Epitaxial growth of tungsten nanoparticles on alumina and spinel surfaces

Isolated tungsten nanoparticles (α-W and β-W phase) were synthesized and epitaxially grown on alumina and spinel particle surfaces with an average tungsten size of ≤20?nm for a low tungsten content (of ≤1.5?vol%). Using tungsten (VI) ethoxide alcoholic solutions, tungsten trioxide hydrated precursors were attached to a ceramic grains surface as a nanoparticle coating. High-resolution transmission electron microscopy (HRTEM) micrographs showed epitaxial interfaces between alumina, spinel and metallic tungsten. This epitaxial growth is assumed to be due to the effect of water vapour on the sublimation of ortho-tungstic acid during the reduction process in a hydrogen atmosphere. The planes involved in the epitaxy were found to be [Formula: see text] and [Formula: see text].     

Size effects on Raman spectra of small CdSe nanoparticles in polymer films

The results of a resonant Raman scattering (RRS) study of polymer-stabilized colloidal CdSe nanoparticles (NPs) are reported. The size-selective nature of the RRS is demonstrated by analysing the NP ensembles with different average size [Formula: see text] and size distribution Δd using a set of excitation wavelengths. The effect of size selection on the estimation of [Formula: see text] and Δd values from the RRS spectra is discussed, as well as some peculiarities of RRS on surface optical phonons. From the experimentally observed small variation of the I(2LO)/I(LO) ratio for 2-5?nm NPs a minor effect of [Formula: see text] on the electron-phonon coupling strength in this [Formula: see text] range is supposed.     

Longer InN phonon lifetimes in nanowires

We present a Raman scattering study of the anharmonic phonon decay of the [Formula: see text], [Formula: see text] and E(1)(LO) phonons in InN nanowires over the 80-400?K temperature range. While the temperature-dependent anharmonic decay in the nanowires is similar to that found for bulk InN, the background contribution to the phonon lifetime is strongly reduced as a result of the improved crystalline quality. High-resolution measurements reveal a remarkably long lifetime of the [Formula: see text] mode. From the comparison between the [Formula: see text] frequencies measured in the nanowires with those of the thin film we obtain the deformation potentials for the [Formula: see text] mode.     

Unique electronic band structures of hydrogen-terminated [Formula: see text] silicon nanowires

Band structure mutation from an indirect to a direct gap is a well-known character of small hydrogen-terminated [Formula: see text] and [Formula: see text] silicon nanowires (SiNWs), and suggests the possible emission of silicon. In contrast, we show that hydrogen-terminated [Formula: see text] SiNWs consistently present indirect band gaps even at an extremely small size, according to our calculations using density functional theory. Interestingly, the band gap of [Formula: see text] SiNWs shows a quasi-direct feature as the wire size increases, suggesting the possibility of using medium SiNWs in optoelectronic devices. This result also indicates that the electronic structures of SiNWs are strongly orientation dependent.     

Analogous mechanical behaviors in [Formula: see text] and [Formula: see text] directions of Cu nanowires under tension and compression at a high strain rate

This study analyzes the plastic deformation on the atomic scale of Cu nanowires (NWs) with [Formula: see text] and [Formula: see text] orientations during uniaxial tension and compression, using a molecular dynamic simulation. The maximum local stress (MLS) method is employed to evaluate mechanical behavior during deformation. Following yielding, the flow stress strongly depends on the variation in the degree of orientation caused by twinning. Both the tension of the [Formula: see text] NW and the compression of the [Formula: see text] NW cause twin deformation and consequent geometrical softening. In contrast, the compression of the [Formula: see text] NW and the tension of the [Formula: see text] NW form twin bands and cause geometrical hardening. These behaviors result in the stress-strain curves that reveal the pseudo-skew-symmetry characteristic. With respect to the difference between the critical resolved shear stress (τ(c)) associated with the distinct orientations, τ(c) depends strongly on the surface critical resolved stress (τ(sc)). Under tension, τ(sc) depends on the degree of lattice distortion. A larger lattice distortion (pre-tensile stress) corresponds to higher τ(sc). However, under compression, a geometrical factor can be used to describe the difference in τ(sc) between the different orientations. A larger geometrical factor corresponds to a larger τ(sc).     

Facet and in-plane crystallographic orientations of GaN nanowires grown on Si(111)

We have determined the in-plane orientation of GaN nanowires relative to the Si (111) substrate on which they were grown. We used x-ray diffraction pole figure measurements to evidence two types of crystallographic orientation, all the nanowires having [Formula: see text] lateral facets. The proportion of these two orientations was determined and shown to be influenced by the pre-deposition of Al(Ga)N intermediate layers. In the main orientation, the GaN basal [Formula: see text] directions are aligned with the [Formula: see text] directions. This orientation corresponds to an in-plane coincidence of GaN and Si lattices.     

High-resolution imaging in a deep turbid medium based on an ultrasound-switchable fluorescence technique

The spatial resolution of fluorescence imaging techniques in deep optically turbid media such as tissues is limited by photon diffusion. To break the diffusion limit and achieve high-resolution and deep-tissue fluorescence imaging, a fundamentally different method was demonstrated based on a concept of ultrasound-switchable fluorescence. The results showed that a small fluorescent tube with a diameter of ～180?μm at a depth of ～20?mm in an optical scattering medium ([Formula: see text] and [Formula: see text] cm(-1)) can be clearly imaged with a size of ～260?μm. The depth-to-resolution ratio is shown to be about one order of magnitude better than other deep-tissue fluorescence imaging techniques.     

Distribution of cations in nanosize and bulk Co-Zn ferrites

The structural and magnetic properties of Co(1-x)Zn(x)Fe2O4 ferrites (Co-Zn ferrites) are investigated in a narrow compositional range around x = 0.6, which is of interest because of applications in magnetic fluid hyperthermia. The study by x-ray and neutron diffraction, M?ssbauer spectroscopy and magnetization measurements is done on nanoparticles prepared by the coprecipitation method and bulk samples sintered at high temperatures. In spite of the known preference of Zn2+ for tetrahedral (A) sites and Co2+ for octahedral [B] sites, the cations are distributed nearly evenly over the two sites of spinel structure and there is also a variable number of [B] site vacancies (see text), making cobalt ions trivalent. In particular for x = 0.6, the cationic distribution is refined to [Formula: see text] and [Formula: see text] for the 13 nm particles (T(C) = 335 K) and bulk sample (T(C) = 351 K), respectively.     

Vertically standing Ge nanowires on GaAs(110) substrates

The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380?°C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the [Formula: see text] direction. Using this unique property, vertical [Formula: see text] Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the [Formula: see text] directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.     

Analysis of the magnetic force generated at a hemispherical microelectrode

An analytical expression is presented for the magnetic force generated during steady-state voltammetry at a hemispherical microelectrode immersed in a uniform magnetic field. Diffusion of electrogenerated ions through the magnetic field results in a magnetic force that induces convective solution flow near the electrode surface. The magnetic force per unit volume,[Formula: see text] (i.e., force density), is shown to decrease as r(-)(2), where r is the distance away from the center of the hemispherical electrode. A consequence of the inverse square dependence of[Formula: see text] on r is that the magnetic force is confined to a microscopic solution volume near the electrode surface (e.g., ～2 × 10(-)(9) L for a 12.5-μm-radius hemispherical electrode). The net magnetic force acting on the diffusion layer volume,[Formula: see text] , is computed as a function of magnetic field strength and orientation and used in an approximate analysis of experimental data obtained at an inlaid 12.5-μm-radius Pt microdisk electrode. Enhancements in voltammetric currents are shown to result from magnetic forces as small as 2 × 10(-)(11) N.     

Pt-Pd alloy nanoparticle-decorated carbon nanotubes: a durable and methanol tolerant oxygen reduction electrocatalyst

We describe the decoration of multiwalled carbon nanotubes (MCNTs) with Pt-Pd alloy nanoelectrocatalysts of three different compositions and their electrocatalytic performance toward the oxygen reduction reaction (ORR). The decoration of the MCNTs involves polymer-assisted impregnation of metal precursors [Formula: see text] and [Formula: see text] and the subsequent reduction of the impregnated precursors by a modified polyol route. The composition of the catalyst was controlled by tuning the molar ratio of the precursors during their impregnation. Electron probe microscopic analysis shows that the catalysts have compositions of Pt(46)Pd(54,) Pt(64)Pd(36) and Pt(28)Pd(72). The Pt(46)Pd(54) and Pt(64)Pd(36) catalysts have truncated octahedral and icosahedral shapes with a size ranging from 8 to 10?nm. On the other hand, the catalyst of Pt(28)Pd(72) composition has a spherical/quasispherical shape with a size distribution of 1-2?nm. The XPS measurement confirms the signature of metallic Pt and Pd. The Pt(46)Pd(54) catalyst has a pronounced electrocatalytic activity toward the ORR with a specific and mass activity of 378 [Formula: see text] and [Formula: see text], respectively at 0.8?V. Moreover, the Pt(46)Pd(54) nanoelectrocatalyst is highly durable and it retains its initial catalytic activity even after 1000 extensive cycles. Interestingly, this catalyst has a very high tolerance toward methanol and it does not favor the oxidation of methanol in the potential window of 0.1-1.4?V. The electrocatalytic activity of the alloy electrocatalyst is compared with commercially available Pt black and MCNT-supported spherical Pt nanoparticles. The catalytic activity of the Pt(46)Pd(54) nanoelectrocatalyst is higher than the other catalysts. The Pt(46)Pd(54) catalyst outperforms the electrocatalytic activity of all other catalysts.     

Efficient coating of transparent and conductive carbon nanotube thin films on plastic substrates

Optically transparent and electrically conductive single-walled carbon nanotube (SWNT) thin films were fabricated at room temperature using a dip-coating technique. The film transparency and sheet resistance can be easily tailored by controlling the number of coatings. Aminopropyltriethoxysilane (APTS) was used as an adhesion promoter and, together with surfactant Triton X-100, greatly improved the SWNTs coating. Only five coats were required to obtain a sheet resistance of 2.05?[Formula: see text] and film transparency of 84?%T. The dip-coated film after post-deposition treatment with nitric acid has a sheet resistance as low as 130?[Formula: see text] at 69?%T. This technique is suitable for large-scale SWNT coating at room temperature and can be used on different types of substrates such as glass and plastics. This paper will discuss the role of the adhesion promoter and surfactant in the coating process.     

Single-crystalline Ni2Ge/Ge/Ni2Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni(2)Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni(2)Ge nanowires exceeds 3.5 × 10(7) A cm(-2), and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni(2)Ge/Ge/Ni(2)Ge heterostructure. The interface epitaxial relationships are determined to be [Formula: see text] and [Formula: see text]. Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni(2)Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10(3) and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.     

Capillary supercritical fluid chromatography/mass spectrometry of phenolic mannich bases with dimethyl ether modified ethane as the mobile phase

The analysis of phenolic Mannich bases [Formula: see text] which are used as hardeners and accelerators for epoxy resins [Formula: see text] by capillary supercritical fluid chromatography (SFC) with dimethyl ether modified ethane as the mobile phase is described. The elution properties of several different mobile phases with respect to amines are shown. SFC with UV detection is coupled via a custom-built interface to a mass spectrometer with atmospheric pressure chemical ionization. Two technically important Mannich bases prepared by different production processes are characterized and compared with respect to their byproducts. The role of dimethyl ether during the ionization process and the fragmentation of phenolic Mannich bases is discussed.     

Implementation of Electrochemically Synthesized Silver Nanocrystallites for the Preferential SERS Enhancement of Defect Modes on Thermally Etched Graphite Surfaces

Highly oriented pyrolytic graphite (HOPG) surfaces, on which atomically well-defined roughness has been introduced via high-temperature gasification reactions, are investigated by noncontact mode atomic force microscopy (NC-AFM) and Raman spectroscopy both before and after the electrochemical deposition of silver nanocrystallites on these surfaces. Exposure of freshly cleaved HOPG surfaces to an O(2)-rich ambient at 650 °C for a few minutes caused the formation of 1-monolayer-deep, circular etch pits on the HOPG basal plane surface. Silver nanocrystallites were electrochemically deposited onto these etched surfaces at two coverages: 0.5 mC cm(-)(2) (or 5 nmol of Ag(0) cm(-)(2)) and 2.4 mC cm(-)(2) (25 nmol of Ag(0) cm(-)(2)). At the lower coverage, NC-AFM images revealed that silver decorated only the circumference of the circular etch pits, forming a uniform annular ring with an apparent diameter of 200-250 ? and a height of ～15 ?. At the higher silver coverage, an increase in the height but not the diameter of this annulus was observed, and additional silver nanostructures [Formula: see text] having dimensions of 300-350 ? diameter and 15 ? height [Formula: see text] were observed on atomically smooth regions of the graphite basal plane. The Raman spectroscopy of these surfaces was investigated and compared with spectra for nanocrystallite-modified but unetched HOPG basal plane surfaces and thermally etched surfaces on which no silver was deposited. For for thermally etched HOPG surfaces at either silver coverage, SERS-augmented Raman spectra were obtained in which defect modes of the graphite surface [Formula: see text] derived from "finite" graphite domains at the surface [Formula: see text] were strongly and preferentially enhanced. In addition, an enhanced band near 2900 cm(-)(1) was assigned to ν(OH) from carboxylate moieties present at step edges based on the basis of the observed pH dependence of the enhancement.     

Nanofabrication of PTCDA molecular chains on rutile TiO(2)(011)-(2 × 1) surfaces

The adsorption of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on a rutile TiO(2)(011)-(2 × 1) surface is studied using ultra-high vacuum scanning tunneling microscopy. The self-assembly process is dominated by the fine interplay between the lateral intermolecular interactions and the binding to the substrate. By means of temperature-induced change in the adsorption configuration and the activation of diffusion, the molecules are assembled into one-dimensional chains oriented along the [Formula: see text] crystallographic direction.     

Ag-assisted CBE growth of ordered InSb nanowire arrays

We present growth studies of InSb nanowires grown directly on [Formula: see text] and [Formula: see text] substrates. The nanowires were synthesized in a chemical beam epitaxy (CBE) system and are of cubic zinc blende structure. To initiate nanowire nucleation we used lithographically positioned silver (Ag) seed particles. Up to 87% of the nanowires nucleate at the lithographically pre-defined positions. Transmission electron microscopy (TEM) investigations furthermore showed that, typically, a parasitic InSb thin film forms on the substrates. This thin film is more pronounced for InSb((111)B) substrates than for InAs((111)B) substrates, where it is completely absent at low growth temperatures. Thus, using InAs((111)B) substrates and growth temperatures below 360?°C free-standing InSb nanowires can be synthesized.     

Fabrication, structural characterization and photoluminescence of single-crystal Zn(x)Cd(1-x)S zigzag nanowires

Large-scale synthesis of ternary Zn(x)Cd(1-x)S zigzag nanowires was achieved in a one-step metal-organic chemical vapour deposition (MOCVD) process with co-fed single precursors of ZnS and CdS. Their morphologies, structures and optical properties were characterized and confirmed by scanning electron microscopy, high-resolution transmission electron microscopy, x-ray spectroscopy, and photoluminescence. The Zn(x)Cd(1-x)S zigzag nanowires are single crystalline, with axis [001], by changing the growth direction from [Formula: see text] to [Formula: see text]. Regarding the formation of zigzag nanowires, we suggest that the shear strain and slight fluctuation of the reaction conditions may be the major factors that make the nanowires change growth direction. In addition, because of the lower temperature and versatility, this new fabrication method might present a new and facile way to form other ternary nanomaterials. Furthermore, the green emission of the nanowires may have potential applications in electronic/optical nanodevices.     

Anisotropic interface induced formation of Sb nanowires on GaSb(111)A substrates

The growth of Sb nanowires on GaSb(111)A substrates is studied by in?situ azimuthal scan reflection high-energy electron diffraction (ARHEED). Bulk and layer contributions can be distinguished in the ARHEED transmission pattern through the Sb nanowires. The three-dimensional structure of the growing Sb nanowires is identified by post-growth atomic force microscopy (AFM) and x-ray diffraction (XRD). The lattice match of the Sb crystal along the [Formula: see text] and the GaSb crystal along [Formula: see text] directions lead to a preferential orientation of the Sb nanowires. The Sb adsorption and desorption kinetics is studied by thermal desorption spectroscopy.     

Adsorption, manipulation and self-assembling of TBrPP-Co molecules on a Ag/Si(111) surface by scanning tunnelling microscopy

Individual adsorption and two-dimensional assembling of 5,10,15,20-tetrakis-(4-bromophenyl)-porphyrin-Co (TBrPP-Co) molecules on a Si(111)-[Formula: see text] Ag reconstructed surface have been studied using low-temperature scanning tunnelling microscopy (STM). All the isolated molecules are observed in a planar shape with slight distortion. The isolated molecules can be controllably rotated with an STM tip to the orientation along the trigonal lattice ([Formula: see text] direction) of the substrate. With an increased coverage (0.07?ML) and appropriate annealing, the molecules assemble to form three types of ordered phase. The long-range ordered structures, however, disappear at higher coverage (0.75?ML).