Showing papers on "Nanoparticle published in 1997"

Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering

Abstract: Optical detection and spectroscopy of single molecules and single nanoparticles have been achieved at room temperature with the use of surface-enhanced Raman scattering. Individual silver colloidal nanoparticles were screened from a large heterogeneous population for special size-dependent properties and were then used to amplify the spectroscopic signatures of adsorbed molecules. For single rhodamine 6G molecules adsorbed on the selected nanoparticles, the intrinsic Raman enhancement factors were on the order of 10 14 to 10 15 , much larger than the ensemble-averaged values derived from conventional measurements. This enormous enhancement leads to vibrational Raman signals that are more intense and more stable than single-molecule fluorescence.

Novel hydrophilic chitosan-polyethylene oxide nanoparticles as protein carriers

Abstract: Hydrophilic nanoparticulate carriers have important potential applications for the administration of therapeutic molecules. The recently developed hydrophobic-hydrophilic carriers require the use of organic solvents for their preparation and have a limited protein-loading capacity. To address these limitations a new approach for the preparation of nanoparticles made solely of hydrophilic polymers is presented. The preparation technique, based on an ionic gelation process, is extremely mild and involves the mixture of two aqueous phases at room temperature. One phase contains the polysaccharide chitosan (CS) and a diblock copolymer of ethylene oxide and propylene oxide (PEO-PPO) and, the other, contains the polyanion sodium tripolyphosphate (TPP). Size (200–1000 nm) and zeta potential (between +20 mV and +60 mV) of nanoparticles can be conveniently modulated by varying the ratio CS/PEO-PPO. Furthermore, using bovine serum albumin (BSA) as a model protein it was shown that these new nanoparticles have a great protein loading capacity (entrapment efficiency up to 80% of the protein) and provide a continuous release of the entrapped protein for up to 1 week. © 1997 John Wiley & Sons, Inc.

Nanosized Particles Made in Colloidal Assemblies

Abstract: In this feature article, syntheses of nanosized particles by using colloidal assemblies as a template are described We asked ourselves the following question: What parameters play a role in the control of the size, shape, and polydispersity? We know that parameters such as the shape of colloidal assemblies, the hydration of the head polar group, the water molecules bounded to the interface, etc play a major role However, there are a number of exceptions preventing any generalization It is shown that the chemical mechanism in nanoparticles production in colloidal assemblies can differ from those usually observed in homogeneous solution This shows that the solution chemistry cannot always be transferred to colloidal systems It is possible to select the size and markedly reduce the polydispersity by a surface treatment of the nanoparticles This favors formation of mono- and multilayers made of nanoparticles, and it is found that these particles form crystals organized in a three-dimensional face-cent

Finite Size Effects in Antiferromagnetic NiO Nanoparticles

Abstract: Antiferromagnetic NiO nanoparticles exhibit anomalous magnetic properties, such as large moments, and coercivities and loop shifts of up to 10 kOe. This behavior is difficult to understand in terms of 2-sublattice antiferromagnetic ordering which is accepted for bulk NiO. Numerical modeling of spin configurations in these nanoparticles yields 8-, 6-, or 4-sublattice configurations, indicating a new finite size effect, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the particle. The relatively weak coupling between the sublattices allows a variety of reversal paths for the spins upon cycling the applied field, resulting in large coercivities and loop shifts.

Synthesis of Highly Monodisperse Silver Nanoparticles from AOT Reverse Micelles: A Way to 2D and 3D Self-Organization

Abstract: A simple method is used to prepare highly monodispersed silver nanoparticles in the liquid phase, which starts from an initial synthesis in functionalized AOT reverse micelles. To narrow the particle size distribution from 43% to 12.5% in dispersion, the particles are extracted from the micellar solution. The size-selected precipitation method is used. The decrease in polydispersity of the silver nanoparticles is followed by transmission electron microscopy, by UV−vis spectroscopy, and by small-angle X-ray scattering. The nanocrystallites dispersed in hexane are deposited on a support. A monolayer made of nanoparticles with spontaneous hexagonal organization is observed. The immersion of the support on the solution yields to the formation of organized multilayers arranged as microcrystal in a face-centered cubic structure.

Experimental Evidence of the Néel-Brown Model of Magnetization Reversal

Abstract: Presented are the first magnetization measurements of individual ferromagnetic nanoparticles (15--30 nm) at very low temperatures (0.1--6 K). The angular dependence of the hysteresis loop evidenced the single domain character of the particles. Waiting time, switching field, and telegraph noise measurements showed for the first time that the magnetization reversal of a well prepared ferromagnetic nanoparticle can be described by thermal activation over a single-energy barrier as originally proposed by N\'eel and Brown. The ``activation volume'' estimated by these measurements was close to the particle volume.

Electrostatic trapping of single conducting nanoparticles between nanoelectrodes

Abstract: For molecular electronics, one needs the ability to electrically address a single conducting molecule. We report on the fabrication of stable Pt electrodes with a spacing down to 4 nm and demonstrate a new deposition technique, i.e., electrostatic trapping, which can be used to bridge the electrodes in a controlled way with a single conducting nanoparticle such as a conjugated or metal–cluster molecule. In electrostatic trapping, nanoparticles are polarized by an applied electric field and are attracted to the gap between the electrodes where the field is maximum. The feasibility of electrostatic trapping is demonstrated for Pd colloids. Transport measurements on a single Pd nanoparticle show single electron tunneling coexisting with tunnel-barrier suppression.

Synthesis of cadmium sulphide superlattices using self-assembled bacterial S-layers

Abstract: Methods for organizing materials at the nanometre scale have advanced tremendously in recent years1,2. One important objective is the synthesis of patterned arrays of inorganic nanocrystals3,4,5,6, whose optical, electronic and magnetic properties might find technological uses, for example as memory elements. Techniques such as colloidal crystallization7,8,9, monolayer deposition10,11,12, multilayer casting13, molecular crosslinking14,15, the use of complementary interactions16,17 and the synthesis of nanoparticles in patterned etch pits18 have been used to organize nanocrystals into superlattices. Here we describe the use of bacterial S-layers — self-assembled, two-dimensionally ordered films of proteins that feature in many bacterial cell walls — as templates for the in situ nucleation of ordered two-dimensional arrays of cadmium sulphide nanocrystals about 5 nm in size. Nucleation of the inorganic phase is confined to the pores between subunits in the S-layers. Two-tier stacks of nanoparticles can be formed in the presence of double-layered protein crystals. The structural diversity of S-layers19,20, their ease of self-assembly on a wide range of substrates and the potential for surface chemical modification suggest that this approach could be exploited to offer a wide range of ordered nanoparticle arrays.

Purification and size‐selection of carbon nanotubes

Abstract: We report on a non-destructive method for the separation and size-selection of carbon nanotubes and nanoparticles produced by arc discharge. A liquid-phase separation of nanotubes and nanoparticles was performed by filtering a kinetically stable colloidal dispersion consisting of the carbon material in a water/surfactant solution, allowing thus to extract the nanotubes from the suspension while leaving the nanoparticles in the filtrate. Further purification was accomplished by size-selection through controlled floculation of the dispersion. Final separation yields as high as 90 % in weight were obtained without any damage to tube tips or tube walls following the separation process.

Electron Dynamics of Passivated Gold Nanocrystals Probed by Subpicosecond Transient Absorption Spectroscopy

Abstract: The electronic dynamics of gold nanocrystals, passivated by a monolayer of alkylthiol(ate) groups, were studied by transient spectroscopy after excitation with subpicosecond laser pulses. Three solution-phase gold samples with average particle size of 1.9, 2.6, and 3.2 nm with size distribution less than 10% were used. The photoexcitation in the intraband (surface plasmon region) leads to the heating of the conduction electron gas and its subsequent thermalization through electron−electron and electron−phonon interaction. The results are analyzed in terms of the contribution of the equilibrated “hot” electrons to the surface plasmon resonance of gold. A different spectral response was observed for different sizes of gold nanoparticles. The results were compared to the dynamics of the large (30 nm diameter) gold nanocrystals colloidal solution. The size-dependent spectral changes are attributed to the reduction of the density of states for small nanoparticles. The observed variation in the kinetics of the ...

Synthesis, Characterization, and Nonlinear Optical Properties of Copper Nanoparticles

Abstract: In this paper, copper nanoparticles were prepared by the reduction of copper(II) acetate in water and 2-ethoxyethanol using hydrazine under reflux. The synthesized nanoparticles exhibit a distinct absorption peak in the region 572−582 nm. The average size variation from 6.6 to 22.7 nm in ethoxyethanol and from 15.5 to 30.2 nm in water was achieved by the addition of various amounts of a protective polymer (poly(N-vinylpyrrolidone)). The nonlinear optical properties of the copper colloids were first measured using the Z-scan technique. The χ(3)/α0 values obtained were found to be of the magnitude of 10-11−10-12 esu cm, which are in good agreement with the reported values obtained for copper nanoparticles embedded in glass.

Structural Characterization of Carbon-Supported Platinum−Ruthenium Nanoparticles from the Molecular Cluster Precursor PtRu5C(CO)16

Abstract: We describe the preparation and structural characterization of carbon-supported Pt−Ru nanoparticles with exceptionally narrow size and compositional distributions. The supported bimetallic particles are obtained by reduction of the neutral molecular carbonyl cluster precursor PtRu5C(CO)16 with hydrogen. A detailed structural model of the nanoparticles has been deduced on the basis of studies by in situ extended X-ray absorption fine structure spectroscopy (EXAFS), scanning transmission electron microscopy, microprobe energy-dispersive X-ray analysis, and electron microdiffraction. These experiments show that the bimetallic nanoparticles have a Pt:Ru composition of 1:5 and an average diameter of ca. 1.5 nm and adopt a face-centered cubic closest packing structure. These results demonstrate a marked sensitivity of the metal particle structure to nanoscale size effects inasmuch as the thermodynamically stable phase for bulk alloys of this composition is hexagonal close-packed. The local metal coordination en...

Novel catalysis of gold deposited on metal oxides

Abstract: Gold exhibits a unique catalytic nature and action when it is deposited as nanoparticles on a variety of metal oxides. Most reactions are noticeably structure sensitive over such supported gold catalysts. Typical examples obtained in Japan of the low-temperature catalytic combustion, partial oxidation of hydrocarbons, hydrogenation of carbon oxides and unsaturated hydrocarbons, reduction of nitrogen oxides, and so forth, are presented. Another line of advance is also introduced in the thin films of gold metal oxide composites, which are applicable to electrical and optical gassensing.

Sonochemical preparation of bimetallic nanoparticles of gold/palladium in aqueous solution

Abstract: Colloidal dispersions of bimetallic nanoparticles composed of gold and palladium were prepared by a sonochemical method, in which Au(III) and Pd(II) ions in an aqueous solution of sodium tetrachloroaurate(III)dihydrate and sodium tetrachloropalladate(II) were reduced by ultrasound irradiation in the presence of sodium dodecyl sulfate (SDS). In addition to the stabilizing effect, SDS remarkably enhanced the reduction rate, probably due to the thermal decomposition that occurs at the interfacial region between cavitation bubbles and bulk solution and provides reducing radicals. Transmission electron microscopy (TEM) photographs showed spherical particles whose size had a fairly narrow distribution with a geometric mean diameter about 8 nm and a geometric standard deviation of 1.1. Analyses with UV−vis spectra indicated that Au(III) ions were first reduced and after their consumption reduction of Pd(II) ions set in. A core−shell structure of the particles, a core of gold and a shell of palladium, was confirm...

Optical and Photochemical Properties of Nonstoichiometric Cadmium Sulfide Nanoparticles: Surface Modification with Copper(II) Ions

Abstract: Nonstoichiometric cadmium sulfide nanoparticles ([Cd2+]/[S2-] = 3) in 2-propanol were surface-modified with Cu2+ ions. Addition of copper(II) perchlorate to CdS nanoparticles leads to binding of copper ions onto the surface of the semiconductor, accompanied by rapid reduction of Cu2+ to Cu+, as confirmed by EPR and absorption spectra. Copper(II) perchlorate also quenches the recombination luminescence of CdS nanoparticles effectively. The quenching data obey a static interaction model, which confirms the binding of copper ions onto CdS. The latter was confirmed also by ultrafiltration and ICP spectroscopy. Copper ions bound onto the surface of CdS lead to formation of a new, red-shifted, luminescence band. The maximum of the new band is at 14 700 cm-1 compared to that of the original band at 17 900 cm-1. It is suggested that, at low copper ion concentrations, copper ions bound onto the surface of CdS nanoparticles exist as isolated Cu+ ions. They create a new energy level in the bandgap at about 1.2 eV be...

Cluster assembled materials: a novel class of nanostructured solids with original structures and properties

Abstract: The low-energy cluster beam deposition technique (LECBD) is applied to produce cluster assembled films with hitherto unknown nanostructured morphologies and properties. Neutral clusters having the very low energy gained in the supersonic expansion at the exit of the inert gas condensation-type source are deposited without fragmentation upon impact on the substrate. Depending on the deposition conditions (nature, size and flux of incident clusters, nature and temperature of the substrate, vacuum conditions), granular nanostructures resulting from the diffusion and coalescence of supported clusters are obtained with materials of any type (covalent or metallic). A critical size for coalescence limits the supported grain size and, finally, highly porous thick films growing by random stacking of nanoparticles are obtained. A recent model developed by combining several dynamical processes simultaneously occurring on the substrate (deposition - diffusion - aggregation, DDA) is used to simulate the cluster assembled film morphology in good agreement with the experimental observations. Examples of novel materials obtained by LECBD are presented to illustrate the interesting potentialities of the technique. In the case of covalent materials such as carbon and silicon, 'amorphon'-type disordered structures, different from the conventional amorphous structures (a-C and a-Si), are obtained with some unique properties. With transition metal (Fe, Co and Ni) cluster assembled films, a specific magnetic behaviour, resulting from the competition between the intrinsic properties of the grains (magnetocrystalline anisotropy) and the interactions between grains, is observed. Also, films of clusters embedded in various co-deposited matrices are produced in order to control the interactions between grains via the matrix materials (insulating, conducting ...). Interesting optical properties (from metallic clusters in ) or giant magnetoresistance effects (from Co clusters in silver) are reported for such systems, emphasizing the future role of LECBD in various fields of applications such as optical and optoelectronic nanostructures, magnetic and magneto-optic nanostructures and quantum devices.

Thiol-Derivatized Nanocrystalline Arrays of Gold, Silver, and Platinum

Abstract: A simple procedure has been developed to prepare thiol-derivatized nanoparticles of metals. The procedure involves the transfer of well-characterized metal hydrosols to a hydrocarbon medium containing the thiol. Thiol-derivatized nanoparticles Au, Ag, and Pt of near spherical shape forming nanocrystalline arrays have been characterized by X-ray diffraction, transmission electron microscopy (TEM), and other techniques. Thiol-derivatized Au particles of mean diameters of 1.0, 2.1, and 4.2 nm from nanocrystalline arrays show characteristic X-ray diffraction patterns. The nanoparticles exhibit Moire fringes when deposited on a flake of MoSe2, showing that each particle is in itself crystalline. Besides spherical nanoparticles, thiol-derivatized particles of other shapes have been prepared, hexagonal Pt nanoparticles being particularly novel. Scanning tunneling microscopy images not only confirm the size and shape of nanoparticles revealed by TEM, but also show evidence for thiol molecules on the surface.

Control of the shape of copper metallic particles by using a colloidal system as template

Abstract: In this paper we show that the use of colloidal assemblies as templates favors the control of the shape of nanoparticles. Cylindrical copper metallic particles having same size can be obtained in various parts of the phase diagram when the template is made of interconnected cylinders. A very low amount of cylinders (13%) is formed when the synthesis is performed in cylindrical reverse micelles. When the colloidal self-assembly is a mixture of several phases, various types of shapes can be obtained. In some cases, the polydispersity in size is so low that metallic particles are able to self-assemble in a hexagonal network. Multilayers can be observed and are arranged in a face centered cubic structure.

Fabrication, Characterization, and Optical Properties of Gold Nanoparticle/Porous Alumina Composites: The Nonscattering Maxwell−Garnett Limit

Abstract: We have been exploring the optical properties of nanoscopic gold particles prepared by electrochemically depositing Au within the pores of nanoporous alumina membranes. Maxwell−Garnet (MG) effective medium theory has been used as a guide for modeling the optical properties of these Au nanoparticle/alumina membrane composites. MG theory is, however, rigorously applicable only in the limit of metal nanoparticles with infinitesimally small diameters. As a result, the position of the plasmon resonance absorption predicted theoretically using MG theory was always blue-shifted relative to the position of the experimental absorption band. The smallest diameter particles investigated in this prior work had diameters of 60 nm. Au nanoparticles with smaller diameters (52 nm down to 16 nm) were prepared, and the optical properties of composites containing these nanoparticles are described here. The λmax values for the smallest diameter particles are essentially identical with the values predicted by MG theory. Hence...

Self-Organization into 2D and 3D Superlattices of Nanosized Particles Differing by Their Size

Abstract: Self-assembly of silver sulfide, Ag2S, nanoparticles is reported for different particles sizes. Monolayers of particles organized in a hexagonal network are formed over very large domains. Small or large aggregates can also be produced. The observations indicate that the formation of self-assemblies of nanoparticles strongly depends on the preparation conditions of the samples. It is also shown that in the aggregates the particles are highly organized and form pseudocrystals with a face-centered cubic structure for various particles sizes.

Plasma spray synthesis of nanomaterial powders and deposits

Abstract: Conventional plasma spraying was used to process atomized liquid droplets of precursor solutions to produce alumina, zirconia and yttria stabilized zirconia nanoparticles and deposits. An electrostatic precipitator collected the plasma synthesized ceramic particles at a rate of ~0.2 mg s −1 , with ~5–20% collection efficiency. Spray processing produced 1–50 nm size ceramic particles. The size, shape and phase composition of the nanomaterials depend on the spray feedstock. Organo-metallic precursors gave rise to a narrow range of fine-grained material, while aqueous solutions produced wider distributions of larger size grains. Spray processing of liquid feedstock produced nanodeposits with a powdery morphology. Plasma spraying of liquid precursors is a viable technique to produce nanoparticles and deposits.

New Technique for Synthesizing Iron Ferrite Magnetic Nanosized Particles

Abstract: Ferrous dodecyl sulfate, Fe(DS)2, micellar solution was used to make nanosized magnetic particles whose size is controlled by the surfactant concentration and by temperature. The average particle size varies from 3.7 to 11.6 nm, with a standard deviation ranging from 0.22 to 0.34. In contrast to what is obtained in homogeneous solution, iron ferrite particles can be obtained when the synthesis is performed at very low reactant concentrations and room temperature. Furthermore, nanoparticles are obtained when the syntheses are performed using Fe(II) as reactant whereas in homogeneous solution particles in the micrometric range are formed. The particle crystallinity varies with the synthesis temperature, going from fairly low values at 25 °C to fairly high values at 50 °C and above. The particles are characterized by superparamagnetic behavior. The saturation magnetization decreases with particle size, which is explained in terms of non-collinear structure at the interface. Particles with low crystallinity a...

Formation of γ-Fe2O3 Isolated Nanoparticles in a Silica Matrix

Abstract: Isolated nanometric particles (D < 30 nm) of γ-Fe2O3 in a silica matrix have been prepared by heating at 400 °C the gel formed in the hydrolysis of an ethanol solution of Fe(NO3)3·9H2O and tetraethylorthosilicate (TEOS). However, when FeCl3·6H2O was used as precursor, well-developed hematite particles were obtained in the final composite. This different behavior was already manifest in the initial gels. Thus, the gel obtained from iron nitrate salt shows a compact appearance as a result of its higher degree of network connectivity (polymeric gel) whereas the one from the iron chloride appears more loose and highly hygroscopic (colloidal gel). In addition, small superparamagnetic nuclei are formed during the hydrolysis and condensation of the gel obtained from the iron nitrate salt. The γ-Fe2O3 nanoparticle formation takes place through a reduction−oxidation reaction which occurs during the burning of the organic species trapped inside the gel pore. The growth mechanism of the γ-Fe2O3 nanoparticles in the ...

EXAFS Studies on the Size Dependence of Structural and Dynamic Properties of CdS Nanoparticles

Abstract: Size-dependent structural and dynamic properties of CdS nanocrystals with 13−120 A diameter and of molecular crystals consisting of three-dimensional superlattices of these nanoparticles have been determined by extended X-ray absorption fine structure spectroscopy (EXAFS) at the Cd K edge at temperatures between 5 and 290 K. It is shown that these properties are governed by the surface-to-volume ratio and the way of surface stabilization of the nanoparticles. Thiol-capped CdS nanoparticles with diameters from 13 to 40 A show an expansion of the mean Cd−S distance whereas mean interatomic distances in polyphosphate-stabilized particles with 30−120 A diameter are slightly contracted with respect to CdS bulk. By measuring the EXAFS spectra between 5 and 290 K, the total mean-square relative displacement could be separated into a static part which is independent of temperature and into a dynamic temperature-dependent part. The temperature-independent mean-square relative displacement (or static disorder) of t...