Symposium CC: Boron and Boron Compounds--From Fundamentals to Applications

2010 MRS Fall Meeting Logo

November 29 - 30, 2010


James H. Edgar
Dept. of Chemical Engineering
Kansas State University
Durland Hall
Manhattan, KS 66506



Martin Kuball
H. H. Wills Physics Laboratory
University of Bristol
Bristol, BS8 1TL United Kingdom


Michael Dudley
Dept. of Materials Science and Engineering
SUNY-Stony Brook
Stony Brook, NY 11794-2275





Symposium Support
National Institute for Materials Science (NIMS), Japan


Proceedings to be published in electronic-only format
(see MRS Online Proceedings Library at
as volume 1307E
of the Materials Research Society
Symposium Proceedings Series.

* Invited paper


SESSION CC1: Cubic BN, Alloys, and Superhard Borides
Chair: Artem Oganov
Monday Morning, November 29, 2010
Room 101 (Hynes)

8:45 AM CC1.1
Syntheis Cubic and Hexagonal Boron Nitride Crystals under High Pressure and Their Impurity Control.Takashi Taniguchi, NIMS, Tsukuba, Ibaraki, Japan.

Among the variety of III-V nitrides, cubic boron nitride (cBN) is a combination of the elements positioned in the highest row in the Periodic Table and is known as the wideband gap material (Eg=6.2eV). Recently, it was also found that hexagonal boron nitride (hBN), low density form of cBN, also exhibits attractive potential as a wideband gap material (Eg=~6eV). Some progresses in the synthesis of high purity cBN and hBN crystals were achieved by using Ba-BN as a solvent material at high pressure (HP) crystal growth. The key issue to obtain high purity crystals is to reduce oxygen and carbon impurities by using the highly reactive alkali base solvent under HP. On the other hand, synthesis of high quality hBN crystals at ambient pressure is an important issue for hBN’s practical application. Since hBN is stable at high temperature and 1atm, crystals growth of hBN should be achievable by finding appropriate solvent. Based upon this concept, high quality hBN crystals with similar optical properties with HPHT products were obtained by using Ni base solvent. Based upon the schemes with impurity control, artificial doping of some elements, such as lanthanide (LA), was carried out to functionalize the opto-electric properties in BN crystals. It was found that addition of LA-fluoride system, such as EuF3, into the growth solvent was effective to fabricate luminous crystals. Optical properties of c-BN:LA (LA=Ce,Gd,Eu,Tb or Sm) crystals exhibit functionalized features by LA in the crystals. With the analogy of this, doping of LA into AlN was carried out. Difference of observed optical properties between cBN:Eu and AlN:Eu suggests that the occupied site of Eu3+ is different between cBN:Eu and AlN:Eu. Ab-initio calculation of those occupied site and structural analysis by using STEM were applied to characterize the doping features of cBN:LA. In this paper, recent above approach for impurity control in BN crystal growth will be described.

9:00 AM CC1.2
Synthesis and Consolidation of Cubic-boron Nitride and Boron Carbide Nanopowders.Jonathan Doyle1, Hsiao-Fang Lee1, Jafar Al-Sharab2, Assimina A. Pelegri1, Bernard Kear2, Oleg Voronov3 and Stephen D. Tse1; 1Mechanical and Aerospace Engineering, Rutgers University, Piscataway, New Jersey; 2Materials Science and Engineering, Rutgers University, Piscataway, New Jersey; 3Diamond Materials Inc., Piscataway, New Jersey.

Nanostructured cubic boron nitride (c-BN) and boron carbide (B4C) are of interest for heavy and light armor, nuclear reactor coatings, and many other applications, which require superhard materials. In this work, c-BN and B4C nanopowders are synthesized using an RF inductively-coupled plasma in stagnation-point geometry injected with vaporized precursors. The chemical precursors dissociate in the hot plasma, and then undergo nucleation, growth, chemical reactions, and quenching while approaching a cold substrate. To understand the synthesis process and achieve repeatable results, the gas-phase synthesis flow field is characterized for temperature and radical/precursor concentrations using advanced laboratory-scale diagnostic techniques, such as spontaneous Raman spectroscopy and laser-induced fluorescence. Nanoparticle characteristics (i.e. composition and crystallinity) are diagnosed in-situ as they are formed during synthesis within flow field using Raman spectroscopy. Reducing the grain size of single- or multi-phase materials to nanoscale dimensions yields significant improvements in mechanical properties. For consolidation purposes, a non-aggregated nanopowder is desired, since otherwise a powder compact contains large pores between the nanoparticle aggregates, which are difficult to remove during sintering. Controlling particle (primary and aggregate) sizes and distributions through thermo-chemical-aerodynamic manipulation is an advantage provided by our setup. For example, average particle sizes of 15nm for the c-BN and B4C nanopowders are readily achieved in our system. The nanopowders are then consolidated utilizing high-pressure sintering (HPS) and spark plasma sintering (SPS) techniques, with grain growth and phase-transformation phenomena carefully examined. The bulk materials are characterized for hardness, wear properties, and impact strength, using various nano-indentation techniques. In addition, all synthesized powders as well as consolidated samples are subject to atomic scale characterization using analytical electron microscopy techniques to correlate processing parameters with final product properties.

9:15 AM CC1.3
Facile Synthesis and Characterization of a Superhard Material: Tungsten Tetraboride.Reza Mohammadi1,2, Andrew T. Lech1, Miao Xie1, Beth E. Weaver1, Michael T. Yeung1, Sarah H. Tolbert1 and Richard B. Kaner1,2,3; 1Chemistry and Biochemistry, University of California at Los Angeles (UCLA), Los Angeles, California; 2Materials Science and Engineering, University of California at Los Angeles (UCLA), Los Angeles, California; 33. California NanoSystem Institute (CNSI), University of California at Los Angeles (UCLA), Los Angeles, California.

Many practical applications such as cutting, forming and drilling involve superhard materials and coatings, traditionally fabricated from diamond or other hard to synthesize materials. Synthesis of these traditional superhard materials usually requires ultra-high pressure and temperature to be accomplished. We report a successful facile synthesis of tungsten tetraboride (WB4) by arc melting from the pure elements (W and B) at ambient pressure. The synthesized material was systematically characterized using x-ray diffraction (XRD), energy dispersed spectroscopy (EDS) and microindentation to measure its purity and mechanical properties. Our XRD and EDS results show that the synthesized material is phase pure with a composition very close to the designed stoichiometry. Also, we report a microindentation hardness of 43.3±2.9 GPa and 28.1±1.4 GPa at low load (0.49 N) and high load (4.9 N) for this material, respectively. Moreover, we obtained a bulk modulus of 285 GPa using high-pressure x-ray diffraction in a Diacell diamond anvil cell with ethylcyclohexane as the pressure medium. This superhard material showed a thermal stability up to ~550°C in air. These superior properties of tungsten tetraboride will be discussed, in more details, based on its structure and bonding of the constituent elements. We believe that this material can be considered as the most inexpensive superhard material, to date, and has a potential to be employed in many industrial applications and processes.

9:30 AM *CC1.4
Epitaxy and Doping of Cubic Boron Nitride Thin Films.Xing Wang Zhang, Zhi Gang Yin, Ya Ming Fan and Jie Ying; Key Lab of Semiconductor Materials Science, Institute of Semiconductors, CAS, Beijing, China.

Cubic boron nitride (c-BN) has many desirable properties including a high hardness second only to diamond, a high-temperature oxidation resistance, a good thermal conductivity, and the ability to machine ferrous-based materials. Furthermore, c-BN is a wide-band-gap semiconductor that can be doped both p and n type. Envisioned applications include use as hard, protective coatings as well as for electronic devices operating reliably in severe environments. Recently, c-BN films have been prepared by various chemical and physical vapor deposition methods. However, up to now the majority of c-BN films is composed of very small grains, and contains a high density of defects and grain boundaries. In order to use c-BN for electronic devices, the epitaxially grown c-BN materials must have a large-area monocrystalline structure and a high degree of crystal perfection. Besides close lattice matching to diamond, c-BN has a linear thermal expansion coefficient similar to diamond. Consequently, the best candidate as a substrate for c-BN epitaxial growth should be diamond. Heteroepitaxial nucleation of c-BN on diamond has been demonstrated using the high-pressure high-temperature technique. Nevertheless, most of c-BN films prepared by low-pressure methods on diamond substrates exhibited the same sequence of amorphous, turbostratic, and cubic layers that is observed on the commonly employed Si substrates. Recently, we have demonstrated that heteroepitaxial c-BN films without any intermediate hexagonal BN layer can be prepared at 900 oC on highly (001) oriented diamond substrates using ion beam assisted deposition (IBAD). A significantly improved crystallinity was obtained on a single crystalline (001) diamond substrate as indicated by a rocking angle of 0.2o for a 500-nm-thick c-BN film. The plasmon energies determined from electron energy loss spectroscopy showed significant radiation damage for diamond substrates under ion bombardment at low substrate temperatures, and such radiation damage will have an unfavorable effect on the c-BN epitaxial growth. Furthermore, the c-BN films with the various Si concentrations were also prepared by in-situ co-sputtering during IBAD, and it is found that the incorporation of a small amount of Si can result in a remarkable relief of stress whereas the c-BN content is nearly unaffected. The introduced Si atoms only replace B atoms and combine with N atoms to form Si-N bonds. The resistivity of the Si-doped c-BN films decreases with increasing Si concentration, and the resistivity of the c-BN film with 3.3 at.% Si is lowered by two orders of magnitude as compared to undoped samples, indicating an electrical doping effect of the Si impurity.


SESSION CC2: Icosahedral Boron and Boron Compounds
Chair: Xing Wang Zhang
Monday Morning, November 29, 2010
Room 101 (Hynes)

10:30 AM *CC2.1
Discovery of a Novel Boron Allotrope with Partially Ionic Bonding.Artem R. Oganov, Stony Brook University, Stony Brook, New York.

Artem R. Oganov, Jiuhua Chen, Carlo Gatti, Vladimir L. Solozhenko Boron is arguably the most complex element in the Periodic Table, and its history is richer in unexpected twists and surprises (see [1]) or even mistakes (see [1,2] for discussion) than history of any other element. For instance, until recently the phase diagram of this element was unknown - and published only in 2009 [3]. As a new surprise, we have found a new thermodynamically stable high-pressure allotrope of boron, which we call γ-B [3]. With the Vickers hardness of 50 GPa [4], this is one of the hardest known solids. The structure of γ-B was found using the evolutionary algorithm for crystal structure prediction [5]. It belongs to the Pnnm space group and contains 28 atoms in the unit cell. The structure contains B12 icosahedra and B2 pair in a NaCl-type arrangement. As we have found, there is a considerable degree of charge transfer between these entitites, a situation that can be described as partially ionic bonding in a pure elemental solid. This partial ionicity has important implications for the physical properties of γ-B - for example, the band gap [3], infrared spectra and LO-TO splitting [3], dielectric constants [3], and spatial distribution of frontier orbitals [1]. This phase occupies a very large stability field on the phase diagram that we have produced on the basis of theoretical and experimental results (see [3]). [1] Oganov A.R., Solozhenko V.L., J. Superhard Materials 31, 285 (2009). [2] Oganov A.R., Solozhenko V.L., Kurakevych O.O., Gatti C., Ma Y., Chen J., Liu Z., and Hemley R.J., 0908.2126 (2009). [3] Oganov A.R., Chen J., Gatti C., Ma Y.-Z., Ma Y.-M., Glass C.W., Liu Z., Yu T., Kurakevych O.O., Solozhenko V.L., Nature 457, 863 (2009). [4] Solozhenko V.L., Kurakevych O.O., Oganov A.R., J. Superhard Mater. 30, 428 (2008). [5] Oganov A.R., Glass C.W., J. Chem. Phys. 124, 244704 (2006).

11:00 AM CC2.2
Self-compensation of Boron Icosahedral Cluster Solids.Hiroshi Hyodo1, Akimitsu Nezu1, Shizuka Hosoi2, Kohei Soga1 and Kaoru Kimura2; 1Material Science and Technology, Tokyo University of Science, Chiba, Japan; 2Advanced Material Science, University of Tokyo, Chiba, Japan.

Elemental boron (B) has a framework crystalline structure built up from B12 icosahedral clusters and these solids are called B-icosahedral cluster solids (B-ICSs). B-ICSs have relatively large interstitial doping sites so that they can accept a large amount of other elements. High TC superconductivity is expected by carrier doping into B-ICSs, however, doping properties of B-ICSs are complex and the control of electrical conductivity by carrier doping based on the rigid band model is difficult and not achieved yet. We attempted Li or Mg doping, which are expected to be doped based on the rigid band model, into β-rhombohedral boron (β-B) and aimed to figure out these electronic structures and the doping properties. High concentration of Li or Mg were successfully doped into β-B, up to 17.8 Li/cell1) and 8.6 Mg/cell2), however, all sample showed the variable range hopping conduction. By electron doping, the disappearance of interstitial B atoms and the defect of B at B4 site were observed. These results are interpreted as follows. β-B is composed of B12 clusters and B28 clusters, and the former is electron deficient and the latter is electron excess. First of all, for undoped β-B, the electron deficient of B12 cluster and the electron excess of B28 cluster are compensated by interstitial B atoms at B16 sites and the defects in B13 sites in B28 cluster, respectively. When less than 8 electrons were doped into β-B, the doped electrons were compensated by dropping out of interstitial B atoms, which is guessed to bond weakly. And then, when more than 8 electrons were doped, the doped electrons were compensated again by the defect newly arisen at B4 site. The self-compensation in B-ICSs is originated from their complex structure. There is no report on the self-compensation in crystalline elemental semiconductors. References 1. H. Hyodo et al., to be published (2010). 2. H. Hyodo et al., Phys. Rev. B 77, 024515 (2008).

11:15 AM CC2.3
Elimination of Degenerate Epitaxy in the Growth of High Quality B12As2 Single Crystalline Epitaxial Films.Yu Zhang1, Hui Chen1, Michael Dudley1, Yi Zhang2, James H. Edgar2, Yinyan Gong3, Silvia Bakalova3, Martin Kuball3, Lihua Zhang4, Dong Su4 and Yimei Zhu4; 1Materials Science and Engineering, Stony Brook University, Stony Brook, New York; 2Department of Chemical Engineering, Kansas State University, Manhattan, Kansas; 3H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom; 4Center for Functional nanomaterials, Brookhaven National Laboratory, Upton, New York.

As a member of icosahedra borides, B12As2 has a wide band gap of 3.20eV, possessing many exceptional properties such as high hardness, high temperature thermoelectric and “self-healing” from radiation damage. Potential applications of B12As2 are for the fabrication of space electronics, neutron detectors, thermoelectronics and beta cells which are capable of producing electrical energy by coupling a radioactive beta emitter into a semiconductor junction. Due to the absence of native substrates, foreign substrates with compatible structural parameters are necessary for the growth of B12As2 epitaxial films. Previous efforts were devoted to deposit B12As2 on (0001) 6H-SiC substrates by chemical vapor deposition. However, growth of three-fold symmetry B12As2 epilayer on six-fold symmetry SiC substrate produces structural variants (rotational and translational) in the film that are related to each other by a symmetry operation that is present in the substrate but absent in the epilayer, which is the phenomenon known as degenerate epitaxy. Thus the films contained high density of B12As2 twin boundaries which may have adverse effects on device performance. In this work, the effects of degenerate epitaxy were successfully eliminated and high quality single crystalline B12As2 epitaxial layers were achieved by using off-axis (0001) 4H-SiC and (1-100) 15R-SiC substrates. Synchrotron white beam x-ray topography (SWBXT) revealed that only one orientation of B12As2 was present in the epitaxial layer and B12As2 diffraction spots have much better-defined shapes compared to those grown on 6H-SiC substrates. This indicates that there are no structural variants in the films and the effects of degenerate epitaxy were fully eliminated. Cross-sectional high resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) both confirmed the presence of only one orientation in the B12As2 grains and revealed the high quality of the films. Ease of nucleation on these unique substrates with ordered step structures overrides symmetry considerations and dominates the nucleation process of B12As2. The detailed mechanism of degenerate epitaxy elimination will be discussed.

11:30 AM CC2.4
Solution Growth and Characterization of Icosahedral Boron Arsenide (B12As2)Clinton E. Whiteley1, A. Mayo1, J. H. Edgar1, Y. Gong2, M. Kuball2, Y. Zhang3 and M. Dudley3; 1Clinton Whiteley, Kansas State University, Manhattan, Kansas; 2H. H. Willis Physics Laboratory, University of Bristol, Bristol, United Kingdom; 3Department of Materials Science and Engineering, SUNY, Stony Brook, Pennsylvania.

Bulk icosahedral boron arsenide (B12As2)crystals were grown by precipitation from molten nickel solutions, and their properties were characterized. Crystals were grown by dissolving either (B12As2) or boron in nickel, under an ambient of arsenic vapor in a sealed quartz tube. Small crystals (<1 mm in their largest dimension) were produced from the (B12As2) source, while larger crystals (5-8 mm) were produced by reacting the boron dissolved in nickel with arsenic. This suggests that (B12As2) is less soluble in nickel than boron. The crystals were formed by first dissolving the boron at 1150 °C for 72-96 hours, then reacting with arsenic vapor (arsenic source at 600 °C), followed by slow 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) precipitates were high structural quality single crystals with relatively low dislocation densities. Furthermore, XRT suggests 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 observed to be edge character growth dislocations with < 001> Burgers vector, and <-110> line direction. In whole, this characterization shows that this is a viable method for producing good quality (B12As2) crystals which have potential use as substrates for boron based devices and their applications.


SESSION CC3: Boron Superconductors and Ferromagnetic Materials
Chair: Takao Mori
Monday Afternoon, November 29, 2010
Room 101 (Hynes)

1:30 PM *CC3.1
Superconducting Properties of MgB2: Implications for Applications.Michael Eisterer, Atominstitut, Vienna University of Technology, Vienna, Austria.

The two band nature of superconductivity in MgB2 is certainly the most interesting property of this material from a theoretical point of view, since it changes the temperature dependence of many fundamental properties, e.g. the upper critical field, the magnetic penetration depth and the electronic specific heat. However, these modifications are of minor importance for applications, since the Cooper pairs related to the smaller energy gap add only little to the critical current. Issues to be addressed are the comparatively small upper critical field, its anisotropy and the difficulties to synthesize materials with clean grain boundaries, which do not harm the current flow. This challenges the material optimization since impurities are needed to improve the upper critical field, which counteracts the formation of clean grain boundaries. A careful characterization of the conductors is mandatory to distinguish between changes in the superconducting parameters, the flux pinning landscape and the connectivity between the grains in order to guide further material developments

2:00 PM *CC3.2
Growth Chemistry and Intentional Carbon-Doping of Superconducting MgB2 Thin Films Grown by Hybrid Physical-Chemical Vapor Deposition.Joan Redwing1,4, Daniel Lamborn3, Alexei Pogrebnyakov2,1, Xiaojun Weng4, Wenqing Dai2, Qi Li2,4 and Xiaoxing Xi2,5; 1Materials Science and Engineering, Penn State University, University Park, Pennsylvania; 2Physics, Penn State University, University Park, Pennsylvania; 3Chemical Engineering, Penn State University, University Park, Pennsylvania; 4Materials Research Institute, Penn State University, University Park, Pennsylvania; 5Physics, Temple University, Philadelphia, Pennsylvania.

The discovery of superconductivity in MgB2 (Tc~39K) has stimulated interest in the development of thin film deposition techniques to facilitate the fabrication of Josephson junctions for superconducting electronics as well as thick coatings on conductive wires and tapes for high-field magnet applications. Hybrid physical-chemical vapor deposition (HPCVD) has emerged as a premier technique for the epitaxial growth of MgB2 films with high Tc and the deposition of carbon-alloyed MgB2 films with high upper critical fields (Hc2). In the HPCVD process, Mg evaporation is used in combination with the thermal decomposition of diborane (B2H6) gas in a hydrogen ambient to deposit MgB2 at growth temperatures of 720-760°C and pressures ranging from 100-700 Torr. In this study, a combination of deposition experiments and computational fluid-dynamics (CFD) simulations were used to investigate the effects of process parameters, growth chemistry and reactor design on the growth rate and properties of MgB2 films. Due to the high evaporation rate, Mg is present in excess in the growth ambient; consequently, the MgB2 growth rate is controlled by the boron precursor. The CFD simulations reveal that B2H6 rapidly decomposes in the gas phase to BH3 under typical HPCVD growth conditions and consequently, the growth rate is limited by the supply of BH3 to the surface. Reduced growth rates (1-3 μm/hr) yield epitaxial MgB2 films on SiC and sapphire substrates. By utilizing reactor designs that minimize gas phase pre-reaction and depletion of B2H6, such as an impinging jet reactor, polycrystalline MgB2 thin films that exhibited a transition temperature of 39.5 K were demonstrated at growth rates up to ~50 μm/hr. The effects of precursor chemistry on the microstructure and superconducting properties of carbon-alloyed MgB2 films were also investigated. Two carbon-containing precursors were investigated; bis(methylcyclopentadienyl)Mg (MeCp2Mg) and trimethylboron (TMB). Transmission electron microscopy characterization of MgB2 films grown with MeCp2Mg indicate that the films consist of nanoscale grains of MgB2 surrounded by an amorphous carbon-rich phase. In contrast, the MgB2 films grown with TMB exhibit a layered structure consisting of thin sheets of MgB2 within a MgB2C2 matrix. The effects of microstructure on the high-field properties of the carbon-alloyed MgB2 films will be discussed.

2:30 PM CC3.3
Magnetic, Mechanical and Microstructure Properties of Fe-B Thin Films. Alfred Ludwig1,2, Hayo Brunken1, Dario Grochla1, Alan Savan1,2 and Michael Kieschnick3; 1Institute of Materials, Ruhr-University Bochum, Bochum, NRW, Germany; 2Research Department IS3/HTM, Ruhr-University Bochum, Bochum, NRW, Germany; 33. Central Unit for Ionbeams and Radionuclides (RUBION), Ruhr-University Bochum, Bochum, NRW, Germany.

Multifunctional nanocomposite thin film systems unifying mechanical, protective and other functional properties are of interest for applications as sensors and actuators in harsh environments. In relation to this, iron borides attract considerable interest due to their potential to serve as hard, wear-resistant and chemically inert coatings while having at the same time interesting ferromagnetic properties. In industry, Fe-B compounds are produced directly on the surface of steels via boriding and are known to improve the tensile strength and hardness of steels. The phases Fe2B and Fe3B have a saturation magnetization of 1.51 and 1.62 T and are soft magnetic. However, to date no studies on Fe-B thin film compounds which have the potential of combining wear-resistance and intrinsic sensor function properties have been published. In this work combinatorial sputter deposition methods were used to create Fe-B composition gradients on thermally oxidized 4” Si (Si/SiO2) wafer substrates. These composition spreads were fabricated from wedge-type multilayer thin films sputtered from elemental targets and subsequent ex situ annealing using different conditions. Nuclear reaction analysis (NRA), Rutherford backscattering (RBS), nanoindentation, vibrating sample magnetometry (VSM) and x-ray diffraction (XRD) measurements were applied for the characterization of the composition spreads. Focused ion beam (FIB) techniques were used for sample preparation for transmission electron microscopy (TEM), later used for the investigation of thin film microstructures. Detailed investigation was made of the phases present in the as-deposited and annealed films within the compositional range of 51 to 82 at.% Fe. The corresponding Young’s modulus and hardness values were determined, and the observed mechanical characteristics were related to the structural and compositional information. It is shown that the magnetic properties are improved and the hardness values increase from 8 to 18 GPa with higher annealing temperatures and after annealing in H2 atmosphere. Here, the appearance of Fe2B phases shows a significant influence on the mechanical properties of the films while heat treatment in H2 atmosphere lowers the coercivity to 0.9 mT. For the Fe-B composition spreads, the results of the magnetic analysis for different annealing parameters, i.e. temperature, time and atmosphere (vacuum and H2) are compared and discussed. The application of the observed multiphase structures of the annealed Fe-B thin films, with a characteristically low coercive field and high hardness, as sensors and actuators in harsh environments will also be discussed.

2:45 PM CC3.4
Compressive Superplastic Behavior in MgB2.John DeFouw1 and David C. Dunand2; 1Materials, University of Wisconsin - Milwaukee, Milwaukee, Wisconsin; 2Materials Science and Engineering, Northwestern University, Evanston, Illinois.

The brittle superconducting ceramic compound MgB2 was studied under uniaxial mechanical compression at temperatures between 900 and 1000 °C and initial strain rates between 9x10-6 s-1 and 9x10-4 s-1. Resulting flow stresses were fitted to a standard Arrhenius equation for creep deformation showing plastic flow occurred under a diffusion-controlled mechanism with activation energies between 250 and 450 kJ mol-1 and stress exponents between 1.4 and 2.0. Activation energies and stress exponents varied due to impurities of MgO and MgB4 present in the hot isostatically pressed commercial powders. Large scale superplastic compressive deformation to 67% (true strain of -1.1) was achieved at 1000 °C without fracture of the specimen. This type of macroplasticity in MgB2 should allow for the mechanical drawing of thin, dense superconducting wires.


SESSION CC4: Thermoelectrics, Diborides, and Metallic Hexaborides
Chair: Michael Eisterer
Monday Afternoon, November 29, 2010
Room 101 (Hynes)

3:30 PM *CC4.1
Advances in Thermoelectric Perspectives of Borides.Takao Mori, National Institute for Materials Science (NIMS), Tsukuba, Japan.

Approximately two thirds of all primary energy (fossil fuels, etc.) being consumed in the world, sadly turns out to be unutilized, with much of it being waste heat. The useful and direct conversion of waste heat to electricity is a large incentive to find viable thermoelectric (TE) materials. One need exists to develop materials which can function at high temperature, T. Boron-rich compounds are attractive materials for their stability, exhibiting melting points above 2200 K. Furthermore, they have been found to possess low thermal conductivity, κ, even for single crystals, which is an inherent advantage for TE application. As a synthesis method we have recently discovered that small amounts of third elements like C, N, and Si can function as bridging sites and result in the formation of novel and varied boron cluster structures [1]. As a result, new borides promising for TE were found. REB44Si2 exhibit Seebeck coefficients, α, greater than 200 μV/K at high T and unlike most compounds, the figure of merit, ZT, shows a steep increase at T>1000 K [2]. A series of homologous RE-B-C(N) compounds; REB17CN, REB22C2N, and REB28.5C4, was discovered to be the long awaited n-type counterpart to boron carbide [3]. Boron carbide is one of the few commercialized TE materials [4], however, lack of an n-type counterpart had been a long standing obstacle, with great efforts made. For example, Slack discovered that heavy vanadium doping of beta-boron results in n-type behavior (recently other groups are repeating this work) [5]. However, it is a simple metallization, and with α trending small at higher T, it is not a viable counterpart. The novel rare earth borocarbonitrides are the first icosahedral borides to exhibit intrinsic n-type behavior and the structural similarities make it a promising compatible counterpart. In this talk I will review the TE properties of all borides, and give an overview on synthesis methods for the novel borides and advancements in research for their application as high T thermoelectric materials. I will also discuss the origin of the low κ observed in borides [6]. Interesting aspects are: 1) crystal complexity, 2) "rattling" of rare earth atoms in the boron framework, 3) disorder, 4) symmetry of the basic building component, the B12 icosahedra, not matching the crystal symmetry, and 5) particular features of the crystal structures, like boron dumbbells, which are found to reduce κ. [1] T. Mori, “Higher Borides” in Handbook on the Physics and Chemistry of Rare Earths, Vol. 38, (North-Holland, Amsterdam, 2008) pp. 105-173 (2008). [2] T. Mori et al., J. Appl. Phys. 97 (2005) 093703, Dalton Trans. 39 (2010) 1027 (Hot Article). [3] T. Mori et al., J. Solid State Chem. 179 (2006) 2908, J. Appl. Phys. 101 (2007) 093714. [4] C. Wood et al., Phys. Rev. B 29 (1984) 4582. [5] G. A. Slack et al., Proc. 9th Int. Symp. Boron, Borides and Related Compounds, Duisberg, Germany (1987), p. 132. [6] T. Mori et al., J. Appl. Phys. 102 (2007) 073510.

4:00 PM CC4.2
Metal Borides - Whether Hard Materials, Thermoelectrics, or Catalysts - They Always Exhibit Fascinating Crystal Structures.Barbara Albert, Eduard-Zintl Institute of Inorganic and Physical Chemistry, Technische Universität Darmstadt, Darmstadt, Germany.

Borides of almost all metals are known to exist and to exhibit an enormous variety of compositions and structures. They also offer a wide range of opportunities in terms of interesting properties and possible functions. Physical properties such as hardness, superconductivity, neutron scattering length and thermoelectricity have made boron-rich compounds attractive to materials research and applications; the biggest challenges to boron chemistry, however, still remain: the synthesis of mono-phasic products as bulk materials and in the form of single crystals, the unequivocal identification and determination of their crystal structure, and a thorough understanding of their electronic situation. This contribution will focus on known and new metal borides, presenting new findings on binary and ternary borides and boride carbides, catalytically active nano-particles (Ni/Fe/Co-B), and boron-rich borides with high Seebeck coefficients. We investigated compounds like BeB2C2, CaB2C2, Mo2B4, W2B4, RuB2, Ru2B3, Os2B3, IrB, ScB30.[1] [1] B. Albert, H. Hillebrecht, Angew. Chem. Int. Ed. 2009, 48, 8640-8686

4:15 PM CC4.3
Crystal Orientations of ZrB2 Thin Films Epitaxially Grown on Si(111).Yukiko Yamada-Takamura1,2, Wenyong Zhang1,2 and Antoine Fleurence1,2; 1School of Materials Science, JAIST, Nomi, Ishikawa, Japan; 2Research Center for Integrated Science, JAIST, Nomi, Ishikawa, Japan.

Zirconium diboride (ZrB2) is known as a conductive ceramic with high hardness, high melting point, high electrical and thermal conductivities. Like most of the diborides, it has a very simple crystal structure, AlB2-structure, which consists of alternating hexagonal close-packed metal layers and honeycomb boron layers. In the form of thin films, diborides could be integrated with other materials easily, and it has been demonstrated that ZrB2, which has a very good lattice matching to a wide-gap semiconductor hexagonal gallium nitride (GaN), serves as a conductive and reflective buffer layer for the GaN growth on an economical and conductive substrate, Si(111) wafer [1,2]. For the growth of ZrB2, chemical vapor epitaxy (CVE) under ultrahigh vacuum (UHV) using zirconium borohydride (Zr(BH4)4) [1] has advantages in source material with high purity and in reducing oxides resulting in pure diboride thin films with low electrical resistivity. A unique UHV-CVE system, which has a capability of monitoring diboride film growth in-situ using reflection high-energy electron diffraction, was constructed. The epitaxial growth of single-crystalline ZrB2(0001) thin films on sapphire(0001) was carried out successfully, driven by a favorable interface between ZrB2(0001) and sapphire(0001) [3]. In comparison to the growth on sapphire substrates, single-crystalline ZrB2(0001) thin film growth on Si(111) substrates turned out to be rather difficult. Although we have observed dominant growth of ZrB2 crystals with singular epitaxial relationship of ZrB2(0001)//Si(111) and ZrB2[1-100]//Si[11-2] under optimized conditions, a small amount of ZrB2 crystallites misoriented from this relationship persisted [4]. In this presentation, we will show that the crystal orientation of ZrB2 thin films on Si(111) substrates is very sensitive to the growth temperature. The crystal orientation changed dramatically from c-axis in-plane to c-axis out-of-plane, with the increase of substrate temperature from 770 oC to 970 oC. Surface reconstructions observed during the growth implies the existence of surfactant, which is likely to be originating from the substrate, and the possible role of the surfactant on the observed crystal orientations will be discussed. [1] J. Tolle, R. Roucka, I.S.T. Tsong, C. Ritter, P.A. Crozier, A. V. G. Chizmeshya and J. Kouvetakis, Appl. Phys. Lett. 82, 2398 (2003). [2] Y. Yamada-Takamura, Z. T. Wang, Y. Fujikawa, T. Sakurai, Q. K. Xue, J. Tolle, P.-L. Liu, A. V. G. Chizmeshya, J. Kouvetakis, and I. S. T. Tsong, Phys. Rev. Lett. 95, 266105 (2005). [3] S. Bera, Y. Sumiyoshi, and Y. Yamada-Takamura, J. Appl. Phys. 106, 063531 (2009). [4] A. Fleurence and Y. Yamada-Takamura, Phys. Status Solidi, to be published.

4:30 PM CC4.4
Defect Processes of Lithium and Helium Within Zirconium Diboride.Simon C. Middleburgh1,2, David C. Parfitt1, Paul R. Blair2 and Robin W. Grimes1; 1Department of Materials, Imperial College London, London, United Kingdom; 2Westinghouse Electric Sweden, Västerås, Sweden.

Simulations using density functional theory (DFT) were carried out to investigate the intrinsic defect properties of zirconium diboride (ZrB2) and also the solution and diffusion of He and Li. Schottky and Frenkel intrinsic defect processes were all high energy. Li and He species, formed by the transmutation of a 10B, are therefore predominantly accommodated at the resulting vacant B sites. Li was found to be considerably more stable at the vacant B sites than He. Further, He was found to di[|#11#|]ffuse as an interstitial species through the lattice with a very low activation energy. This would be consistent with He being lost from the ZrB2 but with Li being retained to a much greater extent.


SESSION CC5: Poster Session: Boron Compound Synthesis, Processing, and Characterization
Chairs: Michael Dudley, James Edgar and Martin Kuball
Monday Evening, November 29, 2010
8:00 PM
Exhibition Hall D (Hynes)

The Local Structure of Transition Metal Doped Semiconducting Boron Carbides.Jing Liu1, Peter A. Dowben1, Guangfu Luo2,3, Wai-Ning Mei2, Orhan Kizilkaya4, Eric D. Shepherd5 and Jennifer I. Brand5; 1Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska; 2Department of Physics, University of Nebraska-Omaha, Omaha, Nebraska; 3State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking Univiversity, Beijing, China; 4Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana; 5College of Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska.

Transition metal (Mn, Fe, Co) doped boron carbides thin films produced by plasma-enhanced chemical vapor deposition of orthocarborane (closo-1,2-C2B10H12) and metallocenes were investigated by performing K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) measurements [1]. The Mn, Fe and Co transition metal atoms dope boron carbide pairwise on adjacent icosahedra. Each transition metal atom occupies one of the icosahedral boron or carbon apical site atomic site within the icosahedral cage on adjacent edge bonded icosahedral cages. Knowledge of the local structure provides the essential information for localized electronic structure calculations for a variety of the transition metal dopants in boron carbide. There is good agreement between the experiment and theoretical modeling of the local structure two adjoined carborane cages each with a Mn, Fe and Co metal atom (forming the pair wise doping). The local spin configurations of all the 3d transition metal doped boron carbides, Ti through to Cu, are compared using theoretical cluster or icosahedral chain calculations [2]. The results suggest that transition metal doping will not only permit fabrication of boron carbide homojunctions, as is now demonstrated, but also may result in materials suitable for spintronic applications, as suggested by other transition metal doped materials [3]. [1] J. Liu, G. Luo, W.-N. Mei, O. Kizilkaya, E.D. Shepherd, J.I. Brand and P.A. Dowben, J. Phys. D: Applied Physics 43 (2010) 085403 [2] Guangfu Luo, Jing Lu, Jing Liu, Wai-Ning Mei, and P. A. Dowben, Materials Science and Engineering B (2010), in press [3] J. A. Colón Santana, R. Skomski, V. Singh, V. Palshin, A. Petukhov, Ya. B. Losovyj, A. Sokolov, P. A. Dowben, and I. Ketsman, J. Applied Physics 105 (2009) 07A930

Abstract Withdrawn

Electron and Hole Traps in Silver-doped Lithium Tetraborate (Li2B4O7) Crystals Being Developed for Neutron Dosimetry. John McClory1, Brant E. Kananen1, Adam T. Brant2, James C. Petrosky1 and Larry E. Halliburton2; 1Engineering Physics, Air Force Institute of Technology, Wright-Patterson AFB, Ohio; 2Physics, West Virginia University, Morgantown, West Virginia.

Lithium tetraborate (Li2B4O7) is being developed for use as a solid-state neutron detector, thus expanding its present role as a versatile thermoluminescence dosimeter. Li2B4O7 is of interest for this new application because of the large cross sections of 6Li and 10B for neutron capture. The 6Li(n,a)3H and 10B(n,a)7Li reactions are the basis for the detection of neutrons and Li2B4O7 crystals enriched with 6Li and 10B are expected to improve detection sensitivity. It is well known that doping Li2B4O7 with Ag significantly increases the light output in dosimeter applications. In the present study, electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques are used to identify electron and hole traps associated with Ag ions in a Li2B4O7 crystal doped with Ag during growth. The crystal was irradiated at room temperature with x rays, and EPR and ENDOR spectra were obtained near 20 K. The resulting EPR spectrum showed two similar Ag2+ (3d9) hole centers and one Ag0 (3d104s1) electron center. The Ag2+ hole centers exhibit small hyperfine splittings due to the 107Ag and 109Ag nuclei (I = 1/2 for both). The electron center exhibits large hyperfine splittings due to the 107Ag and 109Ag nuclei and a smaller hyperfine splitting due to a neighboring I = 3/2 nuclei (either a 11B or 7Li nucleus). Both the Ag2+ and the Ag0 centers occupy Li+ sites in the Li2B4O7 lattice. Heating to approximately 250°C restores the crystal to its pre-irradiated state and reveals different decay temperatures for the hole centers. This suggests that thermoluminescence in Li2B4O7:Ag occurs when holes are thermally released from Ag2+ ions during heating and then move to the trapped electrons at Ag0 ions. The resulting recombination of electrons and holes takes place at the trapped electron site.

Stability and Dynamics of Boron Nitride Nanoscrolls.Eric P. Martins and Douglas S. Galvao; Applied Physics Department, Campinas State University - Unicamp, Campinas, São Paulo, Brazil.

Carbon nanoscrolls (CNS) are extremely radially flexible structures formed by bending single or multiple graphene sheets into a scroll-like fashion [1]. Although known since the 1960s there are very few works on CNSs. This can be explained in part by the intrinsic experimental difficulties in synthesis and characterization. With the recent advances in low-temperature CNS synthesis there is a renewed interest in these materials [2],[3]. Hexagonal boron nitride, also known as inorganic graphite, has been proven to generate similar nanostructures as those generated from graphite, like nanotubes, closed cage structures and, more recently, single layers. Due to this, we investigated the possibility of producing scroll-like boron nitride structures, the BN nanoscrolls (BNNS)[4]. We carried out classical molecular dynamics to investigate the mechanical properties and dynamics of formation of BNNS. Our results show that stable BNSS can be formed in a very similar way that CNSs are produced. Due to van der Waals interactions among overlapping layers, BNNS can be even more stable than planar BN sheets. We also showed that chirality is very important, as zig-zag scrolls present higher stability than armchair ones and that chiral BNNSs evolve to either zig-zag or armchair. The formation path was also investigated and it was shown that it is energetically more favorable to bend the sheet from both ends, instead of bending from one end alone. As recently the BN monolayer has been synthesized [5] by different groups and there are new experimental techniques available for synthesizing CNSs [3], we believe that BNNSs can be now produced using similar methods. [1] Braga S F, Coluci V R, Legoas S B, Giro R, Galvao D S and Baughman R H 2004 Nano Lett. 4 881 [2]Viculis L M, Mack J J and Kaner R B 2003 Science 299 1361. [3]Xie X, et al 2009 Nano Lett. 9 2565. [4]Perim E and Galvão D S 2009 Nanotechnology 20 335702. [5]Jin C, Lin F, Suenaga K, Iijima S 2009 Phys Rev Lett 102 195505.

Effect of Al3Ti Adittion on the Sintering of TiB2.Masashi Yoshida, Mechanical Engineering, Ube National College of Technology, Ube, Yamaguchi, Japan.

TiB2 is a promissing material for high temperature use because it has high hardness, good corrosion resistance and a high melting temperature. However, an extremely high temperature e.g. above 1700C is required to sinter TiB2. In the present study, we investigated the effect of Al3Ti addition on the sintering of TiB2 by using sparc plasma sintering (SPS). It was found that, by the addition of 10wt% Al3Ti powders, the Vicars hardness of sintered TiB2 increased to as high as 2000 by the sintering at 1000C by SPS. On the other hand, no such remarkable improvement has been obtained in the sintering with the addition of Ti or Al powders. Reaction of TiB2 with Al3Ti was observed at the interface to form a new compound, which was supposed to causes the good sintering property of TiB2.

First-principles Study of Ammonia Borane and Alkali-metal Amidoboranes for Hydrogen Storage.Hongbin Huang1,2, Takao Tsumuraya3, Tatsuya Shishidou2 and Tamio Oguchi1,2; 1ISIR, Osaka University, Ibaraki, Japan; 2ADSM, Hiroshima University, Higashihiroshima, Japan; 3Department of Physics and Astronomy, Northwestern University, Evanston, Illinois.

Materials that exhibit high gravimetric and volumetric density of hydrogen, low decomposition temperatures, and reversibility of hydrogen absorption and desorption are required for practical hydrogen-storage applications. Unfortunately, no materials are currently known that possess all these attributes. To obtain the guidelines for design of novel hydrogen-storage materials that fulfill the requirements, it is necessary to understand the bonding nature and the thermodynamic properties of related existing materials. In this context, extensive interest has been focused on ammonia borane, NH3BH3 and alkali and alkaline-earth metal amidoboranes, M(NH2BH3)n (M=K, Na, Li (n=1) and Ca (n=2)) because of their high capacity of hydrogen at initial phase [1-3]. In this study, we investigate the electronic structure of the alkali and alkaline-earth metal amidoboranes by using first-principles density-functional calculations with the all-electron full-potential linearized augmented plane wave (FLAPW) method and molecular orbital (MO) analysis. We have found that the electronic structure of M(NH2BH3)n can be considered as that of an ionic solid phase: Mn+ and [(NH2BH3)-]n since hybridization between cation orbitals and 2a1 MO in NH2 is less significant, compared with that in metal amide M(NH2)n [4]. The anion (NH2BH3)- is basically composed of BH3 and NH2 MOs. It has been seen that hybridization between N-p and B-p state comes from 2a1 MO in NH3 and lowest unoccupied MO in BH3 that may affect the strong bonding between boron and nitrogen atoms in crystal. We also report on the electronic and structural differences between ammonia borane and amidoboranes. Acknowledgements: This work is supported in part by the Grants of the NEDO project ‘Advanced Fundamental Research of Hydrogen storage materials’. [1] W. T. Klooster et al., J. Am. Chem. Soc. 121, 6337 (1999). [2] Z. Xiong et al., Nature Mater. 7, 138 (2008). [3] W. Hu et al., J. Am. Chem. Soc. 130, 14834 (2008). [4] T. Tsumuraya, T. Shishidou, and T. Oguchi, J. Phys.: Condens. Matter 21, 185501 (2009).

Iron-boron Nanowires: A Template Based Approach. Martin Waleczek1, Robert Zierold1, Dieter Lott2, Julien Bachmann1 and Kornelius Nielsch1; 1Institute of Applied Physics, University of Hamburg, Hamburg, Germany; 2Institute of Materials Research, GKSS Research Centre, Geesthacht, Germany.

Amorphous metallic alloys such as iron-boron hold the promise of obtaining nanocrystalline soft-magnetic structures that can be easily produced by electrodeposition on conducting substrates. In the present work the magnetic properties of iron as well as boron's capability to capture neutrons are taken advantage of in order to strike a new path to boron neutron capture therapy (BNCT), an experimental cancer therapy. In this approach electrodeposition is applied using a hexagonal pore array (formed by self-organization in anodic alumina) and commercial polycarbonate filter membranes (pore diameter 200 nm) as template systems. In this way, magnetic nanowires with diameters between 40-300 nm and with widely tunable aspect ratios as well as chemical composition can be fabricated. The biocompatibility of these wires is ensured by a functional silica shell, which is realized by atomic layer deposition (ALD) before electrodeposition. The templates can subsequently be etched away selectively, leaving silica coated iron-boron nanowires. To determine the material's structure and composition, iron-boron thin films are investigated by energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and grazing incidence X-ray diffraction (GIXRD). The electrolyte used for electrodeposition of these thin films is then optimized for the fabrication of nanowires in order to achieve the highest possible boron-to-iron ratio. The magnetic properties are investigated by magnetometry (superconducting quantum interference device (SQUID), vibrating sample magnetometer (VSM) and magneto-optic Kerr effect (MOKE) measurements). Upon irradiation of the samples with thermal neutrons, the isotope boron-10 converts to lithium-7. In this reaction high-energy particles with a very local, highly destructive effect on nearby tissue are produced. This feature is utilized in BNCT. Thus, our iron-boron nanoparticles are applicable as magnetic suspensions for BNCT. With up to 28 % boron embedded in a ferromagnetic carrier and protected by a biocompatible silica shell, basic requirements for a boron delivery agent in BNCT are met in this template based approach. In the next steps, we will investigate the system's potential in vitro. The authors acknowledge the DFG SPP 1165 and the EC Nanotechnology in Medicine for financial support.

Li Intercalation into Hexagonal Boron Nitride.Atsuro Sumiyoshi1, Hiroshi Hyodo2 and Kaoru Kimura1; 1Department of Advanced Materials Science Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Chiba, Japan; 2Department of Materials Science and Technology, Tokyo University of Science, Noda, Chiba, Japan.

Graphite intercalation compounds (GICs) are synthesized by inserting atoms and molecules between the graphene sheets. It is well known that graphite accommodates various materials and some of GICs show an interesting properties including superconductivity. On the contrary, although hexagonal boron nitride (hBN) has the honeycomb layer structure similar to the graphite, it is regarded that the intercalation into hBN interlayer is difficult because of stronger interlayer interaction originating from the partial ionic character of the interlayer B-N bond. Actually, there are few successful reports on the synthesis of hBN intercalation compound (hBNIC). Shen. et al. reported about SO3F-hBNIC. The BN interlayer distance was expanded by the intercalation and the temperature dependence of electrical conductivity showed metallic behavior1).Li-hBNIC was predicted to be a metal by calculation2). It is interesting to confirm the physical property of it experimentally. In this study, powder hBN or bulk pyrolytic BN and Li metal were set into BN or Ta crucible under Ar atmosphere in glovebox and the crucible was sealed into stainless steel tube by arc welding. By use of stainless steel tube as a reaction container, the higher reaction temperature and more Li rich condition than SiO2 container were prepared. The tube was annealed from 1200 K to 1650 K. As a result, Li intercalation to h-BN was succeeded3). The change in the structure was investigated by the synchrotron radiation XRD and the Rietveld method. The expansion of BN lattice which was caused by Li intercalation was observed. The electrical conductivity was measured with Van der Pauw method. The electrical conductivity was improved by several orders of magnitude at room temperature. References 1. C. Shen, et al., J. Solid State Chem. 147, 74 (1999). 2. B.L. Faifel, et al., Sov. Phys. Crystallogr. 31, 497 (1986). 3. A. Sumiyoshi,, J. Phys. Chem. Solids, 71, 569 (2010).

Electronic Structure of the (110) and (100) Surfaces of Li2B4O7.John McClory1, David J. Wooten1, James C. Petrosky1, Yaroslav V. Burak2, Volodymyr T. Adamiv2, Ihor Ketsman3, Jie Xiao3, Yaroslav B. Losovyj3,4 and Peter A. Dowben3; 1Engineering Physics, Air Force Institute of Technology, Wright-Patterson AFB, Ohio; 2Institute of Physical Optics, Lviv, Ukraine; 3Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska; 4Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana.

Apart from a number of interesting opto- and acousto-electronic properties, lithium tetraborate (LTB) is an interesting material with piezoelectric and pyroelectric properties. It is now one of the few pyroelectric materials for which there is a detailed experimental band mapping of both occupied and unoccupied states. The electronic structure of the filled and empty states at the (110) and (100) LTB surfaces was investigated by a combination of angle-resolved ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) studies, while the local electronic structure was probed using electron paramagnetic resonance (EPR) spectroscopy. The experimental density of states from photoemission and inverse photoemission is qualitatively similar to that expected from the model bulk band structure and the experimental band gaps for both surfaces are close to the theoretically predicted values, while the dispersion at the high symmetry points qualitatively follows the predictions of another local density approximation (LDA) calculation. This provides an experimental distinction between the two existing band structure calculations of this material. The light polarization dependent photoemission studies suggest that the (100) surface terminates with more polar character compared to the (110) LTB surface. This is supported by the observed differences in the surface photovoltage charging, much greater in the case of the (100) surface. Another evidence of the polar character of the above surfaces was obtained from the temperature dependent depolarization measurements, which give finite values of pyroelectric coefficient. We conclude that, although the polar axis in the LTB single crystal is in [001] direction, the other surface terminations of the LTB crystal exhibit some polar character as well.

Large Scale Synthesis of α-tetragonal Boron Nanostructures and Their Mechanical Properties.Youfei Jiang, Zhe Guan, Xiaoxia Wu and Terry Xu; UNC Charlotte, Charlotte, North Carolina.

It was reported that α-tetragonal boron nanoribbons could be synthesized by pyrolysis of dibroane (B2H6) at relatively low temperature (630-750 °C) and low pressure (200 mTorr). The yield of the nanoribbons was low so that they could not be characterized by x-ray diffraction (XRD). In addition, the growth mechanism of those nanoribbons (particularly on whether carbon is needed to stabilize the tetragonal boron framework) was not clear.1 In this presentation, we report our recent progress on large scale synthesis of α-tetragonal boron nanostructures. The significant yield increase was realized by co-pyrolysis of B2H6 and small amount of methane (CH4). Keeping the flow rate of B2H6 as constant (i.e., 15 sccm), the flow rate of CH4 was varied from 0 sccm to 20 sccm. 2 sccm CH4 was found to be a threshold, below which the yield of deposition was low. When the flow rate of CH4 was larger than 2 sccm, huge yield of deposition was observed. Scanning electron microscopy (SEM) examination showed those depositions were consisted of numerous nanostructures (e.g., ribbons, platelets and scrolls). XRD examination confirmed the nanostructures were α-tetragonal (B50C2) boron. These experimental results indicate that carbon is necessary to form α-tetragonal boron, even at the nanometer scale. To reveal the fundamental mechanical properties of α-tetragonal boron nanostructures, nanoindentation was performed on individual nanostructures laid on sapphire substrates. Most tested nanostructures were 15-25 nm thick and at least 1 μm wide. The measured Young’s modulus was between 450 GPa and 600 GPa. To correct the measurement errors induced by the substrate and the indenter tip size, finite element modeling was carried out. The corrected Young’s moduli of α-tetragonal boron nanostructures were higher than those of measured values. These boron nanostructures could find applications in nanocomposites where they can be used as reinforcement elements. 1. Xu et al., Nano Lett. 4 (5), p963, 2004.

Synthesis and Chemical Modifications of Few-layer Hexagonal Boron Nitride Nanosheets.Peter Feng, Hongxin Zhang and M. Sajjad; Physics Department, University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico.

Few-layer boron nitride (BN) nanosheets were produced on transition-metal template pads by using super-short-pulse laser produced plasma deposition techniques. The width of boron nitride sheets is up to 30 μm, thickness about 20 nm and length more than 1 mm. Scanning electron microscopy, transmission electron microscopy (TEM), and micro-Raman spectroscopy were used to explore the properties of BN nanosheets. TEM images show the obtained samples are actually few-layer BN and the Raman spectra further confirmed the few-layer hexagonal BN (h-BN) nanosheets. To adjust the band gap of obtained h-BN nanoshees, the chemical modification of h-BN nanosheets were also conducted in an electrical detector system with non-equilibrium hydrogen plasma. Measurements electrical properties of h-BN nanosheets exposed to atomic hydrogen for very low dose 5s, 15 s, 30 s and 50 s, respectively were recorded. Our results suggest that the percentage of hydrogenation can effectively control the band gap of h-BN and plays a crucial role of engineering the electronic properties of h-BN.

Synthesis of Boron Nitride Nanostructures Prepared by Laser Enhanced CVD Technique in Presence of Different Metallic Nano-particles as Catalyst.Muhammad Sajjad, Hongxin Zhang and Peter Feng; University of Puerto Rico, Rio Piedras Campus, San Juan, Puerto Rico.

Boron Nitride nanorods, nanowires and nanobelts (diameter: around 20nm) were synthesized on Silicon and molybdenum substrates in presence of different metallic nano-particles as catalyst. The presence of catalyst helped in formation of different types of boron nitride nanostructures i.e. nanorods, nanowires and nanobelts. Catalytic ions mix with the boron and nitrogen species; make them more reactive in formation of different kind of nanostructure. It was reveled that catalyst can effectively change the BN nanostructures with clear variation in atomic wall layer, size, shape and tip morphology. Scanning Electron Microscopy was used to analyze the surface images of BN nanostructures whereas Energy Dispersive X-Ray spectroscopy showed fewer percentage of impurity in the structure. X-Ray diffraction (XRD) technique was used to confirm the different phases of BN nanostructure. Raman spectroscopy was used to study the vibrational modes of different BN phases. The results from Raman and XRD characterizations were well agreed in analysis of BN nanostructures.

Tunable Growth of Cyclic Twinned Boron-rich Nanostructure by Chemical Vapour Deposition.Jun Yuan1,3, Xin Fu2,3, Jun Jiang2,3, Ziyang Yu2,3 and Lea Steffan1; 1Department of Physics, University of York, York, United Kingdom; 2Department of Materials Science and Engineering, Tsinghua University, Beijing, China; 3Beijing National Center for Electron Microscopy, Tsinghua University, Beijing, China.

Boron-rich nanostructures have attracted many attentions because of their high-temperature stability, excellent mechanical and thermoelectrical properties. To move forward, it is vital that we have adequant understanding and control of the growth process involved. Following our success in selective growth of cyclic twinned boron suboxide and boron carbide nanowires with re-entrant surface morphology [1, 2], we conducted a study of the temperature dependence of the microstructure of the nanowires as well as that of the catalysts. Our result suggests that the growth is promoted by catalytic reaction at the re-entrant surfaces at the twin boundaries and that the production of boron suboxide and carbide are influenced by the composition of the catalytic particles, with oxide assisting the growth of boron oxide nanowires and iron borate assisting the growth of boron carbide nanowires. With the suitable adjustment of the temperature range, we have been able to fabricate the boron-suboxide and boron carbide core-shell nanowires as well as boron-suboxide and iron boride core-shell nanowires. Our result has been replicated on iron seeded silicon wafers, opening ways for large scale studies of these interesting nanostructures. [1] J. Jiang, M. H. Cao, Y. K. Sun, P.W. Wu and J. Yuan (2006) Appl. Phys. Lett. 91, 113107 [2] X. Fu, J. Jiang, C. Liu and J. Yuan (2009) Nanotechnology, 20, 365707

Electronic Structures and Work Functions of Metallic Hexaboride Nanorods. Guangfu Luo1,2, Lu Wang2, Renat F. Sabirianov2, Wai-Ning Mei2 and Chin Li Cheung3; 1Physics, Peking University, Beijing, China; 2Physics, University of Nebraska at Omaha, Omaha, Nebraska; 3Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska.

One dimensional metallic hexaboride (MB6) nanorods have been proposed as efficient field emission materials because of the experimentally measured low work functions of the corresponding bulk materials. Nonetheless, there have been a large range of reported work function values for MB6 materials in the literature due to different experimental conditions used for these measurements. To elucidate the effect of geometry on the effective field emission properties of different MB6 nanorods, it is necessary to understand the origin of their low work function properties. Here we report our systematic modeling study of the electronic structures of quasi one-dimensional MB6 nanorods (M = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Ca, Ba, Y and Sr). The purpose of this study is to investigate metal element specificity and size-dependence of the electronic properties and work functions of these MB6 nanorods. Density functional theory together with minimization scheme based on the ensemble density functional theory was applied to facilitate convergence and optimization of the MB6 nanorods and slab structures in our calculations. Our calculated partial density of states for these materials suggests that a large contribution of the f-electrons in the lanthanide hexaborides is a major factor for the low work functions of these materials.

Synthesis and Formation Mechanism of Hydrogenated Boron Clusters B12Hn+ through Charge Transfer from Ambient Gas Ion.Yuji Ohishi1,2, Kaoru Kimura2, Masaaki Yamaguchi2, Noriyuki Uchida3 and Toshihiko Kanayama3; 1Graduate School of Engineering, Osaka University, Suita, Japan; 2Department of Advanced Material Science, Department of Advanced Material Science, Kashiwa, Japan; 3Nanodevice Innovation Research Center, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan.

Hydrogen-terminated icosahedral B_12H122−(Ih), which has the same structure as the unit in solids, is the most stable molecule among the various polyhedral boranes synthesized thus far. On the other hand, small boron clusters strongly favor planar or nearly planar structures, on the basis of systematic ab initio calculations [1] and experiments [2]. The fact that the stable structure of boron clusters depends on the hydrogen contents is fascinating, because it means that the structure is tunable through the control of the number of hydrogen atoms. We present the formation of hydrogen-content-controlled B12Hn+ clusters through the decomposition and ion-molecule reactions of the decaborane (B10H14) and diborane (B2H6) molecules in an external quadrupole static attraction ion trap. In the process of ionization, a certain number of hydrogen and boron atoms are detached from decaborane ions by the energy caused by charge transfer from ambient gas ion to decaborane molecule. The energy caused by the ion-molecule reactions also induces H atom detachment. The mechanism of the charge transfer and following detachment of hydrogen and boron atoms are investigated by DFT calculations. Parts of this presentation have been already published [3,4]. [1] I. Boustani, Chem. Phys. Lett. 240, 135 (1995). [2] H.-J. Zhai, B. Kiran, J. Li, and L. S. Wang, Nat. Mater. 2, 827 (2003). [3] Y. Ohishi, K. Kimura, M. Yamaguchi, N. Uchida and T. Kanayama, J. Chem. Phys. 128, 124304 (2008). [4] Y. Ohishi, K. Kimura, M. Yamaguchi, N. Uchida and T. Kanayama, J. Phys.: Conference Series, 176, 012030 (2009).

Defect Selective Etching of Icosahedral Boron Arsenide (B12As2) Crystals in Molten Potassium Hydroxide.Clinton E. Whiteley, Ashley Mayo and James H. Edgar; Chemical Engineering, Kansas State University, Manhattan, Kansas.

Semiconductor-based devices require the best electrical properties possible. Single crystals are capable of achieving excellent electrical properties, but there must be high structural quality, ie the defect densities must be low. As a means to estimate the defect densities, the present work reports on the defect selective etching (DSE) of icosahedral boron arsenide (B12As2) crystals using molten potassium hydroxide (KOH). DSE takes advantage of the greater reactivity that exists at the high-energy sites where a dislocation intersects the crystal surface, compared to the surrounding dislocation-free regions. The pits formed by etching are counted as a function of surface area and are indicative of the defect densities in the crystals. Etch pit densities were determined of icosahedral boron arsenide crystals produced from a molten nickel flux as a function of etch time (1-5 minutes) and temperature (450-700°C). The etch pits were triangle shaped, and ranged in size from 5-25μm. Etch pit densities were recorded using a scanning electron microscope (SEM) and an optical microscope. On average, the density of etch pits was 4.4x107cm-2 for crystals that were etched for five minutes at 500°C.

Abstract Withdrawn

Electrical Properties of Silicon Doped Icosahedral Boron Arsenide (B12As2) Epitaxial Layers on Silicon Carbide Substrate.Y. Zhang1, J. H. Edgar1, Y. Zhang2, M. Dudley2, Z. Zhu3, Y. Gong4, S. Bakalova4 and M. Kuball4; 1Chemical Engineering, Kansas State University, Manhattan, Kansas; 2Materials Science and Engineering, Stony Brook University, Stony Brook, New York; 3Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington; 4H.H.Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom.

Icosahedral boron arsenide (B12As2) is a wide bandgap semiconductor with extraordinary radiation resistance and neutron absorption capability. Therefore, it is highly attractive in radioisotope batteries, such as beta-voltaic cells, devices capable of converting nuclear energy into electrical power, and for solid-state neutron detectors due to the high thermal neutron capture cross-sections of 10B. For solid-state device purposes, high quality crystalline B12As2 with tunable electrical properties is desired. In this study, Si-doped epitaxial thin films of B12As2 were deposited on SiC substrates with 7°off-cut towards (1-100) by chemical vapor deposition from diborane (B2H6) and arsine (AsH3). This specific orientation is largely free of rotational twins that plague films on other SiC substrate orientations. Doping with silicon was achieved by adding small amount of Silane (SiH4) in the reactant gases. The Si concentration in the B12As2 was measured by secondary ion on mass spectroscopy (SIMS). A multiple-layer sample with various SiH4 concentrations in the feed was prepared to study the correlation between the feed concentration and the carrier concentration in the p-layer. The resistivity, hole concentration and hole mobility of the doped sample was obtained via temperature variable Hall effect measurements.

Facile Synthesis of Pure Boron Nanotubes and Nanofibers.Jinwen Liu3, Thelma Manning2 and Zafar Iqbal1,3; 1Chemistry, New Jersey Institute of Technolgoy, Newark, New Jersey; 2Propulsion Technology and Direct Fire Branch, US Army RDECOM, Picatinny Arsenal, New Jersey; 3Materials Science and Engineering, New Jersey Institute of Technology, Newark, New Jersey.

A facile, scalable and efficient pyrolysis process using metal boride and borohydride precursors and nickel boride as catalyst and a porous zeolite template has been used to synthesize boron nanotubes and nanofibers (BNTs and BNFs) with diameters ranging from below 10 nm to 20 nm. BNTs and BNFs produced from a metal boride at 950 C are crystalline, rod-like and up to 1 micrometer long. BNTs/BNFs produced from metal borohydride at temperatures down to 650 C are web-like in morphology, thinner, less crystalline and many micrometers long. Detailed scanning and transmission electron microscope imaging together with electron energy loss spectroscopy (EELS) data on these novel boron nanomaterials will be discussed and compared with results from the only known prior report on the synthesis of BNTs [1]. Early results on nitrogen doping of the nanotubes and nanofibers will also be described. [1] D. Ciuparu, R.F. Klie, Y. Zhu, L. Pfefferle, J. Phys. Chem. B108 (2004) 3967.

DFT Study of the Chemical Modification of a Boron Nitride Oxide Nanocluster by Molecular Simulation.Heriberto Hernandez Cocoletzi, Ernesto Chigo Anota and Carlos Solanes Rivas; Facultad de Ingeniería Química, Benemérita Universidad Autónoma de Puebla, Puebla, Puebla, Mexico.

We have previously proposed the two dimensional boron nitride oxide cluster (B27N27H17+O+(OH)3+COOH)[1] using the model CnHm[2]. This system is structurally stable. This system presents higher polarity when compared with graphene oxide, for example; additionally, the gap between the HOMO and LUMO is the range of semiconductors. Now, we modify its chemical composition to analyze the change in electronic properties such as energy gap, chemical reactivity parameters (chemical potential and electrophiliticy index) and geometry. The study is done employing the Density Functional Theory; the exchange-correlation term is treated within the Local Density Approach (LDA). We observe a transition from semiconductor (1.25 eV) to semimetal (0.18 and 0.48 eV) when the carboxyl and hydroxyl groups are removed from the surface. [1] E. Chigo Anota, M. Salazar Villanueva, H. Hernández Cocoletzi, to appear in J. Nanosci. Nanotechnol. (2010). [2] Ernesto Chigo Anota, Sup y Vac. 22 (1), 19 (2009); E. Chigo Anota, M. Salazar Villanueva, H. Hernández Cocoletzi, Phys. Stat. Solidi C (2010). DOI: 10.1002/pssc.200983499 This work was partially supported by VIEP-BUAP (Grant No. CHAE-ING10-I), FIQ-BUAP (2009-2010), and CONACyT, Mexico (Grant No. 0083982).


SESSION CC6: Boride Nanostructures and Nanosheets
Chairs: Wei-Qiang Han and Juerg Osterwalder
Tuesday Morning, November 30, 2010
Room 101 (Hynes)

8:30 AM *CC6.1
Hexagonal Boron Nitride Monolayer Systems and Nanomeshes.Juerg Osterwalder, Physik-Institut, Universitaet Zuerich, Zuerich, Switzerland.

Well defined single layers of hexagonal boron nitride (h-BN) can be produced on transition metal surfaces by thermal decomposition of borazine (BHNH)3. Depending on the type of metal, the lattice constant and the symmetry of the surface, either perfectly flat [1] or strongly corrugated layers [2] result. A particularly interesting system is the boron nitride nanomesh, a superstructure that forms on Rh(111) [2,3]. Two nanometer wide depressions or 'pores', where the h-BN layer bonds closely to the metal surface, are spaced 3.2 nm apart on a regular hexagonal lattice, surrounded by ridges or 'wires' of loosely bonded h-BN. The depth of the pores is 0.05 nm. They can trap single organic molecules [3], an effect which could be explained by electric dipole rings introduced by the edges of the pores [4]. The nanomesh can also be used as a template for the growth of metal cluster arrays, with rather monodisperse clusters forming inside the individual pores [5]. Low temperature adsorption of water leads to the formation of nano-ice clusters with typically 40 water molecules per pore [6]. These h-BN monolayer systems are surprisingly robust: they are stable in air at ambient pressure and in water or in organic solvents [3,7]. Moreover, they exhibit many similarities to single-layer graphene grown on transition metal surfaces, but with complementary behavior due to their hetero-atomic and insulating nature [8.9]. [1] W. Auwärter et al., Surf. Sci. 429 (1999) 229. [2] M. Corso et al., Science, 303 (2004) 217. [3] S. Berner, M. Corso et al., Angew. Chem. Int. Ed. 46 (2007) 5115. [4] H. Dil et al., Science 319 (2008) 1826. [5] J. Zhang et al., Phys. Rev. B 78 (2008) 165430. [6] H. Ma et al., ChemPhysChem 11, 399 (2010). [7] R. Widmer et al., Electrochemistry Communications 9 (2007) 2484. [8] T. Brugger et al., Phys. Rev. B 79, 045407 (2009). [9] T. Greber, e-J. Surf. Sci. Nanotech. 8, 62 (2010).

9:00 AM CC6.2
Methodologies Toward Monolayer Hexagonal Boron Nitride Nanosheets.Yi Lin1 and John W. Connell2; 1National Institute of Aerospace, Hampton, Virginia; 2Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, Virginia.

Hexagonal boron nitride (h-BN) is known to be more inert than their structural analog graphite against oxidative treatments. Such treatments have been used successfully to exfoliate graphite into monolayer graphene sheets. Although several approaches have been proposed to exfoliate h-BN into few-layered nanosheets by using chemical functionalization or polar organic solvents, it has been quite challenging to obtain monolayer h-BN nanosheets. Here we discuss some novel methodologies that lead towards the exfoliation of h-BN into monolayer structures of large quantities. The presence of h-BN monolayers was confirmed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Spectroscopic characterizations and physical properties of these well-exfoliated nanosheets will also be presented.

9:15 AM CC6.3
Growth and Field Emission Properties of Boron Nitride Island Films by Low-energy Ion-assisted Deposition.Kungen Teii1, Yoshiaki Utoda1, Ryota Yamao1 and Seiichiro Matsumoto2; 1Kyushu University, Kasuga, Fukuoka, Japan; 2National Institute for Materials Science, Tsukuba, Ibaraki, Japan.

Wide band-gap semiconductors such as diamond, aluminum nitride, and hexagonal and cubic boron nitride (hBN and cBN) are expected to cause easy emission because of their negative electron affinity and large band bending. hBN is the stable phase under standard and low-pressure conditions, while cBN is the metastable phase. In particular, cBN is believed to have the highest potential for field emission due to the widest band gap of approximately 6.3 eV. However, there are only a few reports working on the emission properties of cBN films. In the rare work, the emission performance of cBN films is far from awesome. There are mainly two problems to identify the potential of cBN films. First, low-pressure deposition of cBN films needs an impingement of energetic ions (typically more than 50 eV) on the growing surfaces. The resulting cBN films have very smooth surfaces with root-mean-square roughness of around 1 nm presumably due to ion impact-induced downhill currents. This suggests that the enhancement of the local electric field is hardly expected. Second, the growth of a cBN layer follows an initial sp2-bonded BN layer, consisting of turbostratic and amorphous phases, and the top cBN layer is not always phase-pure. We demonstrated reccently a way to deposit cBN films with mean ion energies of a few eV to 45 eV by introducing fluorine in a high-density, inductively coupled plasma [1]. For moderate ion energies and high ion fluxes, nanostructured cBN islands can favorably be formed due to increasing migration of adsorbed radicals, thus enhancing the local field for emission. In this study, we examine the influence of cBN phase evolution on the emission properties of BN island films. Nanostructured island films with large surface roughness were grown for initial sp2-bonded BN and subsequent cBN phases by using low-energy (~20 eV) ion bombardment. Ultraviolet photoelectron spectroscopy revealed that the electron affinity was as low as 0.3 eV for both sp2-bonded BN and cBN phases. The evolution of cBN islands reduced the emission turn-on field down to 9 V/μm or less and increased the current density up to 10-4 A/cm2. The potential barrier height was estimated to be about 3.4 eV for emission from the Fermi level. We suggest that the emission is facilitated by the larger field enhancement due to the larger roughness and the higher conduction of cBN islands. [1] K. Teii et al., J. Appl. Phys. 101, 033301 (2007).

9:30 AM CC6.4
Synthesis and Characterization of Boron Carbide One-dimensional (1D) Nanostructures.Zhe Guan, Timothy Gutu and Terry Xu; Department of Mechanical Engineering and Engineering Science, The University of North Carolina at Charlotte, Charlotte, North Carolina.

Boron-based (i.e., boron and metal boride) one-dimensional (1D) nanostructures have recently attracted much attention due to their predicted superior electrical and mechanical properties. Combining with other properties such as low density and high chemical stability, boron-based 1D nanostructures have potential applications in nanoelectronics, in nanocomposites where they may impart stiffness, toughness and strength. In this presentation, we report our recent progress on synthesis and characterization of boron carbide (B4C)-type 1D nanostructures. Catalyst-assisted growth of 1D nanostructures was achieved by co-pyrolysis of diborane (B2H6) and methane (CH4) at elevated temperature (~1050 °C) and low pressure (~1300 mTorr). Nickel (Ni) and iron (Fe) are effective catalytic materials. The as-synthesized 1D nanostructures were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Results show that the nanostructures are B4C-type materials, single crystalline with various growth directions such as that perpendicular to (10-1) planes. Systematic investigation was performed to understand the roles of catalytic materials, reaction temperature and duration, and the amount of CH4 in stabilizing the growth of 1D nanostructures. Results show that the growth of 1D nanostructures is mainly due to the vapor-liquid-solid growth mechanism. In addition, there exists a CH4 threshold below which no 1D nanostructures can be synthesized.

9:45 AM CC6.5
Large Area CVD Synthesis of Few-layer Hexagonal Boron Nitride (h-BN).Aruna Velamakanni1, Alexandru Delamoreanu1,2,3, Jai Ganesh Kameswaran1, Ji Won Suk1, Richard Piner1, Alan Covacevich1, Shanshan Chen1, Columbia Mishra1, Weiwei Cai1, Paulo Ferreira1 and Rodney S. Ruoff1; 1University of Texas at Austin, Austin, Texas; 2Université Joseph Fourier, Grenoble, France; 3Phelma, Grenoble Institute of Technology, Grenoble, France.

Hexagonal Boron Nitride (h-BN) consists of sp2-bonded two-dimensional (2D) layers comprising alternate boron and nitrogen atoms in a honeycomb arrangement; these layers are stacked and weakly bonded to form a highly anisotropic three-dimensional crystal. h-BN is electrically insulating with a large band gap both within and across the layers. Atomically thin h-BN is an interesting system in which to investigate atomic configurations, including defects, edges, and vacancies, of 2D ionic crystals. The possibility of very high thermal conductivity in mono- or few-layer hBN is fundamentally interesting and also makes it a promising candidate for applications in future high speed microelectronic devices. We report a straightforward route to synthesizing thin hBN films on Cu foil substrates in a hot wall furnace by a low pressure chemical vapor deposition (CVD) technique. Characterization techniques such as SEM and AFM have confirmed a uniform coverage over square centimeters. XPS analysis indicates a B:N ratio of 1:1.06. EELS (TEM) edges for boron and nitrogen confirm their presence, and XRD suggests the presence of a very thin film of h-BN. A recently developed diffraction scanning transmission electron microscopy technique (DSTEM) and XRD have also been employed to study the film and will be reported at the meeting. Support from Startup Funds to RS Ruoff from The University of Texas at Austin is appreciated.

10:30 AM *CC6.6
Hexagonal BN Nanotubes, Mono- and Few-Atomic-layer Nanosheets.Weiqiang Han, Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York.

Hexagonal BN (h-BN) and h-C have the same crystal structure with very close cell parameters, but their electronic properties are very distinct. Hexagonal BN is typically an insulator whose bandgap is about 5.4 eV. Although h-BN is less popular than to its analogous h-C, it exhibits some advanced properties and promising applications; thus; further efforts are desired to realize its potential. This field faces many challenges and opportunities. Dimensionality is one of the most momentous material parameters. The same material can exhibit dramatically different physical properties depending on whether it is arranged in zero dimensional, one dimensional, two dimensional, or three dimensional crystal structures. Materials that have the same dimensionality but different numbers of layers also exhibit significantly diverse physical properties. In this talk, the synthesis, microstructure, isotope effect, physical properties, and applications of BN and BCN nanotubes, mono- and few-layer nanosheets will be discussed in detail. This work is supported by the U. S. DOE under contract DE-AC02-98CH10886 and E-LDRD Fund of Brookhaven National Laboratory.

11:00 AM CC6.7
Structural Study of Boron Nanoribbons.Sumit Saxena and Trevor A. Tyson; Physics Department, New Jersey Institute of Technology, Newark, New Jersey.

Density functional theory within the local density approximation has been used to study the atomic and electronic structure of monolayer and bilayered boron nanoribbons derived from the recently predicted stable ‘α-sheets’. The stability of the boron nanoribbons has been found to increase with the increasing width which is also accompanied by an increase in the hole density and hexagonal motifs in the boron nanoribbon lattice. Charge transfer has been studied and is observed to occur from the triangular motifs forming the donor site to the charge acceptor hexagonal motifs in the lattice. The structural stability of bilayered boron nanoribbons has also been investigated.

11:15 AM CC6.8
In-situ Field Emission Characterization of Individual Boron Nitride Nanotubes.Hessam Ghassemi, Chee Hui Lee, Yoke Khin Yap and Reza Shahbazian Yassar; Michigan Technological University, Houghon, Michigan.

Here, we show that boron nitride nanotubes (BNNTs) can be ideal candidates for field emission applications as they demonstrate high emission current and current density, along with reliable mechanical stabilities. Compared to pure carbon nanotube, pure BNNTs start decomposing at relatively higher temperatures, 1000 °C, while their Young’s modulus was calculated and measured in the same range. Field emission experiments were done inside a transmission electron microscope (TEM)using an in-situ electrical holder capable of applying bias voltage of up to 140V. Our results show that individual BNNTs can exhibit current densities above 100 A/cm2, and emission currents up to 2μA. Real-time captured images show the structural stability of individual BNNTs during and after running the field-emission experiments.

11:30 AM CC6.9
Hexagonal Boron Nitride Nanowalls Synthesised by Unbalanced RF Magnetron Sputtering. Boumediene Benmoussa1, Jan D'Haen1,2, Christian Borschel3, Marc Saitner1, Ali Soltani4, Vincent Mortet1,2, Carsten Ronning3, Marc D'Olieslaeger2,1, Hans-Gerd Boyen1 and Ken Haenen1,2; 1Institute for Materials Research (IMO), Hasselt University, Diepenbeek, Limburg, Belgium; 2Division IMOMEC, IMEC vzw, Diepenbeek, Limburg, Belgium; 3Institut für Festkörperphysik, Friedrich-Schiller-Universität Jena, Jena, Germany; 4Institut d'Electronique de Microélectronique et de Nanotechnologie (IEMN), Villeneuve d'Ascq, France.

Since the recent work by Watanabe et al. [1-3] showing remarkable opto-electronic properties of hexagonal BN (h-BN), this material is considered a promising candidate for a wide range of advanced applications based on its luminescent properties. To fulfil this potential, it is desirable that the material can be deposited in thin films on a variety of substrates. While the high pressure, high temperature (HPHT) technique produces high quality crystals, the technique itself is rather cumbersome, leading to small, irregular crystals that are difficult to handle due to their fragile nature. In this work, the easy to handle, scalable and cost effective reactive radio frequency (RF) magnetron sputtering process is used in combination with a hexagonal BN target with the aim of depositing thin h-BN structures. Using different N2/Ar/H2 gas compositions in combination with silicon, quartz or fused silica substrates, films of different thickness are deposited at a rate of 100 to 125 nm/h. The surface morphology, assessed with atomic force microscopy (AFM) and scanning electronic microscopy (SEM), surprisingly revealed the presence of h-BN nanowalls, i.e. vertically positioned 2D structures consisting out of several hexagonal BN sheets [4]. The dimensions and density of these walls have shown to be clearly film thickness dependent. Where a 200 nm thick film shows densely packed walls of ~ 100 nm in width, thicker films become more porous, with ~ 1 µm wide walls that cover the surface more sparsely. By applying x-ray diffraction (XRD) and transmission electron microscopy (TEM), it was established that the (002) planes are vertically positioned with respect to the substrate surface, but that a certain amount of tilt and twist is present when different nanowalls are compared. A TEM image cross-section image confirms the increasing level of porosity with film thickness, with a dense layer of material present at the substrate-film interface, gradually evolving into the 2D nanowall structures observed at the surface with SEM. A recurrent problem in the synthesis of BN is contamination with oxygen and carbon. Here, the addition of H2 during the deposition process clearly suppresses the incorporation of these elements, reducing their combined level below 5 %. As a result, a quasi-perfect stoichiometry of the material was evidenced by combining energy dispersive X-ray analysis (EDX), Rutherford backscattering spectroscopy (RBS), and X-ray photoelectron spectroscopy (XPS). Raman and Fourier transform infrared (FTIR) spectroscopy revealed the sp2 nature of the bonds, confirming the h-BN nature of the nanowalls. [1] K. Watanabe, T. Taniguchi, H. Kanda, Nat. Mater. 3/6 (2004), 404-409. [2] Z. Remes, M. Nesládek, K. Haenen, K. Watanabe, T. Taniguchi, phys. stat. sol. (a) 202/11 (2005), 2229-2233. [3] K. Watanabe, T. Taniguchi, T. Niiyama, K. Miya, M. Taniguchi, Nat. Photonics 3/10 (2009), 591-594. [4] J. Yu et al., ACS Nano 4/1 (2010), 414-422.

11:45 AM CC6.10
Tuning Electronic and Structural Properties of Triple Layers of Intercalated Graphene and Hexagonal Boron Nitride: An Ab-initio Study. Samir S. Coutinho1, David L. Azevedo2,3 and Douglas S. Galvao1; 1Applied Physics, State University of Campinas, Campinas, São Paulo, Brazil; 2Física, Universidade Federal do Maranhão, São Luis, Maranhão, Brazil; 3Física, Universidade Federal do Rio Grande Do Norte, Natal, Rio Grande do Norte, Brazil.

Recently, several experiments and theoretical studies demonstrated the possibility of tuning or modulating bandgap values of nanostructures based on bi-layer graphene [1,2], bi-layer hexagonal boron-nitride(BN) [3] and hetero-layer combinations [4]. In this work, we report results from an detailed study of the electronic properties of triple layers of intercalated graphene and hexagonal boron nitride using ab initio density functional methods. These triple layers present several possibilities of stacking to be analyzed. We observed that an applied external electric field can change significantly the electronic and structural properties of these systems. With the same value of the applied electric field the bandgap values can be increased or decreased depending on the layer stacking sequences. Strong geometrical deformations were observed; even semiconductor-metal transition can be achieved exploiting these aspects. These results show that the application of an external electric field perpendicular to the stacked layers can be effectively used to modulate their inter-layer distances and/or their bandgap values. Possible applications of electro-mechanical devices based on these effects and materials are addressed. [1] Jeroen B. Oostinga et al., Nature Materials 7, 151 - 157 (2007) [2] Yuanbo Zhang et al. Nature 459, 820-823 (2009) [3] Zailin Yang and Jun Ni, J. Appl. Phys. 107, 104301 (2010) [4] J. Slawinska, Phys. Rev. B 81, 155433 (2010)


SESSION CC7: Applications of Boron Compounds
Chair: Joan Redwing
Tuesday Afternoon, November 30, 2010
Room 101 (Hynes)

1:30 PM *CC7.1
Boron Based Thermal Neutron Detectors: Possibilities and Progress.Rebecca Nikolic, LLNL, Livermore, California.

Thermal neutron detection is routinely carried out by utilizing 3helium (3He) tubes. Conventional 3He neutron detectors can achieve very high thermal neutron detection efficiency when appropriately pressurized. However, the use of these proportional counter type devices is encumbered by the required high operation voltage (1000 V), sensitivity to microphonics, large device footprint, and high pressure, all of which result in significant complications for routine deployment and air transport. These operating conditions make 3He tubes difficult to use in the field. To overcome these problems, several solid state thermal neutron detector concepts have been recently developed. Thermal neutrons have a low probability of interacting with conventional semiconductor materials. Only materials with a large thermal neutron cross-section can be used to develop a high efficiency detector. Materials including Boron and Lithium are frequently employed. 10Boron (10B) is an ideal material because of its very high thermal neutron cross-section of 3837 barns and results in the creation of energetic alpha and lithium particles when bombarded with neutrons. The alpha and lithium particles will then loose energy while moving within the host material and can create electron-hole pairs which are responsible for the generation of the collected signal. This talk will include an assessment of the required device physics for high detection efficiency, high gamma rejection and low power. Device architectures being investigated currently include three dimensional (3D) semiconductors filled with 10B and bulk 10B containing semiconductors. The 3D approach being developed at LLNL and elsewhere uses a two-step process using a combination of a 3D semiconductor platform which is filled with the 10B containing neutron sensitive material. First, the thermal neutrons are converted to energetic ions within the 10B material. Second, these ions are collected using a semiconductor diode. Numerous 3D structures being evaluated include pillars, nanowires, ridges, and perforations amongst others. Device and process complexity tradeoffs will be discussed. The two step process can be avoided by using a semiconducting 10B containing material. This is a very elegant approach because the semiconducting material is sensitive to neutrons and can carry out the neutron conversion and electron hole collection within one material. That said, the transport properties of semiconducting boron compounds have in the past shown to be complicated. Boron material growth, deposition methods and etching techniques will be discussed Recent progress on the LLNL Pillar Detector will be included. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-ABS-443011. This work was supported by the Domestic Nuclear Detection Office in the Department of Homeland Security.

2:00 PM CC7.2
Deposition and Etching of Conformal Boron Films for Neutron Detector Applications.Nicholas V. LiCausi1, Justin Clinton2, Yaron Danon2, James Lu1 and Ishwara B. Bhat1; 1Electrical, Computer, & Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York; 2Mechanical, Aerospace, & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York.

There has been a significant research effort surrounding the detection of thermal neutrons. This is primarily because they can be used for the detection of nuclear materials. Typically these detectors work on the principle of converting fission produced thermal neutrons into α-particles, which are then sensed by a p-n junction. The conversion of neutron to α-particles is necessary because the neutron is charge-neutral and cannot be sensed by the standard p-n junction. In order to detect thermal neutron a converter material with high thermal neutron cross-section is desirable. Typically 10B or 6LiF is used. We have selected 10B because it has a higher cross-section and therefore can result in higher detection efficiency. Boron is also compatible with the device fabrication process and thus can simplify the fabrication of the detector device. To maximize the probability that an incident neutron will interact with a 10B atom, the 10B layer must be very thick (45 µm). However, for the generated α-particles to reach the p-n junction, the 10B layer must be thin (2-3 µm), otherwise, the α-particles will be reabsorbed by the 10B. This contradiction of required thickness led us to use 10B filled high aspect-ratio holes and trenches with Si p-n junctions. Hole-depths used for the design are as deep as 60 µm and as narrow as 2 µm. Due to the high aspect-ratio of these features and the availability of processing equipment/materials, low pressure chemical vapor deposition (LPCVD) has been selected for the boron deposition. LPCVD is carried out in a horizontal quartz reactor with pressures ranging from 250-1000 mTorr and temperatures ranging from 400-700 °C. Diborane was used as the boron source, with a typical flow of 100 sccm of 1% diborane in hydrogen. Growth rates as high as 1 µm/hr have been observed with good conformal coverage and good surface morphology, especially for pressure lower than 300 mTorr. As the growth temperature is increased beyond 600 °C, the film surface morphology tended to become rough with grainy surface. Fill factors above 80% have been demonstrated. High film-stress is observed and will be discussed. Stress often leads to spontaneous peeling of the film within minutes or after as long as weeks. Stress for a 0.6 µm film is ~850 MPa at 300K. Boron etching studies have been conducted with both “wet” chemical etching and “dry” reactive ion etching (RIE). Wet chemical etching is important for the selective etching of B over Si. At 75 °C solution temperature, an etching rate as high as 6.3 nm/min has been obtained. Both standard RIE and inductively coupled plasma (ICP) RIE have been studied for various plasma chemistries, including SF6 + O2 and H2. This paper will present the detailed LPCVD conformal boron deposition processes, etching processes and characterizations. The results will be analyzed and discussed. This work is supported by the US Department of Homeland Security under grant award number 2008-DN-077-ARI008-003.

2:15 PM CC7.3
Electrical Properties of B12As2/SiC Heterojunction Diodes and Their Dependence on Microstructure.Yinyan Gong1, Silvia Bakalova1, Yu Zhang2, Michael Dudley2, Yi Zhang3, James H. Edgar3 and Martin Kuball1; 1Department of Physics, University of Bristol, Bristol, United Kingdom; 2Department of Materials Science and Engineering, SUNY, Stony Brook, New York; 3Department of Chemical Engineering, Kansas State University, Manhattan, Kansas.

Icosahedral boron arsenide (B12As2) is a wide band gap semiconductor material (Eg = 3.20 eV at room temperature [1]) with extraordinary radiation tolerance via “self-healing” mechanisms, high melting temperature, and excellent mechanical properties [2]. These unique material properties make icosahedral B12As2 highly attractive for devices working under extreme conditions. One potential application of icosahedral B12As2 is the fabrication of beta-voltaics, devices converting nuclear energy into electricity by combing a radioactive beta source with a semiconductor junction. As radiation can introduce severe degradation to devices based on traditional semiconductors such as silicon, icosahedral B12As2 is a very promising candidate for such applications. In this work, we studied electrical properties of B12As2/SiC heterojunction diodes, and the effect of B12As2 film quality on the diode performance. Unintentionally p-doped B12As2 film was deposited by chemical vapour at 1350 °C on (0001) plane (on-axis) n-type 4H-SiC substrate. Capacitance voltage measurements were carried out on the sample and the extracted built-in potential is ~1.89 V. The band offsets between B12As2 and SiC are calculated using Anderson’s model and were determined to be ~1.06 eV and 1.12 eV for conduction and valence bands, respectively. Current-voltage (I-V) characteristic of the diode shows good rectifying behaviour with an IF/IR ratio of 485 at 2 V (IF and IR represents forward and reverse currents, respectively). The leakage current is ~ 9.4x10-6 A/cm2 under a reverse bias of 0.4 V, which however is higher than for typical single-crystalline 4H-SiC pn junction diode. This higher leakage current can be attributed, at least partially, to present structural defects due to the heteroepitaxial growth, and is expected to be reduced by improving crystalline quality of B12As2 film. Recently, it is reported [3] that twin boundaries, a commonly observed structural defects in B12As2 films, can be greatly reduced by using 7° offcut toward (1-100) (off-axis) 4H-SiC substrates. To investigate this effect on the electrical properties, we deposited a B12As2 film also on off-axis n-type 4H-SiC substrate under the same growth conditions as the previous on-axis sample. It is found that the off-axis diode has a reduced leakage current of ~7.1x10-6 A/cm2 under a reverse bias of 0.4 V and an increased IF/IR ratio of 691 at 2 V, indicating this approach as possible way forward to reducing leakage current further of B12As2 - SiC devices. [1] S. Bakalova, et al. Phys. Rev. B 81, 075114 (2010). [2] D. Emin, Phys. Today 40, 55 (1987) [3] Y. Zhang, et al. Mater. Res. Soc. Symp. Proc. 1246 , Warrendale , PA 2010) 1246-B04-2.

2:30 PM CC7.4
Hierarchical Boron Carbon Nitrides for Hydrogen Storage through Reactive Hard Templating.David Portehault1, Cristina Giordano1, Christel Gervais2, Irena Senkovska3, Stefan Kaskel3, Clement Sanchez2 and Markus Antonietti1; 1Colloid Chemistry, Max Planck Institute for Colloids and Interfaces, Potsdam, Germany; 2Chimie de la Matière Condensée de Paris, UPMC Unic Paris 06, CNRS, UMR 7574, Collège de France, Paris, France; 3Inorganic Chemistry, Dresden University of Technology, Dresden, Germany.

As a bulk material, hexagonal boron nitride (h-BN) with a graphite-like structure is known for its excellent thermal stability and chemical inertness for protective coatings and reinforcing structures, as well as its strong luminescence in the UV range, which makes it a potential material for UV lasing. Novel properties are also expected at the nanoscale, including gas sorption and field emission. Few studies recently demonstrated that nanostructured boron nitrides exhibit high hydrogen uptake because of strong interactions with the H2 molecule due to the dipole moment of B-N bonds and the local curvature of the surface. Furthermore, it is believed that the control of the carbon content in these systems could provide an additional way to control storage properties, as this enables modification of the nature and the energy of the bond between hydrogen molecules and the surface. However, experimental studies on sorption properties of boron carbon nitrides BxCyNz are still scarce because of the difficult texture control. Up to now, the highest surface area reported for porous boron carbon nitrides was 950 m2 g-1 and was obtained by block copolymer templating.[1] The study presented herein reports on a novel hard templating approach towards mesoporous boron carbon nitrides.[2] Mesoporous graphitic carbon nitride (mpg-C3N4) is used as a template. This matrix completely reacts upon heating between 800 and 1400 °C, so that no isolation step is required, providing an easy route towards these materials without purification process. The second advantage of reactive hard templating is the only single reactant which must fill the pores of the template. Moreover, the carbon nitride matrix acts as efficient nitrogen donor and supports boron diffusion, yielding a direct copy of the template, rather than the inverse replica usually observed for common hard templating.[3] Easy tuning of the composition BxCyNzOvHw (0.15 ≤ x ≤ 0.36, 0.10 ≤ y ≤ 0.12, 0.14 ≤ z ≤ 0.32, 0.11 ≤ v ≤ 0.28) as well as of the specific surface area and the pore size distribution is achieved by temperature control. We report values of up to 1560 m2 g-1 for hierarchical materials incorporating micro and mesopores, together with high resistance against oxidation up to 700 °C. To conclude, we highlight the first experimental evidence of the influence of the composition and the textural features of boron carbon nitrides on the hydrogen sorption uptake, ranging from 0.55 to 1.07 wt.% at 77 K and 1 bar. [1] Malenfant, P. R. L.; Wan, J.; Taylor, S. T.; Manoharan, M. Nat. Nanotechnol.2007, 2, 43-46. [2] Portehault, D.; Giordano, C.; Gervais, C.; Senkovska, I.; Kaskel, S.; Sanchez, C. Antonietti, M. Adv. Funct. Mater.2010, 20, 1827-1833. [3] Fischer, A.; Antonietti, M.; Thomas, A. Adv. Mater.2007, 19, 264-267.

2:45 PM CC7.5
Carrier Doping into Boron Crystals and Their Nanostructure by Li Ion Implantation and Neutron Transmutation.Kazuhiro Kirihara1, Hiroshi Hyodo2, Takenori Nagatochi3, Fumitaka Esaka4, Shunya Yamamoto4, Yoshiki Shimizu1, Takeshi Sasaki1, Naoto Koshizaki1, Hiroyuki Yamamoto4, Shin-ichi Shamoto4 and Kaoru Kimura3; 1National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan; 2Tokyo University of Science, Noda, Japan; 3The University of Tokyo, Kashiwa, Japan; 4Japan Atomic Energy Agency, Tokai-mura, Japan.

Carrier doping into α-rhombohedral boron (α-r-B) crystal is expected to realize superconduction with a higher transition temperature (Tc) than that of MgB2. Twelve boron atoms (B12) icosahedral cluster is a building block of α-r-B structure. Theoretical calculation suggested that high electronic density of states at Fermi level could be provided by appropriate carrier doping. Furthermore, high phonon frequency and strong electron-phonon coupling in boron are important factors for high Tc. Recently, we firstly observed superconduction in Li doped α-r-B crystal. The method of carrier doping was Li vapor diffusion. However, the amount of Li in α-r-B is still limited because of the formation of oxide barrier layer or other secondary phases and therefore Tc is still low. Ion implantation is expected to be one of the effective methods of Li doping for realizing higher Tc than ever. In this talk, effect of carrier doping in α-r-B after Li ion implantation and post annealing is presented. Radiation damage is also discussed based on Raman spectroscopy. Thermal neutron transmutation of isotope 10B atom can be another method of carrier doping into boron crystal. The neutron capture reaction, 10B(n, α)7Li, results in significant structural damages in boron crystals. After healing the radiation damage, 7Li atoms can modify the transport property, as in the case of neutron transmutation doping of Si. Previously we reported the electrical transport properties of single crystalline boron nanobelts composed of α-tetragonal boron (α-t-B). Temperature coefficients of electrical conductance revealed a hopping conduction in the BNBs [2]. In this talk, we report the effects of neutron capture reaction of isotope 10B on the structure and electrical transport of 10B enriched single crystalline boron nanobelts. The drastic change of the conductance was discussed based on the distribution of localized states in forbidden band of the crystalline structure. [1] T. Nagatochi et al., to be submitted. [2] K. Kirihara et al., Appl. Phys. Lett., 86, 212101 (2005).


SESSION CC8: Optical Properties of Borides and Their Applications
Chair: Yinyan Gong
Tuesday Afternoon, November 30, 2010
Room 101 (Hynes)

3:30 PM *CC8.1
Hexagonal Boron Nitride as a New Ultraviolet Luminescent Material and Its Device Application.Kenji Watanabe1, Takashi Taniguchi1, Kenta Miya2, Yoshitaka Sato2, Kazuhito Nakamura2, Takahiro Niiyama2 and Masateru Taniguchi2; 1National Institute for Materials Science, Tsukuba, Ibaraki, Japan; 2Futaba Corporation, Chosei-mura, Chiba, Japan.

Hexagonal boron nitride (hBN), which is conventionally used for one of the best-known heat-resistance materials due to its high melting point and high chemical stability, has recently been found as a highly luminous material in far ultraviolet (FUV) region. Strong exciton effects dominate the electronic excitation states near the band gap for highly pure crystals of hBN grown by temperature gradient method. Two series of intense exciton luminescence bands at low temperature show that the exciton luminescence bands are originated from four Frenkel exciton levels arising from Jahn-Teller effects on the exciton series [1]. In order to study potential of hBN as a new FUV fluorescent material, we made a prototype of FUV plane light-emitting device based on hBN. The fabricated devices to make the most of the highly luminous properties were composed of an hBN fluorescent screen and a Spindt-type field emission array [2] as an excitation source. These components were placed in a vacuum case with an FUV transparent window and electrodes. It achieved output power of 0.2 mW at 220 nm with good stability. Low current consumption for exciting the hBN screen makes it possible to operate the device with dry batteries [3]. References [1] K. Watanabe, T. Taniguchi, and H. Kanda, Nature Materials 3, 404 (2004), K. Watanabe, and T. Tanguchi, Phys. Rev. B79, 193104 (2009). [2] C.A. Spindt, I. Brodie, L. Humphrey, and E.R. Westerberg, J. Appl. Phys. 47, 5248 (1976). [3] K. Watanabe, T. Taniguchi, T. Niiyama, K. Miya, and M. Taniguchi, Nat. Photonics 3, 591 (2009).

4:00 PM CC8.2
Optical and Spectroscopic Ellipsometric Study of Indium Boron Nitride Sputtered Thin Films with Low Boron Concentration.Mohammad A. Ebdah1, Martin E. Kordesch1, David C. Ingram1 and Hamad Al-Brithen2; 1Physics and Astronomy, Ohio University, Athens, Ohio; 2Physics and Astronomy Department, King Saud University, Riyadh, Saudi Arabia.

Amorphous indium boron nitride (a-InBN) films have been sputtered using radio frequency (RF) magnetron sputtering onto fused silica and c-Si(100) substrates. Sputtering was made using targets of polycrystalline B and In species under the flow of Nitrogen and Argon. The InBN films were sputtered at room temperature, with thickness of 100 nm and low B at.%. The structure and composition of the films have been investigated by X-ray diffraction (XRD), and Rutherford backscattering spectroscopy (RBS), respectively. The XRD patterns reveal that the sputtered films are amorphous, and the RBS results indicate that B at.% < 10%. The optical absorptions of samples grown on silica were obtained using spectrophometry (SP) technique in the wavelength range (200 - 800) nm. Analysis of the absorption coefficients, using the Tauc linear extrapolation, gives optical band gaps of (1.8-2.0)eV, indicating higher bandgaps comparing to the measured optical bandgap of a-InN of 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 (270-800) nm. The index of refraction and extinction coefficient were obtained by the analysis of the measured ellipsometric spectra of Psi and delta within the framework of the Tauc-Lorentz (TL) and Cauchy models. A roughness top layer in the multi-layered model structure was used for modeling the roughness of the films using the effective medium approximation (EMA). The obtained absorption coefficients were analyzed using the modified Tauc linear extrapolation, and the obtained optical absorption edges, Eg, were in excellent agreement with the values obtained from the TL model fitting parameters. Finally, the morphologies of the films were investigated by atomic force microscopy (AFM), and the obtained roughness was compared to that obtained by SE.

4:15 PM CC8.3
Anisotropic Dielectric Response of Epitaxial B12As2 Films.Silvia Bakalova1, Yinyan Gong1, Christoph Cobet2, Norbert Esser2, Yi Zhang3, James Edgar3, Yi Zhang4, Michael Dudley4 and Martin Kuball1; 1University of Bristol, Bristol, United Kingdom; 2Institute for Analytical Sciences (ISAS), Berlin, Germany; 3Kansas State University, Manhattan, Kansas; 4SUNY, Stony Brook, New York.

The novel wide bandgap semiconductor B12As2 embodying B12 icosahedra can resist degradation caused by radiation via self-healing mechanisms due to the exceptional bonding of boron atoms. This unusual property generates a great interest for advanced nuclear applications, such as betavoltaic cell and solid state neutron detector devices, working under extreme radiation. Prerequisite and fundamental interest is the knowledge of intrinsic semiconductor properties. Previously we have investigated the electronic band structure of B12As2 experimentally by VIS-VUV ellipsometry and theoretically by relativistic plane wave calculations from the first principles. Although B12As2 crystal structure is optically uniaxial, the experimental studies so far have been limited to the in-plane dielectric function due to the orientation of the sample. In the present work, we report spectroscopic ellipsometry results from 7° off-axis B12As2 epitaxial film in the range of 1.24-9.8 eV using Xe lamp and synchrotron radiation. Knowledge of its uniaxial properties, such as electronic band properties and effective masses along principal axes, is essential as it affects devices design considerations. Films were deposited by chemical vapour deposition (CVD) at a temperature of 1350°C on off-axis c-plane 4H-SiC substrates with 7° off-cut toward the [1-100] direction. This growth configuration was found to improve the crystal quality and interfacial properties, and reduce surface roughness. The film orientation was established by transmission electron microscopy (TEM) and was used to determine the optical axis orientation in ellipsometry experimental frame of reference. An optical model was used to resolve the ordinary and extraordinary dielectric function and the results were compared to ab initio calculations. Dispersive features in the spectra were analysed by lineshape analysis in terms of critical point parabolic band theory. Dissimilarities due to the anisotropy have been discussed and their origin was inferred from band structure and wavefunction symmetries. In the fundamental edge the effect is mostly the larger oscillator strength of the first direct transition at 3.46 eV (Z point in the BZ) in extraordinary dielectric response since this is a dipole forbidden transition for the ordinary electromagnetic wave. In addition dielectric response of B12As2 has been experimentally obtained form electron energy loss spectroscopy in the energy range of 0-80 eV, where interband electron-hole and plasmon excitation were determined and electron energy loss function is compared to the results from optical study.

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