Small Silver Particles Research Articles

Electromagnetic force on a metallic particle in the presence of a dielectric surface

By using a method, previously established to calculate electromagnetic fields, we compute the force of light upon a metallic particle. This procedure is based on both Maxwell's Stress Tensor and the Couple Dipole Method. With these tools, we study the force when the particle is over a flat dielectric surface. The multiple interaction of light between the particle and the surface is fully taken into account. The wave illuminating the particle is either evanescent or propagating depending an whether or not total internal reflection takes place. We analyze the behaviour of this force on either a small or a large particle in terms of the wavelength. A remarkable result obtained for evanescent field illumination, is that the force on a small silver particle can be either attractive or repulsive depending on the wavelength. This behaviour also varies as the particle becomes larger.

An Application of the Water/Supercritical CO2 Microemulsion to a Novel “Microreactor”

Abstract Water/supercritical CO2 microemulsions were prepared by using ammonium carboxylate perfluoropolyether (PFPE) as a surfactant. The de-aggregation of the J-aggregate of cyanine dyes and the formation of fairly small silver particles in these microemulsions would indicate the realization of the chemical reactions in the small space. In this sense, the water/supercritical microemulsion is regarded as a novel “microreactor” of the nanometer size.

Reaction of bony tissue to implanted silver glass ionomer and a reinforced zinc oxide-eugenol cement

To evaluate the tissue responses to implants of Ketac Silver and Super EBA cement in the guinea pig mandible. Sixteen guinea pigs were used for 2 experimental periods of 4 and 12 weeks. Both materials were placed in Teflon applicators and implanted into surgically prepared sites in the mandible. A histologic examination for reaction to the material occurred after the animals were killed and processed. After 4 weeks, minimal inflammatory reactions were observed in Ketac Silver implants, whereas the Super EBA implants showed minimal to moderate inflammation. Localized foreign body reaction was present in areas of fragmented small particles of Ketac Silver. At 12-weeks observation, no inflammatory reactions were present around either material. Direct bone contact was observed in 1 Ketac Silver implant. Ketac Silver and Super EBA cement elicited mild reactions under the conditions of this model. From a biologic point of view, these 2 materials may offer equal utility in endodontic surgery.

Photon emission spectroscopy of individual oxide-supported silver clusters in a scanning tunneling microscope

Photon emission spectra of individual alumina-supported silver clusters have been measured for the first time. The light emission stimulated by electron injection from the tip of a scanning tunneling microscope can be assigned to the (1,0) mode of the Mie-plasmon resonance in small silver particles. As cluster sizes decrease, the resonance position shifts to higher energies and the linewidth increases. In the size range examined (1.5-12 nm), intrinsic size effects are discussed as possible origins for the observed size dependence of the Mie resonance.

Role of Poly(amidoamine) Dendrimers for Preparing Nanoparticles of Gold, Platinum, and Silver

Nanoparticles of gold, platinum, and silver were prepared by reduction of their metal salts with NaBH4 in the presence of poly(amidoamine) dendrimers. The dendrimers used were generation 3, 4, and 5 having surface amino group (G3, G4, and G5) as well as generation 3.5, 4.5, and 5.5 having surface carboxyl group (G3.5, G4.5, and G5.5). The metal nanoparticles obtained were characterized by UV−vis spectroscopy and transmission electron microscopy. By using G3−G5, gold nanoparticles in the 1.5−4.0 nm size regime were obtained where their size decreased with increasing concentration of the dendrimers as well as the generation of the dendrimers. Similarly, platinum nanoparticles with a diameter of 2.4−3.0 nm were obtained, and their size was insensitive against the concentration as well as the generation of G3−G5. In the case of silver nanoparticles, very small silver particles using G3.5−G5.5 were obtained. In addition, the dendrimer concentration required for obtaining stable nanoparticles was found to be dependent on the interactions between metal nanoparticles and the dendrimers: when the ratio of [surface group of dendrimer]/[metal salt] was above unity, nanoparticles of gold and silver were obtained, while stable platinum nanoparticles were only obtained above the ratio of 40/1. From infrared spectroscopy it is suggested that the metal nanoparticles adsorb on the exteriors of the dendrimers.

Field enhancement effect of small Ag particles on the fluorescence from Eu3+-doped SiO2 glass

We have investigated the effects of nanometer-sized silver particles on the optical properties of Eu3+ ions in SiO2 glass. Glass samples were prepared by the sol-gel method. The mean particle size and volume fraction of silver particles, which were estimated from UV-VIS absorption spectra using Mie-Drude theory modified by electron mean free path model, rapidly increased with the reduction time of Ag+ up to ∼3 min at 900 °C. The fluorescence from Eu3+ ions for the excitation by N2 laser was greatly enhanced in the presence of silver particles of 4.3 nm size. Our experimental results suggest that the origin of enhanced fluorescence is from local field enhancement around Eu3+ ions, owing to the surface plasmon resonance of small silver particles.

Optical spectroscopy of arsenic- and silver-containing sol-gelcoatings

Sol-gel silica coatings doped with 1 mol% silver and/or 1 mol% arsenic oxide have been investigated by photoluminescence (PL) and optical absorption (OA) spectroscopy. The presence of Ag+ ions in the silica host has been monitored by recording a luminescence peak located between 320 and 330 nm upon excitation with 228 nm light, whereas the formation of small particles of metallic silver has been assessed by recording the absorption band centred at about 405 nm. The luminescence peak has been related to the d10d9 s parity-forbidden transitions in Ag+, which are partially allowed by odd-phonon assistance. On the other hand, the absorption peak at about 405 nm arises from the well known surface-plasmon resonance of silver particles. Coating densification under various atmospheres gives rise to significant effects on the PL and OA spectra. Results indicate that, after coating densification in air, most of the silver appears as Ag+ ions, in contrast to coating densification under a 90% N2-10% H2 atmosphere, which favours the formation of small particles of metallic silver. The presence of arsenic oxide in the silver coatings densified in air has been found to improve the stabilization of Ag+ ions, so that partially prevents the formation of colloidal silver under reducing atmospheres.

Scattering and extinction of evanescent waves by small particles

Expressions have been obtained for the total cross sections for extinction and scattering of evanescent waves by small spherical particles. Due to the different structure of p- and s-polarized waves, cross sections for both extinc- tion and scattering of p-polarised evanescent waves exceed those for s-polarised waves, as long as the Mie coefficients for magnetic multipoles are smaller than those for electric mul- tipoles for each multipolar order, i.e. for sufficiently small particles. The difference increases with the angle of incidence of the totally reflected wave. The definition of cross sections for evanescent-wave excitation with variable intensity over the cross-sectional area of the particle allows quantitative comparison with the case of plane-wave excitation. For ex- ample, within the dipole approximation the cross sections for plane-wave excitation lie between those for p- and s-polarised evanescent-wave excitation, respectively. Due to the inhomo- geneity of the evanescent field higher multipole contributions are strongly enhanced as compared to plane-wave excitation and increase further with the angle of incidence, resulting in corresponding changes in the scattering and extinction spec- tra. These effects are demonstrated in the specific case of scattering of evanescent waves by small silver particles.

Growing Small Silver Particle as Redox Catalyst

Growing small particles of silver have been observed to be more efficient catalysts than stable colloidal particles. These growing particles catalyze the borohydride reduction of several organic dy...

Formation Mechanisms and Aggregation Behavior of Borohydride Reduced Silver Particles

In this work, we examine the formation mechanisms of nanoscale silver particles produced by the reduction of silver perchlorate with sodium borohydride. Evidence is presented that the reaction pathway does not follow classical nucleation and growth theory, but is dominated by colloidal interactions. Upon injection of silver into a sodium borohydride solution, a molecular species absorbing at 220 nm is produced in less than 1 s. We suggest that this species contains borohydride and small particles of reduced silver. The reaction mixture is initially dark as the result of the aggregation of the small silver particles into larger particles which have broad absorption spectra. During an “intermediate” stage, transmission electron microscopy and absorbance data show that even larger (∼6−10 nm) particles grow at the expense of the “monomeric” silver particles. Later in the reaction, electrochemical potential measurements show that the borohydride concentration suddenly decreases. Direct measurement of interparticle forces demonstrate that this change in the solution conditions drives the particle surface potential toward zero and results in increased adhesive forces. The resulting aggregation manifests itself in a darkening of the solution color. At low temperatures this corresponds to a 10 times increase in the particle size whereas, at high temperatures, the increase is minimal. This effect can be linked to the number of monomeric silver particles remaining during the final transition.

THERMOCHROMIC PROPERTIES OF SILVER COLLOIDS EMBEDDED IN SIO2 GELS

By using the sol-gel process, colloids are prepared by the spontaneous reduction of silver ions, where the presence or absence of ethanol plays an important role. Thermochromic properties are observed in these gels in the temperature range between 15°C and 150°C. In the UV-Vis region the transmission spectrum red shift of the cut-off is employed to study these properties as a function of the temperature; a shift from 300 to 750 nm is observed. Originally, a broad shoulder centered at 435 nm is seen in gels prepared in the absence of ethanol and at 425 nm with ethanol, corresponding to surface plasmon resonances of the small silver particles in the colloids. These particles are responsible for the various colors observed in the gels. Additionally, the influence of temperature on the color stability, as well as on the influence shown by ethanol on the rate of displacement of the transmission cut-off wavelength, are studied.

The features of superatomic structure evolution in silver azide by radiolysis

Some details of the superatomic structure evolution in silver azide during its decomposition under the action of soft X-ray radiation are revealed by Small Angle X-ray Scattering techniques (SAXS). The functions of particle size distribution are computed from SAXS intensity curves. It is found that several types of inhomogeneities are formed in the process of decomposition: density fluctuations on atomic scale (probably silver centers), small silver particles with bimodal size distribution and their aggregates.

Formation of amalgam particles in bimetallic silver and mercury colloids

AbstractWe report results gained in the transmission electron microscopy studying of silver and mercury colloids in an attempt to find the alloying behaviours of the two metals in the solution. Amalgam particles are formed, but aggregate and recrystallize to participates. While a large amount of mercury ions are reduced the small silver particles in the solution “disappear”. Then only relative large amalgam particles exist which sediment after some hours.

Influence of surface state on the optical absorption of isolated nanoscale silver particles

The measurement of optical absorption was made on the nanoscale silver particles dispersed in silicon oil and silicon oil with added dressing agent. An absorption peak appears in the nanoscale silver particles dispersed in silicon oil, but the absorption peak disappears when the dressing agent is added. The interaction between external molecules and the surface of small silver particles seems to have strong influence on the absorption peak of small silver particles.

Spectroelectrochemistry of Colloidal Silver

The spectroelectrochemical response of small silver particles was studied in aqueous solution using an optically transparent, thin layer electrode. The position of the surface plasmon band of the colloidal silver was found to depend on the applied electrode potential. It varied from 400 nm in air, corresponding to a redox potential of +0.15 V vs Ag/AgCl, to about 392 nm at −0.6 V vs Ag/AgCl. A value of 80 ± 10 μF cm-2 for the double-layer capacitance of the silver−water interface was obtained on the basis of the spectroelectrochemical shift. The equilibration kinetics of the particles with the electrode obeyed the Cottrell equation. However, the number of electrons transferred at each particle−electrode encounter was found to be potential dependent and reached 1600 ± 300 at potentials more negative than −0.4 V vs Ag/AgCl. The evidence suggests that this particle charging current occurs via electron tunneling across the particle and electrode double layers and not by contact electrification. Changes in the...

Preparation and optical absorption peak of small metal particles dispersed in oil

Small silver and copper particles respectively dispersed in silicon oil were well prepared by a developed vacuum evaporation on running liquid technique. The optical absorption peak resulting from the surface plasmon of the small silver and copper particles was observed, along with its position shifts to shorter wavelengths with decreasing particle size of silver.

SIZE AND SHAPE MANIPULATION OF SILVER AND INDIUM SMALL PARTICLES BY ELECTRON-BEAM IRRADIATION

Small particles of indium or silver were encapsulated in a thin polymer film matrix by simultaneous plasma polymerization and metal evaporation. Electron-beam irradiation inside transmission electron microscopes and with a microfocus electron source was used to induce changes of the encapsulated particle size and shape. At encapsulated indium particles, substantial microstructural changes were observed during the electron-beam irradiation in the electron microscope. Selected area diffraction demonstrates that indium oxide was formed during the electron irradiation. Additional in situ annealing demonstrates that the indium melting point was not reached during electron-beam-induced local heating of the indium particles. At electron-beam irradiation of plasma polymer films with encapsulated silver particles by using a microfocus electron source, the coalescence of the silver particles can be limited to the irradiated areas of the films.

PHOTOINDUCED FORMATION OF AGGREGATED SILVER PARTICLES FROM SILVER SALT OF POLYSACCHARIDE

The thin film of silver salt of alginic acid is available for synthesis of small silver particles and aggregates embedded in the polymer matrix. Variations in the Raman spectrum of the silver alginate film with UV irradiation have been examined. A Raman signal at 1398–1405 cm−1 corresponding to symmetric COO− stretching vibration of silver alginate has been increased, manifesting the contribution of surface-enhanced Raman scattering (SERS).

Erratum: Low-frequency Raman scattering from small silver particles embedded in SiO2 thin films

Erratum: Low-frequency Raman scattering from small silver particles embedded in SiO2 thin films

Characterization of large supported metal clusters by optical spectroscopy

Small sodium and silver particles were generated on dielectric substrates like LiF, quartz and sapphire under ultrahigh vacuum conditions. The optical transmission spectra of the clusters were measured as a function of cluster size and shape, for low and high substrate temperatures as well as for s- and p- polarization of the incident light. Excitation of dipolar surface plasmon oscillations in the directions normal and parallel to the substrate surface could be identified. Furthermore, optical spectra for Na and Ag clusters were calculated with the classical Mie theory. The measured spectra vary strongly if the experimental conditions are changed and can be exploited, for example, to characterize the particles with regard to their size and shape. In particular, the axial ratio of the spheroidal clusters could be determined. Its value is considerably different for the two investigated metals and depends on the substrate material. Furthermore, the temperature of the substrate has a pronounced influence on the shape of the particles. At low temperature of T=100 K two-dimensional island growth is predominant. The particles extend only little in the direction perpendicular to the surface and coalesce readily at small coverage of metal atoms. In contrast, the clusters are truly three-dimensional at T=300 K. At this stage, sodium particles still exhibit a rather small axial ratio whereas silver clusters appear almost spherical. Thus, measurements of the optical spectra permit direct in situ monitoring of cluster growth during the nucleation of adsorbed atoms and of temperature induced shape variations. In addition to investigations of the shape of the particles, the quadrupolar surface plasmon mode was observed for Ag clusters.