Showing papers on "Nitride published in 2011"

A review of dielectric polymer composites with high thermal conductivity

Abstract: The continuing miniaturization of electronic devices and the increasing power output of electrical equipment have created new challenges in packaging and insulating materials. The key goals are to develop materials with high thermal conductivity, low coefficient of thermal expansion (CTE), low dielectric con stant, high electrical resistivity, high breakdown strength, and most importantly, low cost. Polymeric materials have attracted increasing interest because of their excellent processability and low cost; however, most polymers are thermally insulating and have a thermal conductivity between 0.1 and 0.5 W-m-ι-K"1. One approach to increase the thermal conductivity of a polymer is to introduce high-thermal-conductivity fillers, such as aluminum oxide, aluminum nitride, boron nitride, silicon nitride, beryllium oxide, or diamond. In this review paper, we explore how dielectric polymer composites with high thermal conductivity have been developed.

Indirect Auger recombination as a cause of efficiency droop in nitride light-emitting diodes

Abstract: InGaN-based light-emitting diodes(LEDs) exhibit a significant efficiency loss (droop) when operating at high injected carrier densities, the origin of which remains an open issue. Using atomistic first-principles calculations, we show that this efficiency droop is caused by indirect Auger recombination, mediated by electron-phonon coupling and alloy scattering. By identifying the origin of the droop, our results provide a guide to addressing the efficiency issues in nitride LEDs and the development of efficient solid-state lighting.

N2 Reduction and Hydrogenation to Ammonia by a Molecular Iron-Potassium Complex

Abstract: The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N(2)) to ammonia (NH(3)) is an iron surface promoted with potassium cations (K(+)), but soluble iron complexes have neither reduced the N-N bond of N(2) to nitride (N(3-)) nor produced large amounts of NH(3) from N(2). We report a molecular iron complex that reacts with N(2) and a potassium reductant to give a complex with two nitrides, which are bound to iron and potassium cations. The product has a Fe(3)N(2) core, implying that three iron atoms cooperate to break the N-N triple bond through a six-electron reduction. The nitride complex reacts with acid and with H(2) to give substantial yields of N(2)-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N(2) cleavage and N-H bond-forming reactions of iron.

Synthesis of bulk and nanoporous carbon nitride polymers from ammonium thiocyanate for photocatalytic hydrogen evolution

Abstract: Graphitic carbon nitride was synthesized by direct thermal polymerization of ammonium thiocyanate as the precursor. The transfer of this simple thermal-induced polymerization onto hard-templates with various nanoarchitectures enables the fabrication of nanostructured carbon nitridesvia a soft-chemical synthesis, while the involvement of a sulfur species within the reaction cascade offers additional chemical control of the texture and the electronic structures. The catalysts were subjected to several characterizations, and the results obtained revealed that nanoporous carbon nitrides can be obtained by templating with nanosized silica and SBA-15. Photocatalytic activity was evaluated toward hydrogen evolution from proton solution with visible light. Results show that g-C3N4 synthesized from ammonium thiocyanate exhibited improved photoactivity in comparison with g-C3N4 obtained from dicyandiamide. Further improvement in the activity was achieved by creating the nanostructures in g-C3N4. This is due to the enhanced surface area obtained which is favorable for light-harvesting and mass-transfer, as well as to the increased redox potential.

Electron-beam-induced substitutional carbon doping of boron nitride nanosheets, nanoribbons, and nanotubes

Abstract: Substitutional carbon doping of the honeycomb-like boron nitride (BN) lattices in two-dimensional (nanosheets) and one-dimensional (nanoribbons and nanotubes) nanostructures was achieved via in situ electron beam irradiation in an energy-filtering 300 kV high-resolution transmission electron microscope using a C atoms feedstock intentionally introduced into the microscope. The C substitutions for B and N atoms in the honeycomb lattices were demonstrated through electron energy loss spectroscopy, spatially resolved energy-filtered elemental mapping, and in situ electrical measurements. The preferential doping was found to occur at the sites more vulnerable to electron beam irradiation. This transformed BN nanostructures from electrical insulators to conductors. It was shown that B and N atoms in a BN nanotube could be nearly completely replaced with C atoms via electron-beam-induced doping. The doping mechanism was proposed to rely on the knockout ejections of B and N atoms and subsequent healing of vacancies with supplying C atoms.

Mesoporous carbon nitride–silica composites by a combined sol–gel/thermal condensation approach and their application as photocatalysts

Abstract: Mesoporous carbon nitrides, silicas and their composites have been prepared by a combined sol–gel and thermal condensation approach. Precursors for the carbon nitride (cyanamide) and silica (TEOS) are mixed and condensed simultaneously. After condensation and heat treatment it is observed that the carbon nitride and silica formed highly interpenetrating mesophases which leads either to the formation of mesoporous carbon nitride or silica after selective removal of one of the phases. Importantly, the carbon nitride preserves its graphitic stacking even in the spatial confinement introduced by the surrounding silica phase. As both precursors are liquids this approach allows convenient shaping into thin and thick films or monoliths of mesoporous carbon nitrides. Enhanced photocatalytic activity is observed for the production of hydrogen from water when these mesoporous carbon nitrides are applied as photocatalyst in comparison to the bulk, but also to other mesoporous carbon nitrides, prepared by the reported two-step, hard templating approach.

Tunable band gaps in bilayer graphene-BN heterostructures

Abstract: We investigate band gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that graphene−boron nitride heterostructures could provide a viable route to graphene-based electronic devices.

A comparative study of titanium nitride (TiN), titanium oxy nitride (TiON) and titanium aluminum nitride (TiAlN), as surface coatings for bio implants

Abstract: In the present study, the performance of three titanium nitride coatings: TiN, TiON, and TiAlN for biomedical applications were assessed in terms of their surface properties electrochemical corrosion in simulated body fluid and cytotoxicity. Layers of TiN, TiON and TiAlN were deposited onto CP–Ti substrates by DC reactive magnetron sputtering method using a combination of a Ti, Ti–Al targets and an Ar–N 2 mixture discharge gas. The presence of different phases was identified by XRD analysis. The morphology was determined through atomic force microscopy (AFM) imaging. The XPS survey spectra on the etched surfaces of TiN film exhibited the characteristic Ti2p, N1s, O1s peaks at the corresponding binding energies 454.5, 397.0, and 530.6 eV respectively. The characteristic Raman peaks were observed from the Laser Raman spectrometer. Platelet adhesion experiments were done to examine the interaction between blood and the materials in vitro. On Control samples (CP Ti), platelets were seen as aggregates, whereas on coated samples, platelets were seen as singles, without any significant spreading. Cytocompatibility studies of coated samples were carried out with bare titanium (CP Ti — ASTM B 348) as controls. L-929 mouse fibroblast cells were used for samples. All materials showed good cytocompatbility with cell lines used.

Mesoporous coaxial titanium nitride-vanadium nitride fibers of core-shell structures for high-performance supercapacitors.

Abstract: In this study, titanium nitride-vanadium nitride fibers of core–shell structures were prepared by the coaxial electrospinning, and subsequently annealed in the ammonia for supercapacitor applications. These core–shell (TiN-VN) fibers incorporated mesoporous structure into high electronic conducting transition nitride hybrids, which combined higher specific capacitance of VN and better rate capability of TiN. These hybrids exhibited higher specific capacitance (2 mV s–1, 247.5 F g–1) and better rate capability (50 mV s–1, 160.8 F g–1), which promise a good candidate for high-performance supercapacitors. It was also revealed by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) characterization that the minor capacitance fade originated from the surface oxidation of VN and TiN.

Metal Nitride/Graphene Nanohybrids: General Synthesis and Multifunctional Titanium Nitride/Graphene Electrocatalyst

Abstract: A facile, efficient, and general strategy is developed for the fabrication of a new class of nanohybrids consisting of nitrogen-doped graphene functionalized with metal nitride nanoparticles. The graphene decorated with titanium nitride nanoparticles is explored for multifunctional electrocatalytic applications, i.e., as a low-cost counter electrode for I(3)(-) reduction in dye-sensitized solar cells and for nicotinamide adenine dinucleotide (NADH) oxidation in dehydrogenase enzyme-based biosensors.

Tunable band gaps in bilayer graphene-BN heterostructures

Abstract: We investigate band gap tuning of bilayer graphene between hexagonal boron nitride sheets, by external electric fields. Using density functional theory, we show that the gap is continuously tunable from 0 to 0.2 eV and is robust to stacking disorder. Moreover, boron nitride sheets do not alter the fundamental response from that of free-standing bilayer graphene, apart from additional screening. The calculations suggest that graphene−boron nitride heterostructures could provide a viable route to graphene-based electronic devices.

Facile synthesis of mesoporous carbon nitrides using the incipient wetness method and the application as hydrogen adsorbent

Abstract: Highly nitrogen-enriched mesoporous carbon nitride materials with 2-dimensional (2-D) (2D-meso-CN) and 3-dimensional (3-D) mesostructures (3D-meso-CN) were synthesized using mesoporous silica as a hard template and cyanamide as a precursor via the incipient wetness process without using any solvent. The materials were characterized by small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) for the mesostructure analysis, N2 adsorption–desorption isotherms for surface area and pore size distribution, and X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FT-IR) spectroscopy for the composition analysis of frameworks. The mesoporous carbon nitride replicas have graphitic-like stacking of carbon nitride sheets in mesopore walls. The N/C ratio of the mesoporous carbon nitride replicas is 1.13 after the carbonization at 550 °C for 3 h. 2D-meso-CN and 3D-meso-CN have the BET surface area of 361 and 343 m2 g−1, large pore volume of 0.50 and 0.67 cm3 g−1, and pore diameter of 27.8 A (for 2D-meso-CN), 24.5 and 80.3 A (for 3D-meso-CN), respectively. It was found that the 3D-meso-CN has higher capacity of hydrogen uptake of 0.25 wt% than the pure mesoporous carbon FDU-15 (0.16 wt%) at 50 bar under room temperature (298 K).

Copper Nitride Nanocubes: Size-Controlled Synthesis and Application as Cathode Catalyst in Alkaline Fuel Cells

Abstract: Copper nitride nanocubes are synthesized in a facile one-phase process. The crystal size could be tuned easily by using different primary amines as capping agents. Such Pt-free nanocrystals exhibit electrocatalytic activity toward oxygen reduction and appear to be promising cathodic electrocatalysts in alkaline fuel cells.

A carpet cloak for visible light.

Abstract: We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.

Presence and origin of interface charges at atomic-layer deposited Al2O3/III-nitride heterojunctions

Abstract: Unlike silicon and traditional III-V semiconductors, the III-nitrides exhibit high spontaneous and piezoelectric polarization charges at epitaxial polar heterojunctions. In the process of investigating scaling properties of gate-stacks consisting atomic-layer deposited Al2O3/III-Nitride heterojunctions, we find interface charges that appear closely linked to the polarization charges of the underlying nitride substrate. Through capacitance-voltage measurement on a series of samples of varying dielectric thicknesses, we find the presence and propose an origin of benign donor-type interface charges (Qit ∼6 × 1013 cm−2) at the AlN/Al2O3 junction. This interface charge is almost equal to the net polarization charge in AlN. The polarization-related dielectric/AlN interface charge and the role of oxygen in the dielectric as a possible modulation dopant potentially offer opportunities for various device applications.

A Carpet Cloak Device for Visible Light

Abstract: We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nano-porous silicon oxide substrate with a very low refractive index. The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequency.

Decomposition pathways in age hardening of Ti-Al-N films

Abstract: The ability to increase the thermal stability of protective coatings under work load gives rise to scientific and industrial interest in age hardening of complex nitride coating systems such as ceramic-like Ti1−xAlxN. However, the decomposition pathway of these systems from single-phase cubic to the thermodynamically stable binary nitrides (cubic TiN and wurtzite AlN), which are essential for age hardening, are not yet fully understood. In particular, the role of decomposition kinetics still requires more detailed investigation. In the present work, the combined effect of annealing time and temperature upon the nano-structural development of Ti0.46Al0.54N thin films is studied, with a thermal exposure of either 1 min or 120 min in 100 °C steps from 500 °C to 1400 °C. The impact of chemical changes at the atomic scale on the development of micro-strain and mechanical properties is studied by post-annealing investigations using X-ray diffraction, nanoindentation, 3D-atom probe tomography and high-resolution...

Method and apparatus for fabricating crack-free Group III nitride semiconductor materials

Abstract: A method and apparatus for growing low defect, optically transparent, colorless, crack-free, substantially flat, single crystal Group III nitride epitaxial layers with a thickness of at least 10 microns is provided. These layers can be grown on large area substrates comprised of Si, SiC, sapphire, GaN, AlN, GaAs, AlGaN and others. In one aspect, the crack-free Group III nitride layers are grown using a modified HVPE technique. If desired, the shape and the stress of Group III nitride layers can be controlled, thus allowing concave, convex and flat layers to be controllably grown. After the growth of the Group III nitride layer is complete, the substrate can be removed and the freestanding Group III nitride layer used as a seed for the growth of a boule of Group III nitride material. The boule can be sliced into individual wafers for use in the fabrication of a variety of semiconductor structures (e.g., HEMTs, LEDs, etc.).

High Rate Electrochemical Capacitors from Three-Dimensional Arrays of Vanadium Nitride Functionalized Carbon Nanotubes

Abstract: A simple methodology is developed to directly synthesize three-dimensional (3D) electrochemically supercapacitive arrays, consisting of multiwalled carbon nanotubes conformally covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or Inconel electrodes. These nanostructures demonstrate a respectable specific capacitance of 289 F g–1, which is achieved in 1 M KOH electrolyte at a scan rate of 20 mV s–1. The well-connected highly electrically conductive structures exhibit a superb rate capability; at a very high scan rate of 1000 mV s–1 there is less than a 20% drop in the capacitance relative to 20 mV s–1. Such rate capability has never been reported for VN and is highly unusual for any other oxide or nitride. These 3D arrays also display nearly ideal triangular voltage profiles during constant current charge–discharge cycling. Analysis of the post-electrochemically cycled samples indicates negligible changes occurring in the VN nanocrystallite morphology, but a modification in the ...

Surface plasmon modes of a single silver nanorod: an electron energy loss study.

Abstract: We present an electron energy loss study using energy filtered TEM of spatially resolved surface plasmon excitations on a silver nanorod of aspect ratio 14.2 resting on a 30 nm thick silicon nitride membrane. Our results show that the excitation is quantized as resonant modes whose intensity maxima vary along the nanorod's length and whose wavelength becomes compressed towards the ends of the nanorod. Theoretical calculations modelling the surface plasmon response of the silver nanorod-silicon nitride system show the importance of including retardation and substrate effects in order to describe accurately the energy dispersion of the resonant modes.

Development of epoxy/BN composites with high thermal conductivity and sufficient dielectric breakdown strength partI - sample preparations and thermal conductivity

Abstract: The aim of this research is to find a way to achieve the epoxy composites with both high thermal conductivity and acceptable dielectric breakdown (BD) strength. As high thermal conductivity, low permittivity and low thermal expansion coefficient of filler can endow composite with higher thermal conductivity, higher BD strength and lower thermal expansion coefficient respectively, BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient was adopted as main filler in the research. Thermal conductivity was investigated in this part. The BD strength of samples will be discussed in Part II. Neat epoxy and other 25 kinds of epoxy/BN composites were prepared by a hot press method. Most of BN fillers were surface modified with silane coupling agent through ethanol/water reflux method to improve thermal conductivity. The values of 2.91 W/m·K, 3.95 W/m·K and 10.1 W/m·K as thermal conductivity were obtained for the composites that was single-loaded with h-BN(hexagonal boron nitride), c-BN (cubic boron nitride) or conglomerated h-BN, respectively. They were further improved to 5.26 W/m·K, 5.94 W/m·K and 12.3 W/m·K, respectively, by adding extra smaller A1N (aluminum nitride) to fill the voids in sample. Thermal conductivity of samples changes with the ratio of c-BN and h-BN when c-BN and h-BN were co-loaded. A value of 5.74 W/m·K as maximum was obtained at their ratio of 1 to 1 when total filler content is 80 wt%. A much higher value of 7.69 W/m·K was obtained by adding extra AIN. From the experiment data, it is concluded that the filler orientation in vertical direction of sample surface and the decrease of voids in sample are very important to obtain high thermal conductivity, and that the filler surface modification is also necessary to improve thermal conductivity especially for epoxy/c-BN composites, and addition of nano silica in small amount can also increase thermal conductivity if sample is prepared appropriately.

Chemical Vapor Deposition of Boron Nitride Nanosheets on Metallic Substrates via Decaborane/Ammonia Reactions

Abstract: A simple CVD method, employing the thermally induced reactions of ammonia with decaborane, has been found to generate boron nitride nanosheets (BNNS) on polycrystalline nickel and copper foils. Most of the BNNS deposited on Ni were two- or three-layered; however, some regions were thicker, containing up to six BN layers.

Effect of an Ammonia Treatment on Structure, Composition, and Oxygen Reduction Reaction Activity of Fe–N–C Catalysts

Abstract: For the first time, the effect on structure and composition by a second heat treatment in ammonia was studied in detail for Fe–N–C catalysts prepared by oxalate-supported pyrolysis of chloroirontetrametoxyphenylporphyrin (Fe(TMPP)Cl). The oxygen reduction reaction (ORR) activity was determined by rotating ring-disk electrode (RRDE) measurements in acidic solution. To evaluate the origin of the observed increase in ORR activity, bulk elemental analysis 57Fe Mossbauer spectroscopy, N2-sorption measurements, and X-ray diffraction were performed. A second heat treatment in ammonia was found to affect the ORR activity of Fe–N–C catalysts significantly; induced by NH3 treatment, all catalysts contained iron nitride; as more nitride was formed, the effect on ORR activity was more pronounced. A complete removal of the nitride by a subsequent acid leaching was possible but decreased the ORR activity slightly. On basis of these results, we conclude that the nitride itself cannot be the site responsible for the main...

Longitudinal Splitting of Boron Nitride Nanotubes for the Facile Synthesis of High Quality Boron Nitride Nanoribbons

Abstract: Boron nitride nanoribbons (BNNRs), the boron nitride structural equivalent of graphene nanoribbons (GNRs), are predicted to possess unique electronic and magnetic properties. We report the synthesis of BNNRs through the potassium-intercalation-induced longitudinal splitting of boron nitride nanotubes (BNNTs). This facile, scalable synthesis results in narrow (down to 20 nm), few sheet (typically 2–10), high crystallinity BNNRs with very uniform widths. The BNNRs are at least 1 μm in length with minimal defects within the ribbon plane and along the ribbon edges.