Electronic structure of SiC/BN composite segmented nanotubes

The electronic structure of segmented nanotubes composed of the alternating layers of (5,5) and (9,0) BN and SiC nanotubes in armchair and zigzag configurations, which differed in the orientation of the chemical bonds in the segments and the nature of the bonds (Si-N and B-C or Si-B and N-C) at the boundaries of BN and SiC regions, has been calculated using the linearized augmented cylindrical wave method. The calculations have been performed using the local density functional and the muffin-tin approximation for the electronic potential. It has been found that depending on the bonds at the segment boundaries, the (5,5) BN/SiC nanotubes are semiconductors with the energy gap E _g of 1 to 3 eV, whereas the (9,0) BN/SiC nanotubes exhibited a metal, semimetal, or semiconductor (E _g ～ 1 eV) type of band structures.     

Deep UV resonance Raman spectroscopic study on electron‐phonon coupling in hexagonal III‐nitride wide bandgap semiconductors

Electron–phonon coupling (EPC) is an important issue in semiconductor physics because of its significant influence on the optical and electrical properties of semiconductors. In this work, the EPC in wide bandgap semiconductors including hexagonal BN and AlN was studied by deep UV resonance Raman spectroscopy. Up to fourth‐order LO phonons are observed in the resonance Raman spectrum of hexagonal AlN. By contrast, only the prominent emission band near the band‐edge and the Raman band attributed to E_2g mode are detected for hexagonal BN with deep UV resonance excitation. The different behavior in resonant Raman scattering between the III‐nitrides reflects their large difference in EPC. The mechanism for EPC in hexagonal BN is the short‐range deformation interaction, while that in hexagonal AlN is mainly associated with the weak long‐range Fröhlich interaction. Copyright © 2016 John Wiley & Sons, Ltd.     

Electronic structure of collapsed C,BN, and BC3 nanotubes

The electrical properties of single-wall C, BN, and BC₃ nanotubes in ideally rolled-up forms show a wide spectrum from truly metals to large band gap semiconductors. In the presence of radial deformations that collapse tubes, the electrical properties are severely modified such that metals turn into semiconductors and vice versa. Based on first-principles pseudopotential calculations, we find that metallic C nanotubes have a finite band gap if radial deformations break all mirror symmetries of the tubes, and that original finite gaps (∼0.5 eV) of semiconducting C and BC₃ tubes are closed by collapsing deformations. In BN tubes, band gaps can be tuned in the range 2–5 eV. On the other hand, the band gaps of armchair BN and zigzag BC₃ nanotubes are found to be insensitive to radial deformations. These new findings can be applied to design new types of nanotube-based functional devices using radial deformations.     

Nanostructured two-phase nc-TiN/a-(TiB2, BN) nanocomposite thin films

Thin films of Ti-B-N with different N contents were deposited on Si(1 0 0) at room temperature by reactive unbalanced close-field dc-magnetron sputtering using three Ti targets and one TiB₂ target in an Ar-N₂ gas mixture. The effect of N content on bonding structure, microstructure, phase configuration, surface roughness and mechanical properties have been investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), cross-sectional scanning electron microscopy (SEM), plan-view and cross-sectional high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM) and microindentation measurements. It was found that the N content significantly affected phase segregation and microstructure. The nitrogen-free TiB_0.65 films showed an amorphous compound consisting of Ti and TiB₂ (Ti-TiB₂). After adding about 28 at.% N, Ti was preferentially bonded to N to form TiN, accompanying with formation of small amounts of TiB and BN bonds. At this stage they combined TiB₂ to form a two-phase nanocomposite with microstructures comprising of nanocrystalline (nc-) TiN phase in nitrogen-containing amorphous (a-) TiB₂ matrix. Addition of more N promoted formation of BN bonding at cost of TiB₂, which resulted in formation of nanocomposite nc-TiN/a-(TiB₂, BN) thin films. A small grain less than 8 nm in size was found at low N content, and the grain size increased with increasing N content. A low microhardness value of about 20 GPa was obtained in the amorphous Ti-TiB₂ compound, and a maximum microhardness value of about 50 GPa was achieved in nc-TiN/a-TiB₂. A decrease of microhardness took place after formation of BN (i.e. amorphous matrix composed by both TiB₂ and BN) with further increasing N content, and a hardness value of about 35 GPa was followed at high N contents. The surface roughness strongly depended on the phase configuration. The higher the mole fraction of nanocrystalline TiN phase, the rougher the surface became.     

Hydrogen storage in boron nitride and carbon clusters studied by molecular orbital calculations

Possibility of hydrogen gas storage in carbon (C) and boron nitride (BN) clusters was investigated by molecular orbital calculations. Chemisorption calculation was carried out for C₆₀, B₂₄N₂₄ and B₃₆N₃₆ with changing position of hydrogen atom to compare the bonding energy at carbon, nitrogen and boron, tetragonal and hexagonal rings. Chemisorption calculation of hydrogen for BN clusters showed that hydrogen bondings with nitrogen atoms and tetragonal rings were the most stable. Stability of H₂ molecules inside BN and C clusters was also investigated by molecular orbital calculations. C and BN clusters showed possibility of hydrogen storage of 6.5 and 4.9 wt%, respectively.     

Synthesis of boron nitride nanotubes by an oxide-assisted chemical method

We report a new method for the synthesis of boron nitride (BN) nanotubes employing a two-step process in which some oxides have found to catalyze the growth of BN nanotubes. In the first step, a precursor containing B–N–O–Fe/Mg was prepared by ball milling a mixture of B, B₂O₃, Fe₂O₃ and MgO (1:7:2:1 mass ratio) in NH₃ for 3 h. BN nanotubes (diameter: 20–100 nm) were grown in the second step from this precursor by isothermal annealing at 1,350 °C in NH₃ for about 4 h. XRD, SEM and HR-TEM studies elucidated the spindle-like morphology of these nanotubes of hexagonal crystal structure. The Raman spectrum showed the peak broadening and shifts to higher frequency. The present method showed that some oxides assisted the growth of BN nanotubes. A possible reaction mechanism on the formation of BN nanotubes in the presence of these oxides is discussed.     

Synthesis of boron nitride multi-walled nanotubes by laser ablation technique

Carbon nanotubes are known as prospective material for the optoelectronics, vacuum electronics, non-linear optics and for the composite material synthesis. Boron nitride nanotubes (BNNTs) were obtained only recently. It is expected that these nanotubes possess the unique properties. BN is much more chemically inert than carbon. Its oxidation temperature is above 1000°C as the graphite is totally oxidized already at 650°C. BNNTs are wide-gap semiconductors with the 6 eV gap. In case of the introducing of the carbon atoms into the BN nanotube wall its bandgap may be varied in wide range. This property is important for the UV optics.     

磁控溅射方法制备垂直取向FePt/BN颗粒膜

用磁控溅射在热单晶MgO(100)基片上制备了［FePt/BN］多层膜，经真空热处理后，得到具有垂直取向L1₀-FePt/BN颗粒膜.X射线衍射结果和磁性测量的结果表明，［FePt(2nm)/BN(0.5nm)］₁₀和［FePt(1nm)/BN(0.25nm)］₂₀多层膜经700℃热处理1h后，均具有较好的(001)取向.［FePt(1nm)/BN(0.25nm)］₂₀垂直矫顽力达到522kA/m，剩磁比达到0.99，开关场分布S达到0.94，FePt晶粒平均尺寸约15—20nm，适合用于将来超高密度的垂直磁记录介质. 关键词： 磁控溅射 垂直磁记录 0-FePt/BN纳米颗粒膜')" href="#">L1₀-FePt/BN纳米颗粒膜     

Lattice mismatch induced curved configurations of hybrid boron nitride–carbon nanotubes

A unique curved configuration is observed in freestanding hybrid boron nitride–carbon nanotubes (BN–CNTs) based on molecular dynamics simulations, which, in previous studies, was tacitly assumed as a straight configuration. The physical fundamentals of this phenomenon are explored by using the continuum mechanics theory, where the curved configuration of BN–CNTs is found to be induced by the bending effect due to the lattice mismatch between the C domain and the BN domain. In addition, our results show that the curvature of the curved BN–CNTs is determined by their radius and composition. The curvature of BN–CNTs decreases with growing radius of BN–CNTs and becomes ignorable when their radius is relatively large. A non-monotonic relationship is detected between the curvature and the composition of BN–CNTs. Specifically, the curvature of BN–CNTs increases with growing BN concentration when the molar fraction of BN atoms is smaller than a critical value 0.52, but decreases with growing BN concentration when the molar fraction of BN atoms is larger than this critical value.     

Diode laser action in Pb1-xMnxS

Diode laser action has been observed in Pb_1-xMn_xS alloy system for molar fraction of MnS x = 0.014 at the emission energy of 320 meV at 15 K. Combined current and magnetic field tuning has been examined in the range of 2.4–9.0 A and 0–14 kG, respectively and a quasi continuous tuning in the energy range from 320.5 meV to 322.0 meV was achieved. This is a first report about laser action in semiconductors containing paramagnetic ions.     

Fir magnetospectroscopy on (CuIn)1?xMn2xTe2

The paramagnetic resonance of Mn²⁺ in the diluted magnetic semiconductor (CuIn)_1–xMn_2xTe₂ was observed in a pulsed magnetic field up to 15 T at temperatures ranging from 4.2 K to 45 K. The temperature dependence of the line width of the paramagnetic resonance in (CuIn)_1–xMn_2xTe₂ resembles the behaviour of other diluted magnetic semiconductors. A Dzyaloshinski-Moriya exchange constant of approximately 0.62 K was derived. This value fits well with the values reported for II–VI based diluted magnetic semiconductors [1], if we consider the larger degree hybridization of the 3d electrons with the band electrons in chalcopyrite semiconductors.     

毛刺状竹节结构氮化硼纳米管的合成与生长机理

以非晶硼和氧化镍纳米颗粒为原料,在氨气中1100 oC下合成了毛刺状竹节结构的氮化硼纳米管. 利用X射线衍射和透射电镜研究了氮化硼纳米管的结构和形貌. 竹节结构纳米管表面的毛刺是六方氮化硼的纳米薄片. 提出了一种基于固态硼和气态二氧化硼扩散的毛刺形貌生长机理.     

Effects of endohedral element in B24N24 clusters on hydrogenation studied by molecular orbital calculations

Possibility of hydrogen gas storage in boron nitride (BN) clusters was investigated by molecular orbital calculations. Chemisorption calculation was carried out for B₂₄N₂₄ with changing endohedral elements in BN cluster to compare the bonding energy at nitrogen and boron, which showed that Li is a suitable element for hydrogenation to B₂₄N₂₄.     

Phonon spectroscopy measurements at amorphous films

Phonon spectroscopy measurements were used to examine the scattering of high frequency phonons (300 GHz-1 THz) in amorphous materials. The experiments were done with the use of time and frequency resolved measurements of the phonon transmission behaviour through amorphous single films of different thicknesses. The typical film thicknesses were of the order of 10 nm. In contrast to the pure amorphous semiconductors Si and Ge our experiments show inelastic phonon scattering processes in the case of SiO₂ and SiH. This inelastic phonon scattering also occurs when the pure semiconductors Si and Ge are prepared in an O₂ or H₂ atmosphere, but is missing when the preparation process is done in an N₂ atmosphere. In films of the pure semiconductors a-Si and a-Ge we only found evidence to elastic scattering processes. In further experiments at heated a-SiH samples we could examine the atomical bonded hydrogen to be the center of the inelastic phonon scattering.The measurements and investigations described in this work were done in time of preparing a thesis at: Physikalisches Institut Teil 1, Universität Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany     

Boron-nitride and boron-carbonitride nanotubes: synthesis,characterization and theory

We present in this review a joint experimental and theoretical overview of the synthesis techniques and properties of boron-nitride (BN) and boron-carbonitride (BCN) nanotubes. While their tubular structure is similar to that of their carbon analogues, we show that their electronic properties are significantly different. BN tubes are wide band gap insulators while BCN systems can be semiconductors with a band gap in the visible range.     

Effects of germanium incorporation on optical performances of silicon germanium passive devices for group-IV photonic integrated circuits

Optical interconnect in integrated optoelectronic circuits is one of the promising next-generation technologies for replacing metalized interconnect. Efforts have been made to use silicon (Si)-compatible materials such as germanium (Ge) and Ge-buffered III–V compound semiconductors, along with Si, as optical sources for Si and group-IV integrated optoelectronic systems. This opens the possibility that higher fraction of Ge with its high refractive index (n) can be incorporated in Si waveguide for optical interconnect and the graftability between Si and group-IV or III–V materials would be improved in silicon photonics. In this work, advantageous features of nano-structured silicon germanium (Si_1−xGe_x) optical waveguide with different Ge fraction (x) were evaluated by both optical simulations and theoretical calculations, which are mainly found in the enhanced optical confinement and better interfacing capability. Along with the SiGe waveguide, performance of Si_1−xGe_x microring resonator under material loss in the effect of extinction coefficient (k) has been investigated to suggest the necessity of optimizing the Ge content in Si_1−xGe_x passive devices. While carrying out the establish design criteria, n and k have been modelled in closed-form functions of Ge fraction at 1550 nm. Furthermore, by examining high-resolution transmission electron microscopy (HR-TEM) images, process compatibility of Ge with either group-IV alloys or III–V compound semiconductors is confirmed for the monolithically integrated photonic circuits.     

Anomalous coverage behavior of the CsAg distance on CsAg(110)

The position of Cs on the (1 × 2) missing row reconstructed Ag(110) surface was determined by X-ray diffraction for two different Cs-coverages: θ_Cs = 0.2 and θ_Cs = 0.3. The Cs was found to be adsorbed in incommensurate chains in the troughs of the missing row with an average adsorption height of 1.7 Å (θ_Cs = 0.2) and 1.4 Å (θ_Cs = 0.3) above the topmost Ag layer. The apparent contradiction to the classical picture of alkali adsorption, which expects an increase of the Cs adsorption height with coverage, might be partly resolved by introducing a fraction of commensurately adsorbed Cs at θ_Cs = 0.2.     

Thermal conductivity and heat capacity of Si3N4/BN fiber monoliths

This paper reports on measurements within the 5–300-K temperature interval of the thermal conductivity of Si₃N₄ and BN polycrystalline ceramic samples and Si₃N₄/BN fiber monoliths (FM) with different fiber arrangement architecture, [0], [90], and [0/90], with fibers arranged, accordingly, along and across the sample axis and the [0] and [90] layers stacked alternately. In the 3.5–300-K interval, the heat capacity at constant pressure, and at 77 K, the sound velocity have been measured in polycrystalline Si₃N₄ and BN samples and in Si₃N₄/BN [0] fiber monoliths. Our studies suggest that, with a high enough degree of confidence, but for some compositions—with minor assumptions, it can be maintained that, in the case of the Si₃N₄/BN fiber monoliths, one can use for calculation of their thermal conductivities and heat capacities within certain temperature intervals simple models considering mixtures of the Si₃N₄ and BN components with due account of their contributions to formation of the Si₃N₄/BN FM. It has been established that in the low-temperature domain (5–25 K), phonons in Si₃N₄/BN [0], [90], and [0/90] fiber monoliths scatter primarily from dislocations. This effect is not observed in ceramic Si₃N₄ and BN samples. The experimental data obtained on the thermal conductivity, heat capacity, and sound velocity have been used to calculate phonon mean free path lengths in polycrystalline Si₃N₄ and BN samples and the effective mean free path length in the Si₃N₄/BN [0] FM.     

Formation and atomic structures of boron nitride nanohorns encaging boron nitride cluster

Boron nitride (BN) nanohorns were synthesized by arc-melting YB₆ powders. Method, and atomic structure models for BN nanohorns encaging Y@B₃₆N₃₆ were proposed from high-resolution electron microscopy. The molecular mechanics calculation indicated that BN clusters with metal atoms would be stabilized by being encaged in double-walled BN nanohorns.     

Magneto-optical spectroscopy of diluted magnetic oxides TiO2−δ: Co

We report an experimental study on transversal Kerr effect (TKE) in magnetic oxide semiconductors TiO_2−δ:Co. The TiO_2−δ: Co thin films were deposited on LaAlO₃ (0 0 1) substrates by magnetron sputtering in the argon-oxygen atmosphere at oxygen partial pressure of 2×10⁻⁶-2×10⁻⁴ Torr. It was obtained that TKE spectra in ferromagnetic samples are extremely sensitive to the Co-volume fraction, the crystalline structure, and technology parameters. The observed well-pronounced peaks in TKE spectra for anatase Co-doped TiO_2−δ films at low Co (<1%) volume fraction are not representative for bulk Co or Co clusters in TiO_2−δ matrix that indicates on intrinsic ferromagnetism in these samples. With increase of Co-volume fraction up to 5-8% the fine structure of TKE spectra disappears and magneto-optical response in reflection mode becomes larger than that for thick Co films