Phase behavior of organic-inorganic crystal

We have investigated the structure and phase behavior of nonmolecularly layered silver stearate by means of temperature-dependent diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Upon heating the sample, remarkable spectral changes took place. The first phase transition took place that might be associated with a premelting event characterized by the formation of gauche conformers at 390-420 K. A second phase transition took place in which silver nanoparticles with a size of ∼4 nm were formed by thermal decomposition of silver stearate at 520-550 K. These silver nanoparticles, derivatized by stearate, were readily spread as a monolayer at air/water interface, and could be packed in 3-D assemblies by the Langmuir-Blodgett method. Received 29 November 2000     

Temperature dependence of photoluminescence from mono-dispersed Si nanoparticles

The temperature dependence of photoluminescence (PL) from mono-dispersed Si nanoparticles was studied from 4 to 300 K. Si nanoparticles produced by pulsed laser ablation in He background gas were sorted into the 6 nm size range by a differential mobility analyzer (DMA). The spread of the size distribution was narrowed to a geometrical standard deviation _g = 1.05. On decreasing the temperature from 300 to 4 K, the intensity of the PL spectra increased gradually, peaked at about 60 K, and then decreased rapidly. The temperature dependences of the intensity and the full width at half maximum (FWHM) on the PL spectra are discussed in terms of radiative and nonradiative decay rates.     

Size dependence of the plasmon peak position in electron stimulated photon emission spectra of Ag clusters supported on amorphous carbon film

The plasmon energy of Ag clusters produced on an amorphous carbon substrate by gas-aggregation technique has been measured. It has been determined from the plasmon peak position in the light emission spectrum obtained during bombardment of Ag clusters by low-energy electrons. For Ag cluster films with maximum of the cluster size distribution at 30, 8 and 2.5 nm, the plasmon energy comprised 3.76, 4.13 and 4.28 eV (the wavelength was 330, 300 and 290 nm), respectively. The blue shift of the plasmon energy is probably related to the effect of confounding of collective and single-particle excitations.     

Time evolution of Ag-Cu and Ag-Pd core-shell nanoclusters

The possibility of obtaining core-shell nanoparticles by depositing adatoms of a different element B above a cluster of an element A (system B/A) is studied by Molecular Dynamics simulations. We consider the four cases Ag/Cu, Cu/Ag, Ag/Pd and Pd/Ag, which present very different behaviours, and investigate whether it is possible to build up of well-defined core-shell structures.     

Controlled synthesis of CdS nanowires using diamines

We demonstrate a new, simple, inexpensive process and systemic control over CdS nanowires using sulfur powders and organic diamines without any catalysts, surfactants, and templates under atmospheric benchtop conditions. By changing the kinds of amines and reaction temperatures can result in control of the shape and size of the nanocrystals, which are moderately monodispered with unique forms.     

Non-crystalline structures in the growth of silver nanoclusters

The growth of nanometer-size free Ag clusters is studied by Molecular Dynamics simulations. The morphology transition from the icosahedron at the magic size of N = 55 atoms to the Marks truncated decahedron at N = 75 is analyzed in details, in order to single out kinetic trapping and entropic effects. At very low T, the cluster is kinetically trapped in an icosahedral structure. At intermediate T the transition takes place sharply at N≃ 65. At higher T, the transition is smeared out and finally, around 550 K no transition is found because the 75 decahedron is melted. Received 28 November 2000     

69,71Ga NMR studies of a superconducting Ga 84 cluster compound

We present ^69,71Ga-NMR experiments on microcrystalline samples of the recently discovered supramolecular compound Ga ₈₄ [ N ( SiMe ₃ ) ₂ ] ₂₀ Li ₆ Br ₂ ( thf ) ₂₀ ^. 2 toluene, which is composed of ligand-coordinated Ga₈₄ metal clusters, packed together in a fully ordered crystalline matrix. The compound is highly conducting and even shows superconductivity below T _c ~ 7.2 K. Our preliminary results between 10-300 K show a metallic-like behavior: the nuclear spin-lattice relaxation rate T ₁ ^-1 follows the Korringa law ⁶⁹ ( T ₁ T ) ^-1 = 0.36 s ^-1 K ^-1 , but with a relaxation rate approximately three times smaller than in bulk -Ga metal. No quantum-size effects are observed, the Korringa law being followed down to 10 K, whereas the quantum-gaps for individual clusters should amount to ~ 10 ³ K. These results therefore suggest a transport process based on intermolecular charge transfer, similar as in alkali-doped fullerenes and silicon-clathrates.     

Electronic structure of dendrimer-encapsulated Au nanocluster

We have carried out optical and X-ray photoemission studies of the dendrimer-encapsulated Au nanoclusters. The dendrimer-encapsulated Au nanoclusters are prepared by the chemical reduction of Au ions loaded within the dendrimer templates. Photoluminescence spectrum of the dendrimer-encapsulated Au nanoclusters with diameter of about 1.0 nm shows the visible luminescence centered at about 2.8 eV. In addition, we have measured the nanocluster-size dependent photoemission spectra in the valence-band region. From line shape analysis of Au 4f X-ray photoemission spectra, Au 4f core-level spectra of the dendrimer-encapsulated Au nanoclusters reflect the size dependent chemical-states. From these results, we discuss electronic structures and chemical states of the dendrimer-encapsulated Au nanoclusters.     

Titania from nanoclusters to nanowires and nanoforks

A novel method - inverse microemulsion has been developed not only for synthesizing low cost TiO₂ nanoclusters but also for the first time preparing titania nanowires and nanoforks with rutile structure of single crystal. With two microemulsion systems, spherical TiO₂ nanoclusters of 5 nm in average diameter are produced. These nanoclusters are amorphous and turned into anatase at an annealing temperature lower than 750 ^°C, and changed into rutile when annealed at higher temperature. When three microemulsions with TiCl₄, ammonia and NaCl as aqueous phase, are used, the precursor powder containing Ti(OH)₄/NaCl with molar ratio of 1000 are annealed at 750 ^°C and then TiO₂ rutile nanowires with 22 nm in thickness and 4 m in length are formed. At the same time two kinds of nanoforks with defined boundary structures are constructed: one is a bent wire composed of two straight whiskers related by twinning on a (101) plane with the angle of 114^° between the two legs, and the other by twinning on a (301) planes with the angle of 55^° between the legs. Screw dislocations and a periodic structure are found in (301) twin boundary, while edge dislocations are observed in (101) twin boundaries.The experiments demonstrate that the titania rutile nanowires are formed through solid state phase transformation and sodium chloride play an important role in the process.     

Preparation of Cu 2 O nanoparticles dispersed in NH 2 -terminated polyethyleneoxide matrix

Small Copper (I) oxide, Cu₂O, nanoparticles dispersed in diamine-terminated polyethyleneoxide (PEO-NH₂) matrix have been successfully prepared by vacuum evaporation of copper onto the molten PEO-NH₂. The obtained composite were characterized by TEM, electron diffraction, TG-DTA and FT-IR spectroscopy. The stable composite, in which the Cu₂O nanoparticles are stabilized through interaction between NH₂ chain end groups of PEO molecules and Cu₂O nanoparticles was obtained when the samples were heat-treated at 110 ^° C. The mean size of the Cu₂O nanoparticles increased from 2.5 to 3.5 nm in diameter upon increasing the amount of initial Cu deposition. The obtained composite material having a waxy texture was soluble in many solvents without aggregation and can be handled as a simple chemical compound for starting material in various applications. Received 29 November 2000     

Electronic properties of transition-metal-atom doped Si cage clusters

We present a density-functional study of electronic structures of convex-caged Si clusters doped with transition-metal (TM) atoms. First, we show the reason for their peculiar geometries in terms of interplay among the electron orbitals of Si and TM atoms. Then we describe the potential ability of the clusters to serve as charge sources to other objects such as Si crystal surfaces. Millennium Research for Advanced Information Technology (MIRAI) Project.     

Vibrational spectroscopy of large water clusters of known size

The structure of large water clusters of known size distributions 〈n〉 = 20-2000 is investigated by vibrational spectroscopy of the OH stretch mode. The water clusters are predissociated by a pulsed tunable infrared optical parametrical oscillator (OPO) in the frequency range 2800-3800 cm^-1. Their fragments are detected off-axis by electron impact ionization and mass analyzed by a quadrupole mass spectrometer. The largest ion signal stems from the neutral water hexamer fragment. The ion yield is investigated at certain wavelengths while the size of the clusters is varied, and for certain sizes complete absorption spectra of the OH stretch motion are measured. Fingerprints of the different coordination types of the water molecules in the clusters are found and it turns out that our method is especially sensitive to amorphous structures with frequencies shifting in the range of 3300-3400 cm^-1.     

Production of gold nanoparticles-polymer composite by quite simple method

Recently, production methods of metal nanoparticles have been investigated extensively, not only for a research use in laboratory, but also for an industrial use. However, it is difficult to obtain metal nanoparticles in high amounts and concentrations with simple methods. In this study, a gold nanoparticle-polymer composite was prepared with a simple procedure using a gold salt and a melted polymer. The composite, which is in a wax state at room temperature, was highly soluble in water and lower alcohols, moreover the composite was melted at about 50 ^°C.     

Fabrication and structural characterization of nanocomposites consisting of Ni nanoparticles dispersed in polyimide films

Formation and structure of composite layer consisting of polyimide films containing Ni nanoparticles were investigated. The preparation method relies on KOH treatment on polyimide film to form carboxyl acid groups and adsorption of Ni ions by ion exchange followed by hydrogen reduction. The amount of Ni ions adsorbed in polyimide films were found to be systematically controlled by changing initial KOH concentration, subsequent ion exchange time, pH and temperature. Cross-sectional TEM observation revealed that Ni nanoparticles with 3-5 nm in diameter were homogeneously dispersed in the surface modified polyimide layer after heat treatment above 250 ^°C in H₂ atmosphere. The size and distribution of the Ni nanoparticles were strongly dependent on the heat treatment temperature, indicating that this method allows microstructural tuning of metal/polymer nanocomposites.     

Structure and magnetism on iron oxide clusters Fe nO m ( n = 1-5): Calculation from first principles

We have studied structural and magnetic properties in small iron oxide clusters, Fe_nO_m (n = 1-5), by means of the first-principles calculation based on the density functional theory. We have used not only the usual spin polarized scheme, but also the scheme for noncollinear magnetism to carry out efficient optimization in magnetic structure. The result of FeO_m (m = 1-4) is in good agreement with the previous work. We found the stable adduct clusters in FeO₅ and FeO₆. The bridge site of oxygen atom is more favorable in energy than any other site for the clusters of Fe_nO (n = 2-5). As increasing the number of oxygen atoms, the alignment of Fe magnetic moments changes from ferromagnetic configuration to antiferromagnetic one at Fe_nO_n (n = 2-4). Received 10 September 2002 Published online 3 July 2003     

Effect of epitaxial strain on the atomic structure of Pd clusters on MgO(100) substrate

Morphology and atomic structure of supported Pd clusters on MgO(100) substrate are investigated theoretically using a mixed approach: a semi-empirical potential for the metal bonding within the cluster and a potential fitted to ab initio calculations for the metal-oxide interaction. We find that the clusters adopt a truncated pyramidal morphology in agreement with experimental results. The detailed study of the epitaxial relation as a function of cluster size shows the existence of a critical size around 3 nm where elastic strain due to the misfit between the substrate and the deposit is released by the introduction of interfacial dislocations.     

Silicon-riched-oxide cluster assembled nanostructures formed by low energy cluster beam deposition

Free beam of silicon oxide nanoclusters is produced by a gas aggregation source from SiO precursor. Due to the disproportionation reaction during the condensation of SiO vapor the generated clusters are Si-riched. The clusters are collimated to be a fine beam and deposited on the substrate at room temperature. The microstructures of the cluster-based nanofilm are characterized by TEM. It is shown that with appropriate impacting parameters, Si-riched oxide nanofilms assembled from uniformly distributed isolated clusters can be obtained. And the clusters can self-organize into partially densely ordered packing within local domains. XPS spectra are taken to analyze the chemical components of the nanofilms. Photoluminescence from the Si-riched oxide nanofilms has also been observed.     

Photoemission spectra of deuterated silicon clusters: experiment and theory

We compare experimentally measured and ab initio computed photoelectron spectra of negatively charged deuterated silicon clusters ( , 4m10, 0n2) produced in a plasma environment. Based on this comparison, we discuss the kinetics and thermodynamics of the cluster formation and the effect of deuterium on the geometrical and electronic structure of the clusters.     

Energetics and electronic properties of BN nanocones with pentagonal rings at their apexes

The geometric structures, energetics and electronic properties of the recently discovered BN nanocones are investigated using first-principles calculations based on the density-functional theory. We have proposed one particular structure for BN nanocones associated with the 240^° disclination, derived by the extraction of four 60^° segments, presenting as characteristic four pentagons at the apex and termination in two atoms. The cones are simulated by three clusters containing 58 B plus N atoms and additional 12 H atoms to saturate the dangling bonds at the edge. The most stable configuration is obtained when the two terminating atoms are one B and one N. For the cases where the two terminating atoms are of the same kind, the tip with B atoms is determined to have lower binding energy than with N atoms. The local densities of states of these BN nanocones are investigated and sharp states are found in the regions close (below and above) to the Fermi energy. Received 14 October 2002 / Received in final form 6 December 2002 Published online 11 February 2003 RID="a" ID="a"e-mail: ppiquini@smail.ufsm.br     

Silver diffusion and precipitation of nanoparticles in glass by ion implantation

Silver particles in soda-lime glass, less than 10 nm in size, were prepared by ion implantation. The implantation dose was in the range of 0.5 to 2×10¹⁶ Ag ions/cm² and the beam current density was varied from 0.5 to 2A/cm². Here, the beam current density strongly influences ion diffusion and particle precipitation as well as compressive stress generation around the particles due to thermal effects resulting from the deceleration of silver ions. Stress relaxation can be achieved by increased dose rates or thermal processing at elevated temperatures. Based on RBS and HREM results, a possible route to homogeneous distribution of Ag nanoparticles within the glass is discussed with respect to their interesting optical properties.