroll formation and stability as a function of their chirality were also investigated.

Titanium diboride (TiB2) has been sintered using spark plasma sintering (SPS). With the addition of Al3Ti, the Vickers hardness, Hv, of TiB2 increases up to as high as 2100 by the sintering at 1273K, while that of the sample sintered without Al3Ti is as low as 20. Such a remarkable improvement is caused by the formation of rigid direct contacts between TiB2 grains in addition to the effect that Al3Ti fills in the space between TiB2 grains and acts as a binder.

γ-B28 is a recently discovered high-pressure phase of boron, with the structure consisting f icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement: (B2)δ+(B12)δ-, and displaying a significant charge transfer δ~0.48. Boron is the only light element, for which the phase diagram has become clear only a few years ago, with the discovery of γ-B28, and this phase diagram is discussed here among other recent findings. γ-B28 was first experimentally obtained as a pure boron allotrope in early 2004 by J.H. Chen and V.L. Solozhenko (although a similar diffraction pattern was published in a 1965 by R.H. Wentorf, in a paper that until recently was believed to be wrong) and its unique structure was discovered by A.R. Oganov in 2006 with the use of the ab initio evolutionary algorithm USPEX (Oganov & Glass, 2006) and later confirmed by other studies. This allotrope, thermodynamically stable at high pressures, is shown to be also quenchable and dynamically stable upon decompression to 1 atm, and we show its phonon dispersion curves. Present discussion includes also the relative stability of other boron allotropes as a function of pressure. We also discuss more recent publications on the putative isosymmetric phase transition in γ-B28 and the nature of chemical bonding in it. We demonstrate that a qualitative difference in the evolution of the band gap of γ-B28 and the related α-B12 structure, which is due to the partial ionicity of γ-B28.

We present the ionization of decaborane (B10H14) and formation of hydrogen- and boron-contents-controlled B10-yHx+ through the charge transfer from ambient gas ion to decaborane molecules in an external quadrupole static attraction ion trap. The charge transfer energy is estimated from the experimentally observed products. PBE0/6-311+G(d)//B3LYP/6-31G(d) level of DFT calculations are conducted to investigate the mechanism of charge transfer from ambient gas ion. The calculation of the difference of ionization energies and mismatch of orbital energies between decaborane and ambient gas reveals the mechanism of ionization.

The present work reports on the defect-selective etching (DSE) for estimating dislocation densities in icosahedral boron arsenide (B12As2) crystals using molten potassium hydroxide (KOH). DSE takes advantage of the greater reactivity of high-energy sites surrounding a dislocation, compared to the surrounding dislocation-free regions. The etch pits per area are indicative of the defect densities in the crystals, as confirmed by x-ray topography (XRT). Etch pit densities were determined for icosahedral boron arsenide crystals produced from a molten nickel flux as a function of etch time (1-5 minutes) and temperature (400-700°C). The etch pits were predominately triangle shaped, and ranged in size from 5-25μm. The average etch pit density of the triangle and oval etch-pits was on the order of 5x107cm-2 and 3x106cm-2 (respectively), for crystals that were etched for two minutes at 550°C.

The local spin configuration and band structure of chromium doped boron carbide calculated by density functional theory suggests local magnetic ordering. While the long range dopant position appears random in the boron carbide semiconductor, the local position and initial empirical/computational results suggest the promise of large magneto-resistive effects. The chromium doped boron carbide thin films, fabricated by boron carbide-chromium co-deposition, were studied by current-voltage (I-V) characteristics measurements. The results provide some reason to believe that magneto-resistive effects are indeed present at room temperature.

A facile and scalable chemical vapor deposition (CVD) process in flowing argon using a solid instead of a reactive gaseous boron precursor has been carried out to synthesize crystalline boron nanostructures comprising of relatively straight boron nanotubes (BNTs) and nanofibers (BNFs). The synthesis involves the use of solid magnesium boride as the boron and magnesium catalyst precursor, nickel boride as co-catalyst, and MCM-41 zeolite as the growth template. The BNTs and BNFs produced have a narrow distribution of diameters between about 10 nm to 20 nm and lengths from about 500 nm to above 1 μm. Scanning and transmission electron microscope (SEM and TEM) imaging together with electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) have been conducted to characterize the structure, morphology and growth mechanism of these novel nanostructures. High resolution TEM imaging has been used to identify BNTs and BNFs in the nanostructures synthesized.

We report the growth and field emission properties of boron nitride (BN) island films by chemical vapor deposition in inductively coupled plasma. Fine-grained island films with large surface roughness can be grown for initial sp2-bonded BN and subsequent cubic BN (cBN) phases by using low-energy (~20 eV) ion bombardment. Ultraviolet photoelectron spectroscopy indicates that the electron affinity is as low as 0.3 eV for both sp2-bonded BN and cBN phases. The evolution of cBN islands reduces the turn-on field down to around 9 V/μm and increases the current density up to 10-4 A/cm2. The surface potential barrier height is estimated to be about 3.4 eV for emission from the Fermi level.

In this work, the deposition of boron using low pressure chemical vapor deposition (LPCVD) has been investigated on planar and deep reactive ion etched (DRIE) Si substrates. Deposition rate and conformal coverage have been studied. Additional studies of “dry” RIE etching and “wet” chemical etching of the deposited boron films are presented. Deposition rates as high as 1 μm/hr and conformal coverage ratios of ~80% have been achieved. Etching rates for various methods studied range widely from 0.35 μm/hr to 1.2 μm/min.

Amorphous indium boron nitride (a-InBN) thin films were successfully fabricated using radio frequency (RF) magnetron sputtering, and were deposited onto fused silica and c-Si(100) substrates. Sputtering was achieved using a target of polycrystalline B and In species with B/In nominal at.% ratio of 25/75 under the flow of nitrogen. The structure and composition of the films have been investigated by X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), respectively. The XRD patterns reveal that the sputtered films are amorphous, and the XPS confirms the presence of boron in the films in addition to an oxide overlayer. The optical absorption of samples grown on silica was obtained using spectrophotometry (SP) technique in the wavelength range (200 - 800) nm. Analysis of the absorption coefficients using the Tauc linear extrapolation gives an optical bandgap of 2.05 eV, indicating a higher bandgap comparing to the measured optical bandgap of a-InN (1.25 eV) due to doping with boron. Films grown on c-Si(100) were characterized by spectroscopic ellipsometry (SE) technique in the wavelength range of (300-1700) nm. The measured ellipsometric spectra are described well by a two-layer model structure, which consists of a transparent layer on top of an absorbing layer. The thicknesses and optical functions of the transparent and absorbing layers were obtained by analyzing the measured ellipsometric spectra, Ψ and Δ within the framework of the Cauchy–Urbach (CU) and Tauc–Lorentz (TL) models, respectively. While the overlayer is completely transparent over the measured range (k(λ) = 0), the absorbing layer underneath it exhibits a clear absorption above its optical bandgap of 2.15 eV, which is in a good agreement with the SP finding. There was an excellent agreement between the bandgap obtained as a fitting parameter from the optical model and that obtained by linear extrapolation using the empirical Tauc and Cody models for amorphous semiconductors.

A recurrent problem in the synthesis of hexagonal boron nitride (h-BN) is contamination with oxygen and carbon, leading to possible detrimental effects on optical and electronic properties. Here it is shown that the addition of H2 to the N2/Ar mixture used during the deposition process, clearly suppresses the incorporation of these elements, reducing their combined level below 5 %. The surface morphology, assessed with scanning electron microscopy (SEM), revealed the presence of h-BN nanowalls, i.e. vertically positioned 2D structures consisting out of several h-BN sheets. While Fourier transform infrared (FTIR) spectroscopy revealed the sp2 nature of the bonds, confirming the hexagonal nature of the nanowalls, the quasi-perfect stoichiometry of the material was evidenced by combining energy dispersive X-ray analysis (EDX) and Rutherford backscattering spectroscopy (RBS). The dimensions and density of these walls are clearly film thickness dependent and cross-sectional TEM images confirmed the increasing level of porosity with film thickness. A dense layer of material is present at the substrate-film interface, which gradually evolves into the 2D nanowall structures.