Thin Oxide Shell Research Articles

Effect of amine and thiol addition on the surface chemistry and agglomeration of fine Cu powders

The effect of propylamine, sec-butylamine, and benzenethiol addition on the surface chemistry, apparent particle size and agglomeration of fine copper powders (<2 μm mean dia.) prepared by aqueous-phase precipitation was investigated using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), diffuse reflectance infrared spectroscopy (DRIFTS) and scanning electron microscopy (SEM). XPS analysis of the raw, agglomerated copper powders suggests a particle morphology consisting of a metallic core surrounded by a thin oxide shell. Atomic force microscopy (AFM) force measurements on model systems suggest that amines may be effective in preventing agglomeration, whereas the addition of benzenethiol may promote particle aggregation. DRIFTS results indicate that the propylamine, sec-butylamine, and benzenethiol are chemically bound to the copper particle surface in all cases. TGA analysis indicates that at elevated temperatures propylamine is easily removed, whereas butylamine decomposes on the surface leaving a surface hydrocarbon residue. Benzenethiol is removed above 300 °C, however, some atomic sulfur remains on the particle surface which is subsequently removed above 400 °C.

A Novel Approach to Silicon‐Nanowire‐Assisted Growth of High‐Purity, Single‐Crystalline β‐Si3N4 Nanowires

AbstractBundles of single‐crystalline β‐Si3N4 nanowires with high purity are successfully synthesized using a simple CVD process with silicon nanowire(SiNW)‐assisted growth. The β‐Si3N4 NWs obtained have a small diameter of ∼30 nm, a single‐crystalline structure with [100] or [101] direction, and a thin oxide shell. The photoluminescence and Raman scattering spectra confirm the good crystalline structure. The weak blue‐shift of the peaks in Raman scattering compared to bulk β‐Si3N4 is attributed to the phonon confinement effect or laser heating during the measurements. Finally, a possible SiNWs template‐assisted growth model is suggested.

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

An unexpected mechanism for fast reaction of Al nanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before the oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of atomic scale liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). Physical parameters controlling the melt dispersion mechanism are found by our analysis. In addition to an explanation of the extremely short reaction time, the following correspondence between our theory and experiments are obtained: (a) For the particle radius below some critical value, the flame propagation rate and the ignition time delay are independent of the radius; (b) damage of the oxide shell suppresses the melt dispersion mechanism and promotes the traditional diffusive oxidation mechanism; (c) nanoflakes react more like micron size (rather than nanosize) spherical particles. The reasons why the melt dispersion mechanism cannot operate for the micron particles or slow heating of nanoparticles are determined. Methods to promote the melt dispersion mechanism, to expand it to micron particles, and to improve efficiency of energetic metastable intermolecular composites are formulated. In particular, the following could promote the melt dispersion mechanism in micron particles: (a) Increasing the temperature at which the initial oxide shell is formed; (b) creating initial porosity in the Al; (c) mixing of the Al with a material with a low (even negative) thermal expansion coefficient or with a phase transformation accompanied by a volume reduction; (d) alloying the Al to decrease the cavitation pressure; (e) mixing nano- and micron particles; and (f) introducing gasifying or explosive inclusions in any fuel and oxidizer. A similar mechanism is expected for nitridation and fluorination of Al and may also be tailored for Ti and Mg fuel.

A Simple Route to Synthesize Scales of Aligned Single-crystalline SiC Nanowires Arrays with Very Small Diameter and Optical Properties

Scales of aligned single-crystalline SiC nanowires (SiCNWs) arrays with very small diameter were synthesized by a simple thermal evaporation of ZnS and carbon on silicon wafer. The as-received SiCNWs possess a uniform size distribution centered at approximately 8.0 nm, even with a minimum of approximately 3.0 nm. The highly oriented SiCNWs usually grew along [111] direction with a clean surface, very thin oxide shell, and small quantity of stacking faults. A crystalline tube-like SiC nanostructure is also obtained. The optical properties, including photoluminescence and Raman scattering spectra of the SiCNWs, were investigated, respectively. In the end, a growth model on basis of the experimental data is suggested.

Opportunities from the nanoworld: Gas phase nanoparticles

In this paper we present studies related to coalescence and oxidation of transition metal nanoparticles with sizes ranging between 2 and 10 nm. For cobalt and iron exposure to air leads to thin oxide shell formation (thickness ≤2 nm), which is thermally unstable upon annealing above ∼200–300 °C. These particles also undergo structural changes due to electron beam stimulated oxidation. However, for molybdenum and niobium the oxide shell is absent and the nanoparticles are stable under electron beam irradiation. Furthermore, for cobalt nanoparticles oxidation leads to a core–shell formation that causes the appearance of an exchange bias field at cryogenic temperatures due to interface ferromagnetic–antiferromagnetic coupling, which is stabilized due to interparticle exchange interactions. The latter also leads to a correlated spin glass state due to its competition with the random intraparticle anisotropy.

Melt dispersion mechanism for fast reaction of nanothermites

An unexpected mechanism for fast oxidation of Al nanoparticles covered by a thin oxide shell (OS) is proposed. The volume change due to melting of Al induces pressures of 0.1–4GPa and causes spallation of the OS. A subsequent unloading wave creates high tensile pressures resulting in dispersion of liquid Al clusters, oxidation of which is not limited by diffusion (in contrast to traditional mechanisms). Physical parameters controlling this process are determined. Methods to promote this melt dispersion mechanism, and consequently, improve efficiency of energetic nanothermites are discussed.

In situ Transmission Electron Microscopy Studies on Structural Dynamics of Transition Metal Nanoclusters

The structural stability of transition metal nanoclusters has been scrutinized with in situ transmission electron microscopy as a function of temperature. In particular iron, cobalt, niobium, and molybdenum clusters with diameters around 5 nm have been investigated. During exposure to air, a thin oxide shell with a thickness of 2 nm is formed around the iron and cobalt clusters, which is thermally unstable under moderate high vacuum annealing above 200 °C. Interestingly, niobium clusters oxidize only internally at higher temperatures without the formation of an oxide shell. They are unaffected under electron beam irradiation, whereas iron and cobalt undergo severe structural changes. Further, no cluster coalescence of niobium takes place, even during annealing up to 800 °C, whereas iron and cobalt clusters coalesce after decomposition of the oxide, as long as the clusters are in close contact. In contrast to niobium, molybdenum clusters do not oxidize upon annealing; they are stable under electron beam irradiation and coalesce at temperatures higher than 800 °C. In all cases, the coalescence process indicates a strong influence of the local environment of the cluster.

Size-induced stability and structural transition in monodispersed indium nanoparticles

The present study reports the stability and the physical significance of the size-induced crystallographic structural transition in the gas-phase synthesized monodispersed indium nanoparticles. Transmission electron microscopy and x-ray photoelectron spectroscopy studies reveal that the formation of a thin oxide shell results in enhanced stability of indium nanoparticles. These results also show a size-induced structural transition from the bulk tetragonal to face-centered-cubic structure, which is attributed to an increase in the binding energy of core electrons of indium nanoparticles due to quantum confinement effects and the presence of a thin oxide shell.

Single-Crystal Calcium Hexaboride Nanowires: Synthesis and Characterization

Catalyst-assisted growth of single-crystal calcium hexaboride (CaB6) nanowires was achieved by pyrolysis of diborane (B2H6) over calcium oxide (CaO) powders at 860−900 °C and ∼155 mTorr in a quartz tube furnace. TEM electron diffraction and Raman spectroscopy indicate that the nanowires are single-crystal CaB6 and have a preferred [001] growth direction. Analysis of TEM/EDX/EELS data proves the nanowires consist of CaB6 cores and thin (1−2 nm) amorphous oxide shell material. The CaB6 nanowires have diameters of ∼15−40 nm, and lengths of ∼1−10 μm.

Electronic structure of 1 to 2 nm diameter silicon core/shell nanocrystals: surface chemistry, optical spectra, charge transfer, and doping.

Static and time-dependent density functional calculations, geometrically optimized and including all electrons, are described for silicon nanocrystals as large as Si(87)H(76), which contains 163 atoms. We explore and predict the effect that different sp(3) passivation schemes-F or H termination, thin oxide shell, or alkane termination-have on the HOMO and LUMO, on the optical spectra, and on electron transfer properties. Electronegativity comparisons are a useful guide in understanding the observed deviation from the simple quantum size effect model. Nanocrystals containing Al or P impurity atoms, either on the surface or in the interior, are explored to understand electrical doping in strongly quantum-confined nanocrystals. Surface dangling bonds are found to participate in internal charge transfer with P atom dopant electrons.

Nanosized iron clusters investigated with in situ transmission electron microscopy

Transmission electron microscopy is employed for investigating the structural stability of nanosized iron clusters as deposited and after in situ annealing treatments under high vacuum conditions. The thin iron oxide shell that is formed around the iron clusters (upon air exposure) is of the order of 2 nm surrounding a 5 nm core of body-centered-cubic (bcc) iron. The oxide shell breaks down upon annealing at relatively low temperatures (∼500 °C) leading to pure iron particles having a bcc crystal structure. Annealing of clusters, which are in contact, leads to their fusion and formation of larger clusters preserving their crystallographic structure and being free of any oxide shell. On the other hand, isolated clusters appear rather immobile (upon annealing). The truncated rhombic dodecahedron was found as the most probable shape of the clusters which differs from former theoretical predictions based on calculations of stable structural forms.

Lattice contraction in nanosized silicon particles produced by laser pyrolysis of silane

We used laser-induced decomposition of silane for the fabrication of nanosized Si particles and studied in detail their structural characteristics by conventional and high resolution electron microscopy. The silane gas flow reactor incorporated in a molecular beam apparatus was operated without size selection to achieve a broad size distribution. Deposition at low energy on carbon substrates yielded single crystalline, spherical Si particles almost completely free of planar lattice defects. The particles, covered by thin amorphous oxide shells, are not agglomerated into larger aggregates. The lattice of diamond cubic type exhibits deviations from the bulk spacing which vary from distinct contraction to dilatation as with decreasing particle size the oxide shell thickness is reduced. This effect is discussed in terms of the strong Si/oxide interfacial interaction and compressive stresses arising upon oxidation. A negative interface stress, as determined from the size dependence of the lattice spacing, limits the curvature of the interface, i.e., at small sizes Si oxidation must be considered as a self-limiting process.

Characteristic transport properties of CoO-coated monodispersive Co cluster assemblies

We have fabricated CoO-coated monodispersive Co cluster assemblies with the mean cluster size of 13 nm at various oxygen gas-flow rate ${R}_{{\mathrm{O}}_{2}}$ by a plasma-gas-condensation-type cluster beam deposition technique, and studied their electrical conductivity, \ensuremath{\sigma}, and magnetoresistance. For ${R}_{{\mathrm{O}}_{2}}l0.24\mathrm{SCCM}$ (sccm denotes cubic centimeter per minute), the resistivity revealed a minimum and showed $\mathrm{ln}T$ dependence at lower temperatures, probably due to the weak localization of conduction electrons owing to presence of thin oxide shells covering Co cores. A small negative magnetoresistance was observed in this regime. For ${R}_{{\mathrm{O}}_{2}}g0.3\mathrm{SCCM},$ tunnel-type temperature dependence of \ensuremath{\sigma} in the form of ln \ensuremath{\sigma} vs $1/T$ was observed between 7 and 80 K. This differs from the well-known temperature dependence of ln \ensuremath{\sigma} vs ${1/T}^{1/2}$ for disordered granular materials. The magnetoresistance ratio, $({\ensuremath{\rho}}_{\mathrm{H}=30\mathrm{}\mathrm{kOe}}\ensuremath{-}{\ensuremath{\rho}}_{0})/{\ensuremath{\rho}}_{0},$ is negative and its absolute value increases sharply with decreasing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase, by a factor of 6, is much larger than those observed for conventional metal-insulator granular systems. These results are ascribed to a prominent cotunneling effect in the Coulomb blockade regime, arising from the uniform Co core size and CoO shell thickness in the present monodispersed cluster assemblies.

Nanosized silicon particles by inert gas arc evaporation

We have explored the possibilities of inert gas arc evaporation for fabrica tion of nanosized Si particles and studied agglomeration, size, shape, crystallinity, surface roughness and internal structure of these particles by conventional and high resolution electron microscopy. The rarely used technique yielded single crystalline, spherical Si particles 3 – 16 nm in size completely free of planar lattice defects. The particles, covered by thin amorphous oxide shells, are agglomerated into chains and tangles. The lattice of diamond cubic type exhibits contractions -which decrease as with decreasing particle size the oxide shell thickness is reduced. This effect is ascribed to compressive stress at the Si/oxide interface.

Observation of surface mode absorption in oxide coated metal spheres

The infrared absorption coefficient of 5 μm zinc spheres coated with ZnO of variable thickness is reported and compared with theory. The peak absorption by surface phonons in the oxide shell shifts from the long wavelength longitudinal optical mode frequency for thin oxide shells to the Fröhlich frequency for thick oxide shells.