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PubMed2003

Mesoporous polyhedral cages and shells formed by textured self-assembly of ZnO nanocrystals.

We report a new structure, mesoporous structured polyhedral drum and spherical cages and shells formed by textured self-assembly of ZnO nanocrystals, which are made by a novel self-assembly process during epitaxial surface oxidation. The cages/shells exhibit unique geometrical shapes, and their walls are composed of mesoporous and textured ZnO nanocrystals. The structures of the cages and shells are studied, and a growth mechanism is proposed to be a process following solidification of the Zn liquid droplets, surface oxidation, and sublimation.

Journal Article↗
PubMed2003

Asymmetric dimers can be formed by dewetting half-shells of gold deposited on the surfaces of spherical oxide colloids.

Asymmetric dimers consisting of gold microcrystals and spherical silica colloids have been fabricated by depositing thin films of gold onto the spherical colloids to form half-shells, followed by annealing at elevated temperatures. The capability and feasibility of this procedure have been demonstrated with silica and titania beads of 0.2-2 mum in diameter and gamma-Fe2O3/polystyrene@SiO2 core-shell particles 0.5 mum in size. The dimensions of gold microcrystals could be conveniently varied in the range of 100-650 nm by controlling the thickness of gold films and/or the diameter of the spherical colloids. This method provides another route to asymmetric dimers made of colloidal particles that could be different in size, chemical composition, surface functionality, density or sign of surface charge, bulk property, or a combination of these properties.

Journal Article↗
PubMed2003

Synthesis of face-centered tetragonal FePt nanoparticles and granular films from Pt@Fe2O3 core-shell nanoparticles.

This paper describes a new approach for making face-centered tetragonal (fct) FePt nanoparticles with a diameter of 17 nm and granular films from Pt@Fe2O3 core-shell nanoparticle precursors. The core-shell nanoparticles were converted to fct FePt through a reduction and alloy formation process at enhanced temperatures. The Fe and Pt elemental analysis was conducted on both individual nanoparticles and granular films using energy-dispersive X-ray (EDX) spectroscopy. Our convergent evidence from selected area electron diffraction (SAED), powder X-ray diffraction (PXRD), and EDX analysis indicates that the final products are fct FePt alloys. The fct FePt films have coercivities of 8.0-9.1 kOe at 5 K and 7.0 kOe at 300 K measured by a SQUID magnetometer. These values depend on the conversion temperatures of Pt@Fe2O3 nanoparticles. Unlike the previously synthesized disordered face-centered cubic (fcc) FePt nanoparticles with diameters of 4-6 nm (Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989), the FePt nanoparticles presented in this work not only possess the preferred fct phase but also are in a size range that is expected to be ferromagnetic and have high coercivity, which is important to the practical applications in ultrahigh density data storage media and magnetic nano devices.

Journal Article↗
PubMed2004

Determination of the bioavailability of biotin conjugated onto shell cross-linked (SCK) nanoparticles.

Shell cross-linked nanoparticles (SCKs) presenting surface- and bioavailable biotin functional groups were synthesized via a mixed micelle methodology, whereby co-micellization of chain terminal biotinylated poly(acrylic acid)-b-poly(methyl acrylate) (PAA-b-PMA) and nonbiotinylated PAA-b-PMA were cross-linked in an intramicellar fashion within the shell layer of the mixed micelles, between the carboxylic acid groups of PAA and the amine functionalities of 2,2'-(ethylenedioxy)diethylamine. The hydrodynamic diameters (D(h)) of the micelles and the SCKs with different biotinylated block copolymer contents were determined by dynamic light scattering (DLS), and the dimensions of the SCKs were characterized with tapping-mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The amount of surface-available biotin was tuned by varying the stoichiometric ratio of the biotinylated PAA-b-PMA versus the nonbiotinylated PAA-b-PMA, as demonstrated with solution-state, binding interaction analyses, an avidin/HABA (avidin/4'-hydroxyazobenzene-2-carboxylic acid) competitive binding assay, and fluorescence correlation spectroscopy (FCS). The avidin/HABA assay found the amount of available biotin at the surface of the biotinylated SCK nanoparticles to increase with increasing biotin-terminated block copolymer incorporation, but to be less than 25% of the theoretical value. FCS measurements showed the same trend.

Acrylates↗
PubMed2005

Highly ordered arrays of metal/semiconductor core-shell nanoparticles with tunable nanostructures and photoluminescence.

Most of the approaches so far in fabricating core-shell nanoparticles (CSNs) are based on wet-chemical methods. It is usually difficult to achieve highly ordered CSN arrays on substrates from such a wet-chemical method. In this work, highly ordered indium oxide coated indium CSNs, with a structure-dependence photoluminescence, are fabricated on Si substrates using a three-step oxidation process. By controlling the three-step oxidation process, the volume ratio of the oxide shell to the whole CSN can be adjusted continuously from 0 to 1, which results in fine-tuning of the intensity and peak-shift of the photoluminescence from the CSNs. Our work is based on a dry oxidation method for fabricating CSNs, which is capable of achieving highly ordered CSN arrays with tunable nanostructures and optical properties.

Journal Article↗
PubMed2004

Formation of giant solvation shells around fulleride anions in liquid ammonia.

Here, we measure the solvation structure of fulleride C605- anions in potassium ammonia solution using neutron diffraction. We find a very strong solvation structure consisting of two shells of ammonia densely packed around the anion. The system's structure is driven by the propensity of ammonia molecules to direct one of their hydrogen bonds to the center of the anion while retaining axial hydrogen bonding within the shells. This permits high concentrations of solvent separated fulleride anions.

Journal Article↗
PubMed2005

Magnetically responsive carboxylated magnetite-polydipyrrole/polydicarbazole nanocomposites of core-shell morphology. Preparation, characterization, and use in DNA hybridization.

Novel bis-heterocyclic mono- and dicarboxylated dipyrrole and dicarbazole monomers have been synthesized in a modular manner. Their oxidative polymerization around magnetite nanosized particles has been investigated and optimized toward new magnetic magnetite-polydipyrrole/polydicarbazole nanocomposites (NCs) of a core-shell morphology. These NCs were thoroughly characterized by FT-IR, TGA (Thermal Gravimetric Analysis), low- and high-resolution TEM/HR-TEM microscopies, and Mössbauer spectroscopy along with magnetization studies. Exploiting the versatile COOH chemistry (activation by water-soluble diimides) introduced by the polymeric shell, DNA hybridization experiments have been conducted onto NC surfaces using an efficient blue-colored HRP-based enzymatic screening biological system. Highly parallel NC-supported DNA hybridization experimentations revealed that these NCs presented an interesting potential for DNA-based diagnostic applications.

Biocompatible Materials↗
PubMed2005

Inorganic oxide core, polymer shell nanocomposite as a high K gate dielectric for flexible electronics applications.

Organic/inorganic core shell nanoparticles have been synthesized using high K TiO(2) as the core nanoparticle, and polystyrene as the shell. This material is easy to process and forms transparent continuous thin films, which exhibit a dielectric constant enhancement of over 3 times that of bulk polystyrene. This new dielectric material has been incorporated into capacitors and thin film transistors (TFTs). Mobilities approaching 0.2 cm(2)/V.s have been measured for pentacene TFTs incorporating the new TiO(2) polystyrene nanostructured gate dielectric, indicating good surface properties for pentacene film growth. This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.

Electronics↗
PubMed2006

Enhanced magnetic properties of self-assembled FePt nanoparticles with MnO shell.

Self-assembled FePt/MnO nanoparticles with different morphology and size were synthesized with a polyol process. With the MnO coating, FePt nanoparticles exhibit a high blocking temperature and magnetic moment. The low-temperature hysteresis loop of FePt nanoparticles can be shifted through the AFM pinning of the MnO shell. The aggregation of FePt nanoparticles during the L10 phase transformation can be significantly decreased by coating with the MnO shell.

Iron↗
PubMed2006

Tandem synthesis of core-shell brush copolymers and their transformation to peripherally cross-linked and hollowed nanostructures.

Core-shell brush copolymers were prepared on the basis of a tandem synthetic strategy and used as single molecular templates for the preparation of polymeric nanomaterials. An alkoxyamine-functionalized norbornene monomer was prepared and then polymerized by ring-opening metathesis polymerization. The well-defined polymer (Mn = 122 kDa, Mw/Mn = 1.13) contained one alkoxyamine functionality per repeat unit and was then used as a polyfunctional macroinitiator for sequential nitroxide-mediated radical polymerizations of isoprene and tert-butyl acrylate. The resulting well-defined brush copolymer (Mn = 1410 kDa, Mw/Mn = 1.23) was transformed to an amphiphilic core-shell brush copolymer comprising poly(isoprene)-b-poly(acrylic acid) grafts by hydrolysis. Subsequent cross-linking of the poly(acrylic acid) block segments afforded peripherally cross-linked brush copolymer nanostructures, which served, finally, as templates for hollowed nanoscale frameworks by ozonolysis of the poly(isoprene)-based cores. Each transformation led to dramatic changes in the nanoscale composition and structure which were detected by combinations of spectroscopic measurements, atomic force microscopy imaging in the solid state, and/or dynamic light-scattering characterization in aqueous solution.

Journal Article↗
PubMed2005

Ni-V2O5.nH2O core-shell nanocable arrays for enhanced electrochemical intercalation.

We have prepared Ni-V(2)O(5).nH(2)O core-shell nanocable arrays for Li(+) intercalation applications. Ni-V(2)O(5).nH(2)O nanocables were prepared via formation of Ni nanorod arrays through the template based electrochemical deposition, followed by coating of V(2)O(5).nH(2)O on Ni nanorods through electrophoretic deposition. Transmission electron microscopy (TEM) micrograph clearly shows the Ni core was covered completely by a V(2)O(5).nH(2)O shell. Electrochemical analysis demonstrates that in a current density of 1.6 A/g, the Li(+) intercalation capacity of Ni-V(2)O(5).nH(2)O nanocable array is approximately 10 times higher than that of single-crystal V(2)O(5) nanorod array and 20 times higher than that of sol-gel-derived V(2)O(5) film. Both energy density and power density of such nanocable-array electrodes are higher than the V(2)O(5) film electrode by at least 1 order of magnitude. Such significant improvement in electrochemical performance is due to the large surface area and short diffusion path offered by the nanostructured V(2)O(5).nH(2)O.

Letter↗
PubMed2005

Characterization of a closed-shell fluorine-fluorine bonding interaction in aromatic compounds on the basis of the electron density.

A bond path linking two saturated fluorine atoms is found to be ubiquitous in crowded difluorinated aromatic compounds. The bond path is shown to persist for a range of internuclear distances (2.3-2.8 A) and a range of relative orientations of the two C-F internuclear axes. The F. . .F bonding is shown to exhibit all the hallmarks of a closed-shell weak interaction. The presence of such a bond path can impart as much as 14 kcal/mol of local stabilization to the molecule in which it exists, a stabilization that can be offset or even overwhelmed by destabilization of other regions in the molecule. Several other weak closed-shell interactions were also found and characterized including F. . .C, F. . .O, and C. . .C interactions, hydrogen bonding, dihydrogen bonding, and hydrogen-hydrogen bonding. This study represents another example of the usefulness and richness of the bond path concept and of the theory of atoms in molecules in general.

Journal Article↗
PubMed2005

Cylindrical droplet on nanofibers: a step toward the clam-shell drop description.

The existence and shape of a cylindrical (infinitely long) liquid drop on a fiber of arbitrary radius are examined using a microscopic approach based on the interaction potentials between the molecules of the system. A differential equation for the drop profile was derived by the variational minimization of the total potential energy by taking into account the structuring of the liquid near the fiber. This equation was solved by quadrature, and the existence conditions and the shape of the drop were examined as functions of the radius of the fiber, microscopic contact angle theta(0), which the actual drop profile makes with the fiber, and a certain parameter, a, which depends on the interaction potential parameters. Angle theta(0) is defined in the nanoscale range near the leading edge where the interactions between the liquid and solid are strong. It differs from the macroscopically measured wetting angle (theta(m)) that represents the extrapolation of the profile outside the range of liquid-solid interaction to the solid surface. Expressions for both theta(0) and theta(m) are established in the paper. For any given fiber radius, the range of drop existence involves two domains in the plane theta(0) - a, separated by a critical curve a = a(c)(theta(0)). In the first domain, below the curve a = a(c)(theta(0)), the drop always exists and can have any height, h(m). In the second domain, above the curve a = a(c)(theta(0)), there is an upper limit of h, h(m1), such that drops with h(m) > h(m1) cannot exist. The shape of the drop depends on whether the point (theta(0), a) on the theta(0) - a plane is far from the critical curve or near to it. In the first case, the drop profile has generally a circular shape, with the center of the circle not located on the fiber axis, whereas in the second case the shape is "quasi planar", that is, most of the drop profile lies on a circle concentric with the fiber profile. Comparing the potential energies of a cylindrical drop on a fiber and a film of uniform thickness covering the fiber and having the same volume as the drop, the energetically preferred configuration was determined for various conditions. Considering the cylindrical drop as a limiting case of a clam-shell one, and the film as a limiting case of a barrel drop, the obtained analytical results could be employed to examine the barrel-drop-clam-shell-drop transformation (roll-up transition).

Journal Article↗
PubMed2005

Inner-shell excitation spectroscopy and X-ray photoemission electron microscopy of adhesion promoters.

The C 1s, Si 2p, Si 2s, and O 1s inner-shell excitation spectra of vinyltriethoxysilane, trimethylethoxysilane, and vinyltriacetoxysilane have been recorded by electron energy loss spectroscopy under scattering conditions dominated by electric dipole transitions. The spectra are converted to absolute optical oscillator strength scales and interpreted with the aid of ab initio calculations of the inner-shell excitation spectra of model compounds. Electron energy loss spectra recorded in a transmission electron microscope on partly cured adhesion promoter, atomic force micrographs, and images and X-ray absorption spectra from X-ray photoemission electron microscopy of as-spun and cured vinyltriacetoxysilane-based adhesion promoter films on silicon are presented. The use of these measurements in assisting chemistry studies of adhesion promoters for electronics applications is discussed.

Adhesives↗
PubMed2005

An explicit quantum chemical method for modeling large solvation shells applied to aminocoumarin C151.

The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. The computer time used in this new implementation scales approximately linearly (with a low prefactor) with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved.

Journal Article↗
PubMed2005

Efficient synthesis and electronic studies of core-shell nanowires based on colossal magnetoresistive manganites.

We report our recent study on the pulsed laser deposition process in the synthesis of colossal magnetoresistive MgO/LaCaMnO3 and MgO/LaSrMnO3 core-shell nanowires. A highly efficient process has been developed by depositing an epitaxial layer of manganite onto randomly oriented MgO nanowires grown on SiO2/Si substrates. In addition, in-depth studies revealed that the sample-target distance played a critical role in determining the core-shell nanowire quality. The MgO/LaCaMnO3 and MgO/LaSrMnO3 nanowires opened up the unique opportunity to explore a number of intriguing physical properties at the nanoscale. Remarkable metal-insulator phase transitions and pronounced colossal magnetoresistance have been observed in both LaCaMnO3 and LaSrMnO3 nanostructures.

Journal Article↗
PubMed2005

Interactions in open-shell clusters: ab initio study of pre-reactive complex O(3P) + HCl.

Van der Waals interactions between the ground-state triplet O(3P) atom and the closed-shell HCl molecule are investigated in the pre-reactive region. Three adiabatic (two of A'' symmetry and one of A' symmetry) and four non-relativistic diabatic potential energy surfaces are obtained by combining a restricted open-shell coupled cluster approach with the multireference configuration interaction method. The lower A'' adiabatic potential surface has a single minimum (D(e) = 589 cm(-1)) for a linear O...HCl configuration. The upper A'' potential has a weak (D(e) = 65 cm(-1)) minimum for a linear HCl...O configuration. The A' adiabatic potential has a weak (124 cm(-1)) T-shaped minimum. Adiabatic potentials intersect once in the O...HCl linear configuration and twice in the linear HCl...O geometry. The role of electrostatic interactions in shaping these potentials is discussed. The effects of spin-orbit coupling on this interaction are also investigated assuming a constant value of the SO parameter.

Journal Article↗