Coaxial metal-silicide Ni2Si/C54-TiSi2 nanowires

One-dimensional metal silicide nanowires are excellent candidates for interconnect and contact materials in future integrated circuits devices. Novel core-shell Ni(2)Si/C54-TiSi(2) nanowires, 2 μm in length, were grown controllably via a solid-liquid-solid growth mechanism. Their interesting ferromagnetic behaviors and excellent electrical properties have been studied in detail. The coercivities (Hcs) of the core-shell Ni(2)Si/C54-TiSi(2) nanowires was determined to be 200 and 50 Oe at 4 and 300 K, respectively, and the resistivity was measured to be as low as 31 μΩ-cm. The shift of the hysteresis loop with the temperature in zero field cooled (ZFC) and field cooled (FC) studies was found. ZFC and FC curves converge near room temperature at 314 K. The favorable ferromagnetic and electrical properties indicate that the unique core-shell nanowires can be used in penetrative ferromagnetic devices at room temperature simultaneously as a future interconnection in integrated circuits.     

Synthesis of single-crystal GaN nanowires on Si(111) substrate by ammonification technology

GaN nanowires have been synthesized on Si(111) substrate through ammoniating Ga₂O₃/BN films under flowing ammonia atmosphere at the temperature of 900 °C. The as-synthesized GaN nanowires were characterized by X-ray diffraction (XRD), selected-area electron diffraction (SAED), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM). The results demonstrated that the nanowires are hexagonal wurtzite GaN and possess a smooth surface with diameters ranging from 40 to 100 nm and lengths up to several tens of micrometers. The growth mechanism of crystalline GaN nanowires is discussed briefly.     

Modification of SiO2 nanowires with metallic nanocrystals from supercritical CO2

Through hydrogen reduction of metal precursors in supercritical CO2, Cu, and Pd, nanocrystals were deposited onto SiO2 nanowires to form different types of nanostructured materials, including nanocrystal-nanowire, spherical aggregation-nanowire, shell-nanowire composites, and "mesoporous" metals supported by the framework of nanowires. This supercritical fluid deposition technique is an attractive approach for modifying nanowires because of its generality and simplicity; the modified nanowires could be useful as catalysts and for further fabrication of multifunctional composites.     

Stability of three-dimensional metallic glass Ni68B21Si11

Under isothermal and linear heating conditions, the thermal stability of the three-dimensional metallic glass Ni₆₈B₂₁Si₁₁, produced by rapid quenching of the denucleated melt (RQDM), has been systematically studied using PE DSC7 differential scanning calorimetry in relation to denucleation of liquid alloy prior to rapid quenching, pre-anneal treatment of amorphous specimens, and cooling rate. The following results were observed. First, the thermal stability of metallic glass prepared by RQDM is obviously enhanced because of the removal of pre-existing nuclei in advance. This is substantiated by the experimental data showing that the projected life of three-dimensional metallic glass Ni₆₈B₂₁Si₁₁ is increased by an order of magnitude at 400 K. Secondly, pre-anneal treatment of the amorphous alloy leads to a reduction of temperature for the onset of crystallization,T _x, and crystallization heat, H. Finally, quenching rates have little effect on the thermal stability of amorphous alloys.     

Si nanowires synthesized with Cu catalyst

The metal copper which is a newly developed interconnecting material for integrated circuit (IC) has been used as the catalyst to catalyze the formation of the Si nanowires in high temperature tube furnace. The growth direction of the straight Si nanowires is <111> and the polyhedron η″-Cu₃Si alloy is on the tip of the Si nanowires. The synthesis temperature of the Si nanowires is 500 °C. Such a low temperature implies that the vapor-solid (VS) should be the growth method. The cheap Cu catalyst is favorable for the mass synthesis of Si nanowires.     

Analysis of reversible resistivity changes during structural relaxation in Co58Fe5Ni10Si11B16 metallic glass using different kinetic forms

Resistivity changes during structural relaxation in pre-annealed Co₅₈Fe₅Ni₁₀Si₁₁B₁₆ metallic glass were measured as functions of annealing temperature (280 to 400° C) and time and their kinetics were analysed using two different kinetic forms. In the case of the non-linear kinetic form which is expressed as ω = exp [- (t/?_m) ⁿ ], the values of mean activation energy,E _a, and mean pre-exponential factor, ?₀, are 1.84 eV and 2.88 × 10^?13 sec, respectively, and it was found that the quality of data fitting to the calculated curve with a constant value ofn = 0.35 was excellent. In the case of the log normal kinetic form which was developed by Nowick and Berry, the values ofE _a and ?₀ are 1.75 eV and 2.4 × 10^?12 sec, respectively, and the quality of data fitting to the calculated curve with a constant value ofβ = 4.5 was good, except for the slight deviation in the large value of log (t/?_m), whereβ is the width of the distribution of relaxation times. The kinetic parameters obtained in the present study were compared with various reported values.     

Domain wall motion in synthetic Co2Si nanowires

We report the synthesis of single crystalline Co(2)Si nanowires and the electrical transport studies of single Co(2)Si nanowire devices at low temperature. The butterfly shaped magnetoresistance shows interesting ferromagnetic features, including negative magnetoresistance, hysteretic switch fields, and stepwise drops in magnetoresistance. The nonsmooth stepwise magnetoresistance response is attributed to magnetic domain wall pinning and depinning motion in the Co(2)Si nanowires probably at crystal or morphology defects. The temperature dependence of the domain wall depinning field is observed and described by a model based on thermally assisted domain wall depinning over a single energy barrier.     

Al and Ni co-doped ZnO films with room temperature ferromagnetism,low resistivity and high transparence

Zn_0.91Al_0.07Ni_0.02O and Zn_0.90Al_0.05Ni_0.05O films of about 250 nm thick were deposited on glass substrates at 300 K by co-sputtering with ZnO:Al and Ni targets. The films were annealed in vacuum at 673 K for 2 h and then cooled down to room temperature under a magnetic field of 4.8 × 10⁴ A m⁻¹ applied along the film plane. After this process the films showed room temperature ferromagnetism, a resistivity of about 2 × 10⁻³ Ω cm and an average transmittance of 75% in the visible wavelength range. The films have a wurtzite structure with the c-axis orientation in the film growing direction and consist of thin columnar grains perpendicular to the substrate. A temperature dependence of the resistivity from 2 K to 300 K reveals that the carrier transport mechanism is thermally activated band conduction above 150 K and Mott's variable range hopping below 70 K.     

Synthesis of SiO2 covered SiC nanowires with milled Si,C nanopowders

SiO₂ covered β-SiC nanowires were directly synthesized with a novel method, annealing the milled Si,C nanopowders at 900–1100 °C on Si wafer or Al₂O₃ substrate, and there is no any metal catalysts used. The diameters of the nanowires are range of 20 to 50 nm, and the lengths of the nanowires are up to several hundreds of microns. There is a uniform SiO₂ amorphous layer on the surface of SiC nanowires. The axial direction of SiC nanowires is < 111>, and there are stacking faults and twin lamellae in the SiC nanowires. The synthetic mechanism of SiC nanowires includes two solid–solid reactions and one gas–solid reaction between SiO, Si, C and CO.     

Ni-silicide growth kinetics in Si and Si/SiO2 core/shell nanowires

A systematic study of the kinetics of axial Ni silicidation of as-grown and oxidized Si nanowires (SiNWs) with different crystallographic orientations and core diameters ranging from ～ 10 to 100 nm is presented. For temperatures between 300 and 440?°C the length of the total axial silicide intrusion varies with the square root of time, which provides clear evidence that the rate limiting step is diffusion of Ni through the growing silicide phase(s). A retardation of Ni-silicide formation for oxidized SiNWs is found, indicative of a stress induced lowering of the diffusion coefficients. Extrapolated growth constants indicate that the Ni flux through the silicided NW is dominated by surface diffusion, which is consistent with an inverse square root dependence of the silicide length on the NW diameter as observed for (111) orientated SiNWs. In situ TEM silicidation experiments show that NiSi(2) is the first forming phase for as-grown and oxidized SiNWs. The silicide-SiNW interface is thereby atomically abrupt and typically planar. Ni-rich silicide phases subsequently nucleate close to the Ni reservoir, which for as-grown SiNWs can lead to a complete channel break-off for prolonged silicidation due to significant volume expansion and morphological changes.     

Preparation of ultrathin TiO2 single-crystal nanowires for high performance dye sensitized solar cells

Ultrathin TiO₂ anatase nanowires (NWs) were successfully prepared via a rapid and facile hydrothermal route. Consequently, the TiO₂ NWs and TiO₂ nanoparticles (NPs) composites electrodes were prepared with different weight ratios (25, 50 and 100 %) for a dye sensitized solar cell, and the photoelectrical performance has been systematically studied. It is observed that although the amount of absorption dye decreases, the composite solar cells exhibit a higher power conversion efficiency compared to either pure TiO₂ NP or NW solar cells by rationally tuning the weight ratios. The behavior was attributed to a combination of the rapid carrier transport in NW framework and the high dye loading on P25 surface.     

Ultralong zinc-blende ZnS nanowires grown on polar C face of 6H-SiC substrates at low temperatures by metalorganic chemical vapor deposition

Ultralong ZnS nanowires with high purity were grown on Au-coated polar C face of 6H-SiC substrates via metalorganic chemical vapor deposition at low temperatures. The ZnS nanowires have zinc-blende structure and the length is up to tens of micrometers. HRTEM investigations show that the nanowires are well crystalline single crystal grown along [1 1 1] and free of bulk defects. However, sparse straight and curved nanowires with poor crystalline nature are randomly grown on the Au-coated Si face of 6H-SiC substrates. We deduce that the growth of ZnS is related to the substrates and C face can enhance Au-catalytic VLS growth. The CL spectra of an individual nanowire grown on C and Si face reveal different optical properties. Intrinsic sulfur and zinc vacancies are the main reasons for the 458.1 nm and 459.2 nm blue emission detected in the nanowire grown on C face and Si face, respectively. Nevertheless, an unusual green emission at 565.1 nm is observed in the poor crystalline nanowire grown on Si face, which originates from the bulk defects.     

Core and valence excitations in Ni2Si

We present an electron energy loss analysis of Ni₂Si around the NiM_2,3 and NiL_2,3 edges and in the range 0–30 eV (valence excitations). An intensity reduction of the Ni L_2,3 edge of the silicide is observed and interpreted in terms of a depletion of d metal electrons at the Fermi level E_F. The reduction in the Ni M_2,3 Fano line shape with respect to that for pure nickel is also described in a similar way. The near-edge region gives information on the partial density of states in the region above E_F and is related to the structures observed in the valence band range. Particular attention was paid to the study of the Ni₂Si local structure obtained from the analysis of the extended energy loss fine structures beyond the Ni M_2,3 level.     

Thermal expansion and relaxation-crystallization kinetics of metallic glass Co63Ni6Fe4V2Si10B15

The conventional dilatometric technique is applied to study the thermal expansion of the metallic glass Co₆₃(FeNi)₁₀V₂Si₁₀B₁₅ with cylindrical samples rolled from ribbon. The heatingrate effect on expansion and differential thermal analysis (DTA) is investigated. The non-linear region of the temperature variation curves of relative elongation all show a broad peak and a distinct peak which correspond to those in DTA. The broad peak in DTA resolves into two peaks: the first one is associated with the glass transition which is interpreted as a quasi-first-order transformation. On comparing with X-ray diffraction analysis, the crystallization is determined to be a two-stage process in which Co(4F) precipitation is a controlling factor.     

Magnetic properties of metallic glass Fe39Ni39Mo4Si6B12

Magnetic metallic glass Fe₃₉Ni₃₉Mo₄Si₆B₁₂ has been studied by the⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra consist of broad and overlapping six-line pattern below the Curie temperature. The Curie and crystallization temperatures of this metallic glass have been determined to be 575 ± 3 K and 725 ± 3 K, respectively. The hyperfine magnetic field at room temperature is approximately 225 kOe. The reduced hyperfine fields of this sample decrease much faster than that observed for other iron-rich metallic glasses like Fe₄₀Ni₄₀B₂₀, Fe₈₀B₂₀ etc. This behaviour is attributed to the presence of molybdenum atoms in the sample.     

Simultaneous measurement of resistivity and temperature coefficient of resistivity of metallic thin films with the transient hot-strip method

The recently described transient hot-strip method for measuring thermal transport properties of solid insulators has been developed for simultaneously measuring the resistivity ?_F and the temperature coefficient of resistivity (TCR) α_F of metallic thin films. The experimental arrangement is similar to the well-known four-probe technique but the constant current pulse passed through the thin film is sufficiently high to increase its mean temperature by about 1 K. By measuring the time-varying increase or decrease in the resistance, information is obtained not only about the resistivity but also about the TCR.The main advantage of the method is the possibility to measure ?_F and α_F simultaneously without increasing or decreasing the temperature of the film by more than 1 K, i.e. without measurably changing the properties of the film.The method has been tested on unannealed copper films at room temperature.     

On the photoconduction properties of low resistivity TiO2 nanotubes

TiO(2) nanotubes were synthesized by anodic oxidation of titanium foils using dimethyl sulfoxide and hydrofluoric acid as the electrolyte. The electrical properties of individual nanotube-based devices were evaluated and modeled after exposing some of them to different gas and illumination conditions. Resistivity values fully comparable to those of TiO(2) single crystal anatase (ρ(SA) = 1.09 ± 0.01Ω cm) were found, and their photoconductive characteristics, explained in terms of the Shockley-Read-Hall model for non-radiative recombination in semiconductors, were found to be strongly influenced by the applied experimental conditions such as the surrounding atmosphere. These devices may have potential applications in photocatalytic processes, such as CO(2) reduction or H(2)O splitting, avoiding the interfering effects typical of nanotube arrays.     

Interactions of hydrogen with Tb3Ni6M2 (M= Al,Ga, or Si) alloys with structures of Ce3Ni6Si2 type

Summary A crystallochemical analysis has been performed on the structures of Tb₃Ni₆M₂ (M = Al, Ga, Si), which crystallize in the Ce₃Ni₆Si₂ structure type, as they are potential hydrogen absorbers. There are seven types of cavity suitable for hydrogen atoms. When the Tb₃Ni₆M₂ intermetallides interact with hydrogen, hydride phases are formed that contain 0.32-0.95 mass % H and which decompose completely in the range 20–300°C. The hydrogen contents in the hydrides and the thermal stabilities decrease in the aluminide-gallide-silicide sequence. Hydrogenation produces isotropic expansion in the initial body-centered cubic structures. Neutron diffraction indicates that the deuterium atoms in Tb₃Ni₆Al₂D_6.5 occupy the Tb₃Al, TbNi₃, and Ni₄ cavities.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 27, No. 4, pp. 22–25, July–August, 1991.