Preparation and characterization of CdS/Si coaxial nanowires

CdS/Si coaxial nanowires were fabricated via a simple one-step thermal evaporation of CdS powder in mass scale. Their crystallinities, general morphologies and detailed microstructures were characterized by using X-ray diffraction, scanning electron microscope, transmission electron microscope and Raman spectra. The CdS core crystallizes in a hexagonal wurtzite structure with lattice constants of a=0.4140 nm and c=0.6719 nm, and the Si shell is amorphous. Five Raman peaks from the CdS core were observed. They are 1LO at 305 cm⁻¹, 2LO at 601 cm⁻¹, A₁-TO at 212 cm⁻¹, E₁-TO at 234 cm⁻¹, and E₂ at 252 cm⁻¹. Photoluminescence measurements show that the nanowires have two emission bands around 510 and 590 nm, which originate from the intrinsic transitions of CdS cores and the amorphous Si shells, respectively.     

Synthesis of single crystal CuAlS2 nanowires via a low temperature direct polyol route

The direct polyol method is used to prepare copper aluminium sulfide CuAlS₂ nanowires. The lattice constants of CuAlS₂ nanowires calculated from powder X-ray diffraction data indicates chalcopyrite structure. The CuAlS₂ nanowires are uniformly in shape and their dimensions are about 50 nm in diameter and several of micrometers in length. The Photoluminescence (PL) spectrum of CuAlS₂ nanowires show a sharp absorption edge at 358.96 nm and a strong near band edge emission at 3.46 eV. The direct energy gap Eg of the sample has been calculated as 3.48 eV, that corresponding to the photoluminescence study. A possible formation mechanism of copper aluminium sulfide is proposed.     

Electronic and magnetic properties of multishell Co nanowires coated with Cu

The structural, electronic, and magnetic properties of ultrathin Cu-coated Co nanowires have been studied by using empirical genetic algorithm simulations and a tight-binding spd model Hamiltonian in the unrestricted Hartree-Fock approximation. For some specific stoichiometric compositions, Cu atoms occupy the surface, while Co atoms prefer to stay in the interior, forming the perfect coated multishell structures. The outer Cu layers lead to substantial variations in the magnetic moment of interior Co atoms, depending on the structure and thickness of Cu layers. In particular, single Co atom row at the center of nanowire is found to be nonmagnetic when coated with two Cu layers. All the other Co nanowires in the coated Cu shell are still magnetic but the magnetic moments are reduced as compared with Co nanowires without Cu coating. The interaction between Cu and Co atoms induces nonzero magnetic moment for Cu atoms.     

The effect of 3-aminopropyltrimethoxysilane on the formation of NiFe2O4/SiO2 nanocomposites

NiFe₂O₄/SiO₂ nanocomposites were prepared using a sol–gel method with the addition of 3-aminopropyltrimethoxysilane (APS). Different phases and morphologies of NiFe₂O₄/SiO₂ nanocomposites were obtained when different amounts of APS were used. The structural properties of the products were examined by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Sheet-like morphology was observed at higher molar ratio of APS to NiFe₂O₄, while spherical NiFe₂O₄/SiO₂ nanoparticles separated from each other were formed at lower molar ratio of APS to NiFe₂O₄. The magnetic properties of the nanocomposites were also investigated, indicating that the interparticle interactions exhibit strong dependence on the molar ratio of APS to NiFe₂O₄.     

The fluorescence rate of a single molecule close to a spherical metallic nanoparticle

The theoretical study of fluorescence rate of a single molecule close to a spherical metallic nanoparticle is presented. The dielectric function of the metallic nanoparticle and its polarizability is analyzed when the radii of the nanoparticle is rather small. Based on dipole–dipole model, the distance dependence of the excitation rate, radiation rate, nonradiation rate and quantum yield of the emitter molecule are derived out. The results show that the quantum yield is rather small at the vicinity of the metallic nanoparticle surface.     

Laser-assisted growth of gold nanoparticles: Shaping and optical characterization

We demonstrate that supported gold nanoparticles with a given well-defined shape can be produced by laser-assisted growth. For this purpose, gold nanoparticles with average radii ranging from 1.5 to 13 nm, i.e., coverage between 0.45 × 10¹⁶ and 5.6 × 10¹⁶ atoms/cm², were prepared at room temperature by self-assembly of atoms deposited on quartz and sapphire substrates. For analysis of the samples, the optical spectra of the particles were measured with p-polarized light and photon energies in the range of 1.3 to 3.1 eV. Irradiating the particles during growth with laser light of different wavelengths to stimulate surface plasmon excitation made it possible to stabilize mean axial ratios between 0.19 and 0.98. The influence of the laser fluence on the shape of the nanoparticles was also investigated and shows that the position of the surface plasmon resonance shifts to higher energies as the fluence rises. Optimum growth conditions to shape gold nanoparticles with axial ratios close to unity (spheres) with a relatively low laser fluence of 60 ± 5 mJ/cm² have also been found. The results of our experiments show that laser-assisted growth is a powerfultechnique to control the shape of nanoparticles.     

Silicon nanowires grown from Au-coated Si substrate

Amorphous Si nanowires were grown on an Au-coated Si substrate by heat treatment at 1000 °C under an H₂ atmosphere. The nanowires have a length of several tens of a micron and a diameter of 10–20 nm. The growth mechanism of the nanowires was investigated and explained with a solid–liquid–solid model. Received: 11 July 2002 / Accepted: 7 July 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +86/10-62751615, E-mail: yudp@pku.edu.cn     

Large-scale synthesis of neodymium hexaboride nanowires by self-catalyst

Large scale NdB₆ nanowires have been successfully fabricated for the first time using a self-catalyst method with Nd powders and boron trichloride (BCl₃) gas mixed with hydrogen and argon. X-ray diffraction, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) were used to characterize the samples. Transmission electron microscopy (TEM) reveals that the NdB₆ nanowires are single crystals with cubic structure. Our investigation forms part of a series of studies for finding comparatively inexpensive methods to prepare RB₆ nanomaterials.     

Molecular dynamics simulation of the solidification of liquid gold nanowires

The solidification behavior of liquid gold nanowires with about 1.84 nm in diameter has been studied by using molecular dynamics simulation with an embedded atom potential. It is found the cooling rate has great effect on the final structure of the gold nanowires during solidification from liquid. With the decrease of cooling rates, the final structure of the gold nanowires varies from amorphous to crystalline via helical multi-shelled structure. The face-centered cubic structure of the gold nanowires is proven energetically the most stable form.     

Freezing behavior of one-dimensional copper nanowires

The freezing behavior of copper nanowires at different cooling rates was studied by using molecular dynamics (MD) simulation via embedded atom methods (EAM) potential. The simulation results indicate that the structure of the nanowires changed from amorphous to crystalline via helical multi-shelled structure with decreasing cooling rates. The curves of the energy-temperature and the local clusters were used to study the phase transition. The variation of local clusters implies that the order of the system increases as the temperature drops. The fcc crystalline structure of the copper nanowires with the diameter of 1.63 nm was proved to be the most stable form.     

A Mechanism for the Self-Organization of Colloidal Gold Nanoparticles

We report a two-stage mechanism of formation of the two-dimensional surperlattices in colloidal gold nanoparticles. The first stage is the formation and growth of holes. When the film is thinner than the “pancake“ thickness and because the solvent volatility holes nucleate and grow, the dry solid surface is exposed. The second stage corresponds to the reorganization of the colloids in the impacted areas between several holes, in which the particles gathering along the rims order due to the van der Waals-type attraction between colloids and the confinement by the length of dodecanethiol. Then at a compromised distance and perfect position giving the minimum free energy, ordering occurs in small domains along the boundary of the hole. When the ordered domains become closer because of the hole growth, they rotate at certain angles and the further ordering appears.     

Surface defect states of CdTexS1 − x quantum dots

Properties of surface defect states of CdTe_xS_1 − x quantum dots with an average diameter of 7 nm are investigated experimentally. The stoichiometric ratio is found to be for by use of the energy dispersive analysis of x-ray. The photoluminescence spectrum, the photoluminescence excitation spectrum, and the surface passivation are adopted to characterize the properties of surface defect states. The energy levels of surface defect states of CdTe_xS_1 − x quantum dots are also determined.     

High-quality GaN nanowires synthesized using a CVD approach

High-quality GaN nanowires were synthesized on a large-area Si substrate by direct reaction of gallium with ammonia using InCl₃ as a catalyst. The morphology and microstructure of the resulting products were characterized using a field-emission scanning electron microscope (SEM), a high-resolution transmission electron microscope, and X-ray diffraction (XRD). XRD and electron diffraction revealed that the nanowires are of a hexagonal GaN phase with the wurtzite structure. The SEM study showed that the nanowires are straight and have a smooth morphology with lengths up to 500 μm. The present results reveal that InCl₃ is an optimal catalyst in GaN nanowire production. Received: 2 April 2002 / Accepted: 12 April 2002 / Published online: 19 July 2002     

EXAFS study on ultrafine Ni-Co-B amorphous catalysts

\chem{\chemvar{A}C_{60}} with , , . From the temperature dependent line shift of we have extracted information about the electron density distribution on the molecules. The data also support the existence of an like bonding type between the molecules. The magnetic phase transition observed at low temperature in and is monitored by an increase of the second moment as well as an increase of the spin lattice relaxation rates of the and alkali nuclei. The results are compared to NMR data of where no signs of a magnetic ordering are found. The magnetically ordered phases are discussed in terms of antiferromagnetic spin density wave. We also report on NMR measurements of orthorhombic one dimensional and rhombohedral two dimensional polymers of obtained under high pressure. By spinning the samples up to , we are able to identify two resonances at and for the orthorhombic polymer and six resonances at , , , , and for the rhombohedral one. The static distortion of the molecules induced by the transformation under pressure must be at the origin of the observed inequivalent carbons in both systems. The NMR lineshape simulations of the obtained spectra are compatible with the suggested polymeric structures where the molecules are connected by cycloadditions. Received: 10 October 1996/Accepted: 13 November 1996     

Shape tailoring of hexagonally ordered triangular gold nanoparticles with nanosecond-pulsed laser light

In this contribution recent results on selective and precise tailoring of triangular gold nanoparticles (NPs) using ns-pulsed laser light are presented. The NPs were prepared by nanosphere lithography and subsequently tailored with ns-pulsed laser light using different fluences and wavelengths. The method is based on the size and shape dependent localized surface plasmon polariton resonance (SPR) of the NPs. We will demonstrate that the gap size between triangular NPs can be tuned from approximately 102±14 nm to 122±11 nm, due to a shape change of the NP from triangular to oblate. These morphological changes are accompanied by a significant shift of the surface plasmon resonance from λ_SPR=730 nm to λ_SPR=680 nm. Most importantly if the laser wavelength is chosen such that the dipolar SPR is excited, the hexagonal order of the NPs remains intact after irradiation, in contrast to excitation via the quadrupole SPR or within the interband transition. A tuneable gap size and the conservation of the hexagonal order of the NP array is the precondition for applications, where the NPs should serve as anchor points, e.g. for functional molecular nanowires, which can be used to utilize molecular devices.     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Temperature-induced line broadening, line narrowing and line shift in the luminescence of nanocrystalline ZnS:Mn

Emission spectra of nanocrystalline ZnS:Mn²⁺ have been recorded as a function of temperature between 4 and . For the two emission bands observed (a defect-related emission band around and a Mn²⁺-related emission band around ), the temperature dependence of the width and the position of the emission bands has been analyzed. The shift and the broadening of the Mn²⁺ emission can be satisfactorily explained by theoretical models and parameters for electron-phonon coupling that are similar to those for bulk ZnS. For the defect-related emission, the shift to lower energies follows the decrease in bandgap of bulk ZnS with increasing temperature. The width of the defect-related emission decreases as the temperature is raised. This anomalous behavior is explained by inhomogeneous broadening at low temperatures.     

Catalytic and non-catalytic growth of amorphous silica nanowires and their photoluminescence properties

A simple method based on the thermal oxidation of Si wafers in presence of a mixture of MgO and graphite powder was developed for large-scale synthesis of very long amorphous silica nanowires. The synthesis was done with and without gold as the catalyst. Almost aligned uniform nanowires with diameters within 60-90 nm and length up to few hundred micrometers were obtained using gold as the catalyst while bicycle chain like nanowires were obtained in absence of the catalyst. The growth sequence of the nanowires was observed through scanning electron microscope. Both forms of the nanowires emitted blue lights at 414 nm (3 eV) under excitation at 250 nm.     

Optical and nonlinear optical properties of copper nanocomposite glasses annealed near the glass softening temperature

Copper nanocomposite glasses have been prepared by the ion-exchange method, and annealed at different temperatures up to and above the glass softening temperature. The absorption spectra, fluorescence spectra, and nonlinear optical transmission of the samples at 532 nm for nanosecond laser pulses, have been investigated. The optical and nonlinear optical properties of the glasses are found to be distinctly different below and above the glass softening temperature. For instance, thermal annealing up to the glass softening temperature makes the samples behave like saturable absorbers, while annealing at higher temperatures makes them behave like optical limiters. Such flexibility in controlling the optical nonlinearity in these materials makes them potential candidates for photonic applications.     

Patterned aluminum nanowires produced by electron beam at the surfaces of AlF3 single crystals

Cubic- and rectangular-shape single crystals of α-AlF₃ in sizes of 5-50 μm have been synthesized by a solid-vapor phase process. Using the electron beam induced decomposition of AlF₃, a method is demonstrated for fabricating patterned aluminum nanowires in AlF₃ substrate in a scanning electron microscope. By controlling the accelerating voltage, the beam current and scanning time, it is possible to fabricate metallic nanowires of different sizes. The aluminum nanowires may act as nano-interconnects for nanoelectronics. This work demonstrates a potential technique for e-beam nanofabrication.